We have released new tsunami inundation and current speed maps for the Chehalis, Hoquiam, Willapa, and Wishkah Rivers in Grays Harbor and Pacific Counties. The maps show the modeled inundation from a Cascadia subduction zone earthquake scenario.
Tsunami Hazard Maps of the Chehalis, Hoquiam, Willapa, and Wishkah Rivers—Model Results from a L1 Mw 9.0 Cascadia Subduction Zone Megathrust Earthquake Scenario
Publication Abstract
We present results from new tsunami inundation and current-speed modeling for a magnitude 9.0 Cascadia subduction zone earthquake scenario for the Chehalis, Hoquiam, Willapa, and Wishkah Rivers in southwest Washington. This tsunami modeling is a continuation of previous modeling published in 2018 by the Washington Geological Survey for areas surrounding Willapa Bay and Grays Harbor. That modeling needed to be expanded as it did not cover upriver areas. This new study uses the same earthquake scenario as the 2018 publication, with an updated Digital Elevation Model (DEM) to better understand how far up rivers the tsunami inundation would reach. The seismic scenario used here generates subsidence that will affect all coastal elevations in this study area as a result of earthquake-induced land deformation. The impact of subsidence on the first tsunami-driven wave arrival is significant in this study area, as it generally masks the leading trough-phase of the tsunami observed in other parts of the Washington coastline (such as the Salish Sea). The initial tsunami arrives quickly—within 20 minutes at all river mouths modeled in this study area, leaving limited time for official tsunami alerts. Maximum changes in tsunami water heights following the earthquake exceed ~10 feet (~3 meters) at all river mouths, with the highest height of ~15 feet (~4.5 meters) impacting the Willapa River. The tsunami also travels a minimum of ~8 river miles (13 kilometers) upriver in all four river valleys. The farthest modeled upriver flooding extent is up the Chehalis River, where the tsunami travels ~13 miles (21 kilometers) upriver. Although the majority of tsunami inundation in this study area is confined to mapped floodplains, inundation impacting roadways may isolate communities upriver. This study is limited in that modeling does not account for variable tide stage, tidal currents, riverine flow, earthquake-induced landslides, seiches, liquefaction, or minor topographic changes that would locally modify the effects of tsunami waves. In addition, there are many assumptions associated with the scenario earthquake modeled here. Due to these limitations, this modeling may not be suitable for site-specific tsunami inundation assessment or for determining effects on the built environment. This modeling can be used as a tool to assist with emergency preparations and evacuation planning prior to a Cascadia subduction zone event or to determine locations where a tsunami vertical evacuation refuge may be appropriate.
Suggested Citation
Dolcimascolo, Alexander; Eungard D. W.; Allen, Corina, 2023, Tsunami hazard maps of the Chehalis, Hoquiam, Willapa, and Wishkah Rivers—Model results from a L1 Mw 9.0 Cascadia subduction zone megathrust earthquake scenario: Washington Geological Survey Map Series 2023-02, 1 sheet, scale 1:48,000 and 1:166,000. [https://fortress.wa.gov/dnr/geologydata/tsunami_hazard_maps/ger_ms2023-02_tsunami_hazard_sw_wa_rivers.pdf]
Digital Data
Don’t forget to check out the data release that accompanies this publication. View the new tsunami inundation extents on an interactive map by visiting our Geologic Information Portal.
WASHINGTON STATE GEOLOGY NEWS 