Earthquake early warning in northern California

Richard M. Allen
University of California Berkeley

EERI/ISSS First International Conference on Urban Disaster Reduction, Kobe, Japan.

Rapid detection of earthquake nucleation can provide tens of seconds warning of forthcoming ground shaking. Such warnings are sufficient for automated systems integrated into smart-city infrastructure to take mitigation steps and reduce the impact of the earthquake. The Elarms methodology for earthquake early warning [Science 300 p786-789, 2003] uses P-arrivals recorded by broadband velocity and strong motion sensors close to the epicenter to both locate the earthquake and estimate its magnitude. Given these parameters, attenuation relations can be used to map the predicted distribution of peak ground shaking across the affected region and traveltime curves provide the onset time. An assessment of Elarms implementation in northern California is now underway. Here, I present probabilistic distributions of available warning times across the region.

The set of sources used is that provided by the Working Group 2003 report on Earthquake probabilities in the San Francisco Bay Region: 2002-2031 and includes a total of 35 ruptures on the major identified faults each with an estimated probability of occurrence within the next 30 years. Warning times are estimated for each rupture based on a set of assumed operational parameters intended to represent a feasible implementation of Elarms in the region based on the current station distribution. Results 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 useful warnings. 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 with intensities greater than MMI IX in the Marina District it is more likely there will be more than 20 sec warning than less. These observations for the Marina District are typical of any location across the Bay area.