Influence of Shear Heating and Thermomechanical Coupling on Earthquake
Sequences and the Brittle-Ductile Transition
Abstract
Localized frictional sliding on faults in the continental crust
transitions at depth to distributed deformation in viscous shear zones.
This brittle-ductile transition (BDT), and/or the transition from
velocity-weakening (VW) to velocity-strengthening (VS) friction, are
controlled by the lithospheric thermal structure and composition. Here
we investigate these transitions, and their effect on the depth extent
of earthquakes, using 2D antiplane shear simulations of a strike-slip
fault with rate-and-state friction. The off-fault material is
viscoelastic, with temperature-dependent dislocation creep. We solve the
heat equation for temperature, accounting for frictional and viscous
shear heating that creates a thermal anomaly relative to the ambient
geotherm which reduces viscosity and facilitates viscous flow. We
explore several geotherms and effective normal stress distributions (by
changing pore pressure), quantifying the thermal anomaly, seismic and
aseismic slip, and the transition from frictional sliding to viscous
flow. The thermal anomaly can reach several hundred degrees below the
seismogenic zone in models with hydrostatic pressure, but is smaller for
higher pressure (and these high-pressure models are most consistent with
San Andreas Fault heat flow constraints). Shear heating raises the BDT,
sometimes to where it limits rupture depth rather than the frictional
VW-to-VS transition. Our thermomechanical modeling framework can be used
to evaluate lithospheric rheology and thermal models through predictions
of earthquake ruptures, postseismic and interseismic crustal
deformation, heat flow, and the geological structures that reflect the
complex deformation beneath faults.