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| Date | July 9, 869 |
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| Magnitude | 8.6 Ms |
| Countries or regions | Japan, Iwate Prefecture |
| Tsunami | yes |
| Casualties | ≈1,000[1] |
The 869 Sanriku earthquake and tsunami (869年三陸地震 869-nen Sanriku jishin) struck the area around Sendai in the northern part of Honshu on 9 July 869 (May 26, Jōgan 11).[2] The earthquake had an estimated magnitude of 8.6 on the surface wave magnitude scale.[3] The tsunami caused widespread flooding of the Sendai plain, with sand deposits being found up to 4 kilometres (2.5 mi) from the coast.
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Sanriku is a historical name for an area corresponding to today's Aomori, Iwate and parts of Miyagi Prefecture. The earthquake is also known as the Jōgan Sanriku Earthquake (貞観三陸地震 Jōgan Sanriku jishin) in Japanese, where Jōgan is the Japanese era name for the period from 859 to 877.
The Japanese history text, Nihon Sandai Jitsuroku which was compiled in 901, recorded the 869 earthquake and tsunami of Mutsu Province.[4]
The northern part of Honshu lies above the convergent boundary between the over-riding Okhotsk Plate (a proposed microplate within the North American Plate) and the subducting Pacific Plate. This boundary has been associated with a series of large historical earthquakes, originating either from rupture along the plate interface or from deformation within either the over-riding or subducting plates, many of them triggering a destructive tsunami, such as the 1896 Meiji-Sanriku earthquake.
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.[4][5] According to Nihon Sandai Jitsuroku, around 1000 people were killed by the tsunami.[4]
Despite a lack of reliable sources, there are legends about the earthquake from Tōhoku region to Bōsō Peninsula.[4]
The tsunami caused extensive flooding of the Sendai plain, destroying the town of Tagajō. Archaeological investigations have identified the remains of 8th and 9th century buildings beneath the town, covered by sediments dated to the middle of the 10th century.[6]
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 200 kilometres (120 mi) long by 85 kilometres (53 mi) wide with a displacement of 2 metres (6 ft 7 in) is consistent with the observed distribution and degree of flooding.[6] 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 underestimated.[7]
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 4 kilometres (2.5 mi) inland.[6]
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.[6]
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.[8] In 2007 the probability of an earthquake with a magnitude of Mw 8.1–8.3 was estimated as 99% within the following 30 years.[1]
The 2011 Tōhoku earthquake was somewhat larger than the predicted event, but occurred in the same area as the 869 earthquake and caused major flooding in the Sendai area.
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