the effect of earth rheology and ice sheet size on fault slip and magnitude of postglacial earthquakes abstract moderate past and present day seismicity is observed in formerly deglaciated regions of north america and europe an understanding of the occurrence of these earthquakes is important for estimating the seismic risk within these areas as well as areas affected by recent ice sheet melting in particular greenland and antarctica we have developed a new finite element approach that allows us to estimate the fault throw for areas once covered by a continental ice sheet the simulation is initialized by loading a two dimensional earth model with an ice sheet the model incorporates a stress field consisting of rebound horizontal background and vertical stresses as well as a fault that can accommodate slip the sensitivity of fault throw and activation time is tested with respect to lithospheric and crustal thickness viscosity structure of upper and lower mantle and ice sheet thickness and width as well as fault location and angle single event seismic displacements of up to 18 5 m are obtained approximately equivalent to an earthquake with a moment magnitude of 8 5 the thickness of the crust and lithosphere are major parameters affecting the total magnitude of fault slip whereas the size of the ice sheet primarily affects the activation time most faults start to move close to the end of deglaciation and movement typically stops for our simulations after one thrusting reverse earthquake however in our simulations faults with a dip of 60 also show several fault movements before and after the end of deglaciation

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

the relative contribution of orbital forcing and greenhouse gases to the north american deglaciation understanding what drove northern hemisphere ice sheet melt during the last deglaciation 21 7 ka can help constrain how sensitive contemporary ice sheets are to greenhouse gas ghgs changes the roles of orbital forcing and ghgs in the deglaciation have previously been modeled but not yet quantified here for the first time we calculate the relative effect of these forcings on the north american deglaciation by driving a dynamical ice sheet model glimmer cism with a set of unaccelerated transient deglacial simulations with a full primitive equation based ocean atmosphere general circulation model famous we find that by 9 ka orbital forcing has caused 50 of the deglaciation ghg 30 and the interaction between the two 20 orbital forcing starts affecting the ice volume at 19 ka 2000 years before co2 starts increasing in our experiments a delay which partly controls their relative effect

tectonic significance of present day stress relief phenomena in formerly glaciated regions abstract investigations were conducted in several regions of norway with the purpose of detecting and measuring stress relief features and to derive from them valuable information on the shallow crustal stress state stress relief features are induced by blasting and sudden rock unloading in road construction and quarrying operations and are common in norway and probably in other regions of fennoscandia as they are in ne america stress relief at the earth9s surface is diagnostic of anomalously high stress levels at shallow depths in the crust and characterizes the formerly glaciated baltic and canadian precambrian shields the mean orientation of the maximum horizontal compressive stress axis as deduced from the orientation of stress relief features was found to be nw se consistent with north atlantic ridge push forces we show that our determined stress orientations are in excellent agreement with other kinds of stress data as a final step we estimate ridge push force magnitudes from gravity data in the north atlantic region our computations suggest that strong ridge push forces characterize the baltic and canadian shields we conclude from this quantitative analysis and our fieldwork results that present day stress relief in fennoscandia and ne america is mostly triggered by plate scale ridge push forces and not by residual glacial loading stresses

postglacial rebound and the focal mechanisms of eastern canadian earthquakes a detailed numerical model of postglacial rebound for the eastern canadian arctic is described with special attention being paid to baffin island this numerical model is able to account for the observed uplift patterns as recorded by the relative sea level histories of discrete sites distributed throughout the study area these uplift patterns show regional trends so the same model can be used to interpolate between data sites and to estimate the uplift history at any arbitrary site within the study area by treating the lithosphere as a thin elastic plate spatial variations in the uplift pattern can be translated into estimates of lithospheric stress the model predicts the variations in uplift as a function of both space and time since deglaciation and can therefore be used to estimate the temporal evolution of lithospheric stress following deglaciation the stress so calculated is treated as a perturbation to an ambient stress field having its origin in other processes postglacial rebound is shown to

activity along the osning thrust in central europe during the lateglacial ice sheet and lithosphere interactions abstract the osning thrust is one of the major fault systems in central europe with a length of approximately 115 km it underwent a polyphase tectonic evolution in the mesozoic that ranged from extensional movements in the jurassic to reverse faulting and thrusting during inversion in the late cretaceous a series of complex metre scale faults and related fold structures are developed within the upper pleistocene alluvial aeolian complex of the upper senne 1 km away from the thrust the faults dip towards the north and show offsets in a range of several decimetres growth strata indicate a two fold evolution of the structures the faults began as normal faults and were later transformed into reverse faults which resulted in the formation of small scale inversion structures with a typical harpoon shape glaciotectonics can be ruled out as the driving mechanism for these structures because new optically stimulated luminescence osl ages imply that the sedimentary succession was deposited during the late pleistocene between 29 3 2 9 ka and 13 1 1 5 ka at which time the ice margin was much further to the northeast we believe the inversion structures were caused by movement on the osning thrust due to generation of a forebulge of the late pleistocene weichselian ice sheet this led to normal faulting as a consequence of extension in the forebulge area during glaciation the osl ages for the normal fault related growth strata are in a range of 16 13 ka later reverse movements occurred during deglaciation due to the n s directed compressional stress field in northern germany numerical simulations of the deglaciation seismicity point to seismic events with a thrust mechanism in the study area between 15 5 and 12 3 ka bp although normal faulting is also possible in this time period numerous soft sediment deformation structures imply that movement on the osning thrust caused earthquakes with a significant magnitude in the autumn of 1612 an earthquake took place in this area it is very likely that this event is related to background seismicity on this fault although movement due to the influence of the ongoing glacial rebound in fennoscandia is also possible

changes in orientation of near surface stress field as constraints to mantle viscosity and horizontal stress differences in eastern canada observations indicate that the orientation of the stress field in eastern canada has changed significantly during the last 9 000 years if this temporal rotation of stress orientation is due to changes of the dominant stress component from rebound stress 9 000 years ago to tectonic stress at the present then this rotation in stress orientation can be used to constrain mantle viscosity and the difference between the horizontal tectonic principal stresses rebound stress in a maxwell earth due to the application of a realistic ice load is calculated with the finite element method and is superposed on tectonic stress to give the total stress it is demonstrated that large tectonic stress differences 10 mpa or high viscosity 1022 pa s in the lower mantle cannot explain this temporal stress rotation because in the former case stress orientation is determined by the static tectonic stress while for the latter case rebound stress continues to dominate the current stress orientation

