Jōgan earthquake and tsunami in Mutsu

The Nihon sandai jitsuroku, the official history completed in 901, records under the 26th day of the 5th month of the 11th year of Jōgan — 9 July 869 (13 July in the proleptic Gregorian calendar) — that ‘the earth shook greatly in Mutsu’. Light flashed in the sky; people cried out and could not stand; houses fell and the ground split; castle walls, storehouses, gates, towers and fences collapsed ‘in countless numbers’. Then ‘the sea roared like thunder’, the waves surged inland and ‘suddenly reached the foot of the castle’ — usually identified with Taga Castle, the seat of the provincial government; ‘fields and roads all became open sea’, there was no time to take to the boats or to climb the hills, and ‘about a thousand drowned’, while property and crops were almost entirely swept away.

On 15 October the court appointed Ki no Harueda envoy to inspect the earthquake in Mutsu, and on 20 November an edict of Emperor Seiwa acknowledged that ‘in the land of Mutsu the earthquake was most severe: in places the sea overflowed, in others walls and houses collapsed’. The envoys and the provincial officials were to visit the victims ‘without distinguishing between commoners and Emishi’, bury all the dead, give relief to the living and exempt the worst affected from the rice tax and tribute.

Modern estimates put the magnitude at 8.6 or more, possibly as high as 9.0. In 2001 researchers identified sand laid down by the tsunami more than 4.5 km from the coast of the Sendai plain; together with two older tsunami deposits, this suggested that such events recur every 800–1,100 years and that another was due. The Tōhoku earthquake and tsunami of 11 March 2011 struck the same area, and the land it flooded closely matched that flooded in 869.