analysis of 2014 2015 on and off shore passive seismic data on the reykjanes peninsula sw iceland abstract passive seismic techniques are a valuable tool to map active faults follow changes in the local stress field caused by drilling and exploitation processes in geothermal areas and estimate seismic velocity ratios they have however not been used extensively in basaltic crust as we find it in iceland due to the data processing challenges that high impedance contrasts between layers of igneous rocks pose to improve conventional passive seismic techniques and to test new ones a dense network of seismometers recorded seismic activity on and off shore the reykjanes peninsula in southwest iceland from spring 2014 until august 2015 in addition to the existing long term and permanent networks a temporary network consisting of 30 on land stations and 24 ocean bottom seismometers obs was deployed the network was laid out as concentric circles around the reykjanes geothermal area at the tip of the peninsula and was about 100 km in diameter the data of the on land stations were collected every 70 days and data recorded by the obss were read out after one year when they were recovered we used the seiscomp3 software to automatically detect earthquakes and then manually revised the p and s arrival times a total of 2066 earthquakes could be located along the mid ocean ridge and within and around the reykjanes geothermal system earthquake hypocentres associated with the geothermal area occur in the uppermost 2 km while events along the spreading axis occur at 4 6 km depth on land and appear to deepen with increased distance from the shore we see a cluster of about 200 events induced by the drilling and onset of operation of a local injection borehole in the reykjanes geothermal area we find a vp vs ratio from all picks of 1 78 consistent with earlier studies in the area focal mechanisms calculated for selected events reveal a regime dominated by strike slip and normal faulting

the physical basis of mb ms and variable frequency magnitude methods for earthquake explosion discrimination we use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake explosion discrimination we use six three dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials four factors contribute to the success of the mb ms discriminant source spectra focal mechanism near source elastic properties and interference of p spectral differences contribute about 0 5 magnitude unit to the separation between earthquake and explosion populations for large events but this difference vanishes for events with mb 4 0 using a frequency band of 0 5 3 0 hz for small events the vfm discriminant is complementary to mb ms in several ways vfm is improved by p interference while mb ms is degraded vfm is more effective for hard rock explosions while mb ms will preferentially discriminate explosions in low velocity materials and vfm is insensitive to fault orientation vfm is more sensitive to noise and regional attenuation differences than mb ms simultaneous use of both methods should allow improved discrimination of small events

gutenberg s surface wave magnitude calibrating function theoretical basis from synthetic seismograms the surface wave magnitude as originally defined by gutenberg 1945 appears to be the most stable quantity rating the strengths of seismic sources it is based on the ground displacement produced by a nearly monochromatic wave train the calibrating function underlying the magnitude definition was obtained from observations of the amplitudes as a function of epicentral distance for a wealth of earthquakes no distinction was made as to focal depths thus the calibrating function depends on one parameter the epicentral distance only subsequent attempts to introduce a focal depth correction were not successful in practice and one and the same calibrating function continues to be applied to events with focal depths ranging from 0 to about 70 km assuming a strike slip point source located inside a standard earth model synthetic seismograms of the fundamental mode sh love wave are computed for a range of focal depths and azimuths amplitude distance curves are obtained and averaged over the azimuths the curves are the basis for improved calibrating functions average depth corrections are obtained and applied to some populations of seismic events the corrected surface wave magnitudes are internally more consistent than the uncorrected ones usually published applying the depth correction to published surface wave magnitudes and considering themb ms relation for a sample of earthquakes and explosions it has been established that for a given intensity of short periodic radiation the 20 s radiation intensity can vary by orders of magnitude while it is known that the 20 s radiation intensity is generally lower for explosions than for earthquakes it turns out to be even lower if the depth correction is applied yielding a clearer separation between both classes of seismic events

the thermomechanical response of the greenland ice sheet to various climate scenarios using a three dimensional numerical model for land based ice sheets in the shallow ice approximation simulations are performed to determine the velocity and temperature distributions within the greenland ice sheet through time for various climate scenarios the ice is treated as a rheologically nonlinear heat conducting viscous fluid and the substrate is a heat conducting rigid solid this system is fed from above by prescribing as the climatic inputs the atmospheric temperature and the accumulation ablation rate functions at the free surface and from below by the geothermal heat we present the governing equations in the shallow ice approximation discuss the parameterizations used in the descriptions of the ice surface temperature and accumulation ablation functions briefly state how the complicated initial boundary value problem is numerically solved and how the input data that are available from measurements are implemented results of preliminary calculations disclose how the model performs and delimit its validity we study the role played by basal sliding and make clear that sliding should be accounted for where ever the basal ice is temperate and that the frictional heat generated in this sliding is thermomechanically significant we also study the reaction of the greenland ice sheet to various climate scenarios and make clear that today s thermal regime depends significantly upon the prior climate history moreover the thermomechanical properties of the ice are equally significant as is the thermal interaction of the ice sheet with the rockbed beneath

seismicity in a transpressional volcanic arc the liquine ofqui fault system in the puyuhuapi area southern andes chile 44 s

a numerical study of plane ice sheet flow l ecoulement bidimensionnel stationnaire d une nappe de glace reposant sur le sol est analyse numeriquement au moyen du modele simplifie de morland et hutter la glace se comporte comme un fluide visqueux non lineaire dependant de la temperature la nappe de glace est supposee longue et mince

