Two-step interface and velocity inversion—— Study o e of the Tangshan seismic region

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Basement interface structural characteristics beneath Jiashi strong earthquake swarm area in Xinjiang, China

The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interface struc-tural characteristics beneath Jiashi strong earthquake swarm area were determined. The results show that there are two clear basement interfaces at the upper crust in Jiashi strong earthquake swarm area. The first one with buried depth ranging from 2.6 km to 3.3 km presents integral and continuous structure, and it appears an inclined plane interface and smoothly rises up toward Tianshan Mountain. The second basement interface with buried depth from 8.5 km to 11.8 km, is the antiquated crystalline basement of Tarim basin. Near the post number of 37 km, the bur-ied depth of the crystalline basement changed abruptly by 2.5 km, which maybe result from an ultra crystalline basement fault. If taking this fault as a boundary, the crystalline basement could be divided into two parts, i.e. the southwestern segment with buried depth about 11.5 km, and the northeastern segment with buried depth approxi-mately from 8.5 km to 9.0 km. That is to say, in each segment, the buried depth changes not too much. The north-east segment rises up as a whole and upheaves slightly from southwest to northeast, which reflects the upper crustal deformation characteristics under the special tectonic background at the northwestern edge of Tarim basin.     

3-D seismic tomography for velocity and interface structure of the crust and upper mantle (theoretical part)

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Gravity and magnetic data inversion for 3D topography of the Moho discontinuity in the northern Red Sea area, Egypt

The main goal of our study is to investigate 3D topography of the Moho boundary for the area of the northern Red Sea including Gulf of Suez and Gulf of Aqaba. For potential field data inversion we apply a new method of local corrections. The method is efficient and does not require trial-and-error forward modeling. To separate sources of gravity and magnetic field in depth, a method is suggested, based on upward and downward continuation. Both new methods are applied to isolate the contribution of the Moho interface to the total field and to find its 3D topography. At the first stage, we separate near-surface and deeper sources. According to the obtained field of shallow sources a model of the horizontal layer above the depth of 7 km is suggested, which includes a density interface between light sediments and crystalline basement. Its depressions and uplifts correspond to known geological structures. At the next stage, we isolate the effect of very deep sources (below 100 km) and sources outside the area of investigation. After subtracting this field from the total effect of deeper sources, we obtain the contribution of the Moho interface. We make inversion separately for the area of rifts (Red Sea, Gulf of Suez and Gulf of Aqaba) and for the rest of the area. In the rift area we look for the upper boundary of low-density, heated anomalous upper mantle. In the rest of the area the field is satisfied by means of topography for the interface between lower crust and normal upper mantle. Both algorithms are applied also to the magnetic field. The magnetic model of the Moho boundary is in agreement with the gravitational one.     

迭代法在三维网络模型最短路径射线追踪中的应用

目前有许多基于二维网格模型的射线追踪算法,但应用与三维网格模型的射线追踪算法较少,主要是因为三维网格射线追踪算法在算法实现、编程以及图形显示方面都有一定难度.本文提出了一种适用于三维网格射线追踪的迭代算法,并应用于几种理论模型,同时实现了三维绘图显示,通过研究证明迭代算法能在一定程度上提高三维网格射线追踪的精度和速度.     

Conductivity structure of crust in the Tangshan seismic area and the possibility of exploring potential seismic sources by magnetotelluric method

It is shown by the result of digital magnetotelluric soundings in the Tangshan seismic area and its surrounding regions that the crust under the surface conductive sediments is divided into two layers,i. e., the resistive upper crust and the conductive lower crust. The upper crust wherein the Tangshan main shock and most of the aftershocks occurred is a convex lens-like body which is cut by faults at the east, south and west sides. The focus of the mainshock was located at the position of maximum thickness of the resistive upper crust while the spatial variation of Curie point isothermal surface and the deepest limit of the depths of aftershocks coincide with the downward depression of the bottom of the resistive upper crust. Thus, the Tangshan main shock and most of its aftershocks were related closely to the resistive upper crust from the view points of either vertical layering or lateral variations. And there were only a very few aftershocks in the conductive lower crust. The mechanical property of the rocks transforms from being brittle in the upper crust into ductile in the lower crust mainly due to the combination of different factors, e.g., increase of confining pressure, change in minerals, rise in temperature as well as stabilization of slips by high pore—pressure. A small amount of water and a rise in temperature may lead to a decrease of the electric resistivity within the rock while a change in the static pressure and mineral content within the rock causes very little change in the electric resistivity. Thus it is deduced that a resistive upper crust and a more conductive lower crust from the view points of either vertical or lateral variations are related to the brittle and ductile properties respectively. Hence it is possible that there is a relationship between the electric structure of the crust and zones of potential seismic hazard. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 354–363, 1991. The project is supported by the Chinese Joint Seismological Science Foundation, and the Swedish International Development Authority.     

