Internal Structure of the Sambongi Fan - Towada Volcano, Japan: Putting the Theory to the Test, using an Extensive GPR Survey

Christopher Gomez (1, 2) , Kataoka Kyoko (3)

1 Department of Geography
2 Natural Hazards Research Centre
3 Niigata University

The Towada Caldera Volcano is located in Aomori Prefecture - Northern Honshu Island, Japan. The caldera, today filled by a lake, has produced 15,000 years ago a complex eruption emplacing an Ignimbrite deposit topped by the lake outburst flood deposit, through which the present Oirase River cuts. This deposit has shaped the geomorphologic feature named Sambongi fan, on which Towada City extends. Based on outcrops, a few coring and radiocarbon dating, Kataoka (2011) has developed a theory tying the eruption history and the formation of the Sambongi fan. Because of the lack of extended outcrops - typical of the Japanese environment -, the authors have put the theory to the test using GPR (Ground Penetrating Radar) radargram extending along a 640 m length. The GPR used for the survey was a Pulseko-Pro with 50 MHz antennas. The software Reflex was used to process the data. The radargrams have displayed a sole unit, which the GPR could not penetrate and that the authors have interpreted as being the ignimbrite deposit. On top of this deposit a series of subhorizontal layers, with the alternation between a backset and a foreset extend between 5 m and 3 m depth. Above 3 m, the units are regular and subhorizontal. The deposit is also characterized by the extensive presence of blocks, which are located along three bands: (1) on top of the ignimbrite deposit; (2) in the units deposited by the outburst flood, between 3 and 5 m depth; (3) and in the units close to the surface, although part of these punctual elements are most certainly anthropogenic. Compared with the outcrops, the present research confirms that the material located above the ignimbrite material have been deposited by the outburst flood, creating large-sheet patterns, which have transported blocks. These sheets display backsets and foresets patterns, depending on the position of the deposit to the 'high-energy-flow tongues', which may have slightly looked like the ones observed during the 3.11 tsunami in the Sendai Plain (Miyagi Prefecture).

Mots clés: Towada Volcano; Aomori; Japan; Caldera; Sambongi fan; GPR; Ignimbrites; Outburst Flood

Domaines: Environnement et Société; Géophysique [physics.geo-ph]; Géophysique [physics.geo-ph]; Milieux et Changements globaux

Christopher Gomez, Kataoka Kyoko. Internal Structure of the Sambongi Fan - Towada Volcano, Japan: Putting the Theory to the Test, using an Extensive GPR Survey. 2011. ⟨hal-00650834⟩