segmentation of the himalayan megathrust around the gorkha earthquake 25 april 2015 in nepal abstract we put the 25 april 2015 earthquake of nepal mw 7 9 into its structural geological context in order to specify the role of the segmentation of the himalayan megathrust the rupture is mainly located nw of kathmandu at a depth of 13 15 km on a flat portion of the main himalayan thrust mht that dips towards the n ne by 7 10 the northern bound of the main rupture corresponds to the transition towards a steeper crustal ramp this ramp which is partly coupled during the interseismic period is only locally affected by the earthquake the southern bound of the rupture was near the leading edge of the lesser himalaya antiformal duplex and near the frontal footwall ramp of the upper nawakot duplex the rupture has been affected by transversal structures on the western side the judi lineament separates the main rupture zone from the nucleation area on the eastern side the gaurishankar lineament separates the 25 april 2015 rupture from the 12 may 2015 mw 7 2 rupture the origin of these lineaments is very complex they are probably linked to pre himalayan faults that extend into the indian shield beneath the mht these inherited faults induce transverse warping of the upper lithosphere beneath the mht control the location of lateral ramps of the thrust system and concentrate the hanging wall deformation at the lateral edge of the ruptures the mht is therefore segmented by stable barriers that define at least five patches in central nepal these barriers influence the extent of the earthquake ruptures for the last two centuries the 1833 mw 7 6 earthquake was rather similar in extent to the 2015 event but its rupture propagated south westwards from an epicentre located ne of kathmandu the patch south of kathmandu was probably affected by at least three earthquakes of mw 7 that followed the 1833 event a few days later or 33 years 1866 event mw 7 2 later the 1934 earthquake mw 8 4 had an epicentre 170 km east of kathmandu may have propagated as far as kathmandu and jumped the gaurishankar lineament this combined structural approach and earthquake study allows us to propose that the mht in the central eastern himalaya is segmented by stable barriers that define barrier type earthquake families however for each individual earthquake within a family the rupture histories could be different furthermore the greatest earthquakes could have broken the barriers and affected the patches of several families the concept of a regular recurrence of characteristic earthquakes is therefore misleading to describe the succession of himalayan earthquakes

sone megafan a non himalayan megafan of craton origin on the southern margin of the middle ganga basin india abstract researchers in the last few decades have focused on the fluvial megafans at the base of himalayan foothills in the ganga basin no major effort has so far been made to delineate any such large depositional environment at the base of the northern indian peninsular craton at the distal parts of the basin in this work for the first time we document a megafan unusually created by the cratonic sone river in the marginal plains south of the ganga the geomorphology of the megafan surface distribution of palaeochannels sedimentology and areal extent of the megafan are described the study also reconstructs the tectonic and palaeoclimatic conditions which might have helped in sediment supply from the cratonic catchment areas of the sone we suggest tectonic control over the sone river channel dynamics and development of the megafan the oval and fan shaped craton derived sediment body covers an area of around 12 000 km2 in the marginal plains and about 9000 km2 in the central alluvium north of the ganga river the megafan is around 190 km long and 240 km wide the mean slope of the megafan surface is 0 03 an onlap of himalayan sediments has occurred over the megafan at its toe parts caused by southward encroachment of the ganga during the holocene the megafan sediments comprise brownish yellow fine to coarse sand and gravels with an admixture of carbonaceous nodules kankars the thickness of the megafan sediment which directly overlies the precambrian basement varies from only a few meters in the south near the peninsular craton to 1000 m at the north in the central alluvial plain the granular zones form prolific aquifer systems in an otherwise poor hydrogeological setting of the marginal plains

transverse tectonics in the sikkim himalaya a magnetotelluric study abstract the tectonics of seismically active sikkim himalaya as inferred by numerous seismological studies is distinct from the conventional thrust tectonics proposed for the himalayan collision belt here focal mechanisms of several moderate magnitude earthquakes and composite fault plane solutions of microearthquakes have revealed strike slip motion along faults transverse to the northward convergence direction of the indian plate in the present study we analyze broadband magnetotelluric data of 12 sites located along an approximately n s profile cutting across major geological sub domains of sikkim to test whether magnetotelluric strikes also support such transverse tectonic nature of the region we have performed strike analysis of the data by two decomposition approaches as well as by phase tensor method the study has revealed local variations in the strike directions within the region consistent with the geological and tectonic setup and the presence of transverse tectonic features in the region of main central thrust zone mctz where major axis of phase ellipses align in nnw sse to nw se direction this trend coincides with the one obtained by microseismic data recorded after the september 18 2011 earthquake mw 6 9 magnetotelluric strike analysis thus supports the presence of nnw to nw trending transverse tectonic zone in mctz

distribution characteristics of topside ionospheric density irregularities equatorial versus midlatitude regions 1 the global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between 35 geographic latitudes has been studied with the rocsat data during moderate to high solar activity years of 1999 to 2004 the result indicates that the global occurrence distribution of the intermediate scale 0 1 to 50 km density irregularities can be grouped into two different populations one in the equatorial region and the other in the middle to subauroral latitude region the global seasonal longitudinal s l distribution of equatorial irregularities in the current report reproduces the result of mcclure et al 1998 obtained with the ae e observations of the mesoscale 50 to 1000 km plasma bubble structures during high solar activity years of 1978 to 1980 two solar cycles ago this implies that the density irregularities of different scales from multistage cascading process of the large scale 1000 km gravitational rayleigh taylor instability have manifested in same global s l distribution pattern furthermore global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years in addition the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about 30 a different kind of midlatitude irregularity distribution starts to fill in from this dip latitude in other words the equatorial density irregularity inside a depleted flux tube can only rise on statistical average to an apex height of 2000 km different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes the occurrence frequency of equatorial density irregularities increases with solar flux intensity whereas the midlatitude density irregularity is more likely to occur during low solar activity period the equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times on the other hand the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity the local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate scale density irregularities

seismotectonic model of the sikkim himalaya constraint from microearthquake surveys the seismotectonic model in the sikkim himalaya does not fit well with the proposed steady state or evolutionary models the main boundary thrust mbt is seismogenic and is a mantle reaching fault the earthquakes are not confined to shallower depths 25 km above the plane of detachment as proposed in the models the seismic activity continues from surface to the lower crust 0 45 km to the north of mbt and earthquakes are produced by a thrust mechanism the earthquakes to the east of sikkim in the bhutan himalaya on the other hand are produced along a 200 km long northwest southeast trending lineament by transverse tectonics the seismogenic lineament cuts across the himalayan major thrusts and extends to the goalpara wedge in the southeast the earthquakes occur by strike slip mechanism in the midcrust at a depth range of 10 25 km along this long active lineament

