(some of) The Science Behind the Seismometers

Quite a bit of time, money, and effort has into this experiment… but why? Do we really need two ships? What is the science driving this experiment? What do we hope to learn?

Here aboard the CCGS Tully, we are busily deploying ocean-bottom seismometer (OBS) instruments for the R/V Langseth to “shoot” to. The Langseth has been following the Tully around, setting off acoustic signals (sort of “miniature earthquakes”) using a towed, pressurized air system. Some of the crew aboard the Tully have referred to the Langseth as the “bubble blower” ship - adorable, and not entirely inaccurate! The high-energy bubbles send acoustic waves to - and through! - the seafloor, and those waves are then recorded by the OBS instruments the Tully has deployed.

Tully science team and crew getting ready to deploy 2 different kinds of OBS instruments. Photo credit: Maureen Walton

The value of the OBS instruments is that they can record acoustic energy from the Langseth that has traveled deep into the earth’s crust and upper mantle - up to 10s of kilometers. This is one of the best methods available for peering into the physical properties of the crust, the outermost layer of the Earth. For this experiment, we want to image the crust on either side of the Queen Charlotte fault, one of the fastest-moving strike-slip faults in the world. In the past 100 years, the Queen Charlotte fault system has generated 7 earthquakes over magnitude 7; the most recent two events happened in 2012 offshore of Haida Gwaii, British Columbia (M7.8) and in 2013 offshore of Craig, Alaska (M7.5).

Large earthquakes often occur within the Earth’s crust, between tectonic plates. The Queen Charlotte fault separates the Pacific plate from the North American plate, and these plates slide past each other along the fault, causing earthquakes. If we can better understand what the Earth’s crust is made of on either side of the Queen Charlotte fault, we can (hopefully!) better understand why, when, where, and how earthquakes might happen along the fault.

Because the 900-km-long Queen Charlotte fault is located almost entirely offshore, it provides a convenient and ideal setting to deploy OBS instruments along both sides of a large portion of the fault. This makes it easy for a seismic research vessel (in this case, the Langseth) to generate acoustic energy that these instruments will record after the waves travel through the crust and upper mantle.

A map of the OBS station locations (small circles) along the Queen Charlotte fault, which is shown as a thin blue line running in between the longest transects of OBS instruments. Illustration credit: Emily Roland

Our experiment has been designed to image the crust and upper mantle along both sides of the Queen Charlotte fault to help us understand earthquake rupture processes and the relationships between the tectonic plates. We have 2 OBS transects running parallel to the fault on either side, and 3 OBS transects crossing the fault in key locations: 1) the 2013 Craig earthquake area (northern crossing), 2) the 2012 Haida Gwaii earthquake area (southern crossing), and 3) a spot where we can potentially see the extent of underthrusting of the Pacific plate beneath North America (central crossing). While we wouldn’t normally expect underthrusting along a strike-slip plate boundary like the Queen Charlotte fault, the southernmost part of the fault along Haida Gwaii is somewhat convergent. A key science question we have is whether (and how much) the Pacific plate has underthrust beneath North America in response to this convergence.

Ultimately, we hope that the results from this experiment will help scientists to better understand earthquake processes along not only the Queen Charlotte fault, but similar plate boundaries elsewhere - the San Andreas fault in California, for example, is a similar type of strike-slip plate boundary. We have already collected a lot of data, and are looking forward to digging into the analysis after the cruises!

Maureen Walton, research geophysicist and co-chief scientist aboard the CCGS Tully