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## Earthquake cycles on rate-state faults: how does recurrence interval and its variability depend on fault length?

Camilla Cattania, & Paul Segall

Published August 15, 2016, SCEC Contribution #6869, 2016 SCEC Annual Meeting Poster #044

The concept of earthquake cycles consisting of periodic events on the same patch is often invoked or assumed when discussing seismic risk. However, large faults exhibit more complex behavior than periodic stick-slip cycles. Some events, such as the 2004 Parkfield earthquake, are delayed relative to the average recurrence interval; in other cases, the rupture area is larger or smaller than expected. In contrast, small earthquakes can be very predictable: locked patches surrounded by aseismic creep can rupture periodically in events with nearly identical waveforms.

Here we use numerical tools and ideas from fracture mechanics to explore the factors determining recurrence interval (T), rupture size and their variability at different scales. On rate-state faults, T has been estimated by assuming a stress drop given by the difference of steady-state stress at coseismic and interseismic slip speeds, and stressing rate inversely proportional to fault length. However, Werner and Rubin (2013) showed that the scaling of T with fault size in numerical simulations is better explained by an energy criterion: on faults loaded from below, full ruptures occur when the elastic energy release rate at the top of the fault reaches the fracture energy. We run simulations of seismic cycles on rate-state faults, including dynamic weakening due to thermal pressurization. We model a fault loaded from below at constant slip velocity divided in a velocity-weakening section over a velocity strengthening one. We find that T increases with thermal pressurization, and we verify that the energy argument, modified to account for the fracture energy from thermal pressurization, provides a good estimate of T and its scaling with fault size.

We suggest that the recurrence interval is determined by two timescales: the time required to accumulate sufficient elastic energy for full rupture (Tf), and the nucleation time, controlled by the propagation of a creep front into the velocity-weakening region (Tn). Both timescales depend on fault length: Tf increases with length, while Tn decreases. The latter is due to the fact that the velocity-strengthening region experiences faster afterslip on larger faults.
When Tn < Tf, partial ruptures occur; for large enough faults, Tn << Tf multiple partial ruptures are observed. This causes more heterogeneous stress fields and more variability in the recurrence interval. Therefore we suggest that fault size alone, by determining the difference in time scales for nucleation and full rupture, can lead to more variability in seismic behavior without requiring heterogeneity in frictional properties or background stress.

Key Words
Earthquake cycle, rate-state faults, recurrence interval

Citation
Cattania, C., & Segall, P. (2016, 08). Earthquake cycles on rate-state faults: how does recurrence interval and its variability depend on fault length?. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)