San Francisco Bay Area Earthquake Loss Estimation
The USGS, the California Office of Emergency Services, the California Geological Survey, and the Association of Bay Area Governments jointly conducted a loss estimation study focused on the ten most likely damaging earthquakes forecast for the Bay Region by the Working Group. These earthquakes occur on six of the seven major fault systems in the Bay Area and range in size from a magnitude 6.7 event on a blind thrust underlying Mt. Diablo to a magnitude 7.9 repeat of the 1906 rupture on the San Andreas fault in northern California. Their 30-year probabilities range from a high of 15.2% for a M7.0 rupture of the Rodger’s Creek fault to 3.5% for a M7.4 combined rupture of the Peninsula and Santa Cruz Mountains segment of the San Andreas.
|Ten most likely damaging Earthquake scenarios||30-year probability||Magnitude|
|Southern Hayward (possible repeat of 1868 earthquake)||11.3%||6.7|
|Northern + Southern Hayward||8.5%||6.9|
|San Andreas: Entire N. CA segment (possible repeat of 1906 earthquake)||4.7%||7.9|
|San Andreas: Peninsula segment (possible repeat of 1838 earthquake)||4.4%||7.2|
|Northern San Gregorio segment||3.9%||7.2|
|San Andreas: Peninsula + Santa Cruz segment||3.5%||7.4|
Because the ten most likely future earthquakes in the Bay area occur on faults throughout the region, the impact and potential losses reported here reveal significant risk for the entire ten county region.
The study found that all ten of the most likely forecast earthquakes would cause social and economic disruption equal to or greater than the 1989 magnitude 6.9 Loma Prieta earthquake when current building replacement/repair costs in the San Francisco Bay Area are utilized. The M6.9 Loma Prieta earthquake resulted in at least $6 billion in damage to buildings and infrastructure. This amount does not include the resulting economic losses related to this damage.
Our loss estimates were determined using the Federal Emergency Management Agency’s nationally recognized and publicly available earthquake loss estimation model, HAZUS. We used ground-shaking inputs for the scenario events developed using the same methodology employed by the USGS in producing automated, near real-time, web-based ShakeMaps, that depict the degree and distribution of strong shaking following significant quakes in the San Francisco and Los Angeles region. In a post-earthquake setting, FEMA and CA OES will use ShakeMap input into HAZUS for rapid post-event loss estimation and to help guide emergency response. Ground motion inputs in a HAZUS-ready GIS format are available on the web for these ten most likely scenario earthquakes as well as for all potential Bay Area earthquakes characterized by the Working Group.
A repeat of the 1906 magnitude 7.9 earthquake, the worst case scenario for the Bay Area, is estimated to result in about 5800 fatalities if it strikes during working hours. This estimate is comparable to the approximately 6000 deaths caused by the 1995 M6.9 Kobe earthquake that occurred in the afternoon directly beneath an urban area with a population of 1.52 million people. Most scenarios, however, have maximum projected fatalities on the order of several hundred, reflecting the success of earthquake-resistant design and construction practices in California, particularly in residences.
The loss of life is predicted to be highest if an earthquake occurs in the early afternoon when people are working in commercial buildings with varying vulnerability to quakes These predicted mid-afternoon fatalities are generally about 5 times higher than values predicted at 2:00 AM when the population is assumed to be in wood frame residential units.
Because all ten of the scenario earthquakes in the loss estimation study strike much closer to the urban core of the Bay Area than magnitude 6.9 1989 Loma Prieta earthquake, the projected number of uninhabitable residences for all ten scenarios will probably exceed the 16,000 made uninhabitable by the Loma Prieta quake. For example, the Association of Bay Area Governments has estimated that more than 150,000 uninhabitable residences will occur in a M7.9 repeat of the 1906 earthquake or a M6.9 rupture of the entire Hayward fault.
These loss estimates are believed to be minimum ones, in part because of a lack of detailed and representative building inventory in the Bay region, as well as the fact that damage to specific facilities or lifelines, such as the Hetch-Hetchy aqueduct system, Bay Area ports, and transportation systems, is not included. For example, a recent Bay Area Economic Forum study on the seismic vulnerability of the Hetch-Hetchy system estimated potential for tens of billions of dollars of losses related to failure of that system alone in a major earthquake.
The East Bay I-880 corridor along the Hayward fault would experience the most proportional property damage of the entire Bay area, a result of dense development directly along and next to the Hayward fault and the fact that earthquakes on this fault, while lower in magnitude than those on the San Andreas fault, have higher odds of occurring.
For all Hayward fault earthquake scenarios, severe shaking is predicted to extend westward across San Francisco Bay onto the soft soil sites on the San Francisco Peninsula and the made land/artificial fill in San Francisco. In fact, San Francisco’s financial district, which is largely built on pre-1906 bay fill, is approximately equidistant from the Hayward and the San Andreas faults, making it vulnerable to large earthquakes on both sides of the Bay.