crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

seismotectonics in northeast india a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications summary in northeast india three major plates interact along two convergent boundaries the himalayas and the indo burma ranges which meet at the assam syntaxis to clarify this tectonic interaction and the underlying dynamics we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5 we then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements north south compression in a direction consistent with india eurasia convergence prevails in the whole area from the eastern himalayas to the bengal basin through the shillong mikir massif and the upper assam valley e w extension in tibet is related to this n s india eurasia convergence not only does the major n s compression affect the outer segments of the indo burma ranges it also extends into the descending slab of indian lithosphere below these ranges although stresses at depth are controlled by bending of the slab beneath the burmese arc the existence of widespread n s compression in the bengal basin far away from the himalayan front is compatible with the previously proposed convergence between a shillong mikir assam valley block and the indian craton e w compression inside this block supports the hypothesis of a component of eastward extrusion stress inversion of focal mechanism solutions in the indo burma ranges reveals a complex stress pattern the burmese arc and its underlying lithosphere experience nearly arcperpendicular extension with ese wnw trends in the northernmost ne trending segment and ene wsw trends in the main n s arc segment such extensional stress documented from many arcs is likely a response to pull from and bending of the subducting plate at the same time the indo burma ranges are under compression as a result of oblique convergence between the sunda and indian plates the maximum compressive stress rotates from ne sw across the inner and northern arc to e w near the bengal basin this rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors from a ssw directed sunda burma motion to a wsw directed burma india motion as a consequence of this partitioning the major belt parallel fault zones show a variety of movements across the main n s arc segment from right lateral slip in the inner ranges to oblique reverse dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt

crustal structure variations in northeast india from converted phases 1 teleseismic receiver functions from a ten station network deployed in northeast india region sampling the shillong plateau mikir hills himalayan foredeep and the himalayan convergence zone are analyzed to obtain the crustal structure in this seismically active but less studied region the shillong plateau and mikir hills away from the convergent margins reveal remarkably simple crust with thickness 35 km and poisson s ratio 0 25 akin to the indian shield values a surprisingly thin crust for the uplifted shillong plateau may be explained invoking presence of an uncompensated crust that popped up in response to tectonic forces in contrast crustal signatures from assam valley suggest a thicker crust and higher poisson s ratio with evidences for a dipping moho predictably the crust is much thicker and complicated in the eastern himalaya further north with values in excess of 50 km

estimation of source parameters of local earthquakes originated in shillong mikir plateau and its adjoining region of northeastern india abstract we estimate the source parameters seismic moment source radius stress drop and source displacement and scaling laws for local earthquakes that occurred in the shillong mikir plateau assam valley and arunachal himalaya in northeast india during 2001 2008 the source parameters were determined using the spectral analysis of p waves from the vertical component seismograms after correction for attenuation seismic moments are observed within the range from 9 51 10 12 to stress drop ranges from 4 10 5 to 9 10 7 pa for the brune model and 7 10 5 to 1 10 8 pa for the madariaga model seismic events in this study are prominent with an average stress drop of 0 1 10 mpa the effect of site geology may be a contributing factor for such a variation in stress drop source dimensions are however found to be smaller within the major part of the plateau it is suggested that local earthquakes in the region are associated with a brittle shear failure mechanism on fault segments and or the presence of weakened zones and earthquakes are triggered by low deviatoric stress empirical relations between m w m l and m 0 m l are developed leading to the future prediction of calibration coefficients for the local earthquakes in the shillong mikir plateau and its adjoining region online material tables of source parameters

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

shear wave splitting and crustal anisotropy in the shillong mikir plateau of northeast india seismic anisotropy of crust beneath the shillong mikir plateau and the surrounding regions of northeast india have been investigated with the help of splitting analysis of s wave of local earthquakes we estimate a total 83 pairs of splitting parameters and t from 67 local shallow focus earthquakes depth 32 km recorded by the 10 broadband seismological stations operated in the study region the results show delay times ranging from 0 02 to 0 2 s which correspond to anisotropy up to 4 suggesting significant strength of anisotropy in the study region fast polarization direction in the shillong plateau shows mostly nw se trend in the western part and ne sw trend in the northern part s near kopili fault kf follows nw se trend at most of the stations in the study region is consistent with the local stress orientation suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field on the other hand anisotropy observed near the kf is due to aligned macroscopic fracture related to strike slip movement in the fault zone

warming of the polar water layer in disko bay and potential impact on jakobshavn isbrae a number of recent studies have shown enhanced retreat of tidewater glaciers over much of southern and western greenland one of the fastest retreats has occurred at jakobshavn isbrae with the rapid retreat linked to the arrival of relatively warm and saline irminger water along the west coast of greenland similar links to changes in ocean water masses on the coastal shelf of greenlandwere also seen on the east coast this study presents hydrographic data from disko bay additionally revealing that there was also a significant warming of the cold polar water entering disko bay from the mid to late 1990s onward this layer which lies at a depth of 30 200m warmedby 18 28c the heat content of the polar water layer increased by a factor of 3 6 for the post 1997 period compared to the period prior to 1990 the heat content in the west greenland irminger water layer between the same periods increased only by a factor of 2 but contained more total heat the authors suggest that the changes in the polar water layer are related to circulation changes in baffin bay

