Connecting depths of seismicity, fault locking, and coseismic slip using long-term fault models

Abstract:

The depth of faulting and its temporal evolution are important for understanding fault physics and seismic hazard. The seismicity cut-off depth (D_seis) and (interseismic) fault locking depth estimated geodetically (D_geod) give independent constraints on the locked-creeping (LC) transition on faults. How the two are related to each other and to the depth extent D_rupt of large earthquake ruptures is still poorly understood.

We explore the relation between D_seis, D_geod and D_rupt in fault models governed by rate-and-state friction and enhanced dynamic weakening. We find that: (i) transition from locked zones (e.g., creeping rates < 0.1 V_pl) to zones creeping with near-plate rates (> 0.9 V_pl) occurs over a certain depth range, which can be significant; (ii) D_seis is predominantly affected by the location and amplitude of the stress concentration front (SCF) near the top of the LC transition, while D_geod depends on the spatial distribution of fault slip rates; (iii) D_seis either stays in place or becomes shallower due to the up-dip migration of the SCF, while D_geod tends to deepen as the stress shadowing region expands throughout the interseismic period; and (iv) D_rupt and D_seis can differ due to deep seismic slip below the seismogenic zone, while the discrepancy between D_geod and D_rupt depends on the extent and amplitude of postseismic slip. Therefore, D_seis and D_geod reflect different aspects of fault behavior and could diverge, especially in cases of deeper penetration of earthquakes and/or significant afterslip. Assuming D_geod equal to D_seis could lead to an underestimation of fault slip rates and a discrepancy with geological estimates.

Our current work is directed towards (a) establishing whether D_geod could constrain the maximum depth extent of D_rupt, as suggested by our simulations so far, and (b) exploring how the presence of the realistic LC transition affects interpretation of the inferred D_geod and D_seis based on simplified models.