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ABSTRACT: Determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes. The peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate (underplating) at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging, but direct seismic evidence of this process is lacking. By comparing seismic reflection profiles with microearthquake distributions in central Japan, we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting Philippine Sea plate. This region coincides with the location of slow-slip events that may serve as signals for monitoring active underplating.
Science 07/2010; 329(5988):210-2. · 31.20 Impact Factor
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Hiroshi Sato,
Naoshi Hirata,
Kazuki Koketsu,
David Okaya,
Susumu Abe,
Reiji Kobayashi,
Makoto Matsubara,
Takaya Iwasaki,
Tanio Ito,
Takeshi Ikawa,
Taku Kawanaka, Keiji Kasahara,
Steven Harder
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ABSTRACT: Devastating earthquakes occur on a megathrust fault that underlies the Tokyo metropolitan region. We identify this fault with use of deep seismic reflection profiling to be the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 kilometers, is much shallower than previous estimates based on the distribution of seismicity. This shallower plate geometry changes the location of maximum finite slip of the 1923 Kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the Tokyo region.
Science 08/2005; 309(5733):462-4. · 31.20 Impact Factor
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ABSTRACT: Seismic observations to retrieve various information from the Earth are the basis of seismology. A seismic observation system requires various technologies for vibration sensors, analog-and-digital measurement, data transmission, and computing for mass data analysis, for example. New developments in technology are adopted whenever possible in the construction of seismic observation systems. In Japan, after the disastrous Kobe Earthquake in 1995, a high-density and high-sensitivity seismograph network was constructed. The seismic network, called the National Research Institute for Earth Science and Disaster Prevention (NIED) Hi-net, uniformly covers the Japanese Islands with a spacing of 20–30 km. As a result, the detection capability for microearthquakes has been greatly improved, and various research using Hi-net data has indicated that this seismic network has a great potential to resolve the underground structure and various geophysical phenomena as a radar-array oriented toward the Earth. Equipped with modern standard techniques, the Hi-net system is designed to be able to be smoothly upgradeable in the future.
Review of Scientific Instruments 01/2005; 76(2):021301-021301-12. · 1.37 Impact Factor
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ABSTRACT: 1] Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non-volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the source area of tremors. In each episode, the source of the slow slip event and tremor migrate simultaneously. The spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 7230 Seismology: Seismicity and seismotectonics; 8045 Structural Geology: Role of fluids; 8150 Tectonophysics: Plate boundary—general (3040); 8159 Te ct o n op h y s i c s : R he o l o gy — c r u s t a n d l i t h o s ph e r e . Citation: Obara, K., H. Hirose, F. Yamamizu, and K. Kasahara (2004), Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone, Geophys. Res. Lett., 31, L23602, doi:10.1029/2004GL020848.
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Hiroshi Sato,
Naoshi Hirata,
Kazuki Koketsu,
Kiyoshi Ito,
Okaya David,
Takaya Iwasaki,
Tanio Ito, Keiji Kasahara,
Susumu Abe,
Taku Kawanaka,
Makoto Matsubara,
Reiji Kobayashi,
Takeshi Ikawa
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ABSTRACT: Deep seismic profiling was performed in the Kanto and Kinki areas to obtain a better estimation of strong ground motions. In the Kanto area, we identify the seismogenic source fault on the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 km, is much shallower than previously estimated from the distribution of seismicity. This shallower plate geometry changes the location of the maximum finite slip of the 1923 Kanto earthquake, and its location corresponds to a zone of poor reflection on the mega-thrust, namely, a strong reflectivity zone along the mega-thrust coincidences with aseismic slip zone. In the Kinki area, 120-km-long seismic reflection profiling was carried out from Osaka to Suzuka across the Osaka and Ise basins and several active faults. Deep sub-horizontal reflectors are found at 26 and 16km in depth. The shallower re flectors correspond to the base of the seismogenic zone. Dipping reflectors, probably deeper extensions of active faults, merge into the mid-crustal reflectors.
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Shoji Sekiguchi,
Hiroki Hayash,
Masashi Tsukui,
Yo Uesugi,
Tanio Ito,
Yuichiro Tanaka,
Masaki Takahashi,
Yukio Yanagisawa,
Fumio Yamamizu,
Kazushige Obara,
Sadaki Hori,
Katsuhiko Shiomi,
Hisanori Kimura,
Makoto Matsubara, Keiji Kasahara
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ABSTRACT: We carried out borehole drilling in the Yamakita area near an active fault of the arc-arc collision zone in the western part of Kanagawa Prefecture. Slime and core samples were obtained. The borehole geology was established by examined them using petrographic analyses and geophysical logging data. We found that the southwestern slope of Mt. Maruyama was down-faulted from the northern slope of the mountain by an unknown fault. After drilling was completed, the borehole was established as a Hi-net seismic observation station. Data are collected by the Hi-net observation system and made available through the INTERNET.
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ABSTRACT: We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-VP/VS zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-VP/VS zone; however, VP/VS at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-VP/VS zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-VP/VS zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.
Tectonophysics.
