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

simulation of strong ground motion during the 1950 great assam earthquake in this paper ground motion during the independence day earthquake of august 15 1950 mw 8 6 ben menahem et al 1974 in the northeastern part of india is estimated by seismological approaches a hybrid simulation technique which combines the low frequency ground motion simulated from an analytical source mechanism model with the stochastically simulated high frequency components is used for obtaining the acceleration time histories a series of ground motion simulations are carried out to estimate the peak ground acceleration pga and spectral accelerations at important cities and towns in the epicentral region one sample pga distribution in the epicentral region encompassing the epicenter is also obtained it is found that pga in the epicentral region has exceeded 1 g during this earthquake the estimated pga s are validated to the extent possible using the mmi values the simulated acceleration time histories can be used for the assessment of important engineering structures in northeastern india

edge driven mechanical microplate models of strike slip faulting in the tibetan plateau 1 the india asia collision zone accommodates the relative motion between india and eurasia through both shortening and pervasive strike slip faulting to gain a mechanical understanding of how fault slip rates are driven across the tibetan plateau we develop a two dimensional linear elastic two stage deformable microplate model for the upper crust based on the behavior of an idealized earthquake cycle we use this approach to develop a suite of simple india asia collision zone models differing only in boundary conditions to determine which combination of edge forces and displacements are consistent with both the slip rate measurements along major tibetan faults as well as the geodetically observed extrusion of crustal material toward southeast asia model predictions for the altyn tagh 1 14 mm yr kunlun 3 10 mm yr karakorum 5 12 mm yr and haiyuan 3 5 mm yr faults are in agreement with geologically and geodetically inferred slip rates further models that accurately reproduce observed slip rate gradients along the altyn tagh and kunlun faults feature two critical boundary conditions 1 oblique compressive displacement along the himalayan range front west of the shillong plateau and 2 forcing in southeast asia additionally the ratio of internal block potency rate to the total potency rate for each microplate ranges from 28 to 79 suggesting a hybrid view of deformation in tibet as simultaneously localized on major faults and distributed at length scales 500 km

composite structure of plumes in stratus topped boundary layers abstract knowledge of convective plumes within the clear convective boundary layer cbl is quite advanced owing to direct measurements tank experiments and large eddy simulation studies as a result modeling of the cbl is relatively successful progress for the stratus topped boundary layer stbl however is slow this study compares the plume structure of the surface heated cbl with that of the cloud top cooled stbl in the hope of extending our knowledge of the cbl to the stbl a conditional sampling technique is applied to the stbl flow fields that are generated through large simulations so that the structures of typical updrafts and downdrafts may be derived for the purpose of comparing the surface heated cbl and the cloud top cooled stbl an idealized stbl is simulated where the turbulence is maintained solely by cloud top radiative cooling the principal difference between the cbl and this idealized stbl lies in the origin of the plumes primary plumes in the cbl are generated at a rigid surf

3d geo mapping based on surface data for preliminary study of underground works a case study in val topina central italy abstract this paper reports a geological modelling procedure suitable for the reconstruction of three dimensional models and for applications in preliminary tunnelling studies the geological modelling is based on field survey data stored and organized in a database using a geographic information system gis and then exported in a computer aided design cad software system which is used to create the geometrical model the main steps of the elaboration are 1 reconstruction of the digital terrain model 2 calculation of elevation for the 2d geological data 3 reconstruction of geological cross sections oriented perpendicular to the geo structural fold axis geological profiles by means of down plunge projection 4 modelling of the geological surfaces via nurbs interpolation a model built in this way can be used to automatically create sub horizontal geological cross sections at tunnel level or vertical cross sections along the alignments in order to define accurately the geo lithological characteristics along each underground segment during excavation and to calculate the segments lengths in every geological formation along the tunnel the authors delineate the procedure through a simple and yet exhaustive case study carried out in order to choose the optimal path for the foligno nocera umbra railway mainly in tunnel to be realised in val topina umbria region italy

topography and tectonics of the central new madrid seismic zone results of numerical experiments using a three dimensional boundary element program we present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the new madrid seismic zone experiments are constrained by subtle topography and the distribution of seismicity in the region we use a new boundary element algorithm that permits calculation of the three dimensional deformation field surface displacement fields are calculated for the new madrid zone under both far field plate tectonic scale and locally derived driving strains results demonstrate that surface displacement fields cannot distinguish between either a far field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults thus neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present day deformation we have also tested hypotheses about strain accommodation within the new madrid contractional step over by including linking faults two southwest dipping and one vertical recently inferred from microearthquake data only those models with step over faults are able to predict the observed topography surface displacement fields for long term relaxed deformation predict the distribution of uplift and subsidence in the contractional step over remarkably well generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step over region with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked these models suggest that the gently dipping central step over fault is a reverse fault and that the steeper fault extending to the southeast of the step over acts as a normal fault over the long term

late miocene pleistocene evolution of india eurasia convergence partitioning between the bhutan himalaya and the shillong plateau new evidences from foreland basin deposits along the dungsam chu section eastern bhutan the shillong plateau is a unique basement cored uplift in the foreland of the eastern himalaya that accommodates part of the india eurasia convergence since the late miocene it was uplifted in the late pliocene to 1600 m potentially inducing regional climatic perturbations by orographically condensing part of the indian summer monsoon ism precipitations along its southern flank as such the eastern himalaya shillong plateau ism is suited to investigate effects of tectonics climate and erosion in a mountain range broken foreland system this study focuses on a 2200 m thick sedimentary section of the siwalik group strategically located in the lee of the shillong plateau along the dungsam chu at the front of the eastern bhutan himalaya we have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating combined with sedimentological and palynological analyses we show that 1 the section was deposited between 7 and 1 ma in a marginal marine deltaic transitioning into continental environment after 5 ma 2 depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the shillong plateau had an unexpected limited impact on the paleoclimate of the bhutanese foothills and 3 the diminution of the flexural subsidence in the basin and or of the detrital input from the range is attributable to a slowdown of the displacement rates along the main boundary thrust in eastern bhutan during the latest miocene pleistocene in response to increasing partitioning of the india eurasia convergence into the active faults bounding the shillong plateau

ground motion estimation at guwahati city for an mw 8 1 earthquake in the shillong plateau abstract in this paper the ground motion at guwahati city for an 8 1 magnitude earthquake on oldham fault in the shillong plateau has been estimated by stochastic finite fault simulation method the corresponding acceleration time histories on rock level at several sites in the epicentral region have been computed these results are validated by comparing them with the estimates obtained from medvedev sponheuer karnik msk intensity observations of 1897 shillong earthquake using the local soil parameters the simulated rock level acceleration time history at guwahati city is further amplified up to the ground surface by nonlinear site response analysis the results obtained are presented in the form of peak ground acceleration pga contour map the maximum amplification for pga over guwahati city is as high as 2 5 based on the simulated pga the liquefaction susceptibility at several locations in the city has been estimated the results are presented in the form of contours of factor of safety against liquefaction at different depths below the ground surface it is observed that over a large part of the guwahati city the factor of safety against liquefaction is less than one indicating that the city is highly vulnerable to liquefaction in the event of this earthquake the contour maps obtained can be used in identifying vulnerable areas and disaster mitigation

