Aberdeen Landslides and Liquefaction Susceptibility Report Published

The Washington State Geologic Survey announces the publication of Report of Investigations 36. Earthquake-induced landslide and liquefaction susceptibility and initiation potential maps for tsunami inundation zones in Aberdeen, Hoquiam, and Cosmopolis, Grays Harbor County, Washington, for a M9+ Cascadia subduction zone event, by S. L. Slaughter, T. J. Walsh, Anton Ypma, K. M. D. Stanton, Recep Cakir, and T. A. Contreras. This publication is available free online at http://www.dnr.wa.gov/Publications/ger_ri36_aberdeen_liquefaction.zip. Additionally, a limited number have been plotted and will be sold through the Washington Department of Enterprise Services for $38.61.

RI36 image

This report assesses the earthquake-induced ground failure potential, including soil liquefaction and landslides, for the communities of Aberdeen, Cosmopolis, and Hoquiam in Grays Harbor County, Washington.

The probability of soil liquefaction increases with the duration of shaking, and slopes that are stable under static conditions may fail under large ground accelerations. A Cascadia Subduction Zone (CSZ) earthquake of magnitude (M) 9+ could produce ground accelerations on the Washington coast of as much as 0.40 g (g is the acceleration due to gravity) and shaking durations of as much as several minutes, sufficient to initiate soil liquefaction and shallow landslides. Soil liquefaction can damage transportation networks, such as roads and bridges, and landslides; even very small landslides can render a road impassable to automobiles. These ground failures can complicate or prohibit vehicular evacuation, as well as hamper emergency response and recovery efforts.

The objective of this report is to assist city and emergency management officials in evaluating the suitability of existing evacuation routes and assembly areas for potential susceptibility to ground failure from a M9+ CSZ earthquake. Results of this report could indicate the need to modify current evacuation routes and assembly areas. Understanding which areas are more susceptible to ground failure during a large earthquake can help communities prepare for potentially obstructed transportation networks, toppled buildings, and other secondary seismic hazards.

Landslide season is upon us!

As you are no doubt aware, summer sun has now given way to fall clouds and rain in western Washington. Unsurprisingly, this weather transition marks the beginning of landslide season. Landslide events impact many more than just landowners of slide locales; resulting power outages, water supply disruptions, and impassable roads have wide-reaching effects, making it helpful to keep current on the landslide risks in your area.

A test project by DNR and NOAA’s National Weather Service (NWS) is trying a raise awareness of landslide hazards, particularly the ones that occur because of heavy and/or prolonged periods of rain. Our online beta (test) map combines recent rainfall measurements from NWS rain gauges with information about the local soils and slide history. The result is a county-wide risk level rating. The map is not intended to predict landslides at any particular time or location; it only rates the overall risk that one might occur based on the amount of rain that has just fallen.

The map is still in its testing phase, but we have placed it on the DNR website so you can be better informed. The DNR “Ear to the Ground” blog has also recently profiled our landslide hazard map. Take a look and see how your county rates now.