PERALTA HILLS FAULT A PREVIOUSLY UNRECOGNIZED ACTIVE TRANSVERSE RANGE "BLIND THRUST" ORANGE COUNTY, CA

The Peralta Hills fault is an east-west a "blind thrust” except where it surfaces as a low angle thrust in Anaheim Hills. Two tree stumps radiocarbon dated at 3,500 years BP (Fife and Bryant, 1983), have been displaced 15 m. in the lower plate of the fault.

Many structures have been constructed on or near the trace or multiple traces of this active fault as a result of the failure to place the Peralta Hills fault in an Earthquake Hazard Studies Zone.

When the California Geological Survey contracted with the U.S. Geological Survey to digitize their Geological/Landslide Hazard Map of the Orange Quad (Tan, 1995) the Survey “moved” the Peralta Hills fault trace “around” Reservoir 1-A, a 5 million gallon semi-buried hillside steel tank and apparently through or adjacent to a second buried hillside 3.500 gallon steel tank .

The Orange City Department of Public Works continues to fill the 5 million gallon tank, possibly jeopardizing down slope structures should the fault rupture. The fault trace forms a perfect diameter through the middle of the 200-foot diameter tank with differential settlement along the contact with Holocene gravels and Miocene siliceous shale.

The fault passes through portions of the Edison Serrano Substation and a key transmission tower atop the upper plate and will likely sustain damage during a major earthquake. The fault passes through a gas station and Eisenhower Lake above commercial and residential structures.

Many homes in Anaheim Hills are set back from the fault trace to avoid fault rupture, while other homes are built on areas where multiple thrust fault traces and thrust fault-rooted landslides have been buried. The 15 m set-back from the fault trace that is commonly required for structures may not be sufficient when a low angle thrust passes less than 15 m. beneath the structure.

The Peralta Hills fault has a known sinuous trace of about 10 km. The fault consists, in some locations; of multiple imbricate thrusts. Based on fault length it is estimated that the Peralta Hills fault is capable of a 6 to 7 magnitude earthquake and several meters of left oblique surface rupture.