The Loma Prieta, California, Earthquake of October 17, 1989—Main Shock Characteristics

The October 17, 1989, Loma Prieta, Calif., earthquake (0004:15.2 G.m.t. October 18; lat 37.036º N., long 121.883º W.; 19-km depth) had a local magnitude (M_L) of about 6.7, a surface-wave magnitude (M_S) 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.

This publication consists of the following articles:

• Synopsis by Paul Spudich
  

• Rupture history of the earthquake from high-frequency strong-motion data by Gregory C. Beroza

• Source characteristics of the earthquake from global digital seismic data by George L. Choy and John Boatwright

• Comparison of L ₁ and L ₂ norms in a teleseismic-waveform inversion for the rupture history of the earthquake by Stephen H. Hartzell, Gordon S. Stewart, and Carlos Mendoza

• Frequency-domain inversion for the rupture characteristics during the earthquake, using strong-motion data and geodetic observations by Stephen Horton, John G. Anderson, and Andres J. Mendez

• Broadband study of the source characteristics of the earthquake by Hiroo Kanamori and Kenji Satake

• Geodetic measurements of coseismic horizontal deformation by Michael Lisowski, Mark H. Murray, and Jerry L. Svarc

• Elevation changes associated with the earthquake and their use to infer fault-slip geometry by Grant A. Marshall and Ross S. Stein

• Main-shock and very early aftershock activity by Karen C. McNally, Gerald W. Simila, and Judy G. Brown
  

• Downdip geometry of the Loma Prieta rupture from teleseismic-waveform inversion by John L. Nábělek

• Mechanical modeling of fault-fold system, with application to the earthquake by Ze’ev Reches and Mark D. Zoback

• Are rupture models consistent with geodetic measurements? by Jamison H. Steidl and Ralph J. Archuleta

• Long-period surface-wave observations of the earthquake by Aaron A. Velasco, Thorne Lay, and Jiajun Zhang

• Source parameters of the earthquake, determined by using long-period seismic waves by Aaron A. Velasco, Thorne Lay, and Jiajun Zhang

• Strong-motion and broadband teleseismic analysis of the earthquake for rupture-process modeling and hazard assessment by David J. Wald, Donald V. Helmberger, and Thomas H. Heaton

• Coseismic displacements measured with the Global Positioning System by Chesley R. Williams and Paul Segall

• Estimation of the rupture velocity of the main shock from the Love-wave group velocity by Francis T. Wu

• Use of stress-drop models to interpret Geodelite/Global Positioning System and leveling measurements at Loma Prieta by Mingdong Wu and John W. Rudnicki

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