Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Preseismic observations

Professional Paper 1550-C
Prepared in cooperation with the National Science Foundation
Edited by: Malcolm J. S. Johnston



The October 17, 1989, Loma Prieta, Calif., Ms=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.
Publication type Report
Publication Subtype USGS Numbered Series
Title Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Preseismic observations
Series title Professional Paper
Series number 1550
Chapter C
DOI 10.3133/pp1550C
Year Published 1993
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Earthquake Hazards Program, Earthquake Science Center
Description 85 p.
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