Satake, and 3 co-authors, reported in 1996 that the geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the Cascadia subduction zone, off the Pacific coast of North America. The most recent event (or series of events) is dated at about 300 years ago, but the precise date and magnitude have not been determined. Geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two, although this information is important for the assessment of future hazards.
The authors have literally found several tsunami records in Japan from AD 1700 with no indication of a local cause. Historical earthquake records and paleo-seismic evidence indicate the absence of a large earthquake in 1700 in South America, Alaska or Kamchatka, leaving Cascadia as the most likely source of this tsunami. The estimated time of the earthquake is the evening (about 21:00 local time) of 26 January 1700. The magnitude is estimated as 9 from the tsunami heights, in which case the earthquake ruptured the entire length of the Cascadia subduction zone. As indicated above, these estimates are consistent with Native American legends that an earthquake occurred on a winter night.
If the 1700 tsunami came from Cascadia, the authors could then estimate the origin time and magnitude of the earthquake. The earliest documented tsunami arrival time was around midnight on 27 January Japan time, or around 15:00 on 27 January GMT. Because tsunami travel time from Cascadia to Japan is about 10 hours, the earthquake origin time is estimated at around 5:00 on 27 January GMT or 21:00 on 26 January local time in Cascadia. Again, this time is consistent with Native American legends that an earthquake occurred on a winter night.
The authors estimated the moment magnitude of this earthquake to be 9. Although the tsunami magnitude formula has not been established for a Cascadia earthquake owing to lack of instrumental data, the estimated magnitude values for all other possible regions are about 9 as we have already seen. The authors also simulated tsunami propagation across the Pacific Ocean by applying a finite-difference method to real bathymetry data. Preliminary computations indicate that a Cascadia earthquake with Mw ~ 8 produces only a 0.3-m tsunami in Japan, too small to cause damage and easily missed if it happened at night. If the earthquake size is Mw ~ 9, the tsunami height becomes 2 m, large enough to cause damage.
A recent earthquake provides observational support in a reciprocal way; an earthquake (Mw = 8.2) occurred in the Kuril Islands, near Japan, on 4 October 1994, and the tsunami heights were only 0.3 m or less on the Pacific coast of California, Oregon and Washington. A Cascadia earthquake with Mw ~ 8 would therefore produce similar tsunami heights in Japan. (The rupture length of an Mw ~ 8 earthquake is only ~ 100-200 km, so that directivity effects are negligible.) An earthquake with Mw ~ 9 extending along the entire Cascadia zone, on the other hand, would cause a damaging tsunami in Japan, which lies in the direction perpendicular to the fault, because of the directivity effect as well as the larger deformation of the ocean bottom.
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