_SECTIONI
SEISMOLOGICAL IRESEARCH_
Empirical Site Response for POLARIS Stations in Southern Ontario, Canada Charley Murphy and David Eaton 1 University of Western Ontario
INTRODUCTION It is well known that local geology, particularly overburden, can greatly increase ground shaking during an earthquake; the best method for determining the degree of amplification remains controversial, however. The so-called site response is a localized resonant amplification ofseismic wave motion that is largely controlled by the thickness, shear modulus, and viscosity of soft soil layers. Accurate knowledge of site-response spectra is important for understanding potentially destructive amplification that could occur during a large earthquake, as well as for calibration of seismic networks. In the absence of cosited measurements of ground motion in deep bedrock and at the surface, site response must be estimated either by numerical computation, based on detailed knowledge of the properties of near-surface layers, or by empirical means. A number of different empirical methods for determining site response are in common use, with various authors citing advantages of one technique over another. The objectives of this study are twofold. Our first purpose is to report empirical site-response spectra for 18 stations (Table 1) of the recently installed POLARIS seismograph network in southern Ontario, Canada (Atkinson et al., 2003). This network is equipped with Guralp CMG-ESP three-component broadband seismometers and Nanometrics Libra digitizers. Data are digitally sampled at 100 Hz and transmitted in near real-time via satellite to two data-collection centers in London and Ottawa, Ontario. The Ontario POLARIS stations were deployed commencing in late 2001 and cover a roughly rectangular area within the lower Great Lakes region, with a station spacing of 50-1 00 km (Figure 1). Near-surface layers at these sites depend on local conditions and range from unweathered Precambrian bedrock to fractured Paleozoic limestone, to thick Quaternary till deposits. As a consequence of the variable near-surface conditions, we observe a wide range of site-response spectra. 1. Corresponding Author
Our second purpose is to compare results obtained using three popular empirical site-response methods: the horizontal/vertical spectral ratio method (HVSR; Lermo and Chavez-Garcia, 1993); Nakamura's (1989) microtremor analysis method; and the standard spectral ratio (SSR) method of Borcherdt (1970). While the first two methods do not require reference sites, the SSR technique relies on a reference spectrum derived from one (or more) bedrock locations. We have slightly modified the SSR approach by using teleseismic waveforms, in order to mitigate variable source and path effects that apply to local and regional events. In addition, shear-wave velocity models previously obtained by Beresnev and Atkinson (1997) at six of the POLARIS sites were used to compute numerical site-response spectra, using the Equivalent-linear Earthquake Response Analysis (EERA) program of Bardet et ale (2000). These simple numerical siteresponse spectra are used to assess the validity of the empirical estimates.
METHOD 1: HORIZONTALNERTICAL SPECTRAL RATIO The horizontal-to-vertical spectral ratios (HVSR) method was introduced by Lermo and Chavez-Garcia (1993) to study site effects in Mexico City and has since been employed by many authors. This technique divides the spectral amplitudes of the horizontal-component ground motion at a particular site by the corresponding vertical-component spectrum, under the assumption that the vertical ground motion is unaffected by resonant amplification caused by shear-wave reverberations trapped in the low-velocity soil layers. Following Siddiqqi and Atkinson (2002), we applied the HVSR method to event time windows of 50-100 s duration containing the most intense parts of the earthquake record. A pre-event noise window of equal length was also extracted, for signal to noise (SNR) calculations. An example of time-window selection for a well recorded event (the M N 5.1 Au Sable Forks, New York earthquake, on 20 April 2002) is presented in Figure 2.
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Station ACTO ALGO BANO BRCO CLPO DELO ELGO HGVO KSVO L1NO MPPO PECO PEMO PKRO PTCO STCO TYNO WLVO Type: S: soil;
Location Acton Algonquin Park Bancroft Bruce Peninsula
TABLE 1 POLARIS Stations in Southern Ontario Used forThis Study Latitude (0) Longitude (0) 43.60867 -80.06236 45.95443 -78.05093 -77.92802 45.01982 -81.44225 44.24372
Centennial Lake 45.24599 Deloro 44.51769 Elora Gorge 43.67546 Hagersville 42.96066 Keeseville, New York 44.55240 Lindsay 44.35407 Murphy's Point 44.76999 Prince Edward County 43.93402 Pembroke 45.67732 Pickering 43.96431 Port Colborne 42.88437 S1. Catharines 43.20963 Tyneside 43.09498 Wesleyville 43.92356 P: Paleozoic bedrock; B: Precambrian bedrock; B*: Precambrian
Elevation (m) 360 235 360 273
-76.96416 272 -77.61857 213 -80.43743 377 -80.12644 224 -73.68610 388 -78.78020 268 -76.26484 143 -76.99387 92 -77.24660 180 -79.07143 197 -79.31151 180 -79.17053 96 - 79.87018 205 -78.39699 70 bedrock with thin soil cover.
