DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Manuscript approved for publication, January 2, 1997 Library of Congress catalog-card No. 92-32287 For sale by the U.S. Geological Survey Information Services Box 25286 Denver Federal Center Denver, CO 80225 CONTENTS Page Dl Aftershocks of the Lorna Prieta earthquake and their tectonic implications ................................................ By Lynn D. Dietz and William L. Ellsworth Response of regional seismicity to the static stress change produced by the Loma Prieta earthquake --------------- By Paul A. Reasenberg and Robert W. Simpson Spatial variations in stress from the first six weeks of aftershocks of the Loma Prieta earthquake ....................... By John W. Gephart Loma Prieta aftershock relocation with S-P travel times from a portable array ......................................... By Susan Y. Schwartz and Glenn D. Nelson Empirical Green's function study of Loma Prieta aftershocks: determination of stress drop ------------------------ By H. Guo, A. Lerner-Lam, W. Menke, and S.E. Hough U.S. Geological Survey aftershock ground-motion Response of U.S. Geological Survey creepmeters to the Loma Prieta earthquake ........................................ By K. S. Breckenridge and R.W. Simpson Increased surface creep rates on the San Andreas fault southeast of the Lorna Prieta main shock ........................ By Jeff Behr, Roger Bilham, Paul Bodin, Kate Breckenridge, and Arthur G. Sylvester Effect of the Loma Prieta earthquake on fault creep rates in the San Francisco Bay region ............................ By Jon S. Galehouse Postseismic strain following the Loma Prieta earthquake from repeated GPS measurements ------------------ By Roland Biirgmann, Paul Segall, Mike Lisowski, and Jerry L. Svarc Models of postseismic deformation and stress transfer associated with the Loma Prieta earthquake ..................... 253 By M.F. Linker and J. R. Rice A magnetotelluric survey of the Loma Prieta earthquake area: implications for earthquake processes and lower crustal conductivity ......................... 289 By Randall L. Mackie, Theodore R. Madden, and Edward A. Nichols THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: EARTHQUAKEOCCURRENCE AFTERSHOCKS AND POSTSEISMIC EFFECTS INTRODUCTION By Paul A. Reasenberg, U.S. Geological Survey CONTENTS SEISMOLOGICAL STUDIES The earthquake occurred inside the U.S. Geological Survey's Northern California Seismographic Network (NCSN) and is the largest earthquake within the network to be recorded by it. More than 75 high-gain seismographs recorded the main shock and nearly 11,000 aftershocks during the first 2 years of the earthquake sequence. Many of these instruments had been recording for 20 years be- fore the earthquake. The approximately 5,600 earthquakes that they recorded in the region before the earthquake INTRODUCTION allow detailed comparisons of the pre- and post-earth- quake activity. In addition, some 170 records were ob- While the damaging effects of the earthquake represent tained from strong-motion instruments within 200 km of a significant social setback and economic loss, the geo- the epicenter. After the earthquake, 38 temporary seismo- physical effects have produced a wealth of data that have graphs (mostly strong motion instruments) were deployed provided important insights into the structure and mechan- to record the aftershock sequence. The first three papers ics of the San Andreas fault system. Generally, the period in this chapter analyze the seismological data and focus- after a large earthquake is vitally important to monitor. their analyses on the structure of the San Andreas fault, During this part of the seismic cycle, the primary fault its interaction with the major nearby faults, and the con- and the surrounding faults, rock bodies, and crustal fluids temporary tectonics of the greater Pacific-North Ameri- rapidlyreadjust in response to the earthquake's sudden can plate boundary in the San Francisco Bay area. movement. Geophysical measurements made at this time Dietz and Ellsworth provide a comprehensive survey of can provide unique information about fundamental prop- the aftershock locations and magnitudes, the focal mecha- erties of the fault zone, including its state of stress and the nisms for the main shock and the larger aftershocks, and geometry and frictional/rheological properties of the faults the time-evolution of the aftershock sequence. From these within it. Because postseismic readjustments are rapid com- observations they infer possible geometries of the faulting pared with corresponding changes occurring in the structures active in the earthquake and their possible ki- preseismic period, the amount and rate of information that nematic and geometric relationship to the San Andreas is available during the postseismic period is relatively high. fault. The aftershock focal mechanisms were highly di- From a geophysical viewpoint, the occurrence of the Loma verse in orientation and unlike those of the pre-earthquake Prieta earthquake in a section of the San Andreas fault seismicity, leading Dietz and Ellsworth to conclude that zone that is surrounded by multiple and extensive geo- the stress drop in the main shock may have been nearly physical monitoring networks has produced nothing less complete. than a scientific bonanza. At greater distances from the earthquake, Reasenberg The reports assembled in this chapter collectively ex- and Simpson examine the post-earthquake seismicity amine available geophysical observations made before changes in central California. They compare the 20 years and after the earthquake and model the earthquake's prin- of seismicity recorded in central California before the cipal postseismic effects. The chapter covers four broad earthquake to the activity during the 20-month postseismic categories of postseismic effect: (1) aftershocks; (2) period. They find that some regions (for example, the San postseismic fault movements; (3) postseismic surface de- Francisco peninsula) sustained an increase in seismic ac- formation; and (4) changes in electrical conductivity and tivity after the earthquake, while at least one other area crustal fluids. (the Hayward fault) sustained a decrease in activity. These D2 AFTERSHOCKS AND POSTSEISMIC EFFECTS changes are consistent with the stress changes calculated epicenter. These instruments had been recording fault for simple elastic dislocation models of the earthquake movement for periods up to 25 years before the earth- when the effective coefficient of friction on the faults is quake. In addition, small-scale (50-200 m) triangulation assumed to be low (less than 0.3). A surprisingly small networks at about two dozen sites in the San Francisco amount of stress change (0.1 bar) was apparently suffi- Bay Area on the San Andreas, Calaveras, Hayward, and cient to affect the seismicity on these nearby faults. Concord-Green Valley faults had been observed regularly Another view of the stress changes produced by the to measure surface fault displacements for up to 13 years earthquake is provided by the aftershock focal mecha- before the earthquake. These observations form the basis nisms. Gephart uses fault plane solutions for the first 6 for three papers describing and modeling the surface move- weeks of the aftershock sequence to map the spatial varia- ments of the major faults in the Bay Area after the earth- tions in stress in the vicinity of the main shock rupture. quake. Near and northwest of the main shock hypocenter, he finds In a detailed study of surface fault displacements (creep) low shear stress remaining on the main shock fault plane at the eight nearest creepmeter sites on the San Andreas after the earthquake, again consistent with the inference and Calaveras faults, Breckenridge and Simpson find that of a nearly complete stress drop in the earthquake. South- both the sense (right-lateral or left-lateral) and magnitude east of the hypocenter, near Pajaro Gap and the creeping of long-term creep rate changes on nearby faults agree segment of the fault, significant right lateral shear appar- with coseismic changes in horizontal shear stress calcu- ently remained on the fault. lated for elastic dislocation models of the earthquake. In Aftershock observations also were used to hone seis- contrast, the amplitudes of coseismic steps in fault dis- mological technique. Schwartz and Nelson use synthetic placement measured at the same sites apparently bear little calculations and data from portable (PASSCAL) three- similarity to the calculated static stress changes; they pro- component seismographs in order to compare earthquake pose that dynamic stresses (shaking) likely account for hypocenter location methods based on various combina- these coseismic steps. The correlation between the regional tions of P, S, and S-P arrival times. They find that the creep rate changes and the earthquake-induced static stress unsynchronized S-P arrival times observed with a sparse, changes parallels changes observed in the regional seis- portable array provided remarkably good locations rela- micity rates (Reasenberg and Simpson) and suggests that tive to those obtained from the more extensive, permanent over a period of a few years a dominant effect of the NCSN array. The PASSCAL data also were used by Guo earthquake on the major