on post midnight field aligned irregularities observed with a 30 8 mhz radar at a low latitude comparison withf layer altitude near the geomagnetic equator 1 we investigated the relationship between post midnightf region field aligned irregularities fais andf layer altitude by analyzing data of a 30 8 mhz radar installed 5at kototabang indonesia 0 2 s 100 3 e geomagnetic latitude 10 4 s and an ionosonde installed at chumphon thailand 10 7 n 99 4 e geomagnetic latitude 3 3 n chumphon is located near the geomagnetic equator on approximately the same meridian as kototabang case studies show that the altitude of thef layer rose at chumphon a half hour before the post midnight fais appeared at kototabang the doppler velocity of thee region fais observed simultaneously by the 30 8 mhz radar was downward indicating that thef layer uplift was not caused by the electric field we also investigated seasonal variations of the post midnight fai occurrence and thef layer altitude both the post midnight fais and the uplift of thef layer were frequently seen around midnight between may and august the seasonal variation of the midnightf layer uplift around the geomagnetic equator coincided with that of the post midnight fai occurrence at kototabang these results suggest that the uplift of thef layer would play an important role in the generation of post midnight fais we evaluated the linear growth rate of the rayleigh taylor instability based on the altitude of thef layer observed at chumphon the result shows that the uplift of thef layer can enhance the growth rate because gravity driven eastward electric current increases therefore we interpret that the observed fais were accompanied by plasma bubble the growth rate of which was reinforced by the upliftedf layer

seismicity around the mahendragarh dehradun basement fault in the western ganga plain india a neotectonic perspective the ne sw trending mahendragarh dehradun fault mdf is a basement fault in the western ganga plain the earthquake 4 7 mb epicentered on this fault on 2nd june 2017 was focused around 10 km depth and does not show any surface rupture ground penetrating radar gpr survey around the epicenter detects few shallow depth subsurface normal faults parallel to the mdf however it is unclear whether these normal faults are linked with the 2017 earthquake or other previous earthquakes seismic record usgs since 1975 to 2017 suggests occurrence of 22 earthquakes 5 mb around the mdf and majority 63 of these are clustered around delhi and rohtak following the 2017 earthquake frequency of earthquakes occurence sharply increased than previously recorded instrumentation period from 1975 to 1995 majority of the seismicity is focused around 10 km depths shallowing to depths of 25 207 km recorded from 1975 to 2004 earthquakes clustered along the main himalayan thrust mht shows similar temporal trend as near the mdf southward thrust movement of the himalaya orogenic wedge predicts to impose strike slip motion on the orthogonal oriented steep basement faults which is consistent with geologic and geomorphic field relations and seismicity focal mechanisms along the mdf the observed upper crustal deformation associated with such fault deformation pattern may be partly influenced by the mdf and partly by the foreland bulge seismicity potential tectono geomorphic parameters and soil chronosequence may suggest the surface deformation is active through the holocene

modelling role of basement block rotation and strike slip faulting on structural pattern in cover units of fold and thrust belts a series of scaled analogue models are used to study de coupling between basement and cover deformation rigid basal blocks were rotated about a vertical axis in a bookshelf fashion which caused strike slip faulting along the blocks and in the overlying cover units of loose sand three different combinations of cover basement deformations are modelled i cover shortening before basement fault movement ii basement fault movement before cover shortening and iii simultaneous cover shortening with basement fault movement results show that the effect of the basement faults depends on the timing of their reactivation pre and syn orogenic basement fault movements have a significant impact on the structural pattern of the cover units whereas post orogenic basement fault movement has less influence on the thickened hinterland of the overlying belt the interaction of basement faulting and cover shortening results in the formation of rhombic structures in models with pre and syn orogenic basement strike slip faults rhombic blocks develop as a result of shortening of the overlying cover during basement faulting these rhombic blocks are similar in appearance to flower structures but are different in kinematics genesis and structural extent we compare these model results to both the zagros fold and thrust belt in southwestern iran and the alborz mountains in northern iran based on the model results we conclude that the traces of basement faults in cover units rotate and migrate towards the foreland during regional shortening as such these traces do not necessarily indicate the actual location or orientation of the basement faults which created them

modelling of active lineaments for predicting a possible earthquake scenario around dehradun garhwal himalaya india abstract the doon valley which consists of the doon gravels and siwalik rocks is surrounded by the main boundary thrust mbt in the north the main frontal thrust mft mohand thrust and bhimgoda thrust in the south the ganga tear fault in the east and the yamuna tear fault in the west lineaments identified from the aerial photographs show that a few of them trend parallel to the himalayan orogen and the others are transverse to it the effect of the neotectonic activity is clearly reflected in the doon valley implying that the doon valley is tectonically unstable today and that the possibility of earthquakes in this region cannot be ruled out the ruptures along identified active lineaments are modelled three dimensionally peak accelerations are obtained by using a methodology based on the semi empirical method of irikura which was used by midorikawa the efficacy of this modelling technique for the himalayan region is established by simulating peak ground accelerations for the uttarkashi earthquake of 20 october 1991 and comparing it with recorded one a seismic study of the region shows that it is prone to moderate as well as great earthquakes the technique of midorikawa has been applied for the preparation of zonation maps in this seismically active region two different zonation maps showing respectively the effects of moderate as well as great earthquakes in this region are presented in this paper the zonation map for moderate earthquakes shows that the region can experience peak ground accelerations around 310 cm s2 this value is close to the peak ground acceleration of 304 cm s2 recorded for the uttarkashi earthquake ms 7 1 at uttarkashi station which is about 60 km north of dehradun the prepared map shows that among all identified active lineaments rupture along four lineaments can cause major damage in the region among these four three lineaments are transverse to the major tectonic zones viz the mft and mbt in the region and one extends along the mft lying south of dehradun the zonation map for great earthquakes m 8 has been prepared by modelling a rupture in the central gap region of the himalaya seven different possibilities of rupture nucleation within the rupture plane are modelled and a zonation map containing three different zones is prepared the prepared zonation map for this worst possible scenario shows that cities like tehri uttarkashi srinagar deoprayag pauri and dehradun fall within zone 1 and can experience ground accelerations bigger than 600 cm s2 the region like hardwar and rishikesh fall within zone 2 and can experience ground accelerations around 300 cm s2 the zonation map shows that tehri can experience accelerations up to 0 84g and this value is comparable with that obtained by other researchers this study confirms that for very long periods the destruction in the region will be dominantly affected by ruptures having longer lengths and least effected by shorter ruptures therefore special consideration about the longer ruptures should be taken while considering effect of earthquake hazards in this region

