DOGAMI scientist urges Depoe Bay residents to treat long shaking as tsunami warning

Laura Gable, a geologist with the Oregon Department of Geology and Mineral Industries (DOGAMI), told the Depoe Bay Resilience Committee that a full-margin Cascadia subduction-zone earthquake will produce long, rolling shaking and a tsunami that could reach local shores within about 20 minutes.

Gable, who identified herself as working from DOGAMI's coastal field office in Newport, said the nearest portion of the Cascadia Subduction Zone sits roughly 60 miles offshore and that when a large rupture occurs it can shake for minutes. "If there is an earthquake that shakes for more than a minute, you need to assume that a tsunami is on its way," she said. She emphasized that for a local subduction event, early warning systems may be ineffective and personal recognition of the shaking is the primary immediate alarm.

Why it matters: DOGAMI’s briefing focused on life-safety messaging and local consequences for Depoe Bay, a community with limited inland high ground and significant visitor presence. Gable and DOGAMI staff have produced evacuation-zone maps, modeling and a new community impact profile using 2020 census and building data to estimate population, building losses and debris.

Key points from the presentation included: DOGAMI’s modeling uses a set of conservative "T-shirt" scenarios to define local evacuation zones; Depoe Bay’s modeled local tsunami arrival time is about 20 minutes after strong shaking; the community has an estimated 540 permanent residents in the tsunami zone and roughly 1,000 visitors in lodging at peak occupancy assigned to that zone for planning; and the Hazus-derived snapshot used for planning estimated about 10 tsunami fatalities and roughly 80 injuries in the modeled scenario (DOGAMI cautioned these are first-order tool outputs and within tool error margins).

Gable described the geologic mechanism: the oceanic plate subducts beneath the North American plate, strain accumulates for centuries in the locked zone and then the margin can ‘‘rush’’ tens of feet during a large rupture. That uplift of the seafloor generates tsunami waves; she explained the difference between distant tsunamis (hours away, lower energy) and local tsunamis (minutes after long shaking, much higher energy). She also discussed liquefaction as a site-specific shaking effect and noted Depoe Bay’s basalt geology reduces liquefaction risk at many locations.

On maps and public tools, Gable reviewed DOGAMI’s online evacuation tool (enter an address to get a walk route to high ground and written instructions) and DOGAMI’s two-page community profiles that summarize people-at-risk, critical facilities and estimated building losses for counties and towns. She said county-level publications using updated 2020 census data will be released imminently and urged communities to use the outputs when applying for grants or planning caches and assembly-area capacity.

Committee members asked about map changes, arrival-time specifics and what signage or kiosk materials are available for tourists. Gable said the published evacuation zones will not change without a large vetting process and underlined that next-generation products (smaller-scale maps, online turn-by-turn walk routes) supplement but do not replace the core evacuation zones.

What comes next: DOGAMI offered to provide copies of the county report and to meet with local planners to use the modeling outputs for grant justification and supply-cache sizing. Gable encouraged the committee to focus on personal evacuation planning, signage targeted at visitors, and planning for the community’s likely period of post-event isolation.

Ending: Committee members thanked DOGAMI for the briefing and asked staff to circulate the soon-to-be-published Lincoln County/Depoe Bay profile and the OSU/DOMAG-supported evacuation templates used by local hotels and visitor venues.