HR: 17:45h
AN: S42H-08    [PDF]
TI: How do near-source ground motions change when ruptures go super-shear?
AU: * Aagaard, B
EM: baagaard@usgs.gov
AF: U.S. Geological Survey, 525 South Wilson Ave, Pasadena, CA 91106 United States
AU: Heaton, T
EM: heatont@caltech.edu
AF: California Institute of Technology, 104-44 Caltech, Pasadena, CA 91125 United States
AB: We simulate long-period near-source ground motions from kinematic ruptures of Mw 7.4 events on a strike-slip fault with different rupture speeds, including ruptures that propagate below the shear-wave speed (subsonic), between the shear-wave speed and the dilatational-wave speed (intersonic), or above the dilatational-wave speed (supersonic). The strong shear-wave directivity present in scenarios with subshear rupture speeds disappears in the scenarios with ruptures propagating faster than the shear-wave speed. Furthermore, the maximum horizontal displacement and velocities rotate from generally fault-perpendicular orientations at subshear rupture speeds to generally fault-parallel orientations at supersonic rupture speeds. For rupture speeds just above the shear-wave speed, the orientations are spatially heterogeneous as a result of the random nature of the slip assumed in the model. The polarization of the horizontal ground motion time histories provides a much more robust feature than maximum amplitude orientation with which to gauge the rupture speed; subshear ruptures are associated with significant fault-perpendicular motion {\em before} fault-parallel motion and super-shear ruptures are associated with fault-perpendicular motion {\em after} significant fault-parallel motion. Based on these simulation results, we examined the near-source recordings from the 1979 Imperial Valley and 2002 Denali fault earthquakes, for which there is some evidence for super-shear rupture. In the Imperial Valley earthquake, the long-period ground motions contain much more energy in the fault-perpendicular direction than in the fault-parallel direction, suggesting predominantly subshear rupture. The polarity of the velocity waveforms are also consistent with subshear rupture. This is consistent with previous studies that found predominantly subshear rupture but leaves open the question of super-shear rupture over a limited area. On the other hand, in the 2002 Denali fault earthquake, the recording closest to the fault (pump station 10) exhibits characteristics found in the simulations with rupture propagation slightly above the shear-wave speed, including orientation of the peak velocity away from the fault-perpendicular direction and nearly simultaneous arrival of large-amplitude motion in the fault-perpendicular and fault-parallel directions. Thus, these simulations and recordings provide additional support for the existence of super-shear ruptures in earthquakes but illustrate the difficulty in obtaining definitive evidence without an extremely dense seismic network along the surface trace of the fault.
DE: 7200 SEISMOLOGY
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting