Seismic events generate two main wave types: compressional (P) and shear (S) waves. The S-waves carry more energy and provide the base parameter for the design of earthquake-resistant structures. The P-waves are less destructive, propagate faster, and precede the coming S-waves. The difference in the waves’ propagation velocities forms the basis of operation for most earthquake warning systems. The system recognises the P-wave, analyses its characteristics in terms of the probability that a dangerous S-wave is imminent and, if it determines that the probability is high, autonomously triggers alarms and protective measures. Earthquake warning systems that measure P-wave arrivals can be regional (network-based) or on-site (stand-alone) systems. In 2009, our first commercial on-site earthquake early warning system was designed and installed on behalf of the BC Ministry of Transportation and Infrastructure at the George Massey Tunnel on Highway 99. The tunnel provides a critical transportation link beneath the Fraser River within the BC Lower Mainland, with about 100,000 vehicles passing through every day. Because of the high cost of tunnel downtime, the warning system, which is built to ISO 9001:2008 quality standards, must provide total reliability during continuous, autonomous operation, with no false alarms. It must also be accessible over a secure Internet link, and be supported by a comprehensive, round-the-clock maintenance plan. The tunnel earthquake warning system uses triaxial sensors to continuously search for and identify P-wave– associated ground motions. The system’s sensor array consists of three instrumented boreholes, about 50 metres and 10 metres deep, located at the tunnel’s north and south ends. The data acquisition systems, communications, and power supply units are mounted on poles next to the boreholes, and are connected as a local area network (LAN) by a fibre-optic cable running through the tunnel’s service duct. All data channels are synchronised using a GPS timing unit. Real-time data from the sensor array are streamed to the central computer, which fuses and synchronises the data streams, archives storage of data files in a circular buffer, detects potential seismic events using a robust pre-triggering algorithm, and routes the message about pre-triggered events to the P-wave detection module. Configurable P-wave detection software compares real- time data against the parameters of a high-risk seismic event. The software analyses the physical properties of an incoming P-wave—including the polarisation properties of wave-produced, three-dimensional particle motions—against the known properties of both P- and S-waves to calculate the seismic event’s ground-zero and probable S-wave severity and arrival time. If the software determines an earthquake is hazardous, the decision to close the tunnel is made within 0.7 seconds of the threat’s automated identification.
P wave triggering threshold
P wave at 41.5 sec
S wave at 51.1 sec
Local time starting at 23:39:00. [sec]
B elow and previous page : The Massey Tunnel earthquake warning system consists of sensor arrays above the tunnel's north and south entrances. P hoto , this page : W eir -J ones E ngineering C onsultants ; previous page : S tephen R ees , cc by - nc - nd 2.0 A bove : The December 29, 2015, earthquake’s hypocentre ( red ) was only 61 kilometres from the George Massey Tunnel ( green ) , yet records ( top ) obtained from the tunnel early warning system show the intensity of the P-wave was about six percent of the level required to trigger the tunnel’s closure.
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