Analysis of the tsunami generated by the M-w 7.8 1906 San Francisco earthquake

We examine possible sources of a small tsunami produced by the 1906 San Fra ncisco earthquake, recorded at a single tide gauge station situated at the opening to San Francisco Bay. Coseismic vertical displacement fields were c alculated using elastic dislocation theory for geodetically constrained hor izontal slip along a variety of offshore fault geometries. Propagation of t he ensuing tsunami was calculated using a shallow-water hydrodynamic model that takes into account the effects of bottom friction. The observed amplit ude and negative pulse of the first arrival are shown to be inconsistent wi th small vertical displacements (similar to 4-6 cm) arising from pure horiz ontal slip along a continuous right bend in the San Andreas fault offshore. The primary source region of the tsunami was most likely a recently recogn ized 3 km right step in the San Andreas fault that is also the probable epi central region for the 1906 earthquake. Tsunami models that include the 3 I nn right step with pure horizontal slip match the arrival time of the tsuna mi, but underestimate the amplitude of the negative first-arrival pulse. Bo th the amplitude and time of the first arrival are adequately matched by us ing a rupture geometry similar to that defined for the 1995 M-w (moment mag nitude) 6.9 Kobe earthquake: i.e., fault segments dipping toward each other within the stepover region (83 degrees dip, intersecting at 10 km depth) a nd a small component of slip in the dip direction (rake = -172 degrees). An alysis of the tsunami provides confirming evidence that the 1906 San Franci sco earthquake initiated at a right step in a right-lateral fault and propa gated bilaterally, suggesting a rupture initiation mechanism similar to tha t for the 1995 Robe earthquake.