Examining the Risks: A tsunami in Seaside, Oregon
Seaside, a popular Oregon town for beachgoers, sits on the frontlines of one of the Pacific Northwest’s most significant natural hazards: the Cascadia Subduction Zone. In the event of a megathrust earthquake of magnitude 8.0+, the resulting tsunami could devastate coastal communities. Using ArcGIS and data provided by the Oregon Department of Geology and Mineral Industries (DOGAMI), I created a tsunami inundation zone map for Seaside, highlighting critical vulnerabilities and opportunities for improved disaster preparedness.
Understanding the “XXL” Tsunami Inundation Zone
The height of a tsunami, and by extension the expected inundation zone, from a Cascadia megaquake would depend on the size of the earthquake that generated it. DOGAMI elected to use a system of shirt sizes for a variety of modeled events, from Small (Magnitude 8.0) to XX-Large (Magnitude 9.2). This map identifies the extent of flooding modeled to occur under the worst-case XXL scenario, underscoring the urgent need for robust evacuation planning. Key findings include:
- Critical Infrastructure at Risk: Hospitals, schools, law enforcement buildings, and bridges are located within the inundation zone, potentially compromising emergency response and evacuation efforts.
- High Vulnerability of Residents: Many residential areas, especially beachside retirement homes, are situated within this zone, heightening the risk to vulnerable populations such as the elderly and disabled.
Challenges to Evacuation
Seaside’s evacuation challenges are compounded by its geography and infrastructure:
- Limited Evacuation Routes: With few roads leading inland and many bridges likely to collapse in the earthquake, many residents will largely be forced to evacuate on foot.
- Short Warning Time: The tsunami waves are estimated to reach the coastline within 15-20 minutes after the earthquake, leaving minimal time for evacuation.
Tsunami Assembly Areas: Are They High Enough?
This map also examines tsunami assembly areas that have already been designated as safe zones for residents and visitors during a tsunami event. However, my analysis reveals that several of these assembly points might still not be high enough to ensure safety in an “XXL” event. The best example is the four assembly areas in Gearhart to the north, located atop some small ridges in the area. At their tallest, these ridges are 56 feet above sea level as per OpenStreetMap – which would be sufficiently high in a “S” event, but well below the expected wave height from a “XXL” Magnitude 9.2 earthquake. This discrepancy highlights the importance of reevaluating and potentially redesignating evacuation routes to align with worst-case scenario modeling.
Next Steps
The Oregon coast is not the only region of the Pacific Northwest at risk from this megathrust earthquake. In Washington state to the north, there are also many beaches and state park locations that lie within the hypothetical tsunami inundation zone from such a megaquake. My next map will aim to pull data from other sources, such as USGS Earth Explorer, to create maps for other areas of interest at risk which the specific modeled inundation GIS data from DOGAMI would not be relevant for. In doing so, I hope to show my ability to obtain and work with different datasets, rather than limiting myself to just one source.