A new differential creepmeter on the Superstition Hills fault reveals that afterslip at approximately 28 mm/year consists of episodic creep events super-imposed on a slow stable-slip component of 2.4 mm/year. The relative contribution from this background slip (8.5% of the total aseismic surface slip) has remained approximately constant for the past 2 years. To account for this observed behavior, we proposed a two-layer model for aseismic slip in which stable-sliding occurs from the surface to a transition depth, below which episodic creep events are initiated. These creep events propagate to the surface through the stable-sliding layer. From the ratio of background slip velocity to total afterslip velocity, we estimate the ratio of depths of stable-sliding to episodic-slip regions to be approximately 1:10. Thus, assuming that episodic slip extends to 3 km, we infer that background surface creep is moderated by processes in the uppermost 300 m. We speculate that the transition depth is sensitive to applied fault-normal stresses and suggest that the ratio of stable-sliding to episodic-slip velocities may provide an indication of secular variations in tectonic stress. Since the background creep is evidently continuous and little affected by episodic events, its velocity may provide a sensitive indication of applied tectonic strain in the period range days to years.

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