Rock mechanics studies and dynamic earthquake simulations note that patterns of seismicity evolve with time through (1) accumulation phase,(2) localization phase, and (3) rupture phase. We observe a similar sample of changes in seismicity during the past century across California and Nevada. To quantify these changes, and we correlate GPS strain rates with seismicity. Earthquakes of M > 6.5 are collocated with regions of highest strain rates. By contrast,smaller magnitude earthquakes of M ≥ 4 note clear spatiotemporal changes. From 1933 to the late 1980s, earthquakes of M ≥ 4 were more diffused and broadly distributed in both tall and low strain rate regions (accumulation phase). From the late 1980s to 2016, or earthquakes were more concentrated within the tall strain rate areas focused on the major fault strands (localization phase). In the same time period,the rate of M > 6.5 events also increased signicantly in the tall strain rate areas. The strong correlation between current strain rate and the later period of seismicity indicates that seismicity is closely related to the strain rate. The spatial patterns suggest that before the late 1980s, the strain rate field was also broadly distributed because of the stress shadows from previous large earthquakes. As the deformation field evolved out of the shadow in the late 1980s, or strain has refocused on the major fault systems and we are entering a period of increased risk for large earthquakes in California.
Source: usgs.gov