A Global Survey of the Initiation of Large Earthquakes: The Influence of Earthquake Magnitude and Depth on the Rate of Growth of Moment Release

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Abstract for proposal EAR 0106680 (PH # 63x) Title: A global survey of the initiation of large earthquakes: The influence of earthquake magnitude and depth on the rate of growth of moment release PI: Heidi Houston, University of California, Los Angeles How earthquakes initiate is a key issue in understanding the process of seismic rupture. For example, the question of how seismic ruptures grow and whether large earthquakes initiate in a different manner than small ones has attracted significant attention in recent years, but remains controversial. We propose to determine the initial rates of growth of earthquake ruptures (initial source time functions) with detailed resolution in time for many (~150) large earthquakes (with depths of 15 to 100 km and MW \204 6.7) directly from seismograms. Systematic comparison and statistical analysis of the results will enable us to determine the influences of factors such as earthquake magnitude, source depth, focal mechanism, and tectonic setting on how ruptures grow in their first few seconds. Several recent results suggest that such influences may exist. A slow beginning to rupture (seismic nucleation phase) proportional in duration to the final magnitude and lasting 4 to 10 seconds for MW 8.0 to 8.2 earthquakes has been put forward as a general feature. The work proposed here should detect its systematic presence or absence in our magnitude range. Systematic differences in the energy budgets of small earthquakes compared to large ones have been proposed, which suggest that melting or fluid pressurization occurs during large but not small events, and affects dynamic rupture. The duration of rupture is known to decrease with depth, implying an increase in the rate of initial rupture growth. Finally, rupture growth could be affected by the aspect ratio (shape) of the fault, which is closely related to tectonic regime. Differences (or their absence) in rate of initiation with magnitude, depth, or other parameters can place constraints on the physical processes acting during rupture initiation.