Studying seismic activity off Vancouver Island for hazard assessment

An earthquake of enormous magnitude is to be expected on the west coast of North America in the near future. Along the Cascadia Subduction Zone, where an oceanic plate slides under the North American plate into the Earth’s mantle, an earthquake and related tsunami could hit the region on any given day. To better assess the hazard, researchers led by GEOMAR Helmholtz Centre for Ocean Research in Kiel are studying tectonic setting and seismic activity off Vancouver Island where the seismogenic zone is suspected to be.

Where an oceanic plate dips below a continental one, the risk of the planet’s most severe earthquakes and associated tsunamis is high. Such a subduction zone is located off the west coast of North America: Cascadia Subduction Zone is a 1,000 km long dipping fault off the Pacific shore, where the oceanic Juan de Fuca Plate slides under the continental North American Plate. The fault stretches from Canada’s Vancouver Island to northern California and is believed to generate mega earthquakes and giant tsunamis. Currently the subduction zone is locked due to friction but at some point, the strain will be released in earthquakes.

Long-term ocean-bottom seismometers

In order to better assess the future behaviour of the tectonic plates involved and the risk of major earthquakes, scientists led by GEOMAR Helmholtz Centre for Ocean Research in Kiel have been collecting data in the northern Cascadia region since September 2022 as part of expedition SO294 “CLOCKS” (Northern Cascadia: Extent of locked zone, prism deformation, slip-to-toe, and the edge of subduction).

"We wanted to find out more about location and size of the zone where future large earthquakes may nucleate," said the cruise leader, Dr Michael Riedel, geophysicist at GEOMAR. To obtain these data, 26 long-term ocean-bottom seismometers (OBS) had been deployed off Vancouver Island from the research vessel SONNE. These instruments not only record earthquake signals, but by analyzing the data, information on what temperatures prevail at the plate boundary, the amount of liquid contained in the rocks, and how the seabed has already been deformed by former earthquakes can be extracted.

Now all 26 OBS were successfully recovered and data were retrieved from the instruments as part of a follow-up international expedition in collaboration with the Canadian and Japanese research agencies Geological Survey of Canada (GSC) and Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Michael Riedel: “Evaluation of the results will provide a clearer picture of the state of locking and the seismic activity of the fault, helping to better understand large subduction zone earthquakes and their associated tsunamis“.