October 29, 2012

A huge ripple in our own pond – BC quake

Posted by Austin Elliott

Amidst the dire warnings and general alarm about the legitimately fearsome Hurricane Sandy, you may or may not have heard about the massive quake that rocked Canada’s remote Pacific coast on Saturday night, and the modest tsunami is sent racing across the Pacific.

Wave heights from the tsunami generated by a M7.7 quake off the Canada coast Saturday night.

At 8:04pm Saturday (PDT) the M7.7 quake rippled out from beneath Haida Gwaii, an archipelago off the British Columbia coast formerly known as the Queen Charlotte Islands. The quake was partially right-lateral, partially thrust, on a relatively steeply dipping reverse fault plane–just a bit north of the Cascadia subduction zone. At the latitude of the quake, the North American tectonic plate is not overriding the Juan de Fuca oceanic plate, but is grinding sideways along the Pacific Plate , akin to what’s happening in California, but with a bit more convergence.

The earthquake struck a very sparsely populated reach of the North American Pacific coast, but it was large enough to be felt south to Vancouver, BC and Bellingham, Washington; north to Juneau, Alaska, and inland all the way to Edmonton, Alberta. Not much footage of the quake has surfaced, but the Vancouver Sun has plenty of eye-witness accounts, and thanks to IRIS we can watch the quake ripple across the USArray of seismometers, now largely occupying the U.S. Mississippi River Valley.

Screen grab of an animation of seismic data from stations across the U.S. plotted and colored by motion during passage of the Queen Charlotte quake. Click for the video.

In most cities the shaking was just moderate, but because of the rupture’s immense size it went on for over a minute. Although footage is rare, the quake’s location offers a unique vision of the quaking: thousands of miles of mountainous BC forest rustling and swaying in the nighttime mist, with few but bears to watch it happen.

A tsunami was unleashed that was a foot or so high as it lapped the shores of Washington and Oregon, but grew to the better part of a meter–with some reports of up to 5 feet–as it washed ashore in Hawaii.

Three hours after the quake, sirens were sounding along the Hawaiian coastline, in a successful drill of the emergency system. Although the waves were ultimately small, I wouldn’t call this a false alarm: a tsunami was observed, models showed potentially damaging wave heights, and the civil warning system was activated. If you’ve got the technology, use it! Sorry your balmy Saturday evening was interrupted…

Most would agree that this was a great test and an excellent reminder to those living a little farther south along the Pacific Coast: one day this earthquake could be yours. Shaking the coastal forests for two minutes is fine; shaking Vancouver, Seattle, and Portland for two minutes will be a little messier.