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...

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