All of my research right now is located near the northern part of the Juan de Fuca plate, and also a bit on the Explorer plate. These plates are both off the north-west coast of the United States and Canada.
This particular region has been a very interesting area for seismologists, due in no small part to the relatively small size of the Juan de Fuca plate. Having all of the major types of plate boundaries so close to the coast makes it an ideal study area. Research vessels leaving from Seattle can steam out to any part of it in less than a day!
Seismologists measure ground motions on land or at sea to try to gain an understanding of the processes that are happening in the earth. These ground motions can be caused by earthquakes, volcanos, or other seismic sources (like explosions). The instruments that make these measurements are called seismometers.
The type of seismometer data that I look at is from seismometers on the ocean bottom – called (surprise!) Ocean Bottom Seismometers (or OBSs). So far I’ve been looking at data from the KECK experiment, and also from Neptune Canada‘s cabled observatory. The KECK experiment took place between Summer 2003 and Fall 2006, with continuous data being collected at three different locations. The main part of the experiment was a network consisting of 8 OBSs over the Endeavour segment of the Juan de Fuca Ridge.
Since December 2009, the data from the Neptune Canada cabled observatory has been online and publicly available, both archived and also in real time.
Hey, what about the whales?
As I already mentioned, the intended purpose of this OBS data is to monitor seismicity. So where do the whales come in? Well, it turns out that you can actually see some whale calls in seismic data.
OBSs are designed to look at very low frequency ground motions, and the ones from KECK and Neptune Canada are limited to frequencies below 65Hz – some even as low as 25Hz! That’s really low. To give you an idea of just how low, the lowest frequency sound that a typical human ear can hear is about 20hz. There are only a couple of types of whales that make sounds that are in that range – the big ones: fin whales and blue whales.
My focus up to this point has been on fin whale vocalisations. I think fin whale calls are a good place to start. First of all, they show up A LOT in our data. Also, they are a very simple, very repetitive call (whale folks call this “stereotyped”). They are typically a downswept chirp, which means that they start at a high pitch, and sweep down to a low pitch, with the whole thing centered around 20Hz (swept over about 10Hz total). The individual calls last about a second, and are spaced about 25s apart.
Fin whale calls were observed early on in the data processing, during the earthquake analysis. Of course, at that point, they were trying to clean out the fin whale calls – they were the “noise”. But as we all know, one person’s junk is another person’s treasure. So along came Dax, and he was tasked with looking specifically at the fin whale calls, and cleaning out those pesky earthquakes. And lucky for me, Dax really wants to study seismic tomography – so I get to pick up where he left off! Sweet.