Earthquake Hazards in California

California earthquakes are developed by movement of huge blocks of the earth’s crust; the Pacific Plate and the North American Plate. The Pacific Plate is moving northwest, scraping horizontally past the North American Plate at a rate of about 2 inches per year. About two-thirds of this movement occurs on the San Andreas fault and some other parallel faults. Over time, these faults produce about half of the significant earthquakes of the region, as well as many minor earthquakes.

Earthquakes occur suddenly without warning, during any season, and at any time of day or night. Sponsored by the United States Geological Survey (USGS), California Geological Survey (CGS), and Southern California Earthquake Center (SCEC); the 2007 Working Group on California Earthquake Probabilities, released the Uniform California Earthquake Forecast (UCERF). It states that California has 99.7% probability of having a moment magnitude Mw = 6.7 earthquake (same as Northridge event) during the next 30 years. The southern segment of San Andreas fault has the highest probability of generating such an earthquake scenario in southern California with a 67% chance of striking the greater Los Angeles area, while Hayward fault is the most likely earthquake source in northern California with a 63% chance of striking the San Francisco bay area. Larger earthquakes are less likely during the same time frame; 94% probability of Mw = 7.0 event, 46% probability of Mw = 7.5 event, and 4.5% probability of Mw = 8.0 event. For all magnitudes, the earthquake is more likely to occur in southern California than in northern California.

The intensity of ground shaking that a building will experience during an earthquake is a function of three main factors:

1. The earthquake moment magnitude where the larger the earthquake, the stronger is the shaking.
2. The closest distance from the segment of the fault that ruptured to the building, where the nearer the building is to the fault, the greater is the shaking.
3. The type of soil materials beneath the building, where soft soils in San Francisco bay area amplify and prolong the shaking even at great distances from the ruptured fault, while hard bedrocks do not amplify the shaking. Deep soils in Los Angeles sedimentary basin would shake more than bedrock in the hills as a result of the persistent shaking where the seismic waves are trapped and reverberate. Unfortunately most urban development is in basins.

Hazards associated with ground shaking in California include:

1) Liquefaction susceptibility where partially saturated soil deposits may completely lose cohesion during prolonged shaking, behaves like a liquid, and loses their ability to support structures. The highest hazard is in low-lying areas where there are loose, sandy soils or poorly compacted artificial fill. A liquefied sand layer can shoot to the surface through cracks, forming a sand-blow, and depositing sand on the ground. This results in permanent ground deformations such as lateral spreading and settlement, both of which increase the likelihood of damage to buildings and infrastructure. Areas prone to liquefaction include, but not limited to, the southern Coachella valley and the upper Santa Ana river basin in southern California; San Francisco bay area, northwestern Alameda county, and northern Santa Clara valley in northern California as predicted by the USGS.

2) Landslide susceptibility where steep sloping areas underlain by loose or soft rock are most susceptible to earthquake-induced landslides, damaging buildings and other structures. Areas prone to landslides include, but not limited to, eastern San Gabriel mountains.

3) Surface fault rupture where shallow faults can breakthrough the ground surface as they displace against each other during an earthquake causing severe damage to building and infrastructures.

4) Fires where earthquakes in urban areas are often followed by destructive fires because of gas lines break and electrical shorts that may quickly spread due to clogged roads and collapsed bridges that prevent firefighter access, or damaged water tanks that limit water for firefighting. Without fires, the casualty and economic losses would be halved.

5) Tsunamis where major earthquakes occurring in the Pacific Ocean may displace the ocean floor, generating tsunami that could affect the West Coast.

There is a 62% probability that at least one earthquake of Mw ≥ 6.7 will occur in the San Francisco bay area before 2032. Hayward fault has the highest probability of 27% of generating such earthquake event, while the northern segment of San Andreas fault has a probability of 21%, the Calaveras fault of 11%, and the San Gregorio fault of 10%. A repeat of the 1868 Hayward earthquake is expected to cause significant loss of life and extensive damage to homes, businesses, transportation systems and utilities. Several hundred thousand people are likely to be homeless after the earthquake. Economic losses exceeding $120 billion is expected. These losses include damage to buildings and contents, business interruption, and living expenses, with more than 90% of both residential and commercial losses being uninsured.

