GPS-derived deformation rates in northwestern Himalaya and Ladakh

Deformation rates derived from GPS measurements made at two continuously operating stations at Leh (34.1°N, 77.6°E) and Hanle (32.7°N, 78.9°E), and eight campaign sites in the trans-Himalayan Ladakh spanning 11 years (1997–2008), provide a clear picture of the kinematics of this region as well as the convergence rate across northwestern Himalaya. All the Ladakh sites move 32–34 mm/year NE in the ITRF2005 reference frame, and their relative velocities are 13–16 mm/year SW in the Indian reference frame and ~19 mm/year W with reference to the Lhasa IGS station in southeastern Tibet. The results indicate that there is no statistically significant deformation in the 200-km stretch between the continuous sites Leh and Hanle as well as between Leh and Nubra valley sites along the Karakoram fault, whereas the sites in and around the splayed Karakoram fault region indicate surface deformation of 2.5 mm/year. Campaign sites along the Karakoram fault zone indicate a fault parallel surface motion of 1.4–2.5 mm/year in the Tangste and western Panamik segment of the Karakoram fault, which quantifies the best possible GPS-derived dextral slip rate of 3 mm/year along this fault during this 11-year period. Baselines of Ladakh sites show convergence rates of 15–18 mm/year with respect to south India and 12–15 mm/year with respect to Delhi in north India and Almora in the Himalaya ~400 km north-northeast of Delhi. These constitute an arc normal convergence of 12–15 mm/year across the western Himalaya, which is consistent with arc normal convergence all along the Himalayan arc from west to east. Baseline extension rates of 14–16 mm/year between Lhasa and Ladakh sites are consistent with the east–west extension rate of Tibetan Plateau.     

喜马拉雅造山带南北向裂谷的冷缩成因模型

长期以来,学者们普遍认为垂直于喜马拉雅造山带的南北向裂谷是东西向伸展的构造形迹。现代GPS观测数据却显示,喜马拉雅造山带东西位移(分)量很小,甚至为零。综合前人资料,喜马拉雅造山过程可划分为热造山(25~13Ma)及造山后(13Ma)冷却两个时期,热造山期具有受热膨胀,物质向外运移的特点,高喜马拉雅热隆挤出并触发各主要断裂(MCT、STD、GCT)活动,印度板块向北汇聚速率下降。造山后则表现为冷却收缩,前期构造-热活动停止或减弱,印度板块向北汇聚加速。研究认为,南北向裂谷与高喜马拉雅等冷却过程的东西向收缩。且被局限于东、西两个构造结之间有关。并据此建立了裂谷的冷缩成因模型,模型估值与地质事实很吻合。     

Pn wave velocity and Moho geometry in north eastern India

Pn velocity has been computed across the NE India and Moho geometry constrained, using regional earthquake travel times recorded by a network of 30 seismological stations operated during February-May 1993. Using an appropriate velocity model and the arrival times at the network stations, preliminary hypocentres of 16 regional earthquakes provided by NEIC were also improved. The average Pn wave velocity in NE India has been found to be 8.5 ±0.2 km/s. It varies from 8.3 to 8.5 km/s beneath the Shillong Plateau, Mikhir hills and Assam valley, which is significantly higher than those in other parts of India. The crustal thickness in NE India is also high, varying from 45–49 km under the Shillong plateau and the adjoining region to 55–65 km in the convergence zone. The presence of a thick crust and high Pn velocity suggests that the lithosphere in NE India is colder, as also indicated by the observed deeper level (45-51 km) seismicity of the region.     

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.     

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.     

