Long-term fault behavior at the seismic-aseismic transition: space-time evolution of microseismicity and depth extent of earthquake rupture

Junle Jiang, & Nadia Lapusta

Published 2014, SCEC Contribution #6055

What controls the fault behavior at the seismic-aseismic transition? Natural faults feature depth-dependent frictional, hydraulic, and structural properties and local heterogeneities. 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 SL.

Using observations of microseismicity and several recent major earthquakes, we show that different behaviors are observed for the space-time evolution of microseismicity and the depth extent of mainshock coseismic slip: (1) depth of microseismicity unaffected by mainshock (2004 Parkfield); (2) sudden increase and gradual decrease of aftershock depth with overlapping mainshock slip (1989 Loma Prieta); (3) elimination of microseismicity at the bottom of the SL with deeper coseismic slip (1999 Izmit and 2002 Denali).

To understand the physical reasons behind such differences, we use 3D fault models governed by rate-and-state friction with temperature and pore pressure evolution to study the long-term behavior of faults with realistic depth-dependent permeability and shear-zone width. Competition between the two properties determines the depth dependence of co-seismic weakening due to TP. We find that earthquake ruptures can penetrate below the traditionally defined SL due to TP. Microseismicity patterns at the bottom of the SL change in response to the depth extent of earthquake penetration. Our models can explain the observed relation between microseismicity patterns and coseismic slip, potentially allowing for identification of deeper penetration in recent events. Based on fault properties, the rate of change in locking depth in the post- and inter-seismic period could be predicted, placing constraints on the depth limit of the previous deeper events. The behavior of large ruptures, including their depth extent, varies along strike, even though the fault properties are uniform along strike, leading to complexities in the earthquake sequences.

Citation
Jiang, J., & Lapusta, N. (2014). Long-term fault behavior at the seismic-aseismic transition: space-time evolution of microseismicity and depth extent of earthquake rupture. Poster Presentation at 2014 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics