an insight into crack density saturation rate and porosity model of the 2001 bhuj earthquake in the stable continental region of western india abstract the 2001 bhuj earthquake mw 7 6 source zone is examined in the light of crack density e saturation rate and porosity parameter using new data set derived from a large aftershock sequence recorded by the gujarat seismic network gsnet during november 2006 december 2009 processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset pattern of crustal heterogeneities associated with high e high and high anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by mishra and zhao 2003 the anomalous zone is found extended distinctly by 50 60 km in the lateral direction indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 bhuj earthquake in the source area it is inferred that the presence of a fluid filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone anomalous pattern of high e with wider distribution of high indicates the existence of micro cracks in the lower crust while high suggests the cementation of cracks through permeation of residual magma metamorphic fluids into the hypocenter zone the results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

intrinsic and scattering attenuation in western india from aftershocks of the 26 january 2001 kachchh earthquake abstract 176 vertical component short period observations from aftershocks of the mw 7 7 26 january 2001 kachchh earthquake are used to estimate seismic wave attenuation in western india using uniform and two layer models the magnitudes mw of the earthquakes are less than 4 5 with depths less than 46 km and hypocentral distances up to 110 km the studied frequencies are between 1 and 30 hz two seismic wave attenuation factors intrinsic absorption qi 1 and scattering attenuation qs 1 are estimated using the multiple lapse time window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model we first assume spatial uniformity of qi 1 qs 1 and s wave velocity a second approach extends the multiple scattering hypothesis to media consisting of several layers characterized by vertically varying scattering coefficient g intrinsic absorption strength h density of the media and shear wave velocity structure the predicted coda envelopes are computed using monte carlo simulation results show that under the assumption of spatial uniformity scattering attenuation is greater than intrinsic absorption only for the lowest frequency band 1 to 2 hz whereas intrinsic absorption is predominant in the attenuation process at higher frequencies 2 to 30 hz the values of q obtained range from qt 118 qi 246 and qs 227 at 1 5 hz to qt 4000 qi 4600 and qs 33 300 at 28 hz center frequencies being qt 1 a measure of total attenuation results also show that qi 1 qs 1 and qt 1 decrease proportional to f two rates of decay are clearly observed for the low 1 to 6 hz and high 6 to 30 hz frequency ranges values of are estimated as 2 07 0 05 and 0 44 0 09 for total attenuation 1 52 0 21 and 0 48 0 09 for intrinsic absorption and 3 63 0 07 and 0 06 0 08 for scattering attenuation for the low and high frequency ranges respectively despite the lower resolution in deriving the attenuation parameters for a two layered crust we find that scattering attenuation is comparable to or smaller than the intrinsic absorption in the crust whereas intrinsic absorption dominates in the mantle also for a crustal layer of thickness 42 km intrinsic absorption and scattering estimates in the crust are lower and greater than those of the mantle respectively

separation of depth dependent intrinsic and scattering seismic attenuation in the northeastern sector of the italian peninsula summary we investigated the intrinsic dissipation and scattering properties of the lithosphere under the friuli region northeastern italy using two hypotheses i a uniform earth model and ii two reasonable non uniform layered crustal models for case i we measured the coda q and used the multiple lapse time window analysis mltwa technique to obtain separate estimates of intrinsic absorption and scattering attenuation results for the uniform earth model show that the lithosphere in northeastern italy is characterized by a low scattering attenuation small scattering q inverse q 1 and by a relatively high intrinsic attenuation high intrinsic q inverse q 1 a comparison between the investigated region and other areas around the world shows that both q 1 and q 1 for the friuli region are among the lowest values ever measured with the exception of the southern apennines which has the lowest measured q 1 for case ii numerical simulation of the energy envelopes was performed using two layered earth models where the values of the intrinsic and scattering attenuation coefficients are both within reasonable ranges when compared with the geological information the theoretical envelopes calculated for the homogeneous model give a good fit to the synthetic envelopes calculated for the layered models the best fit is obtained for scattering attenuation coefficients of the uniform model always greater than those of the layered model the main result is consequently that scattering q 1 obtained using the mltwa under the assumption of a uniform medium is overestimated on average by a factor 2 finally coda q 1 appears to be closer to the total q 1 than to the intrinsic q 1 a spredicted