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

on the geometric complexity of earthquake focal zone and fault systems a statistical study we discuss various methods used to investigate the geometric complexity of earthquakes and earthquake faults based both on a point source representation and the study of interrelations between earthquake focal mechanisms we briefly review the seismic moment tensor formalism and discuss in some detail the representation of double couple dc earthquake sources by normalized quaternions non dc earthquake sources like the clvd focal mechanism are also considered we obtain the characterization of the earthquake complex source caused by summation of disoriented dc sources we show that commonly defined geometrical fault barriers correspond to sources without any clvd component we analyze the cmt global earthquake catalog to examine whether the focal mechanism distribution suggests that the clvd component is likely to be zero in tectonic earthquakes although some indications support this conjecture we need more extensive and significantly more accurate data to answer this question fully

effects of spatial discretization in ice sheet modelling using the shallow ice approximation this paper assesses a two dimensional vertically integrated ice model for its numerical properties in the calculation of ice sheet evolution on a sloping bed using the shallow ice approxi mation we discuss the influence of initial conditions and individual model parameters on the model s numerical behaviour with emphasis on varying spatial discretizations the modelling results suffer badly from numerical problems they show a strong dependence on gridcell size and we conclude that the widely used gridcell spacing of 20 km is too coarse the numerical errors are small in each single time step but increase non linearly over time and with volume change as a result of feedback of the mass balance with height we propose a new method for the calculation of the surface gradient near the margin which improves the results significantly furthermore we show that we may use dimension analysis as a tool to explain in which situations numerical problems are to be expected

deglaciation of the northern hemisphere at the onset of the eemian and holocene high resolution tims u series dating of sea level events obtained from coral reef complexes suggests that global deglaciation from the saale penultimate glacial to the eem interglacial marine 18o stages 6 5 may have occurred earlier in relation to milankovitch insolation forcing than that from the wisconsinan glacial to the holocene interglacial marine 18o stages 2 1 however the interpretation of these data has been problematic because of the possibility of isotope exchange in order to investigate whether these different lead lag relations between milankovitch forcing and ice volume are feasible from the point of view of large scale ice sheet dynamics and thermodynamics the three dimensional polythermal ice sheet model sicopolis simulation code for polythermal ice sheets is applied to the entire northern hemisphere which gives the major contribution to global ice volume changes due to the relative stability of the antarctic ice sheet and simulations through the last two climatic cycles are conducted the simulations cover the interval from 250 kyr bp until today and are driven by surface temperature reconstructions of deep ice cores grip vostok and simple parameterizations for the change of precipitation with time discussion of the results is focused on the saale eem and the wisconsinan holocene transitions the amount and rate of deglaciation are in good agreement with the specmap record for both cases and the evidence of the data for an early start of the eem interglacial is supported

effect of source depth correction on the estimation of earthquake size the relationship between surface wave magnitude ms and seismic moment mo of earthquakes is essential for the estimation of seismic risk in any region in the hypothesis of constant stress drop theoretical models predict that log mo and ms are related by a linear law the slope most commonly found in the literature is around 1 5 here we show that the application to the ms values of the necessary correction for the focal depth gives a general increment of the correlation coefficient and that a slope around 1 0 is consistent with the global data while for regionalized data it can vary from about 1 0 to 2 0

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

polythermal three dimensional modelling of the greenland ice sheet with varied geothermal heat flux computations over 50 000 years into steady state with greve s polythermal ice sheet model and its numerical code are performed for the greenland ice sheet with today s climatological input surface temperature and accumulation function and three values of the geothermal heat flux 42 54 6 29 4 mw m 2 it is shown that through the thermomechanical coupling the geometry as well as the thermal regime in particular that close to the bed respond surprisingly strongly to the basal thermal heat input the most sensitive variable is the basal temperature field but the maximum height of the summit also varies by more than 100m furthermore some intercomparison of the model outputs with the real ice sheet is carried out showing that the model provides reasonable results for the ice sheet geometry as well as for the englacial temperatures

