The Cascadia Earthquake was a giant megathrust quake that hit the Pacific Northwest in January 26, 1700. It has been the most powerful earthquake in record to ever hit the Seattle area.
It was caused by the Cascadia Subduction Zone going off creating giant shockwaves and tsunamis. Based on estimates, the earthquakes magnitude was around 9.0 on the moment magnitude scale.
The Seattle area was not settled by europeans yet although local natives occupied the land. The only records of the quake came from Japan. It was hit by the tsunami and they recorded the date.
Because it happened so early, the knowledge that Seattle could be hit by the most powerful kinds of earthquakes was not known until recently. This caused has caused early and modern Seattelites to be unaware of past and future threatening Cascadia Subduction Zone events.