Why do continental normal fault earthquakes have smaller maximum magnitudes?

Continental normal fault earthquakes have been reported to have smaller maximum magnitudes (M_max) than continental earthquakes with other fault geometries. This difference has significant implications for understanding seismic hazards in extensional regions. Using the Global Centroid Moment Tensor (GCMT) catalog, we examine how M_max varies with fault geometry in continental regions, whether these trends are robust, and potential physical reasons for the smaller magnitudes of continental normal fault earthquakes. We find that the largest continental normal fault earthquakes are in the low M_w 7 range whereas other fault geometries can reach ~M_w 8. The continental normal fault earthquake magnitude-frequency distribution has a lower corner magnitude (a parameterization of M_max) than other fault geometries. The observed smaller continental normal fault M_max is not an artifact of classification criteria or catalog length. Probability calculations indicate that the GCMT catalog is long enough to capture differences in M_max due to fault geometry. Additionally, our analysis indicates that neither fault length nor width is limiting the size of continental normal fault earthquakes. Fault complexity can limit rupture extent, but it is likely not the primary reason for the smaller continental normal fault M_max. Rather, lithosphere yield stress (strength) appears to be the main factor controlling M_max. In extension, lithosphere is weaker, failing at lower yield stresses than in compression. Although this yield stress difference is consistent with smaller continental normal fault earthquakes, it appears inconsistent with the occurrence of large oceanic normal fault earthquakes. However, the largest oceanic normal fault earthquakes occur near subduction zones where the lithosphere is bending. Laboratory studies indicate that bending lithosphere likely has a higher yield stress than lithosphere in pure extension, which may allow for larger oceanic normal fault earthquakes. Therefore, yield stress—rather than fault geometry alone—appears to be the key factor limiting an earthquake's maximum magnitude.