Integrated description of deformation modes in a sedimentary basin: A case study around a shallow drilling site in the Mizunami area, eastern part of southwest Japan     

Yasuto  Itoh  Kenji  Amano Naoki  Kumazaki 《Island Arc》2006,15(1):165-177

Abstract   Tectonic episodes in a sedimentary basin are described on the basis of an integrated study combining reflection seismic interpretation, drilling survey and paleomagnetism. A shallow inclined borehole penetrated a fault shown by reflection seismic and geological surveys in the Mizunami area, in the eastern part of southwest Japan. Paleomagnetic measurements were carried out on core samples successfully oriented using side-wall image logging of structural attitude. At six horizons, stable characteristic remanent magnetization (ChRM) was confirmed through thermal and alternating field demagnetization tests, which were carried by magnetite with minor amounts of high coercivity minerals, as revealed by experiments of stepwise acquisition of isothermal remanent magnetization. After correction of multiphase deformation inferred from borehole structural analysis, ChRM directions were combined with previous data and confirmed an easterly deflection reflecting the coherent clockwise rotation of the arc before the Middle Miocene. Compilation of reliable paleomagnetic data described differential rotation of the eastern part of southwest Japan raised by collision of the Izu–Bonin Arc since the Middle Miocene. The present study suggests that (i) the Mizunami area is adjacent to a highly deformed zone bounded by the Akaishi Tectonic Line (ATL), and (ii) forearc deformation of southwest Japan is localized around the ATL, which is quite different from gradual bending on the back-arc side without remarkable crustal break related to the collision event.  相似文献   

Estimating the permeability of the Nojima Fault Zone by a hydrophone vertical seismic profiling experiment     

Tsutomu Kiguchi  Hisaso Ito  Yasuto Kuwahara  Teruki Miyazaki 《Island Arc》2001,10(3-4):348-356

Abstract A multi-offset hydrophone vertical seismic profiling (VSP) experiment was done in a 747 m deep borehole at Nojima Hirabayashi, Hyogo prefecture, Japan. The borehole was drilled to penetrate the Nojima Fault, which was active in the 1995 Hyogo-ken Nanbu earthquake. The purpose of the hydrophone VSP is to detect subsurface permeable fractures and permeable zones and, in the present case, to estimate the permeability of the Nojima Fault. The analysis was based on a model by which tube waves are generated when incident P-waves compress the permeable fractures (or permeable zones) intersecting the borehole and a fluid in the fracture is injected into the borehole. Permeable fractures (or permeable zones) are detected at the depths of tube wave generation, and fracture permeability is calculated from the amplitude ratio of tube wave to incident P-wave. Several generations of tube waves were detected from the VSP sections. Distinct tube waves were generated at depths of the fault zone that are characterized by altered and deformed granodiorite with a fault gouge, suggesting that permeable fractures and permeable zones exist in the fault zone. Tube wave analysis shows that the permeability of the fault gouge from 624 m to 625 m is estimated to be approximately 2 × 10⁻¹² m².  相似文献   

Active right-lateral strike-slip fault zone along the southern margin of the Japan Sea     

Yasuto Itoh  Hiroyuki Tsutsumi  Hirofumi Yamamoto  Hiroyuki Arato 《Tectonophysics》2002,351(4):45

We describe an active right-lateral strike-slip fault zone along the southern margin of the Japan Sea, named the Southern Japan Sea Fault Zone (SJSFZ). Onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features, whereas the offshore parts are interpreted from single-/multichannel seismic data combined with borehole information. In an effort to evaluate late Quaternary activity along the fault zone, four active segments separated by uplifting structures are identified in this study. The east–northeast-trending SJSFZ constitutes paired arc-parallel strike-slip faults together with the Median Tectonic Line (MTL), both of which have been activated by oblique subduction of the Philippine Sea plate during the Quaternary. They act as the boundaries of three neotectonic stress domains around the eastern margin of the Eurasian plate: the near-trench Outer zone and NW–SE compressive Inner zone of southwest Japan arc, and the southern Japan Sea deformed under E–W compression from south to north.  相似文献   

Paleomagnetism of a pyroclastic flow deposit and its correlative widespread tephra in central Japan: Possible tectonic rotation since the late Pleistocene   总被引：1，自引：0，他引：1  

