Oct 28, 1997 - Southern California spans a plate bound ... Sciences, University of California, Los Angeles, .... all the way from the Sun to the Earth. The re.
Eos, Vol. 78, No. 43, October 28, 1997 EOS,
T R A N S A C T I O N S ,
A M E R I C A N
G E O P H Y S I C A L
U N I O N
VOLUME 78 NUMBER 43 OCTOBER 28,1997 PAGES 477-492
Crustal Deformation Measured in Southern California PAGES 477, 482
Zheng-kang Shen, Danan Dong, Thomas Herring, Kenneth Hudnut, David Jackson, Robert King, Simon McClusky, and Li-yu Sung Studies at the Southern California Earth quake Center (SCEC) are suggesting that postseismic deformation is significant and long lasting. This s e e m s the c a s e , at least, in a re gion w h o s e dimension is c o m p a r a b l e to the fault rupture length. Researchers at SCEC found strong spatial correlation between the high strain rates and the past large earth quakes at the epicentral areas of the 1952 Kern County, 1971 Imperial, and 1992 Lan ders earthquakes. Southern California spans a plate bound ary c o m p o s e d of hundreds of faults, major and minor, over a region hundreds of km wide. Measuring the crustal deformation field across this broad and c o m p l e x plate boundary poses a great challenge. The Crus tal Deformation Working Group of the SCEC orchestrated a major effort to provide, for the first time, a unified geodetic crustal deforma tion field covering southern California. We have systematically analyzed m u c h of the high-precision geodetic data acquired in southern California over the past 25 years. This work is an important ingredient in fulfill ing the center's goals: to provide a scientific basis for probabilistic seismic hazard estima tion—earthquake probabilities and effects— in southern California and to c o m m u n i c a t e seismic hazard information to users. Analysis of the geodetic measurements reveals the his tory and pattern of crustal deformation, and helps determine slip rates of obscured faults, tests hypotheses of fault interaction, and iden tifies regions where earthquakes may b e likely. The geodetic data include electronic dis t a n c e meter (EDM) measurements s i n c e 1972 by the California Division of Mines and
For m o r e information, c o n t a c t Zheng-kang Shen, Department of Earth and S p a c e Sciences, University of California, Los Angeles, CA 90095-1567.
Geology and the U. S. Geological Survey, very long baseline interferometry (VLBI) measure ments s i n c e 1984 by NASA and the National Geodetic Survey, and both campaign m o d e and continuous Global Positioning System (GPS) measurements s i n c e 1986 by over a dozen universities and government a g e n c i e s including California Institute of Technology; Massachusetts Institute of Technology; Uni versity of California, Los Angeles; University of California, San Diego; U. S. Geological Sur vey; National Geodetic Survey; J e t Propul
sion Lab; California Department of Transpor tation; the city of Los Angeles, and several southern California counties. Most of the data have already b e e n ana lyzed separately [see Dong, 1993; Feigl et al., \993; Bennett etal., 1996; Shen etal., 1996]. The working data were derived from over 100 surveys, 3 - 2 0 days long, between 1986 and 1996. The task of the working group has b e e n to reanalyze them in such a way that the mathematical models are consistent and arbi trary assumptions are avoided. B e c a u s e of the n u m b e r of earthquakes in the region s i n c e 1972 (Figure 1), few stations e s c a p e d c o s e i s m i c disturbance. T o deter mine velocities accurately while a c c o u n t i n g for such displacements, we estimated for af fected stations a three-dimensional step dis placement, constrained for distant stations by seismic models. T h e earthquakes whose c o s e i s m i c displacements are m o d e l e d in clude the 1992 J o s h u a Tree, M6.1; Landers, M 7 . 3 ; Big Bear, M 6 . 4 ; and the 1994 Northridge,M6.7. Near the 1992 Landers
Fig. 1. Station velocities with respect to North America. Velocity error ellipses represent of 95% confidence. Magenta arrows represent station velocities solved from data collected the 1992 Landers earthquake. Stars are epicenters of earthquakes. Original color image at the back of this volume.
