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Numerical model of the hydrothermal system beneath Unzen Volcano, Japan

Author:
Fujimitsu, Yasuhiro  Ehara, Sachio  Oki, Ryosuke  Kanou, Ryohei  


Journal:
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH


Issue Date:
2008


Abstract(summary):

Drilling of the volcanic conduit in the Unzen Scientific Drilling Project (USDP) was completed in 2004. Some conduit materials of the 1990-95 eruption were encountered at the bottom of Well USDP-4 (150 m below sea level), and a bottom temperature of about 200 degrees C was estimated using logging data, although before drilling a temperature of about 700 degrees C had been inferred. Accordingly, conduit cooling from the initial temperature (850 degrees C) to the estimated temperature (about 200 degrees C) was evaluated by numerical simulation. The drilling provides constraints for the numerical model. The drilling indicates that the N-S width of the conduit of the latest eruption is 20 to 30 m and that it occupies a zone of about 300 m, which includes conduits of past eruptions. The process of cooling in the conduit, from an initial temperature of 850 degrees C in 1995 (the end of the eruption) to 200 degrees C in 2004 (completion of the conduit drilling), was replicated in models with permeabilities of I and 10 mdarcys. This result demonstrates that a highly permeable volcanic body surrounding a small conduit is required to explain the estimated bottom temperature. Our study also aimed to use a numerical simulation to construct a comprehensive hydrothermal model beneath Unzen Volcano. There are four large geothermal systems in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure sources ("Sources A", "B" and "C" from shallow to the deep) were determined by geodetic data during the 1990-95 eruption. Source C is located at about 8 km deep at WUHTB, and is considered to be a magma body. We attempted to explain the existence of the four geothermal systems from the large-scale structures (the topography of the Shimabara Peninsula and Unzen Graben) and the various heat sources. We first set a heat source around Source C and changed its position and size. This numerical model produced the upflow zones at the Obama and Shimabara hot springs and WUHTB; however, the Unzen fumarolic field became a recharge area. This result indicates that another heat source is required to explain the Unzen fumarolic field and that two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. (C) 2008 Elsevier B.V. All rights reserved.


Page:
35---44


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