tracking basement cross strike discontinuities in the indian crust beneath the himalayan orogen using gravity data relationship to upper crustal faults the himalaya is the result of the on going convergence and collision of india and asia two plates with complex and protracted geological evolutions the internal configuration and processes that govern the rise of the himalaya and tibetan plateau are crucial to understand continental collision zones however knowledge of the prior configuration of the colliding plates is equally important since inherited preorogenic basement structures can undeniably influence the development of the orogenic architecture throughout the orogen s cycle of collision and eventual collapse three northeast trending palaeotopographic ridges of faulted precambrian indian basement underlie the ganga basin south of the himalaya oil and natural gas commission 1968 valdiya 1976 gahalaut kundu 2012 our study godin harris 2014 illustrates a crustal scale fault origin for these ridges and succeeds in determining how far north beneath the himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern tibet spectrally filtered egm2008 bouguer gravity data and edges in its horizontal gradient at different source depths gravity worms over northern peninsular india the himalaya and southern tibet reveal several continuous himalayan cross strike discontinuities interpreted to represent crustal faults fig 1 gravity lineaments in peninsular india coincide with edges of the precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults the interpreted basement faults project northward beneath the himalayan system and southern tibet our results suggest that several active himalayan cross strike faults such as the ones related to many graben in southern tibet are rooted in the underplated indian lower crust or step en echelon along interpreted basement faults our interpretation thus suggests that south tibet graben are spatially related to deepseated crustal scale faults rooted in the underplated indian crust these major discontinuities partition the himalayan range into distinct zones and could ultimately contribute to lateral variability in tectonic evolution along the orogen s strike

uniformly processed strong motion database for himalaya and northeast region of india this paper presents the first uniformly processed comprehensive database on strong motion acceleration records for the extensive regions of western himalaya northeast india and the alluvial plains juxtaposing the himalaya this includes 146 three components of old analog records corrected for the instrument response and baseline distortions and 471 three components of recent digital records corrected for baseline errors the paper first provides a background of the evolution of strong motion data in india and the seismotectonics of the areas of recording then describes the details of the recording stations and the contributing earthquakes which is finally followed by the methodology used to obtain baseline corrected data in a uniform and consistent manner two different schemes in common use for baseline correction are based on the application of the ormsby filter without zero pads trifunac 1971 and that on the butterworth filter with zero pads at the start as well as at the end converse and brady 1992 to integrate the advantages of both the schemes ormsby filter with zero pads at the start only is used in the present study a large number of typical example results are presented to illustrate that the methodology adopted is able to provide realistic velocity and displacement records with much smaller number of zero pads the present strong motion database of corrected acceleration records will be useful for analyzing the ground motion characteristics of engineering importance developing prediction equations for various strong motion parameters and calibrating the seismological source model approach for ground motion simulation for seismically active and risk prone areas of india

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

active faults stress field and plate motion along the indo eurasian plate boundary abstract the active faults of the himalayas and neighboring areas are direct indicators of recent and sub recent crustal movements due to continental collision between the indian and eurasian plates the direction of the maximum horizontal shortening or horizontal compressive stress axes deduced from the strike and type of active faulting reveals a characteristic regional stress field along the colliding boundary the trajectories of the stress axes along the transcurrent faults and the eastern himalayan front are approximately n s parallel to the relative motion of the two plates however along the southern margin of the eurasian plate they are ne sw in the western himalayan front and nw se to e w in the kirthar sulaiman front which is not consistent with the direction of relative plate motion a simple model is proposed in order to explain the regional stress pattern in this model the tectonic sliver between the transcurrent faults and the plate margin is dragged northward by the oblique convergence of the indian plate thus the direction of relative motion between the tectonic sliver and the indian plate changes regionally causing local compressive stress fields judging from the long term slip rates along the active faults the relative motion between the indian and eurasian plates absorbed in the colliding zone is about one fourth of its total amount the rest may be consumed along the extensive strike slip faults in tibet and china

fluvial dynamics of an anabranching river system in himalayan foreland basin baghmati river north bihar plains india abstract anabranching river systems are now regarded as a separate class in river classifications owing to their distinctive morphological hydrological characteristics and fluvial processes a better understanding of anabranching rivers still needs detailed data from different environmental and geographical settings this paper presents a detailed account of an anabranching river system from the himalayan foreland basin the baghmati river system from north bihar plains eastern india provides a typical example of an anabranching river system located in the interfan area between the kosi and the gandak megafans the river system is braided in upstream reaches and meandering in downstream reaches but the midstream anabranching reach is characterized by low width depth ratio 11 16 gentle gradient 0 00018 0 00015 variable peak discharge frequent flooding and high sediment load the anabranching in the midstream reaches is a response to its inability to transport high sediment load due to gentle channel slope and dominance of aggradation process the development of anabranches is related to rapid and frequent avulsions of the river channels with eight major avulsions observed in the 30 km wide floodplain in the last 230 years the decadal scale avulsion history of the baghmati river system makes it hyperavulsive and the major causative factors for such channel instability are sedimentological readjustments and active tectonics in the basin area

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

earthquake processes of the himalayan collision zone in eastern nepal and the southern tibetan plateau summary focal mechanisms determined from moment tensor inversion and first motion polarities of the himalayan nepal tibet seismic experiment himnt coupled with previously published solutions show the himalayan continental collision zone near eastern nepal is deforming by a variety of styles of deformation these styles include strike slip thrust and normal faulting in the upper and lower crust but mostly strike slip faulting near or below the crust mantle boundary moho one normal faulting earthquake from this experiment accommodates east west extension beneath the main himalayan thrust of the lesser himalaya while three upper crustal normal events on the southern tibetan plateau are consistent with east west extension of the tibetan crust strike slip earthquakes near the himalayan moho at depths 60 km also absorb this continental collision shallow plunging p axes and shallow plunging ew trending t axes proxies for the predominant strain orientations show active shearing at focal depths 60 90 km beneath the high himalaya and southern tibetan plateau beneath the southern tibetan plateau the plunge of the p axes shift from vertical in the upper crust to mostly horizontal near the crust mantle boundary indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate

swarm observations of equatorial electron densities and topside gps track losses the swarm satellites have both upward looking gps receivers and langmuir probes the receivers repeatedly lost track of the l1 band signal in january february 2014 at postsunset hours when swarm was at nearly 500 km altitude this indicates that the signal was disturbed by ionospheric irregularities at this height and above the track losses occurred right at density gradients associated with equatorial plasma bubbles and predominantly where the measured background density was highest the signal showed strong phase scintillations rather than in amplitude indicating that swarm might be in the near field of an ionospheric phase screen density biteouts depletions between steep gradients were up to almost 3 orders of magnitude deep in the background of a more shallow trough centered at the magnetic equator comparison between satellites shows that the biteout structure strongly varied in longitude over 100 km and has in north south steep walls

soft sediment deformation associated with the east patna fault south of the ganga river northern india influence of the himalayan tectonics on the southern ganga plain abstract the geomorphic tectonic and seismic aspects of the ganga plain have been studied by several workers in the recent decades however the northern part of this tectonically active plain has been the prime focus in most of the studies the region to the south of the ganga river requires necessary attention especially regarding the seismic activities the region lying immediately south of the outer himalayas i e the ganga plain responds to the stress regime of the himalayan frontal thrust zone by movement along the existing basement faults extending from the indian peninsula and creating new surface faults within the sediment cover as well as a result several earthquakes have been recorded along these basement faults such as the great earthquakes of 1934 and 1988 associated with the east patna fault large zones of ground failure and liquefaction in north bihar close to the himalayan front have been recorded associated with these earthquakes the present study reports the soft sediment deformation structures from the south bihar associated with the prehistoric earthquakes near the east patna fault for the first time the seismites have been observed in the riverine sand bed of the dardha river close to the east patna fault several types of liquefaction induced deformation structures such as pillar and pocket structure thixotropic wedge liquefaction cusps and other water escape structures have been identified the location of the observed seismites within the deformed zone of the east patna fault clearly indicates their formation due to activities along this fault however the distance of the liquefaction site from the recorded epicenters suggests its dissociation with the recorded earthquakes so far and hence possibly relates to any prehistoric seismic event the occurrence of the earthquakes of a magnitude capable of forming liquefaction structure in the southern ganga plain indicates the transfer of stress regime far from the himalayan front into the peninsular region through these basement faults northward extension of the east patna fault coincides with the region of the himalayan front which corresponds to a less slip potential therefore an association of frequent earthquakes in this region indicates strain release along the east patna fault

