Shallow slip deficit due to large strike-slip earthquakes in dynamic rupture simulations with elasto-plastic response

Yoshihiro Kaneko, & Yuri Fialko

Published 2011, SCEC Contribution #1475

Slip inversions of geodetic data from several large (magnitude ~7) strike-slip earthquakes point to coseismic slip deficit at shallow depths (<3-4 km), i.e., coseismic slip appears to decrease towards the Earth surface. While the inferred slip distribution may be consistent with laboratory-derived rate-and-state friction laws suggesting that the uppermost brittle crust may be velocity strengthening, there remains a question of how the coseismic slip deficit is accommodated throughout the earthquake cycle. The consequence of velocity-strengthening fault friction at shallow depths is that the deficit of coseismic slip is relieved by postseismic afterslip and interseismic creep. However, many seismic events with inferred shallow slip deficit were not associated with either shallow interseismic creep or robust shallow afterslip. Hence the origin of shallow slip deficit remains uncertain. In this study, we investigate whether inelastic failure in the shallow crust due to dynamic earthquake rupture can explain the inferred deficit of shallow slip. Evidence for such failure is emerging from geologic, seismic, and geodetic observations. We find that the amount of shallow slip deficit is proportional to the amount of inelastic deformation near the Earth surface. Such deformation occurs under a wide range of parameters that characterize rock strength in the upper crust. However, the largest magnitude of slip deficit in our models is 2-4 times smaller than that inferred from kinematic inversions of geodetic data. To explain this discrepancy, we further explore to what extent assumptions in the kinematic inversions may bias the inferred slip distributions. Inelastic deformation in the shallow crust tempers coseismic strain near the fault, introducing an additional artificial~ deficit of up to 10% of the maximum slip in inversions of geodetic data that are based on purely elastic models. The largest magnitude of slip deficit in our models combined with the bias in inversions accounts for up to 25% of shallow slip deficit, which is comparable, but still smaller than 30-60% deficit in inferred from kinematic inversions. We discuss potential mechanisms that may account for the remaining discrepancy between slip deficit predicted by elasto-plastic rupture models and that inferred from inversions of space geodetic data.

Citation
Kaneko, Y., & Fialko, Y. (2011). Shallow slip deficit due to large strike-slip earthquakes in dynamic rupture simulations with elasto-plastic response . Geophysical Journal International, 186, 1389-1403. doi: 10.1111/j.1365-246X.2011.05117.x.