dated to the middle of the 10th century. Characteristics Earthquake The estimated magnitude of the earthquake as 8.6 on the surface wave magnitude scale, has been taken from modelling of the tsunami. A source area of long by wide with a displacement of is consistent with the observed distribution and degree of flooding. Analysis of the tsunami deposits associated with the 2011 earthquake suggests that the extent of sand deposition in the earlier events underestimated the degree of inundation. A muddy deposit was found to extend half as far again as the sand sheet. As the topography and cultivation of the Sendai plain has not changed significantly since 869, it has been proposed that the sources of the 2011 and 869 tsunamis were of comparable size, suggesting that the magnitude of the 869 earthquake has been severely underestimated. Thus the magnitude of this quake may have been as high as 9.0. Tsunami The extent of flooding caused by the tsunami of the Sendai plain has been mapped using dated deposits of sand. The tsunami flooded at least inland. The inundated areas closely matched those of the 2011 Tōhoku tsunami. Predicted earthquake hazard Three tsunami deposits have been identified within the Holocene sequence of the Sendai plain, all formed within the last 3,000 years, suggesting an 800 to 1,100-year recurrence interval for large tsunamigenic earthquakes. In 2001 it was reckoned that there was a high likelihood of a large tsunami hitting the Sendai plain, as more than 1,100 years had then elapsed. As for the other two large tsunamis recognized before the 869 tsunami, one was estimated to have occurred between about 1000 BC and 500 BC and the other around 1 AD. In 2007, the probability of an earthquake with a magnitude of 8.1–8.3 was estimated as 99% within the following 30 years. The 2011 Tōhoku earthquake occurred in exactly the same area as the 869 earthquake, fulfilling the earlier prediction and causing major flooding in the Sendai area. See also List of earthquakes in Japan List of historical earthquakes Seismicity of the Sanriku coast References 0869 Sanriku 0869 Sanriku 0869 Sanriku 869 9th century in Japan 9th-century earthquakes