The effect of shear heating on earthquake cycles and the structure of intercontinental shear zones
Kali Allison
Stanford University
- Date & Time
- Location
- Building 3, Rambo Auditorium
- Summary
The interaction between the seismic portion of faults and their deeper roots is central to understanding the mechanics of earthquake cycles. It is well established that faults are highly localized within the cold, brittle upper crust, but less is known about fault structure in the warmer, more ductile, lower crust and upper mantle. Increasing temperature with depth causes two transitions in behavior: a frictional transition from seismic to aseismic fault behavior, and a transition from brittle to ductile bulk deformation. To explore the interaction between these two transitions, we simulate earthquake cycles on a 2D strike-slip fault, representative of an intercontinental fault like the San Andreas. All phases of the earthquake cycle are simulated, allowing the model to spontaneously generate earthquakes, and to capture frictional afterslip and postseismic and interseismic viscous flow. The fault is represented with rate-and-state friction. Off-fault viscoelasticity is represented with a power-law rheology for dislocation creep, using experimentally-based flow laws for feldspar in the crust and olivine in the upper mantle. The power-law rheology involves an effective viscosity that is a function of temperature and stress, and therefore varies both spatially and temporally. Since the behavior of the system is highly sensitive to temperature, we explore a range of ambient geotherms and additionally incorporate heat generation through frictional and viscous shear heating.
In all models, a zone of lower effective viscosity forms near the fault, which is narrow in the lower crust and which broadens with depth, and most bulk viscous flow occurs within this zone. Large earthquakes, such as the Hector Mine earthquake, have sometimes been observed to cause an increase in effective viscosity. In our model, we only see this type of behavior in simulations with the warmest ambient geotherm considered. Additionally, we find that shear heating significantly decreases the depth of the brittle-ductile transition, and that frictional and viscous shear heating contribute roughly equally. Finally, we explore differences in surface deformation and surface heat flux produced by these models.
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