Journal of Geography 116 3/4 576 587 2007 2007 M6.8 Long-period Ground Motions Recorded in the Tokyo Metropolitan Area during the 16 July, 2007 M 6.8 Off Niigata-ken Chuetsu, Japan Earthquake Takashi FURUMURA, Shunsuke TAKEMURA and Toshihiko HAYAKAWA Abstract Large and prolonged shaking with long-period ground motions having periods of about 7 sec were observed in central Tokyo during the Off Niigata-ken Chuetsu, Japan, M6.8 earthquake on 16 July, 2007. The observed ground motions from a dense nationwide strong motion network K- NET and KiK-net demonstrate clearly that the long-period ground motions consist of Rayleigh waves, which developed at the northern edge of the Kanto Basin and were induced by conversion from the S waves radiating from the earthquake source. The amplitude and the duration of the long-period surface waves were enhanced dramatically as they propagated in the Kanto Basin, which has a thick cover of sedimentary rocks overlaying rigid bedrock. Observed ground motions of long-period signals at the center of Tokyo from the 2007 Off Niigata-ken Chuetsu event correlated well with observations from the Chuetsu earthquake on 23 Oct. 2004 M 6.8. By analyzing waveform data from the main shock and aftershocks of the 2007 Off Niigata-ken Chuetsu earthquake and the Chuetsu earthquake in 2004, it is found that the long-period surface wave having a dominant period of about 7 sec at the center of Tokyo is developed efficiently by a large earthquake with a magnitude greater than about M6.5 7, but it is not developed by small earthquakes of less than about M6.5. Key words Kanto Basin, long-period ground motion, strong ground motion, 2004 Niigata-ken Chuetsu Japan earthquake, 2007 Off Niigata-ken Chuetsu Japan earthquake 2004 2007 I 2007 7 16 M6.8 6 40 km 2004 10 23 M6.8 4 200 km 4 3 Earthquake Research Institute, University of Tokyo
2004 7 3 2007 2004 II 1 K-NET, KiK-net K-NET, KiK-net 20 25 km 1800 K-NET KiKnet 696 3,, 1 20 60 120 3 1 9 E-W 2 P S 5 km/s 3 km/s S / 4 7 1.5 km/s SIT010 TKY020 40 60 4000 6000 m 3 7 12
図 1 2007 年 新 潟 県 中 越 沖 地 震 の 揺 れ が 伝 わ る 様 子 地 震 発 生 後 20 60 120 180 秒 後 地 動 速 度 の 強 さ を オ レ ン ジ 色 の 濃 さ と 高 さ で 表 現 し て い る K-NET, KiK-net 強 震 観 測 デー タ 合 計 696 観 測 点 を 使 用 赤 丸 は 震 源 を 示 す Fig. 1 Distribution of ground motions following the 2007 Off Niigata-ken Chuetsu earthquake, Japan. The amplitudes of the ground velocity motions are shown for times of 20, 60, 120, and 180s after the earthquake. Red circle indicates the hypocenter of the earthquake. こ れ ま で 1997 年 伊 豆 東 方 沖 の 地 震 Koketsu の加速度レベルは時間経過とともに急激に小さく and Kikuchi, 2000 や 2004 年新潟県中越地震 なる このため 加速度レベルにより記録の開始 Furumura and Hayakawa, 2007 の観測デー と停止を行う加速度強震計では 長周期地震動の 記録が途中で途切れてしまっていることが残念で タの解析により示されている 堆積平野の厚い堆積層では 長周期の地震動が ある 強く生成する一方で 周期 1 秒以下の短周期地 2 最大加速度 地動分布 震動は 堆積層の非弾性減衰 低 Q 値 や散乱 効果により 強い距離減衰を示すために 地震動 この地震よる 最大加速度の分布と 最大地動 変位の分布の特徴を図 4 に比較する 578
図 2 新潟県中越沖地震による 震源から関東平野への地震波の伝わり方と 関東平野での表面波 長 周 期 地 震 動 の 発 達 の 様 子 9 観 測 点 の 速 度 波 形 東 西 成 分 は 最 大 振 幅 で 正 規 化 し 最 大 速 度 を 右 上 に 表 示 し て い る 右 上 の 地 図 に は 観 測 点 の 位 置 を 示 す Fig. 2 Record section of EW component ground velocity motion associated with the Off Niigata-ken Chuetsu, Japan earthquake. Waveform traces of nine stations are normalized by the maximum amplitude of ground motion at each station. Maximum ground velocity at each station is shown at the top. Locations of seismic stations are shown on the insert map. 最大加速度は 主に周期 1 秒以下の短周期地 加速度レベルに減衰している 波長の短い短周期 震動の強さを表す物理量である 震源近傍の柏崎 の地震動は 地表下数 数十メートルの柔らかい D 15 km では 最大 667 cm/s/s の大加速度 表層地盤の増幅の効果を強く受けるために たと が記録されたが その後震源距離とともにほぼ同 えば埼玉県東部などの利根川の流路とその自然堤 心円状に弱まり 震源距離が 200 km を超える関 防沿いにおいて 10 20 cm/s/s の大きな加速度が 東平野の都心部ではせいぜい 5 10 cm/s/s 程度の 局所的に起きている なお この地域は 東京直 579
