Contexts in source publication

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... Aira caldera is located in the Kagoshima graben of southern Kyushu, Japan, and is one of the major calderas in the region (Fig. 1). The Aira caldera-forming eruptions (Volcanic Explosivity Index (VEI) = 7) occurred approximately 30 ka BP (Okuno 2019;Geshi et al. 2020). The approximate dimensions of 14 km × 14 km extend across ~ 200 km 2 , and its present topographic outline is characterized by steep caldera walls. The postcaldera activity began approximately 26 ka ...

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... × 6 km had a submarine eruption ~ 13 ka BP, is situated in the northeastern part of the Aira caldera ( Moriwaki et al. 2017;Geshi et al. 2020). The area in and around the Aira caldera is widely covered by Quaternary pyroclastic ows and volcanic rocks. Pleistocene sediments, tuff, and tuff breccia can be found under these surface pyroclastic ows ( Fig. 1). Along the eastern inland area of the caldera, Upper Cretaceous Shimanto Group strata, consisting of sandstone and mudstone alterations, are widely distributed and regarded as the geological basement in southern Kyushu. Presently, investigations of the geological structure in the sea area are limited, but the Shimanto Group is ...

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... the RRT, synthetic travel times were calculated using the synthetic model (upper panels in Fig. 10a and b) obtained by inverting the observed data. Synthetic travel times were then inverted to obtain a restored model by tomography (lower panels in Fig. 10a and b). A comparison between the synthetic and restored models demonstrated that the main velocity trends and characteristic structures in both cell models were ...

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... the RRT, synthetic travel times were calculated using the synthetic model (upper panels in Fig. 10a and b) obtained by inverting the observed data. Synthetic travel times were then inverted to obtain a restored model by tomography (lower panels in Fig. 10a and b). A comparison between the synthetic and restored models demonstrated that the main velocity trends and characteristic structures in both cell models were ...

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... Subsurface region characteristics (0-5 km depth) Figure 11 shows the reection section image along the western half of Prole-B by applying common midpoint reection stacking and poststack migration (Yilmaz 2001). Because of the relatively short length of the 500 m streamer cable (40 channels), the reliable depth range was limited to 2-3 km. ...

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... Marked reection events occurred at depths of 0.5-0.7 km, depicting the top of the Kekura layer. The strong reection events at 1.0-1.4 km depths represent a geological boundary between the Kekura layer and the Shimanto Group basement, which is distributed widely around the Aira caldera. Some vertical faults < 1 km in length (white broken lines in Fig. 11) were also recognized in the present study at depth ranges of 1-2 ...

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... and revealed a surface sedimentary layer with a P-wave velocity of 2.3-2.8 km·s − 1 ~1 km in thickness, as well as a LVZ with a Pwave velocity of 4.2-4.4 km·s − 1 over a depth range of 1.4-3.2 km in the center of the Aira caldera. This surface sedimentary layer and LVZ correspond well with the horizontally stratied sedimentary layer shown in Fig. 11, in addition to the shallow, thick, low-velocity region in the AT caldera indicated in Comparatively, this contour line gradually become shallower to the east and reaches 2 km in depth at the eastern end of the caldera. The western end of the Aira caldera rim is sharply dened just west of SP6, while its eastern end remains rather ...

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... the shallow low-velocity region (Fig. 12) over depth ranges < 5 km, and the HVZ ranging 6-11 km depths (Fig. 13) in the present model were not observed in Tameguri et al. (2022), which was likely related to the insucient resolving power of ner anomalies due to their lack of seismic stations in the Aira caldera sea area. Figure 15 shows the distribution of the Japanese-unied hypocenters provided by the Japan Meteorological Agency occurring in and around the Aira caldera from 2000 to 2019. These hypocenters were classied into two categories: volcano-tectonic (VT) and low-frequency (LF) earthquakes. ...

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... the VT epicenters (Fig. ...

