The Seismic Story of the Nile Valley Landslide - Foreshocks, Mainshock and Aftershocks

The Seismic Story of the Nile Valley Landslide - Foreshocks, Mainshock and Aftershocks Lidar image of the Nile Valley Landslide with location of stations installed after the slide (triangles) and best-fit post-slide event locations (circles). The dashed line indicates the approximate boundaries of the landslide and uplifted areas at the toe. Lidar image provided by Washington State Department of Transportation.
The Nile Valley landslide, in Washington State, 11 October 2009, was a translational slide involving a volume of material on the order of 10^7 cubic meters. It damaged a highway and several houses and diverted a river, causing flooding. Fortunately no one was injured because the landslide gave warning “foreshocks,” - rumbling sounds, rocks raveling, and a smaller precursory landslide in the days and hours beforehand.
</p><p>The “mainshock” was a complex ground failure sequence occurring over the course of about 24 hours. The more energetic events generated seismic signals that were captured by two Pacific Northwest Seismic Network (PNSN) short-period regional stations 12 and 29km away. This precise seismic timeline, in combination with detailed eyewitness reports and studies of the geology of the landslide resulted in a detailed account of the unfolding of a landslide unlike any other.
</p><p>After the landslide occurred, we installed 16 temporary seismic stations. Despite a lack of significant continued movements, our instruments detected more than 60 small events, or “aftershocks.” We were able to locate some of the larger events using beamforming techniques. Station locations and event locations are shown on the figure below. The events were at the headscarp north of the slide, most likely slope failures due to instability of the newly created cliff-face.
</p><p>Anderson, D. and G. Taylor (2010), Nile Landslide Timeline Narrative Report, Washington State Dept of Transportation, Unpublished Report.
</p><p>Acknowledgements: Thanks to the Washington State DNR and DOT, in particular to Doug Anderson and Gabe Taylor. Also to IRIS PASSCAL for lending us equipment and Agnes Helmstetter for sharing her event location methods.</p>


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