Eruptions and earthquakes in and around Izu Arc area during the ninth century ; aiming the understandings of the tectonics

为了弄清9世纪伊豆弧北及其周围的独特构造，我们重新审视了伊豆弧北世纪发生的喷发和地震活动。根据地质、考古数据和历史文献，我们证实了那个时期的喷发和地震确实很活跃，到目前为止，我们指出9世纪曾多次发生大喷发，分别发生在小岛火山、新岛火山、宫角火山和大岛火山。此外，富士火山活跃，关东内陆也发生了几次大地震。此外，汇编的结果显示，包括周边地区在内的地震活动也处于较高水平。破坏性地震相继发生在岩川-静冈活动断裂系统、秋田平原、肖奈平原、Echigo平原、长野断裂系统和南开海槽。新泻山火山和卓凯火山也在这一时期喷发。9世纪中叶的这些活动集中在伊豆群岛和日本东西边界沿线。这些迹象表明，跨越日本弧线的800多公里长的大喷发和地震活动之间存在联系。在地质学上，这些事件似乎反映了阿穆里亚板块东缘受板块东移的驱动而发生的东西向缩短超过800公里。我们指出，近50年来的火山和地震活动在震源区和方式上与9世纪的相似。一些科学家认为，岩川-静冈构造线中央的活动断裂系和藤川断裂系的平均重现期是超期的。在这方面，我们不能放弃最糟糕的情况，即在阿穆里亚板块东缘即将到来的大地震之后，富士火山的坍塌和喷发可能会发生。

Aiming to clarify the unique tectonics in and around to northern part of Izu Arc in the 9th century, we re-examined eruptions and seismicity occurred in the northern part of Izu Arc in the century.Based on geological, archaeological data and historical documents, we confirmed that eruptions and earthquakes were really vigorous in that period, as so far pointed out Big eruptions repeatedly took place in the 9th century, at Kozushima, Niijima, Miyakejima and Oshima volcanoes. Moreover, Fuji volcano was active and a couple of big earthquakes also occurred on Kanto inland. In addition, compiled results showed that seismic activity was also at a high level including surrounding area. Destructive earthquakes successively hit along the Itoigawa-Shizuoka active fault system, Akita Plains, Shonai Plains, Echigo Plains, Nagano fault system, and Nankai trough. Niigata-Yakeyama and Chokai volcanoes also erupted in that period. These activities in the middle of the 9th century were concentrated in Izu Islands and along boundary between the East and West Japan. These suggest a linkage of big eruptions and seismic activities over 800km long crossing Japan Arc. Geologically these events seem to express the East-West shortening along eastern margin of Amurian Plate over 800km, driven by the eastward motion of the plate.We point out the similarity of volcanic and seismic activities during the recent 50 years to those happened during the 9th century, in their source areas and manner. Some scientists that the central part of the Itoigawa-Shizuoka tectonic line active fault system and Fujigawa fault system are overdue to the average recurrence period. In this context we cannot discard the worst scenario that the collapse and eruption of Fuji volcano may follow the coming big earthquake along the eastern margin of the Amurian plate.