The Loma Prieta, California, Earthquake of October 17, 1989—Strong Ground Motion

Strong ground motion generated by the Loma Prieta, Calif., earthquake (M_S~7.1) of October 17, 1989, resulted in at least 63 deaths, more than 3,757 injuries, and damage estimated to exceed $5.9 billion. Strong ground motion severely damaged critical lifelines (freeway overpasses, bridges, and pipelines), caused severe damage to poorly constructed buildings, and induced a significant number of ground failures associated with liquefaction and landsliding. It also caused a significant proportion of the damage and loss of life at distances as far as 100 km from the epicenter. Consequently, understanding the characteristics of the strong ground motion associated with the earthquake is fundamental to understanding the earthquake's devastating impact on society. The papers assembled in this chapter address this problem.

Damage to vulnerable structures from the earthquake varied substantially with the distance from the causative fault and the type of underlying geologic deposits. Most of the damage and loss of life occurred in areas underlain by “soft soil.” Quantifying these effects is important for understanding the tragic concentrations of damage in such areas as Santa Cruz and the Marina and Embarcadero Districts of San Francisco, and the failures of the San Francisco-Oakland Bay Bridge and the Interstate Highway 880 overpass. Most importantly, understanding these effects is a necessary prerequisite for improving mitigation measures for larger earthquakes likely to occur much closer to densely urbanized areas in the San Francisco Bay region.

The earthquake generated an especially important data set for understanding variations in the severity of strong ground motion. Instrumental strong-motion recordings were obtained at 131 sites located from about 6 to 175 km from the rupture zone. This set of recordings, the largest yet collected for an event of this size, was obtained from sites on various geologic deposits, including a unique set on “soft soil” deposits (artificial fill and bay mud). These exceptional ground-motion data are used by the authors of the papers in this chapter to infer radiation characteristics of the earthquake source, identify dominant propagation characteristics of the Earth’s crust, quantify amplification characteristics of near-surface geologic deposits, develop general amplification factors for site-dependent building-code provisions, and revise earthquake-hazard assessments for the San Francisco Bay region. Interpretations of additional data recorded in well-instrumented buildings, dams, and freeway overpasses are provided in other chapters of this report.

This publication consists of the following articles:

• Synopsis, by Roger D. Borcherdt

• Strong-motion recordings, by A. Gerald Brady and Anthony F. Shakal

• Effect of known three-dimensional crustal structure on the strong ground motion and estimated slip history of the earthquake, by Vernon F. Cormier and Wei-Jou Su

• Simulation of strong ground motion, by Jeffry L. Stevens and Steven M. Day

• Influence of near-surface geology on the direction of ground motion above a frequency of 1 Hz, by John E. Vidale and Ornella Bonamassa

• Effect of critical reflections from the Moho on the attenuation of strong ground motion, by Paul G. Somerville, Nancy F. Smith, and Robert W. Graves

• Influences of local geology on strong and weak ground motions recorded in the San Francisco Bay region and their implications for site-specific building-code provisions, by Roger D. Borcherdt and Gary Glassmoyer

• Ground response on Treasure Island, by Kyle M. Rollins, Michael D. Mchood, Roman D. Hryciw, Matthew Homolka, and Scott E. Shewbridge

• Ground-response studies at the Alameda Naval Air Station, by Harvey Carlisle and Kyle M. Rollins

• Behavior of young bay mud from the Marina District of San Francisco under static and cyclic simple shear, by Nicholas Sitar and Rodrigo Salgado

• Deep instrumentation array at the Treasure Island Naval Air Station, by Pedro de Alba, Jean Beno”t, Daniel G. Pass, John J. Carter, T. Leslie Youd, and Anthony F. Shakal

• Site response in Oakland, California, near the failed section of the Nimitz Freeway, by E.H. Field, Susan E. Hough, Klaus H. Jacob, and Paul A. Friberg

• Aftershock observations from a dense array in Sunnyvale, California, by Arthur Frankel, Susan E. Hough, Paul Friberg, and Robert Busby

• A three-dimensional simulation of seismic waves in the Santa Clara Valley, California, from an aftershock, by Arthur Frankel and John E. Vidale

• Site-response models from high-resolution seismic reflection and refraction data recorded in Santa Cruz, California, by Robert A. Williams, Edward Cranswick, and Kenneth W. King

• Variation of seismic site effects in the Santa Cruz Mountains, California, by Grant T. Lindley and Ralph J. Archuleta

• A post-earthquake reevaluation of seismic hazard in the San Francisco Bay region, by Janice M. Murphy and Steven G. Wesnousky

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