Using the locations of M ≥ 4 earthquakes to delineate the extents of the ruptures of past major earthquakes

Abstract

Some modern seismicity in the magnitude range of M 4 and M 6 in California and eastern North America preferentially occurs at the edges of past large ruptures. Once a large earthquake rupture has occurred, stress is concentrated at the edges of the rupture, and apparently this stress concentration can trigger earthquakes at or near the rupture edges many decades or even longer after a main shock. Furthermore, the modern M ≥ 4 earthquakes in the vicinity of a past main shock usually have the same focal mechanism as the earlier main shock. There are a number of examples of this in California and Nevada, where there is a statistically significant correlation of the locations of M ≥ 4 earthquakes and the edges of 19th and 20th century fault ruptures in Mw ≥ 6.5 earthquakes. In contrast, the M ≥ 4 earthquakes near the epicentres of future ruptures in California are randomly scattered around the fault with no concentration near the ends of the future fault rupture. The concentration of earthquakes near the ends of earlier large ruptures in California becomes progressively less pronounced as the smallest magnitude in the data set is reduced from M 4.0 to M 3.0. These observations also appear to be true for intraplate regions where aftershock sequences can last millennia. The identification of modern rupture-edge M ≥ 4 aftershocks can be used to help discover where and when past strong earthquakes took place, even if there is no historical record of the main shock. This is of great importance for seismic hazard studies.