Researchers at the 91探花, Portland State University and the University of Oregon have shown that deep-seated landslides in the central Oregon Coast Range are triggered mostly by rainfall, not by large offshore earthquakes.
The open-access was published Sept. 16 in Science Advances.
鈥淕eomorphologists have long understood the importance of rainfall in triggering landslides, and our study is simply driving home just how important it is,鈥� said first author , who did the work as part of his doctorate at the UW. 鈥淥ur results show that more frequent, localized landslide events triggered by rainfall are just as important to consider as less frequent but more far-reaching Cascadia Subduction Zone earthquakes.鈥�
Heavy rains are known to cause landslides that can be disruptive and deadly. A less frequent landslide trigger is a rupture on the geologic fault off the coast of Washington and Oregon that鈥檚 known as the Cascadia Subduction Zone 鈥� among a long list of concerns after a major earthquake. Landslide risks of all types increase if human development or wildfires remove trees, taking away the roots that stabilize the soil.
Recent research in Nepal and Japan, however, suggests that offshore earthquakes might not trigger as many landslides as previously believed. The new study finds a similar situation in the Pacific Northwest.
鈥淲e aren鈥檛 suggesting that the landscape had no response to these magnitude-9 earthquakes, but that the deeper-seated landslide deposits and scars out on the Oregon Coast Range hillslopes today were primarily triggered by precipitation events,鈥� said senior author , a 91探花associate professor of Earth and space sciences. 鈥淲e conclude that past Cascadia Subduction Zone earthquakes triggered no more than a few hundred deep landslides during great earthquake events.鈥�
The researchers used high-resolution aerial laser maps of the Oregon coast to look at 1,000 years of landslide activity. Landslides tended to happen in places with heavier rainfall, they found. But surprisingly, there was no detectable change in the number of deep landslides at the time of the large earthquake that shook the Pacific Northwest in 1700, or for two earlier offshore earthquakes that happened in roughly the years 1150 and 1470.
Duvall, LaHusen and co-author at Portland State University developed a method for dating landslides while studying the site of the deadly March 2014 mudslide in Oso, Washington. In that study, they used high-resolution images to view the surface roughness. Over time, soil settles and exposed rock erodes. The surface gets smoother, so surface roughness can be used to calculate a landslide鈥檚 age.
鈥淭he central Oregon Coast Range offered a massive, 10,000-square-kilometer natural laboratory to explore patterns in deep-seated landslide events through space and time,鈥� LaHusen said. 鈥淚t鈥檚 50-million-year-old sandstone and siltstone that was deposited offshore, buried and compacted, and then uplifted to form the mountains we see today.鈥�
Aerial with less than 3-foot resolution revealed 9,938 landslides inside the study area. Researchers narrowed those down to 2,676 landslides that have happened within the past 1,000 years, and then looked at the landslide frequency during that time.
For the new study, they applied their method to a larger area in the central Oregon Coast Range. To study landslide activity related to the Cascadia Subduction Zone, the researchers needed an area near the Cascadia fault zone with a consistent rock type and publicly available lidar imagery.
Researchers caution that the study doesn鈥檛 apply to shallow landslides, which frequently occur during earthquakes but leave no long-term evidence and can鈥檛 be analyzed with this method, or to different soil types, and so doesn鈥檛 necessarily apply to other regions. The research also didn鈥檛 consider shallower earthquakes from surface faults.
But the paper does support recent findings in Asia suggesting that offshore earthquakes don鈥檛 trigger as many deep landslides as once believed, and that rainfall may be the bigger factor in shaping the landscape over longer timescales.
鈥淭hese data strengthen the point that we don鈥檛 need big earthquakes to trigger large and devastating landslides in Washington and Oregon,鈥� Duvall said. 鈥淪easonal precipitation and large rain events are important to focus on in landslide preparedness planning.鈥�
This research was funded by the National Science Foundation and the Geological Society of America. The team began the work as part of the 91探花, which is studying magnitude-9 earthquakes from the fault that runs parallel to the Washington and Oregon coastlines. Slips along this fault can lead to a so-called 鈥淏ig One,鈥� which last struck the Pacific Northwest in 1700.
Other co-authors are , , and at the UW; and and at the University of Oregon. LaHusen is now working at the U.S. Geological Survey in Mountain View, California.
For more information, contact LaHusen at seanlah@gmail.com or Duvall at aduvall@uw.edu.