late pleistocene holocene development of the tista megafan west bengal india 10 be cosmogenic and irsl age constraints abstract the himalayan proximal foreland is characterized by quaternary megafans of which the formational mechanisms remain debated the tista megafan spreads over more than 16 000 km2 from the mountain front where it is strongly incised to the confluence of the tista river with the jamuna brahmaputra river and stores sediments produced in the sikkim himalaya we propose a scenario for the late pleistocene holocene development of the tista megafan based on new 10be cosmogenic and infra red stimulated luminescence irsl age constraints and discuss the main potential controls on its evolution we suggest that two distal lobes developed successively downstream from a common proximal lobe deposition in the proximal lobe took place since at least 135 ka and incision began at 3 7 0 7 1 0 ka the western distal lobe of the megafan was deposited early in the history of the megafan when the sikkim himalaya catchment was drained by a tributary of the ganga river and was abandoned in the early holocene 10 11 ka the eastern recent

aftershocks of the march 1999 chamoli earthquake and seismotectonic structure of the garhwal himalaya the aftershocks of the chamoli earthquake m b 6 3 of 28 march 1999 are analyzed to understand the seismotectonic structure of the garhwal himalaya the permanent as well as temporary microearthquake network data are used the mainshock occurred on the basement thrust by thrust faulting the aftershocks on the other hand are triggered at the seismogenic faults to the south of the main central thrust mct by thrust faulting as well as by strike slip faulting and all the events occurred above the plane of detachment basement thrust the mct is not the seismogenic fault

active transverse features in the central portion of the himalaya abstract many transverse lineaments or faults across the himalaya are already known interpretation of satellite images of the central portion of the himalaya given here adds to that number and suggests their continuity from the himalaya to its foredeep several such faults appear to be seismically active their shear motion recognized from geologic data and focal mechanism solutions compares well with a classical stress model when applied to northward progress of india under the himalayan collision front

seismic structure of the underthrusting indian crust in sikkim himalaya 1 this study presents the first results of the seismic character of the underthrusting indian crust in the sikkim himalaya deduced through an analysis of 3600 receiver functions rfs abstracted from waveforms registered at 11 broadband stations spanning a 110 km long n s profile from the foothills to the higher himalaya common conversion point stacks of receiver functions prominently trace the northward dipping geometry of the indian moho beneath the himalaya monte carlo inversion of the azimuthal variations of the rfs at individual stations adopting the nearest neighborhood algorithm approach reveals that the crustal thickness varies from 40 km to 61 km from south to north with a dip varying between 4 and 10 among stations a moho doublet prominently seen at a depth of 40 km in the higher himalaya to the north of main boundary thrust has been interpreted in terms of possible partial eclogitization of a granulitic indian lower crust akin to the finding just north of the study region beneath southern tibet a strong layer of anisotropy 17 localized within a low velocity layer between 20 and 30 km has a nw se oriented fast polarization direction counterintuitive to the convergence parallel and range perpendicular alignment expected in a convergent setting due to shear processes midcrustal transcurrent deformation in sikkim and bhutan evidenced by a conjugate system of strike slip faulting with nw to ne trending p axis orientations is the most feasible mechanism for causing a near strike parallel oriented fast axis of anisotropy in this segment of himalaya

orogen transverse structures in the eastern himalaya dextral riedel shear along the main boundary thrust in the garu gensi area arunachal pradesh india implication in hydrocarbon geoscience abstract structural mapping and analyses reveal deformation in the rocks of the lesser himalayan miri and gondwana and lower siwalik dafla successions across the miri thrust mt and the main boundary thrust mbt in the garu gensi area arunachal himalaya india the north dipping gondwana sequence is sandwiched between the miri group of rocks in the north and the lower siwalik dafla sandstones in the south the gondwana rocks folded four times d1 d4 detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the miri the gondwana and the lower siwalik rock sequences in the area are traversed by two sets of nw and ne trending cross fault zones with 70 100 m thickness mappable for 70 km the cross faults trend nw around the locality daring while in the western part around gensi they trend ne the westernmost fault slipped dextrally and the easternmost one at daring sinistrally these transverse faults offset the mbt 0 52 0 59 km for the western and 0 58 1 41 km for the eastern faults the deformation in the gondwana sequence may be constrained as syn himalayan i e india asia collisional event this was followed by a compression from e ese due to the post eocene indo burmese collision the culminating phase of combined india asia and indo burmese compression is manifested in field with a strong ne directed compression with dextral reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards se cluster of the earthquake data indicates record of the events m 6 within 30 80 km depth both n and s of the mbt in the study area further the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the brahmaputra river

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

active tectonics and geomorphology in the sone ganga alluvial tract in mid ganga basin india active tectonics plays a major role in landscape evolution by bringing changes in topography and channel slopes the alluvial plains in ganga basin are divided into a number of tectonic blocks by several active subsurface faults the associated disturbances along these faults have given rise to a distinctive fluvial regime and geomorphology in the region in sone ganga alluvial plain in parts of south ganga plains in middle ganga basin the sone and ganga rivers have responded to the ongoing tectonic deformation in the area the study reports a classical example of tilt induced avulsion and channel migration during holocene from the parts of south ganga plains within the extensional half graben between east patna fault epf and west patna fault wpf the rate of lateral tilt appears to control the style of channel movement with gradual migration occurring at low tilt rates and avulsion at higher rates the river sone lying laterally close to the line of uplift has undergone avulsions at different times of tilting whereas the ganga river located at the lower parts of the tilting block away from the line of uplift has migrated in the down tilt direction various fluvial anomalies such as gradient reversals in the longitudinal profile of channels sinuosity variations channel incision frequency in the braid bar distribution and variation in the cross sectional parameters of the channels most probably created by tectonics have been studied satellite images and maps coupled with relative bank height measurements and other geomorphologic studies in field have helped in identifying such fluvial signatures and their relevance to tectonics the subsurface faults in the sone ganga plains cut across and also run parallel the river channels showing the effects of transverse and lateral tilting manifested in channel avulsions and migrations

