Determining the Optimal Design of a Seafloor Geodetic Observatory on the Cascadia Subduction Zone

Matt Wei,Geology and Geophysics
Jeff McGuire,Geology and Geophysics



The Cascadia subduction zone offshore of Oregon, Washington and British Columbia is capable of generating truly great earthquakes and tsunamis, similar to the March 11, 2011 magnitude 9 Tohoku earthquake.  That destructive earthquake demonstrated that even the best onshore datasets could not tell us the details of what to expect from an upcoming subduction zone earthquake.  The Japanese scientific community was not expecting the Tohoku earthquake to be as large as it was or to generate a truly gigantic, >40m high, tsunami because it did not know the state of the fault far offshore, where most of the fault motion occurred.  The subduction zone fault there produced much greater motion at the shallowest depths (i.e. further offshore) than the canonical model of subduction faults suggested was likely.  The physical processes that induced this spectacularly large fault motion are likely related to a combination of strain build-up in the shallowest part of the fault and the lubrication of the fault-zone by fluids.  The key to understanding these mega-earthquakes is to understand both the strain build up and the fluid flow in the shallowest part of the subduction zone and particularly how these phenomena are coupled to each other.

In December 2011, the W. M. Keck Foundation awarded funds to WHOI to build the instrumentation needed to create a state-of-the-art geodetic observatory offshore of Vancouver Island, part of which would be installed at the existing sub-seafloor hydrological (ACORK) observatory at IODP Site 1364A and provide real-time data to shore via the NEPTUNE Canada cable.  This borehole instrumentation will be combined with a spatially distributed seafloor array of autonomously recording absolute pressure gauges and geodetic benchmarks for monitoring transient fault motion and inter-seismic strain respectively.  All of the hardware, design, and construction of the geodetic instrumentation is funded by the Keck Foundation.  The Keck award does not cover any costs associated with instrument installation.  This proposal requests funds to optimize the geometry of the seafloor geodetic benchmark surveys.  The centerpiece of our effort will be predicting inter-seismic seafloor deformation and determine the best locations to install the geodetic benchmarks.  Our work serves as a preliminary study for future NSF proposal to cover the installation of the instrumentation.  Our array will be installed offshore of Vancouver Island to leverage millions of dollars worth of existing seafloor cable infrastructure that will provide real-time telemetry of the borehole tilt and hydrological data.  We expect to record transient interdisciplinary phenomena that have never before been studied with seafloor geodetic techniques owing to a lack of suitable instrumentation, but are inferred to exist from related hydrological signals.

Ultimately, our project will create the first combined sub-seafloor geodetic and hydrological observatory with real-time data telemetry, enabling new interdisciplinary studies of the connections between strain and fluid flow in the accretionary prism.  Moreover, it will be located directly above the locked zone of one of the most dangerous faults in North America, and our improved understanding of the up-dip extent of the locked zone will improve tsunami hazard models.  The data will be available freely in near real-time to the entire community through the Canadian NEPTUNE program.