地震波速度構造と震源メカニズムによる深部低周波地震の発生機構―岩手山と富士山の例―

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(2) ½ ½ F Y » ¼. Source Mechanism and Seismic Velocity Structure of Source Region of Deep Low-frequency Earthquakes beneath Volcanoes : Case Studies of Mt. Iwate and Mt. Fuji Haruhisa NAKAMICHI. + . &'()*+,-. &/012. 3)45/6*78-. &9. , :;< -. &9=>?@ABC. >DE F+G HI& E&J. '&'(K. L/!MN*7 -. &HO"/P#7Q. E&'(KR, 3)45*S$T&. ;< -. &HI& . U4/%V78WQXY

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(45) }~ Fig. ,. Epicenters and cross-sections of hypocenters of intermediate-depth and deep low-frequency earthquake (solid circles) and high-frequency earthquakes (light grey circles) of Mt. Iwate in the period from December +331 to June ,***. ‘A’, ‘B’ and ‘C’ show groups of deep low-frequency earthquakes. ‘I’ shows a group of intermediate-depth low-frequency earthquakes. The focal mechanisms of the largest deep lowfrequency earthquake (M +.3) and the largest intermediate-depth low-frequency earthquake (M +.0) are also shown. Lower focal hemispheres are shown in equal-area projection, with compressional fields shaded. Open and solid circles on the source mechanism indicate inward motion and outward motion, respectively. The solid star marks the initial rupture point of the M 0., earthquake. The dashed rectangle shows the focal area of the M 0., earthquake. The locations of very-long-period events (Nishimura et al., ,***), dikes and Mogi sources estimated from GPS data (Miura et al., ,***) are also shown. The Conrad and Moho depths are shown with solid lines (Nakajima et al., ,**,). Elevation contours (dotted lines) are at -** m intervals. The cross-section is free of any vertical exaggeration.. P 0 S

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(70) )*+,. 218. Fig. -. Hypocentral distributions (black dots) of deep low-frequency earthquakes of Mt. Fuji. (a) Map view. The source mechanism of the largest deep low-frequency earthquake (M ,.-) is also shown. Lower focal hemisphere is shown in equal-area projection, with compressional fields shaded. Open circles on the source mechanism indicate inward motion. (b) Cross-seciton A-A῍ in (a). (c) Cross-section B-B῍ in (a). Crosssections A-A῍ and B-B῍ include hypocenters projected from +* km either side of the cross-section line.. -./01#234156789 :. !;<=# Nakamichi et al. (,**-) > ?@AB.

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(99) ij#O. Fig. .. S wave velocity perturbations obtained by the receiver function analysis, which are projected onto the south-north cross-section at a longitude of +.+ E, crossing Mt. Iwate. Velocity perturbations are relative the standard velocity model of Tohoku district. Projection is along ray paths of P-to-SV converted waves. Stars show locations of intermediate-depth and deep low-frequency earthquakes. Red triangles denote the location of Mt. Iwate.. POkl YE8YEL#m!V,EL# nY "o /pq#pq$(rCT<0 12. 3 $%& ,**/ . $ , '()*+,- .. s#Ct%S--[ + u + uT<%YE /. &' /pqvw LE Nakamichi et al. (,**1). 0 12!C],xyzc%S- ].) T. () (,**1) C=<7{| / 0 12. <iLE&' /pq# *}V, ~c6

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(145) !2345*0678 7 0 9:;<=/0()>?@0A BCDEF 23 & G HIJ !F23K LMN OP,QR STKUM+VW =A760 &XYF23LZ[KU6+\ ] ^_`K\*0 a6+45*0678 () >?V*+ b"()cd()ef g hi +.*,*+.1 Sb"()cdj() klm `nop ghi +22.**,/ =q()Kr6 760 a6+45*0678. ῌ ῏ ῎ ῍ Aki, K. and Koyanagi, R. (+32+) Deep volcanic tremor and magma ascent mechanism under Kilauea, Hawaii. J. Geophys. Res., 20, 1*3/ῌ1+*3. (|)HIJ (,***) }~*60 ' ./ ,3/ῌ,33. Hasegawa, A. and Yamamoto, A. (+33.) Deep, lowfrequency microearthquakes in or around seismic lowvelocity zones beneath active volcanoes in northeastern Japan. Tectonophys., ,--, ,--ῌ,/,. Hasegawa, A., Zhao, D., Hori, S., Yamamoto, A. and Horiuchi, S. (+33+) Deep structure of the northeastern Japan arc and its relationship to seismic and volcanic activity. Nature, -/,, 02-ῌ023. Knopo#, L. and Randall, M. (+31*) The compensated linear-vector dipole : a possible mechanism for deep earthquakes. J. Geophys. Res., 1/, .3/1ῌ.30-. McNutt, S.R. (+33.) Volcanic tremor from around the world : +33, update. Acta Vulc., /, +31ῌ,**. Miura, S., Ueki, S., Sato, T., Tachibana, K. and Hamaguchi, H. (,***) Crustal deformation associated with the +332 seismo-volcanic crisis of Iwate Volcano, northeastern Japan, as observed by a dense GPS network. Earth, Planets and Space, /,, +**-ῌ+**2. Nakajima, J., Matsuzawa, T. and Hasegawa, A. (,**,) Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Phys. Earth Planet. Int., +-*, -+ῌ.1. &8´µ ,**1 ;¶·o¸¹º» z '

(146) ' «5¬(6®& ¯ 7 ° ± 89 ² : u³¼b"()½ +/+ῌ +/3. Nakamichi, H., Tanaka, S. and Hamaguchi, H. (,**,) Fine S wave velocity structure beneath Iwate volcano, northeastern Japan, as derived from receiver functions and travel times. J. Volcanol. Geotherm. Res., ++0, ,-/ῌ,//. Nakamichi. H., Hamaguchi, H., Tanaka, S., Ueki, S., Nishimura, T. and Hasegawa, A. (,**-) Source mechanisms of deep and intermediate-depth low-frequency earthquakes beneath Iwate volcano, northeastern Japan. Geophys. J.. 221. Int., +/., 2++ῌ2,2. Nakamichi, H., Ukawa, M. and Sakai, S. (,**.) Precise hypocenter locations of midcrustal low-frequency earthquakes beneath Mt Fuji, Japan. Earth, Planets and Space, /0, e-1-e.*. Nakamichi, H., Watanabe, H. and Ohminato, T. (,**1) Three-dimensional velocity structures of Mount Fuji and the South Fossa Magna, central Japan. J. Geophys. Res., ++,, doi : +*.+*,3/,**/JB**.+0+. Nakamura, K. (+311) Volcanoes as possible indicators of tectonic stress orientation principle and proposal. J. Volcanol. Geotherm. Res., ,, +ῌ+0. Nishimura, T., Nakamichi, H., Tanaka, S., Sato, M., Kobayashi, T., Ueki, S., Hamaguchi, H., Ohtake, M. and Sato, H. (,***) Source process of very long period seismic events associated with the +332 activity of Iwate Volcano, northeastern Japan. J. Geophys. Res., +*/, +3+-/ῌ +3+.1. Bs !" t +330 #$uB vwx%&y 8 z'{

(147) ' .+ .-ῌ/3. I ,*** 8 + z'wm_`_ ' ./ .1ῌ0-. Pitt, M. and Hill, D (+33.) Long-period earthquakes in the Long Valley Caldera region, eastern California. Geophys. Res. Lett., ,+, +013ῌ+02,. ,-./ +323 mpm`` k b" /3 +1*ῌ+2+. Tanaka, S., Hamaguchi, H., Ueki, S., Sato, M. and Nakamichi, H. (,**,) Migration of seismic activity during the +332 volcanic unrest at Iwate volcano, northeastern Japan, with reference to P and S wave velocity anomaly and crustal deformation. J. Volcanol. Geotherm. Res., ++-, -33ῌ.+.. 0 +31+ " 1 ? ¡¢ pp+.3. £$2¤¥3s 1

(148) +330 +33/ 4 3 ¦ +* ¦'60'§+'¨*+ v©ªb"() -, ,2/ῌ,3,. Ukawa, M. (,**/) Deep low-frequency earthquake swarm in the mid crust beneath Mount Fuji (Japan) in ,*** and ,**+. Bull. Volcanol., 02, .1ῌ/0. HIJ ,**1 ' «5 ¬(6®&¯7° ±89² :u³ ¼b"()½ +0+ῌ+1,. Ukawa, M. and Ohtake, M. (+321) A monochromatic earthquake suggesting deep-seated magmatic activity beneath the Izu-Ooshima volcano, Japan. J. Geophys. Res., 3,, +,0.3ῌ+,00-. White, R.A. (+330) Precursory deep long-period earthquakes at Mount Pinatubo : spatio-temporal link to a basalt trigger. In Fire and Mud Eruptions and Lahars of Mount Pinatubo, Philippines (Newhall, C.G. and Punongbayan, R.S. eds), -*1ῌ-,1, Philippine Institute of Volcanology and Seismology..

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