UKIAH — A 3.8 magniude shook Ukiah Tuesday just before 2 p.m. and was centered 11 miles north of Ukiah and 10 miles southeast of Willits on the Ma”acama fault zone. The quake was felt from Laytonville down to Hopland; over to Upper Lake and to the coast in Gualala.
The Ma”acama fault, which runs from Mark West Creek in Sonoma County to Laytonville in Mendocino County, was recently identified in a recent study as a fault zone to watch in the near future.
The study, titled, “Crustal deformation along the Northern San Andreas fault system,” is the work of M.H. Murray, who completed it in 2002. Murray”s study, which was discussed at the May Seismic Monitoring Advisory Committee (SMAC) in Lakeport, was supported by the U.S. Geological Survey (USGS). Murray”s study states that the Ma”acama fault, which is part of the San Andreas fault system, has not moved or “slipped” in more than 100 years, meaning it has a “slip deficit.” This deficit cannot last forever, and at some time the fault will try to catch up by rupturing, Murray explains.
“The slip deficit on the Ma”acama fault, based on a high slip rate found by previous geodetic studies, may now be large enough to generate a magnitude 7 earthquake, posing a significant seismic hazard,” Murray states in his report.
At the May SMAC meeting, a member of the public asked David Oppenheimer, a USGS researcher based in Menlo Park if the operations at The Geysers, which cause microearthquakes near the steamfields could cause a large and devasting earthquake on the Ma”acama fault zone.
“It could enhance failure,” Oppenheimer said, “but no calculations have been done on it. More concerning is the slip deficit. It”s supposed to move about 10 millimeters per year, but hasn”t since the 1906.”
“Is the activity at the Geysers enhancing this? I don”t think so,” Oppenheimer said, “but if the earthquake nucleates next to The Geysers, Calpine will be in court. If it happens in Hopland, probably not,” but more studies on the fault need to be completed.
The Rogers Creek fault, which begins near Healdsburg, runs through Santa Rosa down past Petaluma, “Connects (to the Ma”acama fault) via a 6 kilometer right step” according to the USGS and is, according to Oppenheimer, “a locked fault with a 27 percent chance of a 6.9 magnitude in next 30 years.” But the Ma”acama, I”d don”t know, it hasn”t been studied,” Oppenheimer said, and the studies are expensive.
In a press release today, studies show that an earthquake in one area can trigger aftershocks as far as 30 miles away from the epicenter. New research published in the journal Nature by Karen Felzer, a geophysicist at the USGS in Pasadena, analyzed quake data from Northern California and Japan found that even relatively small temblors can, at times, cause aftershocks miles away from the original epicenter.
“It changes where you expect to find aftershocks,” said Felzer in an AP release. The researchers found that in some cases, a small quake produced aftershocks as far as 30 miles away. Researchers believe it was ground-shaking that caused those events, but how the events happen is still unclear. What is clear is that if a large quake ruptures along the Ma”acama fault, it will greatly impact Lake County, whatever the cause.
A 2.8 magnitude quake centered two miles away, shook the residents of Anderson Springs on Saturday. There have been seven quakes since that time.
