Session 2 — Reading the Subsurface#
Format C — Relevance Map Week 2 Relevance: Cascadia site characterization
From ray paths to real decisions · Cascadia Subduction Zone · Seismic hazard
Hook (0 – 7 min)
Show a Cascadia cross-section alongside a Seattle liquefaction susceptibility map overlaid on unreinforced masonry building locations. Ask:
“These maps were made using refraction surveys and borehole data. What don’t we know — and what would change these maps?”
No explanation. Let students speculate.
Discussion (7 – 42 min)
Three groups, three scenarios. Each group has 12 minutes to develop their plan, then 3 minutes to present their reasoning (not the answer).
Group A — Hospital site characterization: Characterize depth to bedrock under a new hospital site in Seattle — one day with a refraction crew, where do your geophones go?
Group B — Buried debris flow: Detect a buried debris flow deposit under a highway — what method, what geometry?
Group C — Soft sediment amplification: Determine if a neighborhood sits on soft sediment that will amplify shaking — what’s your survey plan?
After each group presents, the instructor asks: “What did you assume? What would break your plan?”
Relevance
Hazard: V~s30~ (average shear velocity in the top 30 m) controls ground motion amplification and is determined by refraction/MASW surveys — the exact method from lecture. PNSN has done this work in Seattle and Portland.
Career: This is bread-and-butter near-surface geophysics — every geotechnical report for a building permit in earthquake country involves some version of this analysis. Environmental and geotechnical consulting is one of the largest BS-level geophysics employers in the PNW.
Go Deeper
USGS “Earthquake Hazards in the Pacific Northwest” fact sheet · PNSN hazard maps
One name: Dr. Erin Wirth, USGS — Cascadia megathrust hazard. What does her team publish and what data do they use?