foreshock occurrence before large earthquakes rates of foreshock occurrence involving shallow m 6 and m 7 mainshocks and m 5 foreshocks were measured in two worldwide catalogs over 20 year intervals the overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span the observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in california the aftershock model was extended to the case of moderate foreshocks preceding large mainshocks overall the observed worldwide foreshock rates exceed the extended california generic model by a factor of 2 significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region with the rate before thrust events higher and the rate before strike slip events lower than the worldwide average among the thrust events a large majority composed of events located in shallow subduction zones had a high foreshock rate while a minority located in continental thrust belts had a low rate these differences may explain why previous surveys have found low foreshock rates among thrust events in california especially southern california while the worldwide observations suggests the opposite california lacking an active subduction zone in most of its territory and including a region of mountain building thrusts in the south reflects the low rate apparently typical for continental thrusts while the worldwide observations dominated by shallow subduction zone events are foreshock rich if this is so then the california generic model may significantly underestimate the conditional probability for a very large m 8 earthquake following a potential m 7 foreshock in cascadia the magnitude differences among the identified foreshock mainshock pairs in the harvard catalog are consistent with a uniform distribution over the range of observation

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

permanent fast flow versus cyclic surge behaviour numerical simulations of the austfonna ice cap svalbard a large part of the ice flux within ice caps occurs through spatially limited fast flowing units some of them permanently maintain fast flow whereas others operate in an oscillatory mode characterized by short lived active phases followed by long quiescent phases this surge type behaviour results from intrinsic rather than external factors thus complicating estimates of glacier response to climate change here we present numerical model results from austfonna an ice cap on svalbard that comprises several surge type basins previous studies have suggested a thermally controlled soft bed surge mechanism for svalbard we systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow surge type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure melting point and enhanced sliding of marine grounded ice irrespective of the dynamic regime the absence of considerable volumes of temperate ice both in the observed and simulated ice cap indicates that fast flow is accomplished by basal motion over a temperate bed given an idealized present day climate the equilibrium ice cap size varies significantly depending on the chosen parameters

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

study of the properties of seismicity in the mexico region summary new statistics of seismic properties are investigated in space and time for the mexico region they are main events seismicity number of earthquakes with aftershocks excluded swarming property average magnitude another way to study the slope of the recurrence time law maximum magnitude average focal depth and comparative efficiency of earthquakes in surface and body waves generation known as creepex quite a number of anomalies of the spatial distributions of the statistics were found some of them have evident geophysical meaning the explanation of others is rather controversial and some others have no interpretation as yet the most interesting phenomena are as follows 1 the average focal depth distribution indicates a different trend for the benioff zone west of approximately 96ow longitude due north which fits the position of the neovolcanic belt 2 creepex clearly distinguishes different tectonic areas i e subduction zones with low values transform faults medium and ridges high 3 high swarming is related to places with young sedimentary geology the time pattern of those properties in places of strong earthquakes is quite consistent and includes a phase of concentration of stress indicated by quiescence in average and maximum magnitude and main events seismicity and also more efficient body wave generation and anomalous swarming this phase is followed by an apparent weakening of the material indicated by a higher efficiency in surface wave generation

delayed seismicity rate changes controlled by static stress transfer on 15 june 2010 a m w5 7 earthquake occurred near ocotillo california in the yuha desert this event was the largest aftershock of the 4 april 2010 m w7 2 el mayor cucapah emc earthquake in this region the emc mainshock and subsequent ocotillo aftershock provide an opportunity to test the coulomb failure hypothesis cfs we explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative cfs change imparted by the ocotillo event based on simple cfs calculations we divide the yuha desert into three subregions one triggering zone and two stress shadow zones we find the nominal triggering zone displays immediate triggering one stress shadowed region experiences immediate quiescence and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown we quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of cfs change with the rate state earthquake rate formulation of dieterich 1994 assuming that each subregion contains a mixture of nucleation sources that experienced a cfs change of differing signs our modeling reproduces the observations including the observed delay in the stress shadow effect in the third region following the ocotillo aftershock the delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations we find that rate state models of time dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity

scenarios for earthquake generated tsunamis on a complex tectonic area of diffuse deformation and low velocity the alboran sea western mediterranean the tsunami impact on the spanish and north african coasts of the alboran sea generated by several reliable seismic tsunamigenic sources in this area was modeled the tectonic setting is complex and a study of the potential sources from geological data is basic to obtain probable source characteristics the tectonic structures considered in this study as potentially tsunamigenic are the alboran ridge associated structures the carboneras fault zone and the yusuf fault zone we characterized 12 probable tsunamigenic seismic sources in the alboran basin based on the results of recent oceanographical studies the strain rate in the area is low and therefore its seismicity is moderate and cannot be used to infer characteristics of the major seismic sources these sources have been used as input for the numerical simulation of the wave propagation based on the solution of the nonlinear shallow water equations through a finite difference technique we calculated the maximum wave elevations and tsunami travel times using the numerical simulations the results are shown as maps and profiles along the spanish and african coasts the sources associated with the alboran ridge show the maximum potential to generate damaging tsunamis with maximum wave elevations in front of the coast exceeding 1 5 m the carboneras and yusuf faults are not capable of generating disastrous tsunamis on their own although their proximity to the coast could trigger landslides and associated sea disturbances the areas which are more exposed to the impact of tsunamis generated in the alboran sea are the spanish coast between malaga and adra and the african coast between alhoceima and melilla

