# 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*
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```{dropdown} 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.
```
```{dropdown} 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?"*
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```{dropdown} 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.
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```{dropdown} 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?
```