Large earthquakes are typically preceded by a pattern of irregularly increasing magnitudes of preshocks extending over a period of 10 or more years. The approach of the climax of this growth can be recognized by calculating a characteristic index based on analysis of the largest magnitudes each year. When a pattern of increasing magnitudes occurs over a 10year or longer period accompanied by a steady increase in this index over 4 or more years, including at least a doubling of its value toward the end, a large earthquake typically follows. Five of the six magnitude 6.8 and larger earthquakes in southern California since 1950 could have been anticipated in this fashion. Magnitude 5.9–6.7 earthquakes sometimes but not always exhibit this pattern. Otherwise it rarely occurs. A simple computer program is presented to make this test.

The south end of the Imperial Valley seems to be preparing for a big earthquake.

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