Crustal Shortening on the Margins of the Tien Shan,Xinjiang, China

We present the results of mapping selected cross-sections across the margins of the Chinese Tien Shan, an intracontinental mountain belt that formed in response to the India-Eurasia collision. This belt contains significant lateral variation in topography, structure, and stratigraphy at all scales, and our estimated rates of shortening also reveal a distribution of shortening that varies laterally. At the largest scale, it consists of two major high mountain ranges in the west that merge eastward into a complex, single high mountain belt with several distinct ranges, then separates farther eastward into several low mountain ranges in the south and a single narrow high mountain range in the north. Active fold-and-thrust belts along parts of the north and south flanks of the Tien Shan involve only Mesozoic and Cenozoic sedimentary cover, which varies in both stratigraphy and structure from east to west. The southern fold-and-thrust belt decreases in width and complexity from west to east and ends before reaching Korla. The northern belt begins near the longitude where the southern belt ends, and increases in width and complexity from west to east. Within these two fold-and-thrust belts are both E-W and N-S variations in stratigraphy at the scale of the fold-and-thrust belts and across individual structures. All these variations make it very difficult to generalize either structure or stratigraphy within the Tien Shan or within local areas.

Four maps and cross-sections, two across each of the northern and southern fold-and-thrust belts, imply different magnitudes of shortening. In the eastern part of the northern belt, a cross-section along the southern part of the Hutubi River yields shortening of 6.2 km, and a section to the north across the Tugulu anticline yields shortening of 5.5 km. The two parts of the cross-section cannot be added because the Tugulu anticline lies 20 km west of the Hutubi River, and diminishes greatly in amplitude toward the Hutubi River. In the western part of the northern belt, cross-sections require 4.6 to 5.0 km of shortening at Tuositai and 2.12 to 2.35 km across the Dushanzi anticline. The Tuositai structure lies south of the Dushanzi anticline, but shortening in these two areas also cannot be summed, because they seem to be separated by a N-trending strike-slip fault. In the western part of the southern fold-and-thrust belt, an incomplete cross-section along the Kalasu River suggests shortening of 12.1 to 14.1 km. If the estimated shortening of 6 to 7 km in the Qiulitage anticline, which we did not map, is added, the total shortening in this cross-section would be ～18 to 21 km. To the east, a complete cross-section at Boston Tokar yielded shortening of 10.3 to 13.0 km.

Calculating long-term shortening rates from these four cross-sections is difficult, because the time of initiation of deformation is poorly known. In the Kalasu River area of the southern belt, there is evidence that limited shortening of 2 to 4 km occurred in the early Miocene, if major thickness changes in deposition of conglomerate unit 3b are interpreted to be growth strata. Geological evidence suggests that most of the shortening began in both belts after the beginning of the deposition of the thick conglomerate unit shown as lower Quaternary on Chinese geological maps. Strata within the middle part of these conglomerates were deposited during the growth of the folds. Presence of Equus near the base of similar conglomerates indicates a Quaternary age, but the fossil localities are far from most of our cross-sections, and the contemporaneity of the rocks remains in question. The beginning of conglomerate deposition may be controlled by climate change, and if so, the beginning of conglomerate deposition may be generally contemporaneous throughout the region at ～2.5 Ma. Deformation began at some time after the onset of conglomerate deposition, but this time is not well constrained. Thus we have calculated shortening rates for 2.5, 1.6, and 1.0 Ma that should bracket maximum and minimum slip rates. These calculations yield the following ranges in the northern fold-and-thrust belt: southern Hutubi River = 2.5 to 6.2 mm/yr; Tugulu anticline = 2.1 to 5.5 mm/yr; Tuositai anticline = 1.8–2.0 to 4.6–5.0 mm/yr; and Dushanzi anticline = 0.8 to 2.1–2.4 mm/yr; and in the southern fold-and-thrust belt: Kalasu River = 4.6–5.6 (including the Qiulitage anticline = 7.2–8.4) to 12.1–14.1 (including Qiulitage anticline = 18–21) mm/yr; and at Boston Tokar = 4.1–5.2 to 10.3–13.1 mm/yr. If 2 to 4 km of shortening occurred in the Kalasu River section during early Miocene time, the long-term rates for Quaternary time are 3.2–4.8 (including Qiulitage anticline = 5.6–7.6) to 8.1–12.1 (including Qiulitage anticline = 14–19) mm/yr.

