WHAT CONTROLS NORMAL FAULTING EARTHQUAKES' MAXIMUM MAGNITUDE?

Effective seismic hazard estimation requires knowledge of a potential earthquake’s maximum magnitude. 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 shallow, continental earthquakes reveals that normal fault earthquakes have a maximum magnitude of M_w 7.1 whereas the largest strike-slip and thrust fault earthquakes have ~M_w 8. We find magnitude-frequency differences with both strike-slip and reverse fault events producing lower b-values (~0.8) than normal fault earthquakes (~1). Although historical seismicity catalogs suggest that the intermountain western U.S. may have experienced slightly larger earthquakes in the past, the GCMT catalog indicates remarkable similarity in normal fault earthquake maximum magnitude in a variety of extensional environments over the past 40 years. The largest shallow normal fault earthquakes in the Tibetan Plateau, East Africa Rift, Baikal Rift, Italy, and the intermountain western U.S are all ~M_w 7. By analyzing tectonic and structural proprieties, including fault lengths and extensional rates, of these tectonically diverse extensional regions, we explore the constraints that limit the size of normal fault earthquakes.