a comparative study of gabbro and granite weathering in the piedmont of the southeastern u s a gabbros and granites weather in quite different fashions granites develop extensive weathering rind systems 2 m thick in which plagioclase is altered to a gibbsite kaolinite assemblage in rindlets adjacent to the fresh rock contact in the outer rindlets the clay fraction is dominated by kaolinite and dioctahedral vermiculite not surprisingly granitic weathering rind systems display relative depletions of si na ca rb and sr and relative enrichments of sesquioxides in contrast gabbro weathering rind systems are characterized by many thin 1 cm rindlets whose alteration products are dominated by limonitization of ferromagnesian minerals apart from a decrease in feofe2o3 ratios in the outer rindlets there is little relative depletion or enrichment of major elements in gabbro weathering rinds as compared to their fresh rock cores this contradiction in goldich s stability series is explained if weathering rinds are produced faster on gabbros than in granite outcrops thus the low degree of mineral alteration in gabbro rindlets reflects a shorter time of residence for minerals in this system compared to rindlets on granitic outcrops biotite and perthitic microcline were separated from a fresh granite and its attendant weathering rind system during alteration the albitic blebs of the perthite are dissolved leaving no trace of residual clay the result is a porous microcline whose composition is equivalent to the microcline matrix in the unweathered perthite progressive alteration of biotite results in oxidation of octahedrally coordinated fe2 with concomitant loss of k this results in the formation of dioctahedral vermiculites in outer rindlet samples of granite

late pliocene greenland glaciation controlled by a decline in atmospheric co2 levels it is thought that the northern hemisphere experienced only ephemeral glaciations from the late eocene to the early pliocene epochs about 38 to 4 million years ago and that the onset of extensive glaciations did not occur until about 3 million years ago several hypotheses have been proposed to explain this increase in northern hemisphere glaciation during the late pliocene here we use a fully coupled atmosphere ocean general circulation model and an ice sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the greenland ice sheet in particular we find that greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the late pliocene by contrast our model results suggest that climatic shifts associated with the tectonically driven closure of the panama seaway with the termination of a permanent el nino state or with tectonic uplift are not large enough to contribute significantly to the growth of the greenland ice sheet moreover we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the earth s orbit

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

holocene lowering of the laurentide ice sheet affects north atlantic gyre circulation and climate the laurentide ice sheet which covered canada during glacial periods had a major influence on atmospheric circulation and surface climate but its role in climate during the early holocene 9 7 ka when it was thinner and confined around hudson bay is unclear it has been suggested that the demise of the ice sheet played a role in the 8 2 ka event an abrupt 1 3 c northern hemisphere cooling lasting 160 years through the influence of changing topography on atmospheric circulation to test this hypothesis and to investigate the broader implications of changing ice sheet topography for climate we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9 5 to 8 0 ka between 9 5 and 8 0 ka our simulations show a 2 c cooling south of iceland and a 1 c warming between 40 and 50 n in the north atlantic these surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid north atlantic as the ice sheet lowers the climate response is strongest during the period of peak ice volume change 9 5 8 5 ka but becomes negligible after 8 5 ka the climatic effects of the laurentide ice sheet lowering during the holocene are restricted to the north atlantic sector thus topographic forcing is unlikely to have played a major role in the 8 2 ka event and had only a small effect on holocene climate change compared to the effects of changes in greenhouse gases insolation and ice sheet meltwater

evidence from seismites for glacio isostatically induced crustal faulting in front of an advancing land ice mass rugen island sw baltic sea abstract it is hitherto commonly thought that earthquakes triggered by geologically rapid changes in the pressure caused by the fluctuating extent of thick land ice masses occur exclusively during ice retreat our study of deformed sediment layers within an undeformed glaciolacustrine and glaciofluvial succession exposed in a coastal cliff on rugen island south western baltic sea challenges this widespread idea based on their structural and deformational features the layers under investigation are interpreted as seismites which formed as a result of seismically induced liquefaction the stratigraphic context aided by optically stimulated luminescence osl dating indicate that the documented seismites must have formed shortly before the front of the pleistocene scandinavian ice sheet reached the study area during the last glacial maximum thus during a stage of ice advance this implies that the flexural isostatic response of the earth s crust as a consequence of the ice load during ice advance was accompanied by earthquakes probably due to local re activation of pre existing faults the crustal response to an increasing ice load was consequently less gradual than previous studies about glaciation related tectonics suggest

major earthquake at the pleistocene holocene transition in lake vattern southern sweden lake vattern sweden is within a graben that formed through rifting along the boundary between two precambrian terrains geophysical mapping and geological coring show that substantial tectonic movements along the lake vattern graben occurred at the very onset of the holocene this is evident from deformation structures in the soft sediment accumulated on the lake floor our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a 80 km long fault system if these large displacements are from one tectonic event lake vattern must have had an earthquake with seismic moment magnitudes to 7 5 in addition our geophysical mapping shows large landslides along sections of the steep lake shores pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the younger dryas preboreal transition ca 11 5 ka we suggest that this event is mainly related to the rapid release of ice sheet load following the deglaciation this paleoseismic event in lake vattern ranks among the larger known intraplate tectonic events in scandinavia and attests to the significance of glacio isostatic unloading

