In 1700 a megathurst earthquake struck the northwest coast of America. For centuries all that was known was folk lore past down orally through the Native American tribes of the Pacific Northwest. More precise information only came when investigations began the other side of the Pacific in Japan when scientists began to tie historic tsunamis with the geological events which caused them.
‘The Orphan Tsunami of 1700′ was published in 2005, the authors had set out to discover what caused a major tsunami which deverstated parts of Japan on January 27th. From evidance collected from both sides of the Pacific Ocean it was deduced that the initial earthquake took place at roughly 21.00 on the 26th January 1700. Japan had well documented waves hitting shores 6 to 10 ft high yet no quake had been felt any where that side on the ocean.
Back on the Northwest coast clues were also found in tree rings of tidal devastation from same year indicating the tsunami cause was likely to be local. Linking the two and running wave data through computer simulations gave us the estimated date and even time of the quake although the epicentre is still not certain.
It is believed to be at least a magnitude 8.7 but could have been as high as 9.2. The fault rupture scaled over 1000 kilometers, and there is evidance of at least 20 meteres of slip along the vast majority of the rupture. It’s cause was theJuan de Fuco plate which lays between the Pacific plate to its west and North American plate toothed east. Beneath the Pacific Ocean, this relatively small plate is subduction quicker under the north American plate then it is being produced by the Juan de Fuco spreading ridge, so is slowly disappearing. Subduction zones produce the strongest earthquakes to be recorded including the Indian Ocean quake of 2004 and Sendi 2011, both accompanied by fatal tsunamis
The idea of this happening again is a hot topic amongst geologists world. A ‘megaquake’ along the over populated Pacific Northwest could kill millions. Cities like Seattle, Vancouver and Portland have not been built with seismicity in mind leaving many structures vulnerable.
Also, the 1700 event has not been the only major earthquake in the region leading us to believe the threat is not a case of ‘if’ will happen, but WHEN will it happen again……