Seismicity at White Island volcano, New Zealand: a revised classification and inferences about source mechanism     

Steven Sherburn  Bradley J. Scott  Yuji Nishi  Mituhiko Sugihara 《Journal of Volcanology and Geothermal Research》1998,83(3-4)

The classification of earthquakes at White Island volcano, New Zealand, has been revised to address problems in existing classification schemes, to better reflect new data and to try to focus more on source processes. Seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic, and that generated by fault movement in response to stresses caused by those fluids, regional stresses, thermal effects and so on are referred to as volcano-tectonic. Spasmodic bursts form a separate category, as we have insufficient information to classify them as volcanic or volcano-tectonic. Volcanic seismicity is divided into short-duration, long-period volcanic earthquakes, long-duration volcanic earthquakes, and harmonic- and non-harmonic volcanic tremor, while volcano-tectonic seismicity is divided into shallow and deep volcano-tectonic earthquakes. Harmonic volcanic tremor is related to sub-surface intrusive processes, while non-harmonic volcanic tremor originates close to active craters at shallow depth, and usually occurs during eruptive activity. Short-duration, long-period volcanic earthquakes come from a single source close to the active craters, but originate deeper than non-harmonic volcanic tremor, and are not related to eruptive activity. Long-duration volcanic earthquakes often accompany larger discrete eruptions. The waveform of these events consists of an initial low-frequency part from a deep source, and a later cigar-shaped part of mixed frequencies from a shallow crater source.  相似文献   

Wind-Tunnel Study Of Atmospheric Stable Boundary Layers Over A Rough Surface     

Yuji Ohya 《Boundary-Layer Meteorology》2001,98(1):57-82

Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses.  相似文献   

Seismotectonics of the Median Tectonic Line in southwest Japan: Implications for coupling among major fault systems     

Yuji Kanaori  Shin-ichi Kawakami  Kenji Yairi 《Pure and Applied Geophysics》1994,142(3-4):589-607

The relationship between the slip activity and occurrence of historical earthquakes along the Median Tectonic Line (MTL), together with that of the fault systems extending eastward has been examined. The MTL is divided into three segments, each containing diagnostic active faults. No historical earthquakes have been recorded along the central segment, although the segment has faster Quaternary slip rates compared with the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explained by a microplate model of southwest Japan. The microplate model also provides spatial and temporal coupling of slip on adjacent fault systems. In the context of this model, slip on adjacent faults reduces the normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong ground motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where historical earthquakes were recorded. Alternatively, the earthquakes at the eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that the MTL characteristically does not produce great earthquakes. The microplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough.  相似文献   

Helium-isotope systematics at Nevado del Ruiz volcano, Colombia: implications for the volcanic hydrothermal system     

Yuji Sano  Hiroshi Wakita  Stanley N. Williams 《Journal of Volcanology and Geothermal Research》1990,42(1-2)

We have collected 14 water and gas samples from 9 thermal springs and gas vents near Nevado del Ruiz volcano, Colombia. The ³He/⁴He and ⁴He/²⁰Ne ratios vary significantly from 0.98 R_atm (where R_atm is the atmospheric ³He/⁴He ratio of 1.4 × 10⁻⁶) to 6.30 R_atm, and from 0.37 to 7.0, respectively. The ³He/⁴He ratio (corrected for air contamination) decreases with increasing distance from the central crater of the volcano to the sampling site. The trend is very similar to that observed at Ontake volcano, Japan. A hydrodynamic porous-media dispersion model can explain the ³He/⁴He trend. The temporal variations in the ³He/⁴He ratio at four sites provide useful information on the apparent velocity of the magmatic fluid flow brought on by a volcanic eruption. The estimated value of several tens m day⁻¹ agrees well with the inferred velocity of flow in Oshima volcano, Japan and is comparable to the largest rate of groundwater movement in a deep sedimentary basin.  相似文献   

