圣安德烈斯断层带地震活动的基本特征

美国的圣·安德烈斯断层(以下简称圣安断层)是世界上研究得最详细的几条大活动断层之一,也是目前地震台网和其它地震前兆台网布置得最密集的断层之一。本文试图结合笔者的短期考察和所涉文献归纳其地震活动的基本特征。     

圣安德烈斯断层的强度

在过去25年里,对沿加利福尼亚州的圣安德烈斯断层系的运动速率和长期地震记录的认识已显著提高了。这些认识使得对地震发生概率,尤其是30年周期概率的长期估计成为可能(Working Group on California Earthquake Probabilities,1999)。但是,我们对断裂产生的物理过程的了解仍然非常不足——关于短期预报的可行性问题、一个断层产生地震是否会增加另一断层     

圣安德烈斯断层的钻孔计划

由斯坦福大学的佐巴克(Mark Zoback),美国地质调查局的希克曼和埃尔斯沃思(Steve Hickman and Bill Ellsworth)以及利物摩尔劳伦     

用雷达测绘圣安德烈斯断层的近地表结构

土工技术工程师、土木工程师和考古学家们多年来一直采用了一种被称为电磁地下探测剖面(ESP)的基极接地形雷达图象探测技术。这是用电磁波模拟地震波反射探测剖面的技术,只是在探测的尺度上降低了近乎一个数量级。拉蒙特地质观象台的地震学者比尔哈姆(R.Bilham)等人认为,利用雷达探测技术可在圣安德烈斯断层上选择适当的新地点供挖槽研究用,同时还可用以检验是否所有最近发生     

利用地衣测年法测定圣安德烈斯断层的地震年龄

加利福尼亚内华达山脉地区的区域性岩崩是由当地地震和远处的圣安德烈斯层带北部与南部的地震引起的。五个历史地震所遗贸的岩崩用地衣测年法测定表明，测定地震的准确度达２．２－３．５年。     

圣安德烈斯断层南部微震活动暗示加州地震风险增加

美国加州大学伯克利分校科学家的最新研究表明,地震活动断层区地下震动的增加可能标志着断层闭锁区应力的增加,这很可能增加大地震发生的可能性。     

太原市汾河断裂浅层高分辨率地震探测

通过浅层高分辨率地震探测 ,结合钻孔资料 ,获得了测线处汾河断裂的准确位置 ,几何结构 ,活动性质、时代和速率等参数 ,并对其动力学属性进行了初步的讨论。地震探测揭露出汾河断裂的西支断层 ,为高角度东倾的斜滑断层 ,切割了第四系下部 ,上断点位于地下约 70m处。据钻孔剖面推断有 2条高角度相向而倾的斜滑断层 ,东支位于汾河东岸 ,断至全新统—上更新统 ,其底界面垂直错距约 8m。断裂北段另一个钻孔剖面表明 ,西支断层切割了全新统—上更新统 ,底界面垂直错距约6m。汾河断裂晚更新世以来的平均垂直滑动速率为 0 0 6～ 0 0 8mm/a ,大震平均重复间隔为 5 0～6 7ka     

太原市汾河断裂浅层高分辩率地震探测

通过浅层高分辩率地震探测，结合钻孔资料，获得了测线处汾河断裂的准确位置，几何结构，活动性质、时代和速率等参数，并对其动力学属性进行了初步的讨论。地震探测揭露出汾河断裂的西支断层，为高角度东倾的斜滑断层，切割了第四系下部，上断点位于地下约70m处。据钻孔剖面推断有2条高角度相向而倾的斜滑断层，东支位于汾河东岸，断至全新统-上更新统，其底界面垂直错距约8m。断裂北段另一个钻孔剖面表明，西支断层切割了全新统-上更新统，底界面垂直错距约6m。汾河断裂晚更新世以来的平均垂直滑动速率为0.06-0.08mm/a，大震平均重复间隔为5.0-6.7ka。     

利用地震剖面研究夏垫断裂西南段的活动性

地震方法是针对厚覆盖区城市直下型活动断裂的一种不可替代的探测技术,对于不同的探测深度需采用不同的排列长度。为研究夏垫断裂在远离三河-平谷8.0级地震震源区的活动性,我们在该震源区SW方向约30km处开展了中浅层反射地震探测试验,并跨过中浅层地震探测到的夏垫断裂进行了浅层反射地震探测试验。浅层和中浅层地震探测的试验结果表明,在5m道间距的地震剖面上,在200m深度以下夏垫断裂得到了较好的反映,在该深度以上,该断裂反映不明显;在2m道间距的地震剖面上,夏垫断裂错断明显,但剖面上的最浅一组反射波(深度约30m)却没有发生明显错断。由此得出:距1679年三河-平谷8.0级地震震源位置SW方向约30km处,夏垫断裂的活动性减弱     

