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We conducted an active seismic experiment in Sakurajima volcano in December 2019, three years after the similar experiment that was conducted in 2016. We deployed 185 temporary seismic stations, most of which were installed at the same place as in 2013 and 2016. Eight and two explosive shots with 20 and 100 kg charges, respectively, were detonated in December 5, 2019. The 2019 shot locations differ from 0.4 to 12 m from the corresponding previous shot locations. We successively observed the all explosions during nighttime nine hours continuous recording. We examine reflection phases on the seismic records that obtained by the previous and 2019 experiment to detect temporal change of subsurface seismic structure beneath Sakurajima volcano.

Additional file 1. Discussion of Peacock et al. (2012)’s and Heise et al. (2007, 2008)’s phase tensor residual ellipse natures in a large phase tensor ellipticity case.

Additional file 3. Comparisons between the electrical resistivity and seismic velocity models. Figure S3.1: Plan or map view of the Vp model by Okada et al. (2015) superimposed on the resistivity model (Fig. 5). The white lines are Vp contours with the interval of 0.1 km/s. The background color represents the logarithmic resistivity value. The black and white circles, red character, and black triangles, are observation sites, sampling point of Vp model, Oana Crater, and edifice summits, respectively. The labels A and B for the conductors are referred to in the main text. Figure S3.2: Vertical profiles of the Vp model by Okada et al. (2015) superimposed on the resistivity model (Fig. 6) in the E-W direction. The white lines and circles are Vp contours and sampling point of the Vp model. Other features are the same as those in Fig. 6. Figure S3.3: Vertical profiles of the Vp model by Okada et al. (2015) superimposed on the resistivity model (Fig. 7) in the N-S direction. Other features are the same as those in Figs. 7 and S3.2. Figure S3.4: Plan or map view of the Vp/Vs distribution superimposed on the resistivity model (Fig. 5). The white lines are Vp/Vs contours with the interval of 0.025. The background color represents the logarithmic resistivity value. The black and white circles, red character, and black triangles, are observation sites, sampling point of Vp and Vs models, Oana Crater, and edifice summits, respectively. The label A and B for the conductors are referred to in the main text. Figure S3.5: Vertical profiles of the Vp/Vs distribution superimposed on the resistivity model (Fig. 6) in the E-W direction. The white lines and circles are Vp/Vs contours and sampling point of the Vp and Vs model. Other features are the same as those in Figures 6. Figure S3.6: Vertical profiles of the Vp/Vs distribution superimposed on the resistivity model (Fig. 7) in the N-S direction. Other features are the same as those in Figures 7 and S3.4.

Additional file 2. Comparisons between the observed and calculated MT, GDS responses. Figure S2.1: Map of site number for Figure S2.2. Figure S2.2: MT apparent resistivity and phase responses of each site. Top left number is site number as is shown in Figure S2.1. Observed responses of each component are represented by each character in the legend. Solid curves represent calculated responses with the final resistivity model (Figs. 5 to 7). Figure S2.3: Observed (left column), calculated (center column), and difference between the observed and calculated (right column) in-phase Parkinson vectors (Equation 11) at each site and various period or frequency. The calculated vector is obtained with the final resistivity model (Figs. 5 to 7). Figure S2.4: Observed (left column), calculated (center column), and difference between the observed and calculated (right column) quadrature-phase Parkinson vectors (Equation 12) at each site and various period or frequency. The calculated vector is obtained with the final resistivity model (Figs. 5 to 7).

Additional file 2: Figure S2.Inflation source during the 2014 unrest of the SPCG from March 2014 to October 2015. Optimal crack parameters are summarized at lower right in the figure. We searched for the best-fit combination of four parameters (Z, L, A, and U), assuming a sill-like source (D = 0°) and L/W = 2. The latitude and longitude of the center of the sill were fixed at 36.645° and 138.538°, respectively, representing the center of Mizugama crater. In this analysis, the smallest misfit (root mean square sum of the difference between the observed and calculated values of each tilt vector, and each uplift at KSYG (44 mm) and KSE (25 mm) measured by GNSS) was calculated by the grid-search method as described in Section “Modeling”. The optimal length (L), width (W), and dislocation (U) were 480 m, 240 m (assumed as half of the horizontal length), and 1.00 m, respectively, which resulted in a calculated volumetric expansion of ~1.2 × 105 m3.

