从第22届IUGG大会看地震预测研究

1999年 7月 19～ 3 0日 ,第 2 2届 IUGG大会在英国伯明翰大学举行。从提交的论文中可以看出 ,有关地震预测和地震灾害的研究仍是地学界关注的热点问题。文章介绍了这次大会反映出的有关地震预测研究的情况。     

四川中西部地区地壳结构与重力均衡

汶川MS8.0地震的“孕育”、发生和发展是深部物质分异、调整和运移的产物.为此,在四川中西部地区（松潘—甘孜,龙门山断裂系和四川盆地）应用重力布格异常场资料并通过两条典型剖面的地壳厚度(Moho界面深度）分布与依据Airy均衡理论给出的理论均衡地壳厚度做对比分析,探讨四川盆地、龙门山造山带、邛崃山等川西地域的地壳均衡状态,得到龙门山及其以西地带的深部地壳结构.龙门山造山带恰处于很不均衡的状态,即与其西北和东南部相比差异明显,故地壳处于不稳定状态.为探讨该区重力场均衡与强烈地震活动之间的关系,还必须进行新一轮的高精度观测,以对该区深部精细结构、重力场效应和均衡补偿深入研究.此外,本文研究结果有益于震后确定城镇布局和为不同类型建筑物重建选址提供深部要素.     

Field survey of the East Java earthquake and tsunami of June 3, 1994

A field survey of the June 3, 1994 East Java earthquake tsunami was conducted within three weeks, and the distributions of the seismic intensities, tsunami heights, and human and house damages were surveyed. The seismic intensities on the south coasts of Java and Bali Islands were small for an earthquake with magnitudeM 7.6. The earthquake caused no land damage. About 40 minutes after the main shock, a huge tsunami attacked the coasts, several villages in East Java Province were damaged severely, and 223 persons perished. At Pancer Village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami. The relationship between tsunami heights and distances from the source shows that the Hatori's tsunami magnitude wasm=3, which seems to be larger for the earthquake magnitude. But we should not consider this an extraordinary event because it was pointed out byHatori (1994) that the magnitudes of tsunamis in the Indonesia-Philippine region generally exceed 1–2 grade larger than those of other regions.     

Preliminary seismic anisotropy in the upper crust of the south segment of Xiaojiang faults and its tectonic implications

The Xiaojiang faults,striking north-to-south(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults.     

2021年3月19日西藏比如MS 6.1地震总结

2021年3月19日西藏比如MS 6.1地震发生在班戈—安多地区的安多盆地南缘断裂附近,震源机制解反演结果显示,本次地震为近东西走向断层产生正断兼走滑型错动的结果,与区域构造特征一致;截止至4月15日,地震序列跟踪发现,主震与余震震级差为1.4,主震释放能量占序列的98.96％,表明6.1级主震释放了此次地震序列的绝大...     

鄂尔多斯地块周缘三维地壳速度结构研究

鄂尔多斯地块及周缘位于青藏高原与华北地块之间,其内部无大断裂分布,而周缘却由活跃的地震带——活动断陷带和弧形断裂带包围.因此,鄂尔多斯地块内部无6级以上大地震分布,而周缘地震活动则频度高、强度大(图1),历史上发生多次破坏性大地震,如:1654 年天水南8.0 级、1879 年武都南8.0级、1920 年宁夏海原81/...     

北川县城汶川地震震害特征及其成因探讨

根据笔者在汶川大地震板震区北川县城现场考察的第一手资料,分析了2008年5月12日汶川8．0级大地震后北川县城的震害特征,论述了北川县城地震后建筑物、生命线工程的破坏以及次生地质灾害;并对造成各类震害的原因进行了初步的探讨。最后,在总结震害特征及其成因的基础上,提出了当前应加强研究的相关课题,旨在为今后的工程抗震设防和研究提供参考。     

Enzymatic and cellular responses in relation to body burden of PAHs in bivalve molluscs: A case study with chronic levels of North Sea and Barents Sea dispersed oil

Mytilus edulis and Chlamys islandica were exposed to nominal dispersed crude oil concentrations in the range 0.015–0.25 mg/l for one month. Five biomarkers (enzymatic and cellular responses) were analysed together with bioaccumulation of PAHs at the end of exposure. In both species, PAH tissue residues reflected the exposure concentration measured in the water and lipophylicity determined the bioaccumulation levels. Oil caused biomarker responses in both species but more significant alterations in exposed C. islandica were observed. The relationships between exposure levels and enzymatic responses were apparently complex. The integrated biomarker response related against the exposure levels was U-shaped in both species and no correlation with total PAH body burden was found. For the monitoring of chronic offshore discharges, dose- and time-related events should be evaluated in the selection of biomarkers to apply. From this study, cellular damages appear more fitted than enzymatic responses, transient and more complex to interpret.     

Structure of residual oil as a function of wettability using pore-network modelling

In the water flooding of mixed-wet porous media, oil may drain down to relatively low residual oil saturations (S_or). Various studies have indicated that such low saturations can only be reached when oil layers in pore corners are included in the pore-scale modelling. These processes within a macroscopic porous medium can be modelled at the pore-scale by incorporating the fundamental physics of capillary dominated displacement within idealised pore network models. Recently, the authors have developed thermodynamic criteria for oil layer existence in pores with non-uniform wettability which takes as input geometrically and topologically representative networks, to calculate realistic S_or values for mixed-wet and oil-wet sandstones [16, 21]. This previous work is developed in this paper to include (i) the visualisation of the 3D structure of this residual oil, and (ii) a statistical analysis of this “residual/remaining” oil. Both the visualisation and the statistical analysis are done under a wide range of wettability conditions, which is reported for the first time in this paper.The structure of residual oil for strongly water wet systems is well known (where residual = remaining oil) and our model agrees with this but this structure changes radically for mixed wet systems (where residual ≠ remaining) and this has not yet been visualised experimentally. We find that for more water-wet systems high final residual oil saturations are reached at relatively small amounts of water injected and this oil is present in the pores as bulk oil. On the other hand, for more oil-wet systems we find a slow decrease of the amount of remaining oil with increasing amounts of injected water. During the process, the remaining connectivity of the oil phase is increasingly provided by oil layers only, hence the slow drainage. The final residual oil saturation, only reached in the theoretical limit of an infinite amount of injected water, is almost entirely contained in large number of (relatively low volume) oil layers, which are present in pores of most radius sizes.