What Controls the Maximum Magnitude of Continental Normal Faulting Earthquakes?

Date: 4/24/2019

Time: 08:30 AM

Room: Vashon

Effective seismic hazard estimation requires assuming a potential earthquake’s maximum magnitude (Mmax). Tectonic setting and faulting geometry play a key role in influencing an earthquake’s potential size. In shallow (< 40 km), continental environments, the largest normal faulting earthquakes are approximately one magnitude unit smaller than strike-slip and reverse faulting earthquakes. The mechanisms causing this magnitude discrepancy, however, are unresolved. In this study, we examine why the largest normal fault earthquakes are smaller than other types of earthquakes in similar shallow continental environments.

A review of the Global Centroid Moment Tensor (GCMT) catalog for continental earthquakes reveals that normal fault earthquakes have an Mmax of M_w 7.1 whereas the largest strike-slip and thrust fault earthquakes reach ~M_w 8. The GCMT catalog indicates remarkable similarity in normal fault earthquake Mmax in a variety of extensional environments. Using simulated earthquake histories, generated with input parameters from the GCMT catalog’s earthquake statistics, we find that that the lack of large normal fault earthquakes in the GCMT catalog is not likely due to the catalog’s short length. These results suggest that differences in Mmax may reflect physical differences in faulting mechanics.

Since larger earthquakes generally have longer ruptures, fault length may be limiting Mmax. However, a review of global fault catalogs indicates that there are several normal faults in active extensional environments that are long enough to host M_w 7.5+ earthquakes. These results suggest that the observed Mmax differences may lie in the physics of the rupture process itself. We examine key earthquake parameters including moment tensor principal axes orientations and stress drops to explore why continental normal fault earthquakes have a smaller Mmax.

Presenting Author: James S. Neely

Authors