EVERYTHING BUT THE EARTHQUAKE (Invited Presentation)

In the aftermath of an earthquake, afterslip occurs in and aftershocks decorate the rupture area, damage zone, and areas beyond the rupture tip. Afterslip can be accommodated by a host of deformation mechanisms, fluids move quickly, aided by transient permeability enhanced by co-seismic damage, and a lingering temperature spike enhances fluid-rock interaction. After some period of time, faults relock (begin to accumulate geodetic strain) and the overall slip rate decreases to below the far-field plate rate, reflecting changes in the response of the rocks to the changing stress conditions. Locking and reloading are perhaps the most interesting and poorly understood periods of the earthquake cycle, as they ultimately control loading rate, timing, and magnitude of earthquakes. Some earthquakes seem to be immediately preceded by accelerating creep, and micro earthquakes that coalesce toward the coming rupture. The earthquake cycle can be mimicked with multiple different modeling approaches (e.g. viscoelastic or rate-and-state friction models) but the rock record contains a direct record of the active mechanisms during every stage of deformation.

Recent observations from exhumed faults and shear zones which collectively indicate that at one point in depth, different deformation mechanisms dominate during different periods of the seismic cycle. All the relevant deformation mechanisms – fault slip, granular flow, solution creep, crystal plastic creep – are very sensitive to grain scale conditions such as grain size, phase mixing, pore connectivity, localization, and fault surface and grain packing geometries. Throughout the seismogenic zone and within the brittle-ductile transition zone, the interplay between dilation and solution creep may be responsible for many observed fault behaviors.