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ABSTRACT: Subducting seamounts are thought to increase the normal stress between subducting and overriding plates. However, recent seismic surveys and laboratory experiments suggest that interplate coupling is weak. A seismic survey in the Japan Trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude M approximately 7. Both observed seismicity and the pattern of rupture propagation during the 1982 M 7.0 event imply that interplate coupling was weak over the seamount. A large rupture area with small slip occurred in front of the seamount. Its northern bound could be determined by a trace of multiple subducted seamounts. Whereas a subducted seamount itself may not define the rupture area, its width may be influenced by that of the seamount.
Science 09/2008; 321(5893):1194-7. · 31.20 Impact Factor
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ABSTRACT: We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earth-quake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the sub-ducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12 • dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20 • . Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.
Earth Planets and Space 01/2006; 58:113-119. · 0.80 Impact Factor
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Kimihiro Mochizuki,
Mikako Nakamura,
Junzo Kasahara,
Ryota Hino,
Minoru Nishino,
Asako Kuwano,
Yasuyuki Nakamura, Tomoaki Yamada,
Masanao Shinohara,
Peyman Poor Moghaddam,
Toshihiko Kanazawa
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ABSTRACT: Strong anticorrelation between intensity of plate boundary PP reflection and seismicity had been revealed by a seismic reflection-refraction survey conducted in 1996 in a seismic-aseismic boundary region on the forearc slope of the Japan Trench. Amplitude of the strong reflection was explained by the presence of a thin layer (∼200 m) of low P wave velocity (3–4 km/s) at the top of the plate boundary. We conducted another seismic survey in 2001 in the same region as that of 1996 and verified the strong anticorrelation with planer extension along the plate boundary. Therefore existence of the thin layer of low P wave velocity along the plate boundary is expected within the aseismic regions. Extremely low P wave velocities along the plate interface at depths of around 10–20 km suggest that the layer may include fluid, clay minerals, and/or serpentine-chlorite. Because these materials have low mechanical strength or cause low friction between the overriding and subducting plates, large strain may not be accumulated and aseismic slip may be dominant. As a result of the above speculation, the intense reflection observed within the aseismic regions may imply aseismic slip between the plates. Amplitude of the plate boundary PP reflection phase was found maximum about reflections from deeper plate interface, although the reflected waves propagate for longer distances. One possible explanation to this phenomenon is the presence of serpentinized wedge mantle material that is buoyancy-driven to move along the top of the slab.
Journal of Geophysical Research 01/2005; 110. · 3.02 Impact Factor
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ABSTRACT: Various undifferentiated embryonic stem (ES) cells can grow on mouse embryonic fibroblast (MEF) feeders. However, the risk of zoonosis from animal feeders to human ES cells generally excludes the clinical use of these human ES cells. We have found that human placenta is a useful source of feeder cells for the undifferentiated growth of primate ES cells. As on MEF feeders, primate ES cells cultured on human amniotic epithelial (HAE) feeder cells and human chorionic plate (HCP) cells had undifferentiated growth. The cultured primate ES cells expressed Oct-4, alkaline phosphatase, and SSEA-4. The primate ES cells on HAE feeder cells produced typical immature teratomas in vivo after injection into severe combined immunodeficient mice. Human placenta is quite novel and important because it would provide a relatively available source of feeders for the growth of human ES cells for therapeutic purposes that are also free of ethical complications.
Stem Cells 02/2004; 22(4):433-40. · 7.78 Impact Factor
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ABSTRACT: In August 2000 a seafloor borehole broadband seismometer, WP-2, was installed in the northwestern Pacific basin, and seismic experiments with ocean bottom seismometers (OBSs), a single-channel seismic streamer, and airguns were performed. The WP-2 continued to record data autonomously, and a total of 436 days of broadband seismic records were eventually retrieved. Reflecting the low noise environment, many teleseismic events were recorded. Shallow seismic velocity models just below the OBSs were derived by the tau–p (τ–p) method for the airgun-OBS data and the single-channel seismic data. The crustal structure was estimated from the OBSs and WP-2 data by forward modeling, using a two-dimensional ray tracing method. The results of the seismic surveys show that the crustal seismic structure around WP-2 is laterally homogeneous and corresponds to typical oceanic crust. The uppermost mantle exhibits seismic anisotropy. The velocity variations are about 5% for P-waves and about 3.5% for S-waves, and the fast direction appears to be perpendicular to the magnetic lineations. Travel times of earthquakes recorded by the WP-2 and the previous seismological studies suggest that the lower part of the lithosphere has greater anisotropy than the uppermost mantle. To explain late first arrivals from the earthquakes that occurred in the slow direction with epicentral distances between 1600 and 2200 km, a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are inferred. Receiver function analysis of 16 events with a high signal-to-noise (S/N) ratio from the WP-2 data was performed, and discontinuities were estimated at depths of 416 and 666 km. These discontinuity depths are consistent with those of the average beneath the ocean.
