on the origin of the drumlins of the south german alpine foreland ii the sediments underneath abstract the drumlins of the south german alpine foreland have certain characteristics a distinctly limited occurrence within the formerly glaciated areas a well defined time of formation and a typical rather compact shape the character of the sediments of which they are composed and the relation between drumlin sediments and drumlin form have been a matter of discussion for more than 80 years due to the rareness of good exposures in drumlins a few individual drumlins have become better known during the past decades because they overlie interglacial deposits the observations made at these sites allow the following statements 1 1 the drumlins normally consist of sediments of the last wurm glaciation only in exceptional cases do older pleistocene strata or the pre pleistocene substratum molasse sandstone form part of the drumlin body these exceptions are important in so far as they prove that drumlin formation in principle was an erosional process in the area 2 2 the sediments are mainly till but also a gravel till sequence is not uncommon the rare interglacial beds are mainly lake deposits 3 3 the thickness of the sediments on top of the interglacial deposits varies considerably from some metres to several decametres even underneath a thin cover of younger sediments the fine grained and therefore in principle easily deformable interglacial deposits are glaciotectonically undisturbed altogether these observations allow the conclusion that 1 the process of drumlin formation shaped the glacier bed by highly selective but locally vigorous glacial erosion and 2 this process did obviously not or only poorly affect the sediments below the ice sediment interface even though it was composed of readily deformable material therefore the question is not whether there was erosion or not but why there was not much more erosion and disturbance it is suggested that the answer is likely to be found in the unevenness of the decaying permafrost table at the end of the wurm pleniglacial

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

the 1994 northridge earthquake 3 d crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms a detailed 3 d p wave velocity structure of the crust in the epicentral area of the 17 january 1994 northridge earthquake is determined by using 104 709 arrival times from 1673 northridge aftershocks and 2948 other local earthquakes a test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3 d model are accurate to about 2 km we found that regions with high aftershock activity are generally associated with faster p wave velocities the velocity is high around the main south dipping fault of the 1994 northridge earthquake and the north dipping fault of the 1971 san fernando earthquake a linear distribution of strike slip aftershocks was found along a ne sw boundary between high velocity and low velocity structures to the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low velocity area while to the east aftershocks with thrust mechanisms occurred in a high velocity area these observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the transverse ranges a better understanding of these features is important for long term seismic hazard assessment in the los angeles area

moho geometry and upper mantle images of northeast india 1 images of the crust and mantle beneath northeast india obtained by 2d migration of 1000 broadband p receiver functions clearly trace a northward dipping moho from the himalayan foredeep reaching depths up to 50 km further north beneath the himalayan convergence zone also these images reveal presence of largely coherent 410 km and 660 km discontinuities that conform to the iasp91 model marginal variations in the depth of the 410 km interface are observed that appear region specific the thickness of the mantle transition zone does not deviate significantly from a global average of 250 km interestingly our results reveal consistent presence of a signal from an interface around 300 km origin of such a boundary known as x discontinuity and unrelated to the lehmann discontinuity is discussed possible presence of the x discontinuity from the indian region is reported here for the first time

the effects of vegetative ash on infiltration capacity sediment transport and the generation of progressively bulked debris flows abstract through the alteration of the physical characteristics of a landscape such as the destruction of vegetation and the formation of a hydrophobic layer a fire can dramatically amplify erosion rates on the basis of field observations it has been proposed that the deposition of a layer of ash on the ground surface can enhance the erosion of mountainous terrain by surface runoff and might even be a necessary condition for the generation of progressively bulked debris flows in this study a flume was constructed to investigate the role of ash in increasing both the volume and the transport capacity of runoff the experiments demonstrated that the presence of ash on the soil surface reduces the ability of flowing water to infiltrate this effect is even greater when the ash has been pre wetted in addition the ability of ash slurries to infiltrate decreases with increasing ash concentration the results also indicate that the transport capacity of runoff is enhanced by the incorporation of ash into the flow because of the increased fluid density however the addition of ash reduces the boundary reynolds number such that at high ash concentrations and with fine grained sediment sediment transport declines as the flow becomes hydraulically smooth the experimental results were also used to evaluate the ability of steep flow fronts a common characteristic of debris flows and flash floods to increase sediment transport rates finally it is proposed that ash slurries may evolve into progressively bulked debris flows through a positive feedback between fluid density transport capacity and erosivity

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

mapping of shallow three dimensional variations of p wave velocity in garhwal himalaya abstract a set of short aperture seismic arrays was operated in the garhwal himalaya close to the main central thrust and recorded a large number of small local earthquakes this study pertains to the inversion of the body wave arrival time data of these earthquakes to construct a seismic velocity model for the region the analysis indicates a systematic variation in the p wave velocities of the upper crustal rocks we estimate i significantly lower seismic velocities within the middle lesser garhwal himalaya and ii higher seismic velocities in the interface zone between the middle lesser and higher garhwal himalaya seismic activity is mostly confined to a relatively narrow north east dipping zone in the upper 4 km of the crust characterized by a relatively higher p wave velocity this active seismicity represents reverse thrusting along steep north easterly dipping parallel slip surfaces within this zone forming a ramp in the crystalline formations of higher himalaya the lower velocity zone exhibits a low level of seismicity but appears to be associated with an increased landslide hazard

erosion caused by hyperconcentrated flow on the loess plateau of china abstract hyperconcentrated flows are widely observed in the middle yellow river basin they occur as a result of erosion but once they form on slopes they operate as a strong eroding and sediment carrying force and play an important role in drainage basin erosion and sediment production processes since the energy expenditure is less and sediment carrying capacity is greater hyperconcentrated flows have very high eroding and transporting abilities there exists a strong coupling relation between hyperconcentrated flow transport and gravitational erosion which ensures a rapid removal of mass wasted materials hyperconcentrated flows are an important factor responsible for the high intensity erosion and sediment yield in the middle yellow river basin many previous studies have indicated that the sediment delivery ratio on the loess plateau approaches 1 this can be explained by the action of hyperconcentrated flows as hyperconcentrated flows exhibit some particular behaviors in erosion and sediment production processes the present study suggests that the erosion by hyperconcentrated flows be regarded as a special type of erosion accordingly some countermeasures are proposed to control hyperconcentrated flows and the resulting erosion

