Abstract

Magnitude estimates have been calculated independently by Berkeley and the U.S. Geological Survey for a large number of events in central California. The Berkeley magnitudes have been determined using a stable set of Wood-Anderson seismometers for many decades. This relative stability allowed us to compare CALNET magnitudes with the Berkeley magnitudes in an attempt to evaluate the temporal stability of the CALNET magnitudes. The comparisons were carried out in six different regions of central California for events with 2.5 ≦ M (Berkeley) <3.5. These events make up less than 5 per cent of the CALNET catalog. Magnitude increases of 0.20 to 0.32 units were identified in the Bear Valley, San Juan Bautista, and northern and southern Calaveras regions during 1971 and 1972. Magnitude decreases of between 0.16 and 0.26 units were recognized in the East Bay, Bear Valley, San Juan Bautista, and northern and southern Calaveras regions between 1975 and 1979. In addition, magnitude decreases of 0.10 and 0.12 units were identified in the northern and southern Calaveras regions during 1979 and 1984. The Parkfield region showed no magnitude changes significant at the 99 per cent level. Times of changes in magnitudes were identified by systematically comparing average magnitude differences for different time periods. The significance of the changes was determined using simulations on thousands of normally distributed data sets. The results of this study provide a set of corrections which can be used in studies which rely on temporally consistent magnitudes. However, these corrections may not be appropriate for events outside of the magnitude band considered.

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