Surface displacement estimation along Himalayan frontal fault using differential SAR interferometry

The Himalayan region has been studied extensively during the past few decades in terms of present ongoing deformations. Various models have been proposed for the evolution of the Himalaya to explain the cause of earthquake occurrences and to understand the seismotectonics of the Himalayan collision zone. However, the information on displacements from field geodetic surveys is still too scarce in time and spatial domains so as to provide convincing evidences. Moreover, classical Probabilistic Seismic Hazard Approaches also fail due to paucity of data in higher magnitude range, thus emphasizing the need of spatial level displacement measurements. It is in this context that the present study has been carried out to estimate the surface displacement in a seismically active region of the Himalaya between Ganga and Yamuna Tear using Differential SAR interferometry. Three single-look complex images, obtained from ASAR sensor onboard ENVISAT satellite, have been used. A displacement rate of 8?C10?mm per year in N15°E direction of Indian plate has been obtained in this three-pass SAR interferometry study. It has been noted that the estimated convergence rate using Differential SAR interferometry technique is relatively low in comparison with those obtained from previous classical studies. The reported low convergence rate may be due to occurrence of silent/quite earthquakes, aseismic slip, differential movement of Delhi Hardwar ridge, etc. Therefore, in view of the contemporary seismicity and conspicuous displacements, a study of long-term observations of this surface movement has been recommended in future through a time-series SAR interferometry analysis.     

基于差分干涉雷达的汶川地震同震形变特点

地震同震形变场是认识震源机制、确定发震断裂、分析发震断裂活动方式、评估震害损失及揭示未来发震趋势的重要依据。但在龙门山地区复杂的自然环境中, 现有的地震同震形变场获取方法(GPS测量、构造形迹分析、震源机制解结合数值模拟计算)难以快速、全面的捕捉到这种信息, 这突显出差分干涉雷达技术(D-InSAR, Differential Interferometry SAR)在同震形变场研究中的优势。本文介绍了其观测原理, 在地震同震形变监测中的研究现状, 并重点分析了已获取的D-InSAR汶川地震同震形变场的观测结果。基于差分干涉雷达观测及相关资料, 对龙门山断裂在本次地震中活动性进行了初步分析, 结果表明干涉雷达形变图像与地表破裂范围、逆冲角度的变化、上下盘升降关系及大型余震的展布存在很强的相关性。      

Estimation of strain distribution using GPS measurements in the Kumaun region of Lesser Himalaya

The continuous process of continent–continent collision between the Indian and the Eurasian plates has led to the formation of the Himalayan range and continuously caused earthquakes in the region. Large earthquakes with magnitudes of 8 and above occur in this region infrequently, releasing the elastic strain accumulated over years around the plate boundary. Geodetic measurements can help estimate the strain distribution along the fault system. These measurements provide information on active deformations and associated potential seismic hazards along the Himalayan arc. In order to understand the present deformation around the plate boundary, we collected GPS data during three campaigns in the years of 2005–2007 at 16 sites in the Kumaun region of the Lesser Himalaya. Horizontal velocity vectors estimated in ITRF2000 are found to be in the range of 41–50 mm/yr with an uncertainty level of the order of 1 mm/yr. The velocity field indicates that the present convergence of around 15 mm/yr takes place in the Kumaun Himalaya. Further, we estimate the strain components in the study area for understanding the currently active tectonic process in the region. The estimated dilatational strain indicates that the northern part near the Main Central Thrust (MCT) is more compressional than the southern part. Maximum shear strain is mostly accommodated in the northern part too. The maximum shear and dilatational strain rates are about 1.0 and 0.5 μstrain/yr. It is seen that the distribution of high shear strain spatially correlates with seismicity. The maximum of extensional and compressional strains due to the force acting along the Main Central Thrust (MCT) in the NW–SE direction are found to be 0.4 and 0.1 μstrain/yr, respectively. The maximum shear strain in the northern part of the Himalaya appears to be associated with the convergence of the region proposed by other geophysical studies.     

