Propagating episodic creep and the aseismic slip behavior of the Calaveras Fault north of Hollister, California

Abstract

A detailed kinematic study of fault slip occurring from the surface to a depth of about 7 km on the Calaveras fault north of Hollister was conducted during the summer of 1977. The observations coincided with a period of propagating episodic fault creep activity sensed along the fault trace. Data used in the investigation consist of creepmeter records, near-field strainmeter observations, and high-resolution geodetic measurements, all collected contemporaneously over a period of 4 months. Detailed descriptions and analyses of the creepmeter and geodetic data have been presented elsewhere. The near-field strain measurements are here reported in detail, and their analysis draws upon the previous two data sets for support. The strainmeter observations are most sensitive to slip occurring in the upper 2 km; hence the emphasis of the paper is placed upon the role of propagating episodic creep in the broad-scale behavior of the fault. The results suggest that propagating episodic fault creep as sensed along the fault trace is confined to the upper kilometer or so of the crust and represents the response of the surface layers to a longer-term form of episodic aseismic slip occurring below. The mean form of the advancing rupture front within the upper kilometer is ostensibly the same as that indicated by records from the surface creepmeters. Evidence is presented, however, which suggests that propagating creep events may not always break the surface and may propagate at velocities much slower and at amplitudes significantly larger than those generally observed at the surface.Paper number 80B 1713. 0148-0227/81/080B-1713501.00 (over periods of weeks) fault displacement observed at the surface, and 'propagating episodic fault creep' (or 'episodic creep') will refer to the class of characteristically rapid-onset accelerated slip episodes (creep events) observed at the surface that are thought to result from expanding aseismic shear-failure ruptures along limited sections of the fault surfaces.From the viewpoint of fault mechanics, it is important to establish the physical mechanism that gives rise to propagating episodic fault creep. Several mechanisms have been proposed [Savage, 1971;Ida, 1974;Rice and Simons, 1976;Rutter and Mainprice, 1979]; however, it is difficult to evaluate their applicability without first establishing a thorough kinematic description of the failure process itself, against which the various model predictions may be tested. Clearly, this task demands that the slip behavior of the fault below the weathered free-surface layer be established. The elastic field to which propagating failure gives rise provides a window for viewing subsurface slip. However, as the elastic-field sample points are generafly restricted to the free surface, resolution will generafly deteriorate with increased depth, even for perfect data. This paper presents the results of a kinematic study of the behavior of the Calaveras fault down to depths of •7 km during periods of propagating episodic fault creep monitored during the summ...

Episodic Aseismic Slip at Plate Boundaries

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