My wife used to describe to my kids, What does Daddy do? Daddy plays with dirt.
I’m Greg Brooks, I’m a geomorphologist, and I study evidence of paleo-earthquakes. A paleo-earthquake
essentially is an earthquake that occurred before history. So it’s an ancient earthquake.
Could be an earthquake that happened 500 years ago, an earthquake that happened a thousand
years ago or even 10,000 years ago.
The work that I’ve been doing the past six years has focused on looking for evidence
of earthquakes and looking for that evidence in lakes. And of course, I’m focusing on
underwater landslides that are buried in the bottom of lakes. What we’re looking for
is we’re looking for signatures of multiple landslides that occur at the same buried level
in that lake, which can be interpreted as a signature of a paleo-earthquake or multiple
paleo-earthquakes with multiple layers.
A really obvious question is, Who cares about ancient earthquakes? Well, the study of ancient
earthquakes is really important for the design of critical infrastructure: pipelines, hospitals,
power dams, nuclear power plants and also nuclear waste repositories. You want to have
a really good understanding of the long-term earthquake hazards that that type of critical
infrastructure can be exposed to so that it can be properly designed to be very resilient
to earthquake shaking.
So to understand our earthquake hazard beyond historic records, which in Canada are relatively
short, is we have to look into the past. And we do that by looking for geologic evidence
of earthquakes. Shaking of the earth during an earthquake can trigger landslides on the
surface, and it can trigger landslides in lakes. So, underwater landslides. And the
advantage of looking in lakes is that the failures are very well preserved in the lake
bottom. And it’s also much easier to tell if the landslides have occurred at the same time.
And the main piece of equipment we use is called a sub-bottom profiler. There’s a
very large transducer that’s fixed to the side of the boat. And the best way to explain
what a transducer is, you have to think of a fish finder. A fish finder is a little device
that makes a sound. The sound energy goes from the transducer down to the lake bottom
and the sound bounces off the lake bottom and comes back up to the transducer. And of
course, if you’re if you’re a fisherman, you want to know if there’s fish. Well,
some of that sound energy will bounce off those fish.
Well, the transducer we have, it’s much, much bigger than a transducer that you use
to look for fish. That’s because it contains two transducers. It has the same transducer
that is very similar to a fish finder. And we use that transducer to map the bottom of
the lake. There’s also a much larger lower frequency transducer that makes energy that
will actually penetrate into the lake bottom, and that energy will bounce off the different
layers that are in the lake bottom. And it’ll give us a 2D picture of what the sediments
are in the lake. And from this picture, we can identify buried underwater landslide deposits.
The goal of the geophysical work in the boats is to map the underwater landslide deposits
that are buried in the lake bottom. In order to do that, we do a large number of traverses
in the lake, and we want to make a grid pattern. And we do that by literally sitting in the
boat with our transducer in the water, with the geophysical equipment operating. We stream
GPS data so that we’re actually mapping our route as we go along. And we literally
spend hours and multiple days going back and forth in the lake to make our very detailed
grid pattern, which allows us then, back in the office, to make a very detailed map of
the underwater landslide deposits. And essentially we can make a pseudo three-dimensional map
of those landslide deposits.
And when you study landslides to identify an earthquake you’re looking for a particular
signature. One ancient landslide, and you can date it using radiocarbon dating, that
doesn’t tell you if an earthquake happened. But if you have 10 or 15 landslides that happened
at the same time, well, then the obvious inference is that there was a trigger that caused all
the landslides to occur at the same time. And you’re investigating that trigger. Now,
the trigger, it could be a paleo-earthquake. It could be a climatic event, like a major
storm, like a hurricane that has tracked into eastern Canada. And you have to do the assessment
as to what is the best explanation. And it’s very much detective work. And at the end of
the day, do you absolutely know it was a paleo-earthquake? No. But we come up with the best
explanation, and that’s our working hypothesis.
People who study geology, we are so lucky we get to work outdoors, you know, chug around,
back and forth looking at the deposits in the bottom of the lake. And I get to map the
landslides, and I get to do the detective work to identify an ancient earthquake. I
mean, it’s so cool.
If you liked this video, let us know with a thumbs-up. Click on the logo below to subscribe
to the Simply Science channel, and click the Bell icon to be notified when we upload new videos.