PALEOSEISMIC EVIDENCE FOR PREHISTORIC EARTHQUAKES ON THE NORTHERN MAACAMA FAULT, WILLITS, CALIFORNIA

The right-lateral strike-slip Maacama fault zone (MFZ) is considered to be the northern continuation of the Hayward-Rodgers Creek fault system. This fault zone, which extends from near Santa Rosa in Sonoma County to Laytonville in Mendocino County, is associated with a high rate of microseismicity and ongoing creep. However, it has not generated any known, large, surface-rupturing earthquakes in the past 150 years. Shallow creep has been measured in the town of Willits at an average rate of about 6.5 mm/yr (measured over a 10 year period) (Galehouse, 2002). Modelling of geodetic data across northern California suggests approximately 14 mm/yr of deep strike-slip motion across the MFZ (Freymueller et al., 1999), and in order to reconcile the difference between the creep rate and the total slip rate across the fault we expect the MFZ, like the Hayward fault, to both creep and produce large earthquakes. However, little data bearing on the long-term geologic slip rate of the fault are available so the question of whether or not the MFZ is capable of producing large, surface-rupturing earthquakes remains. We excavated six trenches across the MFZ in Willits. Four trenches crossed a southwest-facing, two- to five-m-high fault scarp on the southwest side of a pressure ridge, and two trenches crossed the southern projection of the fault in an area of younger alluvium lacking surface expression of the fault. The trenches across the scarp exposed a steeply northeast-dipping fault plane, juxtaposing late Pleistocene sediments (dated at 18,810 ± 80 radiocarbon years BP) overlain by a cap of Holocene alluvium to the northeast, against a sequence of Holocene fluvial and colluvial deposits to the southwest. Structural and stratigraphic relations suggest at least four and probably five faulting events during the (late?) Holocene. Slickensides plunging 10-12º to the northwest show a horizontal to vertical slip ratio of about 5:1. The trenches across the area of younger alluvium show evidence for only one faulting event. We interpret the uppermost deformation in these trenches, a series of upward fanning fractures with little displacement, to be the result of ongoing fault creep. Additional radiocarbon dates will provide age constraints on the paleoearthquakes recognized in these trenches.