特提斯构造带和星球级纬向构造体系

东特提斯东段位于现今南海及其周边地区,做为洋壳的岩石学记录——蛇绿岩带的现行展布呈U字形,它是印度板块与欧亚板块的碰撞、南海的扩张以及菲律宾-太平洋板块的向西推挤时,特提斯构造带受到扭曲而形成的。通过古地磁和岩石学记录复原其在白垩纪末古近纪初时的位置和形态,为一位于现今印度次大陆南端经加里曼丹北缘向东延伸的近EW向的挤压构造带,它向西和青藏地区经中亚扎格罗斯山脉和欧洲阿尔卑斯山脉联接成全球性的特提斯构造带,受控于星球级纬向构造体系。研究表明,冈瓦纳的裂散、亚洲的增生和特提斯构造带的演化都受控于星球级纬向构造体系。星球级纬向构造体系是控制板块边界和运动的重要的支配力量之一。      

Active tectonics of the Himalaya

The Himalayan mountains are a product of the collision between India and Eurasia which began in the Eocene. In the early stage of continental collision the development of a suture zone between two colliding plates took place. The continued convergence is accommodated along the suture zone and in the back-arc region. Further convergence results in intracrustal megathrust within the leading edge of the advancing Indian plate. In the Himalaya this stage is characterized by the intense uplift of the High Himalaya, the development of the Tibetan Plateau and the breaking-up of the central and eastern Asian continent. Although numerous models for the evolution of the Himalaya have been proposed, the available geological and geophysical data are consistent with an underthrusting model in which the Indian continental lithosphere underthrusts beneath the Himalaya and southern Tibet. Reflection profiles across the entire Himalaya and Tibet are needed to prove the existence of such underthrusting. Geodetic surveys across the High Himalaya are needed to determine the present state of the MCT as well as the rate of uplift and shortening within the Himalaya. Paleoseismicity studies are necessary to resolve the temporal and spatial patterns of major earthquake faulting along the segmented Himalayan mountains.     

Identification of active seismicity by fractal analysis for understanding the recent geodynamics of Central Himalaya

Seismicity along the Himalayan front is mostly attributed to the processes of collision between the Indian and the Eurasian plates resulting in the under-thrusting of the Indian Peninsula underneath the Himalaya. The dynamics of the region bears very complex components which require in-depth understanding. Here the overall rate of crustal shortening since ∼ 11 Ma is ∼ 21mm/yr, which is comparable to modern rate of under-thrusting of the northern Indian plate beneath the Himalaya. The region experienced a large number of great earthquakes for the last 100–120 years causing massive destruction. Here an attempt has been made to understand the seismicity pattern of the region using fractal correlation dimension and hence used for the detection of active seismicity. Some clusters of seismicity were found to be indicative of seismically very active zones. Such clusters may enlighten the understanding of recent complex dynamics of Himalayan zone.     

Seismological Evidences for the Multiple Incomplete Crustal Subductions in Himalaya and Southern Tibet

SEISMOLOGICAL EVIDENCES FOR THE MULTIPLE INCOMPLETE CRUSTAL SUBDUCTIONS IN HIMALAYA AND SOUTHERN TIBET     

Flexure of the Indian plate and intraplate earthquakes

The flexural bulge in central India resulting from India's collision with Tibet has a wavelength of approximately 670 km. It is manifest topographically and in the free-air gravity anomaly and the geoid. Calculations of the stress distribution within a flexed Indian plate reveal spatial variations throughout the depth of the plate and also a function of distance from the Himalaya. The wavelength (and therefore local gradient) of stress variation is a function of the effective elastic thickness of the plate, estimates of which have been proposed to lie in the range 40–120 km. The imposition of this stress field on the northward moving Indian plate appears fundamental to explaining the current distribution of intraplate earthquakes and their mechanisms. The current study highlights an outer trough south of the flexural bulge in central India where surface stresses are double the contiguous compressional stresses to the north and south. The Bhuj, Latur and Koyna earthquakes and numerous other recent reverse faulting events occurred in this compressional setting. The N/S spatial gradient of stress exceeds 2 bars/km near the flexural bulge. The overall flexural stress distribution provides a physical basis for earthquake hazard mapping and suggests that areas of central India where no historic earthquakes are recorded may yet be the locus of future damaging events.     

