Coyote Dam, an Incidental Fault Alignment Array Across The Central Calaveras Fault, Santa Clara County, California

This paper focuses on the performance of Coyote Dam, a large earthen embankment, and a unique set of field survey data from crest monuments collected during ~75 years. Coyote Dam, located in east-central Santa Clara County, California, has functioned as an incidental fault alignment array for the Central Calaveras fault since its construction in 1936. Dam crest monument survey data provide timelines of fault creep propagation of the Central Calaveras fault through Coyote Dam. The Santa Clara Valley Water District owns and operates Coyote Dam.

The geologic/geotechnical literature has focused on the effects of fault propagation through native soil and rock materials due to single, large seismic events, with very little documentation on how deep soil deposits (> 15 m) respond to bedrock offset. The findings from Coyote Dam provide a unique opportunity to study fault propagation through a relatively thick mass (up to ~30 m) of ductile material as a result of relatively constant fault-induced strain over three-fourths of a century.

A timeline plot of crest monument survey data for the period 1936 to 2010 constrains rates of aseismic right lateral fault creep propagation of the Central Calaveras fault from the embankment foundation to the dam crest. We identify 3 stages of embankment response to right lateral fault creep. During Stage 1, from 1936 to 1954, crest monuments do not exhibit fault-related right lateral displacements. During Stage 2, from 1954 to 1980, crest monuments on either side of the active fault move in a right-lateral sense, but at a rate of ~ 6.5 mm/yr, which is 46% of late Holocene fault creep rate of the Central Calaveras fault (~14 mm/yr). During Stage 3, from 1980 to 2010, crest monuments exhibited right lateral displacement at the approximate late Holocene rate. These displacement data indicate that it took ~44 years (Stage 1 plus Stage 2) for fault movement to fully propagate from the foundation to the dam crest.

Since completion of Coyote Dam in 1936, total measured right-lateral displacement experienced by dam crest monuments is ~0.55 m and the estimated total displacement in the foundation is ~1.0 m (assuming ~14 mm/yr of fault creep over a period of ~75years). Consequently, there is a net shortfall of ~0.45 m between total measured displacement of the dam crest and total assumed displacement of the foundation bedrock. The differential 0.45 m experienced by the foundation was not recorded in the crest during Stages 1and 2. The shortfall can best be explained by responses of clayey materials in the early stages of shear deformation in which strain is taken up by development of Riedel shearing. We speculate that empirical experiments involving Riedel shear tests provide a model that accounts for the lateral and vertical propagation of the Central Calaveras fault through the clay core of Coyote Dam.