Modeling large stress drops and irregular interaction of LA and SF repeaters at Parkfield

Abstract:

Studies of small repeating earthquakes enable better understanding of earthquake source physics. We aim to infer the fault properties in the area of well-studied SF and LA repeating sequences on the creeping section of the San Andreas Fault by numerical modeling, focusing on two intriguing observations: (1) the stress drops of these repeaters are inferred to be much higher, ~30 MPa, than the typical 1-10 MPa range and (2) these sequences appeared to interact irregularly before the 2004 M 6.0 Parkfield event but not after.

Our rate-and-state fault model of the repeaters features two velocity-weakening (VW) patches embedded into a large velocity-strengthening (VS) region. One of our major findings is that interaction between patches is dominated by the effect of accelerated postseismic slip around them, and not by direct static stress changes due to coseismic slip. We are able to numerically reproduce similar seismic events as the SF and LA sequences, with comparable moment magnitude, recurrence time, triggering time, and high stress drop, by incorporating enhanced dynamic weakening of the VW patches due to thermal pressurization. We find that the stress drop of the simulated repeaters depends on the properties of the surrounding creeping segment, since those properties partially control the rupture size.

Our current work is directed towards determining the degree of heterogeneity in the model required for the observed irregular interaction. Many smaller events (Mw < 1.5) have occurred around the two repeating sequences, and we are exploring their effect in our model by adding smaller seismogenic patches. Our preliminary results suggest that some of the observed irregular interaction of SF and LA sequences may be due to their interaction with the smaller patches.