intrinsic and scattering attenuation from observed seismic codas in the almeria basin southeastern iberian peninsula summary the anelastic attenuation in the almeria basin southeastern iberian peninsula is investigated by using seismic data collected during the summer of 1991 a multiple lapse time window analysis is applied to high frequency seismograms corresponding to 20 shallow seismic events with low magnitudes m 12 5 and distances less than 71 km recorded at six short period seismographic stations we have constructed corrected geometrical spreading and normalized energy distance curves for the region over the frequency bands 1 2 2 4 4 8 8 14 and 14 20 hz a theoretical model for body wave energy propagation in a randomly heterogeneous medium has been employed to interpret the observations two parameters describe the medium in this model the scattering attenuation coefficient qs kq3 and the intrinsic attenuation coefficient yi kq where k is the wavenumber and qr1 and q are the intrinsic and scattering attenuation respectively this model assumes that scattering is isotropic including all orders of multiple scattering and predicting the spatial and temporal energy distribution of seismic energy a least squares fitting procedure has been used to find the best estimates of the model parameters the analysis of the spectral amplitude decay of coda waves has provided coda q values at the same frequency bands the results obtained show that q q2 and q decrease with increasing frequency for frequencies lower than 3 hz scattering attenuation is stronger than intrinsic absorption and coda q takes values between intrinsic and total attenuation being very close to qy q is more frequency dependent than q for frequencies greater than 3 hz intrinsic absorption is the dominant attenuation effect and q and q3 have significant frequency dependence in order to correlate the results obtained with the major geological and tectonic features of the region a geotectonic framework for the area is provided and the predominant frequency decay in coda waves is analysed in order to obtain the coda q frequency dependence following a power law qc qo f fo where fo is a reference frequency in this way we have obtained regionalized values of coda q at 1 hz qo finally a first order approach has allowed us to obtain intrinsic and scattering quality factors from the obtained qo and v values leading us to obtain tentative distributions of q qs and qo at 1 hz for the area the derived intrinsic and scattering quality factor distributions are in good agreement with the tectonic history and the main geological features of the region large scattering and intrinsic attenuation qs n 80 qi 100 are found in the sedimentary neogene and quaternary basin while scattering is the dominant effect in the old palaeozoic rocks of the mountains qs 200 qi 1000 intrinsic q shows a higher sensitivity to the geological characteristics than scattering q

study of attenuation mechanism for garwhal kumaun himalayas from analysis of coda of local earthquakes abstract the relative contribution of intrinsic q i 1 and scattering q s 1 attenuation to seismic wave attenuation was estimated for the garwhal kumaun himalayas using multiple lapse time window analysis mltwa method under the assumption of isotropic scattering local earthquake data recorded by an array operated by wadia institute of himalayan geology wihg india was used for this purpose it is observed that scattering attenuation primarily contributes to seismic wave attenuation in this region and its value is much higher compared to that of intrinsic attenuation at around 1 hz frequency as frequency increases the relative contribution of scattering attenuation to total attenuation starts decreasing however as seismic albedo is higher than 0 5 for all the frequencies considered it is concluded that the medium here is highly heterogeneous in nature

attenuation of high frequency p and s waves in the gujarat region india the local earthquake waveforms recorded on broadband seismograph network of institute of seismological research in gujarat india have been analyzed to understand the attenuation of high frequency 2 25 hz p and s waves in the region the frequency dependent relationships for quality factors for p qp and s qs waves have been obtained using the spectral ratio method for three regions namely kachchh saurashtra and mainland gujarat the earthquakes recorded at nine stations of kachchh five stations of saurashtra and one station in mainland gujarat have been used for this analysis the estimated relations for average qp and qs are qp 105 2 f0 82 0 01 qs 74 2 f1 06 0 01 for kachchh region qp 148 2 f0 92 0 01 qs 149 14 f1 43 0 05 for saurashtra region and qp 163 7 f0 77 0 03 qs 118 34 f0 65 0 14 for mainland gujarat region the low q 0 5 as obtained from present analysis indicate the predominant seismic activities in the region the lowest q values obtained for the kachchh region implies that the area is relatively more attenuative and heterogeneous than other two regions a comparison between qs estimated in this study and coda q qc previously reported by others for kachchh region shows that qc qs for the frequency range of interest showing the enrichment of coda waves and the importance of scattering attenuation to the attenuation of s waves in the kachchh region infested with faults and fractures the qs qp ratio is found to be less than 1 for kachchh and mainland gujarat regions and close to unity for saurashtra region this reflects the difference in the geological composition of rocks in the regions the frequency dependent relations developed in this study could be used for the estimation of earthquake source parameters as well as for simulating the strong earthquake ground motions in the region