the 8 2 ka cooling event caused by laurentide ice saddle collapse abstract the 8 2 ka event was a period of abrupt cooling of 1 3 c across large parts of the northern hemisphere which lasted for about 160 yr the original hypothesis for the cause of this event has been the outburst of the proglacial lakes agassiz and ojibway these drained into the labrador sea in 0 5 5 yr and slowed the atlantic meridional overturning circulation thus cooling the north atlantic region however climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial length freshwater forcings such as rerouting of continental runoff and ice sheet melt in combination with the lake release here we show that instead of being caused by the lake outburst the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over hudson bay in north america we forced a general circulation model with time varying meltwater pulses 100 300 yr that match observed sea level change designed to represent the hudson bay ice saddle collapse a 100 yr long pulse with a peak of 0 6 sv produces a cooling in central greenland that matches the 160 yr duration and 3 c amplitude of the event recorded in ice cores the simulation also reproduces the cooling pattern amplitude and duration recorded in european lake and north atlantic sediment records such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability these new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets

late cretaceous to recent tectonic evolution of the north german basin and the transition zone to the baltic shield southwest baltic sea abstract in this study we investigate the late cretaceous to recent tectonic evolution of the southwestern baltic sea based on a dense grid of seismic reflection profiles this area covers the baltic sea sector of the salt influenced north german basin and its transition to the salt free baltic shield across the tornquist zone the upper cretaceous to recent structural evolution is discussed by means of individual seismic sections and derived high resolution time structure maps of the main horizons i e the upper cretaceous tertiary and pleistocene the upper cretaceous and tertiary layers reveal numerous significant faults throughout the study area several of these faults propagate upwards across the unconsolidated pleistocene sediments and occasionally penetrate the surface the salt influenced north german basin reveals three major fault trends nw se n s and nne ssw several of these faults are located directly above basement sub salt faults and salt pillows the majority of these faults are trending n s to nne ssw and parallel the direction of the gluckstadt graben faults in the salt free tornquist zone we identify two major shallow fault trends which are nw se and ne sw the majority of these faults are located above basement faults following the direction of the tornquist zone we conclude that generally basement tectonics controls activation and trends of shallow faults if salt is present the ductile salt layer causes a lateral shift between the sub and supra salt faults major plate reorganisation related to the africa iberia europe convergence and the subsequent alpine orogeny caused reactivation of pre existing faults and vertical salt movement in the late cretaceous the change of stress orientation from ne sw to a nw se during neogene caused another phase of fault and salt tectonic reactivation we explain that the ice sheet loading and or present day stress field may have acted in combination causing the recent tectonics and upward extension of the faults

exploring the ingredients required to successfully model the placement generation and evolution of ice streams in the british irish ice sheet abstract ice stream evolution is a major uncertainty in projections of the future of the greenland and antarctic ice sheets accurate simulation of ice stream evolution requires an understanding of a number of ingredients that control the location and behaviour of ice stream flow here we test the influence of geothermal heat flux grid resolution and bed hydrology on simulated ice streaming the palaeo record provides snapshots of ice stream evolution with a particularly well constrained ice sheet being the british irish ice sheet biis we implement a new basal sliding scheme coupled with thermo mechanics into the bisicles ice sheet model to simulate the evolution of the biis ice streams we find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included we show that the new basal sliding scheme allows the accurate simulation for the majority of biis ice streams the extensive empirical record of the biis has allowed the testing of model inputs and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location spacing and migration of ice streams through millennia simulated ice streams also prompt new empirical mapping of features indicative of streaming in the north channel region ice sheet model development has allowed accurate simulation of the palaeo record and allows for improved modelling of future ice stream behaviour

the deglaciation of the americas during the last glacial termination this paper reviews current understanding of deglaciation in north central and south america from the last glacial maximum to the beginning of the holocene together with paleoclimatic and paleoceanographic data we compare and contrast the pace of deglaciation and the response of glaciers to major climate events during the global last glacial maximum glgm 26 5 19 ka average temperatures decreased 4 to 8 c in the americas but precipitation varied strongly throughout this large region many glaciers in north and central america achieved their maximum extent during the glgm whereas others advanced even farther during the subsequent heinrich stadial 1 hs 1 glaciers in the andes also expanded during the glgm but that advance was not the largest except on tierra del fuego hs 1 17 5 14 6 ka was a time of general glacier thickening and advance throughout most of north and central america and in the tropical andes however glaciers in the temperate and subpolar andes thinned and retreated during this period during the bolling allerod interstadial b a 14 6 12 9 ka glaciers retreated throughout north and central america and in some cases completely disappeared many glaciers advanced during the antarctic cold reversal acr 14 6 12 9 ka in the tropical andes and patagonia there were small advances of glaciers in north america central america and in northern

deglacial rapid sea level rises caused by ice sheet saddle collapses a simulation of ice sheet retreat reveals two meltwater pulses coinciding with mwp 1a and the 8 200 year event caused by saddle collapses between ice domes during the last deglaciation the retreating ice sheets occasionally released massive amounts of fresh water these meltwater pulses are often implicated as important influences on abrupt climate change but the underlying triggering mechanisms are not clear here lauren gregoire et al show that saddle zones that connect adjacent high domes on ice sheets can be lowered by ongoing warming to an elevation at which they rapidly degrade leading to separate domes and a meltwater pulse that is consistent with geological observations although the mechanism itself is not novel the authors establish that two important sudden cooling events meltwater pulse 1a and the 8 200 year event were probably caused by saddle separation the last deglaciation 21 to 7 thousand years ago was punctuated by several abrupt meltwater pulses which sometimes caused noticeable climate change1 2 around 14 thousand years ago meltwater pulse 1a mwp 1a the largest of these events produced a sea level rise of 14 18 metres over 350 years3 although this enormous surge of water certainly originated from retreating ice sheets there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise4 5 6 here we present an ice sheet modelling simulation in which the separation of the laurentide and cordilleran ice sheets in north america produces a meltwater pulse corresponding to mwp 1a another meltwater pulse is produced when the labrador and baffin ice domes around hudson bay separate which could be associated with the 8 200 year event the most pronounced abrupt climate event of the past nine thousand years7 for both modelled pulses the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming the melting then rapidly accelerates as the saddle between the two domes gets lower producing nine metres of sea level rise over 500 years this mechanism of an ice saddle collapse probably explains mwp 1a and the 8 200 year event and sheds light on the consequences of these events on climate