deep geoelectric structure over the lower brahmaputra valley and shillong plateau ne india using magnetotellurics summary magnetotelluric studies over the shillong plateau and lower brahmaputra sediments have delineated the dauki fault as a ne sw striking thrust zone with a dip angle of about 30 along which the low resistivity layer of bengal sediments and the underlying oceanic crust subduct to the northwest at present about 50 km length of these sequences has subducted beneath the shillong plateau and is traced up to depth of about 40 km another thrust zone sub parallel to the dauki thrust is observed in the lower brahmaputra valley corresponding to the brahmaputra fault this is interpreted to be an intracratonic thrust within the indian plate these results suggest that a large fraction of the seismicity over the shillong plateau is associated with the ne sw striking dauki thrust contrary to the earlier belief that this fault zone is relatively aseismic the present studies also suggest that the shillong plateau and the adjoining sedimentary layers act as a supracrustal block not directly participating in the subduction process however in response to the compressive tectonic forces generated by the himalayan and indo burman subduction processes the shillong plateau together with the brahmaputra sediments overlying the indian crust drift eastwards relative to the bengal sediments along the surface expression of the dauki fault leading to a dextral strike slip movement we thus propose that the ne indian crust responds to the compressive forces differently at different depths governed by the rheological considerations at deeper levels the crustal readjustments take place through the subduction along the dauki and brahmaputra thrusts where as at the shallow levels the relative deformability of the supracrustal blocks have a strong influence on the tectonics leading to the strike slip mechanism along the surface expression of the dauki fault

force perturbation analysis of recent thinning and acceleration of jakobshavn isbrae greenland observations between 1997 and 2001 of a 30 velocity increase and up to 60 m thinning of downstream parts of jakobshavn isbrae greenland immediately following calving of about 4 km of its 15 km floating ice tongue suggest that acceleration may have been initiated by the calving assuming that the force perturbation associated with such weakening is swiftly transmitted far up glacier i develop equations to estimate the perturbation initially the observed changes are consistent with the comparatively small perturbation associated with the calving thereafter it was probably sustained by thinning of the remaining ice tongue at rates of about 80 m a 1 otherwise the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned with velocities dropping to their former values the calculated force perturbation increases to a maximum about 10 km inland of the grounding line consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards further inland it progressively decreases probably because marginal drag increased as the glacier accelerated both here and on the floating tongue marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and or fabric more recent observations show continued acceleration and thinning and most of the remaining ice tongue calved away in april 2003 so thinning is likely to continue

regional variation of coda wave attenuation in northeast india an understanding of the physical state of the medium abstract in this study an attempt has been made to estimate spatial variation in attenuation characteristics in northeast india using coda q the entire study region is divided into three sub regions for this purpose estimated average frequency dependencies of coda wave attenuation for 30 s window length are qc f 135 7f0 99 0 03 qc f 109 7f1 10 0 03 and qc f 90 2f1 04 0 02 for shillong plateau mikir hills and surrounding river valley and indo burma ranges respectively it is observed that q0 is greater for the shillong plateau than the other sub regions this indicates lower attenuation due to more rigid high density material present in this area than the other sub regions the depth variations of the qc q0 and n values were also examined it is observed that the rate of increase of q0 with depth is not uniform for all the sub regions indo burma ranges has the smallest q0 and the largest n values at all depth levels among the three sub regions these results indicate that central part of indo burma ranges is the most attenuative seismically active and heterogeneous in nature however this region has smaller qc values than the other two sub regions for all window lengths up to the 6 hz this means at lower frequencies the subsurface beneath this area is more attenuative compared to the other two sub regions similar trends are observed at 8 10 and 12 hz up to 45 s window lengths for window lengths 55 s central part of indo burma ranges has higher qc values at 10 and 12 hz compared to shillong plateau qc values are lower for shillong plateau compared to the other two regions for window length 55 s at 10 and 12 hz which corresponds to depth levels 90 km such a complicated variation in qc values is a manifestation of complex nature of tectonic regime in northeast india

mapping the crustal thickness in shillong mikir hills plateau and its adjoining region of northeastern india using moho reflected waves abstract in this study we have tried to detect and collect later phases associated with moho discontinuity and used them to study the lateral variations of the crustal thickness in shillong mikir hills plateau and its adjoining region of northeastern india we use the inversion algorithm by nakajima et al nakajima j matsuzawa t hasegawa a 2002 moho depth variation in the central part of northeastern japan estimated from reflected and converted waves physics of the earth and planetary interiors 130 31 47 having epicentral distance ranging from 60 km to 150 km taking the advantage of high quality broadband data now available in northeast india we have detected 1607 moho reflected phases pmp and sms from 300 numbers of shallow earthquake events depth 25 km in shillong mikir hills plateau and its adjoining region notably for pmp phase this could be identified within 0 5 2 3 s after the first p arrival in case of sms phase the arrival times are observed within 1 0 4 2 s after the first s arrival we estimated the crustal thickness in the study area using travel time difference between the later phases pmp and sms and the first p and s arrivals the results shows that the moho is thinner beneath the shillong plateau about 35 38 km and is the deepest beneath the brahmaputra valley to the north about 39 41 km deeper by 4 5 km compared to the shillong plateau with simultaneous observation of thinnest crust 33 km in the western part of the shillong plateau in the garo hills region

deep resistivity structure of the northwest indian himalaya and its tectonic implications 1 long period magnetotelluric mt data were collected at 15 stations on a 250 km long profile in the northwest indian himalaya to study the structure of this continent continent collision zone two dimensional mt inversion was used to find a resistivity model that fit the data in the upper crust low resistivity was imaged on the limbs of the tso morari dome and may originate in serpentenization or zones of graphite the indus tsangpo suture was imaged as a sub vertical conductive structure that dips northeast and merges with a mid crustal conductor a north dipping zone of low resistivity is imaged at the top of the underthrust indian plate and is likely due to fluids expelled from the underthrust sedimentary rocks north of the indus suture zone this layer is located at 20 25 km depth with a conductance around 3000 s the resistivity of 5 10 m can be attributed to the presence of fluids likely partial melt or aqueous fluids this layer is underlain by a relatively resistive indian crust that extends from the high himalaya to north of the indus suture a decrease in deep resistivity was observed at the northern end of the profile and is similar to the structures observed further east in the region of the indepth study despite the smaller north south extent of the orogen in the indian himalaya

