Low-Heat and Low-Stress Fault Operation in Earthquake Models of
Statically Strong but Dynamically Weak Faults

Nadia Lapusta
Division of Geological & Planetary Sciences 
Division of Engineering & Applied Science 
California Institute of Technology


Abstract:
Observations suggest that the San Andreas fault generates much less
frictional heat that one would predict based on laboratory static
friction coefficients of 0.6 to 0.8 for most rocks and effective normal
stresses comparable to overburden minus hydrostatic pore pressure.
Hence one concludes that earthquakes there happen under low shear
stress.  Two explanations are most commonly proposed: Either (1)
effective normal stress is very low everywhere on the fault or (2)
static friction coefficients are very low (less than 0.2) and so the
laboratory values are not appropriate for real faults.

There is another possible model, that of a statically strong but
dynamically weak fault with small defect regions to nucleate ruptures.
The idea is that such a fault would operate under low shear stress (and
hence with low heat generation) as follows:  Model earthquakes nucleate
under low shear stress in a defect (weak) region and then propagate into
strong regions due to significant dynamic weakening.   We show that his
idea works in simulations of earthquake sequences in a 2D depth-averaged
elastic model of a faulted crustal plate slowly loaded via coupling to a
steadily moving substrate.  The model uses a modified version of the
classical Dieterich-Ruina rate and state friction law to permit much
stronger weakening at high slip rates V.  In the new law with enhanced
weakening, the steady-state strength varies, essentially, as
1/(1+V/constant) at high V.  Such functional dependence is suggested by
theoretical studies of flash heating of contact asperities at small
slips and behavior of partially drained, thermally pressurized fault
gouge, and perhaps liquefied gouge, at larger slips.  Note that other
weakening processes of dynamic nature such as sliding between different
materials or undrained thermal pressurization of pore fluids may cause
the fault to become dynamically weak, but their descriptions would be
different.