Get Full Government Meeting Transcripts, Videos, & Alerts Forever!
Get email alerts on the Natural Hazards Tsunami topic
No spam. Unsubscribe anytime.
State geologists brief Humboldt County on tsunami hazards: ‘Cascadia tsunami could arrive in 10–15 minutes’
Summary
California Geological Survey staff presented probabilistic tsunami hazard analysis for the North Coast, explained the mapping criteria used to create evacuation lines, and urged communities to plan and drill; CGS recommended using the 1,000‑year tsunami for hazard-area mapping and noted that subsidence and sea‑level rise are included in probabilistic models.
The Humboldt County Planning Commission received an informational workshop on May 7 from the California Geological Survey (CGS) about North Coast tsunami hazards and mapping methods.
Jay Patton of the CGS tsunami unit (speaker 16) explained how Probabilistic Tsunami Hazard Analysis (PTHA) uses logic trees and thousands of simulated earthquake‑slip scenarios to develop hazard curves and inundation models. Patton said roughly “85% of tsunamis are caused by large earthquakes below the sea floor,” and that the CGS used the 1,000‑year tsunami as the basis for conservative tsunami hazard‑area mapping used in the region.
Patton described the state’s mapping criteria: a conservative inundation line derived from modeled flow depths and 1‑meter elevation contours, and a separate evacuation line chosen to be practical for local jurisdictions. He said the state worked with Cal OES and local partners to produce maps and that local agencies retain authority to adjust evacuation destinations to fit real‑world features—examples included moving an evacuation line to a known local high‑ground destination in Crescent City.
Commissioners and members of the public asked about evacuation time, especially for people who must travel on foot or need assistance. Patton said a Cascadia Subduction Zone tsunami could reach the coast in as little as 10–15 minutes and urged regular drills and neighborhood preparedness. He also said CGS included probabilistic subsidence and ran scenario sets with multiple sea‑level rise amounts (0, 0.5, 1, and 2 meters) to assess how hazards change with mean higher high water.
Patton emphasized that evacuation planning is a key mitigation tool and noted state guidance and local examples (Manila school drills, signage) for improving public readiness. The commission thanked CGS and asked staff to share the presentation and underlying data with local planners for further use in the county’s Humboldt Bay Area planning work.
The event was informational only; commissioners asked several follow-ups and encouraged outreach to vulnerable populations and continued local drills.
