Long-Term Fault Slip in Models With Coseismic Weakening: Depth Extent and Spatio-Temporal Complexity of Earthquake Ruptures

What determines the depth extent of slip in large earthquakes? Faults feature depth-dependent frictional, hydraulic, and structural properties. Observationally, faults are separated into seismogenic layers (SL) and deeper creeping extensions based on either microseismicity or inferred locking depth. Slip in large earthquakes is often assumed to be limited to the SL. Physically, this separation can be explained by transition, at slow slip rates, from rate-weakening (RW) to rate-strengthening (RS) behavior. However, as revealed in experimental and theoretical studies, enhanced weakening during rapid earthquake slip – e.g., due to thermal pressurization (TP) of pore fluids - may be critical to rupture propagation. The extent of such weakening need not coincide with the traditionally defined SL.

Using 3D rate-and-state fault models with temperature and pore pressure evolution, we study the effect of depth-dependent permeability and shear-zone width on long-term fault slip. Competition between the two properties determines the depth dependence of co-seismic weakening due to TP, since permeability decreases with depth (due to higher compression), promoting TP, while the shear-zone width likely increases below certain depth (due to increasingly inelastic bulk properties), lowering the co-seismic temperature increase and suppressing TP.

We find that, indeed, large ruptures can penetrate below the traditionally defined SL, into the “stable” fault regions, due to TP. When they do, microseismicity patterns at the bottom of the SL change, potentially allowing for identification of such penetration in recent events. The behavior of large ruptures, including their depth extent, varies along strike, even though the fault properties are uniform along strike. This is because co-seismic weakening is strongly dependent on the local rupture properties (slip rate and slip), setting up a strong feedback loop between the weakening and rupture response. The non-uniform slip during one event leads to spatio-temporal complexity in subsequent events, including large variations of depth extent with time. Our current efforts are directed towards quantifying these effects, including the relation between rupture slip, depth, and length as well as their variability.