low coda qc in the epicentral region of the 2001 bhuj earthquake of mw 7 7 on 26 january 2001 03 46 55 ut a devastating intraplate earthquake of m w 7 7 occurred in a region about 5 km nw of bhachau gujarat 23 42 n 70 23 e the epicentral distribution of aftershocks defines a marked concentration along an e w trending and southerly dipping 45 zone covering an area of 60 40 km2 the presence of high seismicity including two earthquakes of magnitudes exceeding 7 7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the kutch area a site lying in the seismic zone v zone of the highest seismicity for potentially m8 earthquakes on the seismic zoning map of india attenuation property of the medium around the epicentral area of the bhuj earthquake covering a circular area of 61 500 km2 with a radius of 140 km is studied by estimating the coda q c from 200 local earthquakes of magnitudes varying from 3 0 4 6 the estimated q 0 values at locations in the aftershock zone high seismicity are found to be low in comparison to areas at a distance from it this can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone distribution of q 0 values suggests that the local variation in q 0 values is probably controlled by local geology the estimated q 0 values at different stations suggest a low value of q 102 0 80 f 0 98 0 02 indicating an attenuative crust beneath the entire region the frequency dependent relation indicates a relatively low q c at lower frequencies 1 3 hz that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and or intrinsic attenuation due to fluid movement in the fault zone and fluid filled cracks the large q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected based on the attenuation curve estimated for q 0 102 the ground acceleration at 240 km distance is 13 of 1 g i e 0 13 g agreeing well with the ground acceleration recorded by an accelerograph at ahmedabad 0 11 g hence it is inferred that the q 0 value obtained from this study seems to be apt for prediction of ground motion for the region

the attenuation mechanism of s waves in the source zone of the 1999 chamoli earthquake a partial support has been given by italy ingv dpc istituto nazionale di geofisica e vulcanologia and dipartimento di protezione civile projects unrest and speed and by italy s ministry of education prin project seismic hazard in central apennines ur del pezzo

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

estimation of the intrinsic absorption and scattering attenuation in northeastern venezuela southeastern caribbean using coda waves northeastern venezuela has been studied in terms of coda wave attenuation using seismograms from local earthquakes recorded by a temporary short period seismic network the studied area has been separated into two subregions in order to investigate lateral variations in the attenuation parameters coda q 1 q c 1 has been obtained using the single scattering theory the contribution of the intrinsic absorption q i 1 and scattering q s 1 to total attenuation q t 1 has been estimated by means of a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers results show significant spatial variations of attenuation the estimates for intermediate depth events and for shallow events present major differences this fact may be related to different tectonic characteristics that may be due to the presence of the lesser antilles subduction zone because the intermediate depth seismic zone may be coincident with the southern continuation of the subducting slab under the arc

cover stratigraphy and structure of the southernmost external basement massifs in the appalachian blue ridge evidence for two stage late proterozoic rifting

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

attenuation characteristics in eastern himalaya and southern tibetan plateau an understanding of the physical state of the medium abstract attenuation characteristics of the crust in the eastern himalaya and the southern tibetan plateau are investigated using high quality data recorded by himalayan nepal tibet seismic experiment himnt during 2001 2003 the present study aims to provide an attenuation model that can address the physical mechanism governing the attenuation characteristics in the underlying medium we have studied the coda wave attenuation q c in the single isotropic scattering model hypothesis s wave attenuation q s by using the coda normalization method and intrinsic q i 1 and scattering q sc 1 quality factors by the multiple lapse time window analysis mltwa method under the assumption of multiple isotropic scattering in a 3 d half space within the frequency range 2 12 hz all the values of q exhibit frequency dependent nature for a seismically active area at all the frequencies intrinsic absorption is predominant compared to scattering attenuation and seismic albedo b 0 are found to be lower than 0 5 the observed discrepancies between the observed and theoretical models can be corroborated by the depth dependent velocity and attenuation structure as well as the assumption of a uniform distribution of scatterers our results correlate well with the existing geo tectonic model of the area which may suggest the possible existence of trapped fluids in the crust or its thermal nature surprisingly the underlying cause of high attenuation in the crust of eastern himalaya and southern tibet makes this region distinct from its adjacent western himalayan segment the results are comparable with the other regions reported globally