Type S S B* S B B P P S S B P B* S P S S S
km
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100
200
.. Figure 1. POLARIS seismograph stations in southern Ontario. Soil sites, Paleozoic bedrock sites, and Precambrian bedrock sites are indicated by white, gray, and black triangles, respectively. 100 Seismological Research Letters Volume 76, Number 1 January/February 2005
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Time (seconds) ... Figure 2. Three-component recording of the 20 April 2002 MN 5.1 Au Sable Forks earthquake in upstate New York, recorded atstation ELGO. Shaded areas indicate time windows used for method 1. For logarithmically sampled frequency bins between 0.1 and 25 Hz, the horizontal-to-vertical spectral ratio 5 was calculated using
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In Equation 1, the threshold function takes a value of 0 or 1, as follows: (3)
if
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(4)
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We selected a value of 2.0 for SNRthresh' based on testing of a range of values (Murphy, 2003) . In Equation 4, SNRik denotes the ratio of the spectrum in the event window (H) to the horizontal spectrum in the noise window (H N):
SNR=
.
(6)
We computed HVSR site-response functions for 74 local and regional events that occurred between December 2001 and July 2003, spann ing a magnitude (MN) range between 2.0 and 5.1. Event locations are plotted in Figure 3. For each event, we extracted three-component seismograms that included a 2-minute pre-event noise segment. In Figure 4, the HVSR functions are plotted and grouped according to near-surface type (soil, Paleozoic bedrock, and Precambrian bedrock). The soil sites (indicated by white triangles) are generally characterized by relatively simple, single-mode HVSR spectra, although a few of the sites (KSVO, ACTO) show more complex spectral shapes. The apparent peak amplification value for all of the soil stations is greater than 4.7. In contrast, the Paleozoicbedrock stations (indicated by gray triangles) lack distinct spectral peaks. The Paleozoic bedrock station s all exhibit average horizontal amplification
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.. Figure 3. Local and regional events (black circles) employed forsite-response calculation using method 1. POLARIS stations are shown as gray triangles. greater than 2.0. Three of the Precambrian bedrock sites (indicated by black triangles) exhibit nearly flat HVSR spectra, with horizontal amplification values of less than 2.0. The site-response characteristics of these stations (CLPO, DELO, and MPPO) is similar to the average site response for bedrock stations in eastern North America, which typically exhibit frequency-independent amplification by a factor of up to 1.7 (Gupta and McLaughlin, 1987; Atkinson, 1993). Two of the sites (BANO and PEMO) that are indicated in Figure 1 as Precambrian bedrock are actually located on thin soil layers above bedrock. These sites exhibit spectral peaks similar to those of the other soil sites. For the six stations (ACTO, BRCO, PKRO, STCO, TYNO, and WLVO) where velocity profiles are available from the shear-wave refraction surveys of Beresnev and Atkinson (1997) , we computed theoretical site-response functions using the EERA program of Bardet et af. (2000). The theoretical functions are indicated in Figure 4 as dashed lines. The EERA program computes site amplification for transient, vertically traveling shear waves in a stack of horizontal soil layers, based on the ratio of surface ground motion to reference ground motion in a deep bedrock layer. For generality, we specified an impulsive (Dirac 8) source-time function. Since our observations are in the weak ground-motion regime, the soil response is expected to be linear (Shearer and Orcutt, 1987). We suppressed nonlinear soil behavior by specifying strain-independent shear moduli and damping ratios. For consistency with Beresnev and Atkinson (1997), we assigned
a Q value of 30 for soil layers (damping of 1.667%), and infinite Q (damping of 0) for rock layers. The deepest layer in the model was extended to 100 m depth, from the 70 m base of the shear-wave velocity models of Beresnev and Atkinson (1997) . Our choice of 100 m depth for the basal bedrock layer is for consistency only; the values of amplification are effectivelycontrolled by the layer variables in the upper 70 m. Below these near-surface layers, we assigned lis = 3,800 m/s and p = 2.8 g/crrr' to match average values for Precambrian crust in this region (Atkinson and Somerville, 1994). Note that this large value of lis reflects regional high-grade metamorphism and intermediate composition that characterizes the Grenvillian upper crust, which was exhumed from deep crustal levels over a large region during the last major tectonic event (White et al., 2000). Other than the basal bedrock layer, all other layer variables are identical to those reported by Beresnev and Atkinson (1997). As expected, the theoretical site-response curves contain a prominent spectral peak at the fundamental-mode frequency, given by
where h is the total thickness of the soil layers and Va is the average shear velocity of the soil layers. Four of the theoretical spectra (BRCO, PKRO, STCO, and TYNO) also contain prominent spectral peaks associated with harmonic overtones of the fundamental frequency. Overtones such as these are a
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... Figure 7. Empirical site-response spectra obtained using method 3. Dashed lines show numerical results calculated using the program EERA (Bardet etal., 2000) based on velocity models of Beresnev and Atkinson (1997). Station symbols are as in Figure 1. the Canada Foundation for Inn ovation. Numerical siteresponse data can be found at http://www.polonet.ca.