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regions after appears
Eos, Vol. 78, No. 43, October 28, 1997 earthquake we used only postearthquake data, and near Northridge we used only preearthquake data, to avoid any possibility of including c o s e i s m i c offsets there. W e also used velocity constraints from separate analyses of VLBI data and the con tinuous GPS data [Bock etal., 1997] observed simultaneously with the working stations to define a reference frame for the analysis. T h e velocity map shown in Figure 1 represents the average interseismic motion of 287 geo detic stations inferred from EDM, VLBI, and GPS measurements m a d e between 1972 and 1995. T h e s a m e solution has b e e n released electronically by the working group at http://scec.ess.ucla.edu/velmap/welcome.shtml. The estimated crustal velocities provide op portunities to compare results from different studies and test various independent deforma tion models and theories. We detected about 50 mm/yr dextral deformation across the plate boundary in southern California, consistent with a revised NUVEL-1A model prediction of 49 mm/yr [DeMets, 1995] for the Pacific-North America relative plate motion in southern Cali fornia. Therefore the geodetic measurement obtained from 5 to 20 years observation seems to agree with the measure from geological data averaged over a million years. T h e a b o v e observation s e e m s to support the idea that overall crustal deformation rates do not c h a n g e much with time across the full plate boundary. However, we c a n n o t e x c l u d e the possibility that local deforma tion rates c h a n g e significantly with time. Thatcher [1983] showed that crustal deforma tion apparently slows over time following a large earthquake. On the other hand, Savage and Lisowski [1995] c o n c l u d e d from 20 years of EDM data that the deformation rates were constant e x c e p t in o n e c a s e that could b e explained by a single a n o m a l o u s survey. W e derived the geodetic strain rates by in terpolating the velocity map result and c o m pared the strain rates with the pattern of past seismic strains. The result gave us the strong spatial correlation mentioned earlier, be tween high strain rates and past large earth quakes at Kern County, Imperial, and Landers, suggesting significant and long last
ing postseismicdeformation [Jacksonetal., 1997]. Assuming constant fault slip rates, we c o m p a r e d the observed crustal velocities with that predicted by the SCEC Phase II proba b l i s t i c s e i s m i c hazard model [Jackson etal, 1995]. Although the model was developed predominately using geologically estimated fault slip rates, agreement b e t w e e n the two is good in general. Significant discrepancies are detected in the regions of past large earth quakes, with the largest shown at the Landers earthquake epicentral region. T h e deformation around the "Big B e n d " of the San Andreas fault, just north of metro politan Los Angeles, is m u c h b r o a d e r than predicted by the Phase II model with a rea s o n a b l e fault locking depth assumption. Such discrepancy could b e the result of a "thin-skinned" block-fault deformation; that is, motion of the upper crust d e t a c h e d from the lower crust. Alternatively, the broad de formation might reflect postseismic effects from large earthquakes on major faults. High stresses might c a u s e localized strain just after an earthquake, but viscoelastic motions could eventually broaden the strain field. Detailed studies are under way to evaluate the differences between the velocity map ob servations and the Phase II model predictions. Results of such studies will b e incorporated into future seismic hazard models for southern California. Meanwhile, the Crustal Deforma tion Working Group of SCEC will update the ve locity map periodically, as new data provide a longer time span for the existing stations and add new sites to the map. Particularly impor tant data will c o m e from the Southern Califor nia Integrated GPS Network, a group of permanent stations whose number has b e e n growing steadily since the network's founda tion in 1990. Including the new data will allow more detailed analysis of crustal deformation and help further solve temporal changes in the velocity field.
