The Loma Prieta, California, Earthquake of October 17, 1989 - Geologic Setting and Crustal Structure

Although some scientists considered the M_s=7.1 Loma Prieta, Calif., earthquake of 1989 to be an anticipated event, some aspects of the earthquake were surprising. It occurred 17 km beneath the Santa Cruz Mountains along a left-stepping restraining bend in the San Andreas fault system. Rupture on the southwest-dipping fault plane consisted of subequal amounts of right-lateral and reverse motion but did not reach the surface. In the area of maximum uplift, severe shaking and numerous ground cracks occurred along Summit Road and Skyland Ridge, several kilometers south of the main trace of the San Andreas fault. The relatively deep focus of the earthquake, the distribution of ground failure, the absence of throughgoing surface rupture on the San Andreas fault, and the large component of uplift raised several questions about the relation of the 1989 Loma Prieta earthquake to the San Andreas fault: Did the earthquake actually occur on the San Andreas fault? Where exactly is the San Andreas fault in the heavily forested Santa Cruz Mountains, and how does the fault relate to ground ruptures that occurred there in 1989 and 1906? What is the geometry of the San Andreas fault system at depth, and how does it relate to the major crustal blocks identified by geologic mapping?

Subsequent geophysical and geologic investigations of crustal structure in the Loma Prieta region have addressed these and other questions about the relation of the earthquake to geologic structures observed in the southern Santa Cruz Mountains. The diverse papers in this chapter cover several topics: geologic mapping of the region, potential- field and electromagnetic modeling of crustal structure, and the velocity structure of the crust and mantle in and below the source region for the earthquake. Although these papers were mostly completed between 1992 and 1997, they provide critical documentation of the crustal structure of the Loma Prieta region. Together, they present a remarkably coherent, three-dimensional picture of the earthquake source region—a geologically complex volume of crust with a long history of both right-lateral faulting and fault-normal compression, thrusting, and uplift.

This publication consists of the following articles:

• Introduction, by Ray E. Wells
  

• Stratigraphy and structure across the San Andreas fault zone in the Loma Prieta region and deformation during the earthquake, by Robert J. McLaughlin and Joseph C. Clark
  

• Geophysical and geologic setting of the earthquake, inferred from gravity and magnetic anomalies, by Robert C. Jachens and Andrew Griscom
  

• Deep geoelectrical structure of the Loma Prieta region, by Brian D. Rodriguez, Victor F. Labson, William D. Stanley, and Jay A. Simpson
  

• Evidence for oceanic crust beneath the continental margin west of the San Andreas fault from onshore-offshore wide-angle seismic recordings, by Thomas M. Brocher, Michael J. Moses, and Stephen D. Lewis
  

• Three-dimensional tomographic analysis of the Loma Prieta region, by Eylon Shalev and Jonathan M. Lees
  

• Three-dimensional variations in V_P / V_S ratio in the Loma Prieta Region, by Shashank R. Atre and Clifford H. Thurber
  

• Seismotectonics of the Loma Prieta region determined from three-dimensional P-wave velocities, V_P / V_S ratios, and seismicity, by Donna M. Eberhart-Phillips and Andrew J. Michael
  

• Teleseismic tomography of the Loma Prieta region: Implications for strain partitioning, by Yoko Takauchi and John R. Evans

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