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

seismic b values and its correlation with seismic moment and bouguer gravity anomaly over indo burma ranges of northeast india tectonic implications abstract b value is one of the most significant seismic parameters for describing the seismicity of a given region at a definite time window in this study high resolution map of the gutenberg richter b value seismic moment release bouguer gravity anomaly and fault plane solutions containing faulting styles are analyzed in the indo burma ranges of northeast india using the unified and homogeneous part of the seismicity record in the region january 1964 december 2016 the study region is subdivided into few square grids of geographical window size 1 1 and b values are calculated in each square grid our goal is to explore the spatial correlations and anomalous patterns between the b value and parameters like seismic moment release bouguer gravity anomaly and faulting styles that can help us to better understand the seismotectonics and the state of present day crustal stress within the indo burma region most of the areas show an inverse correlation between b value and seismic moment release as well as convergence rates while estimating the b value as a function of depth a sudden increase of b value at a depth of 50 60 km was found out and the receiver function modeling confirms that this depth corresponds to the crust mantle transition beneath the study region the region is also associated with negative bouguer gravity anomalies and an inverse relation is found between gravity anomaly and b value comparing b values with different faulting styles reveal that the areas containing low b values show thrust mechanism while the areas associated with intermediate b values show strike slip mechanism those areas where the events show thrust mechanism but containing a strike slip component has the highest b value

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

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

surface freshening in the arctic ocean s eurasian basin an apparent consequence of recent change in the wind driven circulation 1 data collected by an autonomous ice based observatory that drifted into the eurasian basin between april and november 2010 indicate that the upper ocean was appreciably fresher than in 2007 and 2008 sea ice and snowmelt over the course of the 2010 drift amounted to an input of less than 0 5 m of liquid freshwater to the ocean comparable to the freshening by melting estimated for those previous years while the observed change in upper ocean salinity over the melt period implies a freshwater gain of about 0 7 m results of a wind driven ocean model corroborate the observations of freshening and suggest that unusually fresh surface waters observed in parts of the eurasian basin in 2010 may have been due to the spreading of anomalously fresh water previously residing in the beaufort gyre this flux is likely associated with a 2009 shift in the large scale atmospheric circulation to a significant reduction in strength of the anticyclonic beaufort gyre and the transpolar drift stream

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

3 d seismic structure of the northeast india region and its implications for local and regional tectonics abstract in this study we attempted to estimate 3 d p wave velocity structure of northeast india region using the first arrival data of local earthquakes that were recorded by about 77 temporary permanent local seismic stations the data set the published bulletins include 3494 p wave travel times and 3064 s p travel times from 980 local earthquakes that were located with a minimum of six observations the located earthquakes having a travel time root mean square rms residual 0 49 s and azimuthal gap 180 are selected to compute a 1 d velocity model for the region which is used as initial model for the subsequent 3 d inversions our results demonstrate that the computed 3 d velocity model has significantly improved hypocenter locations of the selected 980 earthquakes by reducing the rms error 0 06 by about 88 with respect to that by the 1 d velocity model the reconstructed p wave velocity v p structure with relocated events reveal strong heterogeneity in lateral as well as in vertical direction corresponding to the local and regional geology tectonics of the region high v p is mapped beneath the shillong plateau mikir hills and in the vicinity of indo burma ranges at shallower crust v p structure is imaged between the shillong plateau and mikir hills at 20 km depth which corresponds to the kopili fault the kopili fault system extends down to 30 km depth as evidenced by the low v p a high v p is imaged below the mikir hills at 40 km depth which is possibly the stress concentrator for high seismic activity along the kopili fault particularly at the fault end the bengal basin south of the shillong plateau is identified as a low v p zone extending down to a depth of about 20 km that indicates the thick alluvium sediments

earthquake source zones in northeast india seismic tomography fractal dimension and b value mapping we have imaged earthquake source zones beneath the northeast india region by seismic tomography fractal dimension and b value mapping 3d p wave velocity vp structure is imaged by the local earthquake tomography let method high precision p wave 3 494 and s wave 3 064 travel times of 980 selected earthquakes m d 2 5 are used the events were recorded by 77 temporary permanent seismic stations in the region during 1993 1999 by the let method simultaneous inversion is made for precise location of the events as well as for 3d seismic imaging of the velocity structure fractal dimension and seismic b value has been estimated using the 980 let relocated epicenters a prominent northwest southeast low vp structure is imaged between the shillong plateau and mikir hills that reflects the kopili fault at the fault end a high vp structure is imaged at a depth of 40 km this is inferred to be the source zone for high seismic activity along this fault a similar high vp seismic source zone is imaged beneath the shillong plateau at 30 km depth both of the source zones have high fractal dimension from 1 80 to 1 90 indicating that most of the earthquake associated fractures are approaching a 2d space the spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region the seismic b value in northeast india is found to vary from 0 6 to 1 0 higher b value contours are obtained along the kopili fault 1 0 and in the shillong plateau 0 9 the correlation coefficient between the fractal dimension and b value is found to be 0 79 indicating that the correlation is positive and significant to the south of shillong plateau a low vp structure is interpreted as thick 20 km sediments in the bengal basin with almost no seismic activity in the basin

