Abstract

The active Casa Loma fault, which closely parallels the San Jacinto fault, was briefly exposed in an aqueduct trench across San Jacinto Valley in Southern California. Physical features, a gravity survey, profile levels, and groundwater data show that San Jacinto Valley is an active graben, which causes concern for the engineer designing large hydraulic structures here. A detail section across the fault reveals dip-slip drag in the bedded sand on the valley side, but contortion folds in the 20-foot thick fault gouge are not compatible. Minor shear planes with offsets to 3½ feet extend 210 feet west of the fault.

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