Abstract

Modern ground failures, ranging from renewed displacement on 2.5-km-long fault scarps to 1-km-long tension cracks, occur in Fremont Valley, California. Most of the failures formed along pre-existing Holocene fault scarps that are part of the tectonically active Garlock fault system. The failures began to form in the 1960s and currently affect an area of ∼17 km2 that is coincident with ground-water-level decline induced by pumping for crop irrigation. Several observations indicate that these failures were caused by water-level decline rather than natural tectonism. First, modern ground failure began after water level started to decline and was restricted to the area affected by decline. Land subsidence was associated with this decline and appears to be caused by aquifer compaction. Second, geodetic monitoring from 1977 to 1982 of faults near the center of the cone of depression demonstrated that fault creep occurred during periods of seasonal water-level decline and stopped during periods of seasonal water-level recovery. By contrast, faults near the margin of the cone of depression, where water levels declined at a constant rate, moved at a steady rate. Third, surface strain fields near failures suggest that subsurface fault-associated deformation was restricted to aquifer-level depths. And fourth, seasonal rebound of the land surface across one fault was caused by elastic expansion of the aquifer system that was induced by large seasonal water-level recovery.

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