spatial heterogeneity of tectonic stress and friction in the crust the complex geometry of faults seismicity and diversity of earthquake mechanisms suggest that the stress and strength in earth s crust are spatially heterogeneous we investigated the degree of heterogeneity using the following two end member models in one end member model we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies in this model the variability of earthquake mechanisms means that friction during faulting must vary for each event we computed friction from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure the values of vary over a large range from 0 to 1 5 in the other extreme model we assumed optimally oriented slip and a constant 0 6 as is suggested by byerlee s law for all the earthquakes and determined the local stress orientation for each earthquake the orientation of the stress changes drastically from one earthquake to another and the assumption of uniform stress field commonly used in stress inversion is not warranted an important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive the actual situation in the crust is most likely to be intermediate between these two end member models from the existing data alone we cannot determine the degree of heterogeneity uniquely but both and the local stress field near earthquake faults are likely to vary substantially and studies on earthquake rupture dynamics must take these heterogeneities into consideration

mechanical evolution of transpression zones affected by fault interactions insights from 3d elasto plastic finite element models abstract the mechanical evolution of transpression zones affected by fault interactions is investigated by a 3d elasto plastic mechanical model solved with the finite element method ductile transpression between non rigid walls implies an upward and lateral extrusion the model results demonstrate that a transpression zone evolves in a 3d strain field along non coaxial strain paths distributed plastic strain slip transfer and maximum plastic strain occur within the transpression zone outside the transpression zone fault slip is reduced because deformation is accommodated by distributed plastic shear with progressive deformation the 3 axis the minimum compressive stress rotates within the transpression zone to form an oblique angle to the regional transport direction 9 10 the magnitude of displacement increases faster within the transpression zone than outside it rotation of the displacement vectors of oblique convergence with time suggests that transpression zone evolves toward an overall non plane strain deformation the slip decreases along fault segments and with increasing depth this can be attributed to the accommodation of bulk shortening over adjacent fault segments the model result shows an almost symmetrical domal uplift due to off fault deformation generating a doubly plunging fold and a positive flower structure outside the overlap zone expanding asymmetric basins subside to negative flower structures on both sides of the transpression zone and are called transpressional basins deflection at fault segments causes the fault dip fall to less than 90 86 89 near the surface 1 5 km this results in a pure shear dominated triclinic and discontinuous heterogeneous flow of the transpression zone

northern hemisphere forcing of southern hemisphere climate during the last deglaciation changes in ocean circulation are the most plausible explanation for the early southern hemisphere deglacial warming and its lead over northern hemisphere temperature milankovitch theory which was developed to explain the relationship between earth s movement and the climate holds that glacial cycles are ultimately controlled by variations in northern hemisphere summer insolation exposure to the sun s rays spectral analyses of palaeoclimate records support this theory but a mechanistic test has been elusive other lines of evidence also show that climate changes in the southern hemisphere may have led those in the northern hemisphere casting doubt on the main theory here feng he and colleagues use a coupled ocean atmosphere model to show that the last deglaciation was indeed triggered by northern hemisphere insolation changes amplified by albedo co2 and ocean circulation changes according to the milankovitch theory changes in summer insolation in the high latitude northern hemisphere caused glacial cycles through their impact on ice sheet mass balance1 statistical analyses of long climate records supported this theory but they also posed a substantial challenge by showing that changes in southern hemisphere climate were in phase with or led those in the north2 although an orbitally forced northern hemisphere signal may have been transmitted to the southern hemisphere3 insolation forcing can also directly influence local southern hemisphere climate potentially intensified by sea ice feedback4 5 6 suggesting that the hemispheres may have responded independently to different aspects of orbital forcing signal processing of climate records cannot distinguish between these conditions however because the proposed insolation forcings share essentially identical variability7 here we use transient simulations with a coupled atmosphere ocean general circulation model to identify the impacts of forcing from changes in orbits atmospheric co2 concentration ice sheets and the atlantic meridional overturning circulation amoc on hemispheric temperatures during the first half of the last deglaciation 22 14 3 kyr bp although based on a single model our transient simulation with only orbital changes supports the milankovitch theory in showing that the last deglaciation was initiated by rising insolation during spring and summer in the mid latitude to high latitude northern hemisphere and by terrestrial snow albedo feedback the simulation with all forcings best reproduces the timing and magnitude of surface temperature evolution in the southern hemisphere in deglacial proxy records8 9 amoc changes associated with an orbitally induced retreat of northern hemisphere ice sheets10 is the most plausible explanation for the early southern hemisphere deglacial warming and its lead over northern hemisphere temperature the ensuing rise in atmospheric co2 concentration provided the critical feedback on global deglaciation9 11

the moho depth distribution in fennoscandia and the regional tectonic evolution from archean to permian times abstract from a variety of seismic refraction and reflection profiling results a new detailed moho depth map for the fennoscandian part of the baltic shield has been compiled a complementary crystalline crustal thickness map was compiled for denmark the skagerrak sea offshore western norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more these maps are discussed in relation to tectonic evolution and crustal ages of fennoscandia seismic velocities bouguer gravity and seismicity areas deviating from the proposed age thickness relationships are the kola peninsula archean and the oslo rift permian the average crustal thickness in the former area is around 44 km while the latter area is associated with a moho elevation of 3 5 km maximum crustal thicknesses in excess of 50 km have been found in southwestern and central finland and correlate with a crust of proterozoic age the caledonides of western norway do not exhibit as elsewhere a crustal root sub moho p n and s n seismic velocities are relatively small in that area and bouguer gravity values are strongly negative intracrustal p velocity variations do not seem to reflect geological age or specific tectonic features although the seismicity is moderate in fennoscandia most of the earthquake activity and in particular the largest earthquakes are confined to areas where the crustal thinning is most pronounced

