Exposure history of separated phases from the Kapoeta meteorite     

Marc W. CAFFEE  Kunihiko NISHIIZUMI 《Meteoritics & planetary science》2001,36(3):429-437

Abstract— The cosmogenic radionuclides, ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁵³Mn were measured in selected clasts and matrix samples from the howardite Kapoeta. Previous measurements of cosmogenic ²¹Ne indicate higher cosmic‐ray exposure ages for bulk samples than for some separated clasts or mineral separates. A possible interpretation for this difference in apparent exposure ages is a complex recent exposure history for Kapoeta. In this scenario some constituents are exposed to cosmic rays in a 2π geometry as part of a larger body immediately preceding its 4π exposure in a smaller body. To test this scenario we measured cosmogenic radionuclides in several clasts from Kapoeta. These measurements are consistent with a simple single‐stage 4π exposure history during which the entire inventory of cosmogenic radionuclides was produced. Taken together, these data are most consistent with a single‐stage 4π exposure lasting ～3 Ma. This scenario is nevertheless consistent with models in which the exposure of some constituents of Kapoeta to energetic particles occurred at an earlier time, as is indicated by ²¹Ne measurements. However, from our data we conclude that insubstantial quantities of cosmogenic radionuclides were inherited from this earlier irradiation; this earlier exposure to energetic particles must have predated the recent exposure by at least ～10 Ma to allow for the decay of the long half‐life cosmogenic radionuclides.  相似文献   

Assessment of the effects of vegetation on soil erosion risk by water: a case of study of the Batta watershed in Tunisia   总被引：2，自引：1，他引：1  

Mohamed Kefi  Kunihiko Yoshino  Yudi Setiawan  Khemaies Zayani  Mohamed Boufaroua 《Environmental Earth Sciences》2011,64(3):707-719

Soil erosion by water is a serious environmental problem which affects particularly the agriculture of developing countries. Due to specific factors, such as high rainfall intensity, steep slopes and vegetation scarcity, Tunisia is prone to soil erosion. Taking this into account, the main objective of this study was to estimate the soil erosion risk in the Batta watershed in Tunisia using qualitative and quantitative erosion model with remote sensing data and geographic information system (GIS). Moreover, a developed method that deals with evaluating the impact of vegetation on soil erosion by water is also applied. This method used multi-temporal satellite images with seasonal variability and the transformed soil adjusted vegetation index (TSAVI) which is appropriate in arid and semi-arid areas. For both erosion models, the results show that a large area of the Batta watershed is seriously affected by erosion. This potentially high risk is due especially to severe slopes, poor vegetation coverage and high soil erodibility in this watershed. Furthermore, the use of multi-temporal satellite images and vegetation index show that the effect of vegetation is a significant factor to protect the soil against erosion. The risk is especially serious in the summer season, but it decreases with the growth of vegetation cover in spring. Erosion model, combined with a GIS and remote sensing, is an adequate method to evaluate the soil erosion risk by water. The findings can be used by decision makers as a guideline to plan appropriate strategies for soil and water conservation practices.  相似文献   

Free oscillation of the Japan Sea excited by earthquakes—I. Observation and wave-theoretical approach     

Kenji Satake  Kunihiko Shimazaki 《Geophysical Journal International》1988,93(3):451-456

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Fall,classification, and exposure history of the Mifflin L5 chondrite     

Noriko T. Kita  Kees C. Welten  John W. Valley  Michael J. Spicuzza  Daisuke Nakashima  Travis J. Tenner  Takayuki Ushikubo  Glenn J. MacPherson  Linda Welzenbach  Philipp R. Heck  Andrew M. Davis  Matthias M. M. Meier  Rainer Wieler  Marc W. Caffee  Matthias Laubenstein  Kunihiko Nishiizumi 《Meteoritics & planetary science》2013,48(4):641-655

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low ²²Ne/²¹Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.  相似文献   

Quantitative mapping of a precursory seismic quiescence to the Izu–Oshima 1990 (M6.5) earthquake, Japan     

Max Wyss  Kunihiko Shimazaki  Taku Urabe 《Geophysical Journal International》1996,127(3):735-743

A highly significant seismic quiescence with a standard deviate Z = 10.1. corresponding to a 99 per cent confidence level, lasted from 1987.7 up to the 1990 February 20 Izu-Oshima M 6.5 earthquake. The quiescent volume had dimensions of 30 km N-S and 10 km E-W and was centred below 14 km depth. Within the recently upgraded seismograph network of the Earthquake Research Institute (ERI), this main shock was the only one with a magnitude M > 5.8 in the upper 30 km of the crust for which the precursory quiescence hypothesis could be tested. Within a radius of 50 km, and during the observation period (1983.5–1995.9), there were no other 1.5 yr or longer periods of quiescence that were rated Z > 6.5 in the declustered earthquake catalogue, except one that was associated with volcanic activity. The total space-time covered by alarms, including the volcanic one, was less than 1 per cent at the Z = 6.5 level. The rarity of highly significant episodes of quiescence, and the correlation in space and time suggest that a precursory seismic quiescence started 2.5 yr before the Izu-Oshima 1990 earthquake in its source volume and to the north of it, and that it can be recognized with an alarm level of Z = 6.0, generating no false alarms. During the 1.5 yr quiescence window, only 10 earthquakes occurred in the quiet volume, whereas 50 events were expected based on the rate seen at other times. In randomly selected volumes containing 50, 100 and 200 events, the anomaly scored Z = 6.1 to 10.1. On the basis of the data from May 1983 to 1995, there is no highly significant quiescence currently present in the Izu-Oshima area.  相似文献   