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

role of the kopili fault in deformation tectonics of the indo burmese arc inferred from the rupture process of the 3 january 2016 mw 6 7 imphal earthquake the burmese arc is a peculiar plate boundary where there exists an eastward subducted indian slab but the current tectonics of the region are predominantly governed by a northward strike slip motion of the indian plate past the burmese plate as evidenced by seismic and global positioning system data the shallower portion of the dipping indian slab is characterized mostly by earthquakes with strike slip type focal mechanisms with roughly northwest and northeast trending fault planes although there is no clear evidence for the choice of rupture planes in the present study we use teleseismic broadband waveforms to model the rupture propagation of an m w 6 7 strike slip earthquake of 3 january 2016 that occurred in the indian city of imphal and infer the fault plane orientation the result indicates a right lateral strike slip motion along a n21 w plane which is confirmed by both waveform fitting and distribution of aftershocks interestingly this earthquake occurred at the intersection of three major faults in this region the churachandpur mao fault bounding the india and burma plates the dauki fault south of the shillong plateau and the kopili fault separating the shillong plateau from the brahmaputra valley this raises various possibilities for interpreting this event however based on the finite fault inversion we infer that this earthquake occurred on the kopili fault running northwest southeast from bhutan himalaya down to the burmese arc and manifested as a right lateral strike slip motion in response to the northward motion of the indian plate and a counterclockwise rotation of the brahmaputra valley we further interpret that the overall deformation in the burmese arc region is accommodated by northward slivering of the eastward subducted indian lithospheric slab along a series of northwest to north south trending right lateral strike slip faults from west to east as the indian plate moves northward past the burma and sunda blocks

late quaternary to decadal velocity fields in asia 1 fields of crustal velocity and strain rate within asia averaged over horizontal distances of 300 km are calculated from estimates of quaternary slip rates on faults and rates of change of lengths of baselines between gps sites the calculated fields fit strain rate components estimated from quaternary slip rates within 95 confidence limits and fit the gps data with an rms residual of 2 5 mm yr 1 the strain rate field constrained only by quaternary slip rates of faults differs from the combined solution at the level of 20 nstrain yr 1 and the differences are explicable by the limited accuracy of geological estimates of slip rate therefore the strain rate fields determined by geodetic measurements taken on the 10 year timescale are consistent with geological observations on the 104 year timescale the combined solution for velocities shows plate or block like motion within deforming asia only in tarim south china and the amurian region elsewhere the kinematics of rigid blocks does not provide a useful description of the motion in particular velocities relative to eurasia point at high angles to contours of their magnitudes not parallel to them as they would if large regions moved as rigid blocks the axes of principal horizontal compressional strain align with topographic gradients in agreement with previous suggestions that asia deforms as a continuum under the influence of gravity

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

geologic site characterization for deep nuclear waste disposal in fractured rock based on 3d data visualization abstract three dimensional 3d visualization of data is an essential step in the development of descriptive hydrologic and rock mechanical models of fractured rock systems data includes geological properties rock mechanical properties and hydraulic responses the 3d visualization approach is applied to characterizing a hypothetical site for a high level nuclear waste repository located at 500 m depth in granitoids of the trans scandinavian igneous belt 1 6 1 8 ga old the study is based on site specific data at the aspo hard rock laboratory hrl on the southeastern coast of sweden the data are extensive and spatially complex and are based on surface information as well as a large number of boreholes penetrating up to 1000 m at this site fractures and fracture zones control both groundwater flow and mechanical stability structures are found within a 2 2 1 km deep model block representing the site 3d locations of fractures and fracture zones are hypothesized based on surface and subsurface geological and geophysical information including borehole radar this results in a 3d geological model structure of the site

revisiting the earthquake sources in the himalaya perspectives on past seismicity abstract the 2500 km long himalaya plate boundary experienced three great earthquakes during the past century but none of them generated any surface rupture the segments between the 1905 1934 and the 1897 1950 sources known as the central and assam seismic gaps respectively have long been considered holding potential for future great earthquakes this paper addresses two issues concerning earthquakes along the himalaya plate boundary one the absence of surface rupture associated with the great earthquakes vis a vis the purported large slip observed from paleoseismological investigations and two the current understanding of the status of the seismic gaps in the central himalaya and assam in view of the paleoseismological and historical data being gathered we suggest that the ruptures of earthquakes nucleating on the basal detachment are likely to be restricted by the crustal ramps and thus generate no surface ruptures whereas those originating on the faults within the wedges promote upward propagation of rupture and displacement as observed during the 2005 kashmir earthquake that showed a peak offset of 7 m the occasional reactivation of these thrust systems within the duplex zone may also be responsible for the observed temporal and spatial clustering of earthquakes in the himalaya observations presented in this paper suggest that the last major earthquake in the central himalaya occurred during ad 1119 1292 rather than in 1505 as suggested in some previous studies and thus the gap in the plate boundary events is real as for the northwestern himalaya seismically generated sedimentary features identified in the 1950 source region are generally younger than ad 1400 and evidence for older events is sketchy the 1897 shillong earthquake is not a decollement event and its predecessor is probably 1000 years old compared to the central himalaya the assam gap is a corridor of low seismicity between two tectonically independent seismogenic source zones that cannot be considered as a seismic gap in the conventional sense

a second order bulk boundary layer model abstract bulk mass flux models represent the large eddies that are primarily responsible for the turbulent fluxes in the planetary boundary layer as convective circulations with an associated convective mass flux in order for such models to be useful it is necessary to determine the fractional area covered by rising motion in the convective circulations this fraction can be used as an estimate of the cloud amount under certain conditions matching conditions have been developed that relate the convective mass flux to the ventilation and entrainment mass fluxes these are based on conservation equations for the scalar means and variances in the entrainment and ventilation layers methods are presented to determine both the fractional area covered by rising motion and the convective mass flux the requirement of variance balance is used to relax the well mixed assumption the vertical structures of the mean state and the turbulent fluxes are determined analytically several aspects of this simple m

