Earthquake early warning times across northern California

Richard M. Allen
University of Wisconsin-Madison

AGU Fall Meeting 2004

The Elarms methodology for earthquake early warning 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. In this study probabilistic distributions of available warning times are calculated for northern California using Elarms.

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. Although only existing stations are used, it is assumed that telemetry upgrades provide data from the stations to a central processing site within 1 sec. An additional telemetry delay of 1 sec to transmit warning to users is also included. It is required that 4 sec of the P-arrival is available from two stations before a warning is issued; studies in southern California show that the average magnitude error when using two stations is 0.56 magnitude units. ShakeMaps of the 35 scenario events are also used to provide an estimate of the intensity (MMI) of ground shaking at any point. Thus warning time probability distribution functions can be estimated for each intensity of ground shaking.

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 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.