Abstract

An anomalous increase of seismicity was recorded at intermediate distances from the Landers, California, earthquake of 28 June 1992 (M = 7.3). Several authors have suggested that enhanced activity was triggered at distances of up to 17 fault lengths. We test a 90-year catalog of shallow world earthquakes (M > 7) for evidence of this effect. It is found that triggering is present even at these high magnitude levels. The pattern of enhanced activity comprises two concentric regions: (a) the aftershock region and (b) the region of triggered seismicity at distances of 300 to 1000 km. The two regions are activated together; they are separated by a ring-shaped gap at a distance of around 300 km. This ``Mexican Hat'' pattern is attributed to recovery flows generated by the stress drop of the earthquake in a steady-state convecting system.

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