SLIP RATE AND AGE OF THE MOST RECENT EVENT ON THE CENTRAL MAACAMA FAULT, NEAR UKIAH, MENDOCINO COUNTY, CALIFORNIA

The 180-km-long Maacama fault zone (MFZ) is the northern extension of the Hayward and Rodgers Creek fault zones, which together comprise a major element of the San Andreas fault system in California. Previous studies indicate that the MFZ has been active in the Holocene and that it continues to produce numerous small-magnitude earthquakes. Fault creep has been reported and measured at several locations, also. However, little slip-rate or earthquake-chronology data are available for the MFZ. Our investigation combined fault trenching, cone penetrometer, and shallow seismic refraction studies on the active trace of the MFZ at a site approximately 3 km northeast of the town of Ukiah. Radiocarbon analyses of detrital charcoal collected from sediments associated with a strath-terrace riser yield a calibrated radiocarbon age range of 4840 to 4550 years BP. Our studies show that this terrace riser is right-laterally offset between 42 and 61 m, giving a slip rate of 8.7 to 13.4 mm/yr. We also determined that the most recent earthquake to cause surface rupture on this section of the Maacama fault, occurred sometime after 1410 AD, but before 1660 AD. If this earthquake is the same earthquake interpreted from evidence exposed in trenches excavated approximately 7 km southeast of this site, then it occurred between 1520 AD and 1660 AD. Thus, the minimum and maximum slip rate values of 8.7 and 13.4 mm/yr, when combined with the minimum and maximum years of elapsed time since the most recent earthquake, yield a strain accumulation of 3.0 to 6.5 m. However, the creep rate of the MFZ approximately 7 km southeast of our site is about 4.5 mm/yr. If this creep rate is representative of the long-term creep rate on this section of the MFZ, then the strain accumulation at our site may be between 1.4 to 4.2 m. If this slip is released in a single earthquake it would correspond to a magnitude ranging between Mw 7.0 to 7.6. These data suggest that the MFZ is a significant seismic hazard that warrants additional study.