Thursday, 15 May 2014

A little digression: Large earthquakes, Alaska in 1964, and why people in Vancouver should be prepared

Time for a little digression. Let's talk about earthquakes! I've recently come back from a conference in Anchorage, Alaska, the Annual Meeting of the Seismological Society of America. I've only ever been to general geophysics/geosciences, or volcano conferences, so this one was quite the change. Since I study a specific type of earthquakes related to volcanoes I'm a bit in between volcanology and seismology, so it made sense to go.
Around 600 or so seismologists met up to talk about earthquakes and related stuff for three days. Overall it was a great conference. The "small" number of attendees was great - it was really easy to meet lots of people with very similar interests. I also liked the fact that they provided breakfast, lunch, and dinners (mostly). One reason for that is - of course, me being a student - the free food aspect, but there is something else: When you find a table to eat you may opt to find people you know, or you can go to a random table, introduce yourself, and start some interesting science talk. Bigger meetings like AGU are great to catch up with friends in different fields, but generally tend to be more anonymous.
A highlight of the conference was the post-conference field trip. Maybe around 1/3 to 1/2 of the conference attendees got on a bunch of busses to head down south towards the Kenai Peninsula. After leaving Anchorage, we stopped in Whittier, this interesting, tiny Alaskan town. We talked about the effects of the 1964 Alaska earthquakes, one of the biggest earthquakes ever recorded.
Around 5:30 in the afternoon on Good Friday, Mar 27, 1964, an earthquake with a magnitude somewhere around 9.2-9.3 struck just East of Whittier at approximately 25 km depth. The shaking was quite intense for a few minutes, but the real damage came from landslides and a tsunami generated by the earthquake. 

Ruins of a house that was abandoned after the earthquake in 1964, close to Girdwood, Alaska.

There was a heartbreaking account of one family's experience of the earthquake in the Anchorage Daily News a few weeks ago. From a seismology perspective, the earthquake is interesting for one specific subfield: paleoseismology. Paleoseismologists can study the change in ground elevation during the 1964 earthquake. A large area reaching from Kodiak island in the West through Anchorage out to Valdez and further East dropped in elevation during the earthquake because of the new plate configuration. Trees in the region that were slightly above sea level before now found their roots in the salt water, and died within a short time. They can be seen as eerie ghost forests until today. 
Ghost forest close to Girdwood, Alaska.

With the trees, a bunch of grass and shrubs ended up in saltwater. They were quickly covered by sand and silt washed up by the tides, and were preserved. During the field trip, we accessed one of the marsh areas. Our field trip guide Peter Haeussler showed us that when you remove the top layer of silt at the edge of the marsh during low tide, you can see a brown peat horizon. That's the grass from the 1964 earthquake!
Peat horizon from the 1964 earthquake. The brown is grass and shrubs that died after they ended up in saltwater after the earthquake, the grey on top is the silt that quickly covered everything.
A piece of grass that died when it was covered in saltwater after the ground dropped in elevation after the 1964 earthquake.

If you dig down deeper you can find more horizons like that, telling tales from previous large earthquakes in the area. Fossils in those peat horizons can be dated, and we thus know approximately at what intervals large earthquakes occur. Offshore BC and the Pacific Northwest, for example, people were speculating whether large earthquakes can occur at all (there aren't many small ones like e.g. in Alaska or New Zealand). Once paleoseismology became established, people found evidence of large earthquakes offshore the West coast of North America. That's how we know! And because we know now, everybody should consider having an emergency kit in the house. Because what we DON'T know is when the next big one is gonna strike.

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