HR: 14:40h
AN: S33C-05    [Abstracts]
TI: Resolution of Fault Processes in Near-source Records: Supershear Ruptures and the 2002 Denali Fault Earthquake
AU: * Dunham, E M
EM: edunham@physics.ucsb.edu
AF: U. California, Santa Barbara, Department of Physics U. California, Santa Barbara, Santa Barbara, CA 93106 United States
AU: Archuleta, R J
EM: ralph@crustal.ucsb.edu
AF: U. California, Santa Barbara, Department of Geological Sciences and Institute for Crustal Studies U. California, Santa Barbara, Santa Barbara, CA 93106 United States
AB: Ruptures propagate in one of two velocity regimes: either less than the Rayleigh wave speed (sub-Rayleigh) or between the S and P wave speeds (supershear). We present an overview of dynamic source representation, the basis of which is a consideration of how waves released by material failure processes in the fault zone transmit shearing forces ahead of the propagating rupture. Depending on the sign of the forces, they will either drive further material failure or act to lock the fault. The allowed velocity regimes follow naturally from this representation. Furthermore, any departure of rupture growth from steady-state conditions generates a set of elastic waves that diffract off of the moving crack edge. These transient diffractions provide the mechanism that allows ruptures to jump between the two propagation velocity regimes. This theory also predicts that the supershear transition should be accompanied by the release of a Rayleigh interface wave on the fault surface, which manifests as a secondary slip pulse trailing the supershear rupture. Until recently, seismic inversions have revealed most, if not all, earthquakes to be sub-Rayleigh. The 2002 Mw 7.9 Denali Fault earthquake offers evidence to the contrary (Ellsworth et al. 2004), in the form of strong ground motion pulses recorded at pump station 10 just 3km from the fault that differ completely from typical near-source motions from sub-Rayleigh ruptures. We present a simple dynamic rupture model of the event, in which an initially sub-Rayleigh rupture accelerates to supershear velocities 30km before the station. The ground motion is characterized by two sets of pulses, one set appearing when the supershear rupture passes the station, and the second when the theoretically predicted Rayleigh interface wave goes by. The interface wave, which appears naturally in our dynamic models, would be a difficult feature to reproduce in kinematic inversions. An exact match to the pulse widths is not possible using a homogeneous stress and strength distribution along with a slip-weakening friction law. Instead, matching the pulse widths requires either including healing on a 1.5s time scale within the friction law, or having stress heterogeneity with a 5km length scale. The ground motion from these two cases are virtually identical. We augment this analysis by calculating synthetic ground motions using 2D analytical solutions for steady-state ruptures, valid for any arbitrary traction function in the breakdown zone, in both the sub-Rayleigh and supershear regimes. These solutions place constraints on which friction law properties can be inferred from near-source records. At supershear rupture velocities, information remains unattenuated as it is transported away from the fault along the S wave Mach front, a situation that does not occur for sub-Rayleigh speeds. Consequently, ground motion from supershear ruptures offers unprecedented insight into such parameters as the rupture pulse length and the extent of the process zone.
DE: 7223 Seismic hazard assessment and prediction
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting