Paleoseismology and earthquake chronology for the Santa Cruz Mountains San Andreas Fault, California
Portland State University
- Date & Time
- Building 3, Rambo Auditorium
- Jack Boatwright
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 19th century surface rupturing earthquakes. Historical records note a few large earthquakes that caused widespread damage in the region in this time period, however it was not previously known with any certainty on which Bay Area faults these earthquakes occurred. Trenches at the site revealed fine-grained interbedded sand and silt above a buried soil that is faulted in the most recent event, 1906, and in and two earlier earthquakes. We discovered hundreds of pieces of cut redwood chips coupled with numerous buried redwood tree stumps just below the ante-penultimate (E3) earthquake surface. This demonstrates that the redwood forest at the site was cut down right before earthquake E3, and thus E3 must be historic. Radiocarbon is frequently used to date pre-instrumental ruptures but can have combined contextual and analytical uncertainty on the order of decades to centuries, especially in the past 300 years. To better constrain age estimates for the onset of logging, we collected a slab from a buried tree stump and radiocarbon wiggle matched samples collected from growth rings to the INTCAL13 terrestrial 14C calibration curve. With these high-resolution age dates we place earthquakes E2 and E3 in the historic period. We use this high-resolution age data to evaluate the impact of C14 sample selection (including detrital charcoal, cones and needles), and use of non-native pollen analysis on our earlier earthquake age models for the Hazel Dell area.