CONTINENTAL NORMAL FAULT EARTHQUAKES AND THEIR MAXIMUM MAGNITUDES

Prevailing wisdom holds that continental normal fault earthquakes have smaller maximum magnitudes (M_max) than continental earthquakes with other fault geometries. This difference has significant implications for understanding seismic hazards of extensional regions. Here, we examine how M_max varies with fault geometry in continental regions using the Global Centroid Moment Tensor (GCMT) earthquake catalog. We explore whether the lower normal fault magnitudes are an artifact of the relatively short earthquake catalog, and - if not - the potential physical reasons for the smaller magnitudes.

We find that the largest continental normal fault earthquakes are ~ M_w 7 while other fault geometries can reach ~ M_w 8. Furthermore, the magnitude-frequency distribution of continental normal fault earthquakes has a higher b-value and lower corner magnitude than the other fault geometries. Our analysis indicates that if significantly larger continental normal fault earthquakes could occur, the GCMT catalog is long enough to capture them. Therefore the common observation that continental normal fault earthquakes have a smaller M_max appears to be true and is not simply due to our relatively short catalog lengths. We also find that normal faults are long enough to host larger earthquakes, so fault length is likely not the primary limiting factor of M_max.

In contrast, oceanic normal fault earthquakes do not exhibit lower M_max or significantly different magnitude distributions relative to other fault geometries. The larger oceanic normal fault earthquakes occur between the trench and outer rise, due to plate bending. Bending produces a different stress field within the lithosphere than pure extension and appears to allow for larger normal fault earthquakes. Because such bending environments do not occur in regions of continental extension, such earthquakes do not arise. Hence although both pure extension and bending lead to normal fault earthquakes, only bending produces the lithosphere stress conditions necessary to produce very large normal fault earthquakes.