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Transcription

1 km : step.5km 2km Tameguri Ishihara, 199 Ishihara1985 et al., Uhira and Takeo, P Rayleigh cavity P cavity 2km

2 Sakurajima KAB KOM N 51-5 m/s V P D LP HAR SVRC HIK A Minamidake B crater SBT ARI KUR 1 2 3(s) 51-5 m/s R P LP Fig. 1 Location map of air-shock and seismic stations. Air-shock is observed at SVRC. Solid circles denote permanent borehole stations. Open squares indicate broadband seismometers installed temporarily m/s T D 3(s) 1 2 3(s) Fig. 2 An example of velocity waveform of explosion earthquake occurred at 4:55 on September 3, The 5.6km three traces represent vertical (V), radial (R), and transverse SVRC (T) components, respectively. 1Hz Fig1 P STS-2 JCP km D.2-12 P D GPS P PD 1Hz LP Rayleigh LP Rayleigh Fig2 HAR P P LP 1 D P 2 2 LP PD P LP Rayleigh Table1 Hisada P 2.5km/s Kikuchi and Kanamori (1991)

3 Table 1. Ranges of search and increment of parameters for waveform analyses. Source parameter Epicenter Depth Rise time of source time function Range X,Y *: Fixed Z *: Fixed P phase Increment Range X,Y : Fixed Z : Fixed D phase Increment Range X,Y : Fixed.51km 13km LP phase Increment.5km.1km.51s.5s.51s.5s.55s.5s Source duration.12s.5s.12s.5s.11s.1s Origin time T *: Fixed T T +2s.5s T T +4s.1s * X, Y, Z and T are hypocenter and origin time calculated from arrival times of P-wave first motions. D P P dipole dipole 2.2 Chouet, 1985 PD D P 1 15 Tameguri et al., 21 1 LP P D LP 15 Fig4 LP 1 Fig5 2km P D Fig.3km Fig3 Fig3 PD 1 PD LP1 1 P LP2 dipole 1 dipole dipole 1

4 1 13 Nms -1 P. M P xx = Nm D. M xx = Nm.4s M yy = 5.7 M yy = -5.4 M zz = 5.8 M zz = -2.8 D M xy =.4 M xy =.4 M xz =-.5 M xz = -.3 M yz =-.4 M yz = s ARI V R T 51-6 m/s P D P D P D HAR SBT KAB KUR KOM (s) (s) (s) Fig. 3 An example of result of the waveform inversion for the P and D phases. The explosion earthquake occurred at 4:14, September 15, Waveforms of vertical (V), radial (R), and transverse (T) components are plotted from left to right. Two triangular source time functions P and D corresponding to the P phase and following D phase are displayed in the upper left. Bars with symbols P and D 5111Nms -1 LP1. M xx = Nm LP2. M xx = Nm M M 1.3 s yy = 2.8 yy = -3. M zz = 2.7 M zz = -.3 LP1 M xy =-.2 M xy = s M xz = -.2 M xz =.3 LP2 PD 4 (s) M yz =.1 M yz =-.1 V R T 21-5 m/s HIK LP1 LP2 ARI HAR KUR KOM 4 8(s) 4 8(s) 1(s) 4 8(s) 1(s) Fig. 4 An example of result of the waveform inversion for the LP phase. Triangular source time functions LP1 and LP2 corresponding to the LP1 phase and following LP2 phase are displayed in the upper left. P and D in the time axis of source time function represent origin times. Bars with symbols LP1 and LP2 under the waveform represent time window of waveform inversion. under the waveform represent time window of waveform inversion. LP1.6.5 LP PD 2km.3 LP km Depth from crater bottom (km) LP km Fig km km Depth from crater bottom (km) LP1 Residual LP2 Residual Fig. 5 Relation between source depths and residual.

5 crater bottom s (km) shallow source.2-.5s deep source.6-1.2s 1 12 Nm/s s s horizontal dipole vertical dipole SVRC S A2 S B2 crater rim S A2 S B2 S B1 A S A1 S A1 A crater B B S B1 crater rim 95m 9m A B 1 m N (s) Fig. 6 Time sequence of origin times and depths of 4 sources generating the P, D, and LP phases. Solid and broken lines indicate source time functions of horizontal and vertical dipoles, respectively. SVRC S A1 = 37m S B1 = 6 m S A2 =529 m S B2 =532 m Fig. 7 Schematic figure for calculation method of path of air-shock (Minakami, 197). S A1 and S B1 represent path from vents to crater rim of direction of the station. S A2 and S B2 represent path from crater rim to the station SVRC. The path of the air-shock from crater to the station is calculated by adding S 1 to S 2. Fig 1 7 AB 4 Minakami et al.197 Fig Fig7 9 phase Ishihara, 1985 Table 2 2km km Sekine et al., 1979 gas bubble 2km 2km gas bubble Fig 8 gas bubble gas flow 2km km

6 Table 2. Origin times of air-shock and each phase of explosion earthquakes. Date Vent v s (m/s) t a T a T 1 (T a - T 1 ) Nov. 16, 1999 B :6: :6: :6:29. (.9) Nov. 21, 1999 A :22: :22: :22:38.1 (1.3) Dec. 12, 1999 A :49:1.7 16:48: :48:52.3 (1.3) Dec. 17, 1999 B :3: :3: :3:32.5 (1.1) T 2 (T a - T 2 ) 11:6:29.3 (.6) 15:22:38.3 (1.1) 16:48:52.5 (1.1) 15:3:33. (.6) T 3 (T a - T 3 ) 11:6:3. (-.1) 15:22:39.2 (.2) 16:48:53.3 (.3) 15:3:33.6 (.) T 4 (T a - T 4 ) 11:6:31.3 (-1.4) 15:22:41. (-1.6) 16:48:54.9 (-1.3) 15:3:34.9 (-1.3) v s : Velocity of sound after atmospheric correction t a : Arrival time of air-shock at SVRC T a : Origin time of air-shock T 1 : Origin time of isotropic expansion at a depth of 2km T 2 : Origin time of cylindrical contraction at a depth of 2km T 3 : Origin time of isotropic expansion at shallow part T 4 : Origin time of horizontal contraction at shallow part Amplitude of air-shock (Pa) Amplitude of air-shock (Pa) Seismic moment of deep isotropic expansion ( 1 11 Nm) Seismic moment of shallow isotropic expansion ( 1 11 Nm) Fig. 8 Relation between the amplitude of air-shock and the seismic moment of deep isotropic expansion (left) and shallow isotropic expansion (right). gas bubble km/s 2km gas flow 2km km/s Murase and McBirney, 1973

7 air-shock (s) crater bottom s s Depth 1 2 shallow source deep source s s km/s 1 12 Nms s 4 Vertical Dipole Horizontal Dipole 3 (km) 2 air-shock gas lava dome gas pocket 3 4 pressure wave 1 2 gas flow (a) (b) (c) (d) Fig. 9 Time sequence of origin times and depths of 4 sources generating the P, D, and LP phases is shown in upper part. Mechanics of explosive eruption is schematically shown in lower part. 2km gas bubble 198

8 23B-1 and T. Utsunomiya (197), Seismometrical studies of pp Chouet, B. (1985), Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor, J. Geophys. Res., 9, pp Hisada, Y. (1994), An efficient method for computing Green s functions for a layered half-space with sources and receivers at close depths, Bull. Seism. Soc. Am., 84, pp Ishihara, K. (1985), Dynamical analysis of volcanic explosion, J. Geodyn., 3, pp Ishihara, K. (199), Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakurajima volcano, Japan, in Magma Transport and Storage, edited by M. P. Ryan, pp , John Wiley, New York. Kikuchi, M. and H. Kanamori (1991), Inversion of complex body waves-, Bull. Seismol. Soc. Am., 81, pp Volcano Asama, Part 1. Seismic and volcanic activities of Asama during , Bull. Earthq. Res. Inst., 48, pp Murase, T and A. R. McBirney (1973), Properties of some common igneous rocks and their melts at high temperatures, Bull. Geol. Soc. Am., 84, pp Sekine, T., T. Katsura, and S. Aramaki (1979), Water saturated phase relations of some andesites with application to the estimation of the initial temperature and water pressure at the time of eruption, Geochim. Cosmochim. Acta., 43, pp Tameguri, T., M. Iguchi, and K. Ishihara (21), Reexamination of moment tensors for initial motion of explosion earthquakes using borehole seismograms at Sakurajima volcano, Japan, Earth Planets Space, 53, pp Uhira, K., and M. Takeo (1994), The source of explosive eruption of Sakurajima volcano, Japan, J. Geophys. Res., 99, pp Minakami, T., S. Utibori, S. Hiraga, T. Miyazaki, N. Gyoda, Mechanics of Explosive Eruption at Sakurajima Volcano Relation between Source Process of Explosion Earthquake and Air-shock Takeshi TAMEGURI, Masato IGUCHI and Kazuhiro ISHIHARA Synopsis Source process of explosion earthquakes that accompany explosive eruptions at Sakurajima volcano, was investigated to clarify mechanical process of explosive eruption. The results of source process showed that the explosion earthquake was initiated by an isotropic expansion and a cylindrical contraction at a depth of 2km beneath the crater bottom. A shallow isotropic expansion at depths of.25-.5km beneath the crater bottom was followed by the explosion earthquake after.9-1.1s. The origin time of shallow isotropic expansion coincided with generation of air-shock at the crater bottom and seismic moments of shallow expansion source were correlated with the amplitudes of air-shock. It is inferred that the isotropic expansion is caused by instantaneous pressure increase in the gas pocket formed at the uppermost part of the volcanic conduit, which generates air-shock Keywords: Sakurajima volcano; explosive eruption; explosion earthquake; air-shock; gas pocket