From coastal traces to earthquake histories
Meeting 3 | Tuesday, September 22, 2026 | 1:00-2:00 PM Pacific
Central question
When do geological traces discriminate a causal history, and when can different processes produce similar deposits?
The paper pair
Pairing type: Discovery claim / diagnostic scrutiny.
Paper A: Evidence for great Holocene earthquakes along the outer coast of Washington state
- Science (1987). Paper / publisher record | DOI
Read Atwater in full.
Access: USGS publication record with publisher link.
Paper B: Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America
- Journal of Geophysical Research: Solid Earth (1996). Paper / publisher record
Read Nelson and colleagues’ abstract, discussion of diagnostic criteria, and conclusions; the discussant examines the detailed examples.
Access: Publisher/DOI page; full text may require institutional access.
Why these papers belong together
Atwater presents evidence for great past earthquakes. Nelson and colleagues examine how to distinguish coseismic subsidence from processes that also occur on passive coasts. This is refinement of the inference, not a blanket rejection of the discovery. (Atwater 1987; Nelson et al. 1996)
Prepare before the meeting
Bring a stratigraphic, seismic, or paleoenvironmental interpretation. Separate observations, proxy assumptions, dating uncertainty, and causal claims.
Discussion questions
- Which observations are directly measured and which quantities are reconstructed?
- What would make a peat-mud contact insufficient evidence by itself?
- What new trace would distinguish competing interpretations?
One-hour meeting
| Time | Activity |
|---|---|
| 0-10 min | Independent first judgments; surface disagreements. |
| 10-25 min | Compare the papers: claim, evidence, assumptions, and limits. |
| 25-45 min | Work through the case exercise below. |
| 45-55 min | Translate the discussion into agent requirements and tests. |
| 55-60 min | Record an output and one unresolved disagreement. |
Case exercise
Construct an evidence table for suddenness, submergence magnitude, lateral extent, tsunami association, and regional synchrony. Add a plausible nonseismic explanation and identify the additional observation needed to discriminate it.
Agent-design or evaluation output
Trace-inference worksheet: measurement -> uncertainty -> alternative processes -> discriminating trace -> provisional conclusion.
Record after the meeting
Record the evidence for your main claim, what changed your mind, what remains unresolved, and one change to the agent or its evaluation. Keep confidential examples in private group notes rather than committing them to this public book.