predicting gully rejuvenation after wildfire using remotely sensed burn severity data abstract the loss of surface vegetation and reduced infiltration caused by wildfires can trigger gully rejuvenation resulting in damage to downstream aquatic resources and risk to human life and property we developed a spatially explicit metric of burn severity the burn severity distribution index bsdi and tested its ability to predict post fire gully rejuvenation in 1st and 2nd order basins burned in the 2000 valley complex fires in the sapphire mountains of western montana the bsdi was derived from burn severity data interpreted from landsat 7 satellite imagery using the normalized burn ratio nbr method and ranged from 0 0 for completely unburned basins to 4 0 for basins burned entirely at high severity in july 2001 rainstorms with peak 30 minute intensities of up to 17 mm h 1 triggered gully rejuvenation in 66 of the 171 basins examined the frequency of gully rejuvenation was higher in basins with higher bsdi values increasing from zero for basins with a bsdi less than 1 3 to 67 for basins with a bsdi greater than 3 0 binary logistic regression indicated that bsdi was a more significant predictor of gully rejuvenation than basin morphometric variables the absence of gully rejuvenation in several basins with a high bsdi was attributed to low gradient dense riparian vegetation or concentration of high burn severity at lower elevations in the basin the presence of gully rejuvenation in several basins with a low bsdi was associated with false negative nbr classification errors in northwest aspects and concentration of severe burn impacts in the drainage headslopes bsdi is a useful metric for predicting gully rejuvenation after wildfire the use of the bsdi in burned area emergency response team assessments could improve the planning implementation and monitoring of burned area recovery treatments

the effect of a low viscosity layer on convection in the mantle summary in this paper we attempt to find a solution of the following problem it appears reasonable to expect that if thermal convection occurs in the earth s mantle it may also occur within the moon and mars the dimensions of these latter two bodies are comparable to the thickness of the earth s mantle presumably the amount of radioactive heat generated per unit mass is similar in all three bodies yet the surface morphology of the earth which many scientists believe arises ultimately from mantle convection differs markedly from that of the moon or mars the explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low viscosity or low creep strength layer it is assumed that the low velocity layer of the mantle is such a low creep strength layer a low viscosity layer changes the amount of coupling between the outer crust and mantle convection the crust is coupled if mantle convection produces stresses in the crust which are large enough to deform it plastically the crust is decoupled from the mantle if these stresses are insufficient to produce plastic deformation the theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer the analysis is similar to that developed earlier for the calculation of stresses within the mantle we find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents if the low creep strength layer lies close to the surface the outer crust is decoupled from the interior according to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle if the temperature profile were kept constant but the pressure reduced by changing g the gravitational acceleration the low velocity zone would be moved to a level closer to the surface it is proposed that because of this interdependence between temperature pressure and depth of the low velocity layer the low velocity layers in mars and the moon if such layers exist in these bodies lie much closer to the surface than does the corresponding layer in the earth hence the outer crusts of mars and the moon are more nearly decoupled from the interior than is the earth s crust

thermal convection and the origin of ice streams ice streams are a fact of ice sheet dynamics draining up to 90 of the ice thermal convection in ice below the density inversion is a speculation an attempt is made to meld the two in such a way that the speculation becomes an explanation for the fact

determining rheological parameters of debris flow material abstract a 2 0 m diameter steel 30 inverted cone and plate viscometer rheometer was designed constructed and used to test the behaviour of coarse grained debris flow materials a 1 5 scale model machine was also constructed and used to test the internal flow dynamics of the viscometer rheometer and to obtain results for fluids grain fluid mixtures and debris flow fines for fluids and grain fluid mixes our results were similar to those obtained earlier using standard viscometric systems derived rheological parameters for debris flow materials and clay slurries agreed well with those determined from calculation direct measurement and field observation apparent viscosities were shear rate dependent extremely sensitive to water content and as high as 6000 pa s instantaneous stresses within shearing debris flow material varied over a wide range debris flow materials with bimodal grain size distributions had a dilatant plastic rheology those with a low content of coarse material and unimodal grain size distribution or exceptionally high fines content had a plastic or viscoplastic rheology

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

seismic b value and the assessment of ambient stress in northeast india seismicity data of northeast india recorded between 1986 and 1999 by a local network are analysed for estimation of b values based on the obtained values viz low b 0 5 moderate 0 5 0 7 the study area is classified into different seismic domains an assessment of stress level is also carried out in identifying seismic domains seismic activities though mostly confined in some sectors are presumably triggered by mutual interaction of the shillong plateau mikir hills indo burman ranges and the easternmost part of the himalayas and the contributions from deep seated fractures cannot be ignored the results resemble the seismic character of a foreland setting adjacent to a convergent margin the b values estimated for 240 square grids of dimension 0 6 0 6 over five seismic domains indicate wide variation an analysis of cumulative seismic moment release m o in different layers also indicates an anomaly in reference to the total seismic energy budget of the five zones the lower b value and higher m o recorded at relatively lower depth 30 km towards the southwest of the study area might be associated with upward bulging of a strong lithosphere the bulging is perhaps regionally compensated by the downward flexing of the descending indian lithosphere beneath the upper assam area features unequivocally observed in any foreland setup towards the north and east of the study area random variations of in both b value and m o along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas

variation in drumlin orientation form and stratigraphy relating to successive ice flows in southern and central nova scotia abstract a strong correspondence exists between striation and drumlin long axes trends in nova scotia the variation in striation trends is largely due to successive ice flow events which inscribed many outcrops with sets of superimposed striations that can be correlated over broad areas of nova scotia a type of drumlin in the nova scotia fields is the lobate or reoriented drumlin which is a form with two principal axes parallel to ice movements established by striations and dispersal of distinctive erratics the internal structure of nova scotia drumlins is generally a layer cake of till sheets the most common situation is a grey core till formed by a southeastward ice flow across grey metasedimentary bedrock and a distinctly red mantle till formed by phases of southward to southwestward ice flow drumlins in the yarmouth field of southern nova scotia exhibit the greatest number of stacked units capped by a coarse textured till that represents deposition by a late stage ice mass centered inland nova scotia drumlins appear to have a multi phase origin the first and major drumlin forming ice flow phase was southeastward nucleating agents for initiation of these drumlins may have been preexisting tills or waterlain deposits individual and lobate drumlin axes imply a late southward drumlin forming ice flow phase parallel to the inferred ice flow trend for the mantle till units based on clast provenance some of these drumlins nucleated from preexisting southeast trending forms

