Seismic reflection survey of the crustal structure beneath Unzen volcano, Kyushu, Japan

Abstract

Unzen volcano is located in the western part of Kyushu, Japan. We carried out a seismic re ection survey at Unzen volcano in order to elucidate the structure of the volcano. Although the survey was conducted in a volcanic area under dif cult conditions, such as arti cial noises and a complex structure, we were able to resolve the structure beneath the pro le using vibrator sources and a large number of stacking signals. The processed depth sections con rmed that Unzen volcano developed in a graben structure, as has been suggested in other geological studies. We imaged many subsurface normal faults shallower than 1 km. These faults, mostly covered with volcanic lava and deposits, were identi ed at the surface. Strong re ectors were found at a depth of approximately 3 km. They were located just above the pressure source of the latest eruption, as inferred from geodetic data. The geometric relationship between the re ection image, the pressure source location, and the lava dome suggests that the conduit from the lava dome could connect to the magma chamber located 4 km away from the lava dome.

Magnetotelluric transect of Unzen graben, Japan: conductors associated with normal faults

“…1a and Fig. 8a; Matsumoto et al 2012). The reflectors are interpreted as the top of the magmatic chamber.…”

“…Furthermore, the site distribution in these studies was too sparse to investigate finer structures such as faults and conduits. To address these shortcomings, we performed a broadband MT survey along the same line as the seismic reflection survey by Matsumoto et al (2012). Although the MT data were obtained along a single profile, we performed 3-D inversion and considered topography because of strong 3-D Fig.…”

“…Although the MT data were obtained along a single profile, we performed 3-D inversion and considered topography because of strong 3-D Fig. 1 a Map showing the location of the seismic reflection survey (solid black line) undertaken by Matsumoto et al (2012). Thin black dashed lines represent faults in the graben, and gray circles indicate pressure sources inferred from leveling and GPS data (Kohno et al 2008).…”

“…Thin black dashed lines represent faults in the graben, and gray circles indicate pressure sources inferred from leveling and GPS data (Kohno et al 2008). Figure courtesy of Matsumoto et al (2012). b Broadband magnetotelluric (MT) observation sites.…”

“…Younger lava domes (No Nodake, My Myokendake, Fg Fugendake; Hoshizumi et al 1999) are indicated by yellow triangles features in the MT data and steep topography. Then, we compared the resistivity profile with the seismic reflection profile of Matsumoto et al (2012). Although MT and seismic reflections have been jointly interpreted in tectonically active or volcanic regions (e.g., Unsworth et al 1997;Brasse et al 2002;Comeau et al 2016), correlating resistivity structure with a seismic reflection profile at fine spatial resolution is rare, especially at shallow levels beneath a volcano.…”