Past Earthquake-Induced Rapid Subsidence along the Northern San ...

Bulletin of the Seismological Society of America, Vol. 92, No. 7, pp. 2612–2636, October 2002

E

Past Earthquake-Induced Rapid Subsidence along the Northern San Andreas Fault: A Paleoseismological Method for Investigating Strike-Slip Faults by Keith L. Knudsen,* Robert C. Witter, Carolyn E. Garrison-Laney,† John N. Baldwin, and Gary A. Carver‡

Abstract Evidence of rapid relative sea level changes preserved in sediment of coastal estuaries along the north coast segment of the San Andreas fault provides information on the dates of past earthquakes. This approach, although successfully used to document large subduction zone earthquakes, has not been used previously to constrain the dates of San Andreas fault or other strike-slip fault earthquakes. Data on prehistoric San Andreas fault earthquakes is needed for development of robust probabilistic hazard assessments in northern California and the San Francisco Bay area. Evidence of earthquake-induced subsidence is preserved in marshes at the northern margin of Bolinas Lagoon and the southeastern margin of Bodega Harbor. These sites occupy structural basins or troughs along the northern San Andreas fault. Radiocarbon dating and identification of the first occurrence of nonnative pollen near abrupt sedimentological changes in cores are used to constrain the dates of San Andreas fault earthquakes over about the last 800 years. Estuarine sediment from northern Bolinas Lagoon preserves evidence of an earthquake that occurred about 750 years ago. Evidence for this earthquake includes a marsh soil that was abruptly buried by tidal flat mud or coarse, poorly sorted deltaic deposits containing a mixture of terrestrial sediment and marine mud. Evaluation of diatom assemblages from above and below the buried soil horizon provides evidence of at least several decimeters of sustained relative sea level rise. Three buried soils in a marsh at the southern end of Bodega Harbor are likely a result of coseismic subsidence. All three buried marsh soils have abrupt upper contacts, although diatom evidence for decimeters of sustained subsidence is not as strong as it is for the buried soil in Bolinas Lagoon. Existing paleoseismic data on the northern San Andreas fault is compared with results of this study in order to evaluate the dates of prehistoric large earthquakes and length of past ruptures. These data indicate that an earthquake occurred about 400 years ago and another earthquake occurred about 700 years ago. The data allow for 1906-type ruptures of the fault from the Santa Cruz Mountains to at least Point Arena. However, a sequence of closely timed, smaller earthquakes would produce a similar set of paleoseismic data. Online material: Tables of fossil diatom analyses. Introduction This article presents the results of the first paleoseismic investigation to employ stratigraphic, radiocarbon, and paleoecological analyses of tidal marsh sediment to assess vertical coseismic deformation due to prehistoric earthquakes *Present address: California Geological Survey, 185 Berry Street, Suite 210, San Francisco, California 94107 ([email protected]) †Present address: University of Washington, Department of Earth and Space Sciences, 63 Johnson Hall, Box 351310, Seattle, Washington 98195 ([email protected]) ‡Present address: 12021 Middle Bay Dr., Kodiak, Alaska 99615, ([email protected])

on strike-slip faults. The methods used in this study have been applied successfully in paleoseismic studies at estuaries within subduction zones such as the Cascadia subduction zone (e.g., Atwater, 1987; Peterson and Darienzo, 1996; Clarke and Carver, 1992; Nelson, 1992a,b; Clague and Bobrowsky, 1994; Mathewes and Clague, 1994), the Peru-Chile trench (e.g., Wright and Mella, 1963; Plafker and Savage, 1970; Nelson and Manley, 1992), and the Aleutian trench (e.g., Ovenshine et al., 1976; Plafker, 1990; Shennan et al., 1999). Widespread distribution of peaty marsh soils abruptly

2612

Past Earthquake-Induced Rapid Subsidence along the Northern San Andreas Fault

buried by lower intertidal mud provides evidence for coseismic subsidence (Darienzo and Peterson, 1990; Clague and Bobrowsky, 1994; Carver and McCalpin, 1996; Nelson et al., 1996; Atwater and Hemphill-Haley, 1997). Evidence for sudden coseismic uplift also may be indicated by rapid changes in lithology and diatom paleoecology (e.g., Bucknam et al., 1992; Mathewes and Clague, 1994). Fault rupture during the 1906 San Francisco earthquake on the northern San Andreas fault caused vertical coseismic deformation within Bolinas Lagoon and at Papermill delta in southern Tomales Bay (Lawson, 1908). G. K. Gilbert (Lawson, 1908) reported an average of 12 inches (⬃30 cm) of earthquake-induced subsidence northeast of the trace of the 1906 surface rupture at Pepper Island in Bolinas Lagoon and a maximum of 17 inches (⬃43 cm) of subsidence at the head of Tomales Bay in the Papermill delta. In this investigation, stratigraphic sequences of interbedded estuarine peat and mud at Bolinas Lagoon and Bodega Harbor (Fig. 1) were identified, described, and dated. Stratigraphic relations provide clear evidence of rapid relative sea level changes in these basins. These submergence events are inferred to be caused by prehistoric earthquakes along the northern San Andreas fault. Relative sea level rise may result from either tectonic movement or compaction of sediment by strong ground shaking. The age of the stratigraphic rapid burial sequence is used to provide age constraints on the dates of the last several pre-1906 earthquakes on the San Andreas fault. Paleoseismic information on the number and dates of large-magnitude, prehistoric earthquakes along the San Andreas fault in the northern San Francisco Bay region is sparse. Data from fault trenching investigations near Olema (Niemi and Hall, 1992), Dogtown (Cotton et al., 1982), Point Arena (Prentice, 1989), and Fort Ross (Noller et al., 1993; Simpson et al., 1996) (Fig. 1) suggest that the penultimate event on the northern segment of the San Andreas fault occurred during the seventeenth century. An earlier event probably occurred between the twelfth and fifteenth centuries based on trenching near Olema (Niemi, 1992; Niemi and Hall, 1992) and near Point Arena (Prentice, 1989; Baldwin, 1996). The paleoseismic data and interpretations presented herein contribute to a better understanding of the dates and recurrence of paleoearthquakes along the northern San Andreas fault. Data on the dates of past events are essential for assessing rupture segmentation of the northern San Andreas fault and for evaluating the probability and size of future earthquakes on the fault.

Tectonic Setting and Vertical Deformation Associated with the 1906 San Francisco Earthquake: Bolinas Lagoon and Bodega Harbor Stratigraphic evidence for coseismic vertical deformation exists at tidal marshes fringing Bolinas Lagoon and Bodega Harbor, both located within the northern San Andreas fault zone (Fig. 1). These shallow estuaries lie along the trace

2613

of the northern San Andreas fault and span approximately 55 km, comprising about 15% of the length of the 1906 MW 7.9 San Francisco earthquake rupture. Surface displacement during the 1906 earthquake occurred on the margins of these marshes and likely extended under each of the water bodies (Lawson, 1908). Bolinas Lagoon Bolinas Lagoon lies at the northwestern apex of an actively subsiding basin related to a complex right-stepping geometry of the San Andreas and San Gregorio faults on the Golden Gate platform (Zoback et al., 1999). The basin is bounded by the east boundary and west boundary faults that were identified by Galloway (1977) based on geologic mapping (Fig. 2). These graben-bounding faults are believed to have been active during the Holocene; however, the 1906 earthquake ruptured the surface along a fault located between the two graben-bounding faults (Fig. 2). Subsidence is controlled, in part, by a component of down-to-the-east dip slip on the 1906 rupture trace that was observed along the fault trace on Pepper Island, across the delta of Pine Gulch Creek and across the Bolinas spit (Fig. 2) (Lawson, 1908). Previous researchers have postulated the existence of an east boundary fault with down-to-the-west normal separation along the northeastern margin of Bolinas Lagoon (e.g., Galloway, 1977), within the lagoon (Bergquist, 1978), and south of the lagoon on the continental margin (Cooper, 1973; Zoback et al., 1999). The inferred east boundary fault has been proposed to control dip-slip displacement on the eastern side of the graben that forms Bolinas Lagoon. Evidence to support mapping an east boundary fault beneath Bolinas Lagoon comes from uniboom seismic reflection experiments (Bergquist, 1978) as well as high-resolution aeromagnetic data (plate 2 of Zoback et al., 1999). In addition, the presence of elevated Quaternary(?) coarse gravelly and cobbly terrace deposits east of Route 1 suggests that a downto-the-west normal fault lies west of the Quaternary terrace deposits. Examination of the terrace deposits suggests that they are in depositional contact with the underlying Franciscan Assemblage. The west boundary fault (Galloway, 1977) likely was mapped based on the alignment of topographic features, including east-facing scarps, saddles, and linear drainages. The west boundary fault lies along strike with offshore faults that extend to the southeast, which may be splays of the San Gregorio fault. Coseismic relative sea level rise associated with the 1906 earthquake is well documented at Bolinas Lagoon (Lawson, 1908). Surface rupture occurred on a strand of the San Andreas fault striking approximately N35⬚W along the west side of the lagoon (Fig. 2). Northeast of the surface rupture trace, measurement of vertical displacement on Pepper Island showed east-side-down subsidence of approximately 12 inches (⬃30 cm) (Lawson, 1908). This estimate was based on the average elevation difference across a vegetation lineament delineated by dead and dying Salicornia (pickleweed) that coincided with the surface trace of the

2614

K. L. Knudsen, R. C. Witter, C. E. Garrison-Laney, J. N. Baldwin, and G. A. Carver

Figure 1.

Tectonic map of northwestern California showing locations of the two study sites. Asterices denote previous paleoseismic trenching studies.

1906 fault rupture (Lawson, 1908). The spit bounding southern Bolinas Lagoon also was lowered by the earthquake such that waves more frequently overtopped the spit (Lawson, 1908). Sudden relative sea level rise caused by the 1906 earthquake also submerged colonies of Salicornia fringing McKennan Island (Fig. 2) by at least 10 inches (25 cm), which caused the plants to perish (Lawson, 1908). McKennan Island lies within 200 m of the inferred location of the eastern boundary fault, based on seismic reflection data (Bergquist, 1978). The fact that McKennan Island, near the east boundary fault, subsided nearly as much as Pepper Island, along the surface rupture trace, suggests that dip-slip displacement may have occurred on the east boundary fault during the 1906 earthquake. The amount of 1906 subsidence that occurred in the marsh at the north end of the lagoon,

where buried marsh stratigraphic sequences were identified during this project, is less constrained. G. K. Gilbert (Lawson, 1908) described stressed Salicornia (pickleweed) and abundant ground cracks at this marsh but did not estimate an amount of submergence or document that coseismic subsidence was the cause of stress on the marsh plants. G. K. Gilbert also investigated whether the block west of the 1906 surface rupture trace was uplifted during the 1906 event and concluded that evidence for such uplift was equivocal. Bergquist (1978) reported long-term tectonic subsidence and sea level rise within Bolinas Lagoon based on characterization of the latest Pleistocene and Holocene sediment beneath the spit. His calculations allow for 5.8–17.9 m of tectonic subsidence of early to middle Holocene deposits now located approximately 33 m below mean sea level.