sensitivity of the modeled present day greenland ice sheet to climatic forcing and spin up methods and its influence on future sea level projections 1 the accuracy of the modeled present day greenland ice sheet gris is crucial for future projections of gris changes in this study we present a detailed evaluation of the modeled present day gris sensitivity to different present day climatology data sets and spin up methods and further assess the influence of the modeled present day gris on future sea level projections our study demonstrates that with present day climatology data sets derived from the regional atmospheric climate model racmo2 the modeled ice volume area and elevation of the gris agree substantially better with observations compared to simulations with present day climatology data sets from temperature parameterizations and era interim reanalysis with transient spin up the simulated rates of ice elevation changes for the 1993 2010 period are closer to observations than those with steady state spin up these results indicate that the racmo2 forcing and transient spin up may be preferable for use in the ice sheet model moreover our results reveal that the present day climatology data sets and spin up methods affect future sea level projections using transient spin up the ice sheet model estimates a range of 26 mm in projected sea level rise by 2098 under the a1b emissions scenario due to different present day climatology data sets compared to the results with transient spin up the estimated sea level rise by 2098 is reduced by 5 21 mm with steady state spin up this discrepancy is attributed mainly to differences in ice thickness and ice velocity between the modeled present day gris with transient and steady state spin ups and the effect of paleoclimatic changes with transient spin up

a model for polythermal ice incorporating gravity driven moisture transport the flow of ice sheets and glaciers dissipates significant amounts of heat which can result in the formation of temperate ice a binary mixture of ice and small amounts of melt water that exists at the melting point many ice masses are polythermal in the sense that they contain cold ice below the melting point as well as temperate ice temperature and melt water or moisture content conversely affect the flow of these ice masses through their effect on ice viscosity and sliding behaviour ice flow models therefore require a component that can solve for temperature and moisture content and determine the free boundary between the cold and temperate subdomains we present such a model based on the theory of compacting partial melts by contrast with other models we describe gravity and pressure gradient driven drainage of moisture while maintaining a divergence free ice flow at leading order we also derive the relevant boundary conditions at the free cold temperate boundary and find that the boundary behaves differently depending on whether ice enters or exits the temperate region the paper also describes a number of test cases used to compare with a numerical solution and investigates asymptotic solutions applicable to the limit of small compaction pressure gradients in the temperate ice regions a simplified enthalpy gradient model is finally proposed which captures most of the behaviour of the full model in this limit

focal mechanisms and size distribution of earthquakes beneath the krafla central volcano ne iceland seismicity was monitored beneath the krafla central volcano ne iceland between 2009 and 2012 during a period of volcanic quiescence when most earthquakes occured within the shallow geothermal field the highest concentration of earthquakes is located close to the rock melt transition zone as the iddp 1 wellbore suggests and decays quickly at greater depths we recorded multiple swarms of microearthquakes which coincide often with periods of changes in geothermal field operations and found that about one third of the total number of earthquakes are repeating events the event size distribution evaluated within the central caldera indicates average crustal values with b 0 79 0 04 no significant spatial b value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt besides the seismicity analysis focal mechanisms are calculated for 342 events most of these short period events have source radiation patterns consistent with double couple dc mechanisms a few events are attributed to non shear faulting mechanisms with geothermal fluids likely playing an important role in their source processes diverse faulting styles are inferred from dc events but normal faulting prevails in the central caldera the best fitting compressional and tensional axes of dc mechanisms are interpreted in terms of the principal stress or deformation rate orientations across the plate boundary rift maximum compressive stress directions are near vertically aligned in different study volumes as expected in an extensional tectonic setting beneath the natural geothermal fields the least compressive stress axis is found to align with the regional spreading direction in the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

