An analytical calculation of two-dimensional dispersion     

B. G. Sanderson  A. Okubo 《Journal of Oceanography》1986,42(2):139-153

The methods of Okuboet al. (1976a) are used to calculate the Lagrangian deformations and diffusivities of a cluster of drifters. A solution of the two-dimensional first-order advection-diffusion equation (Okuboet al., 1983a) is then used to calculate the dimensions and orientation of the cluster from these Lagrangian deformations and diffusivities. The solution is shown to be internally consistent (to give cluster areas that are consistent with the observed cluster areas) to within a 0.5% error. As time progresses a larger portion of the dispersion is caused by the diffusivities rather than the deformations. In the experiments analyzed the Lagrangian deformations and diffusivities are generally observed to increase at a constant rate over time intervals of about one hour. Dimensional arguments suggest that Lagrangian diffusivities increase proportional tot ² and the deformations proportional tot ^1,5 for time intervals large compared to the period required to spread from a point source to the initial cluster dimensions. Small quadratic velocity gradients cause the solution of the first order advection-diffusion equation to overestimate cluster spreading. Most of the displacement (once motion due to the mean velocity and linear deformations is extracted) is caused by scales of motion much smaller than the cluster. This explains the relatively small magnitude of the errors caused by parameterizing quadratic and other statistically significant nonlinear shears as a component of the eddy-diffusivity.  相似文献   

Radium-228 in the Japan Sea     

Takashi Okubo 《Journal of Oceanography》1980,36(5):263-268

The distribution of²²⁸Ra in surface and subsurface waters in the Japan Sea was studied. The concentrations of²²⁸Ra in surface waters were around 100 dpm/1000l which were much higher than those reported for Pacific surface waters. The concentrations of²²⁸Ra decreased with increasing depth to less than 10 dpm/1000l in the Japan Sea Proper Water. Based on the comparison between observed values of²²⁸Ra and calculated profile through the near-surface water mass and the underlying main water mass in the Japan Sea, the apparent vertical eddy diffusion coefficient was estimated to be about 2 cm² s^–1.  相似文献   

Constraints on mechanisms for the growth of gully alcoves in Gasa crater, Mars, from two-dimensional stability assessments of rock slopes     

Chris H. Okubo  Livio L. Tornabene 《Icarus》2011,211(1):207-221

The value of slope stability analyses for gaining insight into the geologic conditions that would facilitate the growth of gully alcoves on Mars is demonstrated in Gasa crater. Two-dimensional limit equilibrium methods are used in conjunction with high-resolution topography derived from stereo High Resolution Imaging Science Experiment (HiRISE) imagery. These analyses reveal three conditions that may produce observed alcove morphologies through slope failure: (1) a ca. >10 m thick surface layer that is either saturated with H₂O ground ice or contains no groundwater/ice at all, above a zone of melting H₂O ice or groundwater and under dynamic loading (i.e., seismicity), (2) a 1-10 m thick surface layer that is saturated with either melting H₂O ice or groundwater and under dynamic loading, or (3) a >100 m thick surface layer that is saturated with either melting H₂O ice or groundwater and under static loading. This finding of three plausible scenarios for slope failure demonstrates how the triggering mechanisms and characteristics of future alcove growth would be affected by prevailing environmental conditions. HiRISE images also reveal normal faults and other fractures tangential to the crowns of some gully alcoves that are interpreted to be the result of slope instability, which may facilitate future slope movement. Stability analyses show that the most failure-prone slopes in this area are found in alcoves that are adjacent to crown fractures. Accordingly, crown fractures appear to be a useful indicator of those alcoves that should be monitored for future landslide activity.  相似文献   

A new method for the computation of global viscoelastic post-seismic deformation in a realistic earth model (II)-horizontal displacement   总被引：1，自引：0，他引：1  

Y. Tanaka  J. Okuno  S. Okubo 《Geophysical Journal International》2007,170(3):1031-1052

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Potential and gravity changes raised by point dislocations   总被引：9，自引：0，他引：9  

Shuhei Okubo 《Geophysical Journal International》1991,105(3):573-586

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Some Attempts for Estimating Rock Strength and Rock Mass Classification from Cutting Force and Investigation of Optimum Operation of Tunnel Boring Machines     

