Reexamination of the subsurface fault structure in the vicinity of the 1989 moment-magnitude-6.9 Loma Prieta earthquake, central California, using steep-reflection, earthquake, and magnetic data

Open-File Report 2018-1093
By: , and 

Links

Abstract

We reexamine the geometry of the causative fault structure of the 1989 moment-magnitude-6.9 Loma Prieta earthquake in central California, using seismic-reflection, earthquake-hypocenter, and magnetic data. Our study is prompted by recent interpretations of a two-part dip of the San Andreas Fault (SAF) accompanied by a flower-like structure in the Coachella Valley, in southern California. Initially, the prevailing interpretation of fault geometry in the vicinity of the Loma Prieta earthquake was that the mainshock did not rupture the SAF, but rather a secondary fault within the SAF system, because network locations of aftershocks defined neither a vertical plane nor a fault plane that projected to the surface trace of the SAF. Subsequent waveform cross-correlation and double-difference relocations of Loma Prieta aftershocks appear to have clarified the fault geometry somewhat, with steeply dipping faults in the upper crust possibly connecting to the more moderately southwest-dipping mainshock rupture in the middle crust. Examination of steep-reflection data, extracted from a 1991 seismic-refraction profile through the Loma Prieta area, reveals three robust fault-like features that agree approximately in geometry with the clusters of upper-crustal relocated aftershocks. The subsurface geometry of the San Andreas, Sargent, and Berrocal Faults can be mapped using these features and the aftershock clusters. The San Andreas and Sargent Faults appear to dip northeastward in the uppermost crust and change dip continuously toward the southwest with depth. Previous models of gravity and magnetic data on profiles through the aftershock region also define a steeply dipping SAF, with an initial northeastward dip in the uppermost crust that changes with depth. At a depth 6 to 9 km, upper-crustal faults appear to project into the moderately southwest-dipping, planar mainshock rupture. The change to a planar dipping rupture at 6–9 km is similar to fault geometry seen in the Coachella Valley.

Suggested Citation

Zhang, E., Fuis, G.S., Catchings, R.D., Scheirer, D.S., Goldman, M., and Bauer, K., 2018, Reexamination of the subsurface fault structure in the vicinity of the 1989 moment-magnitude-6.9 Loma Prieta earthquake, central California, using steep-reflection, earthquake, and magnetic data: U.S. Geological Survey Open-File Report 2018–1093, 35 p., https://doi.org/10.3133/ofr20181093.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Data
  • Previous Modeling of Aeromagnetic Data
  • Interpretation
  • Tectonics
  • Comparison with SAF Structure in Coachella Valley
  • Conclusions
  • Acknowledgments
  • References Cited
  • Appendix 1—Comparison of Results from Broad and Narrow Top Mutes
  • Appendix 2—Steep-Dip Reflection Analysis

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Reexamination of the subsurface fault structure in the vicinity of the 1989 moment-magnitude-6.9 Loma Prieta earthquake, central California, using steep-reflection, earthquake, and magnetic data
Series title Open-File Report
Series number 2018-1093
DOI 10.3133/ofr20181093
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Earthquake Science Center
Description v; 35 p.
Country United States
State California
Online Only (Y/N) Y