Recurrence of great earthquakes at subduction zones

Statistics of the recurrence times of great earthquakes at the Pacific subduction margins are made. The mean return period of great earthquakes is different from zone to zone, ranging from 27 to 117 years. The standard deviation of the return period proves to be very small, several years say, in some cases. The probabilities of a great earthquake recurring in each zone are estimated on the basis of Weibull distribution analysis.The mean return periods thus estimated are combined with the relative plate velocities at respective zones as obtained in the plate tectonics in order to estimate the ultimate displacement to rupture at the interface of the continental plate and the downgoing oceanic plate. It is presumed that great earthquakes at subduction zones occur as a result of a rebound of the continental plate at the time of rupture. The ultimate displacement thus estimated ranges from 2 to 8 m, and seems somewhat larger than that estimated on the basis of seismic observations, although the value of ultimate displacement seems to harmonize roughly with estimates based on geodetic observations on land. However, the ultimate displacement at the Aleutian—Alaska zone as estimated here seems much smaller than that estimated from actual observations.The ultimate strains, which are deduced from the displacements obtained on the assumption that the logarithmic extent of the deformed area is proportional to earthquake magnitude, are then calculated, and compared with those estimated for large inland earthquakes as revealed by repetition of geodetic surveys. The mean ultimate strain is estimated as 4.3 · 10⁻⁵ for subduction-zone earthquakes while that for inland earthquakes has been estimated as 4.7 · 10⁻⁵. As the agreement between both the ultimate strains is fairly good, it is tentatively concluded that the strength of the plate interface under the sea bottom is more or less the same as that in the crust on land.     

西昆仑康西瓦断裂显微构造特征及其地质意义

康西瓦断裂是西昆仑地区一条极其重要的构造缝合带。利用显微构造方法发现：康西瓦断裂明显经历三期构造演化。第一期为晚三叠世一早侏罗世的NE—SW向挤压,由古特提斯洋相继向北持续消减所造成,断裂除挤压碰撞外,还表现为韧性右行走滑;第二期为喜马拉雅运动早期的NE—SW向挤压,由印度板块向欧亚板块北东向俯冲所造成,断裂表现为明显左行平移,形成现代露头的宏观牵引;第三期为喜马拉雅运动晚期的伸展与快速隆升,最终形成现代地貌景观。     

Minimum norm inversion of observed ground elevation changes for slips on the causative fault during the 1905 Kangra earthquake

We estimate the distribution of slip in the dip section of the causative fault for the 1905 Kangra earthquake by applying the minimum norm inversion technique to differences in pre- and post-earthquake levelling data collected along the Saharanpur-Dehradun-Mussoorie highway. For this purpose it is assumed that the causative fault of the 1905 Kangra earthquake was planar with a dip of 5° in the northeast direction and that it had a depth of 6 km at the southern limit of the Outer Himalaya in Dehradun region. The reliably estimated maximum slip on the fault is 7.5 m under the local northern limit of the Outer Himalaya. Using the inverted slip distribution we estimate that the maximum permanent horizontal and vertical displacements at the surface due to the Kangra earthquake were about 4 m and 1.5m respectively. The maximum transient displacements at the surface should have exceeded these permanent displacements. These estimates of maximum slip on the causative fault and the resultant maximum permanent and transient displacements at the surface during the Kangra earthquake may be taken tentatively as being representative of the great Himalayan earthquakes.     

Earthquake slip estimation from the scarp geometry of Himalayan Frontal Thrust, western Himalaya: implications for seismic hazard assessment