a rupture model for the great earthquake of 1897 northeast india we assume that the unusually deep extensive and long lasting floods of 1897 along the section of the brahmaputra river north of the western shillong plateau were due to local ground subsidence associated with the great earthquake which occurred on june 12 of that year in the western part of northeast india numerical simulations of ground level changes due to slip on a buried low angle thrust fault dipping due north then show that the northern limit of the rupture zone of this earthquake should have been along the e w flowing brahmaputra river about 40 km north of the northern edge of the shillong plateau and about 70 km south of the himalayan mountain front a similar interpretation of a ground tilt observation suggests that the western limit of the 1897 rupture zone was along the western margin of the shillong plateau the e w and n s dimensions of the rupture zone are estimated to be 170 km and 100 km respectively so that it enclosed the western half of the shillong plateau and areas north of it up to the brahmaputra river the rupture depth could not be estimated from the available data on ground level changes and was constrained at 15 km beneath the southern margin of the shillong plateau on other evidence the above thrust fault should be of the nature of a detachment at midcrustal depth which arose because the continental crust associated with the indian shield terrains of the shillong plateau and mikir hills immediately to the east could not subduct under the continental crust of the eurasian plate to the north and east it is tentatively suggested that although this detachment may extend under the himalaya it may not be the detachment on which the great earthquakes of 1905 1934 and 1950 have occurred in the northwestern central and eastern himalaya respectively it is also suggested that a distinction should be made between the seismicity of the himalaya and the seismicity of the himalayan convergent plate margin hcpm an earthquake of the himalayan seismic belt is also an earthquake of the hcpm but the converse need not hold true since the inferred northern limit of the 1897 rupture zone is about 70 km south of the himalayan mountain front it is suggested that this earthquake belongs to the hcpm but not to the himalayan seismic belt thus conservatively a seismic gap of about 700 km may exist along the himalayan seismic belt between the eastern and western limits of the ruptures zones of the great 1934 and 1950 earthquakes respectively

the shillong plateau and the great 1897 assam earthquake previous analysis of triangulation data of the survey of india concluded that the great 1897 assam earthquake occurred on a south dipping fault near the northern edge of the shillong plateau which was named the oldham fault this attribution has been questioned on geological and geodetic grounds we refine the triangulation data adding recently discovered observations and demonstrate that they require average slip of 25 5 m on a fault that dips south at 40 beneath the plateau the best fitting solution to the geodetic observations gives a rupture length of 79 km however the chedrang fault immediately to the west of the oldham fault appears to have slipped as a subvertical tear fault during or shortly after the 1897 earthquake with over 10 m of down to the west normal sense slip this observation suggests that the western end of the main rupture approached within a few kilometers of the chedrang fault giving a length of 95 km for the rupture this range of parameters gives a magnitude 8 15 mw 8 35 for the earthquake the triangulation data cannot be satisfied by slip on the north dipping faults that border the southern edge of the shillong plateau nor by slip on a south dipping fault that has been postulated in the brahmaputra valley gps velocities show that up to 5 mm yr of shortening is taken up across the plateau and its borders this suggests via moment frequency relations that the interval between great earthquakes in the region is several thousand years but that earthquakes of magnitude 7 or greater should occur roughly once per century

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

crustal scale detachment in the himalayas a reappraisal summary according to the most popular tectonic model of the himalayas proposed by a number of scientists the indian crustal material underthrusts the himalayas at a low angle and is relatively free of deformation compared to the overlying accreted material that makes up the himalayan mountainchain inthisworkwehavecarriedoutlocalearthquaketomographyforthegarwhalkumaun himalayas to estimate p and s wave velocity variations vp and vs respectively and variation in their ratio vp vs that would indicate the structure of the himalayas and the underlying indian crust in this part of the himalayas the results indicate that there is crustal level folding and faulting in this region indicating that the underlying indian crustal material has also undergone deformation unlike what was postulated for the entire himalayas by some workers before by comparing our tomographic result with that for the eastern nepal southern tibet region it is concluded that there is variation in mode of deformation along the trend of the himalayas this observation matches well with the observed velocity variation in the upper mantle of these two regions reported by others the area under investigation falls within a region where there is more oblique convergence between india and eurasia compared to the nepal himalayas region this may explain why such variation in mode of deformation is observed the ratio vp vs gets affected by strength of material presence or absence of fluid filled fractures or molten material affects it most strongly in the crustal region the variation in vp vs in the study area shows that almost the entire crust here have enough rheological strength such that it can store strain energy that can be released through earthquakes a zone of low vp vs beginning at the higher himalayas and dipping towards sw is observed this zone also has high vp and vs and is observed even when inversion is carried out with very high damping value these observations do not suggest that seismic hazard will be less in the study area

numerical investigation of dip slip fault propagation effects on offshore seabed sediments abstract an enhanced understanding of the phenomenon of dip slip fault propagation through young unconsolidated offshore sediments is essential for the evaluation of the response of shallow offshore structures and subsea oil gas pipelines substantial and permanent seafloor deformation constitutes a severe geohazard for these marine structures and associated offshore elements components this paper addresses the response of seabed sand deposits subjected to underlying dip slip fault movement the resulting propagation of the faulting offset in seabed sediments is explored using 2d finite element modeling in abaqus while accounting for nonlinear soil behavior with strain softening a 2d plain strain fem seabed model is adopted to investigate the effect of different seabed soil properties and layer thicknesses on the extent and magnitude of seabed surface deformation induced by normal and reverse faults with different dip angles the results indicate that 1 the adoption of sand properties that depend on depth is necessary to realistically model the fault propagation mechanism particularly for fault propagation through thick and dense sand deposits 2 the presentation of fault propagation results in a normalized manner with respect to sand cover thickness does not eliminate the dependency on the actual layer thickness and 3 the deformation of the beds in the cover as a result of basement faulting can be modeled realistically using trishear kinematic approaches

the motion and active deformation of india measurements of surface displacements using gps constrain the motion and deformation of india and india eurasia plate boundary deformation along the himalaya the gps velocities of plate interior sites constrain the pole of the angular velocity vector of india with respect to eurasia to lie at 25 6 1 0 n 11 1 9 0 e approximately 6 west of the nuvel 1a pole of 3 ma plate motion the angular rotation rate of 0 44 0 03 myr 1 is 14 slower than the long term rate of 0 51 myr 1 insignificant velocities between plate interior sites indicate that the exposed indian plate is stable to within 7 10 9 yr 1 the observed contraction vector across the himalaya 20 mm yr veers from n20 e in the northwest himalaya to n25 w in east nepal consistent with east west extension of southern 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

separation of intrinsic and scattering seismic wave attenuation in northeast india summary we have analysed the local earthquakes 2 0 ml 5 5 occurred in northeast ne india recorded by a temporary seismic network of 10 stations operated by national geophysical research institute ngri hyderabad to evaluate the relative contributions of scattering loss q 1 and intrinsic absorption q 1 to total attenuation q 1 using the multiple lapse time window analysis assuming multiple isotropic scattering in a medium of uniformly distributed scatterers the results show that q 1 is greater than q 1 at high frequencies f 3hz while the opposite is observed at low frequencies f 3hz the observed frequency dependence of q 1 corresponds to the scale length of lithospheric heterogeneity beneath ne india at least comparable with the wavelength for the lowest frequencies analysed of about 1km the observed q 1 for the study region obtained with single scattering theory is close to q 1 at high frequencies in agreement with theoretical prediction for an idealized case of uniform distribution of scatterers however a discrepancy exists between the two at low frequencies which can be explained by a depth dependent velocity and attenuation structure high value of q 1 is correlated with the geology and tectonic settings of the region characterized by himalayan and burman collision zones with a strong lateral heterogeneity the q 1 estimates obtained in this study can be used to infer the average temperature of the lower crust with an upper limit estimate of 800 c assuming a lower crustal gabbroic lithology