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

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

thermal convection in ice sheets we look but do not see un critere de rayleigh generalise de la convection thermique dans les solides polycristallins chauffes par en dessous a ete developpe et applique a la convection dans les calottes glaciaires une analyse du profil de temperature a l aval du forage de byrd station suggere qu environ mille metres de glace en dessous du point d inversion de la densite entretiendrait la convection

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

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

pn velocity study using a temporary seismographic network in the shillong plateau northeast india abstract seismic p wave velocity pn of the uppermost mantle beneath the shillong plateau has been studied using a temporary seismographic network refracted pn arrivals of shallow distant earthquakes m 3 0 recorded at pairs of stations lying within 2 azimuthal differences with respect to a given epicenter are used in this study since the method is based on relative travel time differences the measurement is independent of origin time and location errors the average pn velocity is found to be 8 57 0 02 km sec which is higher than the average mantle velocity 8 13 km sec under other areas of the indian subcontinent the observed high pn velocity beneath the plateau may be attributed to the presence of high density material

the effective viscosity of slurries laden with vegetative ash abstract after a forest fire ash can blanket the soil surface to depths of tens of centimeters it has been proposed that the incorporation of vegetative ash into hillslope runoff can by increasing the flow s effective viscosity reduce the settling velocity of entrained particles and lead to the development of runoff generated debris flows to investigate how the addition of this material might affect the rheology of runoff in burned areas we measured and compared the effective viscosity of slurries composed of varying concentrations of ash and similarly sized mineral particles i e silt all the slurries were pseudoplastic i e shear thinning and at low weight concentrations 0 3 g g 1 the rheologies of ash and silt slurries were nearly identical at higher concentrations however their rheologies diverged significantly for example whereas the viscosity of the silt slurries increased monotonically with concentration the viscosity of the ash slurries decreased ash incorporated into silt slurries induced this same complex behavior indicating that ash alters the rheological properties of runoff in ways that would not be predicted simply based on its size in addition we found that at high concentrations the intensity of shear thinning in the ash slurries was greater than in the silt slurries finally we compared the settling velocities of small particles in clear water and ash slurries and conclude that the rheological characteristics of ashy runoff could promote runoff generated debris flows in burned landscapes

a morphometric analysis of gullies scoured by post fire progressively bulked debris flows in southwest montana usa in the fall of 2001 an intense thunderstorm in southwest montana triggered many debris flows in the burned area of sleeping child creek in most instances the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor the presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow in this contribution we present a morphometric analysis of six of the gullies to better understand this relatively understudied process we find that the locations of the rill heads and gully heads conform to slope area thresholds that are characteristic of erosion by overland flow our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area thus lending support to an assumption used in a recently proposed debris flow incision law finally the debris flow fans have been relatively unaltered since deposition suggesting that the valley may be currently aggrading while the hillslopes are being denuded

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

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

laboratory analysis of mudflow properties a rotational viscometer has been designed for laboratory measurements of the rheological properties of natural mudflow deposits in colorado the mudflow matrices comprised of silt and clay particles are sheared under temperature controlled conditions at volumetric sediment concentrations ranging from 0 10 0 45 this study stresses the importance of conducting rheological measurements at low rates of shear because 1 those are the conditions found in natural channels and 2 they avoid the slippage problems observed at large sediment concentrations at low rates of shear the bingham model is fitted to the measured rheograms and both the viscosity and yield stress increase exponentially with the sediment concentration of the fluid matrix both the yield stress and the viscosity increase by three orders of magnitude as the volumetric concentration of sediments in the fluid matrix changes from 10 to 40 the addition of sand particles does not significantly alter the rheological properties of the matrix unless the volumetric concentration of sands exceeds 0 20

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

the 1992 landers earthquake sequence earthquake occurrence and structural heterogeneities the june 28 1992 mw 7 3 landers earthquake occurred in the southeastern mojave desert california over 10 000 aftershocks of the earthquake were recorded by the caltech usgs southern california seismic network scsn in 1992 to investigate the relationship between complexities in the crustal structure and variations in seismicity we have used a tomographic method to invert 145 098 p wave arrival times from 3740 landers earthquake aftershocks and 1148 other events recorded by 60 permanent and temporary scsn stations we determined a detailed p wave tomographic image with a spatial resolution of about 5 km and relocated the hypocenters with the obtained 3 d velocity model the results show a correlation between seismicity patterns and velocity patterns and a tendency for regions rich in seismicity to be associated with higher velocities the higher velocity areas are considered to be strong and brittle parts of the fault zone which are apt to generate earthquakes in contrast low velocity areas are probably more ductile and weaker allowing aseismic slippage