Yasuto Itoh  Haruo Kimura 《Island Arc》2004,13(1):110-118

Abstract The late Pleistocene Kamitakara Pyroclastic Flow Deposit (KPFD) and its correlative Kasamori (Ks22) Tephra in central Japan are found to preserve stable thermoremanent magnetization (TRM) and detrital remanent magnetization (DRM), respectively. Untilted site‐mean declinations of the KPFD are characterized by a fairly large scatter with easterly deflection, while those of the Ks22 show significantly smaller deflections. Because northerly paleomagnetic directions consistently characterize shallow marine sediments intercalating the Ks22 layer, the directional discordance is not attributed to different acquisition timing between TRM and DRM, but is probably due to a recent tectonic rotation in central Japan. Large scatter in TRM declinations of the KPFD implies that a number of right‐lateral active faults around the depositional area of the pyroclastic flow raised differential rotation of crustal blocks in central Japan, even during the late Pleistocene.  相似文献   

Effects of temperature and chlorinity on the microdistribution ofFavella taraikaensis Hada (Ciliata) in Maizuru Bay     

Akihiko Kuwahara  Makoto Kashiwai  Masatosi Sinoda 《Journal of Oceanography》1975,31(3):109-114

Continuous sampling of the plankton with a pump sampler was carried out along a 960 m straight-line course in Maizuru Bay. The 25 collections of plankton were successively obtained at a depth of 1.5 m along the course, each of the collections covering a horizontal distance of 38 m. Vertical samplings of the plankton were done at five different depths (90, 120, 150, 180 and 210 cm) at 13 stations along the course. In both kinds of samplings temperature and chlorinity were measured.In the horizontal distribution ofF. taraikaensis, it was demonstrated that the counts of plankton had no correlation with the temperature and chlorinityin situ. There was a correlation between the counts and the differences of temperature and chlorinity between two successive sampling stations. It seemed thatF. taraikaensis was most abundant in the locations where the temperature difference was greater than +0.2C and the chlorinity difference was greater than +0.04 . This species was more abundant around the steep thermocline and the steep halocline. When the vertical profiles of temperature and chlorinity did not show steep gradients, this species was more abundant near the surface.  相似文献   

重庆荣昌诱发地震区精细速度结构及2010年ML5.1地震序列精确定位     

王小龙  马胜利  雷兴林  郭欣  王强  余国政  勾宪斌  桑原保人  今西和俊  蒋霞东 《中国地震研究》2012,26(4):467-478

Based on data collected from a temporal seismic network, and in addition to the data from some nearby permanent stations, we investigate the velocity structure and seismicity in the Rongchang gas field, where significant injection-induced seismicity has been identified. First, we use receiver functions from distant earthquakes to invert detailed 1-D velocity structures beneath typical stations. Then, we use the double-difference hypocenter location method to re-locate earthquakes of the 2010 ML5.1 earthquake sequence that occurred in the region. The re-located hypocenters show that the 2010 ML5.1 earthquake sequence was distributed in a small area surrounding major injection wells and clustered mostly along pre-existing faults. Major earthquakes show a focal depth less than 5km with a dominant depth of ～2km, a depth of major reservoirs and injection wells. We thus conclude that the 2010 ML 5.1 earthquake sequence might have been induced by the deep well injection of unwanted water at a depth ～3km in the Rongchang gas field.  相似文献   

Correlations between electrical and elastic properties of solid–liquid composites with interfacial energy-controlled equilibrium microstructures     

Marina Pervukhina  Yasuto Kuwahara 《Earth and Planetary Science Letters》2008,265(3-4):410-422

Electrical conductivity and seismic velocity are studied for plausible pore geometries in the Earth's interior for reliable quantitative analysis of experimental data such as seismic tomography and magnetotelluric explorations. Electrical conductivity of a two-phase system with equilibrium, interfacial energy-controlled phase geometry is calculated for the dihedral angles θ = 40°–100° that are typical for rock–aqueous fluid and θ = 20°–60° for rock–melt systems of lower crust and upper mantle for the case of tetrakaidecahedral grains. Electrical conductivity vs. seismic velocity correlations are acquired by combining of the simulated electrical conductivities with the seismic velocity calculated with the help of equilibrium geometry model Takei [Takei, Y., Effect of pore geometry on V_P/V_S: From equilibrium geometry to crack. J. Geophys. Res. 107 (2002): 10.1029/2001JB000522.] for the same pore geometries. The results show that electrical conductivity gradually decreases reaching zero when seismic velocities reach seismic velocities of intact rock for rock–melt systems, while for rock–aqueous fluid systems with θ ≥ 60° conductivity drops to zero at velocities up to 10% smaller. This can explain the seeming discrepancy of the low seismic velocity region, attributed to the high fluid fraction, and the low electrical conductivity of the same region, which is sometimes faced at collocated electromagnetic and seismic experiments.  相似文献   

