What will happen when the Ma”acama fault slips? 3.3 temblor hits Tuesday evening along faultline
Terre Logsdon – Record-Bee staff
LAKEPORT ? Tuesday proved a busy day of geologic activity in and around the county.
Following several temblors at The Geysers and, most notably, in Lakeport, a minor 3.3 magnitude quake shook the Cow Mountain Recreation Area in the mountains between Lake and Mendocino counties at 7:01 p.m.
A recently completed study suggests that the Ma”acama fault ? the location of the Cow Mountain quake, which runs from Mark West Creek in Sonoma County to Laytonville in Mendocino county, is due for a much more devastating quake in the near future.
That 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 past Seismic Monitoring Advisory Committee (SMAC), was supported by the U.S. Geological Survey (USGS).
Murray”s study says 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.
The Association of Bay Area Governments (ABAG), of which Sonoma County is a part, has studied what the impacts of a 6.8 magnitude temblor on the Ma”acama fault would be on the Bay Area, based on a 1996 report. It found that nearly 4,500 residents of San Francisco would be displaced and 2,000 in Sonoma County due to damages to homes and 54 roads would be closed in Sonoma County alone – including Highway 101.
The report does not give an estimate for Lake County, as Lake is not a member of ABAG.
It was this information, along with his concern that increasing seismicity due to geothermal energy production at The Geysers may trigger a large quake along the Ma”acama fault, that prompted Upper Lake resident Paul Kolb to attend the last SMAC meeting and ask questions.
Kolb, who has given copies of this report to all five county supervisors, asked David Oppenheimer, a U.S. Geological Service researcher in Menlo Park and member of the SMAC committee, if he was aware of the study and if operations at The Geysers could trigger a devastating quake.
Oppenheimer noted that the Ma”acama fault does not run through The Geysers, and “at first blush, it I don”t think it will effect it. It”s four to five miles away from the [steam] field.”
“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.
Contact Terre Logsdon at tlogsdon@record-bee.com.
