Roughness at the base of the seismogenic zone: Contributing factors

The cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300°C. It can generally be modeled as the transition from frictional to quasi‐plastic behavior in quartzofeldspathic crust. Shear resistance increases with depth through the frictional regime to peak at the transition, beneath which it falls off exponentially with increasing temperature. Larger earthquake ruptures ( M L >5.5) nucleate around this transition depth where the highest concentrations of strain energy may accumulate. Varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone. Factors affecting the depth of the transition include crustal composition, geometry and mode of faulting, fluid pressure levels in the frictional regime, and water content in the quasi‐plastic regime, quasi‐plastic strain rate, and geothermal gradient. Evaluation of their relative importance is complicated because several are interdependent. However, compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance, while regional variations in heat flow look to be particularly effective in creating long‐wavelength heterogeneities in strain energy concentration affecting faulting style.