K. Fukui  S. Okubo 《Rock Mechanics and Rock Engineering》2006,39(1):25-44

Summary. Tunnel face and wall collapse are common during excavations performed by tunnel boring machines (TBMs) due to the difficulty of correctly identifying the properties of the excavated rock. This identification, however, can be simplified by using the cutting force to estimate rock strength, a method that has already proved quite successful in Japanese tunnel excavations. This paper summarizes knowledge relating to the cutting force obtained through tunnel excavation experience, and the relationship between rock strength and TBM operation is discussed. Although TBM operators rely on intuition to set the cutter head speed appropriately, this decision process represents a logical method of operation that takes advantage of the variable speed capability of the cutter head. Selection of appropriate support methods for the excavated face is also a critical issue in tunnel excavation. This selection process is based on the condition of the rock, which is difficult to determine quickly and accurately during tunnel excavation. The present paper uses the excavation of two tunnels to demonstrate that it is possible to assign rock mass classifications accurately based on rock strength when boring a uniform rock type. It is also shown that the rock mass can be classified from the rock strength normalized by the uniaxial compressive strength when boring through mixed rock types.  相似文献   

Green's functions of coseismic strain changes and investigation of effects of Earth's spherical curvature and radial heterogeneity   总被引：2，自引：0，他引：2  

Wenke Sun  Shuhei Okubo  Guangyu Fu 《Geophysical Journal International》2006,167(3):1273-1291

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Inverted channel deposits on the floor of Miyamoto crater, Mars     

Horton E. Newsom  Nina L. Lanza  Sandra M. Wiseman  Giuseppe A. Marzo  Chris H. Okubo  Victoria E. Hamilton 《Icarus》2010,205(1):64-72

Morphological features on the western floor of Miyamoto crater in southwestern Meridiani Planum, Mars, are suggestive of past fluvial activity. Imagery from the High Resolution Imaging Science Experiment (HiRISE) gives a detailed view of raised curvilinear features that appear to represent inverted paleochannel deposits. The inverted terrain appears to be capped with a resistant, dark-toned deposit that is partially covered by unconsolidated surficial materials. Subsequent to deposition of the capping layer, erosion of the surrounding material has left the capping materials perched on pedestals of uneroded basal unit material. Neither the capping material nor the surrounding terrains show any unambiguous morphological evidence of volcanism or glaciation. The capping deposit may include unconsolidated or cemented stream deposits analogous to terrestrial inverted channels in the Cedar Mountain Formation near Green River, Utah. In addition to this morphological evidence for fluvial activity, phyllosilicates have been identified in the basal material on the floor of Miyamoto crater by orbital spectroscopy, providing mineralogical evidence of past aqueous activity. Based on both the morphological and mineralogical evidence, Miyamoto crater represents an excellent site for in situ examination and sampling of a potentially habitable environment.  相似文献   

Geomorphic knobs of Candor Chasma, Mars: New Mars Reconnaissance Orbiter data and comparisons to terrestrial analogs     

Marjorie A. Chan  Jens Ormö  Chris H. Okubo  James J. Wray  James A. McGovern 《Icarus》2010,205(1):138-519

High Resolution Imaging Science Experiment (HiRISE) imagery and digital elevation models of the Candor Chasma region of Valles Marineris, Mars, reveal prominent and distinctive positive-relief knobs amidst light-toned layers. Three classifications of knobs, Types 1, 2, and 3, are distinguished from a combination of HiRISE and Thermal Emission Imaging System (THEMIS) images based on physical expressions (geometries, spatial relationships), and spectral data from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Type 1 knobs are abundant, concentrated, topographically resistant features with their highest frequency in West Candor, which have consistent stratigraphic correlations of the peak altitude (height). These Type 1 knobs could be erosional remnants of a simple dissected terrain, possibly derived from a more continuous, resistant, capping layer of pre-existing material diagenetically altered through recrystallization or cementation. Types 2 and 3 knobs are not linked to a single stratigraphic layer and are generally solitary to isolated, with variable heights. Type 3 are the largest knobs at nearly an order of magnitude larger than Type 1 knobs. The variable sizes and occasional pits on the tops of Type 2 and 3 knobs suggest a different origin, possibly related to more developed erosion, preferential cementation, or textural differences from sediment/water injection or intrusion, or from a buried impact crater. Enhanced color HiRISE images show a brown coloration of the knob peak crests that is attributable to processing and photometric effects; CRISM data do not show any detectable spectral differences between the knobs and the host rock layers, other than albedo. These intriguing knobs hold important clues to deducing relative rock properties, timing of events, and weathering conditions of Mars history.  相似文献   

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars   总被引：1，自引：0，他引：1  

Lynn M. Carter  Bruce A. Campbell  Roger J. Phillips  Ali Safaeinili  Chris H. Okubo  Roberto Seu  Roberto Orosei 《Icarus》2009,199(2):295-302

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.  相似文献