PALEOSEISMIC INVESTIGATION OF THE MAACAMA FAULT AT THE HAEHL CREEK SITE, WILLITS, CALIFORNIA

The Maacama fault zone (Mfz) is one of three major fault zones that comprise the San Andreas fault system in northern California. The fault is creeping near the town of Willits, and determining a long term slip rate for the fault is critical in order to evaluate whether stress on the fault is only released by ongoing creep or whether large earthquakes occur on this fault as well. We investigated Holocene sediments that were deposited across the Maacama fault near the western edge of the Little Lake Valley in Willits, California. The Mfz in Little Lake Valley has been creeping at a rate of 6.6 mm/yr for the last 11 years (Galehouse, 2002). We excavated multiple trenches parallel and perpendicular to the fault to provide exposures of channel stratigraphy and channel margins on both sides of the fault. Detailed study of these excavations allowed us to map the paths of two paleo-channels of Haehl Creek where they are offset by the Maacama fault. The first channel, channel A, is right laterally offset 4.6 m. Based on two radiocarbon ages, the age of channel is less than or equal to 740-640 years, indicating a minimum slip rate of about 6 mm/yr. The second and older channel, channel B, is offset right laterally about 27 m. Based on one radiocarbon age, channel B is less than or equal to 3480-3350 ybp. From the offset and maximum limiting age, we infer a minimum slip rate for channel B of about 8 mm/yr. Because radiocarbon ages are from detrital charcoal that may be older than the age of deposition of the host alluvium, the radiocarbon ages will tend to overestimate the age of the channels. Future dating of more radiocarbon samples may significantly reduce channel ages, which would increase slip rate estimates. Nevertheless, the preliminary results are that over the last 700 years, only fault creep has occurred at the site; however, over the last 3500 years, slip has been accommodate by both creep and earthquakes with surface rupture. The preliminary minimum long-term slip rate at Haehl Creek on the Maacama fault of greater than or equal to 8 mm/yr is consistent with the 8-9 mm/yr rates found on the Hayward and Rodgers Creek segments of this fault system to the south and is consistent with the notion that the Mfz is capable of producing large earthquakes.