Surface Rupture Effects on Earthquake Moment-Area Scaling Relations

Abstract

Empirical earthquake scaling relations play a central role in fundamental studies of earthquake physics and in current practice of earthquake hazard assessment, and are being refined by advances in earthquake source analysis. A scaling relation between seismic moment (M0) and rupture area (A) currently in use for ground motion prediction in Japan features a transition regime of the form M0–A2, between the well-recognized small (self-similar) and very large (W-model) earthquake regimes, which has counterintuitive attributes and uncertain theoretical underpinnings. Here, we investigate the mechanical origin of this transition regime via earthquake cycle simulations, analytical dislocation models and numerical crack models on strike-slip faults. We find that, even if stress drop is assumed constant, the properties of the transition regime are controlled by surface rupture effects, comprising an effective rupture elongation along-dip due to a mirror effect and systematic changes of the shape factor relating slip to stress drop. Based on this physical insight, we propose a simplified formula to account for these effects in M0–A scaling relations for strike-slip earthquakes.