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What Stops Earthquakes?

Ruth A. Harris

Published September 2000, SCEC Contribution #844

Earthquake scientists are still mystified about what controls the eventual size of earthquakes or allows earthquakes to occur in the first place. In one approach to the problem, scientists have been determining how the occurrence of an earthquake may affect subsequent events using models of static Coulomb stress-change (see Harris, 1998 for a review of the subject). These types of calculations are quite simple and have become increasingly popular in the past 10 years. One case where static Coulomb stress-change calculations have done exceedingly well is in explaining the pattern of large aftershocks following a great earthquake. The best example is the stress impact of the great 1906 San Francisco, California earthquake which occurred on the San Andreas fault [Simpson and Reasenberg, 1994; Jaume and Sykes, 1996; Harris and Simpson, 1998]. The great earthquake shut down subsequent large earthquakes for decades and to this date its stress shadow continues to loom over portions of faults in the San Francisco Bay area.

Static stress calculations have shown promise, but we should also examine the role of dynamics in determining earthquake occurrence and size. There are frequent cases where dynamic stress changes greatly exceed static stress changes, and it is of course the dynamic stress changes that do damage to our societal infrastructure. Over the years various ideas have been proposed. Prominent viewpoints are that the nucleation process itself determines the eventual size or that earthquakes run into strong features that prevent further propagation. A variation on the latter is that earthquakes run into zones of varying material and frictional properties that prevent continuing propagation. In all of these cases one is considering the entire physical process of an earthquake and so one must include the waves that are generated during earthquake rupture.

Harris, R. A. (2000, 9). What Stops Earthquakes?. Oral Presentation at 3rd Conference on Tectonic Problems of the San Andreas Fault System.