Interaction of Repeating Earthquake Sequences and Its Relation to Fault Friction Properties

Abstract:

It is widely known that earthquakes can trigger each other. Most explanations attribute this interaction to either static or dynamic stress changes caused by coseismic slip, although other mechanisms of interaction exist. Motivated by the interactive behavior of small repeating earthquake sequences (RES) on the creeping section of the San Andreas Fault, we study, through numerical modeling of rate-and-state faults, how clustered small seismogenic zones embedded into creeping fault areas trigger seismic events on one another.

Our work has shown that, for a wide range of laboratory-motivated fault properties, static stress changes due to postseismic slip is more important in triggering seismic events than the coseismic stress changes. Such finding is based on the comparison of our simulations and the fault failure models that only account for the static coseismic stress changes. The time advance of triggered events is much larger and the interaction of RES extends much farther than would be predicted based on static stress changes from coseismic slip alone.

In this study, we investigate how the interaction of the repeating sequences depends on the properties of the creeping segment. Our results indicate that more velocity-strengthening faults, i.e. faults with larger values of the rate-and-state parameter (a-b), suppress propagating creep fronts and reduce interaction. Further, triggering time is determined by the propagation speed of postseismic creep, which depends on the frictional properties and the co-seismic stress increase on the creeping area. The latter quantity is related to the stress drop of the seismic event and can be estimated from coseismic slip. Hence, with known stress drops and triggering times, it should be possible to infer the frictional properties of the creeping region.

Our models also show that interaction of seismogenic patches of different sizes leads to significant irregularity in the recurrence time and magnitude of the resulting seismic events, even though each patch would produce a perfect repeating sequence on its own. Such irregularity is observed for natural repeating sequences. While spatial and/or temporal variability in fault properties may be responsible for such behavior, our simulations show that the irregularity may be at least in part due to the interaction.