Archaeological studies on the duration and seasonality of two 6^th-century eruptions of Haruna Volcano in central Japan

Abstract

Haruna volcano located in central Japan erupted twice, in early and middle 6th century AD, devastating the northwestern part of Kanto area with airfall tephras, pyroclastic flows and lahars. The Shibukawa tephra formation (Hr-FA), which is divided into S₁-S₁₅, was deposited by the earlier eruption, and is characterized by phreatomagmatic eruptions and pyroclastic flows associated with lava dome formation. On the other land, the Ikaho tephra formation (Hr-FP), divided into I₁-I₁₉, was deposited by the later eruption, and is characterized by Plinian eruptions and pyroclastic flows. With the absence of historical documents on these eruptional events, the seasonality and duration of eruptions were determined by ethnographical analogy of archaeological evidence ; the conditions of paddy fields covered with the tephras, discovered at the Motosoja-Kitagawa site and the Arima-Jori site, were interpreted in line with the annual schedule of modern rice cultivation. It is considered that the S₁-S₇ eruptions, the first half of the Shibukawa tephra, occurred within several days because of the traces of construction of the levees that divide paddy fields, recognized just below and above the S₁-S₇ tephra members at the Motosoja-Kitagawa site. The time between the initiation of eruption (S₁) and the occurrence of the first large scale lahar is estimated to have been less than one month including the rice transplanting period in early summer. On the other hand, it is estimated that the whole eruptional sequence of the Ikaho tephra (I₁-I₁₉) occurred in less than one week, corresponding to the period when divisional levees were built and paddy fields were plowed, as confirmed at the Motosoja-Kitagawa and Arima-Jori sites. The period between the initiation of eruption (I₁) and the first large-scale lahar is believed to have been less than several weeks in early summer.