large and great earthquakes in the shillong plateau assam valley area of northeast india region pop up and transverse tectonics abstract the tectonic model of the shillong plateau and assam valley in the northeast india region the source area for the 1897 great earthquake ms 8 7 and for the four 1869 1923 1930 and 1943 large earthquakes m 7 0 is examined using the high precision data of a 20 station broadband seismic network about 300 selected earthquakes m 3 0 recorded during 2001 2009 are analysed to study the seismicity and fault plane solutions the dominating thrust reverse faulting earthquakes in the western plateau may be explained by the proposed pop up tectonics between two active boundary faults the oldham brahmaputra fault to the north and the dapsi dauki thrust to the south though the northern boundary fault is debated the more intense normal and strike slip faulting earthquakes in the eastern plateau mikir massif and in the assam valley on the other hand are well explained by transverse tectonics at the long and deep rooted kopili fault that cuts across the himalaya and caused the 2009 bhutan earthquake mw 6 3 it is conjectured that the complex tectonics of the shillong plateau and transverse tectonics at the kopili fault make the region vulnerable for impending large earthquake s

crustal structure of bengal basin and shillong plateau extension of eastern ghat and satpura mobile belts to himalayan fronts and seismotectonics abstract gravity data from the east india and the bangladesh are processed and compiled together to provide a composite complete bouguer anomaly map of this region modelling of gravity profiles across the east indian shield chhotanagpur granite gneiss complex cggc the shillong plateau and the bengal basin in bangladesh constrained from seismic studies suggest crustal thickness of 37 38 km and thrusted high density rocks in the middle crust under the east indian shield reducing to 30 32 km with thick sediments of 12 14 km under bangladesh reduced crustal thickness under bangladesh is attributed to oceanic type of crust which is connected to indian continent along hinge zone characterized by linear gravity highs connecting east coast of india to the shillong plateau thrusted high density rocks under the cggc and the shillong plateau are in comparison to those under the eastern ghat and the satpura mobile belts egmb smb respectively suggest the north ward and east ward extensions of these orogenies margins of the thrusted block under shillong plateau when projected on surface coincide approximately with the south dipping dauki and the brahmaputra faults which coincides with central bulge due to lithospheric flexure indicating their surface expositions

oceanic boundary conditions for jakobshavn glacier part ii provenance and sources of variability of disko bay and ilulissat icefjord waters 1990 2011 abstractjakobshavn glacier west greenland has responded to temperature changes in ilulissat icefjord into which it terminates basin waters in this fjord exchange with neighboring disko bay waters of a particular density at least once per year this study determined the provenance of this isopycnic layer for 1990 2011 using hydrographic data from cape farewell to baffin bay the warm atlantic origin core of the west greenland current never filled deep disko bay or entered the fjord basin because of bathymetric impediments on the west greenland shelf instead equal parts of atlantic water and less saline polar water filled the fjord basin and bathed jakobshavn glacier the polar water fraction was often traceable to the east west greenland current but sometimes to the colder baffin current the huge annual temperature cycle on west greenland current isopycnals did not propagate into deep disko bay or the fjord basin because isopycnals over the west greenland shelf were depressed during the warm autumn

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

magnetotelluric studies in puga valley geothermal field nw himalaya jammu and kashmir india abstract puga geothermal field in nw himalaya ladakh district jammu and kashmir india is situated near the junction of the indian and asian plates the thermal activity is attributed to the widespread igneous activity during upper cretaceous to late tertiary age the study area located at an altitude of 4600 m above mean sea level is characterized by springs with temperatures up to 84 c that correspond to the boiling point of water at this altitude in order to image the shallow and deeper parts of the area wideband 1000 0 001 hz magnetotelluric mt measurements have been carried out five component mt data were acquired from the e w trending 15 km long and 1 km wide puga valley at 35 locations the data have been subjected to one 1d and two dimensional 2d modelling the results confirm the presence of a shallow conductive region in the area and also indicate the presence of a deep conductive region 5 15 m commencing from a depth of about 1 5 to 2 0 km and related to the presence of a geothermal reservoir

state of tectonic stress in northeast india and adjoining south asia region an appraisal an attempt is made to map the spatial variation of the tectonic stress pattern in northeast india and its adjoining south asia region using stress tensor inversion of some 516 fault plane solutions the bhutan himalaya and the arunachal himalaya are mapped with north south to north northwest south southeast compression the eastern himalaya syntaxis zone on the other hand shows a clockwise rotation a north northeast compression is dominant to the south in the intraplate part of the region the shillong plateau assam valley bengal basin bangladesh and tripura fold belt exhibit north northwest to north northeast compression orthogonal horizontal extension is dominant in southern tibet bhutan and partly in the syntaxis zone and the same is also observed in the shillong plateau and assam valley area of the intraplate region the indo burma ranges and the sagaing fault in the myanmar region show a northeast southwest compression an orthogonal horizontal northwest southeast extension is also observed in the sagaing fault zone a depth variation of the tectonic stress is observed below the indo burma ranges it changes from north south to northeast southwest in the southern part and from northeast southwest to north northeast south southwest in the northern part in the deeper seismogenic zone the stress inversion results of clusters of events in individual zones though mostly conformable with the average observations indicate a variation in the shillong plateau due to heterogeneity and tectonic complexity

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

a 100 yr record of ocean temperature control on the stability of jakobshavn isbrae west greenland an understanding of the interaction between ice sheet dynamics and forcing mechanisms such as oceanic and atmospheric circulation is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea level change here we report new benthic foraminiferal data from disko bugt west greenland showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier jakobshavn isbrae over the past 100 yr in particular our faunal data show that warm ocean currents entered a bay disko bugt during the retreat phases of jakobshavn isbrae from a d 1920 to 1950 and since 1998 we also show a link between west greenland ocean temperature and the atlantic multidecadal oscillation a key climate indicator in the north atlantic ocean the close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea level change

