The Active Fault Research Center (AFRC) of the National Institute of Advanced Industrial Science and Technology (AIST), one of independent administrative institutions, estimated in collaboration with the United States Geological Survey (USGS) and the Geological Survey of Canada (GSC), the size of the giant earthquake which had hit the Pacific Coast of North America about 300 years ago to be of M9 class on the moment magnitude scale. Based on records of tsunami in the 12th year of Genroku in historical documents found at various parts across Japan, the height of the tsunami waves along the Pacific Coast of Japan was estimated in three ways, taking various factors into consideration.
For six fault models with different fault length and width in the Cascade Subduction Zone, the crustal deformation along the Pacific Coast of North America and the seafloor were computed using the latest three-dimensional plate geometry model. Furthermore, the size of tsunami propagating across the Pacific was estimated and compared with that estimated in Japan. On the basis of 18 combinations, the size of the 1700 earthquake was estimated as moment magnitude (M) 8.7-9.2, fault length 1100 km and slip amount 14 m. These data will be useful for the damage assessment of earthquake and tsunami in the northwestern region of the United States and the southwestern Canada.
*The result was published in the Journal of Geophysical Research (Web edition), November 20, 2003, and press releases were made from USGS and GSC, at 10:30 am Pacific Standard Time, November 20, 2003.
In the Cascade Subduction Zone, off the Pacific Coast of North America (north-west of the US and south-west of Canada), where the Juan de Fuca Plate going down into beneath the North American Plate, the geological setting resembles that of the Nankai Trough of Japan.
Historical events of these areas are documented only from around 1850 on, and the occurrence of earthquakes before had not been recorded at all.
Since around 1990, scientists at USGS and GSC have begun to study geologic traces of coastal movements and tsunami deposits along the Pacific Coast of North America, and found evidence ofgreat earthquake occurred about 300 years ago. Consequently, it was speculated that the earthquake should caused a tsunami in the Pacific, which propagated across the ocean to reach Japan.
The Geological Survey of Japan (GSJ, now reorganized into the AFRC, AIST) proposed in 1996 in collaboration with the Earthquake Research Institute, University of Tokyo, that a tsunami of unknown origin recorded in various parts of Japan on December 8-9, the 12th year of Genroku (1700), under the old Japanese calendar, might be attributed to a huge earthquake in the Cascadia Subduction Zone. The study was published in the journal Nature (Satake et al., Nature, Vol. 379, No. 6562, pp.246-249,1996).
The report that the last giant earthquake occurred in January 26, 1700 in the Cascadia Subduction Zone gave a certain impact to the earthquake planning for the northwest of the US and the southwest of Canada. However, details of earthquake size, such as moment magnitude, fault length and slip amount, are not available, hence the results have not been fully utilized for the disaster prevention against earthquake and tsunami.
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Fig. 1. The Cascadia Subduction Zone extends along the Pacific Coast of North America.
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