Postseismic variations in seismic moment and recurrence interval of repeating earthquakes

In laboratory experiments, longer stationary contact time leads to larger seismic moment during repeated ruptures. However, not all observations in natural fault systems agree with the prediction. We analyze a subset of 34 M-0.4-2.1 repeating earthquake sequences (RES) from 1987 to 2009 at Parkfield to examine the variation of their recurrence properties in space and time. Following a 2004 M6 earthquake, many of the repeating events have greatly reduced recurrence intervals (Tr) that systematically increase with time. In addition to this change in timing, we also find systematic changes in seismic moment (Mo), where most sequences experienced an immediate increase in Mo and subsequent decay as Tr approached pre-quake durations. The RES at shallower depth tend to have a larger range in both Tr and Mo, whereas deeper RES show smaller variation. The shallowest RES with the greatest magnitude (M1.8-2.1) among the events we studied reveal a large variation in Tr but small variation in Mo. These observations are qualitatively consistent with earthquake simulations in 3D continuum fault models with rate- and state-dependent friction. In the models, RES are produced on velocity-weakening patches surrounded by velocity-strengthening fault areas. The models show that the degree of postseismic variation in Mo and Tr is a function of radius (r) and nucleation zone size (h*) of the velocity-weakening patch. A ratio of r/. h* ~ 1 produces negative Mo- Tr slopes, whereas larger ratios of r/. h* yield weak positive slopes. Given the same nucleation size h* (i.e., the same frictional properties and effective normal stress), smaller radii and hence smaller seismic moments result in negative Mo- Tr slopes, whereas larger radii and hence larger moments lead to weak positive Mo- Tr slopes, which are consistent with observations. Conversely, with only a small percentage of its slip accumulated seismically, a small asperity appears to grow in Mo under high loading rate, which is contrary to the view that Mo should decrease due to a reduced strength recovery time.