drumlins and catastrophic subglacial floods shaw j kvill d and rains b 1989 drumlins and catastrophic subglacial floods in j menzies and j rose editors subglacial bedforms drumlins rogen moraine and associated subglacial bedforms sediment geol 62 177 202 a number of lines of evidence are shown to be consistent with drumlin formation by catastrophic meltwater floods drumlins are associated with landforms related to such floods and landscapes produced by outburst floods in extragiacial areas contain drumlin like landforms some drumlins which contain mainly residual materials are compared to similar forms covering a wide range of scales and are interpreted to be scour remnant ridges produced by the action of horseshoe vortices on subgiacial materials these vortices are a product of flow separation in turbulent fluids and this interpretation implies drumlin formation by meltwater other drumlin calied cavity fill drumlins have distinctive shapes that are analogous with infilled erosional marks produced by turbulent separated flows these drumlins are interpreted as the infills of cavities eroded upwards into glacier beds by broad subgiacial meltwater sheetflows the sedimentology and internal structure of the cavity fill drumlins indicate rapid deposition from hyperconcentrated flow clast shape surface texture and lithology are consistent with the short distance of giaciofhivial transport predicted by the cavity fill hypothesis estimates of flow discharge 0 6 6 x 107 m3 s total volume of flow 84 1 103 km3 and flow duration 16 162 days are given for the catastrophic flood proposed to have produced the livingstone like saskatchewan drumlin field subgiacially derived floods are used to explain the sudden drainage of extraglacial lakes and major depositional events in proglacial lakes and it is suggested that they might have contributed to the lid of cold freshwater over the north atlantic held to have caused a late glacial cooling event the flood hypothesis requires extensive subgiacial lakes to have occurred beneath the laurentide and fennoscandian ice sheets which if they existed must have influenced the form and behaviour of these major bodies of ice

the seismic b value and its correlation with bouguer gravity anomaly over the shillong plateau area tectonic implications abstract clues to the understanding of intra and inter plate variations in strength or stress state of the crust can be achieved through different lines of evidence and their mutual relationships among these parameters bouguer gravity anomalies and seismic b values have been widely accepted over several decades for evaluating the crustal character and stress regime the present study attempts a multivariate analysis for the shillong plateau using the bouguer gravity anomaly and the earthquake database and establishes a causal relationship between these parameters four seismic zones zones i iv with widely varying b values are delineated and an excellent correlation between the seismic b value and the bouguer gravity anomaly has been established for the plateau low b values characterize the southwestern part zone iv and a zone zone iii of intermediate b values separates the eastern and western parts of the plateau zones i and ii which have high b values positive bouguer anomaly values as high as 40 mgal a steep gradient in the bouguer anomaly map and low b values in the southwestern part of the plateau are interpreted as indicating a thinner crustal root uplifted moho and higher concentration of stress in comparison the negative bouguer anomaly values flat regional gradient in the bouguer anomaly map and intermediate to high b values in the northern part of the plateau are consistent with a comparatively thicker crustal root and lower concentration of stress with intermittent dissipation of energy through earthquake shocks further depth wise variation in the b value for different seismic zones delineated under this study allowed an appreciation of intra plateau variation in crustal thickness from 30 km in its southern part to 38 km in the northern part the high b values associated with the depth coinciding with lower crust indicate that the shillong plateau is supported by a strong lithosphere

a glaciofluvial origin for drumlins of the livingstone lake area saskatchewan the livingstone lake drumlin field of northern saskatchewan is on the athabasca plains it is draped over high ground underlain by the precambrian athabasca sandstone the drumlins occur in spindle parabolic and transverse asymmetrical forms a map of individual drumlins shows that features of similar form occur together the drumlins are related on a regional scale to even larger landforms there is also an arrangement of drumlins in streams parallel to the assumed flow direction these patterns illustrate spatial organisation of the drumlin forming flows the livingstone lake drumlins are closely related to tunnel valleys large crescentic scours and eskers exposed bedrock reveals widespread scalloping and sichelwannen an argument is made that drumlins are related to these glaciofluvial forms but were created at different times and by different flows than those forming small scale flutings and other lodgment features sediments within the drumlins are mainly primary water sorted deposits some of wh

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

fire related sedimentation events on alluvial fans yellowstone national park u s a abstract we document initiation and flow processes deposit facies and geomorphic effects of forest fire related sedimentation on small alluvial fans in yellowstone national park brief intense convective storm precipitation on steep basins burned in the 1988 fires produced sedimentation events involving a variety of depositional processes on fans over the course of all documented events flows on fans progressed from higher to lower sediment concentration events were often dominated by either debris flows or relatively sediment poor streamflow processes in some events however flows ranging a over wide spectrum of sediment concentration produced significant fan deposits debris flows were generated by progressive sediment bulking involving pervasive surface runoff and rill erosion on st ep upper basin slopes followed by deep incision as flows progressed down channels debris flow deposits show a marked decline in thickness and coarse gravel content downfan often with extensive distal gravel poor facies we recognized a relatively minor percentage of noncohesive debris flow and hyperconcentrated flow facies with sorting and stratification intermediate between muddy debris flow and streamflow facies these were deposited where dilute flows bulked with coarse sediment by eroding channel alluvium or earlier deposits of the event below incised fan channels streamflows expanded as sheetfloods which prograded lower fans with distally fining deposits basins 3 km2 typically produced streamflow events on fans but sediment texture and availability on slopes and in channels are primary factors determining flow processes on fans of smaller basins burned soil surfaces provided abundant silt and clay for debris flow generation but because soil surface sediment was stripped and or compacted over time the lack of available fines resulted in dominance of streamflow processes in later events

flow of ice sheets in the vicinity of subglacial peaks although changes of surface slope across a subglacial peak can be explained at least semi quantitatively on a two dimensional basis in terms of gradients of longitudinal and shear stresses along a flow line we lack appreciation of flow in the third dimension which may tend to cause lower layers to flow around rather than over a sharp peak this paper collects and surveys evidence relevant to flow in the third dimension and discusses the processes involved particularly the evidence from radio echo layering and also discusses possible reasons for the lack of radio echo layers near the base of the antarctic ice sheet

