Anomalous water‐level changes and possible relation with earthquakes

Abstract

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 response 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.