Reducing Earthquake Losses Throughout the United States

##### U.S. Geological SurveyFact Sheet-242-951995

Quake Forecasting—An Emerging Capability

Earthquake forecasts declare that a temblor has a certain probability ofoccurring within a given time, not that one will definitely strike. In thisway they are similar to weather forecasts. Scientists are able to makeearthquake forecasts because quakes tend to occur in clusters that strike thesame area within a limited time period. The largest quake in a cluster iscalled the mainshock, those before it are called foreshocks, and those afterit are called aftershocks.
(Click on image for a full size version - 120K)Aftershock forecasts helped fire departments and other providers of emergencyservices decide how many personnel to keep on duty in the aftermath of the1989 Loma Prieta, California, earthquake. These forecasts also assistedauthorities in allocating critical emergency resources and determining whenit was safe to enter damaged structures. Above, firefighters hose down adamaged building in San Francisco's Marina District during demolition (Photoby George Nikitin); a firefighter (inset) assists residents of the Districtin removing belongings from their homes (Photo courtesy of Michael Mustacchi& Associates). 
In any cluster, most quakes are aftershocks. Most aftershocks are too smallto cause damage, but following a large mainshock one or more may be powerful. Such strong aftershocks can cause additional damage and casualties in areasalready devastated by a mainshock and also threaten the lives of rescuerssearching for the injured. In the first few weeks after the 1994 magnitude6.7 Northridge, California, earthquake, more than 3,000 aftershocks occurred.One magnitude 5.2 aftershock caused $7 million in damage just to electricutility equipment in the Los Angeles area.
The U.S. Geological Survey (USGS) first began forecasting aftershocksfollowing the 1989 magnitude 7.1 Loma Prieta, California, earthquake. Bystudying previous earthquakes, scientists had detected patterns in the wayaftershocks decrease in number and magnitude with time. With such knowledge,scientists can estimate the daily odds for the occurrence of damagingaftershocks following large California temblors. These forecasts are relayeddirectly to the California Office of Emergency Services (OES) and to thepublic.
Aftershock forecasts assist government, industry, and emergency responseteams in deciding when it is safe to demolish, repair, or allow people to usedamaged structures. In the aftermath of the 1989 Loma Prieta earthquake, the"probability of aftershocks given by the USGS was one of the factors ... usedin deciding how many firefighters to keep on duty" (Greg Abell, BattalionChief, San Francisco Fire Department).
(Click on image for a full size version - 120K)Emergency workers in Santa Cruz confer during rescueefforts (U.S. Geological Survey photo).
Some large earthquakes are preceded by foreshocks. Knowledge of pastearthquake patterns allows scientists to estimate the odds that an earthquakestriking today is a foreshock and will soon be followed by a larger mainshockin the same area. These odds depend on the earthquake's magnitude and theseismic history of the fault on which it occurred. When a moderate earthquakehits California, scientists immediately estimate the probability that adamaging mainshock will follow. If the threat is significant, a warning isissued.
This warning process was put into action in June 1988 when a magnitude 5.1shock--one of the largest in the San Francisco Bay region since the great 1906earthquake--struck 60 miles south of San Francisco. Alerted by the USGS thatthere was a 1 in 20 chance of a larger earthquake in the next 5 days, theCalifornia OES issued an advisory to warn the public. (The usual daily oddsof a large quake in the Bay region are 1 in 15,000.) The warning periodpassed without further activity. In August 1989, another earthquake hit thesame area and a similar advisory was issued. Again nothing happened in thespecified warning period. However, 69 days later, the area was rocked by themagnitude 7.1 Loma Prieta earthquake, which killed 63 people and caused $6billion damage in the San Francisco Bay region.
(Click on images for full size versions - 116K, 109K, 84K)The California Office of Emergency Services (OES) operates a communicationsnetwork to issue public warnings and to support disaster response. Thisnetwork includes mobile units (left) and temporary field offices (middle) aswell as a main communications center (right). When earth scientistsmonitoring seismic activity conclude that there is a period of increasedearthquake danger, they alert the OES. If the danger is great enough, the OESissues a public warning. (Photos by Robert A. Eplett, California Office ofEmergency Services.)
Although these warnings did not pinpoint the actual date of the Loma Prietamainshock, local governments still found them useful. For example, in thecity of Oakland during the August 1989 advisory "all ... departments randrills to prepare for an imminent earthquake. This made a tremendousdifference in the city's response when the [Loma Prieta] earthquake struck"(Henry Renteria, Emergency Services Manager, Oakland).Emergency managers and the public  use earthquake advisories for making theswift and often complex decisions that are essential to reduce the risk posedby a forecast shock. Good communication and understanding between scientists,emergency managers, and the public are essential if advisories are to beeffective. Developing such communication and understanding is a goal of theUSGS Parkfield (California) Earthquake Prediction Experiment. 
The town of Parkfield, population 34, was chosen as the site of thisinnovative experiment because six magnitude 6 earthquakes have struck in itsvicinity since 1857. Some of these quakes were preceded by foreshocks andpossibly by other precursors. Because the time between a precursor and amainshock may be short, earthquake warnings must be communicated rapidly. Toaccomplish this, a plan was developed specifying when and how warnings are tobe issued in response to possible precursors to earthquakes in the Parkfieldarea. This plan was activated in 1992 and again in 1993 by possibleforeshocks; state and county officials conducted rapid, well-plannedresponses.
(Click on images for full size versions - 73K, 52K)The San Francisco Chronicle printed this warning on June 29, 1988, inresponse to an advisory from the California Office of Emergency Services(OES). Two days earlier, a magnitude 5.1 earthquake struck 60 miles south ofSan Francisco, and U.S. Geological Survey (USGS) scientists had alerted theOES that a larger shock could follow in the next 5 days. In August 1989,another earthquake hit the same area and the OES, again alerted by the USGS,issued a similar advisory. Sixty-nine days after the second advisory the SanFrancisco Bay region was rocked by the devastating magnitude 7.1 Loma Prietaearthquake. The map shows the epicenters of these three shocks.
The lessons learned from the Parkfield experiment have already enabled earthscientists and emergency response officials to build a framework within whichthey communicate rapidly and effectively. Based on this experience, similaralert plans have been devised for geologic hazards in other areas of theUnited States.With the development of modern seismic monitoring networks and the knowledgegained from past shocks, earthquake forecasts and warnings are now a reality.Continued effective communication of these forecasts to the public will helpreduce loss of life and property in future earthquakes.
**Andrew Michael, Paul Reasenberg,Peter H. Stauffer, and James W. Hendley II**
**COOPERATING ORGANIZATIONS**California Office of Emergency ServicesCalifornia Division of Mines and Geology

For more information contact:Earthquake Information Hotline (650) 329-4085U.S. Geological Survey, MS 977345 Middlefield Road, Menlo Park, CA 94025 USGS Menlo Park Earthquakes Home Page
U.S. Geological Survey Fact Sheet-242-95, March 1995
Web page by Will Prescott - 1996 April 3.  Last revision by Michael Diggles January 5, 2007