mapping of b value beneath the shillong plateau abstract the seismic parameter b has been computed over rectangular grid of dimension 0 3 0 8 at four depths range 0 13 km first layer 13 1 26 km second layer 26 1 39 km third layer and 39 1 52 km fourth layer beneath the shillong plateau area the four depths were carefully selected based on the crustal structure and distribution of hypocentres the dimension of each grid was chosen so as to have enough events that can represent the b value at the respective layer finally two dimensional mapping was done at these depth levels considering the respective b value over each grid this analysis includes viz low b value all through the first layer and a trend of increasing b value which was initially towards north changes to northwest eastern and western parts of the second and third layers document almost moderate b values whereas the north south oriented central part of layer second is apparently dominated by low b values which seems to divide the area broadly into three parallel zones based on b values in the deeper part fourth layer beneath the shillong plateau a moderate b value that was initially trending towards north becomes high near the northeastern part this phenomenon may be associated with higher heterogeneity of the medium and interestingly this region lies between the lower crust and upper mantle possibly documents lower degree of seismic coupling where the shillong plateau is being supported by the strong indian lithosphere at these depths in addition minima were noted towards the southern parts of layers first second and third which may presumably be related with steeply bouguer gravity anomaly it is thus less clear that the occurrence of earthquakes beneath the shillong plateau whether is attributed to faults or lineaments at intermediate to deeper level however a correlation between high b values in few parts of each layer and deep seated minor faults cannot be ruled out

initiation conditions for debris flows generated by runoff at chalk cliffs central colorado abstract we have monitored initiation conditions for six debris flows between may 2004 and july 2006 in a 0 3 km 2 drainage basin at chalk cliffs a band of hydrothermally altered quartz monzonite in central colorado debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14 to 45 the availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes rainfall exceeding i 6 61 d 0 77 where i is rainfall intensity mm h and d is duration h was required for the initiation of debris flows in the drainage basin the approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0 15 m 3 s colluvium in the basin was unsaturated immediately prior to antecedent and during debris flows antecedent volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4 9 and 4 7 respectively during debris flows peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10 20 and 4 12 respectively channel sediment at a depth of 45 cm was unsaturated before and during debris flows antecedent moisture ranged from 20 22 and peak moisture ranged from 24 38 although we have no measurements from shallow rill or channel sediment we infer that it was unsaturated before debris flows and saturated by surface water runoff during debris flows our results allow us to make the following general statements with regard to debris flows generated by runoff in semi arid to arid mountainous regions 1 high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff 2 locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows and 3 rainfall and simulated surface water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment

defining conditions for bulking and debulking in lahars through measurements at semeru volcano east java we define the conditions under which bulking entrainment of sediment and pore water and debulking dilution and sedimentation occur in rain triggered volcanic floods lahars two observation sites were installed 510 m apart along the curah lengkong river 11 5 km southeast of semeru s summit this 30 m wide box valley with a gravel and lava base represents a real world flume analogy pore pressure sensors provided stage measurements a broad band seismograph gave insight into sediment content and frictional collisional behavior video cameras were used to measure surface velocities and direct bucket samples were taken eight rainfall induced lahars were recorded lasting 1 3 h with heights of 0 5 2 m peak velocities of 3 7 m s and discharges of 25 250 m3 s flows ranged from typical 40 wt sediment to coarse and dense hyperconcentrated flows 50 60 wt sediment multiple distinct flow packets occurred within the complex lahars and were used to determine internal changes between sites from the multiparameter data set at each site volumetric bulking and wave shortening due to portions of the lahar accelerating toward the flow front are identified initial debulking of lahars between sites may reflect drainage into the dry substrate estimates of discharge and volume at each site lead to the quantification of bulking and debulking by these actively flowing lahars along the channel reach from this we observe that bulking can be localized to certain parts of lahars resulting in intraevent increases in peak discharge that are greater than what would occur if bulking was evenly distributed throughout the flow such data are essential for the development of numerical descriptions and hazard models for mass flows

genesis of boston harbor drumlins massachusetts abstract drumlins in boston harbor are composed of two superposed silty sandy diamicton units and subordinate sand and gravel the lower compact undifferentiated diamicton is interpreted as a pre wisconsin till based on truncated weathering profiles preserved below an erosion surface the upper less compact diamicton contains numerous discontinuous and poorly sorted lenses and stringers of sand and sandy gravel that tend to parallel the erosion surface the upper diamicton and subordinate sandy gravel lenses are interpreted to date from the late wisconsin glaciation along the exposed sections of the long island drumlin complex the preserved weathering profile on the lower diamicton is thicker beneath the drumlin axes and is either thin or missing in the interdrumlin areas this suggests that glacial erosion and streamlining of pre wisconsin deposits and their weathering profile took place after the weathering profile developed and before subsequent deposition of the overlying diamicton unit the latter drapes the erosional drumlin form without contributing substantially to relief of the drumlin form

grain size characteristics of suspended sediment in the yellow river china abstract based on the data from the yellow river and its tributaries a systematic study has been made of the spatial and temporal variations in grain size feature of suspended sediment the grain size characteristics of suspended sediment in the yellow river basin are closely influenced by precipitation seasonal alternations of wind and water action and the nature of surface materials the lower the mean annual precipitation the stronger the wind action the coarser the surface loess materials the coarser is the suspended sediment in particular grain size characteristics of suspended sediment of the middle yellow river s tributaries are controlled by hyperconcentrated flows with suspended sediment concentrations of 300 000 400 000 mg l or more at a station the relationship between grain size and suspended sediment exhibits two peaks with percentages of the 0 05 mm suspended sediment occurring at both low and very high suspended sediment concentrations based on the data from all the tributaries of the middle yellow river an optimal grain size composition of suspended sediment has been identified that leads the annual suspended sediment concentration to the highest values the long term variation of grain size suspended sediment in the yellow river can be related to variations in precipitation and human activities the 30 year record from longmen station shows that during the 1960s the suspended sediment size increased but since the 1970s it has declined controlled by the influencing factors grain size of suspended sediment in the middle yellow river decreases from northwest to southeast