study of permafrost distribution in sikkim himalayas using sentinel 2 satellite images and logistic regression modelling abstract in this study sentinel 2a s 2a multi spectral instrument msi level 1c l1c products are used for identification and mapping of rock glaciers within a transect under observation in sikkim himalayas the mapped rock glaciers were classified on the basis of their activity intact or relict and their origin glacier derived or talus derived logistic regression models were applied to calculate the probability of permafrost distribution within transect under observation we observed that freely available high resolution s 2a msi l1c products are a valuable addition to the already existing freely available high resolution satellite images that can be exploited to study rock glaciers in himalayan region the distribution of rock glaciers in sikkim himalayas is found to be strongly controlled by elevation and aspect topoclimatic conditions above 5000 m a s l are favourable for the presence of permafrost in sikkim himalayas more glacier derived rock glaciers than talus derived rock glaciers were observed within transect under observation outputs from logistic regression models were capable to reasonably estimate the spatially distributed probability of permafrost occurrence in the region however outputs from these models depended upon the selection of the initiation line location points of rock glaciers and topoclimatic and topographic variables used as predictors for model computation based on probability distribution maps more than 60 800 km2 area of transect under observation could contain permafrost this high areal abundance of permafrost in sikkim himalayas suggests increased risks of thawing permafrost under the influence of climate warming in future

cenozoic exhumation history of the alborz mountains iran new constraints from low temperature chronometry the alborz mountains in the north of the turkish iranian plateau mainly developed in the cenozoic as a consequence of the closure of neo tethys and continental collision between arabia and eurasia cenozoic growth of the fold and thrust belt exploited an older palaezoic mesozoic crustal fabric but the extent to which this governed the overall form of the mountain belt is unclear to determine when and how the alborz mountain belt has grown apatite fission track aft and u th he ahe thermochronometry were performed on 46 bedrock samples collected along 8 transects across the range aft central ages range from 157 24 ma to 10 1 ma with most ages falling between 40 ma and 10 ma ahe ages range from 17 2 ma to 6 1 ma the data revealed enhanced exhumation ca 35 5 ma minor exhumation between ca 30 20 ma and an increase in exhumation thereafter this pattern matches the tripartite mid cenozoic stratigraphy of central iran where upper oligocene lower miocene carbonates are sandwiched between terrestrial clastic formations the most intriguing thermochronometric signal found is a major acceleration of exhumation that initiated in the late miocene to early pliocene recorded by the ahe data there does not appear to be a direct tectonic cause for pliocene intensified erosion as convergence rates between arabia and eurasia have remained relatively constant enhanced exhumation at this time may reflect a climatically induced intensification of erosion during caspian isolation and base level fall or a regional tectonic re organization of the arabia eurasia collision or both

presunrise spread f at fortaleza at fortaleza brazil in the equatorial zone about 400 km south of the magnetic equator a presunrise secondary maximum of spread f occurrence is observed during sunspot minimum and in particular during december solstice the spread f takes the form of patches of irregularities that are convecting eastwards at 50 m s 1 most of the patches are collocated with bottomside bulges of the ionosphere our measurements indicate that these bottomside bulges are unstable due to a gradient drift instability that is slowly growing and produces the spread f the bulges themselves seem to be evidence of a rayleigh taylor instability process

focal depths and fault plane solutions of earthquakes under the tibetan plateau we compare synthetic and recorded p wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts 4000 m of the tibetan plateau fault plane solutions for all 16 events show combinations of normal and strike slip faulting with t axes oriented approximately east west none of these solutions show thrust faulting thus the data corroborate previous inferences that the active tectonics are dominated by east west extension focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km this style of deformation and these depths of faulting are similar to those in the basin and range province of the western united states two intermediate depth events below the crust of southern tibet also show primarily normal faulting with east west t axes the solution for one discussed by chen et al 1981 is unambiguous the solution for the other the event of august 1 1973 27 59 n 89 17 e 85 10 km mb 4 9 is less certain both apparently occurred in the mantle beneath a thick aseismic lower crust and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath tibet

modeling earthquake recurrence in the himalayan seismic belt using time dependent stochastic models implications for future seismic hazards interest in and pertinent studies related to the seismic hazard in the himalayan region have increased significantly since the instrumental era and each damaging earthquake encourages its reestimation in the work presented herein the non poissonian probabilities of exceedance of magnitudes in a specified time in the future were investigated for different elapsed times based on the recurrence time intervals of past earthquakes in the himalaya using the weibull log normal gamma and inverse gaussian stochastic models the whole himalayan arc is divided into four seismogenic source zones viz the northwestern himalayas zone 1 the central seismic gap region zone 2 the eastern nepal and sikkim zone 3 and the eastern himalayas zone 4 by considering approximately 380 years of seismological data the suitability of each stochastic model for each zone was estimated using the kolmogorov smirnov k s test to describe the different physical processes responsible for earthquake occurrence the results show that the gamma inverse gaussian log normal and inverse gaussian models were most suitable for zones 1 4 respectively for m 6 0 the cumulative probability of recurrence intervals reaches up to 90 in 30 years for zones 1 and 2 49 years for zone 3 and 41 years for zone 4 the estimated conditional probability reaches 90 in 30 years in zone 1 35 years in zone 2 and 50 years in zones 3 and 4 the most suitable models for m 7 0 were found to be the log normal for zones 1 2 and 3 and gamma for zone 4 the cumulative probability for m 7 0 reaches 90 in 85 years in zone 1 86 years in zone 2 93 years in zone 3 and 148 years in zone 4 the estimated conditional probabilities reach 90 in 80 years in zone 1 90 years in both zone 2 and 3 and 150 years in zone 4 it is strongly recommended that model suitability be evaluated in complex regions such as the himalaya before proceeding with seismic hazard assessments the heterogeneous and complex tectonics of the himalayas differentiated plate motions different stress release patterns spatially and temporally and locking unlocking of faults thrusts are some of the reasons responsible for the different probabilistic models describing the earthquake occurrence phenomena in these four zones

late cenozoic fluvial successions in northern and western india an overview and synthesis abstract late cenozoic fluvial successions are widespread in india they include the deposits of the siwalik basin which represent the accumulations of the ancient river systems of the himalayan foreland basin palaeomagnetic studies reveal that fluvial architecture and styles of deposition were controlled by himalayan tectonics as well as by major climatic fluctuations during the long 13 ma span of formation the indo gangetic plains form the world s most extensive quaternary alluvial plains and display spatially variable controls on sedimentation himalayan tectonics in the frontal parts climate in the middle reaches and eustasy in the lower reaches close to the ganga brahmaputra delta climatic effects were mediated by strong fluctuations in the sw indian monsoon and himalayan rivers occupy deep valleys in the western ganga plains where stream power is high cut in part during early holocene monsoon intensification the broad interfluves record the simultaneous aggradation of plains fed rivers since 100 ka the eastward increase in precipitation across the ganga plains results in rivers with low stream power and a very high sediment flux resulting in an aggradational mode and little incision the river deposits of semi arid to arid western india form important archives of quaternary climate change through their intercalation with the eolian deposits of the thar desert although the synthesis documents strong variability both spatial and temporal in fluvial stratigraphy climatic events such as the decline in precipitation during the last glacial maximum and monsoon intensification in the early holocene have influenced fluvial dynamics throughout the region

