Gilroy has a bad case of the shakes. Ten earthquakes have hit
South Valley since Friday, and whether a larger magnitude temblor
is on the horizon is up for debate.
Gilroy has a bad case of the shakes.
Ten earthquakes have hit South Valley since Jan. 7, and whether a larger magnitude temblor is on the horizon is up for debate.
Wednesday’s flurry of four quakes in the San Juan Bautista area followed three Jan. 7 earthquakes in San Jose with patches of temblors in Morgan hill during the weekend.
“Every time there is a flurry of earthquakes in a few days, questions arise,” said David Oppenheimer, a U.S. Geological Survey seismologist based in Menlo Park. “They are difficult to answer. The real question is what is happening down in the Earth? But we don’t have instruments at five miles down to know what is going on there.”
Jim Berkland, an 80-year-old retired “maverick” geologist whose public prediction of the devastating 1989 Loma Prieta earthquake – also known as the World Series quake – earned him a two-month suspension from his job as Santa Clara County geologist, predicts more to come with high chances of shakes on March 20.
“This is an extremely strong period here; it’s going to culminate in March,” he said Wednesday from his Glen Ellen home.
There were two noticeable earthquakes at about 8 p.m. Wednesday near San Juan Bautista, according to the USGS.
The first quake measured at magnitude 3.7. It was centered about three miles south of San Juan Bautista, according to the USGS website. The second was magnitude 4.0, according to the USGS. They were at 7:54 p.m. and 8 p.m., respectively.
The quakes, about eight miles from Hollister, were about 4.7 miles deep, according to the USGS.
The two quakes followed a 12:51 a.m. Wednesday temblor. The magnitude 4.5 quake struck six miles southeast of San Juan Bautista, eight miles southwest of Hollister and occurred at a depth of 5.2 miles.
It was followed by 17 aftershocks reported on the USGS website, including a magnitude 3.4 earthquake that occurred at 1 a.m. about five miles from San Juan Bautista. The last aftershock hit at 6:09 a.m.
No damage was reported in the four quakes.
Workers at Mi Pueblo Food Center and Safeway on First Street in Gilroy said the temblor caused no damaged to their facilities and that nothing fell off the shelves.
Sgt. Chad Gallacinao with the Gilroy Police Department said neither the GPD or the Gilroy Fire Department received any report of injuries or significant damage.
The evening quakes were accompanied by several aftershocks, according to the USGS.
The activity was within the realm of normal and don’t necessarily mean there is a greater chance of a big one, according to USGS.
As for March being a month of increased temblor activity, Berkland said it’s a probability, not a certainty.
“I don’t want anybody to panic,” he said, but added March has high chances of significant earthquakes – including large ones.
The swarm of quakes began at 4:10 p.m. Jan. 7 when a 4.1 magnitude temblor hit the Seven Trees area between San Jose and Morgan Hill on the Calaveras Fault, according to the USGS.
The epicenter was about 11 miles north of Morgan Hill. The USGS reported a second magnitude 2.1 quake with the same general epicenter at 4:21 p.m., and a third magnitude 1.1 temblor at 4:29 p.m.
The 4.1 quake was felt 13 miles away at San Jose City Hall. The depth was recorded by the USGS at 4.4 miles.
Friday’s quake was 10.5 miles deep; the ones on Wednesday were all about five miles deep, according to the USGS website.
Morgan Hill got into the act when two earthquakes jolted the area Saturday and Sunday.
A 1.1 earthquake rattled the area about 10:30 p.m. Saturday, while two 3.0 earthquakes struck six miles east of Morgan Hill at 10:13 a.m. and again 10:30 a.m. No damages or injuries were reported.
It’s possible Wednesday’s temblors on the San Andreas were triggered by the earlier one on the Calaveras Fault, Oppenheimer said.
“In the mountains to the west, you have the San Andreas Fault; to the east, you have the Calaveras Fault,” he said.
The two faults meet near San Juan Bautista, he said.
“The 4.1 up by Morgan Hill sent out seismic waves,” Oppenheimer said. “People felt them and so did the rocks everywhere in the Bay Area. It’s possible the seismic activity from Calaveras triggered an event on San Andreas that pushed it along. It would have happened anyway, yet later. But it’s all conjecture.”
All were in areas marked by “creeping faults,” or friction near the fault surface caused by low-friction rocks abutting one another, Oppenheimer said.
Sometimes, the “creep” is barely perceptible, he said. In Hollister, where the Calaveras Fault runs right through town, evidence can be seen in displaced sidewalks and curbs and other spots, Oppenheimer said.
Larger quakes, such as the 1989 Loma Prieta earthquake, occur in “stuck” areas where rock does not glide past adjoining rock, he said.
As for Berkland’s prediction, he explained the moon would make its closest approach to the Earth in the past five years during March.
“Historically, there are three months that are the most prolific for damaging earthquakes in the Bay Area,” he said. “March, April and October.”
Susan Garcia, public information specialist for the Earthquake Science Center at the USGS, said there is a one in five chance of a magnitude 6.7 or greater event on the San Andreas Fault system in the next 30 years.
Berkland told the Dispatch Oct. 13, 1989 he expected an 85 percent chance for a magnitude 6.5 to 7 earthquake to occur in the coming week. Four days later, a magnitude 7.1 quake struck the San Francisco Bay area.
Berkland, who was a Santa Clara County geologist from 1973 to 1994, said the temblor action could be happening between March 19 and March 26.
“It’s an earthquake-prone period,” he said. “The odds are increased.”
McClatchy-Tribune News Service contributed to this report.