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Shear heatinginduced thermal pressurization during earthquake nucleation on planar faults

Stuart V. Schmitt, Paul Segall, & Takanori Matsuzawa

Published 2011, SCEC Contribution #1373

We model earthquake nucleation on faults with coupled rate- and state-dependent frictional weakening and shear heating-induced thermal pressurization.
The latter effect----once considered to be active only in large earthquakes---involves weakening of the fault by increasing pore pressure, thereby decreasing the effective normal stress.
As slip nucleates on a fault with rate/state friction, the resulting shear heating increases pore pressure if it cannot be dissipated by diffusion.
Recent drill cores from mature faults indicate that the permeability of fault zones is on the order of $10^{-21}$ to $10^{-19}$~m$^2$, which is too low to mitigate the thermal pressurization that occurs at subseismic slip speeds.
To explore this process in detail, we perform 2D numerical simulations that couple rate/state friction on a zero-width fault with shear heating and diffusive transport of heat and pore pressure.
We analyze the two most common forms of frictional state evolution, the ``aging'' law and the ``slip'' (logarithmic) law, since they yield qualitatively different nucleation behavior under drained, isothermal conditions.
Under these conditions, nucleation with the aging law is ``cracklike,'' with the interior of the nucleation zone always slipping at nearly the maximum speed.
When thermal pressurization is included, it dominates the fault weakening at speeds of 0.02--2~mm/s (depending on hydraulic diffusivity), which are well below the speed at which seismic radiation occurs.
After thermal pressurization becomes the dominant weakening mechanism, dramatic along-strike localization of slip occurs because of feedback in which the area of maximum slip experiences the greatest weakening, which in turn favors more slip.
With the slip law friction, however, nucleation is pulse-like, in that the fastest-slipping portion of the nucleation zone propagates unidirectionally, with velocity decaying behind.
Thermal pressurization is diminished since most of the frictional weakening occurs in locations with limited amounts of slip, yet we find that it overwhelms frictional weakening at slip speeds in the range of 1--100 mm/s.
At these slip speeds, seismic radiation is either imminent or commencing.
Hence, thermal pressurization should be considered to be already active---and likely the dominant weakening mechanism---at the onset of seismic radiation.

Schmitt, S. V., Segall, P., & Matsuzawa, T. (2011). Shear heatinginduced thermal pressurization during earthquake nucleation on planar faults. Journal of Geophysical Research,. doi: 10.1029/2010JB008035.