Japan's Sendai region, which was devastated by a magnitude-9 earthquake and a tsunami in 2011, experienced similar events in AD 869 — and such disasters could occur in the area more often than previously thought.

Seismic assessments have typically underestimated the earthquake hazard for the Sendai area, in part because of uncertainty surrounding the size of the 869 Jogan earthquake and the frequency of large earthquakes in the region. Yuki Sawai and his colleagues at the Geological Survey of Japan in Tsukuba studied sediment samples from 399 locations along the coast near Sendai, looking for tsunami deposits and signs of seismic activity. They found deposits associated with the Jogan earthquake stretching at least 1.5 kilometres inland — farther than deposits from tsunamis that occurred between 869 and 2011. Earthquake modelling suggests that the Jogan event had a magnitude of 8.4 or higher. Moreover, the authors estimate that such an event could occur every 500–800 years.

The findings, which were reported in preliminary form before 2011, might have improved earthquake hazard assessment for the Sendai region, the authors say.

Geophys. Res. Lett. http://dx.doi.org/10.1029/2012GL053692 (2012)