Observations of the water-level changes in a well drilled into the San Andreas fault zone have been under way since May 1971, with the objective of studying in situ pore-pressure changes in a zone of active tectonic creep and seismicity. Small water-level changes, characterized by a decrease and subsequent rise, have been followed by earthquakes of moderate size on the San Andreas fault zone. Compatibility of these observations with either a dilatancy-type behavior or a dislocation-type behavior for the pre-earthquake process can be demonstrated. Additional water-level observations at other sites in the fault zone are needed to examine the spatial and temporal extent of the actual preseismic process that is responsible for observed creep events, water-level changes, and tilt changes.

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