ElarmS: Earthquake early warning for southern California

R. M. Allen
University of Wisconsin-Madison

H. Kanamori
California Institute of Technology

AGU 2003 fall meeting

The location of faults directly beneath metropolitan areas in southern California requires any earthquake early warning system to make use of the first arriving energy, the P-wave, to estimate the hazard potential of the earthquake. Using the P-wave arrival maximizes the warning time available and may also make it possible to issue a warning before peak ground motion at the epicenter where the warning is most needed. We present a methodology that could potentially provide an earthquake early warning system for southern California. The necessary components include rapid detection and location of earthquakes, magnitude estimation using the P-wave, and prediction of peak ground motion variations across the affected region. As a result of the dense distribution of seismic stations in the region (TriNet), typically two or three stations trigger within one second of the first station to detect an earthquake. This allows for accurate location of the event within a second of the first trigger. Magnitude is estimated from the predominant period of the P-arrival at each triggered station. At any given time after the P-trigger, the maximum period observed thus far is converted to a magnitude. This magnitude represents a minimum estimate; a larger predominant period observation at some later time would indicate a larger magnitude earthquake and the magnitude estimate would be updated. For smaller earthquake (less than magnitude 4.5) the maximum period is usually observed within 1 sec, for larger events up to 4 sec of data is needed to capture the best magnitude estimate. Given the location and magnitude estimates, the distribution of peak ground motion is predicted using simple 1D attenuation relations. As time progresses observations of the peak ground motion at stations close to the epicenter are used to adjust the attenuation relations and improve predictions of ground motion for more distant regions.