2615


EXPLANATION Fault trace (from Galloway, 1976) Approximate trace of the 1906 surface rupture (from Brown and Wolfe, 1972) Approximate zone of coseismic subsidence in 1906 earthquake (interpreted from Lawson, 1908)

Landslides obstructed roads at head of lagoon following 1906 earthquake; “ground cracking” in marsh

Approximate locations of Pepper and McKennon Islands based on U.S. Coast Survey, 1854 (from Berquist, 1978)

s We

0

t

Area of Figure 5

1

2 km

da un Bo ry lt

u Fa

Pepper Island, average subsidence of 12 inches based on dead Salicornia following 1906 earthquake

Alluvium deposited by floods of March 1907 bury fault trace that crossed delta of Pine Gulch Creek

McKennon Island, at least 10 inches of subsidence observed following 1906 earthquake

Ea st “Echelon phase” and vegetation lineament characterize 1906 fault trace

Bo un d

? Sand spit frequently overtopped by waves following 1906 earthquake

“Clam patch” uplifted approximately 1 foot following 1906 earthquake

ar y Fa u

lt ? ?

Figure 2. Bolinas Lagoon, Marin County, California, and location of San Andreas fault traces. Shaded region represents approximate area of coseismic subsidence reported following the 1906 San Francisco earthquake. Fault traces are dashed where approximately located and dotted where inferred. Annotations on effects of earthquake interpreted from Lawson (1908). Bodega Harbor The trace of the San Andreas fault defined by the 1906 surface rupture strikes approximately N35⬚W across the northern margin of Bodega Harbor (Lawson, 1908; Koenig, 1963; Brown and Wolfe, 1972) (Fig. 3). To the south, the fault continues along the eastern edge of the harbor and intersects the marsh that was cored during this study. The tectonic framework that explains the structural low within Bodega Harbor is not well understood. Reconnaissance coring at three locations on the west side of the harbor (Fig. 3, locations 1, 2, and 3) bottomed in sands that are interpreted to be beach deposits because of the presence of shell fragments and pebbles. Because the environment of deposition has changed from a beach to the present fresh to slightly brackish marsh, the western margin of Bodega Harbor has

apparently experienced uplift or emergence. Also, uplifted Pleistocene marine terraces have been mapped on Bodega Head and the west side of the harbor, north of Bodega Head (Knudsen et al., 2000). Thus, reconnaissance investigations suggest tectonic emergence on the west side of the harbor. Local subsidence within the marsh at the southeastern edge of Bodega Harbor can be explained by a structural graben along the east margin of the harbor (Koenig, 1963). To the east, the marsh and harbor are bounded by a 5- to 10m-high, west-facing scarp, which parallels and is east of the probable trace of the 1906 surface rupture on the San Andreas fault (Fig. 3). A sequence of uplifted Pleistocene marine terraces lies east of the scarp (Helley et al., 1979; Knudsen et al., 2000). This scarp may have been formed by an eastern graben-bounding fault. West of the scarp and the

2616


0

0.5

1 km

18 inches of “uplift” observed west of 1906 surface rupture; likely due to right-lateral juxtaposition of topography

3

2

1

No biological evidence for vertical deformation found in Bodega Harbor

Bodega Marsh Study Site (Figure 8)

Geomorphic evidence of 1906 surface rupture obscured by shifting sand

“Craterlets” near east marsh provide evidence for liquefaction following 1906 earthquake

EXPLANATION

BAY GA

Approximate location of fault (from Brown and Wolfe, 1972; Koenig, 1963)

BODE

Surface trace of 1906 rupture (approximate) (from Koenig 1963; interpreted from Lawson, 1908) Quaternary landslide 1

Location of preliminary boring Steep linear scarp Spring

Bodega Head, California 7.5 minute quadrangle

Figure 3. Bodega Harbor, Sonoma County, California, and location of San Andreas fault traces. Fault locations from Huffman and Armstrong (1980), Koenig (1963), and Brown and Wolfe (1972). Fault traces are dashed where approximately located and dotted where inferred, barbs on downthrown side. Annotations on effects of 1906 surface rupture interpreted from Lawson (1908).


marsh, the inferred trace of the 1906 surface rupture crossed the sand spit that bounds the southern edge of Bodega Harbor (Fig. 3). Because drifting sand obscured any geomorphic evidence of surface rupture shortly after the 1906 earthquake (Lawson, 1908), the location of the 1906 fault trace is not well constrained in this part of Bodega Harbor. Previous measurements of coseismic and/or long-term vertical tectonic displacement are sparse in the area of Bodega Harbor. Based on biological surveys of the vertical distributions of mussels and barnacles within Bodega Harbor conducted in the months after the 1906 earthquake, there was no evidence for significant coseismic subsidence within the harbor (Lawson, 1908). Numerous sand craterlets attributed to liquefaction were observed around the margins of Bodega Harbor following the 1906 earthquake (e.g., plate 142 of Lawson [1908]), suggesting that subsidence related to liquefaction may have occurred. Late Pleistocene marine terraces east of the fault and on Bodega Head clearly show relative long-term uplift of the areas southwest and northeast of the harbor area.

Methods To investigate late Holocene, earthquake-induced, vertical deformation within estuaries along the northern San Andreas fault, detailed lithologic, microfossil, and radiocarbon analyses of stratigraphic sequences from cores of marsh sediment were performed. A 2.5-cm-diameter gouge core was used to map and identify buried marsh soils, conspicuous sand beds, and fluvial deposits. Larger 7.6-cm-diameter cores were obtained for detailed lithological description and sampling for diatom, pollen, and radiocarbon analyses. To obtain these larger cores, aluminum irrigation pipe was driven into the marsh with a sledge to depths of 2 to 3 m. Driving the cores into the marsh resulted in 20%–40% sediment compaction, mainly from the friction of the pipe with the core sediment. To compare the evidence of rapid relative sea level rise with changes that occur as a result of storms and extreme high tides, shallow cores were collected from sediment deposited during the 1998 El Nin˜ o winter. The 1998 storm deposits were readily differentiated in the field because they overlie a surface with marsh plants growing on it that had been visited prior to the 1998 storms. Estuarine mud-over-peat contacts, accompanied by changes in fossil diatom assemblages and plant macrofossil communities preserved in the sediment, are produced by abrupt relative sea level rise. Diatom and botanical studies in Pacific coast estuaries of western North America recognize a three-tiered intertidal zonation in estuarine marshes (Atwater et al., 1979; Peinado et al., 1994; Hemphill-Haley, 1995a). Modern diatom assemblages identified for each intertidal zone in the marshes studied for this project were consistent with the intertidal zonation of Hemphill-Haley (1995b) and Atwater and Hemphill-Haley (1997) (Fig. 4).

2617

This intertidal zonation includes high marsh, low marsh, and tidal flat zones that are differentiated, in part, based on modern diatom assemblages common to each ecological zone in North American Pacific coast estuaries (Hemphill-Haley, 1995b) (Fig. 4). Modern diatom assemblages and associated macrophytes from each intertidal zone at the northern end of Bolinas Lagoon were in good agreement with cosmopolitan diatom assemblages for the equivalent zones determined by Hemphill-Haley (1995a). The brackish intertidal diatom index (BIDI) developed by Hemphill-Haley (Atwater and Hemphill-Haley, 1997) relates fossil diatom assemblages to vertical position in the intertidal zone. The BIDI was calculated for all diatom samples analyzed in the latter part (1998) of this study. Diatom counts for samples from earlier cores (pre-1998) were less quantitative, and the interpretations are thus less robust. Diatom slides were prepared and analyzed following the methods of Hemphill-Haley (1995b). ( E Detailed results of the diatom analyses are available online at the SSA website.) Fossil pollen from nonnative plants and accelerator mass spectrometry (AMS) radiocarbon analyses provide age constraints for historical and prehistorical depositional events in the marsh strata. Three exotic pollen types were introduced to the Bolinas area in the middle to late nineteenth century: Plantago lanceolata (English plantain, introduced around 1868–1873), Eucalyptus spp. (introduced in 1875), and Rumex ascetocella (sheep sorrel, introduced around 1857–1859) (Bergquist, 1978). Of the three exotic pollen types, only pollen from R. ascetocella was found in cores taken from Bolinas Lagoon and Bodega Harbor. For purposes of this study, it is assumed that R. ascetocella was introduced in the Bodega area at about the same time that it was introduced in the Bolinas area. Pollen samples from the interior of several cores were processed through standard multistep chemical treatments including hydrofluoric acid (Faegri et al., 1989). Detrital macrofossils, such as seeds from herbaceous marsh plants, delicate twigs, and in some cases fragments of tree bark, were selected for AMS radiocarbon age determination. Although less desirable, where necessary, in situ rhizomes or herbaceous stems, and less commonly, undifferentiated peat, were sampled and dated. Radiocarbon age estimates for both study sites are reported in Table 1. Laboratory reported radiocarbon age estimates were dendrocalibrated to two-sigma standard deviation using an error multiplier of 1 according to the computer program Calib 3.0 developed by Stuiver and Reimer (1993). Identification of plants or plant parts for dating and interpretation of environments of deposition is desirable, although not always easily accomplished. Similarly, identification of entombed, formerly above-ground stems of buried herbaceous plants in growth position is strong evidence for rapid relative sea level change. Such evidence is more likely to be found in surface exposures than in the few drive cores collected from the subsurface for this study.

