THREE HISTORIC GROUND RUPTURING EARTHQUAKES IN THE SANTA CRUZ MOUNTAINS, SAN ANDREAS FAULT: REFINING AGE ESTIMATES WITH 14C WIGGLE-MATCHING AND EARLY HISTORIC REPORTS

The Hazel Dell paleoseismic site on the Santa Cruz Mountains section of the San Andreas Fault provides the first definitive paleoseismic evidence of two pre-1906 19^thcentury surface rupturing earthquakes. Historical records note moderate to large earthquakes that caused widespread damage in the region in 1836, 1838, 1865 and 1890, however it was not previously known with any certainty on which Bay Area faults these earthquakes occurred. Trenches at the Hazel Dell site revealed fine-grained interbedded sand and silt above a buried soil that is faulted in the most recent event, 1906, and in two earlier earthquakes. We discovered hundreds of pieces of cut redwood chips along with buried redwood tree stumps within the buried soil, the ante-penultimate (E3) earthquake surface. This demonstrates that the trees at the site were cut down shortly before earthquake E3.

There are no known ethnographic or historical accounts of pre-contact native people chopping down large trees in the way that European colonists would have, and so earthquake E3 must be historic. The first record of European land use was for pasture in 1803, and the property became a Spanish land grant in 1827, soon after which a whip-saw lumber mill was established in the upper Corralitos area. We corroborate the historic record of logging in the area by determining the felling date of a buried redwood tree stump at the site along with estimating the age of the woodchips using radiocarbon dating and tree ring wiggle matching. Based on 14 radiocarbon dates sampled from annual growth rings taken from the stump, we find a best fit date of 1789 – 1797 for the outer growth ring; a maximum limited age for the felling of the trees. We also wiggle match 2 radiocarbon dates from inner and outer growth rings from two wood chips (with bark); their age is consistent with the tree. Together, these paleoseismic results and historical earthquake accounts for the area lead us to conclude that the San Andreas Fault ruptured in 1838, 1890 and 1906. Earthquakes in 1836 and 1865 likely did not generate rupture on the SAF.

We also correlate earthquakes between Hazel Dell and nearby paleoseismic sites based on revised timing, similarity of stratigraphy and style and size of displacement, and build a composite paleoseismic record with estimated rupture length and magnitude for these Santa Cruz Mountains events.