Collaborative Research: Utilizing GPS Measurements of Postseismic Deformation to Infer Spatial Distribution of Frictional Properties on Faults

Indiana University, Bloomington IN

Investigators

Abstract

This project is seeking to further our understanding of the distribution of frictional properties on faults by comparing afterslip models utilizing laboratory-derived rate and state friction laws with geodetic records of deformation following large earthquakes. The idealized view of fault coupling involving an interseismically locked section that ruptures in earthquakes, with aseismically creeping sections above and below the locked zone, is inconsistent with observations from a number of plate boundary faults. For example, the San Andreas fault is locked above about 15 km depth south of Parkfield, California, but north of Parkfield, the San Andreas is creeping at typically seismogenic depths. Kinematic slip inversions from the 1999 Chi-Chi, Taiwan earthquake and the 2003 Tokachi-oki, Japan earthquake show significant afterslip at typically seismogenic depths, along strike from the mainshocks. This research utilizes GPS data from these two earthquakes and the 2004 Parkfield, California earthquake to investigate the distribution of slip and frictional properties on faults. Numerical models of afterslip implementing laboratory derived rate-state friction laws are being developed and implemented in inversions of postseismic GPS time-series data for frictional parameters. The observations of aseismic slip at seismogenic depths raise important questions about frictional properties on the fault and along-strike variations of slip behavior. Faults within the seismogenic part of the crust are generally assumed to be steady-state velocity weakening. Yet, velocity weakening friction does not always lead to unstable slip and and velocity strengthening friction does not always lead to stable slip. This study investigates the conditions on the fault that allow for the spatial variations in slip behavior. High-rate GPS data recorded at 1 Hz following the Parkfield and Tokachi-oki earthquakes is being used to investigate the transition from dynamic slip during the earthquake to stable creep following the earthquake. The nature of the transition from unstable may provide further constraints on the frictional properties on the fault.