San Francisco

About an hour and a half ago, we had a fairly small earthquake here in the San Francisco Bay Area.

While it only had a magnitude of 4.0, it was interesting to see the reaction on various social networks such as Facebook and Twitter. In fact, when I first felt our building shake, I wasn’t sure if there had actually been an earthquake, so I checked my Twitter account.

Only a few minutes later, this is what my feed looked like.

And here is the shake map for the earthquake:

According to the USGS, it was centered on an unnamed fault located near the Calaveras Fault:

A light earthquake centered near Alamo shook the Bay Area Friday evening. Reported at 9 p.m., it was a magnitude 4.0, said the U.S. Geological Survey.

The Contra Costa and Alameda County sheriff’s offices said they had no reports of damage.

The quake was felt in Petaluma to the north and in Gilroy and Turlock to the south and southeast, said David Oppenheimer, a seismologist with the geological survey. He said it occurred on an unnamed fault about 2.5 miles from the larger Calaveras fault and two miles north-northeast of Alamo, on Serafix Road.

A cluster of quakes occurred on the same fault in April 1990, with 18 registering at 3.0 or higher in a three-week period, Oppenheimer said. He said the larger quakes in that sequence caused minor structural damage to nearby homes.

Friday’s earthquake took place about 10 miles underground, more than twice as deep as the 1990 quakes, and thus is probably less likely to be part of a cluster or cause damage, Oppenheimer said.

Bummer, I predicted that it was actually on the Calaveras Fault itself.

Anyway, not really a big deal in the end. But the fusion of social media and science is always interesting. We’ve written about Twitter before as well, also in terms of earthquakes and disseminating information.

I want to welcome a good friend of mine to the Geology News and geoblogging bandwagon. Peter Polito comes to us from San Francisco, California where he is currently working on his Master’s thesis at San Francisco State University. Peter is studying dendritic river patterns observed on Saturn’s moon Titan and how ice strength affects erosion rates on the moon. Peter received an undergraduate degree in geology from San Francisco State and graduated in 2006.

Welcome aboard Peter! I look forward to the interesting items you’ll be posting about and your perspectives on various geologic topics.

One hundred and two years ago today, at 5:12am, the earth around the San Francisco Bay Area started to violently shake thanks to a ~M7.8 earthquake. A minute and a half later, buildings lay in ruins, while north of the city in Marin and Tomales Bay, fence posts laying across the “newly discovered” San Andreas fault (which didn’t have a name at that time) were offset by nearly 30 feet. The mayhem was just beginning however, as fires from knocked over furnaces and stoves began to rage out of control and completely destroy much of the city.

Despite the tragic loss of life in San Francisco (up to 3,000 people by some estimates), some good did result from the� catastrophe. It marked the dawn of earthquake science and seismology. The Lawson Report pegged the cause of the 1906 earthquake on the San Andreas fault. You can actually read the Lawson Report (published in 1910) online here: Volume I, Part I; Volume I, Part II; and Volume II.

The preliminary report (published May 31, 1906) details the initial recollection of the San Andreas Fault.

One of the remarkable features of the Coast Ranges of California is a line of peculiar geomorphic expression which extends obliquely across the entire width of the mountainous belt from Mendocino County to Riverside County. The peculiarity of the surface features along this line lies in the fact that they are not due, as nearly all the other features of the mountains are, to atmospheric and stream erosion of the uplifted mass which constitutes the mountains, but have been formed by a dislocation of the earth’s crust, or rather a series of such dislocations, in time past, with a differential movement of the parts on either side of the plane of rupture. In general this line follows a system of long narrow valleys, or where it passes through wide valleys it lies close to the base of the confining hills, and these have a very straight trend; in some places, however, it passes over mountain ridges, usually, at the divide separating the ends of two valleys; it even in some cases goes over a spur or shoulder of a mountain. Along this line are very commonly found abrupt changes in the normal slope of the valley sides giving rise to what are technically known as scarps. These scarps have the appearance of low precipitous walls which have been usually softened and rounded somewhat by the action of the weather. Small basins or ponds, many having no outlet, and some containing saline water, are of fairly frequent occurrence and they usually lie at the base of the small scarps. Trough-like depressions also occur bounded on both sides by scarps. [...]

I love living in San Francisco, but it baffles my mind that one day this will happen again. And somehow, my choice of living here means I accept that fact and am okay with it. Crazy.

[Via USGS]

I was going through some old emails from last week and found a press release from the USGS about America’s most dangerous fault; the Hayward fault.

According to a recent study by the USGS and Risk Management Solutions, a M6.8 earthquake on the Hayward Fault could produce upwards of $165 billion dollars, even more expensive than damage and economic losses from Hurricane Katrina.

A sidewalk and curb in Hayward, California offset by aseismic slip along the Hayward Fault.
Image Credit: Dave Schumaker

In marked contrast to Hurricane Katrina where uninsured losses were approximately 60 to 70 percent of total economic losses, more than 95 percent of projected Hayward Fault earthquake residential losses and 85 percent of commercial losses will be uninsured.

“Bay Area residents, businesses and local governments need to take action now to reduce future losses. The public understands this and has repeatedly supported bond measures for well-planned seismic upgrade projects, such as the ongoing retrofit of BART and the Hetch Hetchy system,” said Mary Lou Zoback, earthquake expert from RMS.

“Public and private organizations have already invested over $30 billion to retrofit or replace vulnerable buildings and infrastructure, but more needs to be done,” said Tom Brocher, seismologist with the USGS. Until the Bay Bridge and BART undergo major retrofits, they remain vulnerable to earthquakes and more than 180,000 daily commuters who currently use them could face having to take overtaxed alternate routes for months. Similarly, until the Hetch Hetchy aqueduct system upgrade is complete, earthquake-related activity could cut off water for 2.4 million Bay Area residents, according to a recent report by the Bay Area Economic Forum.

As someone who actively chooses to live and work in the San Francisco Bay Area, this thought is a daily occurrence as I walk through the streets of San Francisco. There is definitely a lot that needs to be done to mitigate potential damage. One of the first that comes to mind is that San Francisco should get off its butt and change it’s fire hydrant connections to the statewide standard. Ridiculous.

The East Bay Express has posted the second part of their series about the earthquake hazards of the Bay Area. This article deals with the aftermath of a major quake on the Hayward Fault.

If the quake hits during the workweek, hundreds of thousands of East Bay residents will be on their own in San Francisco. The city may emerge relatively unscathed if the quake centers on the Hayward Fault’s northern section, according to computer modeling cited by structural engineer and risk-assessment expert Ronald Hamburger. But if the epicenter is on the southern end of the fault and the shock waves pulsate northward? “A rupture on the southern section, going from south to north, just creams San Francisco,” Hamburger said.