Since late 1976 abandoned water wells have been monitored along the San Andreas and San Jacinto faults to detect water-level changes which may be premonitory to earthquakes. An M5.5 earthquake occurred 25 February 1980 within the San Jacinto fault zone near Anza. The water level in a continuously recorded well in Borrego Valley 35 km from the epicenter rose 0.5 m and returned to its prior level during a four-hour period beginning 88 hours before the earthquake. The water level in this well had been remarkably steady compared to other wells being monitored. Another well in Borrego Valley showed a much smaller rise and fall in water level at the same time. Because these changes are unique for the long-term records of these wells, they may represent precursors to the earthquake. Several wells along the Palmdale-Valyermo segment of the San Andreas fault showed possibly anomalous, long-term water-level changes which are not clearly related to rainfall. Between early 1979 and early 1980, water levels in six wells were .6 to 4.5 m higher and levels in two wells were about 3 m lower than would have been predicted based on the previous records. The water-level increases could have been the result of delayed reponse to seasonal rainfall, following a long drought. However, it is interesting that the possibly anomalous water-level changes first occurred at the same time as the reported changes in strain pattern from compression to expansion along the San Andreas fault in the same area. |