contemporary crustal deformation within the pamir plateau constrained by geodetic observations and focal mechanism solutions we used an updated data set of 192 gps derived surface velocities and 393 earthquake focal mechanisms mw 3 0 hypocenter depths 30 km to evaluate the spatial variations in the surface strain rate and crustal stress regime throughout the pamir plateau and its surrounding regions the strain rate field was estimated using the spline in tension approach that solves for the surface velocity in a rectangular grid and the stress field was predicted from focal mechanism solutions using the damped regional scale stress inversion drssi method of hardebeck and michael journal of geophysical research https doi org 10 1029 2005jb004144 2006 the results show that the crustal stress field around the pamir plateau is predominantly characterized by nnw sse compression and e w extension which is consistent with the principal orientations of the two dimensional surface strain rate tensor this agreement supports the notion that the pamir and southwestern tien shan are uniformly strained blocks in particular the fan shaped rotational pattern between text shmax and the strain rate from the western pamir to the tajik basin shows that the counterclockwise rotation of the text shmax orientation is associated with vertical deformation which is consistent with the idea of schurr et al tectonics 33 8 2014tc003576 2014 concerning the gravitational collapse and westward extrusion of the crust in the western pamir we propose that such a stress strain pattern dominated by nnw ese oriented compression and e w trending extension originated from a combination of the northward push of the indian continent and the southward subduction of the tien shan

a model computation of moisture content in polythermal glaciers measurements in boreholes and internal radar horizons confirm the existence of polythermal structures in polar and subpolar glaciers and ice sheets the occurrence of an intraglacial phase boundary makes it necessary to consider two layers and transition conditions between cold and temperate ice the governing field equations and constitutive relations for a two layer model are stated with emphasis on boundary and transition conditions a reduced plane flow model is introduced by using dimensionless variables and performing a scale analysis according to the stated aim of this paper a parallel sided slab model is suitable for rough analyses of borehole measurements its governing equations are deduced and their analytical and numerical treatment is discussed sensitivity tests reveal a strong dependence of moisture content profiles on the numerical value of the water diffusivity coefficient and especially on strain heating

evidence of bias in estimations of earthquake size magnitudes of earthquakes determined using traditional methods show systematic deviations dependent on tectonic setting from accurate estimates of earthquake size magnitudes are overestimated for most of the continental earthquakes underestimated for mid oceanic ridges and differences of varying sign are obtained for subduction zones this is important in evaluating the seismic risk or consideration of effects such as the seismic versus aseismic strain release as seismic moments determined from magnitudes can be wrong by as much as a factor of four

slow earthquakes along oceanic fracture zones evidence for asthenospheric flow away from hotspots abstract the regime of strain release along transform faults of the mid oceanic ridge system is studied it is shown that earthquakes along certain fracture zones exhibit systematic discrepancies between values of their magnitudes measured at short and long periods implying a regime of slower strain release also observed in a pattern of complex body waveshapes these slow fracture zones do not correlate with simple geographic or kinematic properties but usually occur in the neighborhood of hotspot volcanism frequently also characterized by gravity and bathymetry anomalies we propose that regimes of slow strain release may be due to a partial lubrication of the fault along these fracture zones which may itself be due to asthenospheric flow from the nearby hotspots along the pattern of pipelines described by vogt and johnson and by morgan

imaging active faulting in a region of distributed deformation from the joint clustering of focal mechanisms and hypocentres application to the azores western mediterranean region abstract the matching between linear trends of hypocentres and fault planes indicated by focal mechanisms fms is frequently used to infer the location and geometry of active faults this practice works well in regions of fast lithospheric deformation where earthquake patterns are clear and major structures accommodate the bulk of deformation but typically fails in regions of slow and distributed deformation we present a new joint fm and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and fms even in regions of distributed deformation we apply the method to the azores western mediterranean region with particular emphasis on western iberia the analysis relies on a compilation of hypocentres and fms taken from regional and global earthquake catalogues academic theses and technical reports complemented by new fms for western iberia the joint clustering algorithm images both well known and new seismo tectonic features the azores triple junction is characterised by fms with vertical pressure p axes in good agreement with the divergent setting and the iberian domain is characterised by nw se oriented p axes indicating a response of the lithosphere to the ongoing oblique convergence between nubia and eurasia several earthquakes remain unclustered in the western mediterranean domain which may indicate a response to local stresses the major regions of consistent faulting that we identify are the mid atlantic ridge the terceira rift the trans alboran shear zone and the north coast of algeria in addition other smaller earthquake clusters present a good match between epicentre lineations and fm fault planes these clusters may signal single active faults or wide zones of distributed but consistent faulting mainland portugal is dominated by strike slip earthquakes with fault planes coincident with the predominant nne ssw and wnw ese oriented earthquake lineations clusters offshore sw iberia are predominantly strike slip or reverse confirming previous suggestions of slip partitioning