Calculation of the shortening rate across the entire width of the Tien Shan is difficult because of the rapid lateral variations in structure and because of active deformation within the range, which we have not studied. The cross-sections at Boston Tokar in the south and Tuositai in the north lie along the same longitude. Adding the shortening rates in these areas would yield a minimum range (using 2.5 Ma as the initiation time) of 5.7 to 7.2 mm/yr. If deformation began at 1.6 or 1.0 Ma, the range of shortening rates would be 10–11.2 mm/yr to 14.9–18.1 mm/yr, respectively. Because the first indication of structural growth with the mapped areas occurs above the base of the conglomerates at the top of the stratigraphic succession, a minimum shortening rate greater than 5.7 to 7.2 mm/yr is more likely.

Both the marginal fold-and-thrust belts have a thin-skinned geometry with the drcollement at -6 to 10 km and within Mesozoic and Cenozoic sedimentary rocks. Toward the interior of the range the decollement must pass into the Paleozoic basement rocks and steepen beneath the flanks of the range. The structural style is similar to that in the Laramide Rocky Mountains and the California Transverse Ranges. The highest parts of the Tien Shan are adjacent to areas of active shortening. Such a relation might suggest that the major uplift of the Tien Shan is very young, mostly latest Cenozoic or Quaternary in age. The shortening across the Tien Shan is inhomogeneous and spatially distributed.     

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。     

Cooling age record of domal uplift in the core of the Higher Himalayan Crystallines (HHC), southwest Zanskar, India

The cooling and tectonic history of the Higher Himalayan Crystallines (HHC) in southwest Zanskar (along the Kishtwar-Padam traverse) is constrained by K-Ar biotite and fission-track (FT) apatite and zircon ages. A total of nine biotite samples yields ages in the range of 14–24 Ma, indicating the post-metamorphic cooling of these rocks through ∼ 300°C in the Miocene. Overall, the ages become younger away from the Zanskar Shear Zone (ZSZ), which marks the basement-cover detachment fault between the HHC and the Tethyan sedimentary zone, towards the core of the HHC. The same pattern is also observed for the FT apatite ages, which record the cooling of the rocks through ∼ 120°C. The apatite ages range from 11 Ma in the vicinity of the ZSZ to 4 Ma at the granitic core of the HHC. This pattern of discordant cooling ages across the HHC in southwest Zanskar reveals an inversion of isotherms due to fast uplift-denudation (hence cooling) of the HHC core, which is, in turn, related to domal uplift within the HHC. The Chisoti granite gneiss is the exposed domal structure along the studied traverse. Cooling history of two granite gneisses at the core of the HHC is also quantified with the help of the biotite, zircon and apatite ages; the time-temperatures thus obtained indicate a rapid pulse of cooling at ∼ 6 Ma, related to accelerated uplift-denudation of the HHC core at this time. Long-term denudation rates of 0.5–0.7 mm/yr are estimated for the high-grade rocks of the Higher Himalaya in southwest Zanskar over the past 4.0–5.5 m.yr.     

Temperature variability and vertical vegetation belt shifts during the last ∼50,000 yr in the Qilian Mountains (NE margin of the Tibetan Plateau, China)

A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around ∼46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by ∼4-7°C than today. Markedly higher temperatures were inferred from higher arboreal pollen frequencies between ∼13,000 and ∼7,000 yr ago with a Holocene temperature optimum and a maximal Picea-Betula mixed-forest expansion between ∼9,000 and ∼7000 yr ago, when temperatures exceeded the present-day conditions by at least 1-2°C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.     

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.     

Horizontal motions and the generation of strong earthquakes in the North Sakhalin interiors

The recent geodynamics of Sakhalin Island is best described by the convergence of the Eurasian and North American (Sea of Okhotsk) lithospheric plates, which is manifested in the high seismic activity of the island. In North Sakhalin, the plate boundary is thought to correspond to a system of roughly N-S-trending faults, which belong to the North Sakhalin deep fault, and the Upper-Piltun fault; the latter was ruptured by the 1995 M 7.2 Neftegorsk earthquake. This study first confirmed that the stationary motion of the Sea of Okhotsk plate is retarded on this fault to form with time a series of drag folds and stress field anomalies. The latter are released during the subsequent (in a 400⦒o 1000-year period) strong earthquakes by seismic sliding on the flanks of the Upper Piltun fault. The 2003–2006 GPS observations revealed the free state of this fault zone with relative slip rates of 5–6 mm/yr.     