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

response of normal faults to glacial interglacial fluctuations of ice and water masses on earth s surface 1 changes in surface loads during glacial interglacial cycles and the associated flexure and rebound of the lithosphere alter the stress field in actively extending regions and may lead to slip rate variations on normal faults in particular loading may cause a period of seismic quiescence that is followed by temporal clustering of earthquakes during and after unloading here we present a suite of finite element experiments to evaluate how the magnitude distribution and temporal evolution of the load as well as rheological parameters of the lithosphere and asthenosphere influence the response of a normal fault the results show that the duration of the seismically quiet period during loading and the intensity of the slip rate increase during unloading are primarily controlled by the magnitude of the load the time lag between the changes in loading and the reaction of the fault is mainly determined by the viscosity of the asthenosphere parameters that play only a minor role for the fault s response include the rate of load removal fault strength and the thickness of the lithosphere our results imply that earthquake recurrence models derived from holocene slip histories may not be representative for the long term evolution of seismogenic faults

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

using commercial finite element packages for the study of earth deformations sea levels and the state of stress summary modifications to commercial finite element fe packages must be applied before they can be used for geophysical studies involving long wavelength deformation or viscoelasticity this paper provides in detail how and why the commercial codes have to be modified when incompressibility is assumed both the non self gravitating flat earth and self gravitating spherical earth will be considered the latter involves an iterative procedure which converges within 5 iterations this is demonstrated both analytically and numerically in addition implementation of the gravitationally self consistent sea level equation on a self gravitating spherical earth is also described good agreement between numerical results obtained with this coupled finiteelement method and the conventional spectral method is also demonstrated in all cases the interpretation of the outputs of fe models are particularly important in modelling the state of stress

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

astronomical solutions for paleoclimate studies over the last 3 million years three solutions numerical qtd90 semi analytical la90 and analytical ber90 for the planetary and precessional elements of the earth have recently become available they allow the computation of the fundamental astroclimatic parameters eccentricity climatic precession and obliquity the agreement between these different solutions is remarkably good reflecting their accuracy in such a way that they can be considered reliable enough for paleoclimate studies over at least 3 myr the difference between these solutions in eccentricity is less than 0 005 over this time interval while the differences in obliquity and climatic precession are larger but mainly due to phase lag between the solutions at 3 myr bp ber90 leads la90 by 6 x 10 3 yrs and qtd90 by 9 x 10 3 yrs for the obliquity and it leads la90 by 2 x 10 3 yrs and qtd90 by 4 x 10 3 yrs for the climatic precession

evidence of late and postglacial seismic activity in the temiscouata madawaska valley quebec new brunswick canada three types of geological phenomena independently suggest that the temiscouata madawaska valley was affected by one or more seismic events following its deglaciation 1 subbottom acoustic profiling of lac temiscouata and grand lac squatec revealed disturbance of bottom sediments by mass transport processes in both lakes erosional truncation of preexisting acoustically laminated sediments and accumulation of hummocky chaotic deposits over older hummocky surfaces or laminated sequences both result from mass transport processes unidirectional mass flows from several points in these symmetrical basins in situ disruption of laminated sediment beneath flat bottoms and the large area of the lake floors affected suggest strong similarities in sedimentation style with lakes that have been disturbed during strong earthquakes 2 southeast of lac temiscouata in saint jacques new brunswick two separate mass flow deposits made up largely of coarse 0 5 m angular boulders of local bedrock occur on oppos

stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle

ice sheet growth and high latitudes sea surface temperature the response of the lln 2 d climate model to the insolation and co2 forcings during the eemian interglacial is compared to reconstructions obtained from deep sea cores drilled in the norwegian sea and in the north atlantic both reconstructions and modeling results show a decrease of sea surface temperature sst in the higher latitudes 70 75 n zonal belt for the model and the norwegian sea for the proxy records associated with a more moderate cooling at lower latitudes 50 55 n and north atlantic at the middle of isotopic substage 5e several millenia before the beginning of continental ice sheet growth such a comparison between the simulated sst and ice volume of the northern hemisphere has been extended to the whole last glacial interglacial cycle the influence of the insolation forcing on sst and the shortcomings of the model due to its zonal character are discussed

reply to comment by hampel et al on stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle reply to comment by hampel et al on stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle

abrupt bolling warming and ice saddle collapse contributions to the meltwater pulse 1a rapid sea level rise elucidating the source s of meltwater pulse 1a the largest rapid sea level rise caused by ice melt 14 18 m in less than 340 years 14 600 years ago is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change here we quantify how much and by what mechanisms the north american ice sheet could have contributed to meltwater pulse 1a by driving an ice sheet model with two transient climate simulations of the last 21 000 years ice sheet perturbed physics ensembles were run to account for model uncertainties constraining ice extent and volume with reconstructions of 21 000 years ago to present we determine that the north american ice sheet produced 3 4 m global mean sea level rise in 340 years due to the abrupt bolling warming but this response is amplified to 5 6 m when it triggers the ice sheet saddle collapse

fennoscandian earthquakes whole crustal rupturing related to postglacial rebound local and regional earthquake locations provide seismic evidence that large shield earthquakes have occurred in northern fennoscandia these paleoearthquakes with fault lengths of up to 160 kilometers and average displacements of up to 15 meters were triggered by nonisostatic compressive stresses caused by the removal of the ice at the end of the last deglaciation the fennoscandian faults were probably formed by single events that ruptured through most of the crust the largest event moment magnitude mw 8 2 was larger than other known stable continent earthquakes outside failed rifts or extended crust

glacial isostatic adjustment of scandinavia and northwestern europe and the radial viscosity structure of the earth s mantle summary during the last glacial maximum lgm large ice sheets covered scandinavia the barents sea and the northern british isles subsequent to the lgm the ice sheets disappeared and the solid earth readjusted towards a new isostatic equilibrium the glacial isostatic adjustment process is documented in numerous observations for example palaeoshorelines covering the last deglaciation phase and ongoing crustal deformations monitored by gps stations for example the bifrost project in this study we use palaeoshoreline data from scandinavia the barents sea and northwestern europe nw as well as radial crustal velocities from the bifrost campaign to infer the radial viscosity structure of the earth s mantle underneath scandinavia and nw europe a global inverse procedure based on the neighbourhood algorithm allows us to explore the hypothesis of a low viscosity zone in the upper mantle which has been proposed in the literature our results indicate a low viscosity zone underneath the barents sea with viscosities between 1019 and 1020 pa s in a depth interval of 160 200 km no such low viscosity zone is found underneath scandinavia and no clear indication for such a zone underneath nw europe the thickness of the rheological lithosphere increases from 60 to 70 km underneath nw europe and the barents sea towards values exceeding 120 km underneath scandinavia