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

spatial variations in the similarity of earthquake populations the case of the 1997 colfiorito sellano northern apennines italy seismic sequence abstract the spatial properties of events in the 1997 colfiorito sellano seismic sequence northern apennines italy were investigated using coherence a parameter derived from seismic moment tensors that quantifies the kinematic similarity between focal mechanisms the 1997 colfiorito sellano seismic sequence predominantly consists of normal faulting earthquakes with a few strike slip and reverse faulting episodes this kinematic heterogeneity is possibly related to the contemporaneous activity of two different sets of faults nw se normal faults and nne ssw sub vertical faults the latter inherited from the previous miocene compressional phase the study used two independently derived data sets of the same seismic sequence characterized by a different number of events and by different precision of spatial localisation their statistical significances assessed through a reshuffling procedure reveal that data sets with at least some hundreds of events and good positional precision are required to obtain significant results through coherence analysis results from the better quality data set indicate that this seismic sequence is characterized by a rapid decrease in the kinematic similarity between earthquake pairs within 2 km of separation particularly along directions sub perpendicular to the normal fault strike the decrease rate seems to be controlled by the geometric characteristics of the normal faults given that the mean along dip distance between fault segments is 2 km in proximity to pre existing tectonic lineaments the relative abundance of strike slip and reverse faults tends to decrease the kinematic similarity between events but does not influence the coherence decrease rate the presence of mixed focal mechanisms normal reverse and strike slip in a single seismic phase implies that mixed fault types are not restricted to polyphase tectonic histories such heterogeneous kinematics during a single phase may be induced by the presence of inherited discontinuities

insight on the crustal stress state in faial and pico islands azores from analysis of aftershocks of the 1998 earthquake the seismicity in the azores plateau can be of volcanic or tectonic origin the latter usually more destructive especially in the central group of islands in this study we aimed to obtain crustal stress indicators from seismic data collected in july 1998 following the july 9th m5 8 earthquake that struck near faial and pico islands the maximum horizontal stress shmax azimuth was obtained from two methods for the seismogenic volume the shmax was obtained from single and composite focal mechanisms whereas the state of stress beneath the seismic network was derived from shear wave splitting analysis of local earthquakes the composite focal mechanisms were obtained from a waveform clustering method applied to identify identical earthquakes the seismic anisotropy analysis could be applied to only six stations the polarization of the first s arrival being stable and with a secondary perpendicular pulse arriving 0 6 0 24 s later the focal mechanisms provided a nw se shmax with azimuth range of 96 114 and an associated transition of normal fault to strike slip dominant regime consistent with the estimated regional field of 110 120 and coherent with movement along three fault systems inland the shmax azimuths present greater variability in faial the stations located north of the caldeira present a stable azimuth of 151 153 and are consistent with the effect of a nnw sse fault system present in the islands rotating to 50 77 in the eastern sector of the pedro miguel graben probably due to a complex interaction of the wsw ene dominant fault system with old eroded volcanic complexes that segmented the island into several blocks in pico island the shmax direction azimuth ranges from 93 to 135 in a radial pattern around the island s main stratovolcano overlapping the regional stress field

simulating the mid pleistocene transition through regolith removal abstract quaternary 18o ice volume proxy records show a transition from high frequency small amplitude glacial cycles to low frequency large amplitude glacial cycles this reorganization of the climate system termed the mid pleistocene transition mpt is thought to reflect a change in land ice response to orbital forcing despite no significant change in orbital cycles during this period one potential explanation for the mpt proposes that gradual erosion of high latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing here we explore this regolith hypothesis using a complex earth system model we show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice volume response similar to proxy reconstructions of the mpt the simulated change in ice volume response is due to a combination of climate and ice flow changes with crystalline bedrock producing thicker colder ice sheets that accumulate more snowfall and have a smaller ablation zone further experiments that include transient eccentricity amplifying co2 forcing show only small differences in ice response compared to those with orbital forcing only suggesting that cycles of co2 were not the primary cause of the mpt

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

european ice sheet modelling initiative eismint model intercomparison experiments with first order mechanics 1 the european ice sheet modelling initiative eismint intercomparison experiments with thermomechanical coupling are repeated with an ice sheet model that applies the first order approximation for computing the flow field the experiments impose radially symmetric boundary conditions most of the previous results have shown the loss of implied radial symmetry i e the formation of distinct regularly spaced spokes of cold ice in the simulated basal temperatures a similar result is also observed in the presented first order model results the computed velocity components scatter widely along the marginal grid points this indicates that the spokes may be triggered by the poor representation of the margin with a regular grid where the steep gradients in the surface enhance the numerical errors additionally the generally applied second order discretization scheme tends to decouple even and odd numbered grid points thus leading to wavy solutions with a wavelength of two grid cells these patterns strongly suggest that the loss in radial symmetry is a numerical artifact

global scale analysis of seismic activity prior to 2004 sumatra andaman mega earthquake abstract following our experience in intermediate term monitoring of seismicity before large earthquakes worldwide and in some regions we apply a similar approach to the analysis of seismicity on global scale several decades before the december 26 2004 sumatra andaman mega earthquake the earth lithosphere is being considered as a single whole representing the ultimate level of the complex earth s hierarchy the study brings up the following questions are there any anomalies of the global seismicity behaviour observed in advance of the mega earthquake do these anomalies correspond to the global scale seismic patterns similar to those detected on the regional scale before great major and strong events in other words does the earth lithosphere considered as a single whole show up an approach of the mega earthquake in the way typical for events on the smaller levels of hierarchy the results of our investigation favour the positive answer to the questions specifically we found that during the decade before the sumatra andaman mega earthquake the earth lithosphere reveals at least in intermediate term scale classical symptoms of instability which can be depicted by known precursory seismic patterns these are i transformation of frequency magnitude distribution ii change in the rate of seismic activity and iii depth redistribution of activity moreover changes of dependencies between magnitudes of different types are detected at the same time the observed global scale patterns of collective behaviour of seismicity may indicate the state of criticality of the earth lithosphere before the sumatra andaman mega earthquake

dynamics of ice mass deformation linking processes to rheology texture and microstructure 1 prediction of glacier and polar ice sheet dynamics is a major challenge especially in view of changing climate the flow behavior of an ice mass is fundamentally linked to processes at the grain and subgrain scale however our understanding of ice rheology and microstructure evolution based on conventional deformation experiments where samples are analyzed before and after deformation remains incomplete to close this gap we combine deformation experiments with in situ neutron diffraction textural and grain analysis that allows continuous monitoring of the evolution of rheology texture and microstructure we prepared ice samples from deuterium water as hydrogen in water ice has a high incoherent neutron scattering rendering it unsuitable for neutron diffraction analysis we report experimental results from deformation of initially randomly oriented polycrystalline ice at three different constant strain rates results show a dynamic system where steady state rheology is not necessarily coupled to microstructural and textural stability textures change from a weak single central c axis maxima to a strong girdle distribution at 35 to the compression axis attributed to dominance of basal slip followed by basal combined with pyramidal slip dislocation related hardening accompanies this switch and is followed by weakening due to new grain nucleation and grain boundary migration with decreasing strain rate grain boundary migration becomes increasingly dominant and texture more pronounced our observations highlight the link between the dynamics of processes competition and rheological and textural behavior this link needs to be taken into account to improve ice mass deformation modeling critical for climate change predictions

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

seismic evidence for water deep in earth s upper mantle water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone observations of converted seismic waves provide evidence of a 20 to 35 kilometer thick discontinuity near a depth of 410 kilometers most likely explained by as much as 700 parts per million of water by weight

modelling the antarctic marine cryosphere at the last glacial maximum we estimate the sea ice extent and basal melt of antarctic ice shelves at the last glacial maximum lgm using a coupled ice shelf sea ice ocean model the shape of antarctic ice shelves ocean conditions and atmospheric surface conditions at the lgm are different from those in the present day these are derived from an ice shelf ice sheet model a sea ice ocean model and a climate model for glacial simulations respectively the winter sea ice in the lgm is shown to extend up to 7 of latitude further equatorward than in the present day for the lgm summer the model shows extensive sea ice cover in the atlantic sector and little sea ice in the other sectors these modelled sea ice features are consistent with those reconstructed from sea floor sedimentary records total basal melt of antarctic ice shelves in the lgm was 2147 gt a 1 which is much larger than the present day value more warm waters originating from circumpolar deep water could be easily transported into ice shelf cavities during the lgm because the full glacial grounding line extended to shelf break regions and ice shelves overhung continental slopes this increased transport of warm water masses underneath an ice shelf and into their basal cavities led to the high basal melt of ice shelves in the lgm

evolution of the east antarctic ice sheet a numerical study of thermo mechanical response patterns with changing climate an efficient numerical ice sheet model including time dependence and full thermo mechanical coupling has been developed in order to investigate the thermal regime and overall configuration of a polar ice sheet with respect to changing environmental conditions from basic sensitivity experiments in which a schematic east antarctic ice sheet is forced with a typical glacial interglacial climatic shift it is found that i the mutual interaction of temperature and deformation has a stabilizing effect on its steady state configuration ii in the transient mode this climatic transition initially leads to increased ice thickness due to enhanced accumulation after which this trend is reversed due to a warmer base timescales for this reversal are of the order of 103 years in marginal zones and of 104 years in interior regions iii horizontal heat advection plays a major role in damping possible runaway behaviour due to the dissipation strain rate feed back suggesting that creep instability is a rather unlikely candidate to initiate surging of the east antarctic ice sheet the model is then applied to four east antarctic flow lines only the flow line passing through wilkes land appears to be vulnerable to widespread basal melting due to enhanced basal warming following climatic warming time dependent modelling of the vostok flow line indicates that the vostok station area has risen about 95 m since the beginning of the present interglacial due to thermomechanical effects which is of particular interest in interpreting the palaeoclimatic signal of the ice core obtained there

