Refining age estimates for three historic ground rupturing earthquakes in the Santa Cruz Mountains: 14C Wiggle-matching and Non-Native Pollen as age indicators (or not!)

The Hazel Dell site provides the first definitive paleoseismic evidence of two pre-1906 19th century events on the Santa Cruz Mountains section based on the presence of anthropogenic artifacts. Hundreds of pieces of cut redwood chips were found in a stratigraphic horizon just below the ante-penultimate (E3) earthquake surface, suggesting that redwood trees at the site were cut down right before earthquake E3. We correlate our paleoseismic findings with the historic record and the onset of redwood logging in the area by determining the felling date of a buried redwood tree stump at the site and the age of the woodchips. We wiggle match 14 radiocarbon dates sampled from annual growth rings taken from the stump and the known interval between growth rings, with the intercepts of the INTCAL04 terrestrial ¹⁴C calibration curve. Pending ¹³C measurements, we find that the youngest ring we have identified in the tree is A.D. 1800. We also wiggle match 2 radiocarbon dates from inner and outer growth rings from two wood chips (with bark); their ages are consistent with the tree and the youngest woodchip ring is dated to 1813 A.D. There are no known ethnographic or historical accounts of pre-contact native people felling large trees in the way that European colonists did. The first record of European land use was for pasture in 1803. The property became a Spanish land grant in 1827, soon after which a whip-saw lumber mill is documented to have begun operation in the upper Corralitos area. We combine these paleoseismic results with historical earthquake accounts for the area and conclude that the San Andreas fault ruptured in 1838, 1890 and 1906. The Hazel Dell results are in contrast with findings from earlier paleoseismic studies in the Santa Cruz Mountains. The Grizzly Flat site, 6 km to the north, found evidence of 1906 and one 17th century earthquake. Two historic earthquakes were observed at the Mill Canyon site 8 km to the south and at the Arano Flat site 9.5 km south of the Hazel Dell. The Arano Flat and Mill Canyon studies, however, used the lack of non-native pollen, from invasive species introduced by the early Spanish, in near-surface stratigraphy to trim probability distribution functions for both the ages of deposits and the timing of earthquakes. At Hazel Dell we have found that historic sediments lack non-native pollen commonly associated with Spanish cattle migration. So, while the presence of non-native pollen can limit the age of a deposit, the lack of pollen does not require the deposit to occur in the period prior to Spanish settlement, and this age constraint used at Arano Flat and Mill Canyon is invalid. We 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 for the Santa Cruz Mountains that includes a 1906, 1890, 1838. In the 700 years before 1800 the 4 sites have evidence ranging from 1-5 events, suggesting that a complete record has yet to be worked out.