Physics of the Earth and Planetary Interiors.
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力 清水,
宏樹 宮町,
和彦 後藤,
寿喜 角田,
舟一郎 平野,
知郎 山田,
亮太 日野,
肇 塩原,
あずさ 西澤,
利典 佐藤, [......],
Toshinori SATO,
Takahiro NAGAO,
Toshihiro IGARASHI,
Hideyuki HIRAMATSU,
Kazuhiro IWAKIRI,
Shinobu ITO,
Yuko SUGIOKA,
Hideki Shimamura,
Toshihiko Kanazawa,
Junzo Kasahara
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Yuya Machida,
Masanao Shinohara,
Tetsuo Takanami,
Yoshio Murai, Tomoaki Yamada,
Naoshi Hirata,
Kiyoshi Suyehiro,
Toshihiko Kanazawa,
Yoshiyuki Kaneda,
Hitoshi Mikada, [......],
Kenji Uehira,
Narumi Takahashi,
Minoru Nishino,
Kimihiro Mochizuki,
Takeshi Sato,
Ei'ichiro Araki,
Ryota Hino,
Kouichi Uhira,
Hajime Shiobara,
Hiroshi Shimizu
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ABSTRACT: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw = 8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~ 7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock.
Tectonophysics.
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Masanao Shinohara,
Toshihiko Kanazawa, Tomoaki Yamada,
Kazuo Nakahigashi,
Shin ' Ichi Sakai,
Ryota Hino,
Yoshio Murai,
Akira Yamazaki,
Koichiro Obana,
Yoshihiro Ito, [......],
Yuya Machida,
Kimihiro Mochizuki,
Kenji Uehira,
Michitaka Tahara,
Asako Kuwano,
Shin ' Ichiro Amamiya,
Shuichi Kodaira,
Tetsuo Takanami,
Yoshiyuki Kaneda,
Takaya Iwasaki
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ABSTRACT: The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake gen-eration, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the obser-vation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40 • . This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.
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Tomoaki Yamada,
Kimihiro Mochizuki,
Masanao Shinohara,
Toshihiko Kanazawa,
Asako Kuwano,
Kazuo Nakahigashi,
Ryota Hino,
Kenji Uehira,
Takeo Yagi,
Naoto Takeda,
Shinichi Hashimoto
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ABSTRACT: The Noto Hanto earthquake in 2007 (M j 6.9) occurred on March 25, 2007 near the west coast of the Noto peninsula, Honshu, Japan. To study the aftershock activity under the sea, we deployed pop-up type ocean bottom seismometers (OBSs) from April 5 to May 8, 2007. We combined data from ten ocean bottom and four onshore seismic stations located around the rupture area of the earthquake and determined the preliminary distribution of the aftershocks. Most of the offshore aftershocks are located in a depth range between 2 and 10 km, and no earthquakes are observed in the lower crust. Hypocenters of deep events occurring at depths greater than 5 km are confined to an area northeastward from the largest aftershock in offshore region. Most of the aftershocks aligned along a high angle and southeast dipping plane, which is consistent with the geometry of the active faults revealed by previous seismic reflection surveys.
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Hiroshi Shimizu,
Hiroaki Takahashi,
Tomomi Okada,
Toshihiko Kanazawa,
Yoshihisa Iio,
Hiroki Miyamachi,
Takeshi Matsushima,
Masayoshi Ichiyanagi,
Naoki Uchida,
Takaya Iwasaki, [......],
Kazuo Nakahigashi,
Bogdan Enescu,
Kazunari Uchida,
Shin ' Ichi Hashimoto,
Syuichiro Hirano,
Takeo Yagi,
Yuhki Kohno,
Tomotake Ueno,
Masaki Saito,
Mio Hori
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ABSTRACT: On March 20, 2005, a large M JMA 7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of M JMA 5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.
