Toward earthquake early warning in northern California

[1] Earthquake early warning systems are an approach to earthquake hazard mitigation which takes advantage of the rapid availability of earthquake information to quantify the hazard associated with an earthquake and issue a prediction of impending ground motion prior to its arrival in populated or otherwise sensitive areas. One such method, Earthquake Alarm Systems (ElarmS) has been under development in southern California and, more recently, in northern California. Event magnitude is estimated using the peak amplitude and the maximum predominant period of the initial P wave. ElarmS incorporates ground motion prediction equations and algorithms from ShakeMap for prediction of ground motions in advance of the S wave arrival. The first peak ground motion estimates are available 1 s after the first P wave trigger, and are updated each second thereafter for the duration of the event. The ElarmS methodology has been calibrated using 43 events ranging in size from ML 3.0 to Mw 7.1 that occurred in northern California since 2001. We present the results of this calibration, as well as the first implementation of ElarmS in an automated, noninteractive setting and the results of 8 months of noninteractive operation in northern California. Between February and September 2006, ElarmS successfully processed 75 events between Md 2.86 to Mw 5.0. We find that the ElarmS methodology processed these events reliably and accurately in the noninteractive setting. The median warning time afforded by this method is 49 s at the major population centers of the Bay Area. For these events the magnitude estimate is within an average of 0.5 units of the network-derived magnitude, and the ground motion prediction from ElarmS is within an average of 0.1 units of the observed modified Mercalli intensity.