A 2008 comprehensive study, known as the ShakeOut scenario earthquake, by the USGS and CGS of a hypothetical Mw = 7.8 earthquake rupturing the southernmost 200 miles of San Andreas fault has been performed. The fault rupture will produce more than 100 seconds of shaking throughout southern California. The event initiates from the epicenter at the Salton Sea producing very strong shaking along the fault as it ruptures until it ends near Lake Hughes. Soil conditions at sites along the fault will generally tend to amplify earthquake shaking. The intensity of shaking will also be strong in Los Angeles basin, in San Bernardino valley, in areas prone to landslides mainly in the eastern San Gabriel Mountains, and in areas susceptible to significant liquefaction mainly in the southern Coachella valley where the two conditions of liquefaction applies; strong shaking and a high ground-water table. However, localized liquefaction is likely to occur in the upper Santa Ana and Santa Clara river basins. In areas of strong shaking and liquefaction, the ground will shift violently back and forth; collapsing thousands of older buildings, shoving houses off foundations, and sending unsecured furniture and objects flying. Such an earthquake event will cause some 1,800 deaths and 53,000 injuries. Fires are very likely to occur. The estimated economic losses total $213 billion due to earthquake shaking and its consequences specifically fires.

Seismic Retrofit of Home Structure

If your home is constructed before the 1980’s, you probably need to retrofit it, especially if you live within 15 miles from an active seismic fault or within the liquefiable regions mentioned above. Seismic retrofitting measures ensure the structural integrity of your home to withstand earthquakes with little structural damage. Retrofitting is your own decision at your own risk according to your financial situation as it may be costly. However, it may be addressed one step at a time. The best return on the investment of retrofitting efforts is to initially address the foundations issues followed by bracing the cripple walls, then bracing the soft story, and ending with bracing or anchoring the masonry chimney.

Risk Mitigation of Home Contents

You should mitigate the contents of your home whether you retrofit it or not. Earthquake mitigation measures are simple techniques taken to secure non-structural elements and contents of your home to the studs of the interior walls, in order to reduce the risk to lives and investments, using inexpensive hardware tools and materials found in hardware stores and home centers. Items in the garage should also be secured to reduce damage to vehicles. Self-mitigation techniques can be undertaken by homeowners as they are simple to explain and require no special expertise, materials or tools to implement.

Earthquake Insurance

Homeowners insurance does not provide coverage for earthquake damage to your home. It is surprising to know that 33% of homes were covered by earthquake insurance in 1996 when the devastating 1994 Northridge earthquake was still fresh in people's minds, but in 2006 just 12% of homes maintained coverage, according to California Department of Insurance. Californians living in older homes should re-reconsider buying earthquake insurance as an effective way to manage potential costs of future inevitable earthquakes, especially after the 2010 Haiti earthquake.

Earthquake Preparedness Plan

You should review, update, or develop your own earthquake preparedness plan that includes maintaining emergency food and water for 1-2 weeks at all times along with personal survival kits and a household emergency kit. The magnitude 7.0 Haiti earthquake demonstrates the necessity of earthquake preparedness for anyone living in an active seismic region.

Get Ready to Shakeout!

You should participate in the annual Great California ShakeOut Earthquake Drill. You can register at http://www.shakeout.org/ now for the 2010 ShakeOut Drill on October 21 at 10:21 a.m.! It is a great opportunity to learn how to protect yourself and your family during earthquakes, and to get prepared. More than 6.9 million Californians participated in the second annual earthquake drill in 2009 in which Californians were reminded to prepare a personal survival kit for each family member and a household emergency kit, and to stockpile emergency food and water for 1-2 weeks to be prepared for the next inevitable earthquake.

The 2011 Tohoku Earthquake and the 2010 Haiti Earthquake are wake up calls for Californians to develop, update, or maintain their own earthquake preparedness plans. If you live within 15 miles of an active seismic fault and/or liquefiable area, you should re-consider retrofitting your home and mitigating its contents to protect yourself and your family. The benefits of seismic retrofitting and mitigation go well beyond being simply reducing financial losses. It will make your home safer and help in returning your family much more quickly back to their normal life style. In addition, re-considering earthquake insurance would help in managing potential costs of future earthquakes. Finally, develop an earthquake preparedness plan that includes maintaining emergency food and water for 1-2 weeks along with personal survival kits and a household emergency kit.