RESEARCH ON NEOGENE-QUATERNARY STRATIGRAPHIC STRUCTURE AND SHALLOW TECTONIC FEATURES IN THE NORTH SECTION OF DAXING FAULT ZONE BASED ON SHALLOW SEISMIC REFLECTION PROFILING

The Daxing Fault is an important buried fault in the Beijing sub-plain, which is also the boundary fault of the structural unit between Langgu sub-sag and Daxing sub-uplift. So far, there is a lack of data on the shallow tectonic features of the Daxing Fault, especially for the key structural part of its northern section where it joins with the Xiadian Fault. In this paper, the fine stratigraphic classifications and shallow tectonic features of the northern section in the main Daxing Fault are explored by using three NW-trending shallow seismic reflection profiles. These profiles pass through the Daxing earthquake(M6¾)area in 1057AD and the northern section of the main Daxing Fault. The results show that seven strong reflection layers(T₀₁—T₀₃, T_Q and T₁₁—T₁₃)are recognized in the strata of Neogene and Quaternary beneath the investigated area. The largest depth of strong reflection layer(T₁₃)is about 550~850ms, which is interpreted as an important surface of unconformity between Neogene and Paleogene or basement rock. The remaining reflection layers, such as T₀₁ and T_Q, are interpreted as internal interfaces in Neogene to Quaternary strata. There are different rupture surfaces and slip as well as obviously different structural features of the Daxing Fault revealed in three shallow seismic reflection profiles. The two profiles(2-7 and 2-8)show obvious rupture surfaces, which are the expression of Daxing Fault in shallow strata. Along the profile(2-6), which is located at the end of the Daxing fault structure, a triangle deformation zone or bending fracture can be identified, implying that the Daxing Fault is manifested as bending deformation instead of rupture surfaces at its end section. This unique structural feature can be explained by a shearing motion at the end of extensional normal fault. Therefore, the Daxing Fault exhibits obviously different tectonic features of deformation or displacement at different structural locations. The attitude and displacement of the fault at the shallow part are also different to some extent. From the southwest section to the northeast section of the fault, the dip angle gradually becomes gentler(80°~60°), the upper breakpoint becomes deeper(160~600m), and the fault displacement in Neogene to Quaternary strata decreases(80~0m). Three shallow seismic reflection profiles also reveal that the Daxing Fault is a normal fault during Neogene to early Quaternary, and the deformation or displacement caused by the activity of the fault reaches the reflection layer T₀₂. This depth is equivalent to the sedimentary strata of late Early-Pleistocene. Therefore, the geometry and morphology of the Daxing Fault also reveal that the early normal fault activity has continued into the Early Pleistocene, but the evidence of activity is not obvious since the late Pleistocene. The earthquakes occurring along the Daxing Fault, such as Daxing earthquake(M6¾)in 1057AD, may not have much relation with this extensional normal fault, but with another new strike-slip fault. A series of focal mechanism solutions of modern earthquakes reveal that the seismic activity is closely related to the strike-slip fault. The Daxing Fault extends also downwards into the lower crust, and may be cut by the steeply dipping new Xiadian Fault on deep seismic reflection profile. The northern section of the Daxing Fault strikes NNE, with a length of about 23km, arranged in a right step pattern with the Xiadian Fault. Transrotational basins have been developed in the junction between the northern Daxing Fault and the southern Xiadian Fault. Such combined tectonic features of the Daxing Fault and Xiadian Fault evolute independently under the extensional structure background and control the development of the Langgu sub-sag and Dachang sub-sag, respectively.     

由宽角反射地震资料重建壳幔反射结构的相似性剖面

针对人工源宽角地震测深中炮点与接收点稀疏的特点,利用几何散射理论进行壳幔散射体的相似性剖面成像,利用互易性原理计算散射体相似性成像的时间条件,研究基于宽角反射地震资料重建壳幔反射结构的相似性成图方法. 其中,成像点的时间采用快速、高精度的程函方程求解技术. 利用该方法处理东南陆缘区宽角反射剖面实际资料,结果表明,较之壳幔反射结构的CDP成图技术,相似性成图方法具有提高反射同相轴横向连续性的能力.     

