Abstract
<jats:p>Radiocarbon dating of co-seismic subsidence in estuaries in Washington and California define the age range of a megaquake that ruptured at least 900km of the Cascadia Fault Zone to between 1690-1720. Tree ring data further refines the time range of the rupture to between August 1699 and May 1700 (Satake et al 2003).</jats:p> <jats:p>At 9 pm PST on January 26, 1700, a massive tsunami washed ashore on the Pacific Coast of Honshu in Japan (Satake et al 2003). Building on the subsidence and tree ring dating studies, and cross referencing the mapped distribution of the extent of the tsunami, to seismic records in the region including Kamchatka, Aleutian Islands and South America, it was determined that a megaquake on the Cascadia Fault Zone was the source of the Honshu tsunami. Modeling this seismic event suggests that this megaquake was in excess of 9.0 magnitude with a fault rupture exceeding 600 miles, thus correlating it with co-seismic subsidence age dating.</jats:p> <jats:p>The Cascadia Fault Zone is proposed as an analog for the Upper Cretaceous convergent margin where similar sized megaquakes with similar sized fault plane ruptures would have occurred. Such a major rupture in the Upper Cretaceous could serve as a crustal weakness influencing and focusing later tectonic events.</jats:p>