Rupture Process of the April 18, 1906 California Earthquake from near-field Tsunami Waveform Inversion

Abstract

The April 18, 1906 M8 California earthquake generated a small local tsunami that was recorded in the near-field by the Presidio, San Francisco tide-gage, located near the Golden Gate. We investigate the causative, tsunamigenic seismic source by forward modeling and nonlinear inversion of the Presidio marigram. We use existing seismological and geological observations to fix the fault system geometry and the surface slip on the onland portions of the San Andreas fault (SAF). We perform synthetic inversions to show that the single, near-field marigram constrains the main features of the rupture on the portion of the SAF system offshore of the Golden Gate. Finally we perform nonlinear inversions for the slip distribution and the timing of the rupture of the 1906 earthquake. Our results, in agreement with previous studies, identify a dilatational step-over and show a bi-lateral rupture, possibly originating or propagated through the step-over region. We find that little or no co-seismic slip on normal faults in the step-over region is required to fit the marigram, and we obtain adequate fits when allowing delays in the source initiation times of up to 3 minutes on the various fault segments. We constrain slip to be of about 5-6 meters for the onshore portion of the SAF to the northwest of the Golden Gate, in agreement with 1906 surface observations of fault offset. Our results favour the hypothesis of a vertical dip for a currently aseismic SAF to the southeast of the Golden Gate, under the San Francisco Peninsula.