Hologene activity and tectonic setting of the Maacama fault zone, Mendocino County, California

Abstract

The Maacama fault zone is a north-northwest trending, right-lateral zone of structural weakness in eastern Sonoma and Mendocino Counties, California, and fault-related landforms along its trace suggest that it may well have been active during Holocene time. Urbanization and dam construction in the vicinity of the zone have made it especially desirable to determine the extent and history of such geologically recent movements in order to better understand the seismic capability of faults within the zone. The Maacama fault zone is a segment of one of two north-northwest trending zones of right-lateral faulting that diverge as major branches from the easterly side of the San Andreas fault zone north of Hollister. These branches extend at least as far north as Eureka and may mark the eastern boundaries of slivers of a separate tectonic plate. This investigation was conducted to evaluate Holocene activity along the part of the Maacama fault zone in Mendocino County. The investigation shows that this zone comprises discontinuous, subparallel fault strands which extend from points south of the Sonoma—Mendocino county line to points north of Laytonville. Some fault segments are expressed by alignments of topographic features clearly caused by Holocene strike-slip movements. Other topographic features are less distinct and may well represent fault segments with pre-Holocene activity. The Maacama fault zone creeps at a current rate of possibly as high as 2 mm/yr. The total amounts of Holocene offset are unknown. Evidence obtained from exploratory trenches indicates that at least two surface rupture events have occurred within the past 16,200 years, that at least one has occurred within the past 8,310 years, and that there probably has been no surface rupturing within the past 1,140 years. I conclude that surface offsets have occurred along major portions of the Maacama fault zone during Holocene time, and that this fault zone should be regarded as capable of producing a moderate to strong future earthquake with accompanying surface rupture in Mendocino County.