<論説>発掘調査より知られる貞観一一年 (八六九) 陸奥国巨大地震・津波の被害とその復興 (特集 : 災害)

Abstract

貞観一一年(八六九) 陸奥国巨大地震は、多賀城の変遷における第III期と第IV期の画期となっている。多賀城は大被害を受けたが、城内では政庁、外郭区画施設、実務官衙、城外では方格地割を形成する道路、内部の建物群などが復興を遂げている。多賀城跡城内における地震の前後の様子はかなり具体的にわかっている。城外での復興の様子もわかってきたが、巨大津波の襲来が『日本三代実録』貞観十一年五月二十六日癸未条より推定されるにもかかわらず、津波痕跡がよくわからないのが実情である。史料には載っていないが、多賀城付属の多賀城廃寺跡の他、陸奥国分寺跡・国分尼寺跡も被害を受けたが、復興されていることが発掘調査で明らかとなっている。特に多賀城廃寺三重塔跡の復興状況は明瞭で、陸奥国分寺跡の七重塔廻廊跡は貞観地震で倒壊した可能性が高い。多賀城の復興の方式とは異なり、陸奥国分寺・国分尼寺の復興は創建期と同様に国内郡司の協力があったと推定される。

At 14: 46, March 11th, 2011, a massive earthquake of Mw (moment magnitude) 9.0 occurred along the Pacific coast from Sanriku to Ibaraki. The 2011 Tohoku Earthquake has later come to be called the second coming of the Great Mutsu Earthquake of Jogan 11 (869). The Great Mutsu Earthquake of Jogan 11 (869), which had an estimated magnitude 8.3 to 8.4 and occurred along the Pacific coast from Sanriku to Fukushima, was well known to archaeologists, historians of ancient times, seismologists, and tsunami researchers in Tohoku, but was not known by the general public. It is a tragedy that the Great Mutsu Earthquake of Jogan 11 (869) only became well known to the public after the 2011 Tohoku Earthquake disaster occurred. Through the Japanese history Nihon Sandai Jitsuroku (The True History of Three Reigns of Japan), covering the period from 858 to 887 A.D. we can sum up the characteristics of the Great Mutsu Earthquake of Jogan 11 (869) as the follows. On the 26th day of the 5th month of the 11th year of the Jogan era (July 9, 869 A.D.) a massive earthquake occurred in Mutsu Province (present-day Fukushima, Miyagi and Iwate Prefectures). A massive tsunami hit the town outside Tagajo, now an archaeological site. Tagajo was the Mutsu provincial capital and one of the ancient provincial cities from the Nara period to the Heian period. The document reports that about 1, 000 people were killed in the tsunami and that many were killed by collapsed houses and landslides. Enormous wooden buildings, warehouses, gates, turrets and the earthen wall of Tagajo were destroyed. Fields, roads and crops were damaged heavily by the massive tsunami, and there were innumerable cracks in the ground caused by the massive earthquake. It is true that Mutsu Province (present-day Fukushima, Miyagi and Iwate Prefectures) suffered most from the earthquake, but it is assumed that neighboring provinces such as Hitachi Province (Ibaragi Prefecture) were also damaged. As most areas of Mutsu Province were damaged, it can also be assumed that the area north of the Osaki and Ishinomaki plain where the Emishi people lived was damaged heavily. Based on the historical record of the earthquake disaster, the Great Mutsu Earthquake of Jogan 11 (869) must have had a magnitude of more than 8, i.e. above 6 on the seven-point Japanese scale. The first report on the earthquake disaster from Tagajo, the provincial capital of Mutsu, reached Heiankyo (the capital of Japan at the time) long after the event ; the report arrived sometime between the 2nd day of the 7th month of the 11th year of the Jogan era (August 13, 869 A.D.), 35 days after the earthquake, and the 7th day of the 9th month (November 15, 869 A.D.), 98 days after the earthquake. This fact suggests that the damage of the earthquake in Mutsu Province was very extensive. Ninety-eight days after the earthquake, the government sent Harue Kino as an imperial messenger to Mutsu Province in response to the Great Mutsu Earthquake of Jogan 11 (869). He was ranked Jugoinojo (Junior Fifth Rank, Upper Grade), and held the post of Saemon Gon no suke (provisional assistant captain of the Left Division of Outer Palace Guards). After this, the government implemented several measures for the reconstruction of Mutsu Province. Those were as follows. 1) Emperor Seiwa issued an imperial edict on the 13th day of the 10th month of 11th year of the Jogan era (November 20, 869 A.D.). It embodied the fundamental principle of reconstruction based on Saiisetsu (Natural disaster and abnormal phenomenon theory) and stipulated specific measures for reconstruction, such as the dispatching of imperial messengers, relieving the sufferers of the disaster, burial of the dead, exemption from taxes (so and cho) for the earthquake victims, and relief of old people with no one to depend upon. 2) Personnel for reconstruction of Mutsu Province: Harue Ono, a military official, was appointed, Mutsu no suke (assistant governor of Mutsu Province). Soon after, he was promoted to Mutsu no Gon no kami (governor of Mutsu Province). 