A seismic experiment was carried out in east Shikoku, Japan, to detect deep reflections across the Median Tectonic Line (MTL), which juxtaposes low-P/T metamorphic rocks with high-P/T metamorphic rocks. Our experiment employed an unconventional technique: sparse shot spacing, a strong energy source (dynamite) and a dense array of seismometers. The above specifications produce only single fold coverage without common midpoint (CMP) stacking. Nevertheless, the reflection profile provides essential information on the deep structure of the MTL, of other major faults, and of the Moho in east Shikoku. On the MTL, this profile is the first to delineate the MTL from the surface to about 12 km depth. The following three factors were essential to the success of our experiment. First, the receiver interval was sufficiently small to provide horizontal resolution that was able to detect deep reflectors. Second, the simple crustal structure does not require CMP stacking to enhance data quality. Third, a thin weathering layer at the surface reduced the attenuation of seismic waves and minimized the generation of the surface waves that often obscure deep reflectors. In these conditions, the technique can be an effective means of probing the deep crust while substantially reducing survey costs.
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.
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