high density crustal intrusive bodies beneath shillong plateau and indo burmese range of northeast india revealed by gravity modeling and earthquake data abstract modeling and analysis of gravity and earthquake data of a critically dynamic northeastern region of india assumes a special significance for understanding the seismicity and geodynamics in this paper the relation between lateral heterogeneity of gravity interfaces with subsurface structures and seismic characteristics of northeast india are studied in detail to understand the seismicity and dynamics of the region using bouguer gravity earthquake and magnetotelluric data the modern scaled power spectral analysis of gravity data has brought out crustal lateral heterogeneity at a depth of 8 33 km 19 73 km and 37 14 km that correlate well with 10 km upper crust 20 km lower crust and a moho at 28 42 km revealed by 1d seismic crustal velocity and tomography studies the inferred crustal depths support thinner crust over uplifted shillong plateau and gradual increase of depths over subduction interface of indo burmese range 2d gravity modeling constrained by seismic velocity model and 3d euler deconvolution technique along two specific e w and three n s regional gravity transects delineated three significant high density anomalous heterogeneous intrusive bodies presumably due to underplating beneath shillong plateau mikir massif and indo burmese range based on the seismicity and stress pattern analysis through seismic b value variation we infer that the study region exhibits compressive stress concentrations in the form of brittle deformation at shallow depths and weakening of lower crust by high density intrusive mafic ultramafic bodies of ductile nature which may cause intense seismicity besides the ongoing collision of north drifting indian plate with eurasia to the north and the eastern subduction interface of indo burmese range we also infer that the ne to nw trending v shaped areas outlined north of shillong plateau south of mikir massif and ne sw trending thrust belt of schuppen exhibit aseismic nature and may be the future impounding seismic sources in this region

sedimentation and tectonics of the sylhet trough bangladesh the sylhet trough a sub basin of the bengal basin in northeastern bangladesh contains a thick fill 12 to 16 km of late mesozoic and cenozoic strata that record its tectonic evolution stratigraphic sedimentologic and petrographic data collected from outcrops cores well logs and seismic lines are here used to reconstruct the history of this trough the sylhet trough occupied a slope basinal setting on a passive continental margin from late mesozoic through eocene time subsidence may have increased slightly in oligocene time when the trough was located in the distal part of a foreland basin paired to the indo burman ranges oligocene fluvial deltaic strata barail formation were derived from incipient uplifts in the eastern himalayas subsidence increased markedly in the miocene epoch in response to western encroachment of the indo burman ranges miocene to earliest pliocene sediments of the surma group were deposited in a large mud rich delta system that may have drained a significant proportion of the eastern himalayas subsidence rates in the sylhet trough increased dramatically 3 8 times from miocene to pliocene pleistocene time when the fluvial tipam sandstone and dupi tila formation were deposited this dramatic subsidence change is attributed to south directed overthrusting of the shillong plateau on the dauki fault for the following reasons 1 pliocene and pleistocene strata thin markedly away from the shillong plateau consistent with a crustal load emplaced on the northern basin margin 2 the shillong plateau is draped by mesozoic to miocene rocks but pliocene and younger strata are not represented suggesting that the massif was an uplifted block at this time 3 south directed overthrusting of the shillong plateau is consistent with gravity data and with recent seismotectonic observations sandstone in the tioam has a marked increase in sedimentary lithic fragments compared to older rocks reflecting uplift and erosion of the sedimentary cover of the shillong plateau if the dauki fault has a dip similar to that of other himalayan overthrusts then a few tens of kilometers of horizontal tectonic transport would be required to carry the shillong plateau to its present elevation uplift of the shillong plateau probably generated a major 300 km westward shift in the course of the brahmaputra river

recent seismicity rate forecast for north east india an approach based on rate state friction law abstract in this study we have investigated the association of stress field disparities through earthquake production rate in both spatially and temporally for northeast region of india here we have implemented the rate and state dependent friction law for forecasting the seismicity rate over mw 5 0 earthquake events during the period 2013 2015 the most valuable input model parameter of forecasting seismicity rate is coulomb stresses changes cff which promote the seismicity rate the cff distributions that exhibit significant stress increase in the close proximity of the mainshock sources are found to be diminished with the inverse of the distance from the fault rupture plane basically background seismicity depends on the coulomb stress changes these coulomb stress changes amplify the background seismicity so a little change in stress can produce very low or high seismicity rate in areas of high ambient seismicity in several zones the high seismicity rate is revealed by the dominancy of the low b value the observed background seismicity rate lies between the ranges of 0 4 0 the significant b value is achieved in the range of 0 57 1 54 along with average value of 0 93 we have taken the value of constant consecutive parameters i e a 0 05 mpa and constant effective friction coefficient of 0 4 for all the faults however in the forecast model we have obtained the stress perturbations and heterogeneous b value to infer the reliable result in comparison to reference forecast model for the period of 1963 2012 in this study we have also adopted the relm experiment in csep testing centre to verify the consistency of the result related to forecast for the duration of 2013 2015 the statistical test provided the satisfactory result for this study region

active tectonics in eastern lunana nw bhutan implications for the seismic and glacial hazard potential of the bhutan himalaya 1 paleoseismological investigations brittle fault analysis and paleostrain calculations combined with the interpretation of satellite imagery and flood wave modeling were used to investigate the seismic and associated glacial hazard potential in eastern lunana a remote area in nw bhutan seismically induced liquefaction features cracked pebbles and a surface rupture of about 6 8 km length constrain the occurrence of m 6 earthquakes within this high altitude periglacial environment which are the strongest earthquakes ever been reported for the kingdom of bhutan seismicity occurs along conjugate sets of faults trending ne sw to nnw sse by strike slip and normal faulting mechanism indicating e w extension and n s shortening the strain field for these conjugate sets of active faults is consistent with widespread observations of young e w expansion throughout southern tibet and the north himalaya we expect however that n s trending active strike slip faults may even reach much farther to the south at least into southern bhutan numerous glacial lakes exist in the investigation area and today more than 100 106 m3 of water are stored in moraine dammed and supraglacial lakes which are crosscut by active faults strong earthquakes may trigger glacial lake outburst floods and the impact of such flash floods may be worst 80 km downstream where the valley is broad and densely populated consequently tectonic models of active deformation have to be closely linked with glacial hazard evaluation and require rethinking and modification

modeling of strong motion data in northeastern india q stress drop and site amplification abstract in this article point source seismological model is used to model the available strong motion accelerograms in northeastern india key seismic parameters such as quality factor q value kappa factor 0 site amplification and stress drop are derived from the strong motion data a total of 111 three component records coming from seven earthquakes with magnitudes ranging from m w 5 2 to 7 2 are used in obtaining the source and path parameters the q value of the indo burma tectonic domain which is a subduction zone is obtained as 431 f 0 7 for the bengal basin shillong plateau region the quality factor is estimated as 224 f 0 93 the kappa factors for vertical and horizontal components are obtained as 0 013 and 0 033 for soft rock type sites for firm ground sites 0 is estimated as 0 025 for vertical and 0 041 for horizontal components respectively the site amplification functions for soft rock and firm ground conditions are found from the horizontal to vertical ratio the obtained stress drops of the seven events lie between 128 and 282 bars the source parameters obtained in this study provide a useful framework for working out ground motion relations in northeastern india