喜马拉雅造山带东构造结拉布拉多期和格林威尔期造山作用的记录

位于喜马拉雅东构造结西北部的南迦巴瓦复合体,是构造应力最强、隆升和剥蚀最快、新生代变质和深熔作用最强的地区。为厘定该地区早期的变质岩浆作用,本文对南迦巴瓦复合体北部的花岗片麻岩和混合岩进行了岩石学和年代学研究。花岗片麻岩原岩为富钾的偏铝质花岗岩,具有岩浆弧花岗岩的成分特征。花岗片麻岩中的锆石具有岩浆锆石的环带结构,记录了487.9±1.6Ma的一期构造岩浆事件;混合岩的锆石具有明显的核-边结构,核部和边部的不协和线交点年龄分别为1559±13Ma、1154±12Ma。对比印度大陆东部的西隆高原、东高止造山带,发现三者都经历了拉布拉多期、格林威尔期以及泛非期的造山作用。因此,我们认为喜马拉雅东构造结与这两个地区密切相关,可能是他们向北的延伸,这三者可能组成统一的印度大陆东部造山带,一起经历了哥伦比亚超大陆、Rodinia和冈瓦纳超大陆的聚合与裂解过程。     

Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup,eastern Himalaya,Darjiling District,India

The Himalayan fold-and-thrust belt has propagated from its Tibetan hinterland to the southern foreland since ∼55 Ma. The Siwalik sediments (∼20 - 2 Ma) were deposited in the frontal Himalayan foreland basin and subsequently became part of the thrust belt since ∼ 12 Ma. Restoration of the deformed section of the Middle Siwalik sequence reveals that the sequence is ∼325 m thick. Sedimentary facies analysis of the Middle Siwalik rocks points to the deposition of the Middle Siwalik sediments in an alluvial fan setup that was affected by uplift and foreland-ward propagation of Greater and Lesser Himalayan thrusts. Soft-sediment deformation structures preserved in the Middle Siwalik sequence in the Darjiling Himalaya are interpreted to have formed by sediment liquefaction resulting from increased pore-water pressure probably due to strong seismic shaking. Soft-sediment structures such as convolute lamination, flame structures, and various kinds of deformed cross-stratification are thus recognized as palaeoseismic in origin. This is the first report of seismites from the Siwalik succession of Darjiling Himalaya which indicates just like other sectors of Siwalik foreland basin and the present-day Gangetic foreland basin that the Siwalik sediments of this sector responded to seismicity.     

Rigid Indian plate: Constraints from GPS measurements

We analyze GPS data from 26 sites located on the Indian plate and along its boundary. The large spatial coverage of the Indian plate by these sites and longer data duration helped us in refining the earlier estimates of the Euler pole for the Indian plate rotation. Our analysis suggests that the internal deformation of the Indian plate is very low (< 1–2 mm/year) and the entire plate interior region largely behaves as a rigid plate. Specifically, we did not infer any significant difference in motion on sites located north and south of the Narmada Son failed rift region, the most prominent tectonic feature within the Indian plate and a major source of earthquakes. Our analysis also constrains the motion across the Indo-Burmese wedge, Himalayan arc, and Shillong Plateau and Kopili fault in the NE India.     