oceanic boundary conditions for jakobshavn glacier part i variability and renewal of ilulissat icefjord waters 2001 14 jakobshavnglacier west greenland has respondedtotemperaturechangesinilulissaticefjord intowhich it terminates thisstudycollectedhydrographicobservationsinsideilulissaticefjordandfromadjacentdiskobay between2001and2014 thewarmestdeepdiskobaywaterswereblockedbytheentrancesillanddidnotreach jakobshavn glacier in the fjord basin the summer mean temperature was 2 88c from 2009 to 2013 excluding 2010 whenitwas 18c cooler despitethisvariability summerpotential densities in the basinwereinthe narrow range of 27 20 su 27 31kgm 23 and basin water properties matched those of disko bay in this layer each summer this relation has likely held since at least 1980 basin waters from 2009 and 2011 13 were therefore similar tothosein1998 99 whenjakobshavn glacier began toretreat whilebasin watersin2010wereascoolas in the 1980s the 2010 basin temperature anomaly was advected into disko bay not produced by local atmospheric variability this anomaly also shows that ilulissat icefjord basin waters were renewed annually or faster time series fragments inside the fjord did not capture the 2010 anomaly but show that the basin temperatures varied little subannually outside of summer fjord velocity profiles from summer 2013 implied a basin renewal time scale of about 1 month in model simulations of the fjord circulation subglacial discharge from jakobshavn glacier could drive renewal of the fjord basin over a single summer while baroclinic forcing from outside the fjord could not because of the sill at the mouth

the east greenland coastal current structure variability and forcing abstract the subtidal circulation of the southeast greenland shelf is described using a set of high resolution hydrographic and velocity transects occupied in summer 2004 the main feature is the east greenland coastal current egcc a low salinity high velocity jet with a wedge shaped hydrographic structure characteristic of other surface buoyancy driven currents the egcc was observed along the entire greenland shelf south of denmark strait while the transect north of the strait showed only a weak shelf flow this observation in conjunction with water mass considerations and other supporting evidence suggests that the egcc is an inner branch of the east greenland current egc that forms south of denmark strait it is argued that bathymetric steering is the most likely reason why the egc apparently bifurcates at this location repeat sections occupied at cape farewell between 1997 and 2004 show that the alongshelf wind stress can have an influence on the structure and strength of the egcc and egc on timescales of 2 3 days accounting for the wind induced effects the volume transport of the combined egcc egc system is roughly constant 2 sv over the study domain from 68 n to cape farewell near 60 n the corresponding freshwater transport increases by roughly 60 over this distance 59 96 msv referenced to a salinity of 34 8 this trend is consistent with a simple freshwater budget of the egcc egc system that accounts for meltwater runoff melting sea ice and icebergs and net precipitation minus evaporation

structure and variability of the west greenland current in summer derived from 6 repeat standard sections abstract six historical sections across the west greenland current are examined three sections have been regularly occupied since the late 1950s while the three southern ones have been taken since 1984 significant variability is observed for the freshwater core of the coastal current on the shelf with salinity varying by over 3 units between years there is also significant variability in the shape and offshore position of the main shelf break front leading to large variability in eulerian velocities significant presence of irminger water is seen during the 1960s and the 2000s being found right across the sections in recent years maximum mean transport relative to the 34 8 isohaline of 5 5 3 9 sv relative to 700 db for 1984 2005 is observed at the cape desolation section transports decrease to the north with the majority of the exchange with the interior of the labrador sea occurring between cape desolation and fylla bank inter decadal transport variability is observed at fylla bank while a decline in transports since peaks in the early 1990s is seen at cape farewell and cape desolation freshwater transport is largest at cape desolation with a mean summer transport of 60 2 20 5 msv freshwater transport increases slightly between cape farewell and cape desolation and we suggest it is related to local discharge by glaciers into juliannehaab bight as well as the melting of sea ice we also find that years of high greenland ice cap melt are consistently associated with years of high freshwater transport at cape desolation suggesting a portion of the freshwater transport of the west greenland current may be associated with melt from the greenland ice sheet finally significantly enhanced freshwater transports 33 msv at cape desolation compared to the long term mean are seen in 2008 probably a signature of the record arctic ocean ice melt and export in 2007

crustal structure beneath northeast india inferred from receiver function modeling abstract we estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast india by using h vp vs stacking method and a non linear direct search approach neighbourhood algorithm na technique followed by joint inversion of rayleigh wave group velocity and receiver function calculated from teleseismic earthquakes data results show significant variations of thickness shear velocities vs and vp vs ratio in the crust of the study region the inverted shear wave velocity models show crustal thickness variations of 32 36 km in shillong plateau north 36 40 in assam valley and 44 km in lesser himalaya south average vp vs ratio in shillong plateau is less 1 73 1 77 compared to assam valley and lesser himalaya 1 80 average crustal shear velocity beneath the study region varies from 3 4 to 3 5 km s sediment structure beneath shillong plateau and assam valley shows 1 2 km thick sediment layer with low vs 2 5 2 9 km s and high vp vs ratio 1 8 2 1 while it is observed to be of greater thickness 4 km with similar vs and high vp vs 2 5 in rup lesser himalaya both shillong plateau and assam valley show thick upper and middle crust 10 20 km and thin 4 9 km lower crust average vp vs ratio in assam valley and shillong plateau suggest that the crust is felsic to intermediate and intermediate to mafic beneath shillong plateau and assam valley respectively results show that lower crust rocks beneath the shillong plateau and assam valley lies between mafic granulite and mafic garnet granulite