surface deformation in the region of the 1905 kangra mw 7 8 earthquake in the period 1846 2001 1 one century after the mw 7 8 kangra earthquake of 4 april 1905 we report weak constraints on its rupture parameters based on the remeasurement of historic triangulation points in the epicentral region the western corner of the 1905 rupture is constrained to within 20 km and its sw edge strikes n135 15w the down dip width of the rupture is inferred from the location of the interseismic locking line to have been less than 55 km scaling laws and revised msk intensities suggest an along arc rupture length of 100 km with uniform slip of 4 m because the earthquake ruptured 60 of the width of the plate boundary and released a fraction of the inferred slip deficit complacency about its re rupture in the distant future is unwarranted the kangra rupture could fail again today in a mw 7 5 earthquake 1 4 m of slip or as part of a much larger earthquake mw 8 6

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

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

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

collisional emplacement history of the naga andaman ophiolites and the position of the eastern indian suture abstract dismembered late mesozoic ophiolites occur in two parallel belts along the eastern margin of the indian plate the eastern belt closely following the magmatic arc of the central burma basin coincides with a zone of high gravity it is considered to mark a zone of steeply dipping mafic ultramafic rocks and continental metamorphic rocks which are the locus of two closely juxtaposed sutures in contrast the western belt which follows the eastern margin of the indo burma range and the andaman outer island arc broadly follows a zone of negative gravity anomalies here the ophiolites occur mainly as rootless subhorizontal bodies overlying eocene oligocene flyschoid sediments two sets of ophiolites that were accreted during the early cretaceous and mid eocene are juxtaposed in this belt these are inferred to be westward propagated nappes from the eastern belt emplaced during the late oligocene collision between the burmese and indo burma andaman microcontinents ophiolite occurrences in the andaman islands belong to the western belt and are generally interpreted as upthrust oceanic crust accreted due to prolonged subduction activity to the west of the island arc this phase of subduction began only in the late miocene and thus could not have produced the ophiolitic rocks which were accreted in the late early eocene

the status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

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

understanding himalayan erosion and the significance of the nicobar fan a holistic view of the bengal nicobar fan system requires sampling the full sedimentary section of the nicobar fan which was achieved for the first time by international ocean discovery program iodp expedition 362 west of north sumatra we identified a distinct rise in sediment accumulation rate sar beginning 9 5 ma and reaching 250 350 m myr in the 9 5 2 ma interval which equal or far exceed rates on the bengal fan at similar latitudes this marked rise in sar and a constant himalayan derived provenance necessitates a major restructuring of sediment routing in the bengal nicobar submarine fan this coincides with the inversion of the eastern himalayan shillong plateau and encroachment of the west propagating indo burmese wedge which reduced continental accommodation space and increased sediment supply directly to the fan our results challenge a commonly held view that changes in sediment flux seen in the bengal nicobar submarine fan were caused by discrete tectonic or climatic events acting on the himalayan tibetan plateau instead an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation

surface deformation and subsurface slip of the 28 march 1999 mw 6 4 west himalayan chamoli earthquake from insar analysis 1 we report insar observations of coseismic ground deformation caused by the mw6 4 chamoli earthquake of march 28 1999 in the western himalaya near the surface trace of the main central thrust analysis of ers1 2 sar data from ascending and descending tracks reveals surface deformation in a region 30 km by 40 km with a maximum coseismic uplift of 60 mm assuming that the rupture occurred as a planar uniform slip dislocation in an elastic half space reveals slip of 0 55 0 1 m strike of n300 e 15 dip with fault dimensions of 13 3 km along strike and 10 3 km downdip the corresponding moment magnitude is mw 6 2 lower than that derived from seismological methods with the exception of the recent 2005 kashmir earthquake the chamoli earthquake studied in this paper is the only himalayan earthquake for which surface deformation data are available from directly above the epicenter

estimates of interseismic deformation in northeast india from gps measurements abstract estimates of interseismic deformation in northeastern india based on gps measurements at eight permanent stations 2003 2006 and six campaign sites 1997 2006 are presented here the euler pole of rotation of indian tectonic plate in itrf2000 determined from the present data set is located at 51 7 0 5 n 15 1 1 5 e with angular velocity of 0 469 0 01 myr 1 the results show that there is a statistically insignificant present day active deformation within the shillong plateau and in the foreland spur north of the plateau in the brahmaputra valley convergence rate of the northeastern gps sites with respect to the igs station lhasa along baselines that are normal to the himalayan arc in this region is 16 0 5 mm yr this represents the arc normal indo eurasian convergence rate across the northeastern boundary similar to arc normal convergence rates determined in central nepal along the himalayan arc however unlike central nepal in the arunachal himalaya the 16 mm yr shortening is distributed between the lesser as well as higher and tethyan himalayas baselines between sites on the indo burmese fold and thrust belt ibftb and shillong plateau indicate variations in the shortening rate from 1 5 mm yr on the tripura mizoram salient trs south of the plateau to 6 mm yr in the imphal recess ir to the east and 8 mm yr in naga salient ns to the northeast this suggests that the deformation in the ibftb is segmented into n s blocks along e w transverse zones exhibiting dextral slip between ns ir and sinistral slip between ir and trs baselines between the ibftb sites also show 10 0 6 mm yr convergence pointing to the existence of an active transverse zone between aizawl and imphal

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

crustal seismic anisotropy beneath shillong plateau assam valley in north east india shear wave splitting analysis using local earthquakes abstract we present crustal anisotropy estimates constrained by shear wave splitting sws analysis using local earthquakes in the shillong plateau and assam valley area north east india ne india region splitting parameters are determined using an automated cross correlation cc method we located 330 earthquakes recorded by 17 broadband seismic stations during 2001 2014 in the study area out of these 330 events seismograms of 163 events are selected for the sws analysis relatively small average delay times 0 039 0 084 s indicate existence of moderate crack density in the crust below the study area it is found that fast polarization directions vary from station to station depending on the regional stress system as well as geological conditions the spatial pattern of crustal anisotropy in the area is controlled mostly by tectonic movement of the indian plate towards ne presence of several e w and n s trending active faults in the area also play an important role on the observed pattern of crustal anisotropy

damage due to the mw 7 7 kutch india earthquake of 2001 abstract this paper presents a study of the damage due to the m w 7 6 7 7 intraplate kutch earthquake of 26 january 2001 it was a powerful earthquake with a high stress drop of about 20 mpa aftershocks up to m 4 have continued for 2 5 years the distribution of early aftershocks indicates a rupture plane of 20 25 km radius at depths of 10 45 km along an e w trending and south dipping hidden fault situated approximately 25 km north of the kutch mainland fault the moment tensor solution determined from regional broadband data indicates reverse motion along a south dipping by 47 fault the earthquake is the largest event in india in the last 50 years and the most destructive in the recorded history in terms of socioeconomic losses with 13 819 deaths including 14 in pakistan collapse severe damage of over a million houses and us 10 billion economic loss surface faulting was not observed however intense land deformations have been observed in a 40 20 km meizoseismal area these include lateral spreading ground uplifts about a meter ground slumping and deep cracks liquefaction with ejection of sand and copious water was widespread in the banni grassland rann areas salt plains along rivers and also in the coastal areas up to 200 km distance from the epicenter in areas of intensity vii to x stray incidences of liquefaction have occurred up to distances of at least 300 km for the first time in india multistory buildings have been destroyed damaged by an earthquake the maximum acceleration is inferred to be 700 cm s 2 and intensities are 1 3 units higher in soil covered areas than expected from the decay rate of acceleration for hard rock

