Whole lot of shakin' going on beneath our feet in BC

Stephen Hume, Special to the Sun

Published: Monday, May 05, 2008

Most of us wouldn't have known without a seismograph, but another light earthquake rumbled through British Columbia's basement last week.

Ignorance is bliss. "Light" is a relative term. That "pipsqueak" seismic event 375 kilometres west of Vancouver released roughly the same amount of energy as the atomic bomb dropped on Nagasaki.

John Cassidy, who does have a seismograph -- actually, he has scads of them -- did notice. From the Pacific Geosciences Centre near Sidney on Vancouver Island, he keeps track of what's shaking beneath our feet for the Geological Survey of Canada. He's a world expert on earthquake hazards.

"What people forget is how active this region is," Cassidy said when I called to ask about the free public lecture he's giving Thursday at 4 p.m. in the Theatre Foyer of UBC Robson Square. "During the past month there have been 95 earthquakes in the region."

So, nothing unusual, yet "pipsqueak" is also a relative term when talking about earthquakes. It all depends where you are standing, Cassidy said.

Huge earthquakes become geologists' exotic cocktail chat if they are deep and distant. Even small events become unpleasant world newspaper headlines if they are shallow and near a city.

Consider the Nisqually earthquake that shook Seattle and gave Vancouver a scare on Feb. 28, 2001, and the Northridge 'quake in Los Angeles in 1994. Both were virtually the same in magnitude, around 6.8, Cassidy said. But while Nisqually caused damage estimated at about $2 billion, Northridge killed 57 and injured 9,000, and the Southern California Earthquake Center at the University of Southern California estimates damage eventually reached $40 billion.

Why this difference? Nisqually was 52 kilometres deep in the earth. Northridge was only about 18 kilometres down. Without the extra 34 kilometres of buffering rock, shaking in California was far more violent, and a dense urban population exposed more to the consequences.

This should fascinate Lower Mainland residents, who live in one of the least stable places on the planet, a shuddering, juddering zone where huge plates of rock jostle each other like ice floes. Fault lines, shallow and deep, spread everywhere, ubiquitous as cracks in the glaze on an old coffee mug.

Little tremors, smaller than the one nobody noticed last week, occur daily. Yet every decade, there's a whopper like Nisqually, the 7.3 Comox earthquake of 1946 or the 6.9 event that sent frightened Vancouver residents rushing into the streets in 1918.

And over centuries come the great earthquakes, the mega-thrust monsters 10,000 times more powerful than last week's, which equalled only one atomic bomb.

This is National Emergency Preparedness Week, a good time to contemplate that four of the five most powerful earthquakes in Canadian history occurred in B.C. And that of the next five, two were here and a third was just north of the B.C.- N.W.T. boundary.

So that's seven "Big Ones" out of the last 10. Perhaps this subject warrants further education. The Geological Association of Canada's Cordilleran Section is happy to accommodate.

Cassidy, both famous as a seismologist and popular for his infectious enthusiasm and entertaining talks, will discuss the history of earthquakes here, what the risks really are and the astonishing research he and his colleagues are pursuing.

For example, their discovery that Vancouver Island abruptly reverses direction every 62 weeks. Usually it moves toward the mainland at a rate of about one centimetre a year. But satellite tracking shows the Island regularly heads for Japan, nudges the big Pacific plate, then heads for the mainland again. The vast stress along this subduction fault is what generates the hundreds of daily tremors we don't perceive.

"If that [subduction] fault were ready to fail," Cassidy said, "it's likely one of these taps would be the trigger."

Then we might experience another great earthquake like the one at 9 p.m., Jan. 26, 1700. Ground on the Island's outer coast dropped as much as the height of a two-storey house, shaking was so severe that people couldn't stand up and the following tsunami blasted Japan, more than 7,000 kilometres away, which is why we can date it so precisely.

Cassidy can tell you all about it Thursday.

shume@islandnet.com