physical environment of drumlin formation review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth 1 compressive longitudinal and possibly extending transverse strain rates in the ice 2 thin ice such as occurs near the glacier margin and 3 high pore water pressure in the subglacial sediments most drumlin fields display all of these and no fields of well developed drumlins were found that did not on the oilier hand the lithology of drumlin forming sediment appears not to be important in promoting drumlin growth since it varied widely nor are the lithology and large scale topography of the bed

long and short wavelengths of indian ocean geoid and gravity lows mid to upper mantle sources rapid drift and seismicity of kachchh and shillong plateau india abstract spectral analysis of the indian ocean geoid low provides depth to the large wavelength sources as 1300 700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels the three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine spinel transformation of successively increasing fe mg ratio as depth increases known as transition zones the first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field the low density rocks in this section appear to be related to the subducted indian tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments the relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere asthenosphere boundary lab under the indian continent and surrounding oceans respectively a low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows the subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid to upper mantle which is likely to set in baby plumes presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section they would make this region buoyant that appears to be responsible for the central indian ocean deformation zone with large scale folding faulting seismicity and high heat flow the same also appear to be responsible for the rapid drift of the indian plate short wavelength component of the indian ocean geoid low reflects most of the surface shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose kachchh and the shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries tectonics and residual geoid anomalies of the indian continent and adjoining regions suggest that the geoid highs of kachchh and the shillong plateau along the nw and the ne corners of india are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries geoid highs indicating high density mafic rocks suggest their connections extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries besides both these regions are affected by i prominent lithospheric flexure of the indian plate ii large scale mafic intrusions and iii interaction of proterozoic mobile belts with effects of present day himalayan orogeny that make them more vulnerable for seismic activity

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

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

microseismicity and source mechanism study shillong plateau northeast india microearthquakes recorded by two surveys in the shillong plateau during december 1982 july 1983 and january february 1984 respectively have been analyzed for seismicity and tectonic study in the region of 1 000 events recorded during the surveys 180 events with reliable p and s readings which are observed within 50 km radius of the network are relocated by the homogeneous station method it is found that the events are mostly confined within a depth range of 10 to 35 km the seismicity in general may be correlated with the major geological structure lineaments in the area high rates of seismicity are observed in the shillong and tura areas on the other hand an aseismic zone is delineated to the east of shillong composite source mechanisms of two groups of microearthquakes show thrust fault mechanisms one group of microearthquakes 1982 to 1983 in the shillong area shows pure thrust faulting with a se nw compressive stress p axis whereas the other group of microearthquakes 1984 in the tura area shows thrust faulting with a strike slip component and a ene wsw compressive stress p axis the stress orientations are compatible with the known tectonics of the region collision of the indian plate with the eurasian plate at the himalaya and subduction of the indian plate beneath the burmese plate at the arakan yoma range

drumlin formation by subglacial meltwater erosion some drumlins are shown to belong to a family of remnant erosional ridges associated with subglacial meltwater the lengths of these ridges vary in scale from a few tens of millimetres to over 5 km similar form elements are illustrated for three scales of forms it is concluded that some drumlins are composed of preexisting sediment or bedrock left upstanding in ridges as a result of the removal of surrounding materials this erosional theory is related to an earlier theory in which drumlins are held to result from the infilling of subglacial cavities the unifying factor for both theories is the role of sheets of subglacial meltwater

seismotectonics of lower assam northeast india using the data of a dense microseismic network northeast india has been subjected to extensive compressional forces mainly in north south and east west directions resulting from the convergence of the indian plate with the eurasian and burmese plates respectively the area is characterized as one of the most seismically active regions of the world however the lower assam valley s microseismicity has not been monitored and studied intensively by a dense seismic network during the past during this study a seismic network of 76 stations was deployed in northeastern india for one year hundreds of microearthquakes were recorded the most accurately located events moment tensor solutions and focal mechanisms were used in order to define the seismotectonic and stress regime in the area

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

bending shear the rate controlling mechanism for calving ice walls bending shear was observed to produce nearly vertical shear bands in a calving ice wall standin on dry land on deception island iat 63 0 os long 60 6 w and slabs calved straight downward when shear rupture occurred along these shear bands hughes 1989 a formula for the calving rate was developed from the deception island data and we have attempted to justify generalizing this formula to include ice walls standing along beaches or in water these are environments in which a wave washed groove develops along the base of the ice wall or along a water line above the base the rate of wave erosion provides an alternative mechanism for controlling the calving rate in these environments we have determined that the rate at which bending creep produces nearly vertical shear bands along which shear rupture occurs controls the calving rate in all environments shear rupture occurs at a calving shear stress of about i bar our results justify using the calving formula to compute the calving rate of ice walls in computer models of ice sheet dynamics this is especially important in simulating retreat of northern hemisphere ice sheets during the last deglaciation when marine and lacustrine environments were common along retreating ice margins these margins would have been ice walls standing along beaches or in water because floating ice shelves are not expected in the ablation zone of retreating ice sheets

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

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

microseismicity and tectonics in northeast india abstract three microearthquake surveys were carried out in shillong plateau mikir hills and assam valley areas during 1984 to 1986 some 422 events are relocated by the homogeneous station method the microseismicity map reveals intense crustal 10 to 40 km activity beneath the tura area of shillong plateau the areas to the west of shillong and the area around mikir hills also show high activity the microseismicity map further shows major tectonic lineaments that are compatible with the landsat imagery lineaments and the major faults composite focal mechanisms of the microearthquakes show spatial variation of the tectonic stresses in the region an ene wsw horizontal compressive stress is dominant in the tura area whereas a se nw horizontal compressive stress is dominant in the shillong and mikir hills areas

a process for fire related debris flow initiation cerro grande fire new mexico in this study we examine factors that pertain to the generation of debris flows from a basin recently burned by wildfire throughout the summer 2000 thunderstorm season we monitored rain gauges channel cross sections hillslope transects and nine sediment runoff traps deployed in a steep 0 15 km 2 basin burned by the may 2000 cerro grande fire in new mexico debris flows were triggered in the monitored basin during a rainstorm on july 16 2000 in response to a maximum 30 min rainfall intensity of 31 mm h 1 return period of approximately 2 years eleven other storms occurred before and after the july storm these storms resulted in significant runoff but did not generate debris flows the debris flows generated by the july 16 storm initiated on a broad open hillslope as levee lined rills the levees were composed of gravel and cobble sized material supported by an abundant fine grained matrix debris flow deposits were observed only on the hillslopes and in the first and second order drainages of the monitored basin no significant amounts of channel incision were measured following the passage of the debris flows indicating that most of the material in the flows originated from the hillslopes sediment runoff concentrations of between 0 23 and 0 81 kg l 1 with a mean of 0 42 kg l 1 were measured from the hillslope traps following the debris flow producing storm these concentrations however were not unique to the july 16 storm the materials entrained by the july 16 storm contained a higher proportion of silt plus clay sized materials in the 2 mm fraction than the materials collected from storms that produced comparable sediment runoff concentrations but not debris flows the difference in materials demonstrates the critical role of the availability of fine grained wood ash mantling the hillslopes in the runoff dominated generation of post wildfire debris flows the highest sediment runoff concentrations again not unique to debris flow production were produced from maximum 30 min rainfall intensities greater than 20 mm h 1

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

the hydrological properties and the effects of hydration on vegetative ash from the northern rockies usa abstract vegetative ash is known to alter post fire runoff rates in some cases decreasing runoff by protecting the underlying soil and in other cases aiding in the formation of a surface seal increasing runoff variability in this hydrological response of ash may reflect differences in the hydrologic properties of ash caused by shifts in its physical and chemical characteristics this study investigated the physical particle size and shape particle density bulk density and porosity chemical mineralogy and elemental composition and hydrologic water retention hydraulic conductivity and sorptivity characteristics of laboratory and wildfire ash from the northern rocky mountain region of the united states a variety of techniques were used including scanning electron microscope x ray diffraction thermogravimetric analysis and conventional soil science techniques laboratory ash samples were prepared by combusting fuel from three dominant trees species of the region pinus contorta pinus ponderosa and pseudotsuga menziesii at various combustion temperatures 300 c 500 c 700 c or 900 c the main objectives of this study were to determine i how the physical particle size and shape particle density bulk density and porosity and chemical mineralogy and elemental composition properties of ash might explain variations in hydrologic properties water retention hydraulic conductivity and sorptivity ii whether wetting alters the physical chemical and hydrologic properties of ash iii whether ash characteristics could be linked to various stages of thermal decomposition and whether ash produced in the laboratory could be beneficial to understanding wildfire ash characteristics and iv whether ash hydrologic properties can be linked to simple metrics such as calcium carbonate content or color which can be used in rapid post fire hazard assessment results of this study indicate that variation in the hydrological response of ash can be partially explained by differences in ash characteristics associated with combustion temperature depending on ash mineralogy which was dictated by the level of thermal decomposition of organic material ash particles can a contain micro porosity contributing to total porosity values as high as 98 b shift hydraulic conductivity values orders of magnitude from 0 12 0 03 to 0 002 0 0003 cm s 1 and c chemically transform after initial hydration suggesting that ash layers could alter post fire runoff response by creating a chemical crust furthermore while ash color is often used as an indicator of burn severity it may not be an accurate indicator regarding the hydrologic response of ash in post fire systems instead carbonate content is suggested as a more informative aid in predicting post fire hydrologic response

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

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

conditions for generation of fire related debris flows capulin canyon new mexico comparisonoftheresponsesofthreedrainagebasinsburnedbythedomefireof1996innewmexicoisusedtoidentifythe hillslope channelandfirecharacteristicsthatindicateasusceptibilityspecificallytowildfire relateddebrisflow summer thunderstorms generated three distinct erosive responses from each of three basins the capulin canyon basin showed widespreaderosivesheetwashandrillingfromhillslopes andseverefloodingoccurredinthechannel thenorthtributary basin exhibited extensive erosion of the mineral soil to a depth of 5 cm and downslope movement of up to boulder sized material andatleastonedebrisflowoccurredinthechannel negligiblesurfacerunoffwasobservedinthesouthtributary basin thenegligiblesurfacerunoffobservedinthesouthtributarybasinisattributedtothelimitedextent andseverityof the fire in that basin the factors that best distinguish between debris flow producing and flood producing drainages are drainage basin morphology and lithology a rugged drainage basin morphology an average 12 per cent channel gradient andsteep roughhillslopescoupledwithcolluviumandsoilweatheredfromvolcaniclasticandvolcanicrockspromotedthe generation of debris flows a less rugged basin morphology an average gradient of 5 per cent and long smooth slopes mantled with pumice promoted flooding flood and debris flow responses were produced without the presence of waterrepellentsoils thecontinuityandseverityoftheburnmosaic theconditionoftheriparianvegetation theconditionofthe fibrous root mat accumulations of dry ravel and colluvial material in the channel and on hillslopes and past debris flow activity appearedtohavelittlebearingonthedistinctiveresponsesofthebasins publishedin2000byjohnwiley sons ltd

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

wildfire related debris flow initiation processes storm king mountain colorado a torrential rainstorm on september 1 1994 at the recently burned hillslopes of storm king mountain co resulted in the generation of debris flows from every burned drainage basin maps 1 5000 scale of bedrock and surficial materials and of the debris flow paths coupled with a 10 m digital elevation model dem of topography are used to evaluate the processes that generated fire related debris flows in this setting these evaluations form the basis for a descriptive model for fire related debris flow initiation the prominent paths left by the debris flows originated in 0 and 1st order hollows or channels discrete soil slip scars do not occur at the heads of these paths although 58 soil slip scars were mapped on hillslopes in the burned basins material derived from these soil slips accounted for only about 7 of the total volume of material deposited at canyon mouths this fact combined with observations of significant erosion of hillslope materials suggests that a runoff dominated process of progressive sediment entrainment by surface runoff rather than infiltration triggered failure of discrete soil slips was the primary mechanism of debris flow initiation a paucity of channel incision along with observations of extensive hillslope erosion indicates that a significant proportion of material in the debris flows was derived from the hillslopes with a smaller contribution from the channels because of the importance of runoff dominated rather than infiltration dominated processes in the generation of these fire related debris flows the runoff contributing area that extends upslope from the point of debris flow initiation to the drainage divide and its gradient becomes a critical constraint in debris flow initiation slope area thresholds for fire related debris flow initiation from storm king mountain are defined by functions of the form acr tan 3 s where acr is the critical area extending upslope from the initiation location to the drainage divide and tan is its gradient the thresholds vary with different materials

