Megaquakes: The Difference Engine: The really big one

the last time Cascadia let go? Just 311 years ago. We know the time and date precisely, thanks to records kept by diligent Japanese officials on the far side of the Pacific. On January 26th 1700, between nine and ten o'clock at night, the Cascadia fault ruptured along its length, unleashing a megaquake of magnitude 9.0 or more. While the shaking was not felt across the Pacific, the Japanese were alarmed enough by the "orphan tsunami" that inundated their coastal villages to record the details. Knowing the distance and the speed such long waves travel at (around 750 kilometres per hour), the Japanese records allow the time the megaquake struck to be known to within an hour. Numerical models suggest that a seismic event that size would have made the seafloor bounce six metres or more, causing tsunami waves to rise as high as 30 metres along the nearby coasts of Oregon and Washington. This past week Sendai looked devastated enough. Imagine how much worse Portland and Seattle would fare if hit by a tidal surge three times bigger.