Estimating the size of 869 C.E. quake in region hit by 2011 Tohoku quake



The catastrophic 2011 magnitude 9 Tohoku earthquake, which triggered a huge tsunami that led to severe flooding and major destruction, was unexpected based on seismic hazard assessments for the Sendai region—for instance, a Japanese national seismic map from 2010 showed no hazard in the Sendai area from subduction earthquakes along the Japan Trench larger than those of the past 400 years. Seismic hazard assessment relies in part on knowing the size and recurrence intervals of past earthquakes in a given region. In the Sendai area, in 869 C.E. a large quake known as the Jogan earthquake produced a tsunami that affected approximately the same area of coast as the Tohoku earthquake, but scientists have not been certain of the magnitude of the Jogan quake.