The W.M. Keck Foundation has awarded $1 Million to Woods Hole Oceanographic Institution (WHOI) for a new geodesy observatory to be connected to Ocean Networks Canada and the Ocean Observatories Initiative Regional Scale Nodes (OOI RSN). This interdisciplinary project, led by Dr.Jeff McGuire and John Collins of WHOI, will allow scientists to monitor three Cascadia subduction zone segments off the coasts of Vancouver Island, Washington and Oregon. Along this subduction zone, the Juan de Fuca plate is sliding beneath the North American plate and these two plates are locked, which means they are constantly accumulating strain. When this strain is released suddenly, earthquakes are generated. This particular type of fault is prone to very large “megathrust” earthquakes that may subsequently generate tsunamis. The Cascadia subduction zone has not generated one of these megathrust earthquakes for over 300 years; the last one occurred on 26 January 1700, and generated a tsunami that propagated across the Pacific Ocean where it was recorded in Japan.
The fault zone that unleashed the March 2011 Tohoku earthquake in Japan was being monitored by state-of-the-art geodetic instruments, which use GPS to track the movements of the crust. Geodetic measurements revealed that slow, small slip was happening two days before the earthquake occurred. Japan’s geodetic instruments were land-based, as GPS devices do not work underwater. However, autonomous Absolute Pressure Gauges (APGs) in position at the seafloor also detected the slip. APGs record subtle pressure changes on the sea bottom. Unfortunately, none of the data from these instruments were analysed in real time.
The northern-most site for the planned geodesy observatory is located ~2km from our Clayoquot Slope (formerly called ODP 889) site. This proximity will allow the new instruments to be connected to our network in order to obtain their data in real time. But what will they be recording that our instruments already installed there cannot? At Clayoquot Slope, Ocean Networks Canada currently has a seismometer and a bottom pressure recorder (BPR), but these instruments do not measure the small-scale slip events that have been observed to presage other megathrust earthquakes, like the Tohoku earthquake of March 2011.
WHOI’s geodetic array will install two tiltmeters, instruments capable of measuring very subtle changes in tilt. The tiltmeters will be part of an instrument package installed on Advanced CORK 1364A, which extends downward over 300m into a seafloor borehole near our Clayoquot Slope node. Surrounding them, an array of autonomous APGs will enable continuous recording and capture of large signals over short time-scales. Over longer time scales, accumulation of strain within the seafloor crust can be indicated by monitoring slow movements of benchmarks to be placed on the seafloor. Remotely operated vehicles will precisely survey benchmark positions during regular installation and maintenance dives at the location.
These new tools, able to measure very subtle changes in tilt and movements of a fraction of a millimetre in a year, will provide very important information about the state of the Cascadia subduction zone. Data gathered from this new geodesy observatory will help seismologists develop models to determine, if an earthquake were to happen, how much the fault may slip, how much shaking could occur and how large the resulting tsunami might be. It is hoped that the array will be installed in summers of 2013 or 2014.
