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Northwest quakes shake up interest

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On the evening of October 27th, 2012, a magnitude 7.7 earthquake shook coastal British Columbia.

The epicenter of the quake was in Haida Gwaii, formerly known as the Queen Charlotte Islands. It was the third largest recorded earthquake in Canada.

David Oglesby, professor of geophysics at the University of California, Riverside explained why this particular quake caught scientists’ attention.

Professor David Oglesby – Photo courtesy of David Oglesby and http://earthsciences.ucr.edu/oglesby.html

“In Southern Alaska and in Canada is what’s called a transform plate boundary and it typically involves faulting that is horizontal. It’s called strike slip faulting. Very similar to what you get in Southern California and in Central California on the San Andreas Fault. Those are the kinds of earthquakes that we expect to see in your region. This earlier event, this magnitude 7.7 west of Queen Charlotte Island was not that type of earthquake,” said Oglesby.

To understand why this is important, we need to take a look at two different types of earthquakes and how they work.

An earthquake is when the tectonic plates that make up the earth’s crust scrape against each other along fault lines.

Most earthquakes in Southeast Alaska are the type that involve strike-slip faulting.

“The best way to think about it is, take a map and draw a line on it and then cut along that line. And then slide the two portions of the map horizontally past each other. It involves horizontal slipping on a relatively vertical fault,” said Oglesby.

The Himalayas are the highest mountains in the world. They were created by the other type of fault movement, called thrusting. This is the kind of quake that happened off of Haida Gwaii last year.

“You had one plate thrusting underneath the other. So the two plates are actually pushing together,” said Oglesby.

Now, we fast forward to this year’s January 4th quake that happened off the coast of Craig in Southeast Alaska. It was an example of strike-slip faulting. But, Oglesby said, the two quakes might be related.

He said the first thing that tipped scientists off is that the two quakes happened in the same region in a relatively short period of time.

To find out more, scientists are now looking at what happened with the forces inside the earth’s crust during these two events.

“You can look at how the rocks moved around in the first earthquake. When rocks move around inside the earth, they redistribute forces in the earth, so they change the forces on nearby faults,” said Oglesby.

Sometimes the rocks move in such a way that they take pressure off of other faults and reduce the likelihood of more earthquakes happening.

In other cases, the rocks jam up against each other, aggravating the already smashed faults and increasing the chance of a second earthquake.

Oglesby said preliminary analysis of these two recent quakes leads scientists to believe the Haida Gwaii quake did put more pressure on the nearby fault and helped trigger the Alaska quake.

Diagram of the Haida Gwaii quake – Photo courtesy of the National Earthquake Information Center, US Geological Survey, and http://neic.usgs.gov/neis/eq_depot/2012/eq_121028_b000df7n/neic_b000df7n_l.html

“That’s what I mean by there being a connection between the two earthquakes. That’s not to say that the second earthquake wouldn’t have happened by itself eventually. But it’s possible that it was moved forward in time by a certain number of years due to this earlier earthquake a little bit further south,” said Oglesby.

But there’s another reason scientists are paying special attention to the Haida Gwaii quake.

Southeast Alaska’s typical strike-slip faulting can cause a lot of ground shaking.

But it isn’t as potentially dangerous for coastal communities as a thrusting earthquake.

“Because a tsunami is generated whenever you can lift up or throw down a whole bunch of water. So an earthquake that has a significant component of vertical motion can lift up or throw down the sea floor, whereas an earthquake that involves mostly horizontal motion tends not to have as much of a tsunami potential,” said Oglesby.

The Haida Gwaii quake did cause some tsunami activity and Oglesby said the region got lucky…this time. It can be easy to forget about earthquakes when they’re not happening.  But, Oglesby said, it’s important for Southeast Alaska to pay attention.

“I think these two earthquakes that have happened along your plate boundary are a reminder that, in Southeastern Alaska, you are in earthquake country. And so, while we cannot predict when or where the next substantial earthquake is going to take place, we know it has to happen. And we know the effects can be very, very strong. So it’s a little bit of a wake-up call for us, I believe,” said Oglesby.

As long as people are safe, Oglesby said earthquakes can be very interesting natural phenomena to learn more about.

“This is fascinating stuff. It’s always especially fascinating when you can investigate these kinds of effects right after the earthquake happens,” said Oglesby.

He will be presenting his research on the Canadian and Alaskan earthquakes at the Seismological Society of America meeting this spring.

For more information on how you and your family can prepare for an earthquake, click here.

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