Cosmic-ray-produced⁵³Mn in thirty-one meteorites     

Kunihiko Nishiizumi 《Earth and Planetary Science Letters》1978,41(1):91-100

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.  相似文献   

Rupture process of the Miyagi-Oki,Japan, earthquake of June 12, 1978     

Tetsuzo Seno  Kunihiko Shimazaki  Paul Somerville  Ken Sudo  Takao Eguchi 《Physics of the Earth and Planetary Interiors》1980,23(1):39-61

The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 10²⁷ dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km², respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment $\text{M}{}_0\text{= 1.7 × 10}{}^{27}\text{dyne-cm, slip dislocation}\text{u}\text{= 1.9}\text{m}\text{, Δσ = 95}\text{bar, rupture velocity}\text{ν = 3.2}\text{km s}{}^{\mathrm{?1}}\text{,}\text{rise time}\text{τ = 2}\text{s}$, for the first event; $\text{M}{}_0\text{= 1.4 × 10}{}^{27}\text{dyne-cm}\text{,}\text{u}\text{= 2.4}\text{m}\text{, Δσ = 145}\text{bar}$, for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 10¹⁰ erg cm^?2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.  相似文献   

On the theoretical model for vertical ozone density distributions in the mesosphere and upper stratosphere     

Shimazaki  T.  Wuebbles  D. J. 《Pure and Applied Geophysics》1973,106(1):1446-1463

Diurnal variations in the vertical ozone density distribution have been calculated for the height range 40–150 km by extending our existing computer programs. The steady-state profiles were first calculated for fifteen constituents in the original program and three additional constituents (CH₄, CO and CO₂); the result was used as the initial condition for the time-dependent solution. The profile of the eddy diffusion coefficient used in this study was determined by comparing the model profile with the observations for CH₄, whose density distribution is verysenstive to the eddy diffusion coefficient The effects of hydrogen and nitrogen compounds on the ozone density are discussed somewhat quantitatively; they reduce the ozone density mainly in the mesosphere and stratosphere, respectively. Special attention is given to the large depression of the ozone density at around 70–85 km, which has been obtained in many theoretical models but has neither been explained nor definitely confirmed by observations. Our time-dependent model indicates that the depression develops at night by the effect of hydrogen-oxygen and nitrogen-oxygen reactions and of eddy diffusion transports. The latter effect also produces an increase of the ozone density after midnight at some heights in the depression region.  相似文献   

Optimum strong-motion array geometry for source inversion     

Masahiro Iida  Takashi Miyatake  Kunihiko Shimazaki 《地震工程与结构动力学》1988,16(8):1213-1225

Optimum strong-motion array geometry for source inversion is quantitatively determined for each of three types of earthquake faults: strike—slip, dip-slip and offshore subduction thrust. The estimation is done by using the source inversion method^1,2 previously developed and based on Wolberg's prediction analysis.³ Only the far-field S waves are taken into account in source inversion; we test only the ability to estimate seismic moments of subfault elements. A comparison of the optimum array geometries obtained in this study with the ones proposed on the basis of an empirical judgment at the 1978 International Workshop on Strong-Motion Earthquake Instrument Arrays⁴ shows that the Workshop geometries are not the best ones as far as source studies are concerned. The most preferable array involves two different kinds of stations regardless of the target fault type: stations close to the fault and aligned parallel to its strike, and stations surrounding the fault area with good azimuthal coverage. The former stations resolve the later stage of the rupturing process while the latter stations resolve the earlier stage.  相似文献   

洪湖沉积物的磁性特征及其环境意义   总被引：9，自引：0，他引：9  

曹希强  郑祥民  周立旻  杜耘  远藤邦彦 《湖泊科学》2004,16(3):227-232

以洪湖沉积物为研究对象,通过对Hh01-2(B)孔岩芯磁参数的测量发现在岩芯所代表的时间段内洪湖沉积物磁化率呈现了明显的四个主阶段变化;频率磁化率变化与磁化率变化具有较好的相关性,亚铁磁性矿物的变化主导了沉积物磁化率的变化.同时结合年代、孢粉分析以及参考相关历史记载资料,发现磁化率高值段对应湖水动力强、湖面较宽、气候较湿润,而磁化率低值段对应湖水动力弱、湖面较小、气候偏干旱.从而得出了洪湖沉积物磁参数曲线波峰和波谷分别对应洪湖在历史上曾经历了扩张和收缩、气候温湿和冷干的旋回.  相似文献