Short-term and intermediate-term geochemical precursors   总被引：2，自引：0，他引：2  

Hiroshi Wakita  Yuji Nakamura  Yuji Sano 《Pure and Applied Geophysics》1988,126(2-4):267-278

Detection of precursory phenomena in observation data is essential to earthquake prediction studies. Continuous monitoring of radon concentration in groundwater in Japan in one case showed a short-term anomaly related to a nearby earthquake. With the exception of the 1978 Izu-Oshimakinkai earthquake (M7.0), however, no abnormal change has been noted. This may be due partially to difficulty in detecting insignificant precursory signals from observation data, which ordinarily contains a noise-induced fluctuations, and partially to lack of understanding of the mechanism controlling the appearance of precursory phenomena. In order to increase our knowledge of the variation pattern of precursory changes in radon concentration of groundwater, hydrologic precursors with significant features are examined in this paper. Complexity of appearance of precursory phenomena and problems in assignment of the specific earthquake are discussed.  相似文献   

Analysis of light variations of the eclipsing binary system XY ceti with Am spectra     

Akira Okazaki 《Astrophysics and Space Science》1978,56(2):293-302

The photoelectric light curves of XY Cet of Srivastava and Padalia (1975) and of Morrison and Morrison (1968), from which considerably different sets of photometric elements have previously been deduced, were re-analyzed in detail. The elements derived in the present study from Srivastava and Padalia's (1975)UBV observations arer _g=0.157±0.006 (p.e.),r _s=0.142±0.007,i=88°.4±0°.5,L _g=0.600±0.017, 0.602±0.015 and 0.584±0.018 forU, B andV, respectively. It is also found from their light curves that the system has an eccentric orbit withe cos =0.0026±0.0005. The elements deduced from theV observations of Morrison and Morrison (1968) are found to be in satisfactory agreement with those presented above.The existence of theAm characteristics of both components, which was recognized by earlier spectroscopic observations, are supported by theB-V andU-B color indices and [m ₁] value of the system. Both components are also found to have appreciably evolved, as expected for metallicline stars.  相似文献   

Geostrophy in the Intermediate and Deep Layers of the Kuroshio and Its Recirculation Regions South of Japan     

Motohiko Kashima  Shiro Imawaki  Shin-Ichiro Umatani  Hiroshi Uchida  Yuji Hashibe  Hiroshi Ichikawa  Masao Fukasawa 《Journal of Oceanography》2003,59(3):291-301

Theoretically, the geostrophic approximation holds for the low-frequency flow field, but no detailed examination has been done on how well the estimated geostrophic velocity corresponds with the observed velocity. Intensive surveys were carried out during 1993–1995 in the Kuroshio and its recirculation regions south of Shikoku, Japan, including repeated hydrographic surveys and direct current measurements at nominal depths of 700, 1500 and 3000 m. For these depth intervals, vertical differences of estimated geostrophic velocity are compared with those of observed velocity. For the intermediate layer (between 700 and 1500 m depths), the slope of the regression line is 0.99, correlation coefficient is 0.98, and the root-mean-square of difference from geostrophic balance is 2.8 cm/s which is close to the estimated error of 2.1 cm/s. For the deep layer (between 1500 and 3000 m depths), the corresponding values are 0.82, 0.93, 1.2 cm/s and 2.0 cm/s, respectively. The results indicate that the estimated geostrophic velocity compares well with the observed velocity in these regions. This revised version was published online in August 2006 with corrections to the Cover Date.  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   

The contribution of GIS to geographical research     

Yuji Murayama 《GeoJournal》2000,52(3):163-164

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Implications of meso-scale eddies caused by frontal disturbances of the Kuroshio Current for anchovy recruitment   总被引：3，自引：1，他引：2  

Nakata  Hideaki; Kimura  Shingo; Okazaki  Yuji; Kasai  Akihide 《ICES Journal of Marine Science》2000,57(1):143-152

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