Latitude (N) 36.0 TKY007 CHBH10 0 1000 35.0 2000 3000 20 km 139.0 図 3 Basement Depth [m] 140.0 Longitude (E) 4000 5000 6000 141.0 関 東 平 野 の 3 次 元 基 盤 構 造 田 中 ほ か, 2006 a 基 盤 深 度 分 布 と K-NET 観 測 点 TKY007, CHBH10 お よ び b 基 盤 構 造 の 3 次 元 表 示 Fig. 3 a Depth distribution of basement in Kanto Basin Tanaka et al., 2006 and positions of K-NET and KiKnet stations TKY007, CHBH10, and b 3-D view of basement topography. 図 4 2007 年 新 潟 県 中 越 沖 地 震 は 震 源 を 表 す に よ る a 最 大 加 速 度 分 布 と b 最 大 変 位 分 布 図 中 の は 作 図 に 用 い た K-NET, KiK-net 観 測 点 を 表 す Fig. 4 Distribution of peak ground acceleration cm/s/s a and displacement cm b during the 2007 Off Niigata-ken Chuetsu, Japan earthquake. Star and small squares denote hypocenter of the earthquake and recording stations, respectively. 580
5 a PGA b PGV M6.8, 1999 0.5 2 Fig. 5 Attenuation of peak ground acceleration PGA a and peak ground velocity PGV as a function of hypocentral distance. Solid and dashed lines represent the expected PGV attenuation function for a M6.8 inland earthquake in Japan Shi and Midorikara, 1999 and the magnification of such attenuation by factors of 0.5 and 2.0, respectively. 0.5 1,, 2007 4b 1 5 cm 3 3000 4000 m 2 5 cm 2 2 1000 3000 m Furumura and Hayakawa, 2007, 1999 5 2007 10 500 km Mw6.8 0.5 2 1
, 5, 8 9 2 10 3 K-NET TKY007 6 225 km P 30 S 30 40 P S 1 7 Furumura and Hayakawa 2007 2004 SV Rayleigh 2007 Rayleigh S 70 6 cm/s 20 2004 Rayleigh Furumura and Hayakawa, 2007 6 2007 2004 P, S 2 2007 S S1 25 S S2 2 2 40 km S-P 2007 2004 P, S 2 7 13 cm/s 5 6b 2004 2007 0.5 2 2004 1/2 1/10 2007 2004 III Rayleigh 1 ; M
6 a 2007 2004 K-NET TKY007 b Fig. 6 Comparison of observed ground velocity motions recorded at Shinjuku TKY007 of the NS and EW component ground motions from the 2007 Off Niigata-Ken Chuetsu, Japan earthquake and from the 2004 Niigata- Ken Chuetsu, Japan earthquake a. b Velocity response spectra for the two records. P S 7 M M W5 8 S S 2 Aki, 1967 M O dyne cm S T C s Tekemura et al. 1993 log M O 3 log T C 23.28 S M W 7 M W S 7 M W5 8 S Fig. 7 Schematic illustration of velocity spectrum of far-field S-wave expected for magnitudes of M W5, 6, 7, and 8.
8 a 2004 TKY007 EW 3 b 6 2 Fig. 8 a Comparison of observed ground motions recorded at Shinjuku TKY007 of EW component velocity motion for the main shock and three aftershocks of the 2004 Niigata-Ken Chuetsu, Japan earthquake. Each waveform trace is normalized by the amplitude of the S wave. b Velocity response spectra for these records. and denote the 1st and 2nd dominant period of the response spectrum. S 1 M W 6 M W7 3 7 9 10 10 M W7 M W7 2 2004 2004 M6.8; M W6.6 M W5.8 5.0 8 2004 1 F-net M W 8 K-NET TKY007 3 S S M W M W6.8 S 4 M W5.6 S M W5.0 S
9 Fig. 9 2007 2004 1944 a b KiK-net CHBH10 Velocity response spectrum at a Central Tokyo Earthquake Research Institute and Otemachi, Tokyo and at b Chiba KiK-net CHBH10 station and Togane, Chiba during the 2007 Off Niigataken Chuetsu earthquake, 2004 Off Kii Peninsula earthquake, and 1944 Tonankai earthquake. 7 20 cm/s 5 M W 2 7 M W 6.5 7 M5 6 7 2 3 M8 M7 M8 9 2007 M6.8, ERI KiK-net, CHBH10 2004 M7.4 1944 M8.1 ERI CHBH10 5 10 km 2006 M7 8 M7 7
2 4 M 7 2 12 10 12 M7 M8 3000 m 7 M7 7 5000 m 12 M7 2004 M7.4 10 15 cm/s Hayakawa et al., 2005 ; Miyake and Koketsu, 2005, 2005 M7 IV 2007 S 2004 S 2007 2007 M6.9, 2007, 2007 2004 7 10 5 10 km, 2006; Furumura et al., 2007 2007 2004 1 10
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