2618

sp

p.

m ar sh

la ts

(D

60 40

MLHW

(D)

20 0

a ic rn fli o ta ca su a na de a i ith om er c C Ma

Ti da lf

)

80

Tidal Datum MHHW

us m iti osa ata a ca r in ma oli ic os ini ch s f sp rn rg lo pu tina lis ca vi g a i ir r h Tr Sc pa tic ea rni S Dis um lico a J Sa

Lo w

140

(C )

(B )

Elevation Relative to Mean Tide Level (cm)

160

100

ta ta ric ca a a st spi m u a i iti ig a el hlis ar mb alin d c rin ti x s m a s G Dis are ncu eria leja nia C Ju rm stil ke A Ca ran F

Hi gh

180

120

m ar sh

lix ha Sa yp T

sp p sp . p.

la n Up (A )

200

sp sp p. p.

d


MTL

-20

Upland

-40

only rare transported diatoms and freshwater species, in many cases barren

Intertidal marshes High marsh

Cosmioneis pusilla Denticula subtilis Navicula tenelloides Nitzschia brevissima N. pusilla N. terrestris Pinnularia lagerstedtii Tryblionella aerophila

High or low marsh

Caloneis bacillum Diploneis pseudovalis Frustulia vulgaris Luticola mutica Navicula cincta Nitzschia commutata N. fasciculata Tryblionella debilis

Low marsh

Caloneis westii Diploneis interrupta D. smitthii var. rhombica Frustulia linkei Gyrosigma eximium Mastogloia exigua Navicula cryptotenella Nitzschia bilobata N. scapelliformis Rhopalodia musculus Scoliopleura tumida Tryblionella navicularis

Figure 4. Intertidal zones in salt marshes, North American Pacific coast estuaries (after Hemphill-Haley, 1995b; Atwater and Hemphill-Haley, 1997). MHHW, mean higher high water; MLHW, mean lower high water, MTL, mean tide level. Approximate elevation ranges for intertidal zones based on diatom assemblages and macrophytes are as follows: upland, ⬎1.1m; high marsh, 1.1–0.8 m; low marsh, 0.8–0.4 m; tidal flat, ⬍0.4 m. Tidal datums approximated from published tidal benchmark sheets for Drakes Bay at Point Reyes, approximately 19 km west-northwest of Bolinas Lagoon (National Ocean Service, 1994). Diatom species list from Atwater and Hemphill-Haley (1997). Diatoms shown in bold were prevalent in samples analyzed for this study. Dry upland environments are barren of in situ diatoms; high marshes support species that can tolerate long periods of subaerial exposure; low marshes support diatoms that prefer diurnal inundation by tides. Intertidal flats and subtidal channels host diatoms that prefer silty-to-sandy substrate. Listed at the top of the figure are vegetation species common to the upland, high marsh and low marsh zones described by Atwater et al. (1979) and observed during this study. Clam and snail species that live on the modern tidal flat are also listed.

Intertidal flats and shallow subtidal channels Mud

Sand

Bacillaris paradoxa Camplyodiscus echineis Catenula adhaerens Cerataulus turgidus Cocconeis scutellum var. parva Gyrosigma balticum Melosira nummuloides Navicula digitoradiata Navicula tripunctata Nitzchia sigma N. socialis

Odontella aurita Opephora parva Rhaphoneis amphicoros R. psammicola Synedra fasciculata Plagiogramma staurophorum Trachyneis aspera Tryblionella acuminata T. coarctata T. punctata

Achnanthes delicatula Amphora proteus A. ventricosa Cocconeis diminuta C. peltoides Dimeregramma minor Fallacia cryptolyra Lyrella lyra Navicula cancellata Petroneis granulata P. marina Trachyneis australis

Results This research provides strong evidence for sudden relative sea-level rise and subsequent rapid sediment aggradation along the northern San Andreas fault at intertidal marshes of two estuaries: Bolinas Lagoon and Bodega Harbor. Radiocarbon and exotic pollen analyses provide a chronology for these abrupt submergence events, which are inferred to date past large earthquakes on the San Andreas fault. Lithology and Biostratigraphy of a Marsh at the North End of Bolinas Lagoon Stratigraphic successions observed in 10 7.6-cmdiameter drive cores from the marsh at the northwest head of Bolinas Lagoon (Fig. 5) consist of intertidal and fluvial

deltaic deposits overlying at sharp contact a tidal marsh soil (Fig. 6). This peaty buried soil is preserved about 2 m below the ground surface over a horizontal distance of more than 350 m. Sudden burial of the soil, evident as an abrupt mudover-peat contact with a corresponding change in diatom assemblage above and below the contact, indicates submergence due to sudden relative sea level rise (contact S, Fig. 6). The broad extent of this submergence event indicates that it is not the product of localized erosion or deposition from a tributary drainage. Poorly sorted, angular gravelly sand deposited directly above the buried marsh soil in some cores indicates that fluvial aggradation immediately followed soil submergence. A more recent fluvial aggradation episode, unrelated to the oldest submergence event, is indicated by a younger alluvial deposit (contact H, Fig. 6). The lithologic and biostratigraphic aspects of each of these deposits are

2619


Table 1 Summary of Radiocarbon Analytical Data, Bolinas Lagoon, and Bodega Harbor Organic Material

Seeds Cyperaceae Polygonum Apiaceae Charred monocot stem Marsh plant stems Unidentified plant fragment

Sample Depth (cm)§

Lab-reported Age Ⳳ 1r (14C yr B.P.)|

d13C (‰)

Calibrated Age# A.D. at 2r

154–157

1290 Ⳳ 120

ⳮ25

560–960

106–109

740 Ⳳ 40

ⳮ25

190–197

520 Ⳳ 40

ⳮ25

154

850 Ⳳ 40

ⳮ25

1220–1310 1360–1380 1320–1350 1390–1450 1050–1090 1120–1140 1150–1280 1680–1760 1800–1940 1530–1550 1630–1690 1730–1810 1920–1960 890–940 940–1020 1180–1320 1340–1390 1450–1660 1330–1350 1390–1520 1590–1620 690–900 920–960 970–980

Laboratory Number*

Sample Number†

Type of Analysis‡

Bolinas Lagoon LLNL-48893

98 BOL D3 RC6

AMS

LLNL-48894

98 BOL D4 RC2

AMS

LLNL-48895

98 BOL D2 RC2

AMS

LLNL-48896

98 BOL D3 RC3

AMS

LLNL-48897

98 BOL D2 RC1

AMS

Tree bark

132–136

110 Ⳳ 40

ⳮ25

LLNL-48898

98 BOL D3 RC1

AMS

Marsh plant stems

94–96.5

230 Ⳳ 40

ⳮ25

LLNL-48899

98 BOL D6 RC3

AMS

100.5

1070 Ⳳ 40

ⳮ25

Beta-103177

97 BOL D3

173

740 Ⳮ 60

ⳮ28.9

LLNL-54916 LLNL-54918

99 BOL G2 RC5 97 BOL D2 RC1

AMS (LLNL) AMS AMS

Charcoal Asteraceae Wood Plant fragments Plant fragments

203–206.5 127–130

320 Ⳳ 50 470 Ⳳ 50

ⳮ25 ⳮ25

LLNL-54919

97 BOL D2 RC2

AMS

Plant fragments

174–176

1230 Ⳳ 50

ⳮ25

Probability

Context

1

Minimum limiting age, event S

0.96 0.04 0.09 0.91 0.09 0.05 0.86 0.33 0.67 0.03 0.37 0.44 0.17 0.23 0.77 0.83 0.17 1 0.03 0.91 0.07 0.91 0.08 0.01

Minimum limiting age, event S Maximum limiting age, event S Minimum limiting age, event S

Maximum limiting age, event H Maximum limiting age, event H

Maximum limiting age, event E Closely limiting age, event S Minimum limiting age, event S Maximum limiting age, event S

Minimum limiting age, event E

(continued)

described below in stratigraphic order from bottom (oldest) to top (youngest). A peaty soil overlain at sharp contact by a mud or gravelly sand deposit occurs near the base of each core and ranges in thickness from 3 to 50 cm (Fig. 6). In four of the large diameter cores and in many of the reconnaissance gouge cores, this soil also displays an abrupt lower contact with underlying silty-to-sandy mud (contact E, Fig. 6). Fossil diatoms ( E see Diatom analyses online at the SSA website), macrofossils of marsh plants, and estuarine gastropods preserved in the sediment indicate that sudden relative sea level fall accompanied the initial formation of the marsh soil. In cores 98-BOL-D6 and 97-BOL-D2, fossil diatom assemblages preserved in the mud below the soil indicate mudflat and tidal flat paleoenvironments (Fig. 6). Low-to-high marsh diatom assemblages preserved within the overlying soil record a shift to higher intertidal environments, indicating that a sudden fall in relative sea level occasioned the onset of soil formation at contact E. The upper contact of the marsh soil is represented by an abrupt transition from soil to overlying mud in some cores (contact S, Fig. 6). This abrupt transition, together with an abrupt change in fossil diatom assemblages and decrease in

organic content across the contact, indicate rapid submergence. In cores 97-BOL-D1 and 97-BOL-D2, the change from low marsh diatom assemblages present in the marsh soil to tidal flat diatom assemblages present in the overlying mud indicates rapid relative sea level rise (Fig. 6). Furthermore, fossil shells of Cerithidea californica, a lower intertidal snail that inhabits mudflats and deltas of the modern Bolinas Lagoon, and the bent-nosed clam, Macoma nasuta, a mudflat bivalve (Ricketts et al., 1985), is present in mud several decimeters above the buried soil. These tidal flat macrofossils indicate that the rapid rise in relative sea level resulted in a long-lasting change in the depositional environment in this area of the marsh. In other cores (e.g., 97-BOL-D4, 98-BOL-D6, and 98BOL-D3), a matrix supported, angular gravelly sand overlies the soil in abrupt contact. This gravelly sand (alluvium 2, Fig. 6), the older of two alluvial deposits recognized in the stratigraphy, exhibits a similar texture and composition to fluvial deposits in fan deltas on the modern sand flat. In cores 97-BOL-D3 and 98-BOL-D4, alluvium 2 contains estuarine fossils of the tall-spired horn snail C. californica; an estuarine gastropod that inhabits modern mudflat and delta environments. Poorly preserved diatoms within the silty matrix

2620


Table 1 (Continued) Laboratory Number*

Bodega Harbor Beta-104962

Sample Number†

97BB D2

Type of Analysis‡

Organic Material

Sample Depth (cm)§

Lab-reported Age Ⳳ 1r (14C yr B.P.)|

d13C (‰)

Calibrated Age# A.D. at 2r

AMS (Oxford) AMS

Wood

79

340 Ⳳ 40

ⳮ29.2

1470–1650

1

Maximum limiting age, event 2

Twig

125

810 Ⳳ 70

ⳮ32.2

1040–1110 1110–1150 1150–1300 1440–1670 1780–1800 1950–1960 1680–1750 1800–1940 1160–1170 1190–1290 1290–1330 1340–1410

0.12 0.07 0.81 0.97 0.02 0.01 0.3 0.7 0.04 0.96 0.42 0.58

Minimum limiting age, event 3

900–920 980–1160 1170–1190 1160–1170 1190–1200 1210–1330 1350–1360 1370–1390 610–630 640–780 790–810 850–860 1640–1710 1720–1820 1830–1880 1910–1960

0.02 0.95 0.03 0.01 0.01 0.92 0.01 0.05 0.02 0.94 0.03 0.02 0.27 0.48 0.07 0.18

Beta-103176

99BB D1 RC1

Beta-80907

Bodega #1 11/22/94

Conv.

