- © 2005 by the Seismological Society of America
On several occasions in recent memory California has experienced apparent clusters of earthquake activity that are too far apart to be considered related according to a classic taxonomy that includes foreshocks, mainshocks, and aftershocks. During a week-long period in July 1986, California experienced the M 6.0 North Palm Springs earthquake, the M 5.5 Oceanside earthquake, and a swarm of smaller events beneath San Diego Bay. The recent M 6.0 Parkfield earthquake was followed approximately 30 hours later by the M 5.0 Arvin event, which was located well outside the traditional aftershock zone for a M 6.0 mainshock. These periods of apparently heightened activity lead to understandable consternation among California residents, who wonder if activity will build further. The recent, memorably dramatic television mini-series, 10.5, was based on what might be considered an end-member doomsday scenario, culminating in a large part of California literally falling into the ocean. While the public did seem to recognize the gross liberties that were taken with science in this movie, old myths die hard, and seismicity maps showing activity in different parts the state are not reassuring. Neither is what used to be conventional wisdom on the part of the experts, that far-flung earthquakes are not related (even though this might remain a possibility).
Since 1992, however, scientists have come to understand that earthquakes can be related over greater distance and time scales than previously recognized. In addition to developing theories of static stress transfer (e.g., Das and Scholz, 1981; King et al., 1994; Toda and Stein, 2003), remotely triggered earthquakes were first identified in 1992 (Hill et al., 1993) and have subsequently been observed following large (generally M > 7) earthquakes in California as well as in other regions. Investigations of “earthquake interactions” remain at the forefront of earthquake …