Seismic reflection survey of the crustal structure beneath Unzen volcano, Kyushu, Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2012
Received: 8 July 2010
Accepted: 21 November 2011
Published: 28 June 2012
Unzen volcano is located in the western part of Kyushu, Japan. We carried out a seismic reflection survey at Unzen volcano in order to elucidate the structure of the volcano. Although the survey was conducted in a volcanic area under difficult conditions, such as artificial noises and a complex structure, we were able to resolve the structure beneath the profile using vibrator sources and a large number of stacking signals. The processed depth sections confirmed 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 identified at the surface. Strong reflectors 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 reflection 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.
Kohno et al. (2008) proposed a model of the pressure sources in the case of the latest eruption and the post-eruption deformation, using geodetic data such as leveling survey data. They observed four pressure sources beneath Unzen volcano and showed that magma ascended from beneath Chijiwa Bay (west of Shimabara Peninsula) to the crater created by the eruption. The depths of the sources increased towards the west from the lava dome. Umakoshi et al. (2001) determined the cut-off depth of the earthquake distribution from an E-W cross-section and observed that the average depth of the distribution decreased eastwards towards the lava dome. They also determined the variations in the focal mechanism with depth; these variations suggest the presence of high-temperature, ductile, and low-Q, bodies below the cut-off depth. They reported that the seismic activity started beneath Chijiwa Bay, migrated eastwards, and became extremely high just below the lava dome before the eruption. This phenomenon suggests that magma ascended to the lava dome from Chijiwa Bay. The relationship between the seismic activity and the magma sources is shown in Fig. 2.
These above-mentioned studies succeeded in outlining the 1990–1995 eruption and the structure of Unzen volcano. However, the mechanism of the latest eruption and the formation of the Unzen graben are not yet clearly understood. For instance, Kohno et al. (2008) suggested that one of the pressure sources that supplied magma exists in a very shallow region beneath the volcano. This might have had a strong effect on the medium around the pressure source. The effect could be detected as a heterogeneity in the medium, which can prove the existence of the magma body. In addition, subsurface deformation of the layered structure can provide us with valuable information on the growth of the graben and the intrusion of magma. Seismic reflection data can be used to detect the fine velocity contrast beneath the volcano, since the reflection phases are more sensitive to small-scale heterogeneities than are the refracted and/or direct waves. In this study, we have conducted a seismic reflection survey to help understand the subsurface structures of Unzen volcano.
The structures of volcanic regions in oceans have been described in detail by means of reflection surveys (Suzuki et al., 1992). For example, Collier and Sinha (1990), and Singh et al. (2006), detected magma chambers at mid-ocean ridges using reflection surveys. They obtained fine reflection images of the magma chambers, faults, and the structure of the graben formed by crustal accretion at the ridge. In contrast, reflection surveys have rarely been conducted in volcanic regions on land because of the many difficulties posed by factors such as artificial noise and complex geological structures (e.g. lava and volcanic deposits). Despite these difficulties, a seismic reflection survey has been conducted in this study to obtain detailed images of the structures beneath Unzen volcano.
2. Reflection Survey
We conducted a seismic reflection survey to image the subsurface structures beneath Unzen volcano in the period December 10–28, 2001. We set an approximately 15-km-long survey line located 2 km to the west of the lava dome of the latest eruption. Figure 1 shows the locations of the reflection profile and active faults around Unzen volcano (The Research Group for Active Faults of Japan, 1991). The reflection profile cross-cuts the active faults and traverses the Unzen graben from north to south. The detailed reflection profile and the locations of the common midpoint (CMP) line considered in the present reflection analysis are shown in Fig. 3. Analysis has shown that reflectors are present just beneath the CMP line. The region around Unzen volcano is covered with lava and pyroclastic deposits (Hoshizumi et al., 1999, 2003), both having andesitic to dacitic composition. To the east of the profiled region are distributed (1) the lava domes of the volcanic edifices formed during the younger stage, (2) the lava domes of the latest eruption (approximately 2 km east of the region in the reflection profile), and (3) the flow and deposits of the latest eruption. The geological data indicate a relatively older lava flow and dome along the profile—shown by “300–150 ka lava dome/flow” in Fig. 3—than that observed in the eastern part.
Parameters for reflection survey.
Sweeps per point
34 (5 for shallow survey)
50 m (avr.)
201 (80 for shallow survey)
10 Hz vertical
16 s (after correlation)
3. Data Processing
Parameters for deep and shallow section analysis.