Peat

129–133

330 Ⳳ 70

ⳮ25

LLNL-48900

97 BB D2

AMS

73

100 Ⳳ 40

ⳮ25

LLNL-54961

98 BB D1 RC4

AMS

108–109

800 Ⳳ 40

ⳮ25

LLNL-54962

98 BB D1 RC4

AMS

Willow twig Salicaceae Peat aaa residue** Peat humics

108–109

640 Ⳳ 40

ⳮ29

LLNL-54920 LLNL-54921

97 BB D2 RC2 98 BB D2 RC3

AMS AMS

Plant stem Twig

100 124.5

⬎modern 1000 Ⳳ 50

ⳮ25 ⳮ25

LLNL-54922

99 G3 RC10

AMS

Seeds

122–125

750 Ⳳ 50

ⳮ25

LLNL-54923

99 BB G1 RC4

AMS

Wood

161–164

1330 Ⳳ 50

ⳮ25

LLNL-54924

98 BB D2 RC1

AMS

Bark

61–62

210 Ⳳ 50

ⳮ25

Probability

Context


Minimum limiting age, event 2 Maximum limiting age, event 3 Maximum limiting age, event 3 NA Maximum limiting age, event 3




*Radiocarbon laboratory abbreviations: LLNL, Lawrence Livermore National Laboratory; Beta, Beta Analytic. † Sample number includes year, study site, and sample number. BOL, Bolinas marsh; BB, Bodega marsh. ‡ Types of analyses included accelerator mass spectrometry (AMS) and conventional (Conv). § Compacted depth below surface. Noncompacted depth below surface for gouge cores designated by G in second column. | Ages reported by radiocarbon laboratory based upon the Libby half life (5570) for 14C. # Laboratory reported radiocarbon age estimates were dendrocalibrated to 2r standard deviation using an error multiplier of 1 according to the computer program Calib 3.0 developed by Stuiver and Reimer (1993). Values are rounded to the nearest decade. **Alkali-acid-alkali (aaa) pretreatment method for peat samples extracts humic peat from residual more resistant humin material. Humic peat represents the bulk of the organic material in most peat samples. Humin is the most resistant carbon and may represent older redeposited carbon (J. Southon, personal comm., 1999).

of alluvium 2 represent a mixture of in situ estuarine and allochthonous fresh to brackish species. The presence of fossil diatoms from both freshwater and intertidal environments indicates transport of poorly sorted alluvial sediment from higher elevation freshwater environments in the watershed and subsequent deposition in the lower-intertidal mudflats inhabited by C. californica. Alluvium 2 grades upward into clayey mud containing estuarine bivalve and gastropod fossils from mud and sand flat environments, further demonstrating the sustained rise in relative sea level. The younger, poorly sorted, angular gravelly sand (alluvium 1, Fig. 6) occurs near the top of 7 of the 10 cores studied. Chaotically distributed, coarse redwood debris and bark up to 6 cm in longest dimension occur within alluvium

1. In core 98-BOL-D7, a thin 1.5-cm-thick mud layer that grades upward into alluvium 1 abruptly overlies a peat-mud soil. In core 97-BOL-D4, the equivalent to alluvium 1 is a sandy mud that also overlies a peaty soil in sharp contact. The occurrence of the introduced R. ascetocella pollen within a woody debris layer overlying alluvium 1 in core 97-BOL-D3 and the absence of historical pollen in sediment below alluvium 1 suggests that alluvium 1 was deposited around the late 1850s. Because of the chaotic nature, historical age, and preponderance of redwood debris incorporated within alluvium 1, the alluvium is most likely attributed to an increase in sedimentation rate due to logging operations in the Bolinas watershed conducted during the 1850s (Bergquist, 1978).

2621


Explanation Gouge Core Drive Core 97 BOL D4

SW

W

IL

LO

AM

CA

P

X RE

MA

High

RSH

way 1

97 BOL D1-2

W

98 BOL D6 99 BOL G2

SAL

ICOR

R T I N A S A LT N I A & S PA MAR

SH

98 BOL D7 97 BOL D3 98 BOL D2

98 BOL D1

98 BOL D3

MUDFLAT

NIA & S SAL PA TM RT AR IN SH A

MUDFLAT

SALI

COR

CAREX

MAR

SH

98 BOL D4

00

~50 ~50 m m

Figure 5. Tidal marsh site map of northwestern head of Bolinas Lagoon. Highway 1 crosses the north edge of the 1998 air photo. Square symbols indicate drive and gouge core sites corresponding to stratigraphic profiles illustrated in Figure 6. Closed circles denote gouge core sites where a buried marsh soil was observed that exhibited abrupt upper and lower contacts. The Salicornia and Spartina saltmarsh generally inhabits the low marsh intertidal zone. The Carex marsh generally corresponds to the high marsh zone. The freshwater willow swamp lies above the elevation of extreme high water. (1998 aerial photograph from the U.S. Army Corps of Engineers.) The preservation and abundance of diatom species in alluviums 1 and 2 differ from the sparse freshwater diatoms observed in silty sand beds deposited during the 1998 El Nin˜ o winter (Fig. 7). Based on shallow, hand-dug excavations, the loose 1998 deposits overlie a marsh surface that had marsh plants growing on it prior to the 1998 storms. Silty sand beds deposited by streams at the head of Bolinas Lagoon during the winter of 1998 are relatively barren of

diatoms (Fig. 7). This is in contrast to the mixed allochthonous and lower intertidal diatom species observed in alluviums 1 and 2. Poorly preserved freshwater diatoms found in the silty matrix of alluvium 2 suggest transport of sediment from stream valleys that drain into the head of Bolinas Lagoon. The course angular sediment particles and poorly sorted nature of alluvium 2 suggests that a pulse of earthquake-generated (e.g., landslide) sediment was transported

2622


Figure 6. Stratigraphic profiles of marsh sediment from northern Bolinas Lagoon (Figs. 2, 5). Bold dashed lines delineate correlation of rapidly buried soil accompanied by sustained relative sea level change. S, abrupt upper contact of marsh soil; E, abrupt lower contact of marsh soil; H, the abrupt lower contact of historical alluvium; BOC, depth to base of core below surface (not length of compacted core sediment). Left page of figure shows topographically higher cores; right page shows lower cores taken nearer the lagoon (Fig. 5).


2623

Figure 6. (Continued)

by streams into the estuary. The mixed diatom assemblage likely reflects both the transport of sediment into the estuary following rapid relative sea level rise and disturbance of estuary bottom sediment by strong ground shaking. Conversely, the 1998 winter storm deposits are generally barren of diatoms.

Lithology and Biostratigraphy of a Marsh at the South End of Bodega Harbor Sediment in five 7.6-cm-diameter drive cores and three reconnaissance cores from marsh stratigraphic sequences in southeastern Bodega Harbor (Fig. 8) consists of interbedded

2624


Figure 7. Stratigraphic profiles of sediment deposited during the El Nin˜ o winter of 1998 in the north end of the Bolinas Lagoon. Core 98-BOL-ST1 was taken at the location of core 98BOLD1 (Fig. 5), and 98-BOL-ST2 was taken at the location of core 98BOLD2 (Fig. 5). The BIDI values were calculated based on the method of HemphillHaley (Atwater and Hemphill-Haley, 1997).

organic laminated mud, peaty soils, and sand. The sediment at the base of most cores consists of dark gray to black, poorly sorted muddy sand that grades upward into laminated organic mud (Fig. 9a,b). Fossil diatom assemblages preserved in the laminated sediment are typical of low marsh to tidal flat environments (Fig. 9a,b). With the exception of core 97-BB-D1, the laminated mud flat sediment grades upward into a peaty marsh soil. Three buried peaty soils extend 50 to 130 m across the marsh as revealed in core profiles (soils 3 to 1, Fig. 9a,b). The lithologic and biostratigraphic details of each of the three soils that are overlain at sharp contact are reported below in stratigraphic order from bottom (oldest) to top (youngest). In most cores, soil 3 is overlain at sharp contact by a 2to 5-cm-thick sand bed that is found in all cores except 97BB-D1. In two of the cores, 97-BB-D2 and 98-BB-D2, fibrous reddish brown peat of soil 3 is overlain at sharp contact by organic mud that is, in turn, overlain by the continuous sand bed that overlies soil 3 in other cores. Fossil diatom assemblages preserved above and below the sand and mud over peat contact of buried soil 3 indicate sudden relative

sea level rise from a low marsh environment to tidal flat conditions (e.g., cores 98-BB-D1 and 98-BB-D2, Fig. 10). However, fossil diatom assemblages from tidal flat environments above and below the same contact in core 97-BB-D1 do not show a relative sea level change associated with the abrupt change in lithology. Evidence for a lasting relative sea level rise, therefore, is equivocal. In some cores, the sand bed that buries soil 3 grades upward into mud several decimeters thick (e.g., 98-BB-D2 and 97-BB-D1, Fig. 9a,b). However, in other cores, specifically 99-BB-G3 and 98-BBD3, an increase in sand laminae and sand beds in the peat and peaty mud overlying soil 3 suggests an increase in tidal inundation responsible for sand deposition. The possible evidence of more frequent tidal inundation in the sediment overlying soil 3 is consistent with sustained relative sea level rise due to coseismic subsidence. In cores 98-BB-D1, 98-BB-D2, and 97-BB-D2, buried soil 2 consists of laminated peat to muddy peat with an abrupt upper contact with peaty mud (Fig. 9a,b). In cores 98-BB-D3 and 97-BB-D1, at the stratigraphic level of soil 2, there is a 2- to 2.5-cm-thick sand bed. In one core (97-


2625

Figure 8.

Site map showing core locations in southeast corner of Bodega Harbor and surrounding golf course layout. Symbols indicate core sites corresponding to stratigraphic profiles illustrated in Figures 9a,b and 10. Core numbers designated with D indicate 7.6-cm-diameter drive cores; core numbers designated with G indicate 2.5-cmdiameter gouge cores. Solid lines are boardwalks built over marsh.

