Short-period nonseismic tilt perturbations and their relation to episodic slip on the San Andreas Fault in central California

Abstract

Data from four tilt meters and several creep meters along the San Andreas fault in central California 30 km south of Hollister were selected to investigate nonseismic short-period tilt perturbations of minutes to hours duration. The group of short-period tilt amplitude changes with residual deflections was chosen because many nonseismic creep events have similar durations. Although each of the four tilt meters recorded many short-period events, the distribution in space and time suggests that most of these events are due to causes other than slip on the San Andreas fault, such as local strain release. Only at one station used in this investigation (MEL) were creep-related tilt changes observed. A quasi-static dislocation model was constructed to predict the tilt changes associated with propagating slip. Comparing predicted to observed tilt wave forms and amplitudes and comparing the tilt event data to creep event data suggest that (I) a tilt amplitude change of 10 -8-10 -8 rad associated with nonseismic creep events will be observed only if the tilt meter is less than 0.5-1.0 km from the fault, (2) the vertical extent of the slip zone associated with these creep events is greater than about 0.5 km but perhaps not more than a couple of kilometers, (3) nonseismic episodic slip on this part of the San Andreas fault does not appear to propagate uniformly over more than a couple of kilometers, and (4) the model presented in this paper requires 0.5-1.0 mm of dip-slip displacement to reproduce the residual deflection in the creep-related tilt wave forms observed.