variation of seismic coda wave attenuation in the garhwal region northwestern himalaya seismic coda wave attenuation q text c 1 characteristics in the garhwal region northwestern himalaya is studied using 113 short period vertical component seismic observations from local events with hypocentral distance less than 250 km and magnitude range between 1 0 to 4 0 they are located mainly in the vicinity of the main boundary thrust mbt and the main central thrust mct which are well defined tectonic discontinuities in the himalayas coda wave attenuation q text c 1 is estimated using the single isotropic scattering method at central frequencies 1 5 3 5 7 9 12 16 20 24 and 28 hz using several starting lapse times and coda window lengths for the analysis results show that the q text c 1 values are frequency dependent in the considered frequency range and they fit the frequency power law q text c 1 left f right q 0 1 f n the q0 qc at 1 hz estimates vary from about 50 for a 10 s lapse time and 10 s window length to about 350 for a 60 s lapse time and 60 s window length combination the exponent of the frequency dependence law n ranges from 1 2 to 0 7 however it is greater than 0 8 in general which correlates well with the values obtained in other seismically and tectonically active and highly heterogeneous regions the attenuation in the garhwal region is found to be lower than the qc 1 values obtained for other seismically active regions of the world however it is comparable to other regions of india the spatial variation of coda attenuation indicates that the level of heterogeneity decreases with increasing depth the variation of coda attenuation has been estimated for different lapse time and window length combinations to observe the effect with depth and it indicates that the upper lithosphere is more active seismically as compared to the lower lithosphere and the heterogeneity decreases with increasing depth

s wave attenuation characteristics in the galeras volcanic complex south western colombia vertical component short period seismograms from 435 well located volcano tectonic earthquakes are used to estimate s wave attenuation in the galeras volcanic complex south western colombia using coda waves the coda magnitudes mc of the events are less than 2 event depths are less than 10 km and hypocentral distances up to 16 km intrinsic absorption q 1 i and scattering attenuation q 1 s are estimated by means of a fitting procedure between the observed and synthetic coda envelopes in four frequency bands 1 2 2 4 4 8 and 8 12 hz the observations are compared with the theoretical predictions by an accurate approximate analytical solution of the radiative transfer equation which is based on the assumptions of multiple isotropic scattering impulsive isotropic point source and a medium with homogeneous scattering and absorption properties results show that scattering is strong and it constitutes the predominant attenuation effect in this region in the frequency range analyzed in this study the values of the mean free path for scattering of s waves range between 2 7 l 8 1 km which are clearly higher than those obtained in other volcanic regions of the world but about two orders of magnitude smaller than average estimates for the earth s crust the characteristic length scale of intrinsic absorption gives values of 2 5 la 77 km which are on the same order as the usual values for the earth s crust 2010 elsevier b v all rights reserved

intrinsic and scattering seismic wave attenuation in the canary islands the canary islands volcanic complex is studied in terms of coda wave attenuation a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers is applied to the available seismic data in order to obtain the intrinsic absorption qi 1 and the scattering attenuation qs 1 in the canarian lithosphere the analysis is performed for two hypocentral distance ranges from 0 to 80 km and from 0 to 140 km results show that in both cases and for all the studied frequency bands 1 2 2 4 6 8 and 8 10 hz intrinsic absorption dominates the low albedos found in the region indicate the low degree of heterogeneity in the canarian lithosphere at the scale length of the studied frequencies on the other hand the coda attenuation qc 1 calculated on the basis of the single scattering theory gives values near qi 1 for low frequencies and near the total attenuation qt 1 for high frequencies the degree of frequency dependence of the attenuation parameters is strong in all cases a correlation of the observed attenuation parameters with the geological evidence for a hotspot type archipelago and with other geophysical studies is suggested

derived bedrock elevations strain rates and stresses from measured surface elevations and velocities jakobshavns isbrae greenland jakobshavns isbrae 69 10 n 49 59 w drains about 6 5 of the greenland ice sheet and is the fastest ice stream known the jakobshavns isbrae basin of about 10 000 km 2 was mapped photogrammetrically from four sets of aerial photography two taken in july 1985 and two in july 1986 positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation and surface velocity vectors were computed from the positions the two flights in 1985 yielded the best results and provided most common points 716 for velocity determinations and are therefore used in the modeling studies the data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation the field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses using the flow law of ice for rectilinear coordinates x y pointing eastward and northward and curvilinear coordinates l t pointing longitudinally and transversely to the changing ice flow direction ice surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding our calculated ice thicknesses are in fair agreement with an ice thickness map based on seismic sounding and supplied to us by k echelmeyer ice thicknesses were subtracted from measured ice surface elevations to map bed topography our calculation shows that basal sliding is significant only in the 10 15 km before jakobshavns isbrae becomes afloat in jakobshavns isfjord

