Implementation of ShakeMap in Ontario: Challenges of detection and location in a sparse network

ShakeMap is a computer program to map earthquake ground-motion parameters in near real time. It originally was developed and implemented using a dense recording network in California (Wald et al. 1999); typical interstation spacing in southern California is about 5 to 50 km, though some stations in the Los Angeles metropolitan area are less than 2 km from each other, as seen at http://www.data.scec.org/ stations/CI_stationp_map.php). For our implementation of ShakeMap in southern Ontario, we have developed an automated earthquake detection program, "EventDetector," to continuously detect events and trigger ShakeMap in a sparse network where typical interstation spacing is about 100 km. The EventDetector is keyed to ShakeMap objectives in that it detects events based on the exceedance of specified ground-motion amplitudes at five or more stations. The specified threshold is set as peak ground velocity (PGV) = 0.003 mm/s. For our network configuration, this criterion should result in ShakeMap triggering on all events of M > 2.8 within the southern Ontario network. When the program detects an event, EventDetector automatically triggers the ShakeMap program and continues to process the incoming data. The main challenge in event detection is to screen out false triggers caused by teleseismic events or other spurious signals. We distinguish between local/regional and teleseismic events by using the standard deviation of the peak ground velocity, normalized by the mean (denoted NSD) as a discriminant. Events that have NSD < 0.7 are teleseismic events (with a rate of 10% false triggers). To locate the regional earthquake once an event is confirmed by the above criteria, we use the ground-motion centroid concept, originally developed by Kanamori (1993). The centroid provides an estimate of the earthquake moment magnitude (M) and location from the point of view of ground motion, and it may not coincide with the actual earthquake epicenter and magnitude. The estimation is based on fitting the observed amplitude data to groundmotion attenuation relations. An additional check on whether the event is a significant earthquake is based on the determined centroid magnitude; events of M < 2.8 are not considered further. We have compared the accuracy of estimated ground motions obtained using the epicenter location with those obtained using the ground-motion centroid for more than 30 local/regional earthquakes of magnitude 2.5 to 5.0. Overall the centroid performance in terms of estimating recorded ground motions appears to be satisfactory. In fact, the Nuttli magnitude (M_N) 5.4 6 March 2005 Rivière-du-Loup earthquake demonstrates that estimated PGV based on ShakeMap agrees quite well with the recorded PGV (Kaka and Atkinson 2005b). Use of the epicenter might improve estimates of ground motion near the epicenter in cases where no nearby stations are available. However, the centroid is available in near real time while the epicenter is not.