deep structure and tectonics of the burmese arc constraints from earthquake and gravity data abstract active subduction of the indian plate is currently occurring beneath the burmese arc along an east dipping benioff zone which extends to a depth of about 180 km the overriding burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the benioff zone a crustal seismic zone some 60 80 km east of the benioff zone correlates to backarc activity a triangular aseismic wedge in the top part of the crust outlines the central belt molasse basin east of the burmese foldbdt fault plane solutions show that the burmese benioff zone is characterized by shallow angle thrusting at its upper edge whereas down tip tensional events dominate its lower edge most of the backarc seismicity is accounted for by the sagaing transform or by the activity of the shan scarp normal fault zone at the margin of the asian plate a gravity anomaly pair with amplitude of 175 mgal coincides with the 1100 km long burmese arc lying in a north south direction the gravity anomalies along a profile in central burma and in adjacent areas of the bengal basin are interpreted in terms of plate subduction as well as near surface mass anomalies this suggests that sediments below the central belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone the oceanic crust underlying deeper parts of the bengal basin experiences phase transition at about 30 km depth in a benioff zone environment east of the burmese foldbelt several thrust planes are present within the folded and deformed cretaceous tertiary sediments of the fold belt these are often associated with ophiolites and basic to ultrabasic rocks a low density zone at least 60 km wide underlies the andesitic volcanic axis in the overriding plate

effects of sediment pulses on channel morphology in a gravel bed river sediment delivery to stream channels in mountainous basins is strongly episodic with large pulses of sediment typically delivered by infrequent landslides and debris flows identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins in july 2001 intense rainfall triggered numerous debris flows in a severely burnt watershed in the sapphire mountains of montana ten large debris flow fans were deposited on the valley floor and investigations focused on the channel response to these sediment pulses the channel has aggraded immediately upstream of each fan and braided in reaches immediately downstream channel incision through the fans has created sets of coarse grained terraces the deposition upstream of the pulses consists almost exclusively of fine material resulting in a median bed material size d 50 1 2 orders of magnitude lower than the ambient channel material the volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans with 1 2 m of sand deposited across the entire valley floor along a 10 km study reach cross section surveys longitudinal profiles and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal

3 d seismic tomography of the lithosphere and its geodynamic implications beneath the northeast india region we have evolved 3 d seismic velocity structures in northeast india region and its adjoining areas to understand the geodynamic processes of indian lithosphere that gently underthrusts under the himalayas and steeply subducts below the indo burma ranges the region is tectonically buttressed between the himalayan arc to the north and the indo burmese arc to the east the tomographic image shows heterogeneous structure of lithosphere depicting different tectonic blocks though our results are limited to shallower depth 0 90 km it matches well with the deeper continuation of lithospheric structure obtained in an earlier study koulakov 2011 we observe low velocity structure all along the eastern himalayas down to 70 km depth which may be attributed to deeper roots thicker crust developed by underthrusting of indian plate parallel to this low velocity zone lies a high velocity zone in foredeep region represents the indian lithosphere the underthrusting indian lithosphere under the himalayas as well as below the indo burma ranges is well reflected as a high velocity dipping structure the buckled up part of bending indian plate in study region the shillong plateau mikir hills tectonic block is marked as a high velocity structure at shallower depth the eastern himalayan syntaxis tectonic block where the two arcs meet is identified as a high velocity structure the bengal basin tectonic block to the south of shillong plateau shows low velocity due to its thicker sediments based on the tomographic image a schematic model is presented to elucidate the structure and geodynamics of indian lithosphere in study region

reaction of the accumulation zone portions of glaciers to climatic change the response of the accumulation regions of glaciers to changes in accumulation rate and in surface temperature is calculated by considering perturbations to the ice flow the analysis is limited to glaciers in which flow is dominated by internal shear and not by bottom sliding and for which flowlines are geographically parallel in general glaciers begin to alter thickness immediately after a change in accumulation rate but the effect of a change in surface temperature is delayed by the time for this temperature change to penetrate to depth in the glacier a warming leads to glacial thinning the amount and timing of the response is very different for different glaciers characteristic times are on the order of tens of years for mountain glaciers and tens of thousands of years for the east antarctic ice sheet as an example the theory is applied to the ice sheet near byrd station antarctica for hypothetical changes in surface accumulation rate and temperature rates and amounts of thickness change are calculated and it is found that the measured thinning is probably too fast to have been caused by climatic variation alone the effects of hypothetical climatic variations on the depth age and depth temperature relationships through the ice sheet are also calculated it is hoped that the theory will prove valuable in the interpretation of future drill hole results

pleistocene stratigraphy of the boston harbor drumlins massachusetts abstract evidence from the boston harbor drumlins indicates that two superposed tills were deposited during glacier advances which were separated by a long nonglacial interval at long and peddocks islands argillans and truncated clay filled fractures along with discontinuities in clay mineral composition define the till contacts physical indicators separating the tills are not apparent at other exposures where till boundaries were defined solely by discontinuities in clay mineral composition the weathering profile in the upper part of the lower till indicates extensive weathering under a climate similar to that of today and probably similar to that of the sangamon interglaciation the depth of the weathering profile the sequence of clay mineral alteration products and the presence of pedogenic features in the upper part of the lower till are comparable to sangamonian weathering profiles in the midwestern united states implying that the lower till is illinoian or older