REFERENCES Atkinson, G. M. (1993). Earth quake source spectra in eastern Nort h America, Bulletin ofthe Seismological Society of America 83, 1,778-
1,798. Atkinson, G. M . (1996). The high-frequency shape of th e source spectrum for earthquakes in eastern and western Canada, Bulletin ofthe Seismological Society ofAmerica 86, 106-112. Atkinson, G., J. Adams, I. Asudeh, M. Bostock, J. Cassidy, D . Eaton , I. Ferguson, A. Jones, D . Snyder, and D. White (2003). POLARIS Upd ate: Fall 2002, Seismological Research Letters 74,4 1-43. Atkinson, G. M. and P. G. Somerville (1994). Calibration of time history simulation methods, Bulletin of the Seismological Society of America 84, 400-41 4. Bardet, J. P. , K. Iichi, and C. H . Lin (2000). EERA:A ComputerProgram for Equivalent Linear Earthquake Site Response Analysis ofLayered Soil Deposits, University of Southern California, D epartment of Civil Engineering, 40 pp. Beresnev, Igor A. and Gail M. Atkinson (1997). Shear wave velocity survey of seismogra phic sites in eastern Canada: Calibration of ernpir-
ical regression method of estimating site-response, Seismological Research Letters 68,981-987. Borcherdt, R. D . (1970). Effects of local geology on ground motion near San Francisco Bay, Bulletin of the Seismological Society of America 60,29-61. Borcherdt, R. D. and ] . F. Gibbs (1976). Effects oflocal geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake, Bulletin ofthe Seismological Society ofAmerica 66, 467-500. Gupta, I. and K. McLaughlin (1987). Attenuatio n ofground motion in eastern United States, Bulletin ofthe Seismological Society of America 77, 366-383. Lerrno, J. F. and F. J. Chavez-Ga rcia (1993). Site effect evaluation using spectral ratios with only one station, Bulletin of the Seismological Society ofAmerica 83, 1,574-1 ,594. Murphy, c. (2003). Near-surface Characterization and Estimated Site Response at POLARIS Seismograph Stations in Southern Ontario, Canada, M.Sc. th esis, University of Western O ntar io, Londo n, O nta rio, Canada, 122 pp. Nakamura , Y. (1989). A method for dynamic characteristics estimation of the subsurface using microtrem ors on the ground surface, Quarterly Report of the Rdilway Technical Research Institute 30 , 25-33.
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Shearer. P M. and J. A. Orcutt (1987). Surface and near-surface effects on seismic waves: Theory and borehole seismometer results, Bulletin ofthe Seismological Society ofAmerica 77, 1,168-1,196. Siddiqqi, J. and G. M. Atkinson (2002). Ground-motion amplification at rock sites across Canada as determined from the horizontal-tovertical component ratio, Bulletin of the Seismological Society of America 92,877-884. Singh, S. K., J. Lermo, T. Dominguez, M. Ordaz, J. M. Espinosa, E. Mena, and R. Quass (1988). The Mexico earthquake of September 19, 1985: A case study of amplification of seismic waves in the Valley ofMexico with respect to a hill zone site, Earthquake Spectra 4,653-673.
White, D. J., D. A. Forsyth, 1. Asudeh, S. D. Carr, H. Wu, R. M. Easton, and R. F. Mereu (2000). A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec,
Canadian Journal ofEarth Sciences 37, 183-192.
Department ofEarth Sciences University ofWestern Ontario London, ON Canada N6A 5B7
[email protected]
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