characteristics of deformation and past seismicity associated with the 1819 kutch earthquake northwestern india the 1819 earthquake in kutch northwestern india is one of the most significant events to have occurred in a plate interior setting despite being the second largest among the stable continental region scr earthquakes this event has not been analyzed within the context of present day understanding of earthquake seismology coseismic changes related to this earthquake include massive ground deformation in a wide low lying tidal flat area although detailed historic accounts of this earthquake exist many questions regarding the mode of deformation and the seismic history of the region remain unresolved we explored the region nearly 180 years after the earthquake and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone a 90 km long tract of elevated land with a peak height of 4 3 m is the most visible surface expression of this earthquake we surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region the scarp morphology is suggestive of a growing fold related to a buried north dipping thrust rather than a discrete fault that could have resulted from a surface rupture the extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history age data of liquefaction features suggest that a previous event of comparable size must have occurred 800 1000 years ago seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north south compression

scattering and anelastic attenuation of seismic energy in the vicinity of north anatolian fault zone eastern turkey abstract erzincan basin eastern turkey is seismically very active pull apart basin and developed in between the eastern segments of the north anatolian fault zone a study of the regional and site attenuation of seismic waves of earthquakes in this area will contribute in predicting earthquake generated ground motion and becomes vital in making decisions for earthquake regulations and building codes especially for the cities on the north anatolian fault zone in this paper anelastic and scattering attenuation in erzincan region is investigated by using of multiple lapse time window mltw method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatters it is found that the scattering and intrinsic q 1 are almost equal for 3 hz and lower frequencies and intrinsic attenuation is dominant over scattering attenuation for 6 hz and higher frequencies model fitting was quite good for almost all frequencies at long distances more scattered data and observed poor model fitting at low frequencies 1 5 hz and short distances 5 15 km may indicate the complexities in the shallower part of crust in erzincan the seismic albedo is 0 52 for 1 5 hz and 0 32 for 12 hz l e 1 value increases slightly with frequency as 0 058 for 1 5 hz and 0 032 for 12 hz the coda q c 1 determined from the decay rate of coda waves is similar to q s 1 for 1 5 and 3 hz and it is close to q t 1 for frequencies higher than 3 hz we found that the scattering attenuation is strongly frequency dependent decreasing proportional to f 1 48 whereas intrinsic q i 1 is considerably less frequency dependent f 1 1 a decrease of q s 1 faster than f 1 0 with increasing frequency implies that the medium may be characterized by a gaussian rather than exponential auto correlation function

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

monte carlo simulation of seismogram envelopes in scattering media the analysis of the seismogram coda envelopes of local and regional earthquakes is one of the most effective strategies for investigating the heterogeneous lithospheric structure characterized by the seismic scattering and attenuation in order to synthesize the coda envelope we introduce a numerical scheme called the direct simulation monte carlo method which has been used in the field of the kinetic theory of gases because of the simplicity of the algorithm the method has several advantages over previous methods in terms of the flexibility of the numerical calculation to incorporate various factors required to construct realistic seismogram envelopes on the basis of coda envelope simulations including multiple scattering we show that an increase of seismic velocity with depth severely affects the shape of the coda envelope the effects of ray bending due to the velocity increase at the moho and the reflection at the free surface are clearly found in the synthesized envelope for a shallow earthquake our simulation demonstrates that the amplitude of the envelope is magnified by stagnation of seismic energy at shallow depths due to the positive velocity gradient with depth because of this effect for an a priori assumption of a homogeneous velocity model the measurement of the scattering coefficient by conventional methods may be overestimated

