Open-File Report 96-517
Five anomalous BMs are located within or very near fault zones (BM 4, 5, 24, 25, and 34 in Table 3 and the map). Because the material in a fault zone is often quite weak, small amounts of "triggered" or "sympathetic" slip are often found to accompany large earthquakes. BM 4 (EW2515) lies in the Santa Susana fault zone that ruptured during the 1971 San Fernando earthquake. The 3.3-cm disturbance may indicate a small amount of near-surface slip on the Santa Susana fault triggered by the Northridge earthquake. Several studies have pointed out that the 1971 and 1994 earthquakes are in several respects closely related [Mori et al., 1995; Stein et al., 1994]. BM 24 and 25 lie in the San Gabriel fault zone; while the fault itself is believed to be inactive, pulverized rock or "gouge" in the fault zone may settle when shaken.
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U.S. Geological Survey
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