特別セミナー
日時:2008年3月13日(木)15:00-16:30
場所:環境総合館 災害対策室ホール
講演者:Prof. Jonathan M. Lees
     (Department of Geological Sciences, University of North Carolina, Chapel Hill)
講演タイトル:"Waveform Modeling of Volcano Explosions"






講師紹介(セミナー主催者:地震火山・防災研究センター助教 中道治久)

Jonathan Leesさんは,トモグラフィーの研究で学位を取得されて,防災科研でポスドクをされていたこともあり日本通です.最近は世界中の火山における地震や空振観測から火山爆発ダイナミクスの研究をされており,今回の講演をお願いいたしました.

講演要旨:
 Volcanic explosions take place at the boundary between the earth and the atmosphere, and thus represent a significant interface between solid earth and atmospheric environments.  Multi-parameter waveform observations of volcanic activity can provide new and detailed information on the volcano-atmosphere interface and illuminate the physical processes governing the interaction of thermal fluids escaping the earth.  Recent investigations at several volcanoes utilize seismic, acoustic and visual observations to constrain the evolution of explosiveactivity.  We present evidence of non-linear eruptive activity at several exploding volcanoes and relate these to physical models of the conduit systems.  In particular we show that activity can be modeled as choked fluid flow through narrow conduits near surface vents, analogous to a common kitchen pressure cooker.  The conduit models provide a basis for understanding small scale eruptive activity and may be useful for hazard mitigation where long term volcanic activity poses a significant human hazard.  Examples from Karymsky Volcano, Russia, Reventador and Sangay Volcanoes, Ecuador will be used to illustrate the processes. In January, 2007, Santiaguito Volcano, Guatemala, erupted explosively approximately once per hour.  Over a period of several days we recorded numerous explosions at distances of 1-6 km from the active vent.  The active vent erupts frequently,venting gas and pyroclastics for durations of tens of seconds.  The explosions can be classified by cluster analysis into at least two or more groups -- where the strongest explosions share a considerable similarity in wave form.  The explosions have equivalent estimated magnitudes ranging from 1 to 5 based on Tuboi’s (1954) formula and peak frequencies ranging from 0.8 to 1.6 Hz.
 The initiation of these events (explosions/implosions) is associated with LP earthquake generation (of finite source duration: 1-2 s) which we seek to explain using several potential models.  Using synthetic Green’s functions (Nishimura, 1993) we found that a single vertical forcing function source reproduced the basic form of the initial arrival on the observed vertical component velocity seismograms, although radial components were not well synthesized with this simple model.  Since we have available detailed high resolution video of the surface response to explosions, a detailed source time function can be used to refine the synthetic and this is being investigated.


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