Donald Fisher | What Do Observations of Exhumed Tectonic Plate Boundaries Tell Us About Earthquakes?

Dr. Donald Fisher, professor at Pennsylvania State University, gave a talk titled What Do Observations of Exhumed Tectonic Plate Boundaries Tell Us About Subduction Zone Earthquakes?

Abstract: Field and microstructural observations from exhumed examples of the subduction plate interface are incorporated into a model for the slip behavior of active subduction zones. The observations of natural examples lead to a pressure solution flow law, which is combined with a dislocation creep flow law for quartz-phyllosilicate mixtures and incorporated into a numerical model that depicts interseismic creep, seismicity, and fluid flow, including the fluid flow transients that occur during earthquakes. This model (MEFISTO- a Mineralization, Earthquake, and Fluid flow Integrated SimulatOr) includes:1) an earthquake simulator with temperature-dependent increases in cohesion, 2) a fluid flow model coupled to the earthquake simulator through the link between increasing strength (contact area) and permeability, with both low strength and ambient permeability restored by ruptures of the plate interface, and 3) interseismic creep that responds to variations in stress that could drive acceleration in strain rate toward the later part of the seismic cycle. The fluid moves down a pressure gradient driven by fluid production from metamorphic reactions within and downdip of the seismogenic zone. An increase in average shear stress with increasing lithostatic stress along the interface emerges during simulations, with a very low effective coefficient of friction (~0.07), consistent with the shear stress estimates based on heat flow in the forearc. Pressure solution, which is capable of producing measurable strain in mudstones at the updip end of the seismogenic zone (100-150˚C), increases downdip to a point along the interface where the strain rate is capable of accommodating the plate rate. Model results are used to evaluate how coupled seismic slip and fluid flow relate to earthquake size distributions, aftershocks, precipitation associated with veins, fluid pressure transients, slip deficits during the interseismic period, and fault-restrengthening in the aftermath of earthquakes.