HR: 1330h
AN: S52A-0108    [PDF]
TI: The Origin of Supershear Rupture
AU: * Dunham, E M
EM: edunham@physics.ucsb.edu
AF: University of California - Santa Barbara, Department of Physics University of California, Santa Barbara, CA 93106 United States
AU: Favreau, P
EM: pfavre@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252 France
AU: Carlson, J
EM: carlson@physics.ucsb.edu
AF: University of California - Santa Barbara, Department of Physics University of California, Santa Barbara, CA 93106 United States
AB: We discuss four situations in which ruptures make the transition from sub-Rayleigh to supershear propagation velocities, and describe how all are manifestations of a diffraction effect. In 2D, the transition is often discontinuous, with an S-wave stress peak triggering rupture ahead of the main crack edge. For 3D ruptures on homogeneous faults, the rupture smoothly accelerates to supershear velocities. Ruptures breaking the free surface exhibit a stress concentration near the surface that triggers a continuous transition. We describe in detail the fourth mechanism, in which a sub-Rayleigh rupture encounters a high strength obstacle. The rupture front initially decelerates and either partially or totally encircles it. The concave front focuses waves into the center of the barrier, creating slip velocities over an order of magnitude larger than elsewhere. This releases strong elastic waves that manifest as distinctive slip pulses on the fault surface, for which we present an analytical solution. For barriers above a critical strength, the energy concentration triggers a transient supershear burst. Even unbreakable barriers can trigger the transition when the split rupture fronts converge to produce rapid stress drop and high slip velocities (split-front focusing). Scaling laws are given for the time required to break a barrier. The diffraction effect underlying the transition can be understood by considering a rupture front in which the stress breaks down over a finite length scale. The dynamic displacement and stress fields are constructed by a superposition of point shear tractions that appear behind the rupture as it passes a given point and attain their final value (the stress drop) over a finite time scale. The resulting elastic waves cross the broken part of the crack until they reach the moving crack edge, where they diffract into the surrounding medium. Head waves generated within the breakdown zone convert to S-waves at the edge, which, if they are sufficiently strong, trigger a continuous supershear transition. Otherwise, a cumulative effect is necessary to develop this S-wave peak, such as occurs for slowly accelerating 2D cracks, leading to a discontinuous transition. This model also predicts the observed secondary peak in slip velocity moving near the Rayleigh speed behind a supershear front. Rather than having been left behind by the rupture after the supershear transition, it consists instead of surface waves continually emitted from the breakdown zone that never overtake the faster-moving rupture front.
DE: 3210 Modeling
DE: 3230 Numerical solutions
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting