首都圏直下の地震と強震動―安政江戸地震と明治東京地震―

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1 Journal of Geography 116 3/ Large Earthquakes Occurring beneath Tokyo Metropolitan Area and Strong Ground Motions: Ansei Edo Earthquake and Meiji Tokyo Earthquake Takashi FURUMURA and Hiroyuki TAKEUCHI Abstract The Tokyo metropolitan area is known to have been struck by large earthquakes due to the subduction of the Philippine Sea Plate and the Pacific Plate beneath the North American plate. Recent damaging earthquakes that occurred beneath Tokyo include the 1855 Ansei Edo earthquake, the 1894 Meiji Tokyo earthquake, and the 1923 Kanto earthquake. Whereas the Kanto earthquake is known to have occurred at the top of the subducting Philippine Sea Plate, the other events are considered to have occurred in Tokyo bay, but their source depths are unknown. Many researchers have attempted to determine the source mechanisms of these earthquakes through analyses of patterns of seismic intensity distribution in the Kanto area, but the intensity pattern at the center of Tokyo would be considerably affected by the site amplification effect of the shallow, localized structure rather than be related directly to the source itself. In the present paper, we summarize the characteristics of strong ground motions and damage caused by the earthquakes. We then compare the pattern of intensities on local and regional scales with those of recent earthquakes occurring in Tokyo and corresponding computer simulations using heterogeneous crust and upper-mantle structure models below Tokyo to find referable source models for the Ansei Edo and Meiji Tokyo earthquakes. Key words Ansei Edo earthquake, Meiji Tokyo earthquake, seismic intensity, computer simulation I M M7.9 M M7 Earthquake Research Institute, University of Tokyo

2 1894 M SK-net K-NET KiK-net 2 II ,000 14,000 6, 1996; , 2003 Fig. 1 Pattern of regional intensities of the Ansei Edo earthquake in 1855 after Usami, 1996 and local intensity distribution around the Kanto area after Nakamura et al., km

3 400 km , 1976, 1996, M7 7.1, 1976 M7.2, km M7.0, h 0 5 km 6 30 km km km km M km M7.6, 1972;, Bakun km, 1934 S-P , , 1996;, 1972; 2, 1989;, 2004,

4 2, 1996, 1972 Fig. 2 Pattern of regional intensities of the Meiji Tokyo earthquake after Usami, 1996 and local intensity distribution around the Kanto area after Hagiwara, km 60 km 5 4 S-P, S-P km M6.4 M km M M M , S-P 40 km 1999, 1899 M km 3 3, c 2004, cm 1 4 6

5 3 a b c NS Fig. 3 a Ewing-type seismoscope photograph from National Museum of Nature and Science, Japan, b Seismogram observed at Tokyo Imperial University, Hongo, of the Meiji Tokyo earthquake in 1894, c Digitized seismogram of NS motion. 5 4a NS 4b c 4a S S-P mm S-P km P

6 4 a b 2 c 1 Fig. 4 Strong motion seismograms of Meiji Tokyo earthquake, recorded at a 1F of Main building of Tokyo Imperial University solid and dashed lines illustrating ground motions before and after correcting for instrument response, b Department of Technology at 2F, and c Department of Technology at 1F. NS EW S-P 4 P S h 10 km 6 10, Koketsu and Kikuchi, M6.7 S-P S, 2005 S-P

7 5 a 30 m S AVS 30, 2006 b V S 600 m/s Fig. 5 a Average shear-wave velocity of upper 30 m AVS 30 produced by borehole data Miyake et al., 2006, and b expected intensity amplification km III Hz m S AVS ,500 S AVS 30, 2006 AVS 30 ARV 2006 log ARV log AVS

8 ARV S V S 600 m/s PGV I 1999 I log PGV 2 ARV I I I log PGV ARV 3 I 1.72 log ARV 4 V S 600 m/s 5b PGV h 40 km M W6.8 V S 600 m/s PGV 1999 log PGV 0.58 M W h d 1.29 log X M W a PGV d d 0.02 X PGV 5 5a ARV 2 6b 6a 6b km 6c d , km,, 1980;, 2006

9 図 6 a 距離減衰式から求めた深さ東京湾北部の地震の震度分布 b 表層地盤の増幅率を考慮した震度分布 c d 震央印が移動した場合 Fig. 6 a Pattern of intensity distribution of an earthquake occurring in eastern Tokyo star mark, b expected intensity pattern due to the ampliﬁcation of ground motions in a shallow structure, c d same as b but for different hypocenter star marks. 図 7a は 2005 年 7 月 23 日に千葉県北西部のや伊豆の震度が大きくなっていることがわかる深さ 73 km で発生した M 6.0 の地震の震度分このような千葉県北西部の地震に伴う震度分布の布を示したものである震度 4 5 の範囲が一方異常はしばしば観測されており図 7a c 震央より 40 km 以上も西方に大きくずれ横浜千葉県北東部図 7d や東京東部の地震ではこ 439

10 7 a c d Fig. 7 a c Anomalous pattern of intensities observed for an earthquake occurring below northwest of Chiba prefecture, and d normal intensity pattern of an earthquake occurring below northeast of Chiba. Star indicates hypocenter of the earthquake.

11 , 2000 Fig. 8 Hypocentral distribution of man-felt earthquake occurring beneath Kanto region. Solid circle denotes earthquake that shows anomalous pattern of intensities. Hypocentral data is obtained from the JMA Intensity Database (Ishigaki and Takagi, 2000)., a c 23, km 8,, 2006 Q S High-Q; Q S

12 40 km Low-Q; Q S 250 Low-Q Kamiya and Kobayashi, 2000;, 2005 S High-Q S Low-Q km 0.25 km FDM iasp91; Kennett and Engdahl., 1991 High-Q Q S 1000 High-V 5 Ishida, 1992 Kosuga et al., km 80 km, 2006 S V S 1 km/s 1 F-net km T 1 1 Hz 9a V S km/ s 9b c High-Q Q S 1000 Low-Q Q S 150 9d km

13 図 9 千葉県北西部の地震の数値シミュレーション最大地動速度分布 a 標準地球水平成層モデル b 堆積層を加えたモデル c 表層地盤の増幅効果を補正したモデル d High-Q プレートと Low-Q 物体を加えたモデル Fig. 9 Simulated peak ground velocity for the Northwestern Chiba earthquake, using a standard Earth model, b including sedimentary layer, c including superﬁcial layer, and d including High-Q Plate, Low-Q body. 443

14 10 a b Fig. 10 Comparison of intensity patterns of earthquakes occurring a in the Philippine-Sea plate and b in the Pacific plate. 3 Utsu, K-NET 10a 10b

15 図 11 地殻内地震の震度分布の例 a 2004 年新潟県中越地震 b 2003 年宮城県北部の地震 Fig. 11 Example of intensity patterns of shallow crustal earthquakes a Niigata-ken Chuetsu earthquake in 2004 and b Miyagi-ken Hokubu earthquake in に震度が急減した後に震央距離が 150 km を越数 3 Hz までの地震波の伝播を計算したただしえると距離減衰が緩く変化することがわかるこ本計算で用いる粗い格子モデルでは表層地盤にれは震源距離が 150 km を越え S 波が地表とおける地震動の増幅特性を正しく評価することがモホ面との間で全反射を繰り返し起こすようになできないなど求められた震度分布と観測とを直ると地殻内をトラップ S 波 Lg 波として伝接比較することは難しい本計算では震度の絶わるために距離減衰が小さくなるためである対値や詳細なパターンを比較するかわりに震源たとえば Furumura and Kennett, 2001 の深さの変化地震が発生するプレートの違いこのように地震の起きる深さプレートにによる広域の震度分布のパターンの変化を確認よって日本列島全域にわたる広域震度分布が大し観測された震度分布の傾向を比較することをきく変化する様子を地震波動伝播の FDM シ目的とするミュレーションにより以下確認するここでは東京湾北部の北緯 35.7 度東経図 12 に示される中部日本東北日本の度を震央とする a 深さ 10 km 北米プ km の範囲の地殻マントルレート内地殻内 b 50 km フィリピン海直方向に 0.2 km の格子間隔でモデル化し周波ト内の位置に逆断層型のメカニズムを持つ点プレート構造を水平方向に km 鉛プレート内および c 100 km 太平洋プレー 445

16 12 b Fig. 12 a Structural model of regional seismic wave propagation simulation and b vertical slice of model and source positions. Hypocenter distribution is based on the JMA earthquake catalog a M W 6.9 b M W 7.0 c M W CPU Furumura and Chen, TB a c, 2003 h 100 km High-Q S Low-Q

17 447 Fig. 13 Simulated intensity patterns of three events at source depths of a 10 km, b 50 km, and c 100 km, and pattern of intensity during A Ansei Edo earthquake, B Meiji Tokyo earthquake, and C earthquake of 7 Oct 図 13 シミュレーションにより求められた震度分布 a 深さ 10 km, b 50 km, c 100 km の地震の計測震度分布観測された震度分布 A 安政江戸地震 B 明治東京地震 C 1894 年 10 月 7 日の地震

18 13c h 50 km 13b h 10 km km 150 km S Lg 13 13B 13b C 13c S-P A 4 13a IV,, 2005 h 50 km M7 h 40 km h 60 km Hz 2 3 Hz 100 m, Furumura and Kennett, 2005

19 10 7 M6.7 S-P km 3 M M7.2 1, 2005 K-NET KiKnet F-net SK-net 3 Q I 3 JST-CREST Bakun, W.H : Magnitude and location of historical earthquakes in Japan and implications for the 1855 Ansei Edo earthquake. J. Geophys. Res., 110, B02304, doi: /2004jb : : S : B052 Furumura, T. and Chen, L : Large scale parallel simulation and visualization of 3D seismic wavefield using the Earth Simulator. Comput. Model. Eng. Sci., 6, Furumura, T. and Kennett, B.L.N : Variation in regional phase propagation in the area around Japan. Bull. Seismol. Soc. Am., 91, Furumura, T. and Kennett, B.L.N : Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan. J. Geophys. Res., 110, B10202, doi: /2004jb : B : : : 2006 D :

20 70 Ishida, M : Geometry and relative motion of the Philippine Sea plate and Pacific plate beneath the Kanto-Tokai district, Japan. J. Geophys. Res., 97, : : 9, : 127p. Kamiya, A. and Kobayashi, Y : Seismological evidence for the existence of serpentinized wedge mantle, Geophys. Res. Lett., 27, : : Kennett, B.L.N. and Engdahl, E.R : Traveltimes for global earthquake location and phase identification. Geophys. J. Int., 105, Koketsu, K. and Kikuchi, M : Propagation of seismic ground motion in the Kanto basin, Japan. Science, 288, 5469, : S204 P007. Kosuga, M., Sato, T., Hasegawa, A., Matsuzawa, T., Suzuki, S. and Motoya, Y : Spatial distribution of intermediate-depth earthquake with horizontal of vertical nodal planes beneath northeastern Japan. Phys. Earth Planet. Int., 93, : : : : : : : 1855/11/ : : 1899 : : 1894 E : : Hi-net Q : : : 2006 D : 2 S116 P014 Bakun, B. Stein, Ross 2005 : : : : : Utsu, T : Regional difference in absorption of seismic waves in the upper mantle as inferred from abnormal distribution of seismic intensities, J. Fac. Sci. Hokkaido Univ., Ser. 7, 2, : M :

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