During the 24 hr following the great San Francisco, California, earthquake of 18 April 1906, separate seismic events were felt at Paisley, Oregon; Phoenix, Arizona; Los Angeles, California; and Brawley, California (MMIX). Using probability theory, we show that the occurrence of felt earthquakes in each of these widespread locations on the same day would constitute a rare event. Rates of felt-earthquake occurrences over a 9-yr period from 1897 to 1906 were determined for the four different regions that experienced earthquakes within 24 hr after the 1906 event. We modeled the likelihood of occurrence of these aftershocks in the spirit of the “ball-in-the-box” probability problem, and the results indicated a very high probability that the aftershock zone of the great earthquake extended at least 500 km beyond the extent of ground breakage, implying a disturbance of the stress field over an area at least two to three times longer than the fault break itself.

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