Study on Anti-Disturbance and High-Resolution Shallow Seismic Exploration of Active Faults in Urban Regions

The significance of detection of urban active faults and the general situation concerning detection of urban active faults in the world are briefly introduced. In a brief description of the basic principles of anti-disturbance and high-resolution shallow seismic exploration, the stress is put on the excitation of seismic sources, the performance of digital seismographs, receiving mode and conditions, geometry as well as data acquisition, processing and interpretation in the anti-disturbance and high-resolution shallow seismic exploration of urban active faults. The study indicates that a controlled seismic source with a linear or nonlinear frequency-conversion scanning function and the relevant seismographs must be used in data acquisition, as well as working methods for small group interval, small offset, multi-channel receiving, short-array and high-frequency detectors for receiving are used. Attention should be paid to the application of techniques for static correction of refraction, noise suppressing, high-precision analysis of velocity, wavelet compressing, zero-phasing of wavelet and pre-stacking migration to data processing and interpretation. Finally, some cases of anti-disturbance and high-resolution shallow seismic exploration of urban active faults are present in the paper.

Tectonic setting of the seismogeny of the 1976 Tangshan,ChinaM=7.8earthquake

Based on the analysis of multi-temporal and multi-spectral satellite images for North China region, we have found that one year and more before the occurrence of 1976 Tangshan earthquake, the anomalies of electro-magnetic radiation on the satellite images indicated that the NE-trending Tangshan fault zone was dissected by the NNW-trending Nantai-Tangshan fault, and the Changping-Fengnan fault was dragged to form an arcuate bending at Fengnan. All these indicate the right-lateral translation along the Tangshan fault in NE direction. In order to gain an insight into the features of these faults, a shallow seismic exploration along the Tangshan and Changping-Fengnan faults has been carried out. The results have indicated that the NE-trending Tangshan fault is a high angle right-lateral strike-slip normal fault, dipping northwest, while the NWW-trending Changping-Fengnan fault is a southwest-dipping left-lateral strike-slip normal fault.

Study on the Cause of Complex Spatial Distribution of the Tangshan Earthquake Sequence

By analyzing higher-accuracy location data of the Tangshan earthquake sequence,a clear distribution pattern of three aftershock belts in the NE,NWW,and NW directions of has been obtained.The analysis reveals three rupture planes of strong events of MS7.8,MS7.1 and MS6.9 in the sequence.It indicates that the complex pattern is closely related to the earthquake source,and the NE-,NWW-and NW-trending regional fault zones,which have been revealed by the research of the pre-seismicity anomaly.In summary,the source is located in the junction of the three fault zones,and the rupture planes of the three strong events located in the source can be regarded as the locked segments on the three fault zones.On these grounds,the paper explains the complexity of the source and epicentral distribution of aftershocks.

Active Fault Exploration and Seismic Hazard Assessment in Fuzhou City

It has been proven by a number of earthquake case studies that an active fault-induced earthquake beneath a city can be devastating. It is an urgent issue for seismic hazard reduction to explore the distribution of active faults beneath the urban area and identify the seismic source and the risks underneath. As a pilot project of active fault exploration in China, the project, entitled “Active fault exploration and seismic hazard assessment in Fuzhou City”, started in early 2001 and passed the check before acceptance of China Earthquake Administration in August 2004. The project was aimed to solve a series of scientific issues such as fault location, dating, movement nature, deep settings, seismic risk and hazard, preparedness of earthquake prevention and disaster reduction, and etc. by means of exploration and assessment of active faults by stages, i.e., the preliminary survey and identification of active faults in target area, the exploration of deep seismotectonic settings, the risk evaluation of active seismogenic faults, the construction of geographic information system of active faults, and so on. A lot of exploration methods were employed in the project such as the detection of absorbed mercury, free mercury and radon in soil, the geological radar, multi-channel DC electrical method, tsansient electromagnetic method, shallow seismic refraction and reflection, effect contrast of explored sources, and various sounding experiments, to establish the buried Quaternary standard section of the Fuzhou basin. By summing up, the above explorations and experiments have achieved the following results and conclusions ：

