利用1-HzGPS波形数据反演2011年日本北大地震震源破裂过程     

王振  孟国杰  横田佑助  苏小宁 《地震》2013,33(3)

利用1-Hz GPS数据反演2011年3月日本东北大地震的时空破裂过程,通过理论波形图与观测波形图的拟合,以及ABIC准则的判定,反演得到滑移分布的稳定估计.结果表明:破裂集中在震源附近,并在震中以下30 km处达到最大位错量,约72 m,在岩手和福岛靠近海岸线处有两个较小的破裂区,断层的南半段破裂很小,同时也是余震最为密集的地方.整个破裂过程由3个破裂阶段组成,发震后断层从震源向周围缓慢破裂,在50～60 s破裂延伸至地表(海沟);60～90 s在震中下方迅速发生强烈破裂,形成了最大位错区,这一阶段主要是倾向上的双边破裂;90～120 s在断层最北西处和断层南半段发生较小破裂,整个破裂过程持续大约120 s.反演得到的地震矩为3.8×1022 Nm,相应的矩震级为MW 9.0.  相似文献   

Transient wind drift currents in a tidal inlet: Theoretical analysis of Ekman drift current and field experiments in Suonada,the Seto Inland Sea     

Hidekazu Yasuda 《Journal of Oceanography》2009,65(4):455-476

Time-dependent wind drift currents in a basin with finite depth have been solved analytically in order to understand their fundamental behavior in coastal waters. The drift currents due to the land/sea breeze, as a typical example of time-dependent winds, have been examined with attention to the manner of their oscillation in their vertical profiles. The theoretical analysis indicates that the drift current due to the land/sea breeze might be amplified effectively around the southern part of Japan, where the oscillating period of the wind is near to the inertial period. The analysis of the physical process of the drift current reveals the following two important aspects: the Ekman boundary layer in a rotating frame is physically consistent with the Stokes boundary layer due to oscillating currents in an inertial frame, and so the inertial motion due to the wind is dispersed to the deeper level by the vertical viscosity in a rotating frame. The harmonic analysis was performed for the residual data after removal of the four main tidal constituents, M2, S2, K1 and O1, from the raw data observed in Suonada sound, the Seto Inland Sea. The feature of the analytically solved drift currents corresponded well to the observed picture. The vertical viscosity in this field has been estimated at 10⁻³ m²/s by adjusting the harmonically analytical result of the observed data to the vertical profile of the analytically solved drift current.  相似文献   

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)     

Masateru Ishiguro  Ryosuke Nakamura  Naru Hirata  Etsuko Nemoto  Yuta Higuchi  Aya Yamamoto  Yasuhiro Yokota  Tatsuaki Hashimoto 《Icarus》2010,207(2):714-1001

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of ?1% when the signal level was <3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 μm) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flat-field) variation was 3% or less. The uncertainty in background zero level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 ± 0.03 mm, yielding a field-of-view (FOV) of 5.83° × 5.69°. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.  相似文献   

Planetary growth with collisional fragmentation and gas drag     

Hiroshi Kobayashi  Hidekazu Tanaka  Satoshi Inaba 《Icarus》2010,209(2):836-492

As planetary embryos grow, gravitational stirring of planetesimals by embryos strongly enhances random velocities of planetesimals and makes collisions between planetesimals destructive. The resulting fragments are ground down by successive collisions. Eventually the smallest fragments are removed by the inward drift due to gas drag. Therefore, the collisional disruption depletes the planetesimal disk and inhibits embryo growth. We provide analytical formulae for the final masses of planetary embryos, taking into account planetesimal depletion due to collisional disruption. Furthermore, we perform the statistical simulations for embryo growth (which excellently reproduce results of direct N-body simulations if disruption is neglected). These analytical formulae are consistent with the outcome of our statistical simulations. Our results indicate that the final embryo mass at several AU in the minimum-mass solar nebula can reach about ∼0.1 Earth mass within 10⁷ years. This brings another difficulty in formation of gas giant planets, which requires cores with ∼10 Earth masses for gas accretion. However, if the nebular disk is 10 times more massive than the minimum-mass solar nebula and the initial planetesimal size is larger than 100 km, as suggested by some models of planetesimal formation, the final embryo mass reaches about 10 Earth masses at 3-4 AU. The enhancement of embryos’ collisional cross sections by their atmosphere could further increase their final mass to form gas giant planets at 5-10 AU in the Solar System.  相似文献   

Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations     

Masateru Ishiguro  Takahiro Hiroi  David J. Tholen  Sho Sasaki  Yuji Ueda  Tokuhiro Nimura  Masanao Abe  Beth E. Clark  Aya Yamamoto  Fumi Yoshida  Ryosuke Nakamura  Naru Hirata  Hideaki Miyamoto  Yasuhiro Yokota  Tatsuaki Hashimoto  Takashi Kubota  Akiko M. Nakamura  Robert W. Gaskell  Jun Saito 《Meteoritics & planetary science》2007,42(10):1791-1800

Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels.  相似文献   

Radial diffusion rate of planetesimals due to gravitational encounters     

Keiji Ohtsuki  Hidekazu Tanaka 《Icarus》2003,162(1):47-58

We study the rate of radial diffusion of planetesimals due to mutual gravitational encounters under Hill’s approximations in the three-body problem. Planetesimals orbiting a central star radially migrate inward and outward as a result of mutual gravitational encounters and transfer angular momentum. We calculate the viscosity in a disk of equal-sized planetesimals due to their mutual gravitational encounters using three-body orbital integrations, and obtain a semianalytic expression that reproduces the numerical results. We find that the viscosity is independent of the velocity dispersion of planetesimals when the velocity dispersion is so small that Kepler shear dominates planetesimals’ relative velocities. On the other hand, in high-velocity cases where random velocities dominate the relative velocities, the viscosity is a decreasing function of the velocity dispersion, and is found to agree with previous estimates under the two-body approximation neglecting the solar gravity. We also calculate the rate of radial diffusion of planetesimals due to gravitational scattering by a massive protoplanet. Using these results, we discuss a condition for formation of nonuniform radial surface density distribution of planetesimals by gravitational perturbation of an embedded protoplanet.  相似文献   

Thermohaline staircases in the Warm-Core Ring off Cape Erimo,Hokkaido and their fluxes due to salt finger     

Hideo Miyake  Seiji Sasaki  Hidekazu Yamaguchi  Kiyoshi Masuda  Gen Anma  Yoshihiko Kamei 《Journal of Oceanography》1995,51(1):99-109

Thermohaline staircases off Cape Erimo, Hokkaido are described and their physical properties are compared with those in other seas. The mean fluxes for heat and salt across the interface induced by salt finger were estimated as 105 cal cm^–2d^–1, and 0.03 g cm^–2d^–1, respectively. These values were in the same order as those in Caribbean Sea. The effective eddy diffusivities were also in the same order as the Caribbean ones. This suggests that the double-diffusive convection plays on important role on the water mass conversions occurring in the interfrontal zone between the Oyashio and the Kuroshio Waters.  相似文献   

A fundamental study of manganese nodules development by the electromagnetic system     

Y. Inada  K. Yokota  Y. Takabatake 《Ocean Engineering》1984,11(4):403-421

Presently, several methods for development of manganese nodules have received practical application. One of the important problems associated with these methods is contamination of the ocean by stirring pelagic sediments. In this paper, the results of a fundamental and experimental study on manganese nodules development by electromagnetic method are discussed.  相似文献   

Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan: A Hypothesis on the Oxygen-Deficient Water Mass Formation     

Tomoharu Senjyu  Hidekazu Yasuda  Shigehiko Sugihara  Masato Kamizono 《Journal of Oceanography》2001,57(1):15-27

A hydrographic survey and a 25-hour stationary observation were carried out in the western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated the upper layer water from the bottom water over the observational area except for near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M₂ tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer water is controlled by the advection due to the M₂ tidal current. On the other hand, the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due to the M₂ tidal current. The steep thermocline and halocline were maintained throughout the observation period in spite of the rather strong tidal currents. This implies an active intrusion of the low temperature and high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a course of circulation.  相似文献   

A toppled structure with sliding in the Siwalik Hills, midwestern Nepal   总被引：1，自引：0，他引：1  

Naresh Kazi Tamrakar  Shuichiro Yokota  Osamu Osaka   《Engineering Geology》2002,64(4):339-350

Not only the high mountainous regions but also the southern hilly regions of Nepal frequently suffer from landslides and debris flows. An interesting toppled structure with sliding was distinguished on an excavated slope facing the major highway in one such region, the Siwalik Hills. These hills are mostly composed of alternating beds of sandstones and mudstones of the Siwalik Group. A distinct contrast in the direction of dip of the strata was recognized on the excavated slope; the strata in the upper portion of the slope dipped southward, whereas those of the foot slope dipped northward. This indicated that toppling of strata had occurred. The northward direction of toppling and the angle and axis of rotation of the block were determined by examining the distribution of the attitudes of the strata.

Steeply dipping strata influenced by southward thrusting along the major tectonic line, contrasting rigidity between sandstone and mudstone, and rapid dissection by a river were the major causes of the toppling. The bottom surface of the toppled part dipped gently not only northward but also partially westward, showing that the toppled block slid westward after toppling. Evidence for tension cracking and scarplets on the top slope indicated that the block was still actively sliding.

Because steeply dipping sandstone and mudstone strata are widely distributed in the Siwalik Hills, similar structures are inferred to exist in many places. Therefore, the investigation of such slope structures is important, and their instability must be evaluated before road construction and excavation projects are carried out in Nepal.  相似文献