Deep-sea landslides in the Cascadia subduction zone in the Pacific Northwest hold records of earthquakes dating back 7,500 years, and similar evidence may be found at other plate boundaries around the world, a new study shows.
Subduction zones are where oceanic tectonic plates slide beneath continental plates, which can cause large and damaging earthquakes, such as the 2011 magnitude 9.1 Tohoku earthquake in Japan that caused a devastating tsunami. The Cascadia subduction zone, which stretches from northern California to Vancouver Island, British Columbia, has the potential for earthquakes of at least magnitude 9.0, according to the Pacific Northwest Earthquake Network.
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However, turbidites in underwater canyons near the coast can also be caused by storms, ocean currents, and periodic landslides that are unrelated to earthquakes. In a new study published Wednesday (January 14) in the journal Science Advances, U.S. Geological Survey research geologist Jenna Hill and her colleagues decided to dig deeper. They worked with the Monterey Bay Aquarium Research Institute to study the continental slope of southern Cascadia, the steep descent from the North American continent to the deep-sea plains.
Focusing on an area off the coast of Crescent City, California, the researchers used an autonomous, remotely operated vehicle to obtain detailed views of slopes and sediments. They also radiocarbon dated the turbidite deposits using sediment cores from the area and compared their timing to the ages of known ancient Cascadia earthquakes.
Researchers found evidence of at least 10 events over the past 7,500 years, allowing them to link historic earthquakes, landslides, and the turbidites they produced.
“We were able to figure out where and how turbidites are formed,” Hill told Live Science. “So we know they’re coming from landslides that we know are caused by earthquakes.”
It’s unclear how large an earthquake is needed to cause deep-sea turbidites, but it probably has to be large enough to cause damage, Hill said. She and her colleagues also witnessed signs of seafloor shaking in response to seismic turbidites, which could further increase the risk of tsunamis from these types of earthquakes.
Turbidites, found in underwater canyons near the coast of the Pacific Northwest, have already been used to link earthquakes in Cascadia to those on the nearby San Andreas Fault. Turbidites on continental slopes may be more reliable markers of earthquakes because they are less affected by coastal processes such as tides and rainfall, Hill said.
“We think they occur almost everywhere along subduction zones, so you should be able to find these landslide deposits and ocean turbidites all over the world, even in places you’ve never looked for them before,” she said.
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