Seismic analysis of the active character of the Taihang Mountain piedmont fault

The Taihang Mountain piedmont fault is a large-scale structure zone in north and east China which cross Beijing,with the NE-NNE extent spans approximately 620 km.It is very important to determine the fault zone activity due to the close relation of active structures and earthquakes.Regarding the fault activity,there are three different opinions：1） it is a large deep fault zone;2） it is an active fault zone and an earthquake structure belt;and 3） it is not an earthquake structure belt.In order to ascertain the active character of the fault,the deep tectonic setting and the activity since the Quaternary were investigated using recent seismic and drilling data to make a joint interpretation.The investigation results show that the Taihang Mountain piedmont fault is not a large lithospheric fault because the early middle Pleistocene（Q（P2）） layers are offset by the fault and the late middle Pleistocene（Q（P2）） and late Pleistocene layers are not offset by the fault.We determine that the Taihang Mountain piedmont fault in the area is not an active fault and is also not a large lithospheric fault.This study result provides important geological and geophysical data for city planning and construction in Hebei province and, especially,has great significance for seismic hazard assessment of the capital area.

Characteristics of the Zhengzhou Laoyachen Fault Revealed by Seismic Exploration Data

Exploration and research of fault activities are the fundamentals of earthquake prediction and prevention and disaster reduction. In order to determine the location, characteristics and activities of the Zhengzhon-Laoyachen fault, shallow seismic prospecting with different exploration depth across the Laoyachen fault was carried out in the northern suburbs of Zhengzhou city in 2006. The images of the subterranean structure and tectonics at depths of 30m- 6000m have been available by applying the combined methods of explosive seismic sources and vibrator seismic sources, as well as the combination of diverse observation systems with different parameters. The outcome indicates that the Laoyachen fault is a normal fault running NW and dipping NE, which offsets stratums ahead of Neogene （N）. However, no fault displacements are found in the interior stratums of Q ＋ N.

Exploration of Laoyachen fault in Zheng-zhou and its activity investigation

In the paper, an exploration is made by the techniques of shallow seismic prospecting, borehole composite profile analysis, field morphological investigation, and new geological dating for the Laoyachen fault in Zhengzhou that was originally recognized as an active one. The result from shallow seismic prospecting indicates this fault only existed in the stratum prior to the Neogene, dislocations and activity traces of this fault could not be found in the Neogene stratum. At the same time, the surface morphological investigation reveals that the location of the so-called "fault scarp" is not consistent with that of Laoyachen fault. Moreover, seismic drilling and borehole composite profile analysis also indicate that the scarp only developed in Malan Loess. The stratum beneath it is gentle and no dislocation traces can be seen. Therefore, we propose that this scarp is not relevant to the Laoyachen fault, which might be resulted from the diverted course of Yellow River. Laoyachen fault is not an active one.

Three-dimensional high-resolution velocity structure imaging and seismicity study of Yangbi Ms6.4 earthquake

In this study,based on the body wave arrival data of 5506 earthquakes recorded by 32 fi xed stations and 94 temporary stations in Yangbi and surrounding areas,the source parameters of Yangbi Ms6.4 earthquake sequence and three-dimensional(3-D)fi ne Vp,Vs,and Vp/Vs were inverted by using the consistency-constrained double-diff erence tomography method.The results showed that the focal depth after relocation was mostly in the range of 3–10 km,evidently nearly horizontally distributed,and concentrated in the weak area of the high-velocity body or at the side of the high-low-velocity body transition zone toward the high-velocity body,showing a good corresponding relationship with the velocity structure.The velocity structure in the Yangbi area has remarkably uneven characteristics.The seismic activity area is dominated by high-velocity bodies prone to brittle fracture near the surface.As the depth increases,low-velocity anomalies appear.A signifi cant diff erence was observed in the wave velocity ratio between the upper and lower sides of the seismically dense strip.Based on the focal mechanism of the Yangbi Ms6.4 earthquake and the fine 3-D velocity structure,this article concludes that the Yangbi Ms6.4 earthquake was caused by a strong regional tectonic stress concentrated in the relatively weak area by hard high-velocity bodies on the northwest sides.The Ms5.6 foreshock broke the inherent balance of regional stress and promoted the occurrence of the Yangbi Ms6.4 mainshock.Afterward,the stress was adjusted to a new equilibrium state through a large number of aftershocks,forming a foreshock–mainshock–aftershock type of seismic activity model.Based on the activity law of the Yangbi Ms6.4 earthquake sequence and characteristics of the 3D velocity structure distribution,this paper speculates that the seismogenic structure of the Yangbi earthquake was possibly a northwest strike-slip buried fault with a depth of 3–10 km on the southwest side of the Weixi–Qiaohou fault.