surficial geology indicates early holocene faulting and seismicity central sweden in sweden knowledge of the location and timing of glacially induced faulting and seismicity is critical to effective engineering of a long term nuclear disposal facility to improve understanding and modeling of the complex ice induced and tectonic stresses associated with glacially induced faulting field studies detailing the location and timing of movement of such structures are required although the fault has not been confirmed in the bedrock multi proxy surficial geologic evidence indicates that the recently discovered scarp in bollnas is such a structure machine excavated trenches across the scarp reveal landsliding down the scarp and in one location faulted and vertically offset fine grained glacial sediments the presence of water escape structures in trenches excavated on a topographic high strongly suggests a co seismic origin derived from earthquake magnitudes 5 5 numerous landslides in till exist in the region as well four slopes with landslides were examined in detail and the factors of safety for these slopes indicate stable conditions and suggest a seismic trigger basal radiocarbon dates from peat bogs located stratigraphically above the landslides provide minimum limiting ages for the co seismic landslides the oldest date indicates sliding prior to 10 180 calendar years before the present the proposed bollnas fault is 400 km south of the so called lapland fault province to date it is the southernmost confirmed glacially induced fault in sweden the results of this study are consistent with existing modeling results that indicate fault instability in this region of central sweden following deglaciation

dependence of horizontal stress magnitude on load dimension in glacial rebound models summary it has been proposed that the deglaciation of the northern hemisphere triggered large earthquakes within intraplate environments and in this paper we examine this hypothesis by evaluating quantitatively the stress state in the lithosphere produced by time dependent surface loads a series of models demonstrate the dependence of horizontal incremental stress in an elastic plate overlying an inviscid or viscoelastic mantle on the lateral extent of a load applied at the surface the horizontal stress is largest when the dominant wavelength that is twice the diameter of the load is close to eight times the elastic thickness of the plate when the mantle is inviscid and for the particular viscosity model employed in this paper close to 12 times the elastic thickness for a viscoelastic mantle at wavelengths close to the critical wavelength the horizontal incremental stress may be up to six times as large as the vertical incremental stress for appropriate earth model parameters amplification of horizontal stress is close to maximum for ice loads with a radius of 280 km comparable to the dimensions of the former ice sheet over great britain this amplification may be sufficiently large that loading by small ice sheets can lead to failure on marginally stable faults in contrast to the behaviour for large ice sheets the models also predict greater fault instability for fennoscandia than for the larger laurentide ice sheet consistent with the observation of large postglacial faults in northern sweden the model is used to predict the stability of faults and style of faulting due to rebound stresses in the absence of a background tectonic stress field since the last glacial maximum 18 000 years ago in northern europe within the formerly glaciated region thrust faulting is predicted to occur at the end of deglaciation and normal faulting is predicted to occur in peripheral regions for the entire period since the last glacial maximum

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

induced stresses and fault potential in eastern canada due to a realistic load a preliminary analysis summary in order to understand the causal relation between postglacial rebound and earthquakes a simple disc load model is used to 1 calculate stresses induced in the lithosphere and mantle by glacial loading melting and postglacial rebound and 2 evaluate the effect of glacial loading rebound on the failure potential for earthquakes in the upper crust the dependence of the failure potential and the actual mode of failure on the coefficient of friction the ambient tectonic stress magnitude direction the stress due to the overlying rocks and lithospheric thickness are investigated prominent features of this paper are the inclusion of 1 a viscoelastic mantle and thus the migration of stress and 2 the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field it is assumed that throughout the earth there are optimally oriented pre existing virtual faults that are initially close to but not at failure thus a time dependent quantity called dfsm related to the coulomb mohr failure criterion can be defined such that a negative value of dfsm would advocate faulting or earthquake activities whereas a positive value of dfsm would promote stability the results indicate that under all combinations of tectonic stress magnitude and overburden stress crustal loading promotes fault stability directly underneath the load upon the removal of the load thrust faulting is predicted within the ice margin if the horizontal stress s induced by the overburden is greater than or equal to the vertical component s of the overburden stress 121 where sjs under this condition theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation if the horizontal stress induced by the overburden is less than the vertical component of the overburden stress 1 l then theory predicts fault stability within the ice margin the theory predicts fault instability both north and south of the ice margin the mode of failure however is completely determined by the value of i the trade off between the tectonic stress magnitude and the overburden stress parameter 1 is also investigated it is shown that a larger tectonic stress magnitude can be used to compensate a smaller value of 1 the results of this analysis show that variations in the coefficient of friction lithospheric thickness and a ductile zone below the upper crust do not significantly affect the above conclusions

to what extent is the present seismicity of norway driven by post glacial rebound abstract the seismicity of scandinavia is low to intermediate in intensity and with magnitudes that rarely exceed 5 5 even so the seismicity is characterized by a great diversity in space and time related to the fact that its driving forces or sources of stress are multiple and also diverse in nature the dominant driving forces range from plate related such as ridge push through regional to local ones such as topography all however are somehow related to lateral inhomogeneities in lithospheric structure whereas it was earlier assumed that the post glacial uplift was the main cause for the seismicity of scandinavia we now conclude that the uplift may at most be playing a minor role along the coastal stretch of northern norway there are still some indications that the seismicity which is often shallow normal faulting and swarm related may be related to the still remaining glacial isostatic adjustment the flexuring that is needed to explain such behaviour could however also be related to erosion and sedimentation processes it will take more work to separate these potential driving forces