an increase in crevasse extent west greenland hydrologic implications 1 we compare high resolution 1985 and 2009 imagery to assess changes in crevasse extent in the sermeq avannarleq ablation zone west greenland the area occupied by crevasses 2 m wide significantly increased 13 4 over the 24 year period this increase consists of an expansion of existing crevasse fields and is accompanied by widespread changes in crevasse orientation up to 45 we suggest that a combination of ice sheet thinning and steepening are responsible for the increase in crevasse extent we examine the potential impact of this change on the hydrology of the ice sheet we provide a first order demonstration that moulin type drainage is more efficient in transferring meltwater fluctuations to the subglacial system than crevasse type drainage as enhanced basal sliding is associated with meltwater pulses an increase in crevasse extent can therefore be expected to result in a net decrease in basal sliding sensitivity an increase in crevasse extent may also accelerate cryo hydrologic warming and enhance surface ablation

residence time of diffuse waves in the crust as a physical interpretation of coda q application to seismograms recorded in mexico summary we consider a simple model for elastic wave propagation in the earth s litho sphere consisting of a heterogeneous and scattering crust overlying a homogeneous mantle that is we neglect backscattering from the upper mantle this rough condition is not strictly valid everywhere but this model allows us to investigate the effects of variations of diffracting properties between crust and mantle on the coda decay the free parameters describing the wave propagation are the mean free path of the waves and the intrinsic absorption in the crust in the framework of the radiative transfer theory we treat accurately jumps in s wave speed and the mean free path at the moho without intrinsic absorption this configuration yields a synthetic coda decay 1 texp 2 ft qc a form similar to that proposed by aki chouet 1975 to parametrize the observations the exponential decay in our purely elastic model is due to the partial leakage of diffuse energy into the mantle in order to test the applicability of our model we have determined the coda q parameter qc in the 1 15 hz frequency band from data recorded at four stations in mexico in the low frequency regime around 1 hz we suggest that the energy leakage significantly affects coda q whilst at higher frequencies around 10 hz the anelastic attenuation dominates a weak intrinsic absorption expressed by a frequency independent parameter qi of about 1000 accounts satisfactorily for the observed qc values in the whole frequency range our very simple model has only two free parameters and offers an attractive alternative physical interpretation of coda q in mexico this model is sufficient to interpret the coda observations according to our model in regions where the mean free path is much larger than the crustal thickness weakly heterogeneous crust the leakage effect should be small and the observed qc will mainly be due to the intrinsic absorption on the other hand when the mean free path is of the order of the crustal thickness strongly heterogeneous crust we predict a strong sensitivity of the decay rate of the coda to the crustal thickness this dependence may be important for the interpretation of the regional variations of coda q we propose introducing a new parameter d the residence time of diffuse energy in the crust which is physically more meaningful than a quality factor to quantify the leakage effect

characterization of the causative fault system for the 2001 bhuj earthquake of mw 7 7 abstract precise hypocenters erh m w 2 0 5 3 delineate a east west trending blind thrust dipping 45 towards south named as north wagad fault nwf about 25 km north of kachchh main land fault kmf as the causative fault for the 2001 bhuj earthquake of m w 7 7 the aftershock zone involves a crustal volume of 60 40 35 km 3 lying between kmf and nwf the waveform inversion of 45 aftershocks of m 3 0 5 3 suggest that the e w trending south dipping reverse faulting mainly characterizes the aftershock zone however some reverse faulting along nw se as well as ne sw planes and some strike slip faulting along nw se trending vertical plane are also noticed the estimated p axes point on an average towards n s while t axes orient in e w agreeing well with the prevailing regional plate tectonic stress directions the estimated velocity model from one dimensional inversion of 8000 p and 5000 s travel times from 600 aftershocks deciphers a detailed crustal structure of the region upper 0 6 km on an average suggests a low velocity zone characterizing the jurassic and younger sediments the depth range 6 42 km is characterized on an average by larger values of v p 6 31 to 6 98 km s with an average of 6 71 km s v s 3 64 to 4 05 km s with an average of 3 85 km s and v p v s 1 69 to 1 81 with an average of 1 75 the high crustal velocities can be attributed to the existence of a high velocity mafic intrusive rift pillow structure beneath the region probably emplaced during the rifting time this brittle competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of m w 7 5 in the lower crust further within the overall higher velocity depth range a layer characterized by large values of v p 6 98 km s v s 3 854 km s and v p v s 1 81 is found in the depth range of 20 5 30 km which is inferred to be fractured and saturated high crack density and probably fluid filled presence of numerous fractures in this layer is confirmed by a high b value 0 75 1 1 in this layer this layer might have facilitated the nucleation process of the 2001 bhuj earthquake the major rupture and 47 of the aftershocks have occurred within this layer