BB-D1), the sand deposit consists of two fining-upward beds separated by a thin peaty parting. The base of this sand bed is tentatively correlated with the abrupt contact for soil 2. Diatom samples analyzed from above and below the upper contact of soil 2 suggest that a sudden but brief rise in relative sea level accompanied soil burial (98-BB-D1, Fig. 10). Fossil diatoms within the laminated peaty soil indicate low marsh conditions, whereas, diatom assemblages preserved within detrital organic deposits directly overlying the buried soil are from a tidal flat environment. However, the environment rapidly returned to low marsh conditions based on diatom assemblages preserved in the peaty mud overlying the detrital organic deposit. The youngest soil in the section, soil 1, is overlain at sharp contact by mud and can be correlated in all cores across the entire marsh (Fig. 9a,b). Soil 1 consists of peat to muddy peat that is overlain at sharp contact by mud to peaty

mud that contains abundant roots. Diatom analyses of sediment samples across the mud-over-peat contact do not indicate a change in the elevation of relative sea level. Near the top of most cores the sediment became sandier, and the poorly sorted nature of the deposits suggests that the rate of hillslope erosion surrounding the marsh increased with a corresponding increase in the rate of fluvial deposition in the marsh. These changes likely result from historical land use changes. Chronology of Soil Burial and Sediment Aggradation Events The chronology of the depositional events described above is based on radiocarbon analyses of peat and detrital macrofossils sampled from specific horizons (Table 1) and by the first occurrence of the introduced R. ascetocella pollen (ca. 1857–1859) in the stratigraphy (Figs 6, 9a,b). The

2626


Figure 9.

Stratigraphic profiles of marsh sediment from the marsh in the southeast corner of Bodega Harbor (Figs. 3, 8). Numbers and bold dashed lines denote correlative buried soil horizons identified in the stratigraphy. Light dashed lines tentatively correlate other stratigraphic units that are found in multiple cores. BOC, depth to base of core below surface (not length of compacted core sediment). Left page of figure shows cores from north side of marsh; right page shows cores from the south side of the marsh (Fig. 8).

rapid submergence events are inferred to correspond to past large earthquakes on the northern San Andreas fault (Fig. 11). Bolinas Lagoon. Emergence and initial formation of the widespread marsh soil recorded in sediment at the head of Bolinas Lagoon probably occurred between A.D. 890 and

980 based on bracketing ages of charcoal and marsh plant fossils from below and above the lower soil contact (Fig. 11). However, a stratigraphically inverted radiocarbon age on seeds from within the buried soil in core 98-BOL-D3 provides a minimum limiting age for soil emergence of A.D. 560 to 960. The abrupt submergence of the widespread buried soil


Figure 9. (Continued)

2627

2628


Figure 10. Cores from Bodega Harbor with BIDI information. The BIDI values were calculated based on the method of Hemphill-Haley (Atwater and Hemphill-Haley, 1997). Refer to Figure 9a for explanation of core description nomenclature.

preserved in the Bolinas stratigraphy occurred between A.D. 1050 and 1450 based on maximum and minimum limiting radiocarbon ages from samples below and above the upper soil contact (Fig. 11). The deposition of alluvium 2, consisting of poorly sorted, gravelly sand that abruptly buried the marsh soil, attended or closely followed the submergence of the soil based on radiocarbon age estimates of marsh plant remains preserved within the deposit. The age of sudden burial of the soil is difficult to evaluate because it is based on four radiocarbon ages: a closely limiting age on a twig deposited on the upper contact of the buried soil; a maximum limiting age on a fossil Scirpus rhizome preserved within the buried soil; and two minimum limiting ages on plant stems and fragments recovered from the sediment overlying the buried soil (Fig. 11). A more conservative age range that estimates the time of soil submergence that incorporates all bracketing ages analyzed is A.D. 560–1660. However, by using only the high-probability parts of three dendrocalibrated radiocarbon dates that most tightly constrain the rapid relative sea level rise, a time of about A.D. 1150–1330 is estimated for this earthquake (Fig. 11).

Deposition of alluvium 1 probably occurred around the late 1850s. The first appearance of pollen from sheep sorrel (R. ascetocella) occurred above alluvium 1 in core 97-BOLD3. No pollen from historically introduced exotic plants was observed in sediment samples below alluvium 1, suggesting that the deposition of alluvium 1 occurred around or before A.D. 1857–1859, the time that the exotic plant R. ascetocella was reportedly introduced to the Bolinas area (Bergquist, 1978). This poorly sorted, probably rapidly deposited alluvium is readily recognized in the cores, based on abundant redwood debris associated with the gravelly sand deposit, the apparent change in sediment supply, and the variable and poorly sorted nature of the sediment above the lower contact of alluvium 1. Bodega Harbor. Burial of soil 3 and subsequent deposition of the overlying sand bed recorded in the Bodega marsh cores probably occurred after A.D. 900–1190 but prior to A.D. 1050–1390 based on calibrated radiocarbon ages of twigs and seeds from above and below the upper buried soil contact (Fig. 11). The anomalously young age estimate for

2629


Figure 11.

Event chronology based on dendrocalibrated radiocarbon ages for abrupt sedimentologic contacts observed in stratigraphy of Bolinas Lagoon and Bodega Harbor. Each cluster of boxes approximates the probability distribution for a single dendrocalibrated radiocarbon age estimate. The area of each box corresponds to the probability (noted to the right of the box) that the age of the sample falls within the age interval represented by the vertical span of the box. Diagonally shaded areas represent conservative age range estimates for submergence events evident at the marshes at Bolinas Lagoon and Bodega Harbor. Cross-shaded ranges represent age estimates for earthquakes based on radiocarbon age estimates that most tightly constrain the event. Refer to Table 1 for radiocarbon ages and calibration information.

the peat sample Bodega #1 is probably the result of rejuvenation by young downward penetrating roots (e.g., Wells, 1998). By using the two dendrocalibrated radiocarbon dates that most tightly constrain this event, a date of about A.D. 980–1300 is estimated for this earthquake (Fig. 11). Burial of soil 2 probably occurred after A.D. 1470 but prior to 1680–1860 AD based on bracketing ages on wood specimens collected from above and below the upper soil contact and the first appearance of historical pollen of R. ascetocella observed within peat overlying the sand bed associated with the soil burial in 97-BB-D1. By using the younger of two maximum limiting ages, the earthquake can be constrained to have occurred after about A.D. 1600 (Fig. 11). R. ascetocella pollen appeared in the sedimentary record prior to the formation of the youngest buried soil in the section. Based on this evidence, the burial of soil 1 probably

occurred historically, sometime after A.D. 1857–1859, assuming that R. ascetocella was introduced to the Bodega Harbor area at approximately the same time that it was introduced to the Bolinas area. Although there is no historical report of measurable coseismic subsidence at Bodega Harbor, the youngest buried soil is attributed to possible liquefaction-related or tectonic subsidence accompanying the 1906 San Francisco earthquake.

Discussion Stratigraphic sequences of interbedded estuarine peat and mud within marshes at Bolinas Lagoon and Bodega Bay are consistent with sedimentation in response to vertical deformation within right-releasing (transtensional) steps of the San Andreas fault. Based on historical observations of subsidence immediately following the 1906 San Francisco

2630


earthquake and characterization of estuarine sediment, a model of sedimentary response to the San Andreas earthquake deformation cycle is proposed that predicts sudden submergence and burial of marsh soils followed by gradual shoaling and redevelopment of new marsh soils. Rapid fluvial aggradation, within the months or years following the earthquake, responds to an increase in the base level of coastal streams supplying sediment to the marsh and probable increased sediment supply immediately following an earthquake. Gradual shoaling and redevelopment of a new marsh soil occurs in the interseismic period prior to the next earthquake. To substantiate this model of sediment response to the earthquake deformation cycle, a set of criteria proposed by Nelson et al. (1996) for Cascadia-type events is modified to identify sudden submergence events caused by earthquakes on strike-slip faults (Table 2). Nelson et al. (1996) proposed five criteria to assess the likelihood that marsh stratigraphic sequences were formed by coseismic subsidence during earthquakes on the Cascadia subduction zone: (1) suddenness, (2) magnitude and duration of relative sea level change, (3) spatial extent, (4) presence of tsunami or liquefaction deposits, and (5) synchroneity between sites. Nelson et al. (1996) recognized that multiple lines of evidence are necessary to infer that stratigraphic sequences reflect coseismic vertical displacement. A case for coseismic formation of a particular stratigraphic sequence becomes stronger as more criteria are satisfied at several study sites. Deformation in a strike-slip earthquake can be expected to be different than the deformation accompanying a Cascadia-type event. For instance, the magnitude of deformation accompanying a Cascadia event should be greater and more widespread (less localized) than the deformation that might accompany a San Andreas fault event. Coseismic subsidence along a strike-slip fault must be localized and related to grabens and releasing step-over geometries along the fault. Another criterion useful in Cascadia that may not aptly indicate coseismic subsidence along a strike-slip fault is a tsunami deposit coincident with evidence of marsh submergence. Although the 1906 earthquake generated a small tsunami off-

shore of the Golden Gate (Geist and Zoback, 1999), there were no reports of sand deposits resulting from inundation of the coastline by the tsunami. Kelsey et al. (1998) concluded that the Sixes River near Cape Blanco, Oregon, responded to coseismic subsidence during a Cascadia earthquake about 300 years ago by rapid and voluminous deposition of terrestrial sediment. Thus, evidence for rapid fluvial aggradation coincident with stratigraphic sequences indicating relative sea level rise may be a reasonable criterion for identifying coseismic subsidence along strike-slip fault systems (Table 2). Alternative, nontectonic processes that can produce short-term rises in relative sea level could cause stratigraphic sequences similar to those produced by coseismic subsidence (Carver and McCalpin, 1996; Nelson et al., 1996). These climatic and oceanic processes include (1) short-term changes in ocean currents and sea-surface temperatures; (2) changes in coastline morphology including breaching or growth of barrier spits; (3) migration of tidal channels and delta distributary systems; and (4) gradual local subsidence associated with compaction of sediment. Several of these mechanisms may explain particular stratigraphic sequences that only satisfy one or two of the criteria for coseismic subsidence in Table 2. However, it is unlikely that any buried soil evident in stratigraphic sequences formed by nontectonic processes could satisfy more than two of the coseismic criteria proposed by Nelson et al. (1996) (Carver and McCalpin, 1996). Breaching of the sand spit that protects the respective water bodies might cause a change in marsh depositional environments. If a spit were breached, an increase in wave energy within the lagoon or harbor might cause erosion of marsh deposits. Breaching of the spits likely would not cause a short- or long-term change in relative sea level unless greater than 0.5 m of erosion were to occur over a large area as a result of the breaching. Erosion of an areally extensive marsh soil would leave an unconformity in the sedimentary record, and coarse, sandy sediment would likely be left behind if such erosion occurred. The unconformities observed in Bolinas Lagoon and Bodega Harbor sediment, which are

Table 2 Evaluating Coseismic Subsidence Explanation for Buried Soils along the Northern San Andreas Fault Stratigraphic Horizon Bolinas Lagoon Criteria

1. Suddenness: abrupt lithologic contact? 2a. Magnitude of change: diatom evidence for relative sea level change? 2b. Duration of change: sustained submergence? 3. Graben or releasing fault geometry and minimum lateral extent of submergence? 4. Rapid aggradation of coarse material coincident with burial? 5. Synchronous with other paleoseismic events?

