Application of ElarmS across California

Richard M. Allen
University of California Berkeley

Earthquake Early Warning Workshop, Caltech, June 2005.

The ElarmS methodology combines data streams from a regional seismic network to rapidly predicted ground shaking maps for an earthquake within a few seconds of its nucleation. P-wave trigger times, predominant frequencies and amplitudes are gathered from stations and combined in real-time. The methodology allows for the first hazard map to be generated one second after the first P-wave arrival at the closest station. This map is then updated as more data becomes available from stations in the epicentral region. The accuracy of the hazard predictions increase as more data becomes available meaning there is a trade-off for users between the certainty of the ground shaking prediction and the warning time available.

I will present an assessment of the accuracy of ground shaking predictions and the likely warning times that will be available given implementation of ElarmS across California. A study of 32 earthquakes in southern California using the current distribution of seismic stations shows that hazard maps would be available for more than 50% of earthquakes at the time of the S-wave arrival at the epicenter, and for 97% of earthquakes within 5 sec. The greatest source of uncertainty in the hazard predictions is not due to errors in the rapid magnitude estimates, but is due to the accuracy of the attenuation relations used to translate earthquake magnitude and location into peak ground shaking maps.

Using the earthquake probability estimates for the San Francisco Bay Area [Working Group '02], estimates of the warning times for the set of likely earthquakes in the region can be made. As probabilities are assigned to each of these events and estimates of the ground shaking distribution are available (scenario ShakeMaps), probabilistic warning time distributions can be calculated for any given location and associated with the an intensity of ground shaking. Using the current distribution of seismic stations, the warning times for regions that would experience damage is between 0 and ~80 sec. For the earthquakes that would be most damaging to the city of San Francisco, it is more likely there will be more than 20 sec warning than less.