Band-pass filter (Hz)
Deconvolution filter operator length (s)
CMP stack offset limit (m)
Interval of velocity analysis (m)
fx prediction filter operator length (trace)
3.1 Data processing for the deep section
3.2 Data processing for the shallow section
4. Interpretation of Seismic Section
We imaged many strong reflectors with a heterogeneous distribution in the deep section (Fig. 7(a)), and observed several distinctive features. The prominent reflectors are found above sea level and have a deflecting shape and horizontal segmentation structure, as shown in Fig. 7(b), suggesting the existence of many faults in this region. The dip angles of many of the faults beneath the profile are greater than 60°. These faults are recognized either in sections in which the reflector disappears, or in sections in which there are gaps in the longitudinal direction of the reflector. Most of the faults are classified as normal faults on the basis of the relationship between the direction of dip of the fault and the direction of subsidence of the reflector. The existence of these normal faults suggests that this area was transformed into a graben under a tensile stress field, as suggested by previous geological studies (e.g., Hoshizumi et al., 2005). Some of the fault traces coincide with active faults at the surface (see Figs. 1 and 3). The largest active fault in this area, called the Chijiwa fault, is observed in a seismic section as a reflector gap over a wide depth range (i.e., from 1 to 1.5 km at least). In this study, active faults as well as many subsurface faults are observed at the surface.
The lower depth limit of the reflective layer, shown in Fig. 7(b) as a dashed line, becomes shallower with increasing distance from the center of the profile. The depth of the lower limit varies from about 1.5 km below sea level to about 0.3 km above sea level. This feature is consistent with a typical graben structure and supports the geological model that Unzen volcano is located at the center of the Unzen graben. The shape traced by the dashed line in Fig. 7(b) is similar to the shape of the basement of the Unzen graben as derived in the geological studies of Hoshizumi et al. (1999, 2005). They estimated the basement structure from a geological survey, and well-log data, and suggested that the basement formed by Unzen volcanic activity after 0.5 Ma is about 1.0-km deep in the eastern and western parts of Shimabara Peninsula. Below the depth indicated by the dashed line in Fig. 7(b), the reflectors disappear, indicating that the layer formed before 0.5 Ma is relatively homogeneous. The depth of the basement, as determined in the present study, is a few hundred meters greater than the depth of the basement as determined in the geological study. This could be due to the uncertainty in the determination of the basement depth in the geological study. Another possible reason is that, in the geological study, the well-log geological cross-section was determined for the eastern and western parts of Shimabara peninsula (i.e., away from the profiled region) and not around the region profiled in the present study.
Strong reflectors can be observed at a depth of about 3 km towards the north of the lava dome. These reflectors were spread over approximately 4 km in a direction parallel to the profile (i.e., N-S). The arrival of many strongly reflected phases suggests that the structure at a depth of 3 km is complex and is composed of layers with strong impedance contrasts. Kohno et al. (2008) estimated the locations of the pressure sources activated by the latest eruption from geodetic data using a point source assumption. According to their results, the source B is located just beneath these reflectors (Fig. 7(b)). Source B is not located directly beneath the lava dome; rather, it is located to the north of the dome just beneath the profiled region.
In the shallow section, some detailed structures are observed directly beneath the center of the profile. The shallow section, and its interpretation, are shown in Fig. 8. Reflectors that are subparallel to the surface can be found in the shallowest part of the section. Beneath these reflectors, a vertical undulation of reflectors between depths of about −0.5 km and −0.2 km is noticeable (e.g., reflectors between the red line and the dashed blue line in Fig. 8(b)). The location of this undulation in the N-S cross-section is just above the pressure source B, estimated from the geodetic study, and is beneath the latest lava dome. Many faults with a high dip angle are seen in these undulated reflectors. The type of these faults is considered to be a normal fault due to the extension force contributing to the growth of the Unzen graben. Similarly, the fact that the faults have high dip angles suggests that the normal faulting may have occurred due to a kind of intrusion accompanied by anticline formation of the stratified layer.
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.
Magma intrusion into Unzen graben occurred concomitantly with the growth of the graben. This intrusion led to the formation of an anticline structure.
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.
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).
The Unzen graben is dominated by normal faults with dips greater than 60°, suggesting the existence of a subsurface graben structure.
Strong reflectors occur at depths of approximately 3 km. This could be related to the material from the magma body based on the geometrical correspondence between locations of the reflectors and of the magma body deduced from geodetic study.
The reflecting zone continues up to sea level. The base of the zone appears to correspond to the basement of Unzen volcano inferred from geological studies.
The undulation structure was formed by an activity related to source B.
The small change in the volume of source B in the latest eruption could not deform the subsurface layer. Therefore, the magma intrusion that led to the formation of source B did not occur during the latest eruption but during a past event.
Source B might be approximately 3 km in length and 1 km from the center of the source to the top according to the shape of the reflector and the relation between the source and the reflector.
Magma supply during the latest eruption from source B to source A, as suggested in previous geodetic studies, could not be found in any image of the conduit from source A to B.
We wish to thank all the staff and students of the Institute of Seismology and Volcanology, Kyushu University, who helped in carrying out this survey. We are grateful to the anonymous reviewers for their insightful and critical discussion of this paper. We also thank Drs. Uraki and Higashinaka of the Japex Geoscience Institute for their efforts in carrying out the reflection analysis. We are grateful to Dr. Umakoshi for his valuable comments and discussions and for giving us access to hypocenter data. This study was supported by Unzen Scientific Drilling Project, funded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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