大陆碰撞构造剖析

 首先对青藏高原分布的蛇绿岩带的地质特征进行剖析,认为这些蛇绿岩是陆内裂谷环境产出的初始洋壳而不是大陆缝合线。然后对喜马拉雅山系的构造特征及其造山体制加以讨论,认为以喜马拉雅山系作为碰撞造山的样板并无地质事实的根据,而是一种想当然的臆说,后来也得不到参验。结论:碰撞造山带在大陆是不存在的,是看不到的。     

Optically stimulated luminescence chronology of terrace sediments of Siang River,Higher NE Himalaya: Comparison of Quartz and Feldspar chronometers

Four levels of terraces located along Siang River, north of Main Central Thrust at Tuting, NE Himalaya are dated using Optically Stimulated Luminescence (OSL). The dating technique is applied using (1) Blue LED stimulation on Quartz (2) Infrared Stimulated Luminescence (IRSL) stimulation on Feldspar at 50 °C and (3) Infrared Stimulated Luminescence stimulation on Feldspar at an elevated temperature of 225 °C. The results indicated that the later two protocols on feldspars yielded overestimated ages that suggested incomplete bleaching of luminescence signals in feldspar. The ages derived using quartz suggested a nearly continued valley aggradation from >21–8 ka with three phases of bedrock incision. The phase of aggradation coincides with a climatic transition from cold and dry Last Glacial phase to warm and wet Holocene Optimum. The bedrock incision phases centered at <21 ka, ∼11 ka and ∼8 ka indicate towards major episodes of tectonic uplift in the region around Tuting.     

青藏高原东部及周边现时地壳运动

通过1991—2001年期间在青藏高原东部及周边地区的GPS测量,获得该地区不同参考框架下的地壳运动速度场,其测量的速度精度高于2mm／yr。印度板块与华北地块之间的地壳形变分为喜马拉雅及高原南部、高原中部(拉萨—格尔木)和高原北部(格尔木—金塔)三部分,它们分别吸收了印度板块与欧亚板块汇聚速率的43％、24％和32％。在欧亚框架下和相对于成都,印度板块和华南地块之间存在着以东喜马拉雅构造结为轴心的顺时针巨型涡旋构造——滇藏涡旋构造,运动速度分别为26～6mm／yr和24～7mm／yr,总体上从北东方向转变为南东和南西方向,有别于青藏高原中部的北东方向。滇藏涡旋和东喜马拉雅构造结的形成与南迦巴瓦—阿萨姆“犄角”的楔入作用有关。     

大陆碰撞构造剖析

首先对青藏高原分布的蛇绿岩带的地质特征进行剖析,认为这些蛇绿岩是陆内裂谷环境产出的初始洋壳而不是大陆缝合线。然后对喜马拉雅山系的构造特征及其造山体制加以讨论,认为以喜马拉雅山系作为碰撞造山的样板并无地质事实的根据,而是一种想当然的臆说,后来也得不到参验。结论:碰撞造山带在大陆是不存在的,是看不到的。     

Controls of surface erosion on the evolution of the Alps: constraints from the stratigraphies of the adjacent foreland basins

 The combined information about the stratigraphies from the foreland basins surrounding the Swiss Alps, exhumation mechanisms and the structural evolution of the Alpine orogenic wedge allow an evaluation of the controls of erosion rates on large-scale Alpine tectonic evolution. Volumetric data from the Molasse Basin and fining-upward trends in the Gonfolite Lombarda indicate that at ∼20 Ma, average erosion rates in the Alps decreased by >50%. It appears that at that time, erosion rates decreased more rapidly than crustal uplift rates. As a result, surface uplift occurred. Because of surface uplift, the drainage pattern of the Alpine hinterland evolved from an across-strike to the present-day along-strike orientation. Furthermore, the decrease of average erosion rates at ∼20 Ma coincides with initiation of a phase of thrusting in the Jura Mountains and the Southern Alpine nappes at ∼50 km distance from the pre-20-Ma thrust front. Coupled erosion-mechanical models of orogens suggest that although rates of crustal convergence decreased between the Oligocene and the present, the reduction of average erosion rates at ∼20 Ma was high enough to have significantly influenced initiation of the state of growth of the Swiss Alps at that time. Received: 8 June 1998 / Accepted: 30 October 1998     

Fine structure of seismotectonics in western Shillong massif,north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.