Late Holocene Slip Rate and Ages of Prehistoric Earthquakes along the Maacama Fault Near Willits, Mendocino County, Northern California

Abstract

The Maacama fault is the northward continuation of the Hayward-Rodgers Creek fault system and creeps at a rate of 5:7 0:1 mm=yr (averaged over the last 20 years) in Willits, California. Our paleoseismic studies at Haehl Creek suggest that the Maacama fault has produced infrequent large earthquakes in addition to creep. Fault terminations observed in several excavations provide evidence that a prehistoric surfacerupturing earthquake occurred between 1060 and 1180 calibrated years (cal) B.P. at the Haehl Creek site. A folding event, which we attribute to a more recent large earthquake, occurred between 790 and 1060 cal B.P. In the last 560-690 years, a buried channel deposit has been offset 4:6 0:2 m, giving an average slip rate of 6:4-8:6 mm=yr, which is higher than the creep rate over the last 20 years. The difference between this slip rate and the creep rate suggests that coseismic slip up to 1.7 m could have occurred after the formation of the channel deposit and could be due to a paleoearthquake known from paleoseismic studies in the Ukiah Valley, about 25 km to the southeast. Therefore, we infer that at least two, and possibly three, large earthquakes have occurred at the Haehl Creek site since 1180 cal B.P. (770 C.E.), consistent with earlier studies suggesting infrequent, large earthquakes on the Maacama fault. The short-term geodetic slip rate across the Maacama fault zone is approximately twice the slip rate that we have documented at the Haehl Creek site, which is averaged over the last approximately 600 years. If the geodetic rate represents the long-term slip accumulation across the fault zone, then we infer that, in the last ∼1200 years, additional earthquakes may have occurred either on the Haehl Creek segment of the Maacama fault or on other active faults within the Maacama fault zone at this latitude. Online Material: Trench logs, discussion of channel B, photographs of compressional features, and table of radiocarbon analyses.

Widespread Fault Creep in the Northern San Francisco Bay Area Revealed by Multistation Cluster Detection of Repeating Earthquakes

“…More intriguingly, the shallower REs in the area, located at depths of 1-5 km, define a subvertical trend that projects to the surface ∼ 5 km NE of the main Maacama surface trace, suggesting that there is a subvertical shallow splay fault at this location that may also be creeping (profile A3-B3; Figure 4). This structure is variously referred to as the "East Willits fault" (Prentice et al, 2014) or the "East Valley fault" (Woolace, 2005) and was recognized in the 1970s (Simon et al, 1978). This structure is variously referred to as the "East Willits fault" (Prentice et al, 2014) or the "East Valley fault" (Woolace, 2005) and was recognized in the 1970s (Simon et al, 1978).…”

Regional‐Scale Detection of Fault Scarps and Other Tectonic Landforms: Examples From Northern California

“…The mapped fault zone follows a series of oriented ridges, slope breaks, and a shutter ridge in the Willits area (supporting information Figure S5). Few tectonic features along the MF were identified at any template window size, including slope breaks mapped by Prentice et al () along the East Willits Fault near Berry Canyon (supporting information Figure S5). Many of these landforms are curvilinear and modified by channels that drain nearby high topography, complicating their detection by a linear scarp template.…”

“…Regional‐scale Quaternary mapping of the MF is restricted to the western strand of the fault zone; the East Willits section proposed in Prentice et al () is not included in the Quaternary faults and folds database. The mapped fault zone follows a series of oriented ridges, slope breaks, and a shutter ridge in the Willits area (supporting information Figure S5).…”

“…The MF lies to the east of the northernmost NC segment (Figure ). It displays surface creep, seismicity, and geodetic displacement and has been mapped in detail near Willits, CA, and in the Little Lake Valley area (Prentice et al, ). A M L = 4.8 earthquake occurred on the Willits section of the MF, followed by aftershocks on the MF and its Rocktree Valley strand (Warren et al, ).…”

“…Together, the NC segment and MF accommodate approximately 45% and 35% of the total plate boundary motion of 39.6 mm/year in this area, with a geodetic slip rate estimate of about 13.9 mm/year for the MF (Freymueller et al, ). Several prominent geomorphic features, including a pressure ridge and abandoned channels in the Willits area were mapped by (Prentice et al, ), and a trench study at the Haehl Creek site provides evidence for multiple prehistoric earthquakes in the area (Prentice et al, ). Near Little Lake Valley, Prentice et al () used the 2009 GES data set to map a series of lineations, slope breaks, and deflected drainages that define the East Willits Fault (supporting information Figure S5).…”