Most landslides in western Oregon are caused by heavy rainfall, not large earthquakes

Coastal Oregon Mountain Range extends from around Newport, Oregon, south to Port Orford. Located near the Cascadia Subduction Zone, the low mountain range is a sea fault that can cause earthquakes of magnitude 9. Credit: LaHusen et al / Science Advances

Researchers at the University of Washington, Portland State University and the University of Oregon have shown that deep landslides in the Oregon Coast Chain are mostly caused by precipitation, not large marine earthquakes.

An open access paper was published Sept.16 in Science Advances.

“Geomorphologists have long recognized the importance of precipitation in triggering landslides, and our study simply leads home how important it is,” said first author Shawn Lahussen, who conducted the work as part of his PhD at the University of Wisconsin. “Our results show that more frequent and localized landslides caused by precipitation are just as important as the less frequent but far-reaching Cascadia subduction zone earthquakes.”

Heavy rains are known to cause landslides that can be devastating and deadly. A less frequent cause of a landslide may be the rupture of a geological fault off the coast of Washington and Oregon known as the Cascadia Subduction Zone – adding to a long list of concerns after a major earthquake. The risk of landslides of all kinds increases if human development or forest fires remove trees and remove roots that stabilize soil.

However, recent research in Nepal and Japan indicates that marine earthquakes may not cause as many landslides as previously thought. The new study found a similar situation in the Pacific Northwest.

“We’re not suggesting that the landscapes had no response to these magnitude 9 earthquakes,” said senior author Alison Duvall, “but that deep landslide deposits and scars on the slopes of the Oregon Coast Range today are mainly caused by precipitation events.” , Associate Professor at the University of Washington in Earth and Space Sciences. “We conclude that previous earthquakes in the Cascadia Subduction Zone did not cause more than a few hundred deep landslides during the events of the Great Earthquake.”

Researchers used high-resolution aerial laser maps of the Oregon coast to examine 1,000 years of landslides. And they found that landslides tend to occur in places with heavy rain. But surprisingly, there was no detectable change in the number of deep landslides at the time of the Great Earthquake that rocked the Pacific Northwest in 1700, or of two previous marine earthquakes in about 1150 and 1470.

Within the study area, shown on the left, analyzes of the aerial lidar images revealed nearly 10,000 landslides. In the subgroup on the right, landslides of the past 1,000 years are shown in red. Credit: LaHusen et al / Science Advances

Duval and Lahuysen and co-author Adam Booth at Portland State University developed a method for dating landslides while studying the site of a fatal mudslide in March 2014 in Osu, Washington. In that study, they used high-resolution images to display the surface roughness. Over time, the soil settles and erodes the exposed rocks. The surface becomes smoother, so the surface roughness can be used to calculate the lifetime of the landslide.

For the new study, they applied their method to a larger area in the Central Oregon Coast Chain. To study landslide activity related to the Cascadia Subduction Zone, researchers required an area near the Cascadia fracture zone with stable rock type and lidar images available to the public.

“The Central Oregon Coastal Range provides a massive 10,000-square-kilometer natural laboratory for exploring patterns in deep landslide events across space and time,” said Lahussen. “It is a sandstone and a 50 million-year-old silt stone that was deposited in the sea, buried, compressed and then raised to form the mountains that we see today.”

Aerial lidar maps with a resolution of less than 3 feet revealed 9,938 landslides within the study area. Researchers reduced the number of landslides to 2,676 landslides that occurred during the past 1,000 years, and then studied the frequency of landslides during that period.

The researchers warn that the study does not apply to shallow landslides, which often occur during earthquakes but do not leave long-term evidence and cannot be analyzed in this way, or to different soils, and thus not necessarily apply to other soils. Regions. The research did not take into account surface earthquakes caused by surface faults.

But the study supports recent findings in Asia that indicate that marine earthquakes do not cause as many deep landslides as previously thought, and that rainfall may be the largest factor in shaping landscapes over longer periods of time.

“These data reinforce the point that we do not need large earthquakes to cause large and devastating landslides in Washington and Oregon,” Duval said. “Monsoon rains and major rain events are important to focus on in landslide preparedness planning.”

The history of historical activity at the Oso site shows frequent major landslides. More information: “Rainfall causes more deep-seated landslides than Cascadia earthquakes in the Oregon Coast Series, USA” Science Advances (2020). advances.sciencemag.org/lookup….1126 / sciadv.aba6790 provided by the University of Washington

Quote: Most of the landslides in western Oregon have been recovered due to heavy rainfall, not large earthquakes (2020, September 16) on September 16, 2020 from https://phys.org/news/2020-09-landslides-western-oregon- triggered-heavy. programming language

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