3d geological modelling and geotechnical characteristics of phnom penh subsoils in cambodia abstract subsoil characterisation is an important task in both geotechnical and geological engineering in recent years a number of subsurface investigations have increased as a result of infrastructure construction in phnom penh city in cambodia although there had been investigations on the geotechnical characteristics of the subsoils understanding soil characteristics and 3d modelling of geological structures of phnom penh subsoils have not yet been tackled this research aims to conduct a 3d geological modelling by means of commercial software called groundwater modelling system gms as well as provide an insight into the geotechnical properties of phnom penh subsoils over 1 200 soil boring log data were employed to build the 3d soil stratigraphic system in phnom penh city the engineering properties of the phnom penh subsoils such as physical properties i e grain size distribution and atterberg limits and engineering properties i e compressibility undrained shear strength and internal friction angle have been highlighted in this paper finally the analysis results of geotechnical data can be used to construct some useful empirical correlations such as the relationship of the undrained shear strength to the spt n value

eastern mediterranean tectonics and tsunami hazard inferred from the ad 365 earthquake historical accounts describe an earthquake and tsunami on 21 july ad 365 that destroyed cities and drowned thousands of people in coastal regions from the nile delta to modern day dubrovnik the location and tectonic setting of this earthquake have been uncertain until now here we present evidence from radiocarbon data and field observations that western crete was lifted above sea level by up to 10 m synchronously with the ad 365 earthquake the distribution of uplift combined with observations of present day seismicity suggest that this earthquake occurred not on the subduction interface beneath crete but on a fault dipping at about 30 within the overriding plate calculations of tsunami propagation show that the uplift of the sea floor associated with such an earthquake would have generated a damaging tsunami through much of the eastern mediterranean measurement of the present rate of crustal shortening near crete yields an estimate of 5 000 yr for the repeat time of tsunamigenic events on this single fault in western crete but if the same process takes place along the entire hellenic subduction zone such events may occur approximately once every 800 yr in the year ad 365 an earthquake and tsunami destroyed much of the eastern mediterranean coastal regions the distribution of uplift at the time suggests that the earthquake occurred on a fault within the overriding plate at the subduction zone beneath crete and not on the subduction interface itself

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

simple approach to dynamics of entrainment interface layers and cloud holes in stratocumulus clouds this paper presents simple conceptual models of phenomena essential to the entrainment processes at the top of inversion capped stratocumulus clouds mixing processes lead to formation of an interface layer between pure cloud and the top of the inversion as well as downdraughts of dry air that penetrate the cloud interior and form cloud holes spaces that have reduced liquid water content or are completely devoid of it these processes are illustrated with one dimensional models permitting estimation of interface layer thickness as well as the cloud hole parameters and conditions of formation results are in rough agreement with observations copyright 2009 royal meteorological society

the 2016 mw 6 5 nura earthquake in the trans alai range northern pamir possible rupture on a back thrust fault constrained by sentinel 1a radar interferometry abstract on june 26 2016 an mw 6 5 earthquake occurred in the trans alai range about 37 km west of a 2008 mw 6 7 earthquake both near the town of nura china we use interferometric synthetic aperture radar insar images from sentinel 1a to image coseismic surface displacements from the most recent nura earthquake we invert the interferograms for both fault geometry and slip distribution we find that the insar data can be described by either a steeply north dipping fault or shallowly south dipping fault although only when slip on the south dipping fault is constrained to be blind the insar misfit for the north dipping fault plane is slightly lower and the north dipping fault better describes the gradient of deformation in the near field we prefer the north dipping fault geometry 67 dip and 266 strike as the source for the 2016 nura earthquake not only due to the better fit to the insar but the resulting coseismic slip model is more consistent with the focal mechanism solutions and regional seismogenic thickness however it is important to reiterate that the insar data cannot uniquely determine which model is the definitive representation of the nura earthquake the maximum inferred coseismic slip is about 0 75 m at 14 km depth beneath the trans alai range with the largest slip in the depth range of about 8 20 km slip does not reach the surface and is dominated by thrust displacement with a right lateral strike slip component the geodetic moment of our preferred slip model is 6 61 1018 n m equivalent to mw 6 5 the surface projection of our preferred fault is to the south of the pamir frontal thrust pft and we argue it is a back thrust of the listric pft suggesting that the trans alai range is a pop up structure

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

lithological control on the geomorphic evolution of the shillong plateau in northeast india abstract the shillong plateau in northeast india is a block of raised topography in the himalayan foreland which consists of crystalline basement rocks partially covered by a cretaceous to miocene sedimentary succession it is dominated by a mature low relief landscape surrounded by high relief fluvially dissected margins particularly along its southern flank which is bounded by the dauki thrust fault we use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat we show that initiation of the wave of incision does not require surface rupture on the dauki fault or an increase in fault slip rate at the end of the miocene as suggested by previous studies we propose that incision has been spatially controlled by the slope of the basement palaeosurface likely moderated by an incision threshold river profiles in the shillong plateau cannot be interpreted as simple records of surface uplift the observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility such dynamics have implications for fluvial geomorphology highlighting that near horizontal lithological contacts can strongly influence river profiles and topography even when no longer physically preserved the topography of the northern shillong plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes consistent with its surface exposure during late cretaceous times

homogenization of earthquake catalog for northeast india and adjoining region a catalog for northeast india and the adjoining region for the period 1897 2009 with 4 497 earthquakes events is compiled for homogenization to moment magnitude m w gcmt in the magnitude range 3 8 7 relations for conversion of m b and m s magnitudes to m w gcmt are derived using three different methods namely linear standard regression inverted standard regression isr and orthogonal standard regression osr for different magnitude ranges based on events data for the catalog period 1976 2006 the osr relations for m s to m w gcmt conversion derived in this paper have significantly lower errors in regression parameters compared to the relations reported in other studies since the number of events with magnitude 7 for this region is scanty we therefore considered whole india region to obtain the regression relationships between m w gcmt and m s isc a relationship between m w gcmt and m w neic is also obtained based on 17 events for the range 5 2 magnitude 6 6 a unified homogeneous catalog prepared using the conversion relations derived in this paper can serve as a reference catalog for seismic hazard assessment studies in northeast india and the adjoining region

