Beyond elastic rebound: extracting permanent strain from interseismic deformation

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One of the main uses of GPS data in studying the boundaries of tectonic plates is to understand current rates for fault movements and understand how faults interact. This kind of understanding can all feed into a better assessment of earthquake hazards because it helps us better understand the longer term behavior of fault systems. The researchers in the project will try to reconcile two kinds of models, those that characterize single faults with those that look at long-term evolution of a full plate boundary. The results of this work have important implications for how fast plate boundaries are moving and how the ground around faults deforms over time. The work will use two examples, on in Taiwan, and the other in Southern California. Comparing and contrasting the two areas will improve overall understanding of earthquakes processes.The purpose of the proposed research is to develop dynamic models of geodetic data that extend traditional elastic models to include permanent deformation. The approach is to develop viscoelastic-plastic boundary element models in which the deforming crust is loaded by far-field stresses and faults slip and the surrounding crust deforms in response to the loads. They will employ geodetic data, stress state inferred from focal mechanism data, and geologic and thermochronologic data to constrain the deformation models. In Taiwan they will use the GPS-derived horizontal and vertical velocity fields, an island-wide leveling network, and stress orientations throughout the crust to address several key outstanding issues about the fault architecture responsible for mountain building processes in Taiwan. They will test several models of deep fault geometry and a proposed deep-seated underplating processfor uplifting the core of the mountain range. Model results will be compared with low-temperature thermochronology. In southern California the research will examine models of long-term deformation across the strike-slip fault system to determine the relative contributions of various pate-driving forces (plate boundary forces, gravitational potential energy, mantle drag) and compare predicted long-term fault slip rates with geologic slip rate estimates. They will focus on the important outstanding problem of the portion of deformation that occurs as off-fault inelastic yielding distribution through the crust versus deformation that is accommodated by slip on faults.