No damage or injury, but schools weren’t in session
By Peter Vogel
The B.C. Catholic



Schools weren’t in session at the time, but a recent earthquake felt across the Lower Mainland proved to be a successful test of an early warning system.

Shortly after 11:39 p.m. Dec. 29, an earthquake centred near Sidney, B.C., sent ripples across Vancouver Island and the region around Vancouver. A number of school sites in these areas have detectors that form part of the UBC Earthquake Early Warning System (EEWS).

These detectors, coupled with alarm systems in all the Catholic Independent Schools in the Vancouver Archdiocese (CISVA), form part of a network designed to provide an audible warning from a few seconds to perhaps a minute before an earthquake, depending on the location of the quake.

The system provides what is known as p-wave warnings across the entire network. P-waves, lower in intensity than s-waves and arriving sooner, are generally not noticed by people.

In the case of the Dec. 29 earthquake, a school in Victoria was closest to the epicentre, and it detected the p-waves about six seconds before the arrival of the stronger s-waves. This initial detection occurred at 11:39:39 p.m. An internal system warning was triggered but not an alarm, as the magnitude of these p-waves was fairly low.

At 11:39:42 p.m. the p-waves arrived at Star of the Sea School in south Surrey. Two seconds later those waves hit St. Pius X School in North Vancouver.

Just one second later the sensor in the Victoria school recorded the arrival of the s-wave cluster with an average magnitude about 25 times as big as the p-waves. That triggered a system-wide alarm that reached as far as Campbell River to the north and to St. Mary’s in Chilliwack to the east.

Had the p-waves been much stronger, a harbinger of even bigger s-waves, then the alarm in Victoria would have triggered sooner, providing an additional six seconds of warning.

At Star of the Sea the system was able to deliver a warning of about seven seconds, at Holy Rosary Cathedral about eight seconds, and at St. John Brebeuf in Abbotsford about 12 seconds.

Of course the longer warning times were for sites where the measured ground shaking was less than for those closer to the epicentre of the earthquake.

In my own school, Notre Dame, the alarm was triggered, and had students been in the school they would have heard a verbal recorded warning about an imminent earthquake followed by a siren.

Notre Dame does not have an earthquake sensor. All the schools have alarms and are part of the EEWS, but each school does not need a sensor. In the graph accompanying this piece you will see the data that was recorded at Our Lady of Sorrows School (OLOS), just a couple of blocks away.

Kent Johansen of Extrasensory Systems Inc., a member of the UBC Earthquake Engineering group who developed the EEWS, kindly sent me several minutes worth of data from the many sensors that form part of the network.

Here is data recorded by the system just as the ground shaking at OLOS reached its peak.

This data was recorded at 11:39:55 p.m., about 12 seconds after the alarm was issued. From the graph you can see the earthquake arriving at the parish, p-waves first (noted by the first, small peak in motion), and the larger s-waves (noted by the second, big spike in motion), shortly thereafter.

There are three plots, for three directions, showing that lateral shaking (North/South and East/West) predominated over vertical motion (Up/Down). At OLOS the maximum shaking reached around 3000 micro gs (or three thousandths of a G, or one gravity – the amount of gravity the earth exerts on you when you fall).

This earthquake was, to be sure, a small one, about magnitude 4.8 at the epicentre. By way of reference, a vehicle increasing in speed from zero to 60 km/h in 16 seconds experiences about 100,000 micro gs, some 30 times the peak shown in the OLOS graph.

Notre Dame science students, many of whom will have felt this temblor, will be receiving some of the raw data from the EEWS and using it to learn a little more about earthquakes than they might otherwise from a textbook.