Geometric and kinematic analyses of minor thrusts and folds, which record earthquakes between 1200 AD and 1700 AD, were performed for two trench sites (Rampur Ghanda and Ramnagar) located across the Himalayan Frontal Thrust (HFT) in the western Indian Himalaya. The present study aims to re-evaluate the slip estimate of these two trench sites by establishing a link between scarp geometry, displacements observed very close to the surface and slip at deeper levels. As geometry of the active thrust beneath the scarp is unknown, we develop a parametric study to understand the origin of the scarp surface and to estimate the influence of ramp dip. The shortening estimates of Rampur Ghanda trench by line length budget and distance–displacement (D–d) method show values of 23 and 10–15 %, respectively. The estimate inferred from the later method is less than the line length budget suggesting a small internal deformation. Ramnagar trench shows 12 % shortening by line length budget and 10–25 % by the D–d method suggesting a large internal deformation. A parametric study at the trenched fault zone of Rampur Ghanda shows a slip of 16 m beneath the trailing edge of the scarp, and it is sufficient to raise a 8-m-high scarp. This implies that the Rampur Ghanda scarp is balanced with a single event with 7.8-m-coseismic slip in the trenched fault zone at the toe of the scarp, 8–15 % mean deformation within the scarp and 16-m slip at depth along a 30° ramp for a pre-1400 earthquake event. A 16-m slip is the most robust estimate of the maximum slip for a single event reported previously by trench studies along the HFT in the western Indian Himalaya that occurred between 1200 AD and 1700 AD. However, the Ramnagar trenched fault zone shows a slip of 23 m, which is larger than both line length and D–d methods. It implies that a 13-m-high scarp and 23-m slip beneath the rigid block may be ascribed to multiple events. It is for the first time we report that in the south-eastern extent of the western Indian Himalaya, Ramnagar scarp consists of minimum two events (i) pre-1400 AD and (ii) unknown old events of different lateral extents with overlapping ruptures. If the more optimistic two seismic events scenario is followed, the rupture length would be at least 260 km and would lead to an earthquake greater than Mw 8.5.     

Seismic hazard assessment of Chennai city considering local site effects

Chennai city suffered moderate tremors during the 2001 Bhuj and Pondicherry earthquakes and the 2004 Sumatra earthquake. After the Bhuj earthquake, Indian Standard IS: 1893 was revised and Chennai city was upgraded from zone II to zone III which leads to a substantial increase of the design ground motion parameters. Therefore, a comprehensive study is carried out to assess the seismic hazard of Chennai city based on a deterministic approach. The seismicity and seismotectonic details within a 100 km radius of the study area have been considered. The one-dimensional ground response analysis was carried out for 38 representative sites by the equivalent linear method using the SHAKE91 program to estimate the ground motion parameters considering the local site effects. The shear wave velocity profile was inferred from the corrected blow counts and it was verified with the Multichannel Analysis of Surface Wave (MASW) test performed for a representative site. The seismic hazard is represented in terms of characteristic site period and Spectral Acceleration Ratio (SAR) contours for the entire city. It is found that structures with low natural period undergo significant amplification mostly in the central and southern parts of Chennai city due to the presence of deep soil sites with clayey or sandy deposits and the remaining parts undergo marginal amplification.     

云南川西地区地震地质基本特征的探讨

云南川西地区是多震的地方(图1),从公元前116年到1974年6月,据记载共发生M≥4.75级的地震481次,其中6≤M<7的99次,7≤M<8的14次,M≥8的3次。震源深度一般小于30公里,均为浅源地震。本区强震多发生在深大断裂带上,而且都有一定的地质标志可循,与近代地壳运动、板块构造在成因上有着密切关系。     

Compressive Tectonics around Tibetan Plateau Edges

Various earthquake fault types, mechanism solutions, stress field, and other geophysical data were analyzed for study on the crust movement in the Tibetan plateau and its tectonic implications. The results show that numbers of thrust fault and strike-slip fault type earthquakes with strong compressive stress near NNE-SSW direction occurred in the edges around the plateau except the eastern boundary. Some normal faulting type earthquakes concentrate in the Central Tibetan plateau. The strikes of fault planes of thrust and strike-slip faulting earthquakes are almost in the E-W direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. This implies that the dislocation slip vectors of the thrust and strike-slip faulting type events have quite great components in the N-S direction. The compression motion mainly probably plays the tectonic active regime around the plateau edges. The compressive stress in N-S or NE-SW directions predominates earthquake occurrence in the thrust and strike-slip faulting event region around the plateau. The compressive motion around the Tibetan plateau edge is attributable to the northward motion of the Indian subcontinent plate. The northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins.     

Human Losses Expected in Himalayan Earthquakes

Quantitative estimates of potential losses that may be caused by future great earthquakes along the Himalaya suggest that as many as 150,000 people may die, 300,000 may be injured and typically 3,000 settlements will be affected in a single event. Scenario results used here vary and are based on ruptures of 150 km segments of the plate boundary at seven positions, where sufficient elastic energy is believed to be stored for magnitude eight earthquakes. The method of calculating these results was calibrated, using the 17 disastrous Indian earthquakes, which have occurred since 1980. About 50 settlements in the region are considered most at risk because in each more than 2000 fatalities may occur.     

