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In January 1700, villages along Japan's Pacific coast recorded flooding, damaged houses, and fires from a tsunami, yet no one had felt an earthquake. Without a local shock to explain it, the wave stayed an 'orphan' in the records for nearly three centuries. It became traceable only once geologists across the Pacific found evidence of a matching event.
When a subduction-zone earthquake drops the coast by a meter or more, seawater floods lowland forests and kills the trees. Their dead trunks stay standing and their buried roots are preserved. Ring-counting on Washington's dead cedars showed they died after the 1699 growing season and before the 1700 one, which narrows the date to a single winter.
They then worked backward from the wave's known arrival time in Japan, using its roughly 10-hour trans-Pacific travel time. That placed the rupture off Cascadia around 9 p.m. on 26 January 1700, local time.
A tsunami still several meters high after crossing an entire ocean requires a huge rupture that displaced an enormous area of seafloor. Modeling the Japanese flood heights implies a Cascadia earthquake of about magnitude 9. That size means a rupture roughly 1,000 km long, spanning most of the subduction zone rather than one segment.
Coastal First Nations accounts describe a night-time shaking and flood that destroyed a winter village at Pachena Bay. These oral traditions agree with the geological and Japanese evidence on season, time of day, and effects. Read this way, oral history becomes an independent data stream that can confirm and enrich findings from written and physical records.
Cascadia had no great earthquake in its brief written history, so some scientists judged it possibly aseismic or slipping quietly. Buried marsh soils, sand sheets, and dead forests showed that great earthquakes recur there every few centuries, a cycle longer than the local written record. Silence in a short record reflects the record's length, not an absence of hazard.