a freshwater jet on the east greenland shelf in august 1997 rrs discovery cruise 230 world ocean circulation experiment woce section a25 ran a hydrographic section into cape farewell on the southern tip of greenland the closest approach to the shore was 2 nm in a water depth of 160 m over the east greenland shelf analysis of the hydrographic data conductivity temperature depth ctd vessel mounted acoustic doppler current profiler and thermosalinograph has revealed a current flowing southwestward 15 km wide 100 m deep and centered 10 km offshore we believe it to be driven by meltwater runoff from greenland this feature which we call the east greenland coastal current egcc carries a little less than 1 sv 106 m3 s 1 with peak current speeds of 1 m s 1 at the surface the center of the egcc lies on a salinity front with maximum salinity contrast 4 practical salinity units psu between coast and shelf break and between surface and bottom a spot value of freshwater transport is 0 06 sv 1800 km3 yr 1 which is equivalent to 30 of the arctic freshwater gain the presence of the egcc and its continuity up the east greenland coast as far as denmark strait is confirmed in satellite sea surface temperature images and surface drifter tracks we estimate the sensitivity of its freshwater flux to changes in melt season mean surface air temperature to be 25 per 1 c

pop up tectonics of the shillong plateau in the great 1897 earthquake ms 8 7 insights from the gravity in conjunction with the recent seismological results 1 it was reported that a hidden 110 km long south dipping fault named oldham fault was responsible for the 1897 great earthquake ms 8 7 in the shillong plateau by reverse faulting due to pop up tectonics of the plateau in northeast india here we report the results of our geophysical investigations where we critically examine the crustal structure of the plateau on the basis of gravity modeling and attempt to shed light on the hitherto debated hidden oldham fault at the northern boundary of the shillong plateau our gravity model constrained by broadband seismological data suggests that the moho beneath the shillong plateau is at a shallower depth of about 35 km when compared to the bengal basin to the south and the assam valley to the north which is about 42 km thinning of the crust under the plateau may be a consequence of the pop up mechanism to examine the possible pop up of the plateau during the great 1897 earthquake we have estimated the energy released by this earthquake and compared it with the energy required for the pop up of the plateau using a simplistic rigid model it is found that the shillong plateau between the oldham fault and dauki dapsi thrust would require an energy of 4 5 1019 j which does match well with the energy released by the great earthquake of ms 8 7

oceanographic regime shift during 1997 in disko bay western greenland data from a long time series of temperature salinity and nutrient measurements in disko bay west greenland reveal a marked change in the water characteristics during recent years seasonal dynamics in the upper 150 m of the water column were highly affected by the seasonality in meteorological conditions while the deeper water strata were more stable and were primarily influenced by large scale circulation patterns there was a marked increase in the average water temperatures at 200 m depth in spring 1997 with the long term average increasing from 1 30 c to 2 25 c weekly data from 1996 to 1997 show that the sudden change in bottom water occurred in april 1997 due to the inflow of a larger proportion of north atlantic water which propagated north along the coast before entering the bay further support for this inflow was found when tracing the relative proportion of atlantic water in the bay using inorganic nutrients these changes in the oceanography of the bay will not only lead to further glacial retreat but will also affect the local marine ecosystem by changing the relative dominance of major copepod species that overwinter in bottom waters of the bay

interpreting the style of faulting and paleoseismicity associated with the 1897 shillong northeast india earthquake implications for regional tectonism 1 the 1897 shillong assam northeast india earthquake is considered to be one of the largest in the modern history although oldham s 1899 classic memoir on this event opened new vistas in observational seismology many questions on its style of faulting remain unresolved most previous studies considered this as a detachment earthquake that occurred on a gently north dipping fault extending from the himalayan front a recent model proposed an alternate geometry governed by high angle faults to the north and south of the plateau and it suggested that the 1897 earthquake occurred on a south dipping reverse fault coinciding with the northern plateau margin in this paper we explore the available database together with the coseismic observations from the region to further understand the nature of faulting the geophysical and geological data examined in this paper conform to a south dipping structure but its location is inferred to be in the brahmaputra basin further north of the present plateau front our analyses of paleoseismic data suggest a 1200 year interval between the 1897 event and its predecessor and we identify the northern boundary fault as a major seismic source the shillong plateau bounded by major faults behaves as an independent tectonic entity with its own style of faulting seismic productivity and hazard potential distinct from the himalayan thrust front a point that provides fresh insight into the regional geodynamics

crustal shear wave velocity structure beneath northeast india from teleseismic receiver function analysis abstract we investigated the seismic shear wave velocity structure of the crust beneath nine broadband seismological stations of the shillong mikir plateau and its adjoining region using teleseismic p wave receiver function analysis the inverted shear wave velocity models show 34 38 km thick crust beneath the shillong plateau which increases to 37 38 km beneath the brahmaputra valley and 46 48 km beneath the himalayan foredeep region the gradual increase of crustal thickness from the shillong plateau to himalayan foredeep region is consistent with the underthrusting of indian plate beyond the surface collision boundary a strong azimuthal variation is observed beneath shl station the modeling of receiver functions of teleseismic earthquakes arriving the shl station from ne backazimuth baz shows a high velocity zone within depth range 2 8 km along with a low velocity zone within 8 13 km in contrast inversion of receiver functions from se baz shows high velocity zone in the upper crust within depth range 10 18 km and low velocity zone within 18 36 km the critical examination of ray piercing points at the depth of moho shows that the rays from se baz pierce mostly the southeast part of the plateau near dauki fault zone this observation suggests the effect of underthrusting bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the ew ne striking dauki fault dipping 300 toward northwest

injections multiples de magmas dans un conduit nourricier implications sur le remplissage des plutons et l extraction des magmas for some plutons periodic replenishment is required to explain trends in magmatic differentiation the houghton pluton located in the northern abitibi greenstone belt contains breccias interpreted as feeder dykes that document the periodic replenishment of the chamber and the expulsion of the interstitial liquid between the enclaves interpretation of the textures in these breccias provides a mechanism for efficient magma extraction which maybe applicable for example to partially molten rocks there are two types of breccia i polymictic breccias made of various enclaves with an interstitial leucotonalite of 5 20 and ii foliated block breccias made of foliated tonalite and deformed hornblendite enclaves with an interstitial leucotonalite of 0 5 dykes ending in enclaves enclaves indented together and separated afterwards and enclaves rimmed by a magma different from the interstitial one all argue in favour of periodic replenishment magma expulsion is explained by compaction of enclaves u