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... VT earthquakes occurred at depth ranges of < 11 km, and the LF earthquakes were primarily distributed across depths of 17-30 km. From the distribution of the VF and LF earthquakes, an apparent aseismic region was distributed over a depth range of 12-18 km (Fig. 15b), corresponding strongly to the deep LVZ observed in this study (Fig. 14). Alternatively, VT earthquakes rarely occurred in or around the HVZ. As discussed in Section 5 − 3, this fact is important when understanding the role of the HVZ for the magma transportation path. The vertical distribution of LF earthquakes deeper than 18 km may ...

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... observed in this study (Fig. 14). Alternatively, VT earthquakes rarely occurred in or around the HVZ. As discussed in Section 5 − 3, this fact is important when understanding the role of the HVZ for the magma transportation path. The vertical distribution of LF earthquakes deeper than 18 km may be interpreted as magma ascent in the lower crust (Fig. ...

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... deep LVZ at approximately 15 km depth (the area enclosed by a white solid curve in Figure. 17) is in nearly the same location as the aseismic region in Fig. 15 and is located near the high Vp/Vs region detected by Tameguri et al. (2022). These facts suggest that the deep LVZ may represent the magma reservoir, hereafter referred to as the deep magma reservoir. (Fig. 15). Nonspherical pressure sources were proposed by Hickey et al. (2016), who identied an oblate pressure source at a depth of 13 ...

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... 15 km depth (the area enclosed by a white solid curve in Figure. 17) is in nearly the same location as the aseismic region in Fig. 15 and is located near the high Vp/Vs region detected by Tameguri et al. (2022). These facts suggest that the deep LVZ may represent the magma reservoir, hereafter referred to as the deep magma reservoir. (Fig. 15). Nonspherical pressure sources were proposed by Hickey et al. (2016), who identied an oblate pressure source at a depth of 13 km in the northeastern Aira caldera by applying nite element analysis to geodetic GNSS data from 1996 to 2007. Notably, a portion of the deep magma reservoir in the present model was included within their oblate ...

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... magma transportation from the inclined solidied magma reservoir to the Sakurajima area was proposed. For example, Hotta et al. (2016) mentioned that the magma in pressure source-A moved to shallow pressure source-K (Fig. 15), which is located at a depth of 3.3 km in the northern Sakurajima volcano; however, they could not dene a detailed magma transportation path between the two pressure sources. Comparatively, the present study can reveal the detailed velocity model in the Aira caldera along the E-W section; however, no information was obtained regarding ...

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... in the Aira caldera was distributed concentrically around the center of the caldera (Yokoyama and Ohkawa 1986;Yamamoto and Shichi 2004). Therefore, it is reasonable to assume that the underground structure had a general concentric geometry about the center. Accordingly, if this solidied magma also extended in the N-S direction (as shown in Fig. 15a), and pressure source-K was situated near its southern end, the spatial relationship may have led the magma to move along the lower boundary of the solidied magma reservoir and ascend vertically beneath pressure ...

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... the present velocity model, the mean thickness of the solidied magma reservoir was estimated to be 2 km. Then, the resultant volume of the cylindrical magma reservoir of the solidied magma reservoir was 140 km 3 (Fig. 19), and the pyroclastic ejecta from the AT eruption was estimated to be ~ 350 km 3 DRE (Ueno 2007), corresponding to the 5 km-thick cylindrical magma reservoir. As a result, the total thickness of the cylindrical magma reservoir was estimated to be 7 km, while the total volume of the AT magma reservoir was estimated to be 490 km 3 DRE. ...

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... the AT eruption ( Kobayashi et al. 2013). The diameters of the spherical magma reservoirs corresponding to the P14, and Taisho eruptions were ~ 3 and ~ 1.5 km, respectively. Considering the spatial resolution in this study, a spherical magma reservoir with a diameter of > 3 km in the upper crust was expected to be detected in the present model (Fig. 19). Therefore, there was no magma reservoir with a diameter > 3 km; thus, the present probability of a VEI = 6 class eruption is likely small. For a smaller reservoir with a diameter < 2 km, the present seismic data do not have enough resolving power, particularly in the deeper (> 5 km) crust; however, if such a small reservoir is in a ...