[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 M L 3.0 to M w 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 M d 2.86 to M w 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.
We derive empirical magnitude scaling relationships for southern California using a dataset of 59 past earthquakes recorded in southern California by the Southern California Seismic Network (SCSN) between 1992 and 2003. The events range in magnitude from 3.0 to 7.3. We use the maximum predominant period (τpmax) and the peak displacement amplitude (Pd) measured from the first 4 seconds of P‐wave arrivals to determine period‐magnitude and amplitude‐magnitude scaling relationships respectively. Our calibration study shows that the scaling relationships are similar to those derived for northern California. The average error in magnitude estimates is 0.2 magnitude units for events with magnitudes smaller than 4.5 (M < = 4.5), 0.3 magnitude units for events with magnitudes ranging from 4.5 to 6.5 (4.5 < M < = 6.5), and 0.5 magnitude units for events with magnitudes greater than 6.5 (M > 6.5).
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