A re-examination of the subsurface fault structure in the vicinity of the 1989 Loma Prieta Mw 6.9 earthquake, central California, from analysis of steep reflections, earthquakes, and potential-field data

Gary S. Fuis, Edward Zhang, Rufus D. Catchings, Daniel S. Scheirer, Mark R. Goldman, & Klaus Bauer

Submitted August 12, 2017, SCEC Contribution #7502, 2017 SCEC Annual Meeting Poster #155 (PDF)

We reinterpret the causative structure of the 1989 Mw 6.9 Loma Prieta earthquake, in central California, using reflection, earthquake, and potential-field data, prompted by recent interpretations of a two-part dip of the San Andreas fault (SAF) accompanied by a flower structure in Coachella Valley, in southern California. Initially, the prevailing interpretation of fault structure at Loma Prieta was that the mainshock did not rupture the SAF, but rather a secondary fault, because network locations of seismicity did not define a vertical plane nor a fault plane that projected to the surface trace of the SAF. Subsequent double-difference (DD) relocations of Loma Prieta seismicity clarified the structure at Loma Prieta, supporting a flower structure and possible connection of steep faults in the upper crust 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, indicates only 3 robust fault-like features, but these agree approximately in shape and location with upper-crustal DD-aftershock clusters. Subsurface shape of the San Andreas, Sargent, and Berrocal faults are interpretable from these reflections and aftershock clusters, and the San Andreas and Sargent faults appear to dip initially northeastward in the uppermost crust. Prior modeling of potential-field data on nearby profiles confirms a steep SAF with an initial northeastward dip in the uppermost crust, and also suggests strongly that a steep SAF in the upper crust is continuous with the more moderately dipping mainshock rupture in the middle crust, just as interpreted for the southern SAF in Coachella Valley.

Key Words
San Andreas fault structure, active source, potential field, earthquake relocations

Citation
Fuis, G. S., Zhang, E., Catchings, R. D., Scheirer, D. S., Goldman, M. R., & Bauer, K. (2017, 08). A re-examination of the subsurface fault structure in the vicinity of the 1989 Loma Prieta Mw 6.9 earthquake, central California, from analysis of steep reflections, earthquakes, and potential-field data. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
San Andreas Fault System (SAFS)