Magnitude 6.0 earthquake strikes California’s Bay Area

There are reports of structural damage to unreinforced masonry (URM) buildings, with toppled brick parapets being more widespread, in Napa, California’s historic downtown following a magnitude 6.0 earthquake that struck 80 km west-southwest of the state capital of Sacramento Sunday, AIR Worldwide reports.

“While it is too soon to assess the extent of damage from today’s 6.0-magnitude earthquake and its aftershocks, the temblor is a reminder that those living in earthquake-prone areas of the country need to have proper coverage,” Dr. Robert Hartwig, president of the Insurance Information Institute (III) and an economist, said in a statement issued Sunday.

III reports that earthquake insurance provides protection from the shaking and cracking that can destroy buildings and personal possessions. Coverage for other kinds of damage that may result from earthquakes, such as fire and water damage due to burst gas and water pipes, is generally provided by standard homeowners and renters insurance policies, the statement adds.

CoreLogic EQECAT, however, has issued a loss estimate. “It is expected that insured losses for this event could range from US$500 million to US$1 billion,” notes an EQECAT statement. “However, it should be noted that there is a fair amount of uncertainty associated with this given the unknown extent of (business interruption) BI and contents losses, which are still being fully understood,” the statement adds.

“Residential losses are approximately from one half to one quarter of this loss estimate. If the loss exceeds US$1 billion, it will be from uncertainty in commercial losses. It is anticipated that losses to the wine industry could increase this estimate,” EQECAT adds. BI “losses are a major concern. As this is a very popular tourist area, many businesses – including wineries and restaurants – have sustained damage, both non-structural and structural.”

The strongest earthquake to hit the San Francisco Bay area since the Loma Prieta magnitude 6.9 quake in 1989, the Napa event occurred at 3:20 a.m. local time, notes a statement from AIR Worldwide. “Shaking, which last about 30 seconds, was felt in San Francisco, some 40 miles from the epicenter,” the catastrophe modelling firm reports.

Hartwig points out that the 1989 Loma Prieta quake “resulted in US$1.8 billion in insured claims (in 2013 dollars) being paid to policyholders.”

The U.S Geological Survey (USGS) reports the Napa quake occurred at a depth of just 6.7 miles (about 11 km), making it a shallow event.

The earthquake had a strike-slip mechanism with an almost vertical dipping angle, AIR Worldwide explains in the statement. The epicentral location is very close to the West Napa fault, which lies between two major traces of the San Andreas fault system — the Hayward-Rodgers Creek fault zone to the west and the Calaveras-Concord Green fault zone to the east, the statement adds.

EQECAT reports that the strongest ground motions were limited to a radius of approximately 25 km to 50 km of the epicenter. Geologically, the San Francisco Bay region is underlain by many softer soils. Historically, it has been observed that softer soils amplify ground motions and contribute to a disproportionate amount of damage, EQECAT adds.

As a general rule, aftershocks can be expected and could reach within one degree of the magnitude of the main earthquake (i.e. in the magnitude 5 range for this event) within one week, EQECAT notes. It cites the Northern California Seismic System operated by the University of California at Berkeley and the USGS, which have indicated there is a 36% probability of a strong and possibly damaging aftershock (smaller than the main shock) in the next seven days and a 5% to 10% probability of an earthquake of equal or larger magnitude.

Beyond the reported structural damage, Pacific Gas and Electric Company has announced power outages to about 10,000 households in Napa and surrounding towns, notes the AIR Worldwide statement. There have also been reports of gas leaks, buckled highways and a fire breaking out in a mobile home park, but no reports to date of structural damage to bridges or The Bay Area Rapid Transit.

“The potential cost of earthquakes has been growing in many parts of the world, including the U.S., because of increasing urban development in seismically active areas and the vulnerability of older buildings, which may not have been built or upgraded to current building codes,” Dr. Hartwig adds. 

Citing USGS information, III notes that the United States has about 20,000 earthquakes annually, mostly small, and 42 states are at risk of quakes.

The 1994 Northridge, California earthquake was the most costly in U.S. history, causing an estimated US$44 billion in total property damage, including US$15.3 billion in insured losses, US$24.1 billion in 2013 dollars.

The San Francisco Earthquake of 1906 is estimated to be second on the list, with AIR Worldwide reporting it would have caused insured losses of US$96 billion, were the quake to hit under current economic and demographic conditions.

Top Photo: A pedestrian passes a building with a collapsed wall after an earthquake Sunday, Aug. 24, 2014, in Napa, Calif. Officials say an earthquake with a preliminary magnitude of 6.0 has been reported in California’s northern San Francisco Bay area. (AP Photo/Ben Margot)

Bottom Photo: Barrels filled with Cabernet Sauvignon are toppled on one another following an earthquake at the B.R. Cohn Winery barrel storage facility Sunday, Aug. 24, 2014, in Napa, Calif. Winemakers in California’s storied Napa Valley woke up to thousands of broken bottles, barrels and gallons of ruined wine as a result of Sunday’s earthquake. (AP Photo/Eric Risberg)