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

moment tensors state of stress and their relation to post glacial rebound in northeastern canada summary in the stable craton of northeastern canada moderate seismicity as well as glacial isostatic adjustment gia has been observed we investigate five earthquakes with moment magnitudes between 3 6 and 4 1 that have occurred in northern hudson bay since 2007 which may be triggered by gia focal mechanisms of the earthquakes are determined using a waveform fitting procedure for surface waves in which the best double couple mechanism is obtained through a grid search over strike dip and rake all events exhibit a thrust fault mechanism in general agreement with mechanisms of previously analysed earthquakes in that area stress inversion results incorporating our new mechanisms as well as previously published results exhibit significant differences from previously calculated stresses from gia models for the first time maximum horizontal stress direction shmax is compared to the modelled stress values for northeastern canada we find that the maximum horizontal stress direction strikes roughly nnw sse in contrast to previously inferred ne sw orientation of shmax our results indicate that the local stress field in northern hudson bay deviates from regional stresses resulting from the existence of a roughly e w oriented zone of faulting which is not included in existing gia models

the baltic shield earthquakes abstract the baltic shield area is a low seismicity area with few seismic events exceeding m l 4 since 1979 dense regional seismic networks have been in operation and have allowed accurate location including focal depths and fault plane solutions of more than 200 earthquakes in the range m l 0 6 4 5 the location of the southern sweden seismicity shows that a long part of the protogine zone the border between the younger western and older eastern crustal rocks marks a change in seismic activity the focal depths of the earthquakes indicate three seismically different crustal layers upper crust 0 13 km in northern sweden 0 18 km in southern sweden middle crust down to 35 km and the seismically quiet lower crust the gutenberg b value is smaller for the middle crust than for the upper crust the fault plane solutions show that strike slip is dominant along the tornquist line the southwestern border of the baltic shield area normal faulting occurs in northern sweden reverse faulting is more common than normal faulting the stresses released by the earthquakes show a remarkable consistency considering the small sizes of the earthquakes the data are consistent with a n60w direction of the regional principal compression supporting the view that the baltic shield seismicity is related to plate tectonic processes rather than to uplift the spatial distribution of the baltic shield seismicity is consistent with a model in which the earthquakes are considered as sudden breakdowns of asperities of normally stably sliding faults aseismic sliding is estimated to be more than 20 000 times more extensive than seismic sliding the rate at which southern sweden presently deforms horizontally is estimated to be 1 mm yr

long aftershock sequences within continents and implications for earthquake hazard assessment one of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries plate tectonics gives no insight however into where and when earthquakes will occur within plates because the interiors of ideal plates should not deform as a result within plate interiors assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes here however we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago we present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries these predictions are in accord with observations so the common practice of treating continental earthquakes as steady state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere

significant contribution of insolation to eemian melting of the greenland ice sheet during the last interglacial period the volume of the greenland ice sheet was up to 60 smaller than today climate and ice sheet modelling suggests that about 55 of this change was caused by higher ambient temperatures and the remaining 45 was a result of higher insolation and the associated climate feedbacks

north american ice sheet reconstructions at the last glacial maximum abstract the areal extent of the last glacial maximum lgm ice sheets is well known in north america but there is no direct geological proxy for ice sheet thickness or volume uncertainties associated with glaciological and geophysical reconstructions give widely varying estimates of north american ice sheet nais volume at lgm in an effort to quantify the uncertainty associated with glaciological reconstructions we conducted a suite of 190 numerical simulations of the last glacial cycle in north america prescribing different climatic mass balance glaciologic and isostatic treatments for the least constrained model variables lgm ice sheet reconstructions were evaluated using the well established geologic record of ice sheet area and southern extent at lgm dyke and prest 1987 these constraints give a subset of 33 simulations that produce reasonable lgm ice cover in north america ice sheet dispositions and the associated parameter settings in this subset of tests provide insight into the plausible range of nais thickness form and mass balance regime at lgm ice volume in this subset of tests spans a range of 28 5 38 9 10 15 m 3 at lgm with a predominant cluster at 32 36 10 15 m 3 taking floating ice and displaced continental water into account this corresponds to 69 94 m eustatic sea level msl more than 75 of the accepted tests fall in the range 78 88 msl we argue that this is a plausible estimate of the volume of water locked up in the nais at lgm

tectonic regime and slip orientation of reactivated faults the slip orientation of reactivated faults is predicted by assuming that one principal stress direction is vertical that faulting occurs on pre existing planes of weakness with a slip direction along the applied shear stress and by extending the original concept of tectonic regime so as to include the stress tensor aspect ratio the predicted rake of slip is then analysed so as to reveal the respective influence of the fault plane orientation strike and dip and of the tectonic regime the analysis of this direct problem allows one to describe the geometry of the slip directions within any given tectonic regime its evolution as the tectonic regime varies and the constraints provided by slip direction data on the tectonic regime however the question of what fault orientations would satisfy a failure criterion and thus be preferentially reactivated is not addressed in any tectonic regime pure strike slip occurs on vertical planes and only on them except in wrench regimes where it also occurs on all fault planes along four strikes that are determined by the tectonic regime pure dip slip occurs on planes striking along one of the horizontal principal stress directions and only on them except in radial compression and radial extension regimes where it occurs on all planes except vertical ones at fixed strike but varying dip the slip direction is closest to dip slip for zero dip and pure strike slip for vertical dip at fixed dip and varying strike the slip direction is pure dip slip for strikes along one of the horizontal principal stress directions and closest to strike slip for four strikes that are determined by the tectonic regime the horizontal principal stress directions separate four quadrants two where the slip direction is dextral and two where it is sinistral the slip directions of all fault planes of the same dip can be easily deduced from those of subhorizontal planes and can be constructed geometrically as the tectonic regime varies continuously from radial compression to radial extension the domain of fault plane orientations where steep reverse slip directions are possible shrinks and disappears when the extensional regime is entered conversely the domain where steep normal slip directions are possible appears when the wrench regime is entered and grows thereafter the strike slip domain first grows towards shallow dip until the wrench regime is entered and then shrinks after the extensional regime is entered whereas in the model of failure in isotropic rocks dip slip or strike slip data fully determine the original tectonic regime in the model of failure on pre existing planes of weakness constraints on the extended tectonic regime require additional information the sense of shallow slip direction data which are to be found on steeply dipping planes constrains the horizontal principal stress directions and the steepness of slip direction data on shallow dipping planes striking away from the principal stress directions constraints the extended tectonic regime in the case of wrench regimes an extra constraint arises from the fact that the extended tectonic regime also controls the strike at which the dip component of slip changes from normal to reverse these constraints can be quantified by superposing a simple plot of the data rake versus strike with an abacus that is called breddin s graph for tectonic regimes while this graph does not replace inverse techniques it may help in detecting tectonic phases in complex data sets without using this graph or resorting to inverse techniques the transition from wrench to either compressional or extensional regime is difficult to infer from data that display a limited range of fault plane strikes and the same sense of dip component of the slip direction