We conducted an active seismic experiment in and around Aira caldera in December 2013, five years after a similar experiment conducted in 2008 in order to detect temporal changes of seismic structure beneath Sakurajima volcano and Aira caldera. We deployed 280 temporary seismic stations, 90% of which were at the same locations used in 2008. Six explosions (S1 to S6) of 200 kg or 300 kg of dynamite were sequentially detonated during the night of 5 December 2013. All of the shot locations expect S3 are less than 60 m from the 2008 locations. We undertook continuous seismic recording for 9 h during the night: the obtained records contain both the waveforms excited by the six shots and those from an explosive eruption and volcanic tremor. To evaluate the changes of subsurface structure beneath Sakurajima volcano since 2008, we calculated cross-correlations of waveforms recorded at all of the recording stations that duplicated those of 2008. The most cross-correlations are equal to or greater than 0.7.

The fifth round of repetitive seismic survey in Sakurajima volcano and a qualitative forward modeling of the annual variation in reflectivity at the depth 4.9km are described. The seismic survey has been conducted on December 2013, as a round of repetitive seismic project since December 2009. The seismic survey includes six shot points and 74 temporary stations. The deviation seismic sections are obtained and the profile shows that there are significant variation in the horizons at 4.9km depth and 8.0km depth. A qualitative forward modeling of the horizon at 4.9km depth is discussed on the basis of the compiled deviation profiles for five years. A sandwich layer model is introduced and its parameter study suggests that the annual variation of the relative reflectivity may explained by decreasing of both density by several percent and velocity down to 80% in the sandwich layer of several tens meter thickness.

The fourth round of repetitive seismic experiment in Sakurajima Volcano on 2012 and the analysis of compiled data set are presented. The repetitive surveys have been carried out since 2009 after the pilot survey on 2008. Two lines, NS and EW were deployed in the eastern foot and the northern flank of the volcano, respectively. The survey lines include 14 shot points and 273 temporary stations, those are the same specification as those of the previous observations. More than 90% of stations are placed at the same points with the previous observations. Data retrieval was successful and 98.5% of stations were retrieved. Reference seismograms are derived through stacking the single-year seismograms and the differential seismograms are also calculated with the subtraction of those single-year seismograms. Differential sections are derived from the differential seismograms. Some systematic changes in reflectivity are found in the sections and deeper changes below 3km in the depth are consistent with development of the edifice inflation. Changes in the shallow part can be correlated with explosions at the crater and can alternate in short cycle.

桜島火山北方の姶良カルデラ下10kmにおいてマグマの蓄積が進行している。2008年11月にこれらの地域において人工地震探査を行った。本調査の目的は(1)姶良カルデラの基盤構造、(2)南岳直下のマグマ供給系の構造、(3)姶良カルデラ下のマグマ溜りから南岳へ至るマグマ供給路明らかにすることである。15点の爆破点から放射された地震波を646台の陸上に設置した地震計と32台の海底地震計により観測した。, In order to make clear (1) basement structure of Aira caldera, (2) magma supply system of magma reservoirs beneath Aira caldera and Minamidake crater and volcanic conduit and (3) magma path from the magma reservoir beneath the Aira caldera to that beneath the Minamidake, seismic exploration using artificial sources was conducted at Sakurajima volcano and Aira caldera, southwest Japan in November 2008. A total of 646 temporal stations equipped with a 2 Hz or 4.5 Hz vertical component seismometer and a portable data logger were deployed at land area. Sea area was covered by 32 Ocean Bottom Seismometers. Dynamite shots with charges of 20-300 kg were detonated at 15 locations.

本報告書中の、桜島火山における反復地震探査(2013年観測)(2014-06) 京都大学防災研究所年報. 57(B): 138-149の最終版はhttp://hdl.handle.net/2433/196150からご覧いただけます。関連リンクをクリックしてください。

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