The Loma Prieta, California, Earthquake of October 17, 1989—Preseismic Observations

The October 17, 1989, Loma Prieta, Calif., M_s=7.1 earthquake provided the first opportunity in the history of fault monitoring in the United States to gather multidisciplinary preearthquake data in the near field of an M=7 earthquake. The data obtained include observations on seismicity, continuous strain, long-term ground displacement, magnetic field, and hydrology. The papers in this chapter describe these data, their implications for fault-failure mechanisms, the scale of prerupture nucleation, and earthquake prediction in general.

Of the 10 papers presented here, about half identify preearthquake anomalies in the data, but some of these results are equivocal. Seismicity in the Loma Prieta region during the 20 years leading up to the earthquake was unremarkable. In retrospect, however, it is apparent that the principal southwest-dipping segment of the subsequent Loma Prieta rupture was virtually aseismic during this period. Two M=5 earthquakes did occur near Lake Elsman near the junction of the Sargent and San Andreas faults within 2.5 and 15 months of, and 10 km to the north of, the Loma Prieta epicenter. Although these earthquakes were not on the subsequent rupture plane of the Loma Prieta earthquake and other M=5 earthquakes occurred in the preceding 25 years, it is now generally accepted that these events were, in some way, foreshocks to the main event.

This publication consists of the following articles:

• Introduction, by Malcolm J.S. Johnston

• Seismicity in the southern Santa Cruz Mountains during the 20-year period before the earthquake by Jean A. Olson and David P. Hill

• Analysis of low-frequency-electromagnetic-field measurements near the epicenter by Anthony C. Fraser-Smith, Arman Bernardi, Robert A. Helliwell, Paul R. McGill, and O.G. Villard, Jr.

• Seismomagnetic effects by Robert J. Mueller and Malcolm J.S. Johnston

• Near-source short- to intermediate-period ground motions by Randall A. White and William L. Ellsworth

• A reported streamflow increase by Evelyn Roeloffs

• Near-field high-resolution strain measurements by Malcolm J.S. Johnston and Alan T. Linde

• A shear-strain precursor by Michael T. Gladwin, Ross L. Gwyther, and Rhodes H.G. Hart

• No convincing precursory geodetic anomaly observed by Michael Lisowski, James C. Savage, William H. Prescott, Jerry L. Svarc, and Mark H. Murray

• Detection of hydrothermal precursors to large northern California earthquakes by Paul G. Silver, Natalie J. Valette-Silver, and Olga Kolbek

• Borehole strain measurements of solid-earth-tidal amplitudes by Alan T. Linde, Michael T. Gladwin, and Malcolm J.S. Johnston

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