imaging the indian lithosphere beneath the eastern himalayan region summary lithospheric thickness is an important parameter to understand the nature of collision and subduction between the indian and asian tectonic plates in this study we apply the s receiver function technique to data from a network of broad band stations in the northeast india and eastern himalayan regions and image the geometry of indian plate collision this analysis reveals clear s to p conversions from the moho and lithosphere asthenosphere boundary lab in the various tectonic units of the study region the indian lithosphere is found to be only 90 km thick beneath the shillong plateau deepening to 135 km on either side suggestive of a lithospheric upwarp related to the plateau uplift the lithosphere thickens northward with values reaching 180 km beneath the eastern himalaya the trend of the lab north of the foredeep region indicates that the indian plate plunges beneath the eastern himalaya the consistent northward dipping character of the indian plate suggests that the indian plate is traceable until it gets subducted beneath tibet just south of bangong suture zone the deepening of the lab and its correlation with the topographic elevation is in agreement with homogeneous thickening of the lithosphere in response to compressive forces due to the continental collision of india with asia

electrical structure across the indus tsangpo suture and shyok suture zones in nw himalaya using magnetotelluric studies 1 magnetotelluric studies in the nw himalayan region have shown anomalously high conductance of about 20 000 siemens in the crust beneath the indus tsangpo suture its and the adjoining tso morari dome in the depth range of 1 20 km high heat flow and high attenuation of the seismic waves in the himalayan crust together with the observed high conductance indicate presence of wide spread partial melt generated from the subducted indian crust the ladakh batholith appears as a resistive block to the north of its a moderately conductive zone demarcates the ladakh batholith from karakoram batholith to the north the similarity in the resistive structure with the results reported from the tibetan region by wei et al 2001 about 1500 km to the east is rather significant indicative of a two dimensional nature of the himalayan collision belt at least to the first order

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

estimation of crustal discontinuities from reflected seismic waves recorded at shillong and mikir hills plateau northeast india in this study an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the shillong mikir hills plateau in northeast india region the principle of the technique is to relate seismic travel times with crustal thickness above the conrad and moho discontinuities broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities the results show that the conrad discontinuity is at 18 20 0 5 km beneath the shillong mikir hills plateau and the moho discontinuity is at 30 1 0 km beneath the shillong plateau and at 35 1 0 km beneath the mikir hills

three dimensional seismic structure beneath shillong plateau and assam valley northeast india abstract northeast india is bounded by the himalayan arc to the north and the burmese arc to the east the shillong plateau and assam valley lie at the boundary zone of the two arcs three microearthquake surveys were conducted in this area from 1984 to 1986 we have applied a tomographic method to about 2800 high quality p and s wave arrival times from 364 local earthquakes that were recorded in the magnitude range 2 0 to 4 0 by 22 temporary seismic stations during the surveys to determine the 3d velocity structure of the crust and upper mantle in this region the result reveals significant lateral heterogeneities in the study area the tomographic images obtained in this study are compatible with the major tectonic features such as active faults and seismicity trends

north atlantic oscillation induced changes of the upper layer circulation in the northern north atlantic ocean 1 hydrographic data of the 1990s along world ocean circulation experiment sections a1e greenland ireland and a2 newfoundland france indicate a redistribution of cold low saline subarctic waters and warm saline subtropical waters in the upper layer of the northern north atlantic within about 2 years after the north atlantic oscillation had turned from a period with strong westerlies until 1995 to a period with weak westerlies in 1996 and 1997 in the latter period subarctic waters spread preferably southward along the continental slope east of newfoundland rather than eastward with the north atlantic current nac into the irminger iceland and west european basins as a consequence the subarctic front and the associated nac shifted eastward in the newfoundland basin where subarctic waters accumulate and westward in the iceland basin where a large warm and saline anomaly was found in 1996 and 1997 indicating a contraction of the subpolar gyre in the irminger basin the anomaly occurred in 1998 and 1999 whereas the cyclonic circulation in the irminger and iceland basins weakened it intensified in the newfoundland basin where a strengthened labrador current retroflection joined the nac this and the increased anticyclonic recirculation of subtropical waters in the newfoundland and west european basins caused a considerable reduction of the northward heat transport across 47 n in the upper layer in 1997 the anticyclonic circulation occurring in the west european basin suggested a northward expansion of the subtropical gyre

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

crustal p wave velocity and velocity ratio study in northeast india by a microearthquake survey the upper crustal 20 km p wave velocity beneath the shillong plateau and nowgong area has been studied by the time distance plot method thep arrival data of the shallow 20 km microearthquakes from three temporary networks are used and the average velocity is found to be 5 55 km s the velocity ratio v p v s for the upper crust 0 20 km as well as for the lower crust 21 40 km are determined by the wadati plot method and station by station method the average value obtained by the two methods is compatible thev p v s ranges between 1 74 to 1 76 a generalized seismic velocity model of the area is suggested by this study which has been very useful for microearthquake location