Recent Study of the Changjiang Fault Zone

The Changjiang fault zone,also known as the Mufushan-Jiaoshan fault,is a famous fault located at the southern bank of the Changjiang River,near the Nanjing downtown area.Based on multidisciplinary data from shallow artificial seismic explorations in the target detecting area(Nanjing city and the nearby areas),trenching and drilling explorations,classification of Quaternary strata and chronology dating data,this paper provides the most up-to-date results regarding activities of the Changjiang fault zone,including the most recent active time,activity nature,related active parameters,and their relation to seismic activity.

Late Quaternary Activity of the Central-North Segment of the Taihang Mountains Piedmont Fault Zone

The location and late Quaternary activity of the Central-North Segment of the Taihang Mountains Piedmont fault zone have been studied by shallow seismic survey and combined drill exploration.Our results show that the Baoding-Shijiazhuang fault and the Xushui fault were active in the late Pleistocene,but the south Xushui fault has been inactive since the late Pleistocene.The maximum magnitude of potential earthquake of the faults is 6.0.

不同勘探方法对浅层小断距隐伏断裂探测效果

采用浅层地震勘探、钻孔联合剖面勘探、槽探3种方法联合探测临汾盆地郭家庄断裂的空间位置,并研究其活动特征。结果表明,不同探测方法对断层信息的揭示存在很大差异,浅层地震勘探在探测具有一定断距的断层,同时地层存在一定物性差异的情况下能取得理想的效果,且不同的工作参数探测效果不相同;钻孔联合剖面勘探能提高对小断距断层的分辨率,但受地形及地层主观认别等因素的影响,可能会遗漏断距小、标识层薄的断层;槽探能较好地识别地表断层的微小断距,但受施工深度和施工条件影响。3种方法相结合,可以精确确定断裂的空间位置和活动特征。

基于多层次探测研究茅山断裂带北延段空间展布特征及第四纪活动性

本文介绍了从深至浅完成对茅山断裂北延立体式综合探测方法。使用重力场资料解译和大地电磁勘探初步定位断裂,随后通过浅层地震勘探进行精确定位,最后根据跨断点的小间距钻孔联合地质剖面探测法捕获了上断点并研判其第四纪活动性。研究发现:①茅山断裂带继续向北延伸入镇江市区,茅山断裂带北延段切割深度至少延伸至中地壳区域;②茅山断裂带北延段浅部区域表现出正断性质,倾向东,走向北东,视倾角约为60°,视断距约为3~5 m;③钻孔联合地质剖面探测结果显示,断点两侧基岩面落差较小,上覆第四纪地层未发现较大的起伏,未见第四纪有明显活动迹象,综合判断茅山断裂带北延段为前第四纪断裂。本项研究初步表明,茅山断裂在地理空间上可能与江苏陆域历史上两次显著的地震事件存在联系。这一发现为进一步研究该地区的发震构造特征和地震活动规律提供了新的视角,并可能对今后的相关研究有所启发。

用高分辨率地震反射剖面揭示临颍断裂浅部构造特征

为研究临颍断裂浅部构造特征,完成2条跨断裂高分辨率地震反射剖面。根据探测获取的地震反射剖面,结合石油地震剖面以及区域地质资料,分析临颍隐伏断裂位置、浅部构造特征及活动性。结果表明,临颍断裂是一条整体走向北西、倾向北东的正断层,最新活动时代为早更新世,断裂上盘发育一个次级断裂,与主断层成反“Y”字形态。研究结果可为许昌市及邻区地震危险性评价、城镇规划及国土利用提供地震学依据。

邯东断裂浅部特征及第四纪活动性分析

邯东断裂为太行山山前断裂以东的一条隐伏断裂,是华北平原坳陷区内邯郸凹陷的东界断裂,位于邯郸东部新区规划建设范围内。研究其空间分布特征和活动性对于地震危险性评价及城区重大工程项目选址具有重要意义。本文采用浅层地震勘探、钻孔联合剖面探测和年代学测试方法,对邯东断裂的展布特征及第四纪活动性进行系统研究,共布设跨断裂浅层地震勘探测线6条、钻孔联合地质剖面3条。综合分析表明,邯东断裂为一条近NNE-SSE走向的高角度正断层,倾向W,视倾角约50°~75°。断裂上断点埋深南浅北深,最新活动时代为中更新世晚期。中更新世早期,南段与北段相比较为活跃。

