Sanriku in this context is a name roughly corresponding to the Pacific front northeastern coastal area of Honshu island.

The Japanese history text, Nihon Sandai Jitsuroku which was compiled in 901, recorded the 869 earthquake and tsunami of Mutsu Province.

[7] Although this earthquake occurred in the frontier region of the ancient Japanese Empire based at Kyoto, a short and precise official record of this catastrophe was left.

The description in Nihon Sandai Jitsuroku [True History of Three Reigns of Japan] (901) reads as follows:[8] 五月廿六日癸未　陸奧國地大震動　流光如晝隱映　頃之　人民叫呼　伏不能起　或屋仆壓死　或地裂埋殪　馬牛駭奔　或相昇踏　城（郭）倉庫　門櫓墻壁　頽落顛覆　不知其數　海口哮吼　聲似雷霆　驚濤涌潮　泝洄漲長　忽至城下　去海數十百里　浩々不辨其涯諸　原野道路　惣爲滄溟　乘船不遑　登山難及　溺死者千許　資産苗稼　殆無孑遺焉 On the 26th day of the 5th month (9[4] or 13[5] July 869 AD) a large earthquake occurred in Mutsu province with some strange light in the sky.

In the area which the earthquake struck, the Imperial Court of Japan battled with an indigenous people of the Tōhoku region, Emishi, at that time.

[7] The tsunami caused extensive flooding of the Sendai plain, determined to have reached at least 4 km (2.5 mi) inland, destroying the town of Tagajō and its castle.

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.

[11] 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.