Friction and stress drop controlled by fault roughness and strength heterogeneity

Olaf Zielke, Martin Galis, & Paul M. Mai

Published August 6, 2016, SCEC Contribution #6468, 2016 SCEC Annual Meeting Poster #052

An earthquake's stress drop is a fundamental quantity of the rupture process, intimately related to the decrease of frictional resistance to sliding during rupture evolution. A number of recent high-speed laboratory friction experiments have shown a nearly complete breakdown in frictional resistance when coseismic slip velocities are reached. Such large changes in friction coefficient imply large stress drops that exceed those reported for natural earthquakes by an order of magnitude or more. Laboratory friction experiments and seismologic observations seem to contradict each other.
Using large-scale numerical simulations, we address this discrepancy between laboratory- and field-based stress drop estimates. We show that it can be resolved when considering fractal fault geometries and spatial heterogeneity of fault strength. We find that stress drop estimates for natural earthquakes are consistently too low because the underlying analytical expressions assume a planar instead of a geometrically complex source. Fault roughness serves as a bulk frictional agent. To achieve compliance in laboratory- and field-based stress drop estimates additionally requires a heterogeneous distribution of fault strength: Laboratory friction experiments represent characteristics of strong fault asperities (stress drop estimates of high-strength regions) while field observations provide space-time averaged estimates of stress drop, containing asperities and non-asperities. The distribution of strength asperities across a rupture surface has a profound impact on an earthquake’s moment release. Unilateral ruptures – with near-the-edge strength asperities – release less seismic moment per stress drop than bilateral ruptures with well-centered strength asperities.

Zielke, O., Galis, M., & Mai, P. M. (2016, 08). Friction and stress drop controlled by fault roughness and strength heterogeneity. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)