综合勘探技术在公路桥梁活动断裂地震危险性评估中的应用——以西宁高速天久大桥为例

则木河断裂与拟建西宁高速为同一走廊带,对拟建西宁高速影响极大。在断裂隐伏地段,首先利用高密度电法对断裂进行定位,然后利用浅层地震反射波法进一步追踪断层并利用反射波同相轴追踪确定错断层位,结合近场区地震地质调查综合分析判断断层活动性质。最后利用钻探工程验证综合推断解释结论,最终确定则木河断裂(塘河-八家村段)为一条全新世活动断裂。评价了工程场地地震稳定性,为高速公路桥梁设计提供了可靠资料。研究表明通过地质调查结合综合物探技术对断层进行定位后采用钻探验证的综合勘探技术是进行公路桥梁活动断裂地震危险性评估的一种有效方法。

浅层地震与高密度电法在断裂带探测中的应用--以川西理塘毛垭坝盆地西缘断裂为例

毛垭坝盆地西缘断裂,作为川西理塘断裂带的分支,是控制大毛垭坝盆地的西边界断裂,关于其几何展布和深部结构还未见报道。本文采用高密度电法和浅层地震勘探技术在毛垭坝盆地西缘断裂的南北两段分别进行了地球物理探测,分析了毛垭坝盆地西缘断裂在高密度电法和浅层地震勘探剖面上的响应特征,毛垭坝盆地西缘断裂在浅层地震反射剖面上表现为标志层横向错断,存在断层绕射波等现象,层析反演速度剖面反映断层两侧速度横向上呈现出明显变化,推测出上断点埋深位于15 m以浅,通过高密度电法反演的视电阻率剖面上的低阻条带特征,并结合野外调查和区域地质资料初步确定了断层产状、几何结构及平面展布。探讨了文中两种物探方法在活动断层探测方面的优缺点和应用前提,结合国内外类似实例,提出多种方法组合可以更为有效地探测断层结构。

综合地球物理方法在活断层探测中的应用研究

提出活断层探测综合地球物理方法为遥感、地震勘探、高密度电法、放射性测量,对新乡-商丘断裂(东段)进行了探测,探测结果:研究区第四系覆盖层厚度约160 m,上断点埋深30 m左右,断距由上到下逐渐加大,所探测的断层最新活动时代为晚更新世(Q3p)。结合本次新乡-商丘断裂东段的探测成果,总结出了一套活断层探测技术方法体系。

综合物探和钻探技术在第四纪隐伏断层探测中的应用

采用以高分辨率地震勘探为主,配合瞬变电磁和高密度电阻率探测的方法,快速、准确圈定了北坞断裂的上断点位置、埋深等信息;通过跨断层联合剖面钻探和地球物理测井工作,查明了晚第四纪地层特征和层序,精确定位了目标断层的空间位置和断层位移量,进而揭示了目标断层的活动性。结果表明:综合物探技术的解释结果和钻探结果基本吻合,证明了方法的有效性。

浅层地震勘探与钻孔探测揭示的安宁河东支断裂特征

已有资料证实安宁河东支断裂为一条隐伏断裂,为探明其位置、分析其活动性,跨断裂完成了两条高分辫率的浅层勘探剖面。在浅层地震勘探成果的基础上,于断裂两侧各布设了一个钻孔。根据浅层地震、钻孔并结合该区域地质资料,对该断裂的位置、活动性等开展了分析和讨论。结果显示,安宁河东支断裂为走向NNE-SN,倾向E的逆断层,错断埋深约38~42 m的第四系沉积层,为一条具晚更新世活动性的隐伏活动断裂,初步估算其晚更新世滑动速率为0.17 mm/a。

汾东断裂甘亭段浅部构造特征

利用浅层人工地震勘探技术对临汾盆地汾东断裂进行探测,获得2条高分辨率地震勘探剖面,分析汾东断裂甘亭段浅部地层和断裂基本特征。结果表明,甘亭段浅部地层均呈现西倾形态,整体西厚东薄;汾东断裂甘亭段走向北东,倾向北西,视倾角67°~72°,断层落差26~182m,可辨别的最浅上断点埋深85m,为第四纪活动断裂。对比研究汾东断裂北部甘亭段、南部临汾段剖面特征,认为甘亭段新生代活动强度高于临汾段,汾东断裂活动强度由北向南逐渐降低。