04 Mid Niigata earthquake Earthquake Research Institute, University of Tokyo. 04 23 17 6 M6.8 7 18 12 M6.018 34 M6. 6 27 40 M6.1 11 8 11 1 M.9 30km M6 1 14 1) 2 RMS P 0.17 0.074 S 0.476 0.166 900 P 3 S 1 719 3km 17km 3 M6 km 27 M6.1 Double-Difference 2)
4 km 3 4 km (M6.8) (18:34 M6.) /27 6 /27 1 7 6,000m tear fault - 6 30km 2mm/ 1mm/ 4 7 8 7 A-A' 8 1 11) 23 6 34 M6. 78 4
27 40 M6.1 78 2 8 9 1) Takeda, T., H. Sato, T. Iwasaki, N. Matsuta, S. Sakai, T. Iidaka and A. Kato, Crustal structure in the northern Fossa Magna region, central Japan, from refraction/wide-angle reflection data, Earth Planets Space (04). 2) Zhang, H., and C., Thurber, Double-Difference Tomography: the method and its application to the Hayward Fault, California, Bulletin of the Seismological Society of America, 93, 187-1889 (03). 3) 2 (1992). 4) : 2463-7(04) ) : 1 132p.1991. 6) : 1 4p.198. 7) : 1 177p.1986. 8) 799p1990. 9) :, 60p, (02). ) :1:2,000 D.1-No.388.01. 11) :04 23 04 http://unit.aist.go.jp/actfault/niigata/report/index.html, : (Wed, 17 Nov 04 08:14:41 +0900), http://www.seis.nagoya-u.ac.jp/info/niigata/reportaf24.html. 12) p.1992.
138 00' 38 00' 138 30' 139 00' 139 30' 37 30' 37 00' 36 30' 138 00' 138 30' 139 00' 139 30' 1 Fig.1Location of permanent stations ( ) and temporal stations(, ). 0 0 2 4 6 8 30 40 2 Fig.2Model of velocity structure (W:thick line, E:thin line).
37.4 0 37.2 30 138.6 138.8 139.0 1 1 1 1 1 1 1 1 3 Fig.3Hypocentral distribution. The main shock ( ) and some large aftershocks ( ) are shown.
4 Double-Difference Zhang ;Thurber, 03 P km/s Fig.4Depth section showing the P wave velocity structure and the relocated aftershocks derived from the double-difference tomography (Zhang and Thurber, 03). Gray circles denote the locations of the aftershocks, and the color contour shows the P wave velocity.
Mw 30 km km 1 m 9.0 **18 Nm 6.6 km km 0. m 3.0 **18 Nm 6.3 Fig.Change of Coulomb stress due to the main shock and the largest aftershock.
6 a: b: :04 :03 1992 04 03 Fig.6Bouguer anomaly map of Niigata and southern Tohoku district and the hypocentral location of the 04 Mid Niigata earthquake. a: Shibata-Koide tectonic line, b: Kashiwazaki-Choshi tectonic line, red star: hypocenter of the 04 Mid Niigata earthquake, Blue star: hypocenter of the 03 Northern Miyagi earthquake, Bouguer anomalies are after Geological Survey of Japan (1992). 7 1: 23 17 6 M6.82: 23 18 3 M6.33: 23 18 11 M6.04: 23 18 34 M6.: 27 40 M6.1a: b: c: 6)7)8) 9 4 Fig.7Geological structure and hypocentral distribution around the source area of the 04 Mid Niigata earthquake. 1: Main shock (October 23, 17:6' M6.8), 2: Aftershock (Oct. 23, 18:03' M6.3), 3: Aftershock (Oct. 23, 18:11' M6.0), 4: Largest aftershock (Oct. 23, 18:34 M6.), : Aftershock (Oct. 27, :40 M6.1), a: Obirou fault, b: Mikamachi fault, c: Suwa-toge flexure. Geological map is compiled after ), 6), 7) and 8), active fault is 9) and ), tilting of river terraces is after 4).
8 1 4 7, 12 Fig.8Schematic diagram showing the crustal structure across the source faults of the 04 Mid Niigata earthquake. Thick solid line represents an estimated source fault. Circle: aftershock, triangle: site of drill hole projected onto cross section. The data of drill holes is after ) and 12). 9 Fig.9Schematic diagram showing the 3D view of the source faults (not in exact scale).