Acknowledgments This project is o n e of the major products of the Crustal Deformation Working Group, SCEC. Members of the group are Kenneth
Scientists Track Solar Event All the Way to Earth PAGES 477, 483
Mauricio Peredo, Nicola Fox, and Barbara Thompson For the first time ever, the satellites of the International Solar-Terrestrial Physics (ISTP) "Observatory" have tracked a solar eruption all the way from the Sun to the Earth. The re
sulting c o r o n a l mass ejection (CME) traveled 4 days through interplanetary s p a c e before ar riving at Earth, where it c a u s e d violent distur b a n c e s of the magnetic environment and
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Hudnut (Chair), Duncan Agnew, Y e h u d a B o c k , Andrea Donnellan, Bradford Hager, T h o m a s Herring, David J a c k s o n , Robert King, Zheng-kang Shen, Ross Stein, and Steven Ward. Other major contributors to GPS data collection, archiving, and analysis include Richard Bennett, Michael Cline, Peng Fang, Javier Gonzalez, David Potter, Robert Reilinger, and Stephen Salyards.
References Bennett, R. A., W. Rodi, and R. E. Reilin ger, Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico, J. Geophys. Res., 101, 21,943-21,960, 1996. Bock, Y., et al., Southern California per manent GPS geodetic array: Continuous measurements of regional crustal defor mation between the 1992 Landers and 1994 Northridge earthquakes, J. Geo phys. Res., 102, 18,013-18,033, 1997. DeMets, C , A reappraisal of seafloor spreading lineations in the Gulf of California: Implications for the trans fer of Baja California to the Pacific plate and estimates of Pacific-North America motion, Geophys. Res. Lett., 22, 3 5 4 5 - 3 5 4 8 , 1995. Dong, D., The horizontal velocity field in southern California from a combina tion of terrestrial and space-geodetic data, Ph.D. Thesis, Mass. Inst, of Technol., Cambridge, 1993. Feigl, K. L., Space geodetic measurement of crustal deformation in central and southern California, 1984-1992, J. Geo phys. Res., 98, 21, 677-21712, 1993. Jackson, D. D., K. Aki, A. Cornell, J . Dieterich, T. Henyey, M. Mahdyiar, D. Schwartz, and S. Ward, Seismic haz ards in southern California: Probable earthquakes, 1994 to 2024, Bull. Seismol. Soc. Am., 85, 3 7 9 - 4 3 9 , 1995. Jackson, D. D., Z.-K. Shen, D. Potter, X.-B. Ge, and L. Sung, Earthquakes and strain in California, Science, in press, 1997. Savage, J. C , and M. Lisowski, Interseis mic deformation along the San Andreas fault in southern California, J. Geophys. Res., 100, 12,703-12,717, 1995. Shen, Z.-K., D. D. Jackson, and B. X. Ge, Crustal deformation across and be yond the Los Angeles basin from geo detic measurements, J. Geophys. Res., 101, 27,957-27,980, 1996. Thatcher, W., Nonlinear strain buildup and the earthquake cycle on the San Andreas fault, J. Geophys. Res., 88, 5 8 9 3 - 5 9 0 2 , 1983.
s p e c t a c u l a r auroral displays. T h e initial ex pulsion o c c u r r e d on the Sun on January 6, 1997, and the resulting magnetic cloud hit the Earth on January 10. T h e Sun often erupts. It flings out whitehot ionized gas (actually hotter than whitehot, to where it glows in X rays) with explosive v i o l e n c e . Only o c c a s i o n a l l y is this gas aimed at Earth, however, and it is even more unusual for scientists to b e watching the potentially disruptive mass ejection (as they were in J a n u a r y ) just as it leaves the Sun. This m a d e it possible to alert other scien tific teams of possible activity they might ob serve 2 to 3 days later. It normally takes that long for such ejecta to travel the 150-million-
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Fig. 1.
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Station velocities with respect to North America. Velocity error ellipses represent regions
of 95% confidence. Magenta arrows represent station velocities solved from data collected after the 1992 Landers earthquake. Stars are epicenters of earthquakes.
