ASSESSING TECTONIC STRAIN AT COYOTE DAM DUE TO ACTIVE CREEP ALONG THE CALAVERAS FAULT, SANTA CLARA COUNTY, CALIFORNIA

Coyote Dam, a 42-m high earth-rock embankment built in 1935/36 is one of two dams in the State of California knowingly built across actively creeping faults. The dam is owned and operated by Santa Clara Valley Water District in Santa Clara County, California. Coyote Dam was designed with special zoning and materials to accommodate up to ~4.5 meters of lateral displacement from either ongoing creep or a single earthquake-induced offset along the Calaveras fault, which is located in the left abutment area where the dam is 15 to 18 meters thick (vertically).

In the vicinity of Coyote Dam the predominantly right-lateral Calaveras fault system is marked by a 6-km long elongate structural basin probably formed by down-dropping of a block along a releasing bend, or “pull-apart” structure. The dam straddles the northernmost end of the pull-apart basin. Here, the active Calaveras fault trace marks the northwestern boundary of the pull-apart basin and a fault known as the “Right Abutment fault” marks the northeastern boundary.

We evaluated crest monument survey data for the period 1936 to 2004 to determine a timeline for the propagation of fault creep displacement from the embankment foundation to the dam crest. Deformation of the embankment due to right-lateral creep on the Calaveras fault falls into three general periods. From 1936 to ~1954 there is general absence of right lateral deformation between crest monuments. From ~1954 to ~1980 there is a relatively low rate (~4 mm/yr) of right-lateral separation of monuments, indicating that fault displacement had begun propagation upward through the embankment. Since ~1980, the creep rate of lateral separation of the monuments has increased to ~14-16 mm/yr, which is similar to the range at other locations along this segment of the fault. We conclude that fault-induced deformation of the dam core has fully developed and took ~44 years to propagate through ~18 vertical meters of embankment.