Cascadia Subduction Zone Earthquakes by Cascadia Region Earthquake ...

Recent work has improved earthquake-landslide volume scaling relationships by accounting for rupture depth and topographic steepness these models predict total coseismic landslide volumes of 0.6 to 2.0 km 3 for moment magnitude ( M w) 9.0 CSZ earthquakes with rupture depths of between 20 and 10 km, respectively ( 8). The few complete coseismic landslide inventories in subduction zones challenge assumptions about the number and volume of landslides triggered by these earthquakes. However, predictive models for earthquake-induced landslides from a potential CSZ event have high uncertainty and often fail to match the observations from similar subduction zone events. Empirical estimates for the number of triggered landslides for earthquakes of this magnitude range from 50,000 to 1,000,000+, with total predicted volumes of 10 to 110 km 3 ( 6, 7), representing a risk to the millions of people living in the region. In the Pacific Northwest of the United States, magnitude 8 ( M8) to M9 earthquakes occur every 300 to 500 years along the Cascadia Subduction Zone (CSZ) ( 5), a 1100-km-long megathrust fault that accommodates subduction of the Juan de Fuca Plate beneath the North American plate ( Fig. Hillslope failures triggered by shaking can affect homes and infrastructure and dam rivers and cause flooding, sometimes in catastrophic outbursts ( 1– 4). Landslides are one of the greatest secondary hazards of large earthquakes. Our results agree with findings about other recent subduction zone earthquakes where relatively few deep-seated landslides were mapped and suggest that despite proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall. We find landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Here, we map 9938 deep-seated bedrock landslides in the Oregon Coast Range and use surface roughness dating to estimate that past earthquakes triggered fewer than half of the landslides in the past 1000 years. Little is known about the number of landslides triggered by these earthquakes because the last magnitude 9 rupture occurred in 1700 CE.

Historic landslides have primarily been triggered by rainfall, but the region is also prone to large earthquakes on the 1100-km-long Cascadia Subduction Zone megathrust.

The coastal Pacific Northwest USA hosts thousands of deep-seated landslides.

Cascadia Subduction Zone Earthquakes by Cascadia Region Earthquake ...