On the resolving power of tomographic images in the Aegean area

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The imaging of upper mantle heterogeneity by seismic tomography is strongly limited by the uneven global distribution both of seismic recording stations and earthquake sources. This can result in a loss of resolution and significance in the final image, particularly when a sparse data set contains few ray paths which intersect at sufficiently high angles in the volume of interest. In order to investigate the theoretical resolving power of a previously published tomographic image of the Aegean area, synthetic tests of the inversion procedure using a ray-path matrix obtained in this previous study for local and teleseismic P -waves were carried out. The aim was to examine the extent to which the shape of a synthetic lithospheric slab penetrating to different depths is inherently distorted by the tomographic imaging procedure, and to compare the synthetic tomographic images with the results from the actual inversion. The distortion is found to take the form of an artificial stretching of the lithospheric slab. The maximum 'stretching factor', as indicated by the downdip displacement of the peak amplitude of the synthetic high-velocity anomaly, is found to be a factor of 2 or so, though the distortion is usually less than this. The peak amplitude of the tomographic image of a lithospheric slab is found from the inversion of traveltime data to be at depths at or below 400 km. This indicates that the high-velocity lithospheric slab in the Aegean penetrates deeper than the Benioff zone seismicity of about 200 km. However, no constraints of the maximum depth of penetration could be established with the data set used in the present work.     

Precursory seismic quiescence in the Mudurnu Valley, North Anatolian fault zone, Turkey

The seismicity rate in the Mudurnu Valley of Turkey was studied using an earthquake catalogue that reports events homogeneously down to magnitude 2.3 for the years 1985–1989, and covers the area between latitudes 40.2° and 41.0°N, and longitudes 30.0° and 31.5°E. During this period the only two main shocks, M = 4.0 and M = 4.3, occurred on 1988 September 6 and 1988 December 9 within about 30km of each other. A highly significant seismic quiescence is evident in the area surrounding these main shocks, while the seismicity rate in the rest of the area covered by the catalogue remains constant. the quiescence becomes more pronounced the smaller the area around the main shocks that is studied. the smallest areas that can be studied contain about 60 earthquakes and have dimensions of approximately 25km on each side. the decreases in seismicity rates are 50–80 per cent depending on the volume and period used for defining the quiescence. the quiescence started in 1988 January and lasted about seven months, with approximately 4.5 months of normal activity separating it from the main shock of December. the precursor time of 12 months for an M = 4.3 main shock is similar to those observed in California. It is concluded that it is possible to resolve precursory quiescence before moderate and large earthquakes in the Mudurnu area with the existing seismograph network.     

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

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.     

The first and third asymptotic distributions of extremes as applied to the seismic source regions of India and adjacent areas

The seismic source regions are identified on the basis of spatial and temporal distributions of shocks (1900–1989), recurrence relations and the tectonic architecture of the Indian subcontinent and adjoining areas. The probable occurrence of the maximum magnitude earthquake is estimated using the theory of extreme values of Gumbel. The parameters of the first and third asymptotic distributions of extremes and their uncertainty values are computed for the seven identified seismic source regions of India and adjacent areas. The third-type distribution curve is preferable to the first type in all the regions, as revealed by the χ² test. The results of the third asymptotic distribution indicate the upper bound to earthquake magnitude w is equal to 8.94 ± 0.21 for Assam, 8.56 ± 0.29 for Bihar-Nepal, 8.43 ± 0.10 for Kangra, 8.97 ± 0.27 for Hindukush, 7.61 ± 0.24 for Pakistan-Cutch, 7.34 ± 0.12 for Koyna and 8.98 ± 0.27 for Andaman Sea seismic source regions. The predicted most probable largest earthquake magnitude is computed for return periods of 10, 20, 50, 75 and 100 yr in each source region.     

东部三大经济区城市流动人口的多维度社会融入及其地区差异

以流动人口集中的京津冀、长三角和珠三角三大经济区35个城市2012年流动人口动态监测数据为基础,采用均方差权值法,综合经济、社会、心理、制度和公共服务5个维度构建衡量流动人口社会融入的指标体系,在此基础上计算和比较了三大经济区及其35个城市流动人口各维度社会融入指标和社会融入综合水平得分及其地区差异。研究结论显示,社会融入综合水平得分由高到低依次是京津冀、长三角、珠三角。同一地区流动人口的融入状况在各个维度不均衡,京津冀地区流动人口社会维度的融入程度最高,制度维度融入程度最低;长三角地区流动人口的心理维度融入居前列,但社会维度融入位列最后;珠三角地区流动人口的社会维度融入程度最高,公共服务维度融入不足。此外,就整体而言,三大地区流动人口的工作强度大、交往对象固化于流动人口、在流入地长期居住意愿多数仍处于举棋不定的状态;三大地区都存在落实居住证/暂住证制度和流动人口享有城镇医保等方面较为薄弱等诸多问题并共同制约着流动人口真正融入流入城市。为此,需要在政策制定中予以关注。     

Numerical simulation of roadbed slope under seismic action in permafrost regions

The deformation and strength characteristics of roadbed slope under seismic loading in permafrost regions are simulated numerically. The seismic response of roadbed at different positions and inclinati...     

Analysis of permanent deformations of railway embankments under repeated vehicle loadings in permafrost regions

By large-scale dynamic tests carried out on a traditional sand-gravel embankment at the Beilu River section along the Qinghai-Tibet Railroad, we collected the acceleration waveforms close to the railway tracks when trains passed. The dynamic train loading was converted into an equivalent creep stress, using an equivalent static force method. Also, the creep equation of frozen soil was introduced according to the results of frozen soil rheological triaxial tests. A coupled creep model based on a time-hardening power function rule and the Druker-Prager yield and failure criterion was established to analyze the creep effects of a plain fill embankment under repeated train loads. The temperature field of the embankment in the permafrost area was set at the current geothermal conditions. As a result, the permanent deformation of the embankment under train loading was obtained, and the permanent deformation under the train loads to the total embankment deformation was also analyzed.