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More than a thousand acoustic-emission (AE) hypocenters were determined in a cylindrical andesite specimen under two-stage uniaxial creep at stresses of 204 and 214 MPa. Strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen's wall. The strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and, therefore, biased stress distribution within the specimen. The correlation between dilatant strain and AE hypocenters was investigated for whether or not tensile cracks emit AE. The region with high AE activity shows only a small dilatant strain. This negative correlation between AE and the dilatant region may eliminate tensile cracks as possible AE sources. A composite focal-mechanism solution of local AE events, covering a wide solid angle of the focal hemisphere, indicates that shear fractures emit AE waves. The direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force, suggesting that the local stress field is governed by preexisting weak zones that are, presumably, produced by tensile cracks within the specimen. AE hypocenters tended to form clusters during steady creep under the constant compressional stress. During acceleration creep caused by a small step increase of the external stress, the preceding clusters disappeared while a new cluster appeared in an incipient fault plane. This suggests that changes in seismicity pattern such as migrations or quiescences of swarm—important clues for earthquake predictions—may be caused by an instantaneous change in the tectonic-stress levels. 相似文献
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We investigate the electron-positron pair concentration in an optically thin mildly relativistic plasma which is supposed to exist in active galactic nuclei. Firstly the equilibrium concentration is calculated when copious soft photons are supplied through the cyclotron higher harmonics. It is shown that the attainable states of the plasma are strongly restricted. Secondly we examine the pair production in a hot accretion plasma around a massive black hole, comparing relevant time scales. We find that significant pair production occurs when the accretion rate is moderately high and the infall velocity is slow compared to the free fall.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984 相似文献
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Possible solar neutron emission associated with five SMM gamma-ray events on 7 June 1980, 21 June 1980, 6 November 1980, 26 November 1982 and 25 April 1984 was found from analysis of 10-minute records of the ground-based neutron monitor at Tokyo. Of these the two events on 21 June 1980 and 25 April 1984 have been already known as neutron events. The time histories of the neutron monitor count rate are compared with those of the gamma-ray count rate and the possibility of energetic neutron emission at the flare site is discussed. 相似文献
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Solar Physics - Possible solar neutron emission associated with five SMM gamma-ray events on 7 June 1980, 21 June 1980, 6 November 1980, 26 November 1982 and 25 April 1984 was found from analysis... 相似文献
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Hitoshi Oda Osamu Nishizawa Kin'ichiro Kusunose Takayuki Hirata 《Pure and Applied Geophysics》1990,133(1):73-85
Velocity as well as attenuation factorQ
–1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5
f
, where
f
is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ
–1 increases. Particularly, the change ofQ
–1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ
–1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ
–1 is approximated asf
n
in the present frequency range, the exponentn takes a value from 0.63 to 0.77. 相似文献
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Fractal structure and characteristic scale in the distributions of earthquake epicentres, active faults and rivers in Japan 总被引:3,自引:0,他引:3
Using a box-counting method, we examined the fractal structures of the spatial distributions of three geological systems in Japan, namely those of earthquake epicentres, active faults, and rivers. Our results show that, in the scale range from 1 to 100 km, all geological systems have a common characteristic scale of ~13 km, which divides the spatial distribution into two bands: a smaller scale r < 13 km and larger scale r > 13 km (where r is the box size). In both bands, the three systems obey a power law distribution, and therefore it is proposed that all geological systems have a band-limited fractal structure. Since the characteristic scale of ~13 km is in agreement with the depth of the brittle–ductile transition zone of the crust, we suggest that it is a common feature of the heterogeneity of the crust. 相似文献
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