UVic’s ocean observatory helps BC prepare for The Big One

- Krista Zala

When the 9.0 earthquake struck Japan on March 11, UVic’s world-leading marine observatory helped BC residents know what to expect when the tsunami hit our shores. Now, scientists are using the data to improve our understanding of how earthquakes near and far affect the BC coast.

Sensors on the NEPTUNE Canada ocean network—part of UVic’s Ocean Networks Canada Observatory—detected the earthquake and tsunami as it swept toward BC. Because the measurements are available in real-time, details about the tsunami size and speed were online moments after each observatory site measured them.

Scientists at Fisheries and Oceans Canada (DFO) are using the data to get a better picture of how future tsunamis will affect each bay and inlet up and down the BC coast. DFO advises Emergency Management BC, which is responsible for issuing warnings to BC’s coastal communities.

“We know we’ll get our big quake anywhere from one to 400 years from now,” says Dr. Richard Dewey, associate director of research at the observatory’s VENUS network. The tectonic plates off BC have a subduction zone similar to the coast of Japan, and the violent rebound of a major quake will be the problem. “The kind of earthquake we’ll get, like the one in Japan, is particularly effective at generating large tsunamis.”

Most people on the coast are familiar with the risk that a tsunami may follow an earthquake. But each fjord, cove and point has a unique shape, and each tsunami has its own features like origin, speed, size and direction, so every massive wave acts differently in each case.

Knowing how various kinds of tsunamis affect parts of our coast prepares British Columbians for how to respond when one happens, and it helps determine the materials, size and location of new construction for everything from schools to tsunami walls.

The ONC observatory has two networks: the NEPTUNE Canada regional network and the VENUS coastal network. Each added different pieces to the picture.

NEPTUNE Canada and the tsunami

The NEPTUNE Canada regional network has five study areas stretching 300km from the coast, down the continental slope, and across the Juan de Fuca tectonic plate. Sites vary in depth: research nodes are as shallow as 20 metres in Folger Passage (in Barkley Sound between Bamfield and Ucluelet) and as deep as 2,660 metres on the abyssal plain.

NEPTUNE Canada registered both the earthquake and tsunami at all of its pressure sensors. The quake traveled fast. The seismometers felt the tremors a mere 12 minutes after it happened more than 7,000 km away, which means the pulses surged through the Pacific plate at hundreds of metres per second.

The tsunami first registered on a NEPTUNE Canada node about nine hours after the earthquake struck. A tsunami travels at an extremely long wavelength and rises very little in deep water. The 2,660 metre-deep tsunami sensor registered this one as just under 15 cm high.

In less than 10 minutes, a sensor at the node 120 km offshore recorded the wave as it passed overhead. As this node is also in water that is quite deep (1,250 metres), the wave was only a few centimetres higher.

Then, 40 minutes later, the wave arrived at a sensor near the coast between Ucluelet and Bamfield, where it had risen to 40 cm. When it hit the shore, the tsunami raised water levels by 1 metre.

“The network of tsunami sensors gives us a very precise measurement of how the waves change as they approach the coast from deep water. This in turn refines the models and helps predict behavior of big tsunamis based on information from smaller ones,” says Dr. Mairi Best, the associate director of science at NEPTUNE Canada.

Scientists such as Rick Thomson and colleagues at the Institute of Ocean Sciences in Sidney, BC, use the data to test their predictions and refine their models, explains Best.

“The real-time data are being integrated into national and international tsunami warning systems, as well as refining models that help predict and plan ahead of an event.” she says.

The tsunami reaches VENUS

VENUS has four study sites branching from three nodes, including one in sheltered Saanich Inlet north of Victoria. 

Once the tsunami reached Vancouver Island, part of it pushed through Juan de Fuca Strait and into Saanich Inlet about two hours after NEPTUNE Canada sensed it. As is common with tsunamis, the sea level in the inlet dropped first as the trough of the wave led the approach. After dropping 10 cm, it rose to about the same height and sloshed around in the inlet’s basin for 12 hours.

The measurements augment models created by DFO that predict how long a tsunami would take to reach Saanich Inlet after entering Juan de Fuca Strait

“This one wasn’t a concern to us, but it will help us see which towns are most vulnerable to the kinds of tsunamis that are most likely to occur,” says Dewey. “The question of whether to build a 3-metre or a 4-metre dyke at a town is going to make all the difference.”

At a March 17 public forum at the University of Victoria on understanding and responding to the devastating earthquake and tsunami in Japan, Dr. Chris Barnes, director of the NEPTUNE Canada ocean network, presented a series of graphs, charts and images of the devastating earthquake in Japan and the subsequent tsunami’s journey across the Pacific. For his presentation, visit: http://bit.ly/fw1tMr


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Keywords: Ocean Networks Canada, emergency, tsunamis, oceans

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