Application of Elarms in California

Richard M Allen
University of Wisconsin-Madison

International Workshop on seismic early warning for European cities, held in Napoli from September 23 to 25, 2004

The metropolitan areas of both Los Angeles and San Francisco are dissected by the complex San Andreas Fault system. Maximizing the warning time provided by any future early warning system requires a methodology that uses as few P-wave arrivals as possible recorded at seismic stations closest to the epicenter. The Elarms methodology generates hazard maps showing the distribution of peak ground shaking based on P-wave arrivals only. This approach has the potential to issue a warning before peak ground shaking initiates even at the epicenter.

Using data from past earthquakes in southern California, an assessment can be made of how the methodology could operate in realtime. Based on system delay times and accuracy statistics generated from these past events, predictions of how Elarms would perform in future earthquakes in both northern and southern California are also possible.

Tests to determine probabilistic warning times for major earthquakes in northern California show that for any given location there is a much higher probability that a warning can be issued than not. While the intensity of ground shaking is dependent on the distance from the fault, the warning time is dependent on the distance from the epicenter. It is therefore the large lateral extent of strike-slip faults in the region that allow for substantial warning times. For example, consider the Marina District of San Francisco that suffered severe damage in the Loma Prieta earthquake with an epicenter 100 km southeast. For earthquakes that result in shaking at MMI > IX it is more likely there will more than 20 sec warning than less. These observations for the Marina District are typical of any location across the Bay area.