"The Big One" in Seaside, OR

        Nearly every summer since before I can remember, my family has rented a beach house on the Oregon Coast, in the town of Seaside. Every time we arrived, like clockwork, my parents would give a verbal pop quiz on our tsunami evacuation route and would often accompany this ritual with an ominous mention of “The Big One.”

        The Cascadia Subduction Zone is a 700-mile fault line where the Juan de Fuca tectonic plate dives beneath the North American plate. “The Big One” refers to the earthquake and tsunami that will ensue when the Cascadia Subduction Zone inevitably ruptures. This rupture is bound to happen, but the severity of the fallout depends on whether it is a full or partial rupture. A full rupture, with an earthquake upwards of magnitude 9.0 (estimated to have a 16–22% chance of occurring within the next 50 years), could generate a 100-foot tsunami that would wash away coastal towns, destroy Highway 101, cause tens of billions in damages, and likely kill thousands.

        While Seaside has a number of evacuation routes, our preferred location on the southern end of town near the beach would have us going south along the boardwalk, around the cove, and up Tillamook Head. This route can be seen on the map provided by the City of Seaside and via the red line on the picture below.

Pedestrian Walking Map - City of Seaside

Seaside - Oregon Coast Visitors Association

         The Northwest Association of Networked Ocean Observing Systems provides a free online interactive map that shows detailed information about the optimal evacuation route from any address on the Oregon and Washington coasts. I encourage you to try out the tool found at this link: https://nvs.nanoos.org/TsunamiEvac. Using the example address of 1441 S. Promenade in Seaside, the interactive map gave a 1.2-mile route to safety along Tillamook Head and a minimum speed to reach safety of 16 minutes per mile, which it classified as a “jog.” While I could cover this distance easily, I am frightened for less mobile individuals and saddened by the sheer destruction to communities and ecosystems that will occur here.

Sources

Bourgeois, M. (2025, September 9). Ahead of a Cascadia earthquake, Oregon Gov. Kotek signs order for
        building safety. KOIN 6 News. https://www.koin.com/news/oregon/ahead-of-a-cascadia-earthquake-
        oregon-gov-kotek-signs-order-for-building-safety/

Goff, J., Terry, J. P., Chagué-Goff, C., & Goto, K. (2014). What is a mega-tsunami? Marine Geology, 358,

        12–17. https://doi.org/10.1016/j.margeo.2014.03.013

Lin, R.-G., II. (2025, October 7). West Coast’s two monster faults could trigger back-to-back earthquakes,
        new research suggests. Los Angeles Times. https://www.latimes.com/california/story/2025-10-
        07/what-could-trigger-a-massive-quake-on-californias-san-andreas-fault