separation of intrinsic and scattering attenuation using single event source in south korea abstract simultaneous use of numerous events in the multiple lapse time window analysis mltwa method is the most effective way to obtain and values such simultaneous use of numerous events normally shows a large observational scatter which may be caused by different radiation pattern and focal depth of each earthquake and regional alteration of the local structure to avoid these scattering factors the current study attempted to use single event source as the input data in mltwa for south korea the direct simulation monte carlo dsmc method was used to simulate the observations which showed realistic results when the number of recorded stations was eight or more the obtained values of seismic albedo b 0 were less than 0 5 which were in accordance with the same in the previous results three envelopes constructed by combination of three regional values of at 1 hz produced regional variations of which the ones at high frequencies of and at low frequencies of were remarkable this result showing the local variation indicated an advantage of the study using single source event over the previous studies in south korea that involved extensively distributed data because dsmc was known to be applicable to 3d structure due to its simple algorithm the current study also tried to correct radiation pattern of the observations by inserting focal mechanism algorithm to the code of the dsmc method such insertion however generated extremely low values of because the geometrically shrunk receiver could not capture most of the scattered particles this receiver problem is most likely to be a major obstacle of the 3d study of dsmc in addition the significant depth effect of q 1 was identified for the prior mltwa studies resulting from different measurement range and focal depth

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

internal geometry and evolution of moulins storglaciaren sweden depart ment of physical geog raphy university of stoc kholm s 106 91 stockholm sweden abstract the initial conditions needed for formation of moulins are crevasses and a supply of melt water water pouring into a crevasse may fill it until it ove rflows at the lowest point which is normally near the margin however as the crev asse deepens it intersects englacial chann els through which the water can drain these chann els may be finger tip tributaries in a dendritic system such as that described by shreve 1972 and observ ed by raymond and harrison 1975 when the cre vasse closes heat in the melt water keeps the conne ction open and a moulin is formed the englacial channel enlarges rapidly by melt ing utilizing mechanical energy released by the descending water descents into moulins and mapping of stru ctures exposed at the surface after many years of melting demonstrate that the drainage channels leading down from the bottoms of the moulins have incli nations of 0 450 from the vertical these channels trend in the dire ction of the original crevasse but appear to be deeper than the expected depth of the crevasse they have not

estimation of scattering and intrinsic attenuation based on multiple lapse time window analysis in sikkim himalayan region india abstract attenuation of seismic wave energy was estimated in and around the source region of the sikkim earthquake of 18 september 2011 using aftershocks the main shock had a moment magnitude 6 9 and after this main shock approximately 283 aftershocks with a magnitude distribution ranging from 1 02 to 5 0 were recorded by 8 temporary broad band seismometers in the present study the relative contributions of scattering attenuation and intrinsic absorption are estimated by multiple lapse time window analysis under the hypothesis of isotropic scattering and a uniform distribution of scatterers in the crustal part of the study area the coda wave attenuation is also estimated using single backscattering method all of the attenuation parameters are estimated as a function of frequency in the range 1 16 hz the results show that scattering attenuation is greater than intrinsic absorption for all of the frequencies the relative contribution of scattering attenuation decreases with increasing frequency around the 1 4 hz frequency band scattering attenuation primarily contributes to seismic wave attenuation in the sikkim region the estimated values of seismic albedo ranges from 0 65 to 0 79 as seismic albedo is higher than 0 5 for all frequencies it is concluded that the medium in the sikkim himalayas is highly heterogeneous and tectonically active our knowledge of the attenuation mechanism of the sikkim region is enhanced by the application of the multiple lapse time window analysis method and results can be useful for the hazard assessment

spatial and temporal correlation between coda q and seismicity in china a map of coda q at 1 hz q was constructed for china by analyzing more than 1000 seismograms from 80 seismic stations distributed throughout china the resultant contour map of q shows a strong correlation with the epicenters of historic great earthquakes in china the high q regions are devoid of the epicenters and the low q regions are full of them a remarkably linear relation was found between the logarithm of coda q and the maximum earthquake within the area sampled by coda waves radius of about 150 km from each station when we consider earthquakes which occurred from 1700 to present roughly speaking if q is 1000 m max is about 4 and if q is 100 m max is about 8 this relation will be very useful for siting critical facilities such as nuclear power plants and waste disposal sites we also found evidence in support of the migration of low q region coincident with the well known migration of high seismicity region over the distance of several hundred kilometers during the period of several hundred years if the migration is due to a diffusion process the diffusion constant is of the order of 100 km2 yr which agrees very well with those associated with the aftershock migration fluid injection and vp vs anomaly comparison of coda q with q1 estimated from the intensity data shows that they agree very well for q1 150 supporting the validity of the s to s single backscattering model for coda waves for q1 150 however coda q becomes constant independent of q1 this behavior is qualitatively consistent with numerical experiments of frankel and clayton and suggests that coda q may be measuring the intrinsic q of about 150 for these regions where q1 150 this limiting coda q value may depend on regions because coda q lower than 100 has been observed in some active regions in this study and also coda q as low as 30 to 40 has been reported in california mexico and italy

