A technique to express changes in seismicity patterns by means of an objective criterion has been investigated. A parameter v2=(Δ¯)2/Δ¯2, where Δ is the distance between two adjacent earthquakes in space and time, was derived on the basis of the Weibull distribution function. This parameter can be related to an apparent interaction in the earthquake sequences and used to monitor changes of the seismicity patterns which may reflect the state of the crustal stress. The patterns are classified as regular, completely random, and clustered according to the value of v2.

The technique was applied to the U.S. Geological Survey microearthquake catalog for central California. A total of 9740 earthquakes (M ≧ 1.5) was selected from along the 250-km-long creeping zone of the San Andreas and Calaveras faults for the period 1971 to 1981. The seismicity pattern was found to be generally a combination of highly clustered ones overlapped with a background, which was characterized as almost random or slightly clustered.

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