利用重复地震观测重庆巫山地区地壳介质变化

利用波形互相关技术,对重庆数字地震台网记录的巫山地区90次地震波形资料进行相关计算,识别出24对同时被2个地震台站记录且各台波形互相关系数不小于0.8的重复地震对;利用射线追踪方法,对巫山地区重复地震位置做归一化处理,得到所选台站记录地震P波走时差变化,结果显示,在2013年重庆巫山M_L 3.7地震前,建坪、双龙、荆竹地震台记录到该地区短期地壳介质速度有明显升高现象。     

Synthesis of accelerations of the 1976 Tangshan earthquake ( M S 7.8) in near- and far-field by using semi-empirical method

In this paper a heterogeneous fault model of the Tangshan earthquake is suggested, which consists of two southern sub-faults striking N30°E and two northern sub-faults striking N50°E. Total length of the main shock fault is 114 km and seismic moment is about 1.4 × 10²⁰ N·m. The epicentre of the main shock is located at the southern part, near the intersection of the two bands. Accelerations of two aftershocks (M _L 5.5, M _S 6.9) were used as empirical Green’s functions to synthesize the accelerations of the main shock in near and far field. A method that small events and main shock are considered not satisfying the similarity relationship in the improved empirical Green’s function is also applied in this paper. Peak values, duration and response spectra of synthesized accelerations in far field are in agreement with the observed records. The synthesized results in near field are also in agreement with the epicentral intensity distribution of the main shock. The results show that the peak acceleration of Tangshan earthquake in epicentral region exceeds 1.1 g. It is consistent with the peak accelerations recorded in some large earthquakes occurred in recent years. The research is supported by the National Science Foundation of China.     

Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

Moment tensor inversion and source rupture process of the September 27, 2003 M_S=7.9 earthquake occurred in the border area of China, Russia and Mongolia

IntroductionOn September 27, 2003, an earthquake of MS=7.9 struck the border area of China, Russia and Mongolia. According to the field investigation from the Earthquake Administration of XinjiangAutonomous Region, the whole northern Tianshan region felt the hit. Buildings and structures within six counties and one city in Altay region, which is total about 0.11×106 km2 area, were damaged to different extent and caused certain economic losses. The epicenter determined by China National …     

1927年古浪8级大震及其周边地区三维地壳P波速度结构特征

本文联合利用甘肃及周边测震台网记录的古浪及周边地区4592次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法反演了古浪震源区高分辨率的三维P波速度精细结构.结果显示,浅部P波速度分布与地表地质之间具有很好的对应关系.皇城—双塔断裂带在6 km以上深度表现为高速异常带,而在6~15 km逐渐转换为明显的低速特征,之后再次转换为高速体.震区下部在10~20 km深度有一个尺度约200 km²的低速异常体,地震发生时破裂首先在该低速体发生,与主震空间位置非常吻合.主震区的岩石结构主要由奥陶纪变质砂岩、石英岩和加里东期的花岗岩等坚硬岩体组成.这种坚硬岩体对应的P波速度结构为高速体,有利于能量积累.武威盆地在20 km以上深度表现为明显的低速异常,在25 km深度之下,整体显示为高速体,表现出稳定块体的特征.表明武威盆地中下地壳和上地幔顶部已插入到冷龙岭隆起带之下.震区小震重新定位发现,皇城—双塔断裂带东、西两段表现出不同的力学运动性质,西段以逆冲运动为主,地震主要发生在断裂的下盘.而东段地震却主要发生在上盘,断层活动以局部拉张为主.我们还首次发现在皇城—双塔断裂带的中段与主破裂呈垂直方向存在有在主震发生时新产生的一条共轭断层,基于小震的断层面参数反演显示该断裂是一高倾角运动性质以右旋为主兼具正断的断裂.

     

2012年唐山ML5.2地震前后的速度结构对比分析

针对2012年5月28日唐山M_L5.2地震发生在老震区5级地震平静17年的背景,且其震级与唐山大地震序列强度衰减趋势相比明显偏高这一现象,采用双差层析成像方法,以2年为时间间隔对唐山地区2010年5月～2016年5月的地震数据进行反演,获得该区2012年唐山M_L5.2地震前后地壳的三维地震P波和S波速度的空间分布,结合此次地震的精定位震源参数,对比分析地震前后震源区地下介质波速的变化特征。研究结果发现此次地震发生前,震中处在高低速过渡带的位置,震后震中附近的波速出现一定程度的降低。     

Moment tensor inversion and source rupture process of the September 27, 2003 M S=7.9 earthquake occurred in the border area of China, Russia and Mongolia

We conducted moment tensor inversion and studied source rupture process for M _S=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic P-wave seismograms recorded by long-period seismograph stations of the global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the M _S=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process inversion result of M _S=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M ₀=0.97×10²⁰ N·m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the M _S=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence of spatial distribution of slip on the fault plane with the distribution of aftershocks also supports that the rupture is a heterogeneous process owing to the presence of barriers.     

福建区域地震前兆台网观测系统升级改造

介绍福建区域地震前兆观测台网观测系统升级改造建设情况,阐述具体的实施改造以及仪器设备在系统接入改造中的一些关键技术问题,在并行期间通过数据对比总结改造效果,“九五”系统升级改造建设为前兆台网观测提供更完善的管理及数据服务.     

