Pulse-like and crack-like dynamic shear ruptures on frictional interfaces: experimental evidence, numerical modeling, and implications
Destructive large earthquakes occur as dynamic frictional ruptures along pre-existing interfaces (or faults) in the Earth's crust. One of the important issues in earthquake dynamics is the local duration of relative displacement or slip. Seismic inversions show that earthquakes may propagate as self-healing pulse-like ruptures, with local slip duration being much shorter than the overall rupture duration. Yet many classical models produce crack-like ruptures, with local slip durations comparable to the overall rupture duration. We study rupture modes in an experimental set up designed to mimic a fault prestressed both in compression and in shear. Our experiments demonstrate systematic variation from crack-like to pulse-like rupture modes as nondimensional shear prestress is decreased. The results of our experiments are consistent with theories of ruptures on interfaces with velocity-weakening friction. To consider the possibility that slip-weakening friction can also result in such rupture mode transition in the presence of the dynamic nucleation procedure employed by the experimental setup, we conduct numerical simulations with linear slip-weakening friction. In the simulations, we use the parameter regimes that were shown in previous studies to reproduce supershear transition distances obtained in the same experimental setup. We find that simulations with linear slip-weakening friction are unable to reproduce pulse-like ruptures, even in the presence of the dynamic initiation procedure. Our experimental results and simulations imply that velocity-weakening friction plays an important role in dynamic behavior of shear ruptures and needs to be included in earthquake models.
© 2010 Springer Science+Business Media B.V. Received: 16 November 2009. Accepted: 11 March 2010. Published online: 20 April 2010. Nadia Lapusta gratefully acknowledges the support of NSF (Grant EAR 0548277) for this study. Ares J. Rosakis also gratefully acknowledges the support of NSF (Grant EAR 0207873), the US Department of Energy (Grant DE-FG52- 06NA 26209) and MURI (Grant N000140610730, Dr. Y.D.S. Rajapakse, Program Manager). The numerical simulations for this research were performed on Caltech Division of Geological and Planetary Sciences Dell cluster.