Unzen volcano has developed within a graben structure due to an N-S oriented extensional stress field (Hoshizumi et al., 2003). The growth of the graben could have been accompanied by syncline formation and normal faulting. Many normal faults with high dip angles, which are characteristic of a graben structure, are predominant in the target region, as shown in Fig. 8. Generally, the reflective area at depths shallower than 1 km corresponds to the graben structure in the younger stage (< 120 ka) as described in the previous section by the geological study. Many reflectors, however, are deformed just beneath the cones formed in the younger stage. In the shallow section, where the depth ranges from approximately −0.5 km to −0.2 km, the reflectors are undulated and anticlinal beneath the lava domes. The lava domes in the younger stage are distributed in the region between the profile and the most recent lava dome, as shown in Fig. 3. It is reasonable to consider that the conduit contributing to Unzen volcano formation developed in the region around the profile; thus, conduits in both the latest eruption and the younger stage exit there. If magma intrudes into a syncline structure due to graben formation, the basement of the structure would be deformed and a local anticline would develop just above the position of magma intrusion. This feature can be observed in the shallow section in Fig. 8(a); here, the basement structure seems to be deformed due to magma intrusion. However, this intrusion did not occur during the latest eruption, since geodetic analysis did not reveal any large subsurface uplift. Without considering the growth of the lava dome, the maximum vertical deformation of the latest eruption was less than 10 cm, although the height of the local anticline shown in Fig. 8(b) appears to be 0.1 km at most. Therefore, this deformation of the layer could have occurred in the past. Moreover, this deformation might have occurred during past eruptions; this is supported by the presence of multiple dykes and veins in the vicinity of this region, as determined by USDP-4 (Nakada et al., 2005). The undulation of the reflectors can be observed in the 3D velocity structure of this area, as described by Nishi (2002). He obtained the velocity structure from refraction experiments involving artificial explosions in and around our target area. The locations of the reflectors are plotted in Fig. 9 on the velocity structure reported by Nishi (2002).
Four pressure sources under Unzen volcano were detected by Kohno et al. (2008). The sources A and B are within our target depth range, as shown in Fig. 2. The source A, located beneath the latest lava dome, could not be found in the seismic sections in the present study since it is located approximately 2 km east of the profile and could not be detected by seismic waves. In other words, the radius of A must be smaller than at least 2 km. The reflector located at a depth of about 3 km corresponds to the horizontal location of the source B. The depth of this source was estimated to be 4 km and, therefore, there is a difference of 1 km between the depths of the source and the reflector. In their analysis, the pressure sources were assumed to be small spherical regions. The locations of the pressure sources can be considered to be the centers of the pressure sources and do not depend on material properties as long as the medium under consideration is a Poisson solid. Thus, we can assume that the location of B coincides with the center of the pressure source. Because the size of the source could not be estimated, we could not directly compare the results of Kohno et al. (2008) with our reflector distribution. However, it is straightforward to consider that the reflectors correspond to the magma chamber, since the estimated pressure sources activated in the latest eruption contrast strongly, in terms of velocity and/or density, with the background rocks. Therefore, the reflectors could image either the top of the magma chamber, which possibly contributed to the latest eruption, or the water distribution in a hydrothermal system related to the magma chamber. Assuming that the upper edge and center of the magma chamber are located around the reflector and the pressure source, the size of the chamber (i.e., pressure source B) in the vertical direction can be estimated to be approximately 1 km. The width of the chamber is estimated to be approximately 3 km on the basis of the horizontal extension of the reflector. This chamber is comparable in width to that of the Campi Flegrei volcano, as determined by Zollo et al. (2008). They analyzed the reflection records obtained at the Campi Flegrei caldera in detail and, through amplitude versus offset analysis, they showed that the melt zone spread laterally over several kilometres. In the case of Mount St. Helens, where effusion of dacitic lava similar to the case of Unzen volcano was reported, the size of the magma reservoir was approximately 2–3 km, as inferred from the P-velocity structure (Lees, 1992). Magma chambers with similar dimensions were found in Izu and Kilauea (Owen et al., 2000; Toda et al., 2002). Therefore, we can conclude that the magma chamber at Unzen volcano is not extremely large, but standard in size.
We consider a probable model of the magma supply system at Unzen volcano on the basis of the results obtained in this study, and in recent studies conducted by other researchers. A six-year drilling project, USDP, was conducted in the target region, starting April 1999. The trace of the USDP-4 drill hole and the merged deep and shallow sections are shown in Fig. 10. In this figure, the USDP-4 trace, shown in orange, corresponds to the conduit zone reported by Nakada et al. (2005). The conduit zone contains multiple dykes of different ages, including materials deposited during the latest 1990–1995 eruption. The conduit zone is located away from the undulation structure in the reflection section towards the south. The horizontal distance (i.e., in the E-W direction) from the borehole to the profile is approximately 1 km. Therefore, we cannot conclude that the conduit discovered by the USDP-4 is identical to that in our survey because of the difference in the horizontal locations. However, there is a possibility that the conduit found in the drilling project connects to the magma chamber because a method of transporting magma from the reservoir beneath the profile to the lava dome is required.
There is abundant information about the past eruptions of Unzen Volcano. Some of the information can be summarized as follows: (1) Mt. Fugen was formed in eruptions that occurred after 0.5 Ma. (2) Multiple conduits and veins were found near Mt. Fugen during USDP-4. (3) The anticline structure observed was not formed during the latest eruption. (4) The pressure sources B and A were activated during the last eruption. Accordingly, a conduit from B to Mt. Fugen should have existed to feed magma during the last eruption. If the source B existed during the past eruptions and formed the anticline structure, the conduit must also have existed since these past eruptions. We can then consider that the magma spread and ascended from the reservoir B beneath the profile to A. This implies that the path of magma in the last eruption was similar to that in previous eruptions. On the basis of the above features (i.e., (1) to (4)), we can create a qualitative model of the magma supply system at Unzen volcano. This model is schematically shown in Fig. 10.
-
(a)
Magma intrusion into Unzen graben occurred concomitantly with the growth of the graben. This intrusion led to the formation of an anticline structure.
-
(b)
The magma ascended and spread horizontally as a dyke intrusion (yellow part in Fig. 10). Some part of the magma reached Mt. Fugen and erupted.
-
(c)
During the latest 1990–1995 eruption, the magma supplied from pressure source B to the lava dome via pressure source A followed a similar path to that of past eruptions. This model suggests that the conduit should connect B to A. In this case, the shortest path to A from B is in the eastwards direction.
The magma chamber and the layered structure beneath the Mid-Atlantic Ridge were found by Singh et al. (2006). The structure is similar to that obtained in the present study. The reflection from the top of the magma chamber occurs approximately 3 km beneath the sea floor; the layered structure also has the characteristics of a graben. Singh et al. (2006) observed the axial valley bounding faults that appeared to penetrate down to the depth of the magma chamber. The characteristics of the faults is similar to those observed in our result. Therefore, the graben structure obtained in this study can be considered typical for a volcano growing in an extensional field, even though the compositions of the rocks at Unzen volcano and at mid-ocean ridges are different (namely dacitic in the case of the former and basaltic in the case of the latter).