粤桂琼交界地区地震趋势综合分析

应用地震活动、台站前兆观测、流动重力、海平面变化和统计分析等多种方法,绘合分析了粤桂琼交界地区近斯的地震活动趋势。认为近３年内或稍长一些时间,该区发生６级以上地震的可能性很小,但有可能发生５级左右地震。     

The inversion of 3-D crustal structure and hypocenter location in the Beijing-Tianjin-Tangshan-Zhangjiakou area by genetic algorithm

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门源地区地壳三维体波速度结构及地震重定位研究

本文使用甘肃、青海数字地震台网及中国地震科学探测台阵记录到的门源地区地震的P波和S波到时资料,应用双差层析成像方法联合反演了该地区的地壳三维速度结构和震源位置参数.结合地质构造背景,研究了门源M_S6.4地震孕育发生的深部介质环境及该地区速度结构与地震活动性之间的关系.结果表明：反演之后地震的走时残差均方根显著降低,重定位后的地震在垂直方向上呈现出与断层位置有关的条带状分布.门源地区地壳速度结构存在明显的不均匀性,浅层P波和S波速度结构与地表地质构造及地形特征密切相关.研究区内地震活动性与地壳速度结构具有很强的对应关系,地震主要分布在高速异常区域及其边缘.门源M_S6.4地震震中附近的P波和S波速度结构表现出明显的高速异常,且在震源区下方存在P波低速层,这种特殊的构造条件可能是导致此次地震发生的重要原因.

     

庐枞矿集区矾山-将军庙地区AMT三维反演及地质结构解释

庐江-枞阳矿集区是长江中下游成矿带内的一个重要的多矿种成矿区.为揭示矾山-将军庙地区的地质结构以指导找矿,首先,在庐枞矿集区北部的矾山-将军庙地区开展了三维音频大地电磁数据采集工作,采用Rhoplus方法进行AMT数据死频带校正保证数据质量,利用相位张量分析地下介质的维性特征;其次,利用模块化三维反演并行代码ModEM开展了实测数据的三维反演,获得了一个典型的具有双层结构特征的火山岩盆地三维地下电性模型;最后通过电性模型及其他地质资料的综合解释,依据反演模型中近地表的低阻层和深部的高阻隆起区,分别勾画出了火山岩地层的厚度和深部侵入岩的分布范围,并在小岭地区发现地表浅部下方存在一个含矿次级火山机构的高导体.     

三维地震属性技术在湖区的应用效果

微山湖湖区表层地震地质条件十分复杂,芦苇丛生,三维地震勘探野外数据采集采用自行研制的钻井平台加套管护壁的成孔工艺,沼泽检波器加上加长尾椎,提高了检波器的耦合效果,获取了高信噪比的三维地震数据。资料解释应用了三维可视化、相干数据体、地震属性技术进行三维构造精细化解释,明显提高了地震解释识别地下小地质构造及地震异常体的能力,它完善和发展了三维高分辨率地震勘探技术,扩大了地震勘探的应用范围,为解决煤矿开采阶段所遇到的构造、煤层等地质问题提供了一种综合应用的解释方法,在湖区的勘探开发中,取得了明显的地震地质效果。     

汶川地震发震构造带过WSFD-2钻井深反射地震数据3-D处理技术研究

2008年5月12日汶川发生里氏8.0级地震的发震机制与龙门山断裂带的构造特征紧密相关,应用反射地震探测方法精细探测发震构造内部的结构特征,对研究汶川地震的发震机理有重要意义.5.12地震后在地表错断最剧烈的区域之一虹口乡白庙村穿过发震构造和深钻科研井WSFD-1和WSFD-2布置了兼顾浅中深层信息的反射地震探测剖面,该区域断裂带硬岩出露,地形地貌复杂多变,起伏剧烈,只能采取弯线地震测线布设方式采集二维反射地震数据,而且弯曲度大,造成CDP点分散严重.区域内构造复杂,褶皱逆推构造发育,地层和构造倾角大,采用常规二维地震数据处理方法进行叠加处理时,易将不同地层的反射信息叠加到同一反射层.弯线叠加剖面上侧面波,混波干扰严重,会显著地降低原本低信噪比数据的分辨率,或剖面可靠性低,容易在二维剖面中造成解释陷阱.本文利用弯线地震采集的三维特性,用拟三维地震叠加技术处理汶川地震科学钻井附近横跨北川—映秀断裂带的二维弯线地震数据,弥补常规二维弯线地震数据处理技术的不足,将不同地层的反射信息分离归位到不同的三维叠加剖面上,解决二维弯线地震数据处理时混波干扰严重的难题.通过理论分析和实验选取合适的共中心点面元,获得了高分辨率的三维叠加剖面.相比于二维弯线叠加剖面,三维叠加剖面切片成像更真实,剔除了不同地层反射信息混叠的影响,能得到更准确的断点信息,并可以获得沿断层走向横向的信息,显著提高了构造解释的可靠性和精度.应用拟三维地震数据处理方法处理龙门山断裂带的二维弯线地震数据,获取高分辨率的构造信息,有利于断裂带内汶川地震发震机制和龙门山隆起机制的解释.