Complex fault interactions in a restraining bend on the San Andreas Fault, southern Santa Cruz Mountains, California

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Abstract
The unusual oblique thrust mechanism of the October 18, 1989 Loma Prieta earthquake focused attention on the complex tectonic setting of this segment of the San Andreas Fault. Near the mainshock epicenter, the San Andreas Fault curves to the left defining a restraining bend. The large thrust component of the mainshock focal mechanism is consistent with the horizontal compression expected across restraining bends. However, repeated “Loma Prieta type” earthquakes cannot exclusively produce the observed topography of the southern Santa Cruz Mountains, the highest point of which experienced subsidence during the 1989 earthquake. In this paper, we integrate seismic, geomorphic and tectonic data to investigate the possibility that motions on faults adjacent to the San Andreas Fault play an important role in producing the observed topography. The three‐dimensional geometry of active faults in this region is imaged using the Loma Prieta preshock and aftershock sequences. The most conspicuous features of the fault geometries at depth are: 1) the presence of two distinct zones of seismicity corresponding to the San Andreas and the Sargent‐Berrocal Fault Zones 2) the concave upward shape of the Loma Prieta rupture surface, 3) the reduction in dip of the deepest portions of the rupture plane as the mainshock hypocenter is approached, 4) the apparent transfer of shallow slip in some areas from faults in the San Andreas Fault Zone to those in the Sargent‐Berrocal Fault Zone, and 5) the presence of a deep northeasterly dipping plane associated with the Sargent‐Berrocal Fault Zone. We find that a model of fault interactions which involves displacement on faults in both the San Andreas and the Sargent‐Berrocal Fault Zones is consistent with Loma Prieta coseismic displacements, preshock and aftershock seismicity and observed topography.

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