INNOVATION March-April 2016

If a potentially hazardous situation is detected, the computer triggers tunnel-closure and alert measures. The system signals the traffic lights at both entrances to the tunnel to turn red and posts closure messages on the electronic signs along Highway 99, immediately closing the tunnel’s north- and southbound lanes to new traffic. The tunnel system analysed the December 29, 2015, earthquake that occurred at 11:39 PM (Pacific Standard Time), 19 kilometres north–northeast of Victoria, in less than one-half second. Despite the epicentre being only 61 kilometres from the tunnel and the magnitude being 4.8, the system determined the quake presented no risk to the tunnel. The authors have since installed earthquake early warning systems elsewhere in the Pacific Northwest, and at most US nuclear power plants. To date, no false alarms have occurred, while the systems have continuously monitored ground motion, and have recorded and analysed many non-hazardous regional earthquakes. Warning time, based on the travel-time difference between the P- and S-waves, can range from zero when an earthquake’s epicentre is very close to where an earthquake warning system is installed, up to dozens of seconds when the epicentre is located hundreds of kilometers away but still generates dangerous ground motion. Therefore, even with reliable early warning systems, populations located close to an earthquake’s focus may have only a few seconds to react before the destructive S-wave arrives. Those seconds could be enough to save lives and limit damage to equipment and infrastructure in the immediate area—if people are aware of the alert signal, know what to do when it sounds, and are prepared. As distance from the epicentre increases, greater advance warning becomes possible. Even then, however, early warning cannot compensate for individuals’ lack of preparation. The practical benefit of having earthquake early warning stations located close to an earthquake’s epicentre and integrating their data with data from stations near monitored facilities was demonstrated with the December 29 earthquake. The event’s hypocentre, or point of origin within the

Site #19: Hypocentral distance: ~420 km

~83 sec

Site #7: Hypocentral distance: ~80 km Ampitude Ampitude 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 10 sec 40 sec P 1 P 1 P 2 S 1 S 1 S 2

Local time starting at 23:39:00. [sec]

The December 29, 2015, earthquake’s hypocentre ( red ) was less than 80 kilometres from Site #7 ( orange + green ) in the Lower Mainland, and about 420 kilometres from Site #19 ( green ) north of Kamloops. By combining the P-wave arrival at Site #7 with that at Site #19 ( top ) , the total warning time of the arrival of the earthquake’s S-wave at Station #19 became 84 seconds. The amplitudes shown are normalized to compensate for the loss of seismic energy with distance. (D ata collected during the D ecember 29, 2015, earthquake at two commercial S hake A larm ® earthquake early warning system installations .)

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