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Today is our earthquake anniversary

January 26, 2010

(Listen to me talking about this subject today with Long Beach Radio’s Geoff Johnson – click here to listen to podcast, or right-click/control-click to download mp3. Runs 22 min).

No one else pays attention to this date, but I always do. The anniversary of our earthquake is perhaps not pleasant to think of, but it is important. Kind of like Remembrance Day.

It was 310 years ago today – around 9pm on the night of January 26th, 1700, that the last big quake hit. How do scientists know that? Native people up and down the coast have earthquake stories in their oral history – but as non-written cultures, they are not able to provide exact calendar dates for these events.

Geologists can recognize ancient tsunami deposits by taking core samples in the mudflats – some of their data come from right here beside Tofino, in Browning Passage (pictured in photo). Whenever there is a tsunami, a layer of sand gets thrown up on the mudflats, smothering and killing the vegetation below (eelgrass, algae, etc.) Geologists can carbon-date that killed vegetation layer, thereby dating the tsunami-lain sand layer.

That’s how they know the last big earthquake/tsunami event was in the year 1700, with an error of plus or minus 20 years. By the way, the 1964 tsunami was nothing, in comparison. Click here for a map that shows the reach of both the 1700 and 1964 tsunamis in Port Alberni. You can see that the 1700 one (thick black line) went much higher than the 1964 one (shaded area).

Lower down, the geologists found more sand layers that represent other large earthquake/tsunami events that took place in the years 1310, 810, 400, 170BC and 600BC. All together, they have identified a total of 13 earthquake/tsunami events here, and that is how they know that the average recurrence interval is 500 years. (That is just the average – they have been anywhere from 200 to 900 years apart; this means that, at 310 years, we are in range right now).

So, from carbon-dating below the sand deposits, they had the date narrowed down to between 1680 and 1720. Then they found a trees that were knocked down around that time, preserved under a lake. By dendrochronology – looking for patterns in the sequences of tree rings (that relate to seasons, e.g. harsh winters, droughts, good growing seasons) and comparing them to the ring patterns in living trees today that are greater than about 350 years old, they could count back on the rings and narrow the earthquake down to the winter of 1699-1700.

Then they went to Japan, which has had written tsunami records for centuries (or perhaps millenia, I am not sure), and they found a record that winter of a big tsunami that hit Japan, but with no known earthquake related to it. Back-calculating from the time of the tsunami in Japan, and knowing how fast a tsunami wave travels, they figured out that this big magnitude 9 earthquake hit here around 9pm on January 26th, 1700.

Wondering why we have earthquakes here? North America’s Pacific Rim region is an area where tectonic plates are colliding. The oceanic plate is subducting under the continental plate.

There is no such thing as a fault “line” – you have to think of the Earth in 3D. People think of a fault is a “line” only because that’s how you draw it on a map – but the map only shows the Earth’s surface, and you need to imagine what is going on below the surface. A fault is really a plane – which represents the boundary between two masses of rock – in this case, between the two tectonic plates.

That boundary comes to the Earth’s surface roughly 75 km off the shore of Vancouver Island – under the ocean. You can draw where it comes to the Earth’s surface on a map, and that’s why people looking at maps think of it as a line. But that plane actually angles downward to the northeast, dipping right under Washington State.

This map, from the United States Geological Survey, shows the plate boundaries. The thin red line with the triangles on it (labelled Cascadia Subduction Zone) is where that fault plane hits the surface. The triangles show which side it is angling down towards. Here in Tofino, that fault plane – the top of the subducting oceanic plate – is 25 km below the Earth’s surface, and getting deeper as it moves eastward. Eventually it melts… and that magma rises up to form volcanoes. That’s why all the volcanoes are located in a defined arc, 150 or so km to the east of where the fault “line” is on the map.

So, if you can imagine in 3D that oceanic plate sliding downward, below the coasts of northern California, Oregon and Washington and below Vancouver Island, you’ll find that it makes sense why the earthquakes occur where they do, in the region outlined in black. That is the region directly above where the two plates are moving against one another. The high pressure deep in the Earth makes them get stuck. When the pressure build up to the point that they move, that’s the earthquake.

Subduction earthquakes are the strongest type of earthquakes. (Other types of earthquakes occur from plates moving sideways past one another, rather than downward – such as the San Francisco earthquakes). The earthquake we are expecting here will likely be a magnitude 8 or 9 event – that is between 10 and 100 times stronger than the Haiti earthquake earlier this month.

I hope that this info helps people to understand why we get earthquakes, and how scientists know these things. I don’t think this is talked about enough. If people are not informed, they cannot prepare. When this earthquake comes, it will affect a huge region and the damage (and casualties) will be extensive. We will experience what the people of Haiti are living through right now. If you want to hear more about my views on our emergency preparedness, please check out my post today on the Tofino Residents blog.

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