seismic landscape from sarpang re entrant bhutan himalaya foredeep assam india constraints from geomorphology and geology abstract geomorphic landscape and late quaternary geological attributes from the raidak manas interfluve in the bhutan himalayan foothills kokrajhar district assam led towards documenting the east west trending south dipping 30 km long active frontal back thrust fbt well within the foredeep south of the main frontal thrust mft spectacular north facing 6 50 m high tectonic scarp generated by the north propagating emerging thrust front along with a complementary subdued south facing scarp defines the terrain as a pop up structure the entire belt is made up of 5 to 8 km wide six distinct blocks separated by antecedent rivers streams scarp parallel east west drainage along with linear lakes characterises the emerging thrust front field evidence for a major fault propagation fold structure along with thrust faulting within the late quaternary fluvial sediments is ubiquitous clay beds deposited in lakes along the footwall of fbt have formed due to blockade of south flowing rivers by episodic upliftment of the hanging wall block three such episodes of uplift since 16 k years correspond to three morphogenic earthquakes of magnitude 6 9 rupturing the fbt during late pleistocene holocene in light of geomorphological and geological studies neotectonic activity has been modelled as an active south dipping backthrust that originates at shallow crustal depth from south vergent basal himalayan decollement in response to the advancing himalayan wedge

microearthquakes at the main boundary thrust in eastern himalaya and the present day tectonic model abstract two microearthquake surveys were carried out at the main boundary thrust mbt eastern himalaya temporal as well as spatial variation in microseismicity was observed the vertical section of the hypocentres shows subcrustal earthquakes down to a depth of 80 km to the south of the mbt and composite focal mechanisms show strike slip solutions or transverse tectonics which do not agree with the conceptual tectonic model

seismotectonics of bhutan evidence for segmentation of the eastern himalayas and link to foreland deformation abstract the instrumental record of bhutan is characterized by a lower seismicity compared to other parts of the himalayan arc to understand this low activity and its impact on the seismic hazard a seismic network was installed in bhutan for 22 months between 2013 and 2014 recorded seismicity earthquake moment tensors and local earthquake tomography reveal along strike variations in structure and crustal deformation regime a thickened crust imaged in western bhutan suggests lateral differences in stresses on the main himalayan thrust mht potentially affecting the interseismic coupling and deformation regime sikkim western bhutan and its foreland are characterized by strike slip faulting in the indian basement strain is particularly localized along a nw se striking mid crustal fault zone reaching from chungthang in northeast sikkim to dhubri at the northwestern edge of the shillong plateau in the foreland the dextral dhubri chungthang fault zone dcf causes segmentation of the indian basement and the mht between eastern nepal and western bhutan and connects the deformation front of the himalaya with the shillong plateau by forming the western boundary of the shillong block the kopili fault the proposed eastern boundary of this block appears to be a diffuse zone of mid crustal seismicity in the foreland in eastern bhutan we image a seismogenic flat portion of the mht which might be either related to a partially creeping segment or to increased background seismicity originating from the 2009 m w 6 1 earthquake in western central bhutan clusters of micro earthquakes at the front of the high himalayas indicate the presence of a mid crustal ramp and stress buildup on a fully coupled mht the area bounded by the dcf in the west and the seismogenic mht in the east has the potential for m7 8 earthquakes in bhutan similarly the dcf has the potential to host m7 earthquakes as documented by the 2011 sikkim and the 1930 dhubri earthquakes which were potentially associated with this structure

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

more major earthquakes at the nepal himalaya study on coulomb stress perspective abstract on april 2015 a major earthquake of 7 9 mw occurred in the nepal himalaya followed by 553 earthquakes of local magnitude greater than 4 0 within the first 43 days including another major event of 7 3 mw we resolve the static coulomb failure stress cfs change onto the finite fault models of 7 9 mw after elliott et al 2016 and galezka et al 2015 and its effect on associated receiver faults correlation of aftershocks with the enhanced cfs condition shows that the elliott et al 2016 model explains 60 4 and the galezka et al 2015 model explains about 47 7 of the aftershocks in high stress regions aftershocks were poorly spatially correlated with the enhanced cfs condition after the 7 9 mw main shock and can be explained by correlation with release of seismic energy from the associated secondarily stressed prominent thrust planes and transverse faults stress resolved on the associated receiver faults show increased stress on both transverse and thrust fault systems with the potential of triggering significant aftershocks or subsequent main shocks

causative source of mw 6 9 sikkim nepal border earthquake of september 2011 gps baseline observations and strain analysis abstract the recent earthquake of mw 6 9 which occurred on september 18 2011 in sikkim nepal border region epicenter 27 72 n 88 06 e depth 20 7 km 68 km nw of the capital city gangtok is the strongest earthquake in the instrumentally recorded history of the region the fault plane solution of this earthquake indicates a strike slip motion however the seismological and geological studies carried out so far after the earthquake could not confirm the causative fault plane in the present study gps observations are used to ascertain causative source in the generation of earthquake and its correlation with the observed seismic data of the region the co seismic displacements recorded by gps show maximum displacement of 11 mm at phodong and 9 mm at taplejung station near the epicenter a simple rigid cross fault model using gps baseline observations was employed to figure out the causative fault plane and seismological characteristic of the region it is inferred that the movement represents the kinematic adjustment of the subsidiary faults as a result of the displacement along the nw se principal plane strain analysis using gps baseline inferred that the region southeast of epicenter has undergone large deformation in addition a significant part of the measured deformation across the surface fault zone for this earthquake can be attributed to post seismic creep

one dimensional modeling of a recent ganga avulsion assessing the potential effect of tectonic subsidence on a large river abstract river avulsion as studied in small sized and medium sized rivers is partly explained by the water surface gradient advantage of a new channel course over the old course caused by spatial differences in aggradation and compaction recently the effect of meandering upstream of the avulsion node or bifurcation was shown to have an equally large effect on avulsion duration as gradient advantage these effects remain poorly understood for the largest rivers on earth where gradients are very small subtle gradient advantages are affected by tectonics and often several anabranches remain active simultaneously our objective was to assess the relative importance of these factors in the river ganga in determining the pacing of an avulsion we used a combination of historical data remote sensing and one dimensional modeling the course of the ganga in historical times was through the present ganga bhagirathi system but then there was either a gradual or sudden shift to the present ganga padma system historical evidence and remnant paleochannels as observed in satellite sensor data corroborate the changing pattern of the ganga river system but the exact causes of the shifting and of the short avulsion duration remain unclear based on generalized data using a one dimensional model we ran idealized scenarios bracketing different tectonic subsidence estimates for long term morphodynamic evolution of the upstream channel and the two downstream bifurcates the model predicts flow and sediment partitioning at the bifurcation node and includes the effect of migrating meanders at the bifurcation and width adjustment of the bifurcates our modeling demonstrates that the old and the new branches can remain open and morphologically active for a long time because of the large backwater effect and the high mobility of the sediment the bifurcation stabilizes at an asymmetrical flow and sediment division which in smaller rivers such as the river rhine would be followed by residual channel filling but in the much larger ganges results in morphologically active anabranches the model results reveal that neither a gradient advantage nor a bend upstream of the bifurcation leads to an avulsion within centuries as has been observed in some large rivers in tectonically inactive regions on the other hand a realistic tectonic uplift of the old branch or subsidence of the new branch may force an avulsion to take place quickly and historical data show that the study area is seismically active the combination of these factors leads to a realistic modeled avulsion duration of less than three centuries historical data indicate that these general conclusions might also apply to other large rivers in this region e g the brahmaputra and the teesta we conclude that large rivers may avulse quickly in response to tectonics but attain an anabranching pattern because of the large dimension of the residual channel and backwater effects

