Abstract

Paleoseismic investigations at the Hazel Dell site on the Santa Cruz mountains section (SAS) of the San Andreas fault provide the first definitive geologic evidence of two pre‐1906 nineteenth‐century earthquakes based on the presence of anthropogenic artifacts at the antepenultimate earthquake (E3) horizon. We review historic accounts of candidate events and interpret the penultimate earthquake and E3 to be the April 1890 and June 1838 earthquakes, respectively. These new data suggest more frequent surface‐rupturing earthquakes within historical time than previously recognized and highlight variability of interseismic intervals on the SAS of the San Andreas fault. We correlate earthquakes between Hazel Dell and nearby paleoseismic sites based on revised timing, similarity of stratigraphy, style, and size of displacement, and build a composite paleoseismic record. The composite record requires at least two modes of behavior in strain release on the SAS through time. One mode is through great multisegment earthquakes, like that in 1906. Historic records and geologic studies suggest that prior to 1906 the Santa Cruz mountains region was characterized by a second mode of moderate seismicity, with three M≥6 earthquakes between 1838 and 1890, including two that caused surface rupture at Hazel Dell. In the 700 years prior to 1800, individual sites have evidence ranging from 1 to 5 events, suggesting that the longer record remains unresolved.

Online Material: Additional Hazel Dell trench logs, figure of age ranges for wood chips, Arano Flat OxCal model, and table of radiocarbon samples.

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