Dynamic Compaction as a Simple Mechanism for Fault Zone Weakening

Elevated pore fluid pressures have long been thought to contribute to the apparent weakness of large plate bounding faults such as the San Andreas Fault. Sleep and Blanpied (1992) propose a mechanism in which compaction during interseismic creep reduces available pore space and hence increases fluid pressure. Assuming that the fault zone is not fully compacted during the interseismic period, we invoke a similar concept in this study, however in this case the compaction process occurs dynamically via the stresses associated with earthquake rupture. Microstructural analysis of fault zone rocks reveals intragranular fragmentation that is developed dynamically (e.g. Rempe et al., 2009) and is consistent with the style of fracture observed in individual quartz grains during bulk compaction (Chester et al., 2004), which thus lends some justification to our proposed mechanism.
We incorporate undrained compaction into a dynamic rupture model of a strike-slip fault by using an end-cap failure criterion (e.g. Wong et al., 1997). We show that dynamic stresses associated with rupture propagation cause the fault zone to compact, leading to elevated pore pressure in the undrained fault zone and low effective shear stress on the fault, consistent with the heat-flow constraints. An important result is that radiated S-waves propagating ahead of the rupture front can cause compactant failure and consequently lower the static friction as well as the dynamic friction, leading to a low strength drop on the fault. This may present a possible advantage over other dynamic weakening mechanisms such as thermal pressurization, which involves large strength drops accompanying rupture propagation.