The Loma Prieta, California, Earthquake of October 17, 1989—Aftershocks and Postseismic Effects

While the damaging effects of the earthquake represent a significant social setback and economic loss, the geophysical effects have produced a wealth of data that have provided important insights into the structure and mechanics of the San Andreas Fault system. Generally, the period after a large earthquake is vitally important to monitor. During this part of the seismic cycle, the primary fault and the surrounding faults, rock bodies, and crustal fluids rapidly readjust in response to the earthquake's sudden movement. Geophysical measurements made at this time can provide unique information about fundamental properties of the fault zone, including its state of stress and the geometry and frictional/rheological properties of the faults within it. Because postseismic readjustments are rapid compared with corresponding changes occurring in the preseismic period, the amount and rate of information that is available during the postseismic period is relatively high. From a geophysical viewpoint, the occurrence of the Loma Prieta earthquake in a section of the San Andreas fault zone that is surrounded by multiple and extensive geophysical monitoring networks has produced nothing less than a scientific bonanza.

The reports assembled in this chapter collectively examine available geophysical observations made before and after the earthquake and model the earthquake's principal postseismic effects. The chapter covers four broad categories of postseismic effect: (1) aftershocks; (2) postseismic fault movements; (3) postseismic surface deformation; and (4) changes in electrical conductivity and crustal fluids.

This publication consists of the following articles:

• Introduction, by Paul A. Reasenberg

• Aftershocks of the Loma 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 data, by Leif Wennerberg

• 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 Loma 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 BŸrgmann, Paul Segall, Mike Lisowski, and Jerry L. Svarc

• Shallow, postseismic slip on the San Andreas fault at the northwestern end of the Loma Prieta earthquake rupture zone, by John Langbein.

• Models of postseismic deformation and stress transfer associated with the Loma Prieta earthquake, by M.F. Linker and J.R. Rice

• A shear strain anomaly following the Loma Prieta earthquake, by M.T. Gladwin, R.L. Gwyther, and R.H.G. Hart

• A magnetotelluric survey of the Loma Prieta earthquake area: implications for earthquake processes and lower crustal conductivity, by Randall L. Mackie, Theodore R. Madden, and Edward A. Nichols

This publication is part of the set of four multi-chapter USGS Professional Papers on the Loma Prieta, California, Earthquake of October 17, 1989:

Professional Paper 1550, Earthquake Occurrence , Coordinators: William H. Bakun and William H. Prescott

Professional Paper 1551, Strong Ground Motion and Ground Failure, Coordinator: Thomas L. Holzer

Professional Paper 1552, Performance of the Built Environment, Coordinator, Thomas L. Holzer

Professional Paper 1553, Societal Response, Coordinator: Dennis S. Mileti