Location and Fault Mechanics of the Focal Region of the California Earthquake of April 18, 1906

Anthony Lomax

Anthony Lomax Scientific Software, Mouans-Sartoux, France


We investigate the location and tectonics of the focal region of the 18 April 1906 California Earthquake. Applying modern, probabilistic relocation to arrival observations from the Lawson report, we place the 1906 hypocenter to the west of San Francisco near the offshore San Andreas Fault system; we associate this focus with a dilatational right-bend in the San Andreas Fault system, in agreement with previous work. Using seismological, geological and marine evidence from the Lawson report and from later studies, we propose a
10-15 km long focal region in a zone of sea floor subsidence and complex faulting extending northwestwards from offshore Daly City to offshore of the Golden Gate. We characterize the geometry of 1906 rupture in this region by normal faulting on a steeply west-dipping structure trending about 20º clockwise to the San Andreas Fault zone, and by strike-slip rupture along the San Andreas
Fault system around and away from the west-dipping structure: rupture to the north on a vertical, currently a-seismic fault structure under the Golden Gate Fault, and rupture to the south under the San Francisco Peninsula along a steeply
southwest-dipping structure showing present day extensional tectonism. All three of these structures extend from the near-surface to about 10 km depth. We also identify a 20 km long, linear trend of clustered seismicity at 10-13 km depth which connects the northern part of the proposed focal region with the south end of the southwest-dipping, San Francisco Peninsula structure. We suggest that
this trend reveals faulting at the base of a brittle, upper crust in response to an underlying shear zone in a ductile, lower crust. Motion across this hypothesized, deep shear zone, rotated about 6º clockwise relative to the strike of the San Andreas Fault, may explain the present day extensional tectonism found along the northern San Francisco Peninsula. These new interpretations on the focal region of the 1906 earthquake have implications for seismotectonic understanding, earthquake monitoring and seismic hazard assessment in the San Francisco Bay Area.

Location of the 1906 focal region and hypocenter

Fault mechanics of the focal region

3D visualization using the Java Seismicity Viewer and screen images of NonLinLoc (NLL) location results.  Each Java visualization and screen image shows the NLL relocation for the corresponding figure in the printed paper. ...

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