Bodega Harbor

Contact H (⬃1860s)

Soil S

Soil 1

Soil 2

Soil 3

Yes No

Yes Yes

Yes No

Yes No

Yes Yes

No NA, 350 m

Yes Yes, 350 m

Possible Yes, 50–130 m

Possible Yes, 50 m

Possible Yes, 50–130 m

Yes NA

Yes 1200s?

No 1906?

No 1600s?

Yes 1200s?


herein attributed to earthquake-induced subsidence, have preserved marsh soil that is abruptly overlain by tidal flat mud. Also, marshes studied for this project lie at corners of their respective lagoons and are relatively well protected from wave energy. The change in diatom assemblages from marsh to tidal flat and associated change in sediment organic content indicate that breaching of the spit alone is not responsible for rapid burial of the soil horizons in either Bolinas Lagoon or Bodega Harbor. Migration of the marsh channels in Bolinas Lagoon (Fig. 5) and Bodega Harbor (Fig. 8) might cause an abrupt sedimentological and diatom change when the channel erodes into the adjacent sediment. However, the channel would have to erode greater than 0.5 m of sediment to account for the change in diatom assemblages and would have to migrate across the entire area over which evidence for rapid burial is preserved. In Bolinas Lagoon, evidence for rapid burial of soil S is found over at least a 350-m-wide area. Additionally, channel, or lag, sediment would be expected at the erosional unconformity. Perhaps the coarse material in several of the Bolinas Lagoon cores at the S horizon is evidence of such channel migration. However, most of the cores from Bolinas Lagoon do not have this type of coarse material at horizon S, and none of the event horizons in Bodega Harbor have a coarse deposit that might be related to channel migration. Short-term changes in ocean currents and/or sea surface temperature would not affect relative sea level within protected areas, at least not in the sustained way observed in sediment from Bolinas Lagoon and Bodega Harbor. Similarly, gradual local subsidence associated with compaction of sediment would not result in the abrupt changes in sediment texture, organic content, and diatom assemblages observed in the cores. Changes in diatom assemblages across the mud-over-peat contacts are consistent with sudden relative sea level rise. A long duration of submergence is indicated by the overlying mud deposits of several decimeters that contain fossil shells of Cerithidea californica and Macoma nasuta, mollusks common to mudflats in Bolinas Lagoon (Ricketts et al., 1985). Buried tidal-wetland soils extend across at least 350 m at the northern end of Bolinas Lagoon and from 50 to 130 m across the marsh at the southeast end of Bodega Harbor. Both of these basins lie adjacent to the San Andreas fault where geomorphic evidence of transtensional deformation is present. Deposits of poorly sorted alluvium that abruptly overlie the buried soil at Bolinas Lagoon are consistent with dramatic changes in sedimentation rate caused by rapid (months to years) fluvial aggradation responding to sudden areally extensive changes in sediment throughout the north end of Bolinas Lagoon and the southeast end of Bodega Harbor. Widespread, Synchronous Subsidence about A.D. 1250 On the basis of sedimentology, biostratigraphy, and radiocarbon analyses, rapid submergence events at Bolinas La-

2631

goon and Bodega Harbor are consistent with the hypothesis that surface rupture on the San Andreas fault occurred about A.D. 1250. The dates of the Bodega Harbor and Bolinas Lagoon submergence events may be correlative with an approximately A.D. 1250 event hypothesized by Niemi and Hall (1992), Prentice (1989), and Baldwin (1996). Abrupt contacts of estuarine mud and sand over peaty soil from the northern margin of Bolinas Lagoon and the southeast edge of Bodega Harbor suggest a rapid rise in relative sea level approximately 750 years ago (Fig. 11, Table 2). The mudover-peat contacts indicate rapid lowering of marsh surfaces to mudflat elevations. The sedimentological evidence for rapid relative sea level change is corroborated by (1) change in diatom assemblages across the event horizon from shallowwater, low-marsh species to deeper water mudflat and sandflat species; (2) energetic mixing of peaty mud from existing marsh deposits in Bolinas Lagoon with overlying mud and sand of deeper water deposits, resulting in mixed deposits of mud or sandy mud with internal rip-ups of peat or peaty mud (cores 97-BOL-D1, 98-Bol-D1, and 98-BOL-D2, Fig. 6); and (3) rapid fluvial aggradation at Bolinas Lagoon indicated by coarse, poorly sorted gravelly sand abruptly deposited over a low marsh soil. The subsidence during this event is constrained to have occurred between A.D. 180 and 1450 (Fig. 11). The postulated A.D. 1250 buried soil preserved at Bolinas Lagoon and Bodega Harbor satisfies the five criteria proposed herein for distinguishing earthquake-induced from nonseismic relative sea level changes (Table 2). The abrupt upper contact between marsh soil and overlying tidal flat mud or sand indicates sudden submergence. Changes in diatom assemblages across the mud-over-peat contacts are consistent with sudden relative sea level rise. A long duration of submergence is indicated by the overlying mud deposits of several decimeters that contain fossil shells of Cerithidea californica and Macoma nasuta, mollusks common to mudflats in Bolinas Lagoon (Ricketts et al., 1985). Buried tidal-wetland soils extend across at least 350 m at the northern end of Bolinas Lagoon and from 50 to 130 m across the marsh at the southeast end of Bodega Harbor. Both of these basins lie adjacent to the San Andreas fault where geomorphic evidence of transtensional deformation is present. Deposits of poorly sorted alluvium that abruptly overlie the buried soil at Bolinas Lagoon are consistent with dramatic changes in sedimentation rate caused by rapid (months to years) fluvial aggradation responding to sudden base level rise at the head of the lagoon. Finally, synchroneity of submergence events at widely spaced sites is demonstrated by the consistent radiocarbon results from mud-over-peat contacts in Bodega Harbor and Bolinas Lagoon (Fig. 11). Diatom analyses across the abrupt lower contact of the peaty soil in the marsh at Bolinas Lagoon provide preliminary evidence of rapid emergence prior to the inferred A.D. 1250 earthquake. Bucknam et al. (1992) cited similar evidence for earthquake-induced abrupt emergence of tidal flats to freshwater meadows about 1000 years ago in the

2632


southern Puget Sound. Although, in most cores the sudden formation of the soil at Bolinas Lagoon is indicated by a peaty soil containing low marsh diatoms abruptly overlying estuarine mud that contains mud and sand flat diatoms, no other evidence was found that is consistent with earthquakeinduced uplift prior to A.D. 1250 at Bodega Harbor. Furthermore, the coseismic uplift required to suddenly form emergent marshes from what once was a tidal flat environment is inconsistent with the structural setting of Bolinas basin as an inferred right step in the right-lateral San Andreas fault system. Other processes such as local erosion or shoaling in the lagoon related to changes in the tidal range or morphology of the barrier spit also may explain the apparent rapid development of the intertidal soil. Therefore, coseismic emergence represents one of several mechanisms that could explain the abrupt lower bounding contact of the buried soil beneath Bolinas Lagoon. Equivocal Stratigraphic Evidence for Submergence approximately A.D. 1650 Possible evidence for marsh submergence during a San Andreas fault earthquake approximately 300–350 years ago exists in the stratigraphy of Bodega marsh. In four of the eight cores sampled at Bodega, mud to peaty mud abruptly overlies a buried peaty soil (Fig. 9a,b). This relationship extends for at least 50 m laterally. In one core (98-BB-D1), a detrital organic deposit preserved between the buried soil and overlying peaty mud contained tidal flat diatoms, whereas, low marsh diatom assemblages were preserved in the buried soil. The presence of tidal flat diatoms overlying a low marsh soil suggests sudden relative sea level rise may have occurred. However, low to high marsh diatom assemblages in the peaty mud overlying the detrital organic deposit indicate that the duration of relative sea level rise may have been brief. Deposits of sand in cores 98-BB-D3 and 97-BBD1 in the southwestern part of the transect may be correlative with buried soil 2 observed in the northeastern part of the transect (Fig. 9a,b). The 2- to 2.5-cm-thick sand bed in core 97-BB-D1 exhibits two fining-upward beds of sand and is a candidate tsunami deposit possibly associated with the buried soil observed in other cores. Although these cores are located several tens of meters closer to the Pacific Ocean and barrier dunes southwest of the marsh than the cores that exhibited buried soil 2, neither of the sand deposits were directly coincident with buried soils themselves. Finally, bracketing radiocarbon ages that constrain the date of submergence for soil 2 (Fig. 11) are consistent with age constraints proposed for a prehistoric San Andreas fault earthquake around A.D. 1650 (Prentice, 1989; Niemi and Hall, 1992; Simpson et al., 1996; Schwartz et al., 1998). Evidence for a seventeenth-century earthquake in Bolinas Lagoon may have been eroded during the middle to late nineteenth century’s logging. Contact H is an erosional unconformity and likely represents a period of anthropogenic erosion. An indication of missing stratigraphy is suggested by the lack of sediment with radiocarbon age estimates be-