block and fault dynamics modelling of the himalayan frontal arc implications for seismic cycle slip deficit and great earthquakes abstract a numerical block and fault dynamics model bafd of the himalayan frontal arc india is developed to understand the long term patterns of strain accumulation and occurrences of great earthquakes in the himalaya the morphostructural scheme outlines twelve major crustal blocks and external driving motions are prescribed using gps data the bafd model reproduces essential features of the geodynamics and seismicity of the himalayan frontal arc the locations of the large synthetic earthquakes and their maximum magnitudes are consistent with the information available from the instrumental and historical earthquake catalogues we model the evolution of the slip deficit and seismic cycles for different sections across the himalaya frontal arc the modelled seismic cycles are found to be varying from 700 to 2100 years and are in good agreement with the return periods estimates from the recent paleoseismological studies we notice that the accumulation of the slip deficit depends not only on the rate of shortening rheology and structure but also on the dynamics of the confining crustal blocks further we observe that tectonic motions of the shillong plateau and assam basin microplates play a significant role in controlling the seismicity patterns of the bhutan block which resulted in the decreased seismic activity and increased rate of aseismic displacement thus we infer that the regional seismicity patterns are a consequence of dynamics of the entire regional fault and block system rather than dynamics of individual fault our bafd modelling predicts the maximum earthquake hazard associated with future large great earthquakes for the central himalayan gap region which lies between the 1905 kangra and the 2015 gorkha earthquake ruptures but relatively less hazard in kashmir and assam

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

possible influence of subducting ridges on the himalayan arc and on the ruptures of great and major himalayan earthquakes abstract subduction of bathymetric features such as ridges seamounts fractures etc on the subducting plate influences the arc morphology and earthquake ruptures we analyse their effect on the development of the arcuate shape of the himalayan arc and on the ruptures of great and major himalayan earthquakes besides the two most prominent ridges in the indian ocean namely the chagos laccadive deccan ridge and the 90 e ridge which are assumed to extend up to the himalayan arc at least three major subsurface ridges have been mapped on the underthrusting indian plate under the indo gangetic plains it appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the himalayan arc the interaction and subduction of the other subsurface ridges probably influenced the himalayan arc morphology by causing a localised cusp in the frontal topography also these ridges probably acted as barriers to the ruptures of the major and great himalayan earthquakes

evidence for right lateral strike slip environment in the kutch basin of northwestern india from moment tensor inversion studies abstract the kutch region located in northwestern part of india is an ancient rift basin that was active until cretaceous period the region falls close to the india arabia and the india eurasia plate boundaries and has experienced devastating earthquakes in the past namely the 1819 allah bund earthquake the 1956 anjar earthquake and the 2001 bhuj earthquake to understand the tectonics of this region with respect to the adjacent plate boundaries we invert seismic waveform data of 11 earthquakes in this region recorded by a network of the institute of seismological research isr during 2007 2009 the study yields focal mechanism solutions of reverse fault and strike slip type mechanism the inferred fault planes correlate well with the local trends of the known tectonic faults while the principal stress directions derived from stress inversion based on a linearized least squares approach trend agreeably with the ambient stress field directions a consistently right lateral sense of shear is found on all the local faults as derived from each of the matching planes of the focal mechanism solutions computed in the present study it is inferred that in the kutch region a right lateral strike slip environment prevails along predominantly ew to nw se oriented deep seated pre existing faults in an otherwise compressive stress regime this in conjunction with the left lateral movements along the girnar mountain in southern saurashtra inferred from previous studies indicates a westward escape of the kutch saurashtra block as a consequence of the northward collision of the indian plate with respect to the eurasian landmass

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

seismic event of the dauki fault in 16th century confirmed by trench investigation at gabrakhari village haluaghat mymensingh bangladesh abstract the dauki fault which is inferred to go through the southern margin of the shillong plateau is an e w trending reverse one inclined towards the north the dauki fault was believed to be active during the late quaternary time by the geomorphic features of the shillong plateau the gravity anomaly data and uplifted tertiary and quaternary deposits on the southern foothills of the shillong plateau however previous studies did not show any specific evidence that the dauki fault is active since active fault survey such as satellite photo interpretation and trench investigation was not performed hitherto we carried out corona satellite photo interpretation and trench investigation across the dauki fault at gabrakhari village for the first time consequently small fault strands and clear unconformity accompanied with faulting were confirmed in the trench however these small fault strands may be secondary ones branched from a main fault which may be concealed under the trench the time of seismic event which is inferred from unconformity is dated back to a d 1500 1630 the seismic event during a d 1500 1630 may correspond with the 1548 earthquake which is currently a first recorded large earthquake in bangladesh furthermore sand dikes due to paleo liquefaction which reach near the ground surface were confirmed at the trench according to radiocarbon age the sand dikes were formed during the 1897 ms 8 0 great assam earthquake

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

a major intraplate normal faulting earthquake the 1739 yinchuan event in northern china a large continental normal faulting earthquake occurred in the yinchuan graben in northern china on 3 january 1739 this event is of significant interest for two reasons first it has been suggested on the basis of historical records of shaking that this was a magnitude 8 0 event if this is true the 1739 earthquake would be one of the largest continental normal faulting earthquakes ever recorded and this should prompt us to consider the factors contributing to its unusual size second there has been some debate in the literature about the causative structure with at least three different faults posited as the seismogenic source we use five 14 c ages to date a series of scarps on the east helanshan fault which lies to the west of the graben and bracket the scarp formation to the last 350 years we use high resolution stereo imagery from the pleiades satellites to build a 1 to 2 m resolution dem of the fault and combine this with field observations to map the east helanshan rupture in detail we then construct a throw distribution determine the slip vector azimuth and reassess the earthquake magnitude using simple scaling relationships we conclude that the 1739 earthquake occurred on the east helanshan fault had a rupture length of approximately 87 km a maximum throw of 5 1 m and an average throw of 3 0 m yielding a best estimate magnitude in the range m w 7 1 to m w 7 6 we suggest that the previous magnitude estimates are biased by enhanced shaking of the sediments within the yinchuan graben

active transverse faulting within underthrust indian crust beneath the sikkim himalaya summary deep focus earthquakes within the underthrust indian lower crust beneath the himalaya occur in very specific regions and have distinct source characteristics the study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust indian plate and its influence on the active deformation of the overlying himalayan wedge one of the most significant regions of these deep focus earthquakes is beneath the sikkim and bhutan himalaya we study the source characteristics of the 2011 september 18 mw 6 9 deep focus sikkim main shock and its major aftershocks using global regional and local waveform data we determined the focal mechanism of the main shock using moment tensor inversion of global p and sh waveforms and ascertained the earthquake fault plane using rupture directivity from regional p wave spectra the main shock originated at 53 4 km depth and ruptured at least 20 km thickness of the underthrust indian lower crust faulting occurred on a near vertical dextral strike slip fault oriented nw se strike 127 d ip 81 and rake 167 oblique to the local strike of the himalayan arc the rupture initiated from the se end of the fault and propagated to the northwest the main shock was followed by 20 small to moderate aftershocks mb 3 0 which we relocated using phase arrival times we computed the focal mechanisms of the larger ones mb 3 5 using local waveform inversion we find that all aftershocks originated se of the main shock between depths of 12 and 50 km and have dominantly strike slip mechanisms our results combined with the source mechanisms of earthquakes from previous studies reveals that the entire underthrust indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the sikkim and bhutan himalaya these active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating microplates in northeast india observed from gps geodesy

