New Tsunami Maps for Puget Sound

Click map for a larger version

In January, the Washington Geological Survey (WGS) published a new set of maps derived from their “inundation models […] based on the shape of the ocean floor, the topography of the land, and the expected size and shape of tsunami waves. These properties are derived from what we know about past earthquakes and tsunamis that are found in the geologic record of the Pacific Northwest. The result of the modeling is a series of maps that show the expected depth of water and the speed of the currents from an earthquake-generated tsunami.”

Click here to download DNR’s new tsunami hazard maps of the Puget Sound and adjacent waters (Map Series 2021-01). The last file in the zip includes the report.

Note that this is a large (70MB) file and may take some time to download.

New tsunami modeling using a large earthquake scenario along the Cascadia subduction zone is now available for all areas within the Puget Sound and parts of the Strait of Georgia, some of which had not been modeled before.

This modeling uses a simulated magnitude 9.0 earthquake that produces a tsunami that is unlikely to be exceeded in the next great earthquake. The intent of the modeling is to encourage hazard planning and increase community resilience. The earthquake scenario presented here is based on one that was recently used to estimate the hazard posed to Oregon by tsunamis. However, because tsunamis generated offshore must enter through the Strait of Juan de Fuca to reach Washington’s inner coastline, our scenario adopts a fault geometry that extends farther north than the one used for Oregon. This extension better represents a tsunami generated offshore of northern Washington.

The updated subduction zone geometry assumes a full-length rupture that spans ~775 mi from northern California to the northern end of Vancouver Island, Canada. Scientists inferred that this scenario encompasses ~95 percent of the variability of Cascadia tsunami simulations. Modeling results indicate that the tsunami would first arrive in all inner coastal waterway locations as a trough, with sea level gradually receding.

The first tsunami wave generated by the earthquake would arrive on the west side of Whidbey Island approximately 1 hour and 30 minutes after the earthquake, with large wave crests in excess of 16 ft (5 m) traveling north into the Strait of Georgia and south through Puget Sound.

Most other locations within Puget Sound and parts of the Strait of Georgia would encounter this first tsunami wave within 2–4 hours of the earthquake, leaving little time to issue official warnings, although any felt earthquake shaking is an immediate warning. Fast moving currents from the tsunami waves could locally exceed 9 knots in multiple harbors, inlets, and passages within the narrow waterways of Washington’s inner coastline, presenting a significant navigational hazard to the maritime community.

Tsunami wave inundation would likely continue over 14 hours and remain hazardous to maritime operations for more than 24 hours. This study is limited in that modeling does not account for tide stage, tidal currents, 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 and its probability of occurrence modeled here. Due to these limitations, this modeling is unsuitable for site-specific tsunami inundation assessment or for determining effects on the built environment. Instead, we recommend using this modeling as a tool to assist with emergency preparations and evacuation planning prior to a Cascadia subduction zone event.”

– Abstract from Tsunami Hazard Maps of the Puget Sound and Adjacent Waters—Model Results from an extended L1 Mw 9.0 Cascadia Subduction Zone Megathrust Earthquake Scenario  ger_ms2021-01_tsunami_hazard_puget_sound_pamphlet.pdf

A little more dramatic is their 2019 tsunami videos, available on DNR’s YouTube channel at: https://bit.ly/2UakcB8

More tsunami information is at: https://www.dnr.wa.gov/tsunami
…or at VashonBePrepared’s tsunami page.