On what date is the 1700 Cascadia earthquake believed to have occurred?

The 1700 Cascadia earthquake is believed to have occurred on January 26, 1700.

The date was reconstructed rather than recorded in a contemporary written account from the Pacific Northwest. Researchers matched the event to Japanese records describing an “orphan tsunami”—waves that arrived on Japan’s coast without a locally felt earthquake. The tsunami struck communities in what is now Iwate Prefecture during the night of January 27–28, Japan time, corresponding to January 26 in Cascadia.

Geological evidence supports the reconstruction. Drowned coastal forests, tsunami sand deposits, submerged soils, and tree rings show that coastal land suddenly sank near the end of the 1699 growing season. Together with Japanese observations, these clues identify a major Cascadia subduction-zone earthquake at about 9 p.m. Pacific time.

A common mix-up is the date conversion: Japanese records place the tsunami on January 27–28, while the earthquake itself occurred on January 26 in North American local time. The earthquake is estimated at roughly magnitude 8.7–9.2 and ruptured a section of the fault about 1,000 kilometers long.