Four years of continuous water level data from a 1,900 m deep well near the San Andreas fault in southern California show a 21-month period of unusual rapid fluctuations. The anomalous water level activity ceases with the occurrence of the 8 July 1986, ML = 5.6 North Palm Springs earthquake on the San Andreas fault, approximately 120 km southeast of the well site. The water level data correlate with signals from two San Andreas fault zone strain sensors. The long-term water level variations are similar in shape to those recorded by a borehole dilatometer 30 km from the well site, while the rapid water level fluctuations correlate in time with anomalies in the record of a fault zone creepmeter located 35 km from the water level monitor. The observations are interpreted in terms of strain occurring in a horizontally and vertically heterogeneous crust. Tectonic deformation is concentrated along weak block-bounding fault zones. The blocks are sufficiently rigid to generate correlated displacement phenomena along their weak boundaries. As a consequence, fault zone strain events can be related over large distances as in the case of the North Palm Springs earthquake and the coincident creep and water level anomalies. The observed creep and water level events occur in a weak aseismic shallow (<3 km) material overlying stronger asperities along which the fault is locked and regional strain loading occurs. Local failure of the fault within the weak shallow material has no far-field effects and is not important in the crustal stress budget, but may signal accelerated strain accumulation at depth. In this context, the reported observations demonstrate the possibility of joint detection of fault zone anomalies that relate to, and possibly anticipate, major tectonic strain events on large faults.

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