electrical anisotropy in the main central thrust zone of the sikkim himalaya inference from anomalous mt phase abstract the sikkim himalaya in the eastern sector of the himalayan collision belt shows some distinct features such as the occurrence of the main central thrust as a several km wide zone mctz of intense ductile deformation and its deviation from the general east west strike of the himalayan thrusts suggesting that the tectonics of this region could be different from the regional himalayan tectonics we have carried out a mt survey across mctz to understand the tectonics of the region the broadband mt data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90 for frequencies smaller than 1 hz whereas other sites further north of this zone show normal behavior such anomalous phase behavior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors we have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high grade crystalline gneisses at the northern boundary of mctz among various models analyzed a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data we thus infer that the presence of a localized strongly anisotropic block at the northern boundary of mctz could be a viable explanation for the observed anomalous phase behavior

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere

the condor equatorial spread f campaign overview and results of the large scale measurements during the condor campaign a number of instruments were set up in peru to support the rocket experiments in this series of papers we report on the results of the experiments designed to study the equatorial f region in this overview paper we summarize the main results as well as report upon the macroscopic developments of spread f as evidenced by data from backscatter radas from scintillation observations and from digital ionosonde meaurements in this latter regard we argue here that at least two factors other than the classical gravitational rayleigh taylor plasma instability process must operate to yield the longest scale horizontal organization of spread f structures the horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow although detailed comparison with theory seems to require shear frequencies a bit higher than observa tions indicate on the other hand the largest scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions using simultaneous rocket and radar data we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over jicamarca are primarily spatial structures advecting overhead the detailed rocket radar compari son verified several other earlier results and speculations particularly those made in the plumex experiments in particular companion papers discuss and extend some of the plumex results to include the role of anomalous diffusion labelle et al this issue in the theory of equatorial spread f and to shed light upon the shallow spectral form often observed in the intermediate scale regime labelle and kelley this issue

global distribution of density irregularities in the equatorial ionosphere we analyzed measurements of ion number density made by the retarding potential analyzer aboard the atmosphere explorer e ae e satellite which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km large scale 60 km density measurements in the high altitude regions show large depletions of bubble like structures which are confined to narrow local time longitude and magnetic latitude ranges while those in the low altitude regions show relatively small depletions which are broadly distributed in space for this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation n n over a path length of 18 km as an indicator of overall irregularity intensity seasonal variations of irregularity occurrence probability are significant in the pacific regions while the occurrence probability is always high in the atlantic african regions and is always low in the indian regions we find that the high occurrence probability in the pacific regions is associated with isolated bubble structures while that near 0 longitude is produced by large depletions with bubble structures which are superimposed on a large scale wave like background considerations of longitude variations due to seeding mechanisms and due to f region winds and drifts are necessary to adequately explain the observations at low and high altitudes seeding effects are most obvious near 0 longitude while the most easily observed effect of the f region is the suppression of irregularity growth by interhemispheric neutral winds

longitudinal variability of annual changes in activity of equatorial spread f and plasma bubbles global distribution maps of equatorial spread f esf activity for various annual periods were derived from the topside soundings by the ionosphere sounding satellite b iss b in 1978 1980 the esf activity during the northern winter period reveals maximum enhancement at the atlantic longitudes of large westward geomagnetic declination and during the northern summer at the pacific longitudes of large eastward declination on several orbits passing over the region of the esf activity enhancement abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km it has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced esf activity and asymmetric in the region of the suppressed esf activity these observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the rayleigh taylor type plasma instability

earthquakes in the garhwal himalaya of the central seismic gap a study of historical and present seismicity and their implications to the seismotectonics the historical and the present seismicity catalogues of the garhwal himalaya have been studied for their spatio temporal variations and their implications on the seismotectonics of the region the micro seismicity fractal dimensions dc and frequency magnitude distribution b value coupled with the available literature on geology geomorphology and geophysics have been used to derive the seismotectonics and stress level changes in the region the seismic cross sections for the relocated micro seismicity focal mechanisms and the swath profiles for the presence of physiographic transition 2 pt2 at the foothill of the higher himalaya indicate the constant presence of the mid crustal ramp mcr in the detachment plane and its active seismogenic nature the comparison of this scenario suggests the constant presence of seismogenically active mcr structure throughout the central seismic gap the seismic cross sections further reveal that the sensu stricto main central thrust munsiari thrust is also a site of generation of the micro seismicity in few segments due to its reactivation by thrusting along the mcr the high fractal dimension value dc 1 47 suggests the heterogeneous nature of the region owing to the presence of local faults and transverse tectonics the high stress accumulation in the garhwal himalaya with low b value b 0 70 suggests the high probability of occurrence of a larger or greater earthquake in the near future further the study also reveals that the 2011 chamoli earthquake of m 5 0 preceded by a quiescence period of nearly a year shows different stress levels before and after its occurrence which is well constrained with the increased moderate earthquake activity around the chamoli region this increased seismic activity and stress conditions in the chamoli region suggest the high possibility of the occurrence of major earthquakes hence the study recommend for a detailed seismic hazard evaluation of the region

effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread f we use radar observations from the jicamarca observatory from 1968 to 1992 to study the effects of the f region vertical plasma drift velocity on the generation and evolution of equatorial spread f the dependence of these irregularities on season solar cycle and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts in the early night sector the bottomside of the f layer is almost always unstable the evolution of the unstable layer is controlled by the history of the vertical drift velocity when the drift velocities are large enough the necessary seeding mechanisms for the generation of strong spread f always appear to be present the threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux the geomagnetic control on the generation of spread f is season solar cycle and longitude dependent these effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields the occurrence of early night spread f decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset the generation of late night spread f requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour these irregularities occur most often at 0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes the occurrence of late night spread f is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

on monthly seasonal longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities 1 monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by rocsat 1 at the 600 km topside ionosphere from march 1999 to june 2004 during high to moderate solar activity years this global distribution of monthly irregularity occurrence rate not only provides the best spatial temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern pacific region where no ground observation is available the 5 1 2 year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern pacific longitudes excellent agreement is noted for the current results with aarons 1993 conjectured sketch of the global scintillation distribution furthermore the seasonal longitudinal s l variations of quiet time postsunset vertical drift velocities are found to track closely with the s l variations of irregularity occurrences linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons this implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination the smooth variation of monthly seasonal longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator there seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences

mapping of coda wave attenuation and its frequency dependency over eastern indian shield the study area comprises two tectonic blocks namely the chotanagpur microplate and the singhbhum microplate waveform data of 112 local earthquakes of magnitude mw 2 5 were considered for the present analysis coda q qc for each event at eight different central frequencies 1 14 hz and four different coda windows was estimated using back projection algorithm the average q0 qc at 1 hz and exponent values were regionalized by projecting each value in selected square grid of dimension 0 1 0 1 back projection algorithm was used for inversion of qc and the final values were computed after ten iterations when the residual error was reduced to acceptable level inversion was stabilized by a nine point spatial smoothening gaussian filter to compute the qc values for each frequency and subsequently combined values of q0 and were also computed the final power law equation of the coda wave attenuation for the concerned region was found to be qc 336 38 16 50 f 0 83 0 036 q0 varies from 180 to 255 270 to 360 297 to 386 and 340 to 420 and vary from 0 9 to 1 02 0 79 to 0 89 0 79 to 0 88 and 0 79 to 0 86 for 30 35 40 and 45 s coda windows increase in q0 and decrease in values with increase of coda window length shows the depth dependency of seismic wave attenuation at 1 hz frequency the central part surrounding the receiver lying at the junction of the chotanagpur and the singhbhum tectonic provinces documents lower q0 and higher values and presumably caused by more heterogeneities present in the area the higher concentration of lower magnitude earthquakes in this area support this observation the mapping values of attenuation can be utilized for hazard assessment of the area

crustal geoelectric structure of the sikkim himalaya and adjoining gangetic foreland basin abstract we present the results of a broadband magnetotelluric survey along a 200 km long profile across the sikkim himalaya the data were acquired at average station spacing of 5 6 km and transfer functions of 31 sites in 0 01 1000 s period range have been used for 2 d joint inversion of te and tm modes the composite model incorporating the effect of transverse strike reveals several features that correlate with the available seismic and kinematic models of the region a major result of the present study is that the main himalayan thrust forms the base of several resistive blocks within the wedge and that a ramp structure is present south of the main central thrust zone mctz another significant result is that the crust and mantle lithosphere beneath the mctz and the higher himalayan crystallines hhc seem to be compositionally geologically different from the lithosphere south of the mctz a steep crustal scale fault with the moho offset of 14 km is inferred to be separating these two blocks the deep crustal seismicity could be related to this fault whereas shallow seismicity can be linked to the deformation within the wedge the results also reveal the presence of some more conductors we relate the conductor within the hhc to the sedimentary rocks of the tethyan sequence exposed in a window about 40 km west of the profile and north of the south tibetan detachment system stds the conductor at 90 km profile location is linked to the gondwana rocks exposed in the rangit window a 4 6 km thick sedimentary layer overlies the basement in the gangetic foreland basin we also delineate a sub crustal conductor at 50 60 km depth beneath the foreland basin at the southern end of the profile the cause of which is not apparent and needs to be explored

response of active tectonics on the alluvial baghmati river himalayan foreland basin eastern india active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope the baghmati an anabranching foothills fed river system draining the plains of north bihar in eastern india has responded to ongoing tectonic deformation in the basin the relatively flat alluvial plains are traversed by several active subsurface faults which divide the area in four tectonic blocks each tectonic block is characterized by association of fluvial anomalies viz compressed meanders knick point in longitudinal profiles channel incision anomalous sinuosity variations sudden change in river flow direction river flow against the local gradient and distribution of overbank flooding lakes and waterlogged area such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through dem and field observations to understand the nature of vertical movements in the area the sub surface faults in the baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes

quiet time equatorial f region vertical plasma drift model derived from rocsat 1 observations 1 we have used five years of measurements on board the rocsat 1 satellite to develop a detailed quiet time global empirical model for equatorial f region vertical plasma drifts this model describes the local time seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions the model results are in excellent agreement with measurements from the jicamarca radar and also from other ground based and in situ probes we show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier especially during december and june solstice the late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons the morning and afternoon december solstice drifts have significantly different longitudinal dependence and the daytime upward drifts have strong wave number four signatures during equinox and june solstice the largest evening upward drifts occur during equinox and december solstice near the american sector the longitudinal variations of the evening prereversal velocity peaks during december and june solstice are anti correlated which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere

rapid pliocene exhumation of the central greater caucasus constrained by low temperature thermochronometry 1 constraining the timing of onset and rates of deformation within the greater caucasus mountains is key to understanding their role in accommodating deformation across the arabia eurasia orogen we present new low temperature thermochronometric constraints on the cenozoic thermal evolution of the central greater caucasus that elucidate a three phase cooling history between 50 and 30 ma cooling within the range was negligible in oligocene time cooling rates throughout the range increased to 4 c myr these rates remained constant until the early pliocene time when they increased again reaching 25 c myr along the axial part of the range rates and timing of oligocene exhumation are consistent with previous results from the western greater caucasus and are proposed to result from onset of subduction of the greater caucasus back arc basin rapid exhumation of the greater caucasus beginning in pliocene time contrasts with previously reported thermal histories for other portions of the range pliocene exhumation of the central greater caucasus appears to be tectonically driven and coincides with widespread evidence for a major reorganization of the arabia eurasia plate boundary we hypothesize that this exhumation and regionally observed plate reorganization results from the collision of the lesser caucasus with eurasia completing the subduction of oceanic lithosphere across this segment of the arabia eurasia plate boundary

spatial analysis of mutual fault fracture and slope controls on rocksliding in darjeeling himalaya india in this paper the fry analysis was applied to study the spatial patterns in terms of trends of rockslides proportion analysis was applied to study the spatial associations between rockslides and slope aspects and distance distribution analysis was applied to study the spatial associations of rockslides with different sets of faults fractures based on trends in a study area in the darjeeling district india the results of applications of these spatial analytical techniques support a proposition of mutual fault fracture and slope controls on rocksliding in different parts of the study area deep seated rockslides are associated with a ne trending faults fractures and sse facing slopes b nw trending faults fractures and ene facing slopes and c nne trending faults fractures and se or ene facing slopes whereas shallow translational rockslides are associated with a nne trending faults fractures and se nw wnw or wsw facing slopes b wnw trending faults fractures and sw facing slopes c nnw trending faults fractures and ese or sw facing slopes and d nw trending faults fractures with sw facing slopes creating and integrating spatial evidence layers representing mutual fault fracture and slope controls on rocksliding can be achieved satisfactorily via application of evidential belief functions a predictive map of rockslide susceptibility derived by using slope aspects slope inclinations and proximity to the faults fractures that are spatially associated with rockslides is superior to that derives by using slope aspects slope inclinations and proximity to all faults fractures the proposed analytical methods are suitable for first pass regional scale assessment of rockslide susceptibility