http://danvillesanramon.com/print/story/print/2008/09/05/were-surrounded-by-earthquake-faults


DanvilleSanRamon.com

Newsfront - September 5, 2008

'We're surrounded by earthquake faults'

Danville experiences 'Quake Swarms,' says expert

by Geoff Gillette

Californians have been hearing about it for years. The Big One. The earthquake that cracks the state off the continental U.S. and drops it into the ocean, leaving neighboring states with some nice beachfront property.

The question is: Could that really happen?

Earthquake specialists say no. But an earthquake doesn't have to be of Hollywood disaster proportions to have a devastating impact.

Earthquake Geologist David Schwartz with the United States Geological Survey says quakes happen on an almost daily basis in the Bay Area, and the San Ramon Valley in particular sees its fair share as a result of its location.

"We're fundamentally surrounded," Schwartz said. "Starting in the western part, the Hayward fault is a concern for the entire Bay Area. If it produced a 6.8 or 6.9 magnitude earthquake it would produce widespread damage throughout the area."

To the east are the Calaveras and Greenville fault lines. North there is the Concord/Green Valley fault line. Schwartz, himself a Danville resident, said there is one fault that everyone in Danville can see every day.

"Mount Diablo isn't a volcano; it's a fold in the earth's crust caught between the Concord/Green Valley and Greenville faults," he said.

Schwartz said Mount Diablo also differs from the other faults in the area in that it is what is called a "thrust" fault.

"These others are strikeslip faults. These are faults that move sideways, one side of the crust moves parallel to the other," he explained. "That's the principal type of fault we have in the Bay Area."

But with a thrust fault, the crust moves up over the other side. Mount Diablo grows by a few millimeters per year as one plate slides up over the other. Schwartz said it is the same kind of fault that produced the Northridge earthquake of 1994. The 6.6 magnitude quake, which struck near Los Angeles, caused 72 deaths and more than $12 billion in damage.

"So when people look around at Mount Diablo they should look at it with a new degree of respect," he said. "What they're seeing is the earth's crust deforming right before their eyes."

Danville's location in the center of four major faults also creates interesting seismological behaviors. Schwartz said that in the past 30 years U.S.G.S has charted several earthquake "swarms" in the San Ramon Valley.

"We have a series of smaller and unmapped faults that sit between Calaveras and Mount Diablo," he said. "These get active and create earthquake swarms. The largest swarm was in 1990. It lasted for 42 days and there were 350 earthquakes reported. The largest was a 4.4."

The most recent swarm was reported in 2003, lasted 31 day, and created some 120 earthquakes.

"These small faults get turned on and move for a few days or weeks and then they turn off again. Not truly a hazard, but a major nuisance. And a reminder to people that they live in earthquake country," Schwartz said.

So what are the chances of one of these major faults triggering a major seismic event? According to statistics released by the U.S.G.S, the entire Bay Area has a 63 percent chance of a magnitude 6.8 or greater earthquake in the next 30 years.

The majority of that percentage lies west, toward San Francisco, where the San Andreas and Rodgers Creek/Hayward fault lines have a 21 percent and 31 percent chance. In the San Ramon Valley, the Calaveras fault has the highest probability at 6 percent. Greenville has a 3 percent chance, as does the Concord/Green Valley fault. Mount Diablo only has a 1 percent chance in the next 30 years.

Schwartz said that while those numbers are not large, any event in the local area will be felt and residents should be prepared.

"People should be ready for an earthquake on any one of these faults," he said. "There's a common thought that the closer you live to a fault the greater the hazard."

He said that is not the case. It is largely dependent on the construction of your home and the ground conditions.

"We like to say you can run but you can't hide." He added, "The real cause of damage in a quake is shaking and to a large degree it's the amplification of that shaking by local ground conditions."

Estimates show that the San Ramon Valley experiences several quakes per week. Most are very small. Schwartz said most people won't even feel a quake less than a 2.0 unless they are right on top of it.

Prediction of earthquakes is still in its infancy, with no reliable method of determining when a large event may occur, but Schwartz said there are some warning signs the U.S.G.S looks for.

"Five to 10 percent of magnitude 5.0 quakes are followed by something larger," he said. "Sometimes it's a signal of something bigger, and sometimes it just triggers something else."

The safest course is for residents to work on making their homes quake resistant and to make plans and set aside food and equipment in the event of a quake along one of the many faults in the area.

Comments

Posted by LK, a resident of San Ramon
on Sep 5, 2008 at 9:45 pm

just experienced a strong but short M 4.0 earthquake near San Ramon

lk


Posted by Alice, a resident of Walnut Creek
on Sep 5, 2008 at 9:54 pm

We really shook from the 4.0 that just hit 9/5 around 9:45PM We're in south WC off Rudgear.


Posted by Alice Valle-Riestra, a resident of Walnut Creek
on Sep 5, 2008 at 9:54 pm

We really shook from the 4.0 that just hit 9/5 around 9:45PM We're in south WC off Rudgear.


Posted by CBS, a resident of another community
on Sep 5, 2008 at 10:44 pm

Felt the quake at our place, Altamont pass area


Posted by ami, a resident of Walnut Creek
on Sep 5, 2008 at 11:07 pm

Pleasant Hill Bart really strong


Posted by mde, a resident of Diablo
on Sep 6, 2008 at 1:53 am

I concur. I was startled by a sound (the house shifting) and a second or two later felt the ground beneath me shift as though it had been hit by a truck. It got the adrenaline pumping, for sure.


Posted by Willie Nelson, a resident of San Ramon
on Sep 6, 2008 at 3:25 am

My wife and I live in San Ramon but were eating dinner at the Outback Steakhouse in Dublin, about 12 miles from the epicenter of this quake. It was felt by everyone in the restaurant, but was a rolling motion (no hard jolts) characteristic of a distant quake, as opposed to one you're right on top of. It lasted maybe 2 to 3 seconds and was no big deal. Everyone went about eating their dinners like nothing had happened.


Posted by Sam Andreas, a resident of another community
on Sep 8, 2008 at 3:53 pm

It would seem that if we are surrounded by earthquake faults, we could quite accurately say, "It's our fault!" Such serious humor takes us back to a product planned as a sequel to the Pet Rock. "It's your FAULT" was a box containing a core sample of the San Andreas Fault and the history of California fault activity. It was intended to welcome visitors to our local airports and tourist attractions.

I hope all is well with all neighbors after our very local earthquake!

Sam