tween about the fourteenth century and the middle nineteenth century in Bolinas lagoon sediment (Fig. 6). Stratigraphic Evidence of Subsidence during the 1906 Earthquake The youngest buried soil (soil 1) at Bodega marsh may record local submergence and subsequent burial of the marsh during the 1906 San Francisco earthquake. Although Gilbert (Lawson, 1980) did not report evidence for coseismic subsidence in this area following the 1906 rupture, an abrupt mud-over-peat contact occurs in the upper part of six of the eight Bodega cores studied. Diatom analyses of sediment from above and below the soil 1 contact indicate no change in relative sea level (e.g., cores 98-BB-D1 and 98-BB-D2; Fig. 10). The burial of soil 1 is post-1850 based on the first appearance of R. ascetocella pollen, which occurs below soil 1. An alternative explanation for soil 1 is that it reflects widespread changes in hillslope erosion and basin sedimentation rates due to anthropogenic activities since about the late 1850s. The 1906 earthquake did not leave an unambiguous sedimentologic record in the marsh at the north end of Bolinas Lagoon despite evidence for previous coseismic subsidence 750 years ago and historical reports of coseismic subsidence during the 1906 earthquake (Lawson, 1908). Possible reasons for the lack of clear stratigraphic evidence for the 1906 earthquake include the following: (1) Extensive redwood logging, which began in the middle 1800s, occurred on the nearby hillslopes. The logging led to increased rates of hillslope erosion and rapid sedimentation in the marshes, possibly masking sedimentological evidence of coseismic subsidence in 1906. (2) There may not have been as much subsidence in northern Bolinas Lagoon as there was to the south. Gilbert (in Lawson, 1908) reported minor Salicornia stress in the northern Bolinas Lagoon area. He suggested that the stress resulted from either liquefaction or shaking. Implications for Earthquake History of the Northern San Andreas Fault The 1906 MW 7.8 San Francisco earthquake produced surface rupture from San Juan Bautista in the south to near Cape Mendocino in the north (Fig. 1) (Lawson, 1908). The 1996 Working Group on Northern California Earthquake Potential (WGNCEP 1996) divided the 1906 rupture segment into three subsegments. The north coast segment extends from the Golden Gate area, north of the San Francisco Peninsula, northward to at least Point Arena (Fig. 1). The other two segments defined by the WGNCEP (1996) lie south of the north coast segment. The 1999 Working Group further subdivided the north coast segment into north and south segments (WGCEP, 1999). The earthquake dates presented herein, when combined with paleoseismic trench data from previous studies conducted along the northern San Andreas fault, can be used to evaluate whether the north coast seg-

2633


ment typically ruptures in its entirety or is composed of shorter rupture segments that produce smaller earthquakes. Given the limits of resolution of radiocarbon age estimates for prehistoric earthquakes, it is not possible to conclusively evaluate whether the north coast segment of the San Andreas fault ruptures in a series of closely timed smaller earthquakes or whether a single large earthquake always ruptures the entire segment, as proposed by the 1996 WGNCEP and included as a likely scenario by the 1999 WGCEP. Figure 12 is a compilation of all available paleoseismic data along the northern San Andreas fault, including the results of this study. Two aspects of this figure require further discussion. First, it is important to note that the vertical bars represent permissible limits for past surface ruptures and are based on maximum and minimum limiting dendrocalibrated radiocarbon age estimates. The radiocarbon age estimates used to constrain the vertical bars are proba-

bility distributions. Thus, the probability that individual points along the vertical bar represent the true age of the earthquake is highest near the middle of each vertical bar. Second, the shaded horizontal bars represent windows in time when rupture of the entire north coast segment of the San Andreas fault might have occurred, if the entire segment ruptured in a single earthquake. The data presented in Figure 12 allow the interpretation that the entire north coast segment characteristically ruptures in single events. However, the data also allow an equally plausible interpretation that several smaller earthquakes occurred on shorter subsegments of the north coast segment. The fact that the shaded horizontal bars (Fig. 12) lie near or at the ends of the permissible event age ranges for many of the sites, suggests that it is unlikely that the entire north coast segment always ruptures in single earthquakes. Multiple smaller earthquakes are more likely, each rupturing shorter

Santa Cruz Mtns.

(Heingartner, 1998)

Santa Cruz Mtns.

(Schwartz et al., 1998)

(This Study)

Bolinas Lagoon

Dogtown

(Cotton et al., 1980, Niemi and Hall, 1992)

(Niemi, 1992) Niemi and Hall, 1992)

Olema

Bodega Harbor

(This Study)

Fort Ross

(Simpson et al., 1996)

Point Arena

(Prentice, 1989)

(Baldwin, 1996)

2000

Point Arena

"North Coast Segment" (WGNCEP, 1996)

(Halle et al., 1995)

Southeast

San Francisco Peninsula

Northwest

1906 1906a,e

1900

1850c

1800 1700

1660aef 1640a

1640b

1650b

1690a1

1870d 1610b

1660a

1600

1670f

1560 1530

1500

1520

1450c

1470 1390c

1400

1300

1290b

1300

1450

1430

1330a

Calender years, AD

1200 1170

1100 1100 1050

1040

1000

1020

950b

900

920

800

900

Time window for penultimate event, if entire North Coast segment ruptured

700 680

600

560

Time window for earlier event, if entire North Coast segment ruptured

560

500 400 300 200 100

a – Stratigraphic truncation (i.e., unconformity) a1 – Reinterpretation of existing geologic and radiometric data b – Fault termination (i.e., buried surface trace) c – Indirect evidence (coseismic subsidence inferred from marsh stratigraphy) d – Indirect evidence (inferred based on geometrical constraints of offset channel) e – Geologic evidence (fissure fill) f – Geologic evidence (warped deposits within fault zone)

0

Figure 12. Space–time diagram for the northern San Andreas fault. Locations of studies are shown in Figure 1. Vertical bars represent calibrated age limits for surface rupture events based on minimum and maximum limiting radiocarbon age estimates. Shaded horizontal bars represent windows in time when 1906-type surface rupture of the entire San Andreas fault might have occurred.

2634


reaches of the fault. Examples of faults that have experienced closely timed historical earthquakes that would not be distinguishable using today’s radiocarbon dating and paleoseismic methods include subduction zone earthquakes in 1944, 1946, and 1854 along the Nankai trough in southwest Japan (Ando, 1975) and multiple middle twentieth-century earthquakes along the North Anatolian fault in Turkey (Stein et al., 1997). Conducting paleoseismic investigations where smaller rupture segments could be expected to overlap and where two closely timed events would be recorded in the stratigraphy may be the only way to confidently resolve this issue.

Conclusions This approach to the study of prehistoric strike-slip earthquakes, which includes identifying and dating estuarine sedimentary evidence for rapid relative sea level change, has proven to be a fruitful method of paleoseismic investigation. Evidence of earthquake-induced subsidence from the northern margin of Bolinas Lagoon and the southeastern margin of Bodega Harbor is described. Each of these sites occupy structural basins or troughs along the northern San Andreas fault. Radiocarbon dating and identification of the first occurrence of nonnative pollen near abrupt sedimentological changes in the cores allow the dates of coseismic subsidence during San Andreas fault earthquakes to be constrained. Estuarine sediment from northern Bolinas Lagoon preserves evidence of an earthquake that occurred about 750 years ago. Evidence for this earthquake includes a marsh soil that was abruptly buried by either tidal flat mud or coarse, poorly sorted deltaic deposits containing a mixture of terrestrial sediment and marine mud. Diatom evidence for at least several decimeters of sustained relative sea level rise corroborates the interpretation of earthquake-induced subsidence for this abruptly buried soil. Three buried soils in a marsh at the southern end of Bodega Harbor are likely also a result of coseismic subsidence. All three buried marsh soils have abrupt upper contacts, although diatom evidence for decimeters of sustained subsidence is not as strong as it is for the buried soil in Bolinas Lagoon. The dates of rapid relative sea level rise estimated for the Bodega Harbor marsh correspond to estimates for dates of earthquakes based on nearby paleoseismic investigations. Existing paleoseismic information describing the northern San Andreas fault has been compiled and interpreted and compared with results of this study in order to evaluate the dates and length of past large earthquakes. Data from most of the paleoseismic investigations indicate that an earthquake occurred about 400 years ago and another earthquake occurred about 700 years ago. The data allow for 1906-type ruptures of the fault from the Santa Cruz Mountains to at least Point Arena. However, a sequence of closely timed, smaller earthquakes would have produced a similar set of paleoseismic data that would likely be indistinguishable using present radiocarbon dating techniques. Researchers and

those interested in characterization of earthquake hazard and/or risk are encouraged to give equal weight to the possibility of non-1906-type ruptures occurring on the northern San Andreas fault.

Acknowledgments This research was supported by U.S. Geological Survey National Earthquake Hazards Reduction Grant 1434-HQ-97-GR-03009. For assistance in the field, we thank Bill Lettis and Christopher Hitchcock of William Lettis & Associates, Inc., and Rick Koehler, Kevin Ryan, and Chandler Tucker of Humboldt State University. For assistance in preparing this report, we gratefully acknowledge the contributions of Laura Paella, Rick Zeeb, and Richard Burmeister of William Lettis & Associates, Inc. C. Garrison-Laney specifically wishes to thank Eileen Hemphill-Haley for her guidance. Many landowners and agency representatives allowed us access to property, including Sarah Koenig and Kim Cooper of the National Park Service, the Giacomini family of Point Reyes Station, Skip Schwartz, Ray Peterson, Katy Etienne and John Kelly of Audubon Canyon Ranch, Ed Ueber and Jan Rolleto of the Gulf of the Farallons National Marine Sanctuary, Dennis Kalaowski from Bodega Harbour Golf Links, Chris Branham, Marin County Department of Parks, Open Space and Cultural Services, Renee Pasquinelli, Brek Parkman, and Dave Boyd of the California State Parks System. Also, a hearty thanks to Bill Lettis for his discussions and reviews. This article benefited from the review comments kindly provided by Brian Atwater and Lisa Grant.

References Ando, M. (1975). Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan, Tectonophysics 27, 119– 140. Atwater, B. F. (1987). Evidence for great Holocene earthquakes along the outer coast of Washington State, Science 236, 942–944. Atwater, B. F., S. G. Conard, J. N. Dowden, C. W. Hedel, R. L. MacDonald, and W. Savage (1979). History, landforms, and vegetation of the estuary’s tidal marshes, in San Francisco Bay: The Urbanized Estuary, Investigations into the Natural History of the San Francisco Bay and Delta with Reference to the Influence of Man, J. T. Conomos (Editor), Pacific Division of the American Association for the Advancement of Science, San Francisco, 347–385. Atwater, B. F., and E. Hemphill-Haley (1997). Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington, U.S. Geol. Surv. Prof. Pap. 1576, 108 pp. Baldwin, J. N. (1996). Paleoseismic investigation of the San Andreas fault on the north coast segment, near Manchester, California, M.S. Thesis, San Jose State University, California, 127 pp. plus plates. Bergquist, J. R. (1978). Depositional history and fault-related studies, Bolinas Lagoon, California, U.S. Geol. Surv. Open-File Rept. 78-802, 164 pp. plus plates. Brown, R. D., and E. W. Wolfe (1972). Map showing recently active breaks along the San Andreas fault between Point Delgada and Bolinas Bay, California, San Francisco Bay Region Environment and Resources Planning Study, Basic Data Contribution 1, scale 1:62,500. Bucknam, R. C., E. Hemphill-Haley, and E. B. Leopold (1992). Abrupt uplift within the past 1700 years at southern Puget Sound, Washington, Science 258, 1611–1614. Carver, G. A., and J. P. McCalpin (1996). Paleoseismology of compressional tectonic environments, in Paleoseismology, J. P. McCalpin (Editor), Academic Press, San Diego, 583 pp. Clague, J. J., and P. T. Bobrowsky (1994). Evidence for a large earthquake and tsunami 100–400 years ago on western Vancouver Island, British Columbia, Quat. Res. 41, 176–184.

