Anomalous Rayleigh-Wave Propagation Along Oceanic Trench     

Yomogida  Kiyoshi  Okuyama  Riko  Nakanishi  Ichiro 《Studia Geophysica et Geodaetica》2002,46(4):691-710

A clear later phase of amplitude larger than the direct surface wave packet was observed at stations in Hokkaido, Japan, for several events of the December 1991 off-Urup earthquake swarm in the Kuril Islands region. From its particle motion, this phase is likely to be a fundamental Rayleigh wave packet that arrived with an azimuth largely deviated from each great-circle direction. As its origin, Nakanishi (1992) proposed that the sea-trench topography in this area as deep as 10 km may produce a narrow zone of low velocity for Rayleigh waves of periods around 15 sec. Following this idea, we compute ray paths and estimate how Rayleigh waves would propagate if we assume that lateral velocity variations are caused only by seafloor topography. We confirm that thick sea water in the trench indeed produces the phase velocity of Rayleigh waves to be smaller than in a surrounding area by the degree over 100%. Such a low-velocity zone appears only in a period range from 12 to 20 sec. Although this strong low-velocity zone disturbs the direction of Rayleigh wave propagation from its great circle, the overall ray paths are not so affected as far as an epicentre is outside this low-velocity zone, that is, off the trench axis. In contrast, the majority of rays are severely distorted for an event within the low-velocity zone or, in other words, in the neighborhood of the trench axis. For such an event, a part of wave energy appears to be trapped in this zone and eventually propagates outwards due to the curvature or bend of trench geometry, resulting in very late arriving waves of large amplitude with an incident direction clearly different from great circles. This phenomenon is observed only at a very limited period range around 16 sec. These theoretical results are consistent with the above mentioned observation of Nakanishi (1992).  相似文献   

Inner-core anisotropy beneath Australia and differential rotation     

Takehi Isse  Ichiro Nakanishi 《Geophysical Journal International》2002,151(1):255-263

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Phase velocity and Q of mantle Rayleigh waves   总被引：1，自引：0，他引：1  

Ichiro Nakanishi 《Geophysical Journal International》1979,58(1):35-59

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Structural characteristics off Miyagi forearc region, the Japan Trench seismogenic zone, deduced from a wide-angle reflection and refraction study   总被引：4，自引：0，他引：4  

Seiichi Miura  Narumi Takahashi  Ayako Nakanishi  Tetsuro Tsuru  Shuichi Kodaira  Yoshiyuki Kaneda 《Tectonophysics》2005,407(3-4):165-188

The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab.  相似文献   

Interpretation of Sa on a continental structure     

K. K. Nakanishi  F. Schwab  E. G. Kausel 《Geophysical Journal International》1976,47(1):211-223

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An Extension of the Mellor–Yamada Model to the Terra Incognita Zone for Dry Convective Mixed Layers in the Free Convection Regime     

Junshi Ito  Hiroshi Niino  Mikio Nakanishi  Chin-Hoh Moeng 《Boundary-Layer Meteorology》2015,157(1):23-43

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Millennial‐scale stratigraphy of a tide‐dominated incised valley during the last 14 kyr: Spatial and quantitative reconstruction in the Tokyo Lowland,central Japan          下载免费PDF全文

Susumu Tanabe  Toshimichi Nakanishi  Yoshiro Ishihara  Rei Nakashima 《Sedimentology》2015,62(7):1837-1872

Spatial and quantitative analysis of infilling processes of the tide‐dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post‐Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea‐level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet‐like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea‐level rise of 4 to 7 mm yr^?1. Before migration of the Tone River at 5 ka, the tide‐dominated bay in the Nakagawa Valley was filled by upward‐fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide‐dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward‐coarsening deltaic sediments.  相似文献   

Direct ascent to the surface of asthenospheric magma in a region of convex lithospheric flexure     

Yuki Sato  Naoto Hirano  Shiki Machida  Junji Yamamoto  Masao Nakanishi  Teruaki Ishii 《International Geology Review》2018,60(10):1231-1243

The stress field of oceanic lithosphere controls the distribution of submarine petit-spot volcanoes. However, the eruption sites of these petit-spot volcanoes are considered to be limited to concavely flexed regions of lithosphere off the outer rise. Here, we present new data for a recently identified petit-spot lava field on a convexly flexed section of the lithosphere adjacent to the subduction zone offshore of northeast Japan in an area containing more than 80 volcanoes. This area is marked by strongly alkaline lavas that were erupted on the convexly flexed region. As for the concavely flexed region where the petit-spots previously reported, the base of the lithosphere beneath the eruption sites is under extension, whereas the upper part of the lithosphere is under compression. This change in the stress field, from the lower to upper lithosphere, causes ascending dikes to stall in the mid-lithosphere, leading to metasomatic interaction with the surrounding peridotite. The new geochemical data of rocks and xenocrysts presented in this study indicate that strongly alkaline magmas erupted on the convexly flexed region would have ascended more rapidly through the mid-depth of lithosphere because of the extensional regime of the upper lithosphere and decreasing the degree of metasomatic reaction with the surrounding mantle peridotite. The results indicate that the degree of metasomatism and the compositional variations of petit-spot magmas are controlled mainly by the stress field of the lithosphere.  相似文献   

Center-of-mass corrections for sub-cm-precision laser-ranging targets: Starlette,Stella and LARES   总被引：2，自引：2，他引：0  

Toshimichi Otsubo  Robert A. Sherwood  Graham M. Appleby  Reinhart Neubert 《Journal of Geodesy》2015,89(4):303-312

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Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy – II. Analysis by the single-station method     

Ichiro Nakanishi  Don L. Anderson 《Geophysical Journal International》1984,78(2):573-617

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