One of the most dangerous seismic faults in North America is the Pacific Northwest’s Cascadia fault – an offshore, subduction zone capable of producing a magnitude 9 earthquake that would damage Portland, Tacoma, Seattle, and Victoria, British Columbia, and generate a large tsunami. Yet until 2011 there were no instruments installed offshore, directly above the fault, for measuring the strain building up along it.
A 2011 grant of $1 million from The W. M. Keck Foundation to scientists at the Institution changed that. An interdisciplinary project led by geologist Jeff McGuire, an expert in global earthquake seismology and geodesy, and John Collins, director of WHOI’s Ocean Bottom Seismometer Lab, used the funding to build and install the first seafloor geodesy observatory above the expected rupture zone of the next great Cascadia earthquake.
“I think all scientists agree there will be another magnitude 9 earthquake off Oregon and Washington,” McGuire said. “What we’re doing is trying to understand what that will look like. This information is critically important for modeling how much the fault will slip – and hence how much the ground will shake — and for predicting the maximum height of the tsunami that could be generated.”
The Cascadia subduction zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California. For many years, according to conventional wisdom, the Cascadia subduction zone slipped without earthquakes. But in the last 30 years, geologists have uncovered sedimentary records as well as historical records in Japan showing that “indeed, the fault repeatedly had these huge earthquakes with big tsunamis,” McGuire said.
Cascadia’s last big event occurred in 1700 and was likely very similar to the March 2011 Japanese earthquake – a magnitude 9 quake and tsunami that traveled all the way across the Pacific. This similarity is foreboding for earthquake scientists, as a key scientific lesson of the Japanese earthquake has been that the standard datasets collected onshore are completely inadequate for characterizing the upcoming ruptures on an offshore subduction zone thrust fault.
To collect data, McGuire and Collins installed tiltmeters at a location approximately 4 kilometers above the Cascadia subduction zone thrust interface. Tiltmeters are standard instruments on land – most volcano observatories have them, according to McGuire. “These instruments are very, very sensitive to tiny deformations that occur in the rock,” adds Collins. “The movements can be subtle.”
WHOI is grateful for the support from The Keck Foundation, which has a long track record of supporting important research and technology innovations to understand the Earth and its systems for the benefit of society. The real-time data flowing from the fault on the seafloor will not only advance our understanding of earthquakes, but also can help city planners and emergency response managers.