3) Prayers to gods, Buddha and imperial mausoleums: 4) Dispatch of ten Shiragi pirates to Mutsu Province: They were ordered to teach people how to make Shiragi style roof tiles. 5) There had been a succession of disasters during the reign of Emperor Seiwa, so Emperor Seiwa abdicated in 876 (the 18th year of Jogan), passing his title to Imperial Prince Sadaakira (Emperor Yozei) in order to put an end to the Rational disasters that had occurred one after another. The Great Mutsu Earthquake of Jogan 11 (869) is one of the most important points in the chronological division of the Tagajo Fort Site. The date marks the divide between stage III and stage IV. Through continual excavation and research over 50 years, it has been estimated that the Tagajo Fort Site suffered great damage from the Great Mutsu Earthquake of Jogan 11 (869). But the Tagajo government district and adminisrative offices within the walls of the castle were reconstructed. And outside the walls of the Tagajo Fort Site, roads and buildings were also reconstructed. Within the walls of the Tagajo Fort Site, the actual situation before and after the Great Mutsu Earthquake of Jogan 11 (869) was thus known concretely. On the other hand, the picture of reconstruction outside the walls of the Tagajo Fort Site is gradually being learned. It has been assumed from the entries from the Nihon Sandai Jitsuroku for the 26th day of 5th month of the 11th year of Jogan (July 9, 869 AD) that the massive tsunami of Jogan 11 (869) struck outside the walls of the Tagajo Fort Site, but actual traces of the Jogan Tsunami were not known clearly. I reconsidered excavation and research reports, and pointed out five locations near the Ancient Sunaoshi River that are possibly deposits from the Great Jogan Tsunami. Before they undergo post-disaster rebuilding, large-scale excavations have been underway since last year on the Sanno Site and lchikawabasi Site that are located outside the walls of the Tagajo Fort Site. I have been engaged in this excavation as part of my official duties since last year. To get a clearer picture of the deposits from the Great Tsunami of Jogan 11 (869), I will continue to exert myself in this excavation and anticipate the results of future excavation by the Tagajo City Board of Education. In spite of the absence of records in historical sources, it has become clear through excavations and research that the abandoned temple at Tagajo attached to the Tagajo Fort Site, the Mutsu Provincial Monastery, and the Mutsu Provincial Nunnery suffered great damage in the Great Mutsu Earthquake of Jogan 11 (869), and that they were also later reconstructed. We can clearly know the status of the restoration of the Sanjunoto (three-storied pagoda) of the abandoned temple at Tagajo, and it is highly likely that the Nanajunoto (Seven-storey pagoda) of the Mutsu Provincial Monastery was destroyed by the massive 869 (Jogan 11) Mutsu Earthquake. It can be assumed that the Mutsu Provincial Monastery and the Mutsu Provincial Nunnery were reconstructed through the cooperation of many gunji (district managers) in Mutsu Province, as they had been at the time of their founding. And this manner differed from the reconstruction of structures at the Tagajo Fort Site. As a result of the Great East Japan Earthquake, study of the Great Mutsu Earthquake and Tsunami of Jogan 11 (869) began to be fully advanced in various fields. To discover the reality of the Great Mutsu Earthquake and Tsunami of Jogan 11 (869), interdisciplinary research in a variety of closely related fields --archeology, ancient history, seismology, geomorphology and tsunami engineering-- are needed. To perform this task, it is necessary to discover the reality of the Great Mutsu Earthquake and Tsunami of Jogan 11 (869) in each area of study at the outset. This paper is one attempt in preparation for such interdisciplinary collaborative research.