improving surface boundary conditions with focus on coupling snow densification and meltwater retention in large scale ice sheet models of greenland snowpack changes during the melt season are often not incorporated in modelling studies of the surface mass balance of the greenland ice sheet densification of snow accelerates when meltwater is present due to percolation and subsequent refreezing and needs to be incorporated in ice sheet models for ablation calculations in this study simple parameterizations to calculate surface melt snow densification and meltwater retention are included as surface boundary conditions in a large scale ice sheet model of greenland coupling the snow densification and meltwater retention processes achieves a separation of volume and mass changes of the surface layer in order to determine the surface melt contribution to runoff experiments for present day conditions show that snow depth at the onset of melt mean annual near surface air temperature and the mean density of the annual snow layer are key factors controlling the quantity and spatial distribution of meltwater runoff above the equilibrium line on the greenland ice sheet

high frequency seismogram envelope inversion using a multiple nonisotropic scattering model application to aftershocks of the 2008 wells earthquake abstract in this paper we use the sato 1995 nonisotropic scattering formulation to describe an intensity spectral density with a spherical impulse wave radiated from the source a characteristic source time is introduced to define the initial impulse width we apply the modified multiple nonisotropic scattering model to analyze velocity seismogram envelopes of the 2008 wells earthquake in nevada a series of scattering patterns is generated by a nonisotropic scattering pattern function higher order spherical functions are included in the expansion of the scattering pattern function so that stronger nonisotropic scattering patterns can be represented a variety of synthesized envelopes from backward dominated forward dominated and isotropic scattering processes are investigated by comparing synthesized envelopes with observed data we found that the backscattering pattern fails to predict the observations of high frequency s waves and envelopes from a forward dominated scattering pattern provide the best fit to the data based on these results we offer a new interpretation for the origination of high frequency s coda waves the broadening effect of the envelope is successfully explained by the process being dominated by forward scattering for all the aftershocks the frequency dependence of intrinsic attenuation and scattering attenuation is discussed for frequency bands of 1 2 2 4 4 8 and 8 16 hz both types of attenuation decrease when frequency increases and the intrinsic attenuation dominates over scattering attenuation at higher frequencies

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

persistent englacial drainage features in the greenland ice sheet 1 surface melting on the greenland ice sheet is common up to 1400 m elevation and in extreme melt years even higher water produced on the ice sheet surface collects in lakes and drains over the ice sheet surface via supraglacial streams and through the ice sheet via moulins water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally here we use ice penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred we find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years we find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years our observations indicate that once established these persistent moulins might be capable of establishing well connected meltwater drainage pathways

attenuation of high frequency p and s waves in garhwal himalaya india abstract tectonics of a region plays important role on the attenuation characteristics of the region attenuation characteristics have been estimated to understand the effect of the heterogeneity of the region in a tectonically active garhwal region of himalaya seismic body wave attenuation characteristics in the region is studied using 234 short period seismic observations from local events the p wave attenuation qp 1 and s wave attenuation qs 1 were estimated by applying the state of the art extended coda normalization method for the frequency range from 1 5 to 24 hz estimates of qp 1 and qs 1 decrease from 15 86 0 90 10 3 and 5 35 0 51 10 3 at 1 5 hz to 0 54 0 11 10 3 and 0 34 0 06 10 3 at 24 hz respectively the values of qp 1 and qs 1 show strong frequency dependence and fit the power law relation qp 1 f 27 43 3 15 10 3f 1 16 0 06 and qs 1 f 8 05 0 74 10 3f 0 93 0 05 respectively obtained results are in the range of those reported for qp 1 and qs 1 of the other seismically active regions the ratio of qp 1 qs 1 is larger than unity in the entire analyzed frequency range and may suggest high degree of heterogeneity in the region