IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS007 Poster presentation 1976
Self-potential anomalies around Ontake volcano, Central Japan - the
earthquake swarm and summit areas -
Dr. Ryokei Yoshimura
Disaster Prevention Research Institute Kyoto University IAGA
Ken'Ichi Yamazaki, Yasuaki Okada, Naoto Oshiman, Makoto Uyeshima
Ontake stratovolcano is located in the southern end of the Norikura Volcanic Chain, central Japan, close
to the junction of the Izu-Bonin and Mariana and Southwestern Japan volcanic arcs. It is almost conical
and made of andesite. Earthquake swarm activity has been continuously observed around the eastern
flank of Ontake since 1976. A phreatic explosion occurred in 1979 at a fissure on the southwestern
slope of the Kengamine, the main peak of Ontake. And a large earthquake with the depth about 2 km
and a magnitude of 6.8 occurred in 1984 in the southeastern flank of the volcano. Recently, Kimata et
al. (2004) revealed uplift ground deformation above the earthquake swarm area by using repeated
leveling. Furthermore, Magnetotelluric soundings estimated a low resistivity region with the depth about
2km beneath the uplift area [Kasaya et al., 2002]. In order to investigate a relationship between
tectonic movements and subsurface low resistivity zone, we carried out self-potential (SP)
measurements from 2003 and 2006 around the earthquake swarm and the summit areas of Ontake
volcano. As the result of SP measurements around the seismic swarm area, a torus-shape positive SP
anomaly has been detected at the eastern part of survey profile. This anomaly is located between
recent active clusters of earthquakes and near the ground uplift detected by Kimata et al. [2004]. They
suggest that the uplift is associated with a region of low resistivity [Kasaya et al. 2002] and anomalous
increases in chemical compositions of springs [Tanaka et al. 2003], imaging potential shallow
hydrothermal activity. Generally, an upflow caused by a hydrothermal convection produces positive
current sources in the direction of flow. The comprehensive positive sense anomaly supports potential
shallow hydrothermal activity. Recently, we established a continuous SP observation network with the
aim of monitoring the hydrotheral activity by reference to the obtained SP distribution. This network
uses metallic telephone lines for measuring SP with 1 sec sampling. We also found the positive sense
anomalies up to about 2V p-p around the northern part of summit area. This large anomaly may be not
irrelevant to recent eruption vents. Younger vents are located near the positive anomalies. In this
presentation, we will report a detail of SP results and introduce outline of continuous SP observation
network which has been started since late February, 2007.
Keywords: self potential, ontake volcano, earthquake swarm