抚顺活断层浅层反射横波地震勘探资料解释与分析

应用多种途径对抚顺目标区的浅层反射横波活断层勘探资料进行的解释与分析,证实了浑河断裂总体上为北倾、逆断的性质,且只是在采空区表现为新的活动段,本身不是由构造活动原因引起的活动断裂。在第四系埋深浅的地区,反射横波法是具有较强抗干扰能力的活断层探测技术,可靠程度及解释精度均较高。本文为抚顺市活断层的地震危险性评价工作奠定了基础。     

THE INTERPRETATION OF GRAVITATIONAL AND MAGNETIC DATA FOR THE VOLCANIC ROCKS DISTRIBUTION IN KE-BAI FAULT ZONE,JUNGGAR BASIN

This paper firstly discusses the feasibility of delineating the volcanic rocks distribution by gravitational and magnetic methods on the basis of the statistical results of the magnetic susceptibility and density of the different lithologies in the study area. After the separation of gravitational and magnetic fields by bandpass filter, we determined the residual gravitational and magnetic anomalies caused by volcanic rocks. The results of potential field separation show that the residual anomalies are in beaded NE-directed distribution. In the meantime, the boundary enhancement techniques such as horizontal total gradient and vertical derivative are employed to delineate the distribution of the faults, as the result, fifteen faults including four NE-striking main faults which appear as an arc protruding to the southeast and other secondary faults are distinguished. Furthermore, two fault systems with deep and superficial attribution are revealed from the inversed section of the telluric electromagnetic sounding, and their properties, characteristics and roles in the volcanic activity are fully discussed by combining with the tectonic background. Based on the comprehensive analysis of the correlation between the distribution characteristics of the residual gravitational and magnetic anomalies and the location of the faults, three volcanic activity zones are reasonably delineated. Finally, we carry out the inversion of apparent density by taking advantage of the residual gravitational anomalies and acquire the apparent density anomalies of the top part of Paleozoic. Integrated with the apparent density anomalies, the lithologies exposed from the boreholes, the observed density data of different rocks and the residual gravitational-magnetic anomalies, we elaborately delineate the lithologies of the top of Paleozoic and further define the distribution range and the lithology combination of Kekesayi group of Middle Ordovician, Tailegula group of Lower Carboniferous and Jiamuhe group of Lower Permian.     

东秦岭陆壳反射地震剖面

河南省叶县一邓州的反射地震剖面给出了秦岭地壳构造模型．东秦岭深部构造可分为３个区域：华北克拉通、扬子克拉通和秦岭碰撞缝合带．华北克拉通是稳定的地壳;扬子地壳要比稳定的华北地壳更具流变性质,有多层滑脱,至少可分辨出４个滑脱面：陡岭滑脱面、武当滑脱面、扬子滑脱面和地壳底部滑脱面;秦岭碰撞缝合带宽约１００ｋｍ,向南倾,倾角约１５°,地壳结构呈菱形块体样式．秦岭地区的上部地壳为巨型推覆构造,可分为北秦岭和南秦岭两个推覆体,各由主推覆体和前缘叠瓦扇组成．前印支期,秦岭地壳向南俯冲,秦岭古生代海盆闭合．在碰撞的后期,秦岭下部地壳向扬子作Ａ型俯冲,而上部地壳则发生大规模由北向南的推覆。     

东秦岭陆壳反射地震剖面

河南省叶县一邓州的反射地震剖面给出了秦岭地壳构造模型．东秦岭深部构造可分为３个区域：华北克拉通、扬子克拉通和秦岭碰撞缝合带．华北克拉通是稳定的地壳；扬子地壳要比稳定的华北地壳更具流变性质，有多层滑脱，至少可分辨出４个滑脱面：陡岭滑脱面、武当滑脱面、扬子滑脱面和地壳底部滑脱面；秦岭碰撞缝合带宽约１００ｋｍ，向南倾，倾角约１５°，地壳结构呈菱形块体样式．秦岭地区的上部地壳为巨型推覆构造，可分为北秦岭和南秦岭两个推覆体，各由主推覆体和前缘叠瓦扇组成．前印支期，秦岭地壳向南俯冲，秦岭古生代海盆闭合．在碰撞的后期，秦岭下部地壳向扬子作Ａ型俯冲，而上部地壳则发生大规模由北向南的推覆。     

电剖面资料解释方法的探讨

在直流电剖面法的实际工作中,因受各种干扰因素的影响,使之ρ_5曲线复杂化,采用信息理论来处理直流电剖面资料,可以消除一些干扰因素,突出地电异常,为直流电剖面的资料解释提供了新的尝试。