The M w 8.6 Indian Ocean earthquake on 11 April 2012: Coseismic displacement, Coulomb stress change and aftershocks pattern

The M _w 8.6 Indian Ocean earthquake occurred on April 11, 2012 near the NW junction of three plates viz. Indian, Australian and Sunda plate, which caused widespread coseismic displacements and Coulomb stress changes. We analyzed the GPS data from three IGS sites PBRI, NTUS & COCO and computed the coseismic horizontal displacements. In order to have in-depth understanding of the physics of earthquake processes and probabilistic hazard, we estimated the coseismic displacements and associated Coulomb stress changes from two rectangular parallel fault geometries, constrained by Global Positioning System (GPS) derived coseismic displacements. The Coulomb stress changes following the earthquake found to be in the range of 5 to ?4 bar with maximum displacement of ～11 m near the epicenter. We find that most of the aftershocks occurred in the areas of increased Coulomb stress and concentrated in three clusters. The temporal variation of the aftershocks, not conformed to modified Omori’s law, speculating poroelastic processes. It is also ascertained that the spatio-temporal transient stress changes may promote the occurrence of the subsequent earthquakes and enhance the seismic risk in the region.     

青藏高原现今构造变形特征与GPS速度场

文章以青藏高原的GPS观测数据为基础 ,结合活动地质构造资料 ,研究了青藏高原的现今构造变形状态和机制 ,并探讨青藏高原现今构造变形所反映的大陆内部动力学过程。GPS观测的速度矢量揭示了青藏高原整体向北和向东运动的趋势 ,平行于印度和欧亚板块碰撞方向上的地壳缩短量约是 38mm/a ,而青藏高原周边主要断裂带的滑动速率均在 10mm/a以下。大约 90 %的印度与欧亚板块相对运动量被青藏高原的地壳缩短所吸收和调节。GPS速度矢量由南向北逐渐向东偏转 ,向东的分量也增加 ,形成了以羌塘地块北部 (或玛尼—玉树—鲜水河断裂 )和祁连山中部为中心的两个地壳物质向东流动带。青藏高原的向东挤出实际上是地壳物质在印度板块推挤下和周边刚性地块阻挡下围绕东构造结发生的顺时针旋转。     

极具潜力的空间对地观测新技术——合成孔径雷达干涉

“合成孔径雷达干涉(InSAR)”是近十年发展起来的空间对地观测遥感新技术。它具有从覆盖同一地区的星载(或机载)合成孔径雷达复数图像对提取干涉相位图,借助于雷达成像时的姿态数据重建地表三维模型(即数字高程模型)的巨大潜力。尤其是基于多幅雷达复数图像处理的差分干涉技术(D-InSAR)可以用于监测地表形变,精度可达厘米级甚至更高,其监测空间分辨率是前所未有的。介绍了InSAR和D-InSAR的基本原理,对影响干涉结果的一些重要因素做了分析,重点回顾和展望了差分干涉技术在与地表形变有关的地震监测和震后形变测量、地面下沉和山体滑坡、火山运动监测等方面应用的现状和前景。     

On the seismic structure of Indian upper-mantle regions

Several velocity models on upper mantle regions of the world have been postulated during the last two decades. There has been a broad agreement amongst seismologists that upper mantle has got two transition zones, though the models differ in detail. These zones have been found to occur around ‘400 km’ and ‘650 km’ depth ranges with varying thicknesses of the zones. A limited number of such studies have been made on the upper mantle structure of the Indian subcontinent. High positive velocity gradients were reported to exist around the above depth range. Evidence for lateral heterogeneities has also been found. We address some problems like refinement of Indian upper mantle velocity models specially after considering the effect of scattering and attenuation on the short periodP-waves. The study of proper positioning of the cusps of the travel-time branches and their extension is essential as well. In our opinion, analysis of such problems would help in the better understanding of the nature of propagation of seismic waves and mechanism of earthquakes. Complexity of seismic signatures observed is another major problem and may also be taken into consideration.