Variability in the Relative Penetration of Ultraviolet Radiation to Photosynthetically Available Radiation in Temperate Coastal Waters, Japan   总被引：1，自引：0，他引：1  

Victor S. Kuwahara  Tatsuki Toda  Koji Hamasaki  Tomohiko Kikuchi  Satoru Taguchi 《Journal of Oceanography》2000,56(4):399-408

UVR and PAR wavelengths are attenuated to different extents within the water column, causing variations in spectral composition with depth. The present investigation (a) describes the variability of UVR and PAR penetration at a station in the temperate coastal waters of Sagami Bay and determines (b) the characteristics of relative UVR penetration to the euphotic zone. Examination of the seasonal irradiance profile measurements indicated eight measurements displaying two distinct attenuation coefficients (K _d) for specific UVR wavelengths and PAR. The two attenuation coefficients observed from specific wavelengths in the water column may be caused not only by chlorophyll pigments, but also by dissolved organic material in the upper layer. The 1% depth of surface UVR at 305, 320, 340, and 380 nm averaged 10.8 ± 5.7, 14.9 ± 9.5, 19.8 ± 12.1, and 30.4 ± 17.6 m, respectively. The depth of euphotic layer displayed less variability averaging 62 ± 15 m throughout the entire study. Relative UVR penetration within the euphotic zone averaged 17.8 ± 8.1, 22.9 ± 10, 30.5 ± 13.8, and 46 ± 46.9% for 305, 320, 340, and 380 nm, respectively. A large variation of the relative transmission of UVR within the euphotic zone was found although the spectral composition was relatively stable in the air throughout the study.  相似文献   

SPATIO-TEMPORAL STRESS FIELD VARIATION IN THE CENTRAL LONGMENSHAN FAULT ZONE AFTER THE 2008 WENCHUAN EARTHQUAKE FROM FOCAL MECHANISM SOLUTIONS OF SMALL EARTHQUAKES          下载免费PDF全文

JIANG Min  CHEN Jiu-hui  Yasuto Kuwahara  Reiken Matsushita 《地震地质》2018,40(2):310-322

We determined the focal mechanism solutions(FMS)of aftershocks of the 2008 Wenchuan earthquake using the waveform data recorded by the Western Sichuan movable seismic array. We further obtained the spatio-temporal variation of the stress field by inverting the stress tensors from these data. The results show that the FMSs of the small earthquakes are primarily reverse faulting with considerable number of normal and strike-slip. The proportion of reverse type earthquakes clearly increases with time, and the spatial distribution of the FMSs is closely related with local geology and the characteristics of the deep faults. The stress tensor inversion results reveal that the orientations of the maximum horizontal compressive stress(S_H)shortly after the mainshock in each area are mainly in EW to NWW-SEE directions, while in the area along Xiaoyudong-to-Lixian aftershock branch it is in NE-SW direction in shallow crust. This implies that the stress field at the early stage after the Wenchuan earthquake is mainly controlled by the dynamic stress change caused by the rupture propagation, and the conspicuous factor determining the S_H direction is the characteristics of the rupture plane. Temporal variations of the stress tensors show that the stress regimes at depth changed from a mixture of reverse and strike-slip faulting to pure reverse, implying that local Coulomb stress caused by the main shock is released through strike-slip faulting and gradually recovers into the background stress field. The change of stress in the shallow subsurface follows that in the deep subsurface with observable time delay.  相似文献   

Optimum strength ratio of hysteretic damper     

Kazuo Inoue  Susumu Kuwahara 《地震工程与结构动力学》1998,27(6):577-588

This paper deals with steel moment frames combined with hysteretic dampers. Specifically, it examines the strength of dampers required to minimize damage to moment frames under earthquake loading. The important structural variables are the ratio of the dampers' shear strength to the maximum resistance, termed β in this study, and the relative stiffness between the damper and the main frame, termed k. The equivalent viscous damping ratio of the entire structural system is affected by the quantities β and k and the ductility ratio of the main frame. The optimum dampers' strength ratio (β_opt), which maximizes the equivalent viscous damping ratio, was formulated as a function of k, and it was found that β_opt is independent of the ductility ratio of the main frame. Earthquake response analysis confirmed that damage to the main frame can be significantly reduced by hysteretic dampers and minimized at the value of β_opt. © 1998 John Wiley & Sons, Ltd.  相似文献