Past Earthquake-Induced Rapid Subsidence along the Northern San Andreas Fault Clarke, S. H., and G. A. Carver (1992). Late Holocene tectonics and paleoseismicity, Southern Cascadia subduction zone, Science 255, 188– 192. Cooper, A. K. (1973). Structure of the continental shelf west of San Francisco, U.S. Geol. Survey Open-File Rept. 73-0048, 72 pp. Cotton, W. R., N. T. Hall, and E. A. Hay, (1980). Holocene behavior of the San Andreas fault at Dogtown, Point Reyes National Seashore, California, U.S. Geol. Survey Open-File Rept. 80-1142, 33 pp. Darienzo, M. E., and C. D. Peterson, (1990). Episodic tectonic subsidence record in late Holocene salt marshes, Northwest Oregon Central Cascadia Margin, Tectonics 9, no. 1, 1–22. Faegri, K., and J. Iverson (1989). Textbook of Pollen Analysis, Fourth Ed., K. Faegri, P. E. Kaland, and K. Krzywinski (Editors) John Wiley & Sons, New York, 328 pp. Galloway, A. J. (1977). Geology of the Point Reyes Peninsula, Marin County, California, California Division of Mines and Geology Bulletin 202, 72 pp., plus 1:48,000-scale plate. Geist, E. L., and M. L. Zoback, (1999). Analysis of the tsunami generated by the Mw 7.8 1906 San Francisco earthquake, Geology 27, 15–18. Hall, N. T., R. H. Wright, and K. B. Clahan (1995). Final Technical Report: Paleoseismic investigations of the San Andreas fault on the San Francisco Peninsula, U.S. Geol. Surv. Final Tech. Rept. NEHRP award no. 14-08-001-G2081, 53 pp. Heingartner, G. F. (1995). Paleoseismic recurrence investigation of the Santa Cruz Mountains segment of the San Andreas fault near Watsonville, California, M.S. Thesis, San Jose State University, California, 136 pp. Helley, E. J., K. R. LaJoie, W. E. Spangle, and M. L. Blair (1979). Flatland deposits of the San Francisco Bay Region, California: their geology and engineering properties, and their importance to comprehensive planning, U.S. Geol. Surv. Prof. Pap. 943, 88 pp. plus plates. Hemphill-Haley, E. (1995a). Intertidal diatoms from Willapa Bay, Washington: application to studies of small-scale sea-level changes, Northwest Sci. 69, 29–45. Hemphill-Haley, E. (1995b). Diatom evidence for earthquake-induced subsidence and tsunami 300 years ago in southern coastal Washington, Geol. Soc. Am. Bull. 107, 367–378. Huffman, M. E., and C. F. Armstrong (1980). Geology for planning in Sonoma County, Calif. Div. Mines Geol. Spec. Rept. 120, 31 pp. plus plates. Kelsey, H. M., R. C. Witter, and E. Hemphill-Haley (1998). Response of a small Oregon estuary to coseismic subsidence and postseismic uplift in the past 300 years, Geology 26, 231–234. Knudsen, K. L., J. M. Sowers, R. C. Witter, C. M. Wentworth, E. J. Helley, R. S. Nicholson, H. M. Wright, and K. H. Brown, (2000). Preliminary maps of Quaternary deposits and liquefaction susceptibility, ninecounty San Francisco Bay region, California: a digital database, U.S. Geol. Surv. Open-File Rept. 00-444. Koenig, J. B. (1963). Santa Rosa sheet, Calif. Div. Mines Geol. geologic map of California, scale-1:250,000. Lawson, A. C. (Chairman). (1908). The California earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission (Reprinted 1969), Vol. 1, Carnegie Institution of Washington Publication, Washington, D.C., 451 pp. and atlas. Mathewes, R. W., and J. J. Clague (1994). Detection of large prehistoric earthquakes in the Pacific Northwest by microfossil analysis, Science 264, 688–691. National Ocean Service (NOS) (1994). National Ocean Service tide gage data, http://www.co-ops.nos.noaa.gov/ benchmarks/9415020.html (last accessed September 2002). Nelson, A. R. (1992a). Discordant 14C ages from buried tidal-marsh soils in the Cascadia subduction zone, southern Oregon coast, Quat. Res. 38, 74–90. Nelson, A. R. (1992b). Holocene tidal-marsh stratigraphy in south-central Oregon: evidence for localized sudden submergence in the Cascadia subduction zone, in Quaternary Coasts of the United States: Lacustrine and Marine systems, C. P. Fletcher and J. F. Wehmiller (Editors),

2635

Society of Economic Paleontologists and Mineralogists Special Publication No. 48, 287–301. Nelson, A. R., and W. F. Manley (1992). Holocene coseismic and aseismic uplift of Isla Mocha, south-central Chile, Quat. Int. 15/16, 61–76. Nelson, A. R., I. Shennan, and A. J. Long, (1996). Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America, J. Geophys. Res. 101, no. B3, 6115–6135. Niemi, T. M. (1992). Late Holocene slip rate, prehistoric earthquakes, and Quaternary neotectonics of the northern San Andreas fault, Marin County, California, Ph.D. Thesis, Stanford University, Palo Alto, California, 199 pp. Niemi, T. M., and N. T. Hall, (1992). Late Holocene slip rate and recurrence of great earthquakes on the San Andreas fault in northern California, Geology 20, 195–198. Noller, J. S., K. I. Kelson, W. R. Lettis, K. A. Wickens, G. D. Simpson, K. Lightfoot, T. Wake, (1993). Preliminary characterizations of Holocene activity on the San Andreas fault based on offset archaeologic sites, Ft. Ross State Historic Park, California, U.S. Geol. Surv. NEHRP Final Tech. Rept. Ovenshine, A. T., D. E. Lawson, and S. Bartsch-Winkler (1976). The Placer River silt–intertidal deposit caused by the 1964 earthquake, U.S. Geol. Surv. J. Res. 4, 151–162. Peinado, M., F. Alcatraz, J. Delgadillo, M. De La Cruz, J. Alvarez, and J. L. Aguirre, (1994). The coastal salt marshes of California and Baja California: phytosociological typology and zonation, Vegetatio 110, 55–66. Peterson, C. D., and M. E. Darienzo (1996). Discrimination of climatic, oceanic and tectonic mechanisms of marsh burial, Alsea Bay, Oregon, in Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest, A. M. Rogers, T. J. Walsh, W. J. Kockelman, and G. Priest (Editors), U.S. Geol. Surv. Profess. Pap. 1560, 115–146. Plafker, G. (1990). Regional vertical tectonic displacement of shorelines in south-central Alaska during and between great earthquakes, Northwest Sci. 64, 250–258. Plafker, G., and J. C. Savage, (1970). Mechanism of the Chilean earthquakes of May 21 and 22, 1960, Geol. Soc. Am. Bull. 81, 1001–1030. Prentice, C. S. (1989). Earthquake geology of the northern San Andreas fault near Point Arena, California, Ph.D. Dissertation, California Institute of Technology, Pasadena, California, 252 pp. Ricketts, E. F., J. Calvin, J. W. Hedgpeth, and D. W. Phillips (1985). Between Pacific Tides, Fifth Edition, Stanford University Press, Stanford, California, 652 pp. Schwartz, D. P., D. Pantosti, K. Okumura, T. J. Powers, and J. C. Hamilton (1998). Paleoseismic investigations in the Santa Cruz Mountains, California: implications for recurrence of large-magnitude earthquakes on the San Andreas fault, J. Geophys. Res. 103, no. B5, 17,985–18,001. Shennan, I., D. B. Scott, M. M. Rutherford, and Y. Zong (1999). Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood Flats, Alaska, Quat. Int. 60, 55–73. Simpson, G. D., J. S. Noller, K. I. Kelson, and W. R. Lettis (1996). Logs of trenches across the San Andreas fault, Archae camp, Fort Ross State Historic Park, Northern California, U.S. Geol. Surv. NEHRP Final Tech. Rept. Stein, R. S., A. A. Barka, and J. H. Dieterich (1997). Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering, Geophys. J. Int. 128, 594–604. Stuiver, M., and P. J. Reimer (1993). Extended 14C data base and revised Calib 3.0 14C age calibration program, Radiocarbon 35, 215–230. Wells, L. (1998). Radiocarbon dating of Holocene tidal marsh deposits: applications to reconstructing relative sea level changes in the San Francisco estuary, in J. M. Sowers, J. S. Noller, and W. R. Lettis (Editors), Dating and Earthquakes: Review of Quaternary Geochronology and its Application to Paleoseismology, NUREG/CR-5562, pp. 3–35 to 44. Working Group on Northern California Earthquake Potential (WGNCEP),

2636


(1996). Database of potential sources for earthquakes larger than magnitude 6 in Northern California, U.S. Geol. Surv Open-File Rept. 96705, 40 pp. plus figures. Working Group on California Earthquake Probabilities (WGCEP), (1999). Earthquake probabilities in the San Francisco Bay Region: 2000 to 2030—A summary of findings, U.S. Geol. Surv. Open-File Rept. 99517, online version 1.0, 44 pp. plus figures (http://geopubs.wr.usgs. gov/open-file/of99-517/). Wright, C., and A. Mella, (1963). Modifications to the soil pattern of southcentral Chile resulting from seismic and associated phenomena during the period May to August 1960, Bull. Seism. Soc. Am. 53, 1367–1402. Zoback, M. L., R. C. Jachens, and J. A. Olson (1999). Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula, J. Geophys. Res. 104, 10,719–10,742.

William Lettis & Associates, Inc. 1777 Botelho Drive, Suite 262 Walnut Creek, California 94596 (K.L.K., R.C.W., J.N.B)

Humboldt State University Geology Department Arcata, California 95521 (C.E.G., G.A.C)

Manuscript received 19 August 2001.