Re-examination of the Subsurface Fault Structure in the Vicinity of the 1989 Loma Prieta Mw 6.9 Earthquake, Central California, Using Steep Reflection, Earthquake, and Aeromagnetic Data

Abstract:

We re-examine the geometry of the causative fault structure for the 1989 Mw 6.9 Loma Prieta earthquake in central California, using seismic reflection, earthquake hypocenter, and aeromagnetic data. Our study is prompted by recent interpretations of a two-part dip of the San Andreas fault (SAF) 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 reveal in cross section three upper-crustal steeply dipping, slightly curved clusters that are located within a seismic-velocity depression centered 2-3 km northeast of the SAF zone. Examination of steep-dip reflection data, extracted from a 1991 seismic-refraction profile through the Loma Prieta area, reveals reflective zones that are located slightly above the three aftershock clusters described above. We have remodeled aeromagnetic data along the Loma Prieta profile interpreting a SAF that initially dips northeastward, into the velocity depression. The interpreted SAF reverses its dip at 2- to 4-km depth to dip southwestward, and it and joins the Berrocal fault and the main rupture at ~ 10-km depth. We have no conclusive image of the Sargent fault, between the SAF and Berrocal faults. We interpret that the Loma Prieta rupture occurred on the SAF itself. The geometry interpreted for the SAF largely divides a reflective and seismicity-rich North American plate from a more sparsely reflective and seismicity-poor Pacific plate.