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

seismological evidence of the hales discontinuity in northeast india the crust and upper mantle shear wave velocity structure beneath the northeast india is estimated by joint inversion of rayleigh wave group velocity and receiver function calculated from teleseismic earthquakes data recorded at nine broadband seismic stations the assam valley and the shillong mikir plateau are the two important tectonic blocks in the northeast india which are surrounded by the himalayan collision zone in the north indo burma subduction zone in the east and by the bengal basin in the south the joint inversion followed by forward modeling reveal crustal thicknesses of 30 34 km beneath the shillong plateau 36 km beneath the mikir hills and 38 40 km beneath the assam valley with an average shear wave velocity vs of 3 4 3 5 km s the estimated low upper mantle shear wave velocity vsn 4 2 4 3 km s may be due to the rock composition or grain size or increased temperature and partial melt 1 in the upper mantle or an effect of all also we report for the first time the existence of the hales discontinuity at depths 56 74 km with vs 4 4 4 6 km s variable depth of the hales discontinuity may be explained by the geotherm and or addition of cr3 and fe2 in the spinel garnet system

substantial thinning of a major east greenland outlet glacier aircraft laser altimeter surveys in 1993 and 1998 over kangerdlugssuaq glacier in east greenland reveal thinning over the 5 year interim of several meters for all surveyed areas within 70 km of the seaward ice front rising to 50 meters in the final 5 km such rapid thinning is best explained by increased discharge velocities and associated creep thinning most probably caused by enhanced lubrication of the glacier bed the calving ice front over the past decade has occupied approximately the same location as in 1966 velocity estimates for 1995 96 are about the same as those for 1966 and 1988 but significantly less than for 1999 suggesting that major thinning began after 1995

acoustic mapping of the ilulissat ice fjord mouth west greenland a ship based acoustic mapping campaign was conducted at the exit of ilulissat ice fjord of west greenland and in the sedimentary basin of disko bay west of the fjord mouth submarine landscape and sediment distribution patterns represented by five acoustic facies types represent glaciomarine sediment facies types that are related to variations in the past position and relative motion of the glacier front asymmetric ridges on the shelf that form a curved entity and a large sill at the fjord mouth represent moraines that depict at least two relatively stable positions of the ice front in the disko bay and at the fjord mouth comparable ice end features are not observed seaward of the east greenland kangerdlugssuaq glacier although both glaciers are comparable in their latitudinal position present size and present role for the ice discharge from the inland ice sheet apparently the retreat of the greenland inland ice after the last maximum expansion was a more discontinuous process on the west greenland shelf than on the east greenland shelf the iceberg bank a prominent sill at the fjord exit appears to play an important role for the sedimentation after the retreat of the ice front from the shelf was completed the retreat of the glacier behind the iceberg bank into the inner fjord is marked by a reorganization of sediment delivery in disko bay as most of the till is now deposited within the fjord two linear clusters of pockmarks in the center of the sedimentary basin seem to be linked to methane release due to dissociation of gas hydrates a process driven by fast crustal uplift of the greenland shelf the orientation of these clusters appears to reflect a migration path that is defined by a buried structure which we could not resolve

source parameters of earthquakes and intraplate deformation beneath the shillong plateau and the northern indoburman ranges we determined the fault plane solutions and focal depths of 17 earthquakes beneath the shillong plateau and the northern indoburman ranges by combining results from the inversion of long period p and sh waveforms and amplitudes from polarities of first motions and from the identification of pp and sp phases on short period seismograms fault plane solutions of 15 earthquakes show mixtures of thrust and strike slip faulting but the p axes for these events are nearly horizontal and consistently oriented north northeast south southwest all of these earthquakes occurred at depths greater than 29 km beneath the shillong plateau one event occurred at a depth of 52 km the relatively large depths for earthquakes in an intraplate setting suggest that the indian lithosphere in this area is especially cold earthquakes beneath the northern indoburman ranges define a gently east southeast dipping zone from 30 to 45 km beneath the bengal basin to 40 to 90 km beneath the ranges this zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath burma these earthquakes cannot have occurred along the interface between a subducting indian plate and the overriding indoburman lithosphere because the p axes not the nodal planes are parallel to the north south trending seismic zone although a couple of the earthquakes might have occurred within the indoburman lithosphere most of this seismicity seems to have occurred within the indian plate recently and currently being subducted eastward beneath the indoburman ranges the consistent north northeast trend of the p axes implies that the orientation of maximum compressional strain in the indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north south trending folds of the indoburman ranges thus either recently in geologic time since 1 ma the orientation of maximum compression changed dramatically or more likely the deformation in the indoburman ranges is decoupled from that in the underlying indian plate meanwhile the seemingly identical northward displacement of india and the indoburman ranges with respect to south china must be accommodated farther east along the sagaing and other faults

the circulation water masses and sea ice of baffin bay abstract the oceanographic meteorological and sea ice conditions in baffin bay are studied using historical hydrographic satellite and meteorological data and a set of current meter data from a mooring program of the bedford institute of oceanography baffin bay is partially covered by sea ice all year except august and september the interannual variation of the ice extent is shown to be correlated with winter air temperature available hydrographic data were used to study the water masses and the horizontal and vertical distribution of temperature salinity three water masses can be identified arctic water in the upper 100 300 m of all regions except the southeast west greenland intermediate water at 300 800 m in most of the interior of baffin bay and deep baffin bay water in all regions below 1200 m the temperature and salinity in baffin bay have limited seasonal variability except in the upper 300 m of eastern davis strait northern baffin bay and the mouth of lancaster sound summer data have a temperature minimum at 100 m which suggests winter convection does not penetrate deeper than this depth current meter data and results of a circulation model indicate that the mean circulation is cyclonic the seasonal variation of the currents is complex overall summer and fall tend to have stronger currents than winter and spring at all depths among the different regions the largest seasonal variation occurs at the mouth of lancaster sound and the baffin island slope model generated velocity fields show a basic agreement with the observed currents and indicate strong topographic control in the vicinity of davis strait and on the greenland shelves the model also produces a southward counter current on the greenland slope which may explain the observed high horizontal shears over the greenland slope estimates of the volume and fresh water transports through lancaster jones and smith sounds are reviewed transports through davis strait are computed from the current meter data the balance of freshwater budget and sensitivity of the thermohaline circulation to freshwater transport are discussed