Chapter A. The Loma Prieta, California, Earthquake of October 17, 1989 - Main shock characteristics

Professional Paper 1550-A
Prepared in cooperation with the National Science Foundation
Edited by: Paul Spudich



The October 17, 1989, Loma Prieta, Calif., earthquake (0004:15.2 G.m.t. October 18; lat 37.036? N., long 121.883? W.; 19-km depth) had a local magnitude (ML) of about 6.7, a surface-wave magnitude (MS) of 7.1, a seismic moment of 2.2x1019 N-m to 3.5x1019 N-m, a source duration of 6 to 15 s, and an average stress drop of at least 50 bars. Slip occurred on a dipping fault surface about 35 km long and was largely confined to a depth of about 7 to 20 km. The slip vector had a large vertical component, and slip was distributed in two main regions situated northwest and southeast of the hypocenter. This slip distribution caused about half of the earthquake's energy to be focused toward the urbanized San Francisco Bay region, while the other half was focused toward the southeast. Had the rupture initiated at the southeast end of the aftershock zone, shaking in the bay region would have been both longer and stronger. These source parameters suggest that the earthquake was not a typical shallow San Andreas-type event but a deeper event on a different fault with a recurrence interval of many hundreds of years. Therefore, the potential for a damaging shallow event on the San Andreas fault in the Santa Cruz Mountains may still exist.

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Chapter A. The Loma Prieta, California, Earthquake of October 17, 1989 - Main shock characteristics
Series title Professional Paper
Series number 1550
Chapter A
DOI 10.3133/pp1550A
Year Published 1996
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Earthquake Hazards Program, Earthquake Science Center
Description 297 p.
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