global creepex distribution and its relation to earthquake source geometry and tectonic origin summary the ms mb ratio conventionally used for the discrimination of earthquakes and underground nuclear explosions has been studied for a pure earthquake catalogue in an attempt to classify earthquakes into two categories one related to extremely lowfrequency events of creep and the other to relatively high frequency explosions ms and mb measured at 20 s and 1 s respectively are considered as estimates of source spectra at two separate points thus allowing one to compare large samples of earthquakes from the spectral point of view creepex or the deviation of ms mfc points from the orthogonal regression of ms on mb is by definition a parameter independent of the size of seismic events it has been estimated from neic magnitudes and is studied in relation to parameters of central moment tensor solutions of the harvard group for this purpose the parameters of central moment tensors have been factorized to make classes of equivalence of events with the same dip and rake and arbitrary strike and size the resulting triangle representation of source mechanisms has been used to correct the global distribution of creepex for the type of source mechanism dip slip events have consistently lower creepex than strike slip ones however the global distribution of creepex either corrected or uncorrected for source mechanism has a clear tectonic pattern negative values in most of the subduction zones and positive ones in mid ocean ridges the dependence of creepex on focal depth is negligible according to empirical data for events shallower than 40 km limitation of the sample by considering only events in this depth range does not diminish considerably the amount of data and does not change the tectonic pattern of the creepex

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

total moment spectra of fourteen large earthquakes total moment spectra mt m where m is the moment rate tensor are computed for 14 large earthquakes recorded by the international deployment of accelerometers ida network using the scalar moment retrieval method proposed by silver and jordan 1982 for each event we obtain estimates of mt averaged over the 10 disjunct 1 mhz intervals in the low frequency band 1 11 mhz typical ida record sets from events with mt 0 2 a 1 a 1027 dyne cm yield standard errors on the 1 mhz averages that are generally less than 20 our multiple band estimates of mt are usually consistent with comparable single band values found by other investigators from the total moment spectra we derive the zero frequency static moment mt 0 and the characteristic source duration c 2 var f t where f t is the time function of the moment rate tensor assumed to be the same for all components the parameter c formally depends on the second degree temporal spatial and mixed spatial temporal moments of an extended source but calculations with realistic source geometries indicate that the interpretation of tc strictly in terms of the second temporal moment leads to very little error a plot of c against shows considerable scatter some events lie significantly above the empirical scaling curve of kanamori and given 1981 slow earthquakes and some below fast earthquakes examples of the former include all three deep focus events analyzed here as well as the 1970 colombia and 1963 peru bolivia deep focus earthquakes whose total moment spectra are calculated from the moment tensor solutions of gilbert and dziewonski 1975 an example of the latter is the great sumbawa earthquake of august 9 1977 24 3 a which is inferred to have a relatively shallow spatial centroid 20 km and a nearly flat moment spectrum suggesting that the faulting was confined to the upper more brittle portion of the oceanic lithosphere three other large earthquakes tonga june 22 1977 23 2 a kuril islands december 6 1978 3 6 0 2 a and tumaco december 12 1979 25 2 a exhibit moment spectra which significantly decrease toward higher frequencies indicating larger values of c the aftershocks of both the tonga and kuril islands events are distributed over a considerable range of depths consistent with rupture into the lower more ductile portions of the oceanic lithosphere we speculate that the characteristic source duration may be causally related to the depth of coseismic rupturing within the lithosphere

influence of depth focal mechanism and tectonic setting on the shape and duration of earthquake source time functions source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by tanioka and ruff 1997 and coworkers were compared in a systematic way they were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration time function durations picked by tanioka and ruff 1997 are proportional to the cube root of seismic moment if moments from the harvard centroid moment tensor catalog are used the average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones with a distinct change occurring at 40 km depth the complexity of the time functions as quantified by the number of subevents appears to decrease below 40 km depth furthermore among events shallower than 40 km the average duration of scaled time functions is shorter and their average shape has a more abrupt termination 1 for events with strike slip focal mechanisms compared to thrust events and 2 for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate subduction boundary in each of these cases events in more technically and seismically active settings have a longer duration and a more gradual termination this can be interpreted in terms of lower stress drops and or slower rupture velocities at active plate boundaries suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates furthermore differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones supporting data table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on seraching and submitting electronic supplements is found at http www agu org pubs esupp about html