coherent structures in large eddy simulations of a nonprecipitating stratocumulus topped boundary layer abstractthe properties of coherent convective structures are analyzed in a nonprecipitating marine nocturnal stratocumulus topped boundary layer stbl with a series of high resolution large eddy simulations less a new classification method based on octant analysis using vertical velocity and two passive scalars is introduced to systematically define convective structures in both the cloudy and the cloud free regions it is therefore possible to detect and track updrafts downdrafts and their turbulent shells both ascending and subsiding together with the entraining air from the inversion layer or the free troposphere the geometrical and thermodynamical characteristics e g areal fraction temperature liquid and total water mixing ratio buoyancy of those structures are then accurately described and particular attention is given to their respective contributions to the turbulent transport of mass heat and moisture it is shown that updrafts downdrafts and entrainment are equally important

timing and return period of major palaeoseismic events in the shillong plateau india abstract the close temporal occurrence of four great earthquakes in the past century including the great assam earthquake of 1897 in the shillong plateau necessitated examination of the palaeoseismicity of the region the results from such investigation would definitely aid in addressing the problem of the earthquake hazard evaluation more realistically our recent palaeoseismological study in the shillong plateau has led us to identify and provide geological evidence for large major earthquakes and estimate the probable recurrence period of such violent earthquakes in parts of the shillong plateau and the adjoining brahmaputra valley trenching along the krishnai river a tributary of the river brahmaputra has unravelled very conspicuous and significant earthquake induced signatures in the alluvial deposits of the valley the geological evidence includes 1 palaeoliquefaction features like sand dykes and sand blows 2 deformational features like tilted beds 3 fractures and syndepositional deformational features like flame structures caused by coeval seismic events chronological constraints of the past large major earthquakes are provided from upper and lower radiocarbon age bounds in the case of the palaeoliquefaction features and the coeval timing of the palaeoseismic events is obtained from the radiocarbon dating of the organic material associated with the deformed horizon as well as buried tree trunks observed wide distances apart our palaeoseismic measurements which are the first from the area indicate that the shillong plateau has been struck by large major earthquakes around 500 150 1100 150 and 1500 150 yr bp in addition to the well known great seismic event of 1897 thereby the 14 c dates indicate a recurrence period of the order of 500 yr for large earthquakes in the shillong plateau

seismic site classification and empirical correlation between standard penetration test n value and shear wave velocity for guwahati based on thorough subsoil investigation data guwahati the gateway of india in the northeast is a large business and development center past seismic scenarios suggest moderate to significant effects of regional earthquakes eqs in guwahati in terms of liquefaction as well as building damages considering the role of local soil in amplifying eq generated ground motions and controlling surface damages present study attempts seismic site classification of subsoil of guwahati subsoil is explored based on 43 geophysical tests and 244 borelogs gathered from different resources based on the borehole data 4 numbers of 2d cross sections are developed from different parts of guwahati clearly indicating that a majority of the locations are composed of clay of intermediate to high plasticity while at specific locations only layers of sand are found at selective depths further seismic site classification based on 30 m average spt n suggests that a major part of guwahati falls under seismic site class ssc d such as balaji temple and airport however assam zoo pan bazaar iit campus dhol gobinda and maligaon show ssc e clearly indicating the presence of soft soil deposits at these locations similar site classification is also attempted from masw test based 30 m average shear wave velocity vs30 vs30 based site classification also categorizes most of guwahati under ssc d however there are locations in the southern part of guwahati which belong to ssc c as well mismatch in ssc based on two different test findings for indian soil found here are consistent with previous studies further three empirical correlations based on both spt n and vs profiles at 22 test locations are developed for 1 clayey 2 sandy and 3 all soil types proposed correlation for all soil types is validated graphically and is found closely matching with similar correlations for turkey and lucknow

the underthrusting indian crust and its role in collision dynamics of the eastern himalaya in bhutan insights from receiver function imaging most of the convergence rate between the indian and eurasian plate is assumed to be absorbed along a major basal thrust beneath the himalaya the main himalayan thrust mht deformation along this basal thrust in combination with frontal accretion results in the formation of the upper crustal fold thrust belt the role of the underthrusting indian crust and its impact on the long term growth of the himalaya are only poorly understood partly due to the lack of high resolution seismic images of the crust to improve the imaging of lithospheric structures we developed an 3 d migration scheme for receiver functions using seismic data from the temporary gansser network in bhutan extending the 2 d high frequency ray approximation and common conversion point stacking to 3 d including linear phase weighting and a quality assessment we reveal significant along strike differences in the lithospheric structure beneath bhutan in western bhutan the moho geometry shows an increased dip south of the higher himalaya reaching almost 70 km depth thereafter whereas in eastern bhutan the moho is almost subhorizontal at 50 km depth across our network the appearance of distinct listric structures beneath the mht indicates intra crustal deformation up to crustal imbrication down to the lower crust we propose that these variations in the crustal thickness and in intra crustal structures influence the upper crustal kinematics of the bhutan himalayan orogeny and are primarily driven by an indian mantle slab northwest of bhutan and its absence northeast of bhutan

comment on interpreting the style of faulting and paleoseismicity associated with the 1897 shillong northeast india earthquake by c p rajendran et al

speleogenesis of the world s longest cave in hybrid arenites krem puri meghalaya india abstract meghalaya nw india is one of the regions in the world with highest recorded rainfalls because of these peculiar climatic conditions and intense solutional weathering karstic caves are widely reported in the numerous limestone areas of this part of india likewise the extremely high rainfall and the tropical monsoon climate have fostered the formation of several caves in silicate rich lithologies the most important of these caves is krem puri over 25 5 km long characterized by a fracture controlled labyrinth forming a bi dimensional maze with mostly phreatic morphologies and some recent still active vadose entrenchments a detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession xrf elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate rich layers geochemical analyses carried out on water flowing in the cave drips streams have shown that carbonate dissolution is the dominant chemical weathering process contributing to krem puri water solutes whereas the dissolution of quartz feldspars and clays play only a secondary role in rock weathering the morphology of the cave points to a lithology guided dissolution of the carbonate rich layers and piping of the residual quartz sand in phreatic conditions when the surface river network was not yet evolved and the water table was at the level of or higher than the cave this phreatic speleogenetic phase predates the start of river network entrenchment that started not earlier 5 ma ago and probably accelerated around 3 5 ma which deactivated the karst maze with a low denudation rate of 0 15 mm yr this active phreatic cave level which currently lies beneath a more or less 120 m overburden was originally covered by a sedimentary sequence at least 1 2 km thick after its decoupling from the underground drainage system denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits this caused the formation of the vadose entrenchments probably