triassic to recent tectonic evolution of a crestal collapse graben above a salt cored anticline in the gluckstadt graben north german basin abstract in this study we investigate faulting above a salt wall in the gluckstadt graben north german basin two supra salt faults are mapped from coast to coast over a distance of 6 9 km based on offshore and onshore seismic data these faults form a ca 2 km wide crestal collapse graben and pierce the seafloor salt wall evolution started in the early late triassic to early jurassic due to regional extension and resulting sub salt faulting the salt wall was eroded following exposure to costal and sub aerial erosion by the regional mid late jurassic to early cretaceous uplift late cretaceous to early paleogene compressional tectonics reactivated the vertical salt movement and shortened the salt wall creating a salt cored anticline with the crestal collapse graben above the supra salt faults were reactivated between the late eocene and middle miocene when the principal horizontal stress orientation changed from a ne sw to a nw se the present day orientation stratigraphic data indicate that these faults moved mainly in the cenozoic several observations strongly suggest that the faults continued developing during the pleistocene until today i the pleistocene unconformity is concave upwards and cut by faults ii growth strata within the marine holocene deposits above the graben imply recent tectonic movements iii onshore high resolution p wave vibroseis data of the south eastern eckernforde bay suggest about 10 m of faulted holocene strata and iv marine seismic data show the faults piercing the seafloor we suggest that the recent salt tectonics and upward propagation of supra salt faults resulted from differential ice sheet loading that effect on the salt wall stopped once ice grew over the whole structure at which time the wall subsided because of ice loading the salt wall and faults were reactivated again once the ice front retreated so that the ice loaded only one side of the structure

how much does slip on a reactivated fault plane constrain the stress tensor given a fault plane and its slip vector the stress tensor which caused the displacement is sought two constraints are considered first a geometrical constraint that the shear stress applied to the fault plane is parallel to the slip wallace 1951 bott 1959 second a frictional constraint that the shear to normal stress ratio equals tan 0 coulomb 1776 in a first step the stress tensors that satisfy the geometrical constraint are sought for tensors belonging to the vectorial space of solutions shear and normal stress magnitudes become a function of the orientation of the principal stresses and are mapped extending a study by mckenzie 1969 in a second step it is investigated which of these tensors also satisfy the frictional constraint within this more restricted vectorial space there is a relationship between the magnitudes 1 2 1 3 and s 1 3 1 and the orientations of the principal stresses both the range of s and the spatial distribution of are more restricted than when the geometrical constraint alone is considered as when the geometrical constraint is solely considered mckenzie 1969 the orientations of the principal stresses may lie significantly away from and up to right angle to the p b t axes however this can happen only in two cases 1 either the effective stress difference s has reached a high value which is unlikely to happen if enough preexisting fractures are available to release the stress or 2 2 becomes close to either 1 or 3 and therefore barely distinguishable from it in that case the delocalization of the orientations of the principal stresses is best described by a tendency for to exchange role with either or when the stress difference remains small and 2 reasonably away from 1 and 3 approach positions that we define as the pf b tf axes and that are obtained from the p b t axes by a rotation of angle 0 2 around b and toward the slip vector this explains why the p b t axes gives reasonable estimates of the orientations of the principal stresses scheidegger 1964 despite objections mckenzie 1969 however whenever the fault plane can be distinguished from the auxiliary plane pf b tf should give a better estimate raleigh et al 1972 in an area where many fault planes are available and a uniform tensor is assumed the scatter in the plane orientations contains information about both the relative position of 2 represented by and the relative stress difference s the higher s or the closer to either 0 or 1 the more scatter this information could then be extracted by inverse methods because a friction law would constrain these inverse method more tightly it may show the necessity of nonuniform tensor to explain scattered fault plane

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

suppression of earthquakes by large continental ice sheets the interior regions of antarctica and greenland are aseismic no earthquake larger than body wave magnitude 4 5 5 0 is known for either except along coastal zones or continental shelves here i advance an explanation for this lack of seismic activity in terms of pressure effects produced by the continental ice sheets that mantle both continents

effect of viscosity structure on fault potential and stress orientations in eastern canada summary previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern canada have focused on the mode of failure and the observed timing of the pulse of earthquake faulting activity following deglaciation in this study the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times it is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 1021 pa s mantle the effects of viscosity structure on the above predictions are also examined it is shown that since most of the above observations are found within the ice margin they are not very sensitive to lithospheric thickness also the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust high viscosity 1022 pa s in the lower mantle is rejected by the stress orientation and rotation observations a low viscosity 6 times 1020pa s upper mantle with 1 6 times 1021 pa s in the upper part of the lower mantle and 3 times 1021 pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations

ocean loading effects on stress at near shore plate boundary fault systems 1 changes in eustatic sea level since the last glacial maximum create a differential load across coastlines globally the resulting plate bending in response to this load alters the state of stress within the lithosphere within a half flexural wavelength of the coast we calculate the perturbation to the total stress tensor due to ocean loading in coastal regions our stress calculation is fully 3 d and makes use of a semianalytic model to efficiently calculate stresses within a thick elastic plate overlying a viscoelastic or fluid half space the 3 d stress perturbation is resolved into normal and shear stresses on plate boundary fault planes of known orientation so that coulomb stress perturbations can be calculated in the absence of complete paleoseismic indicators that span the time since the last glacial maximum we investigate the possibility that the seismic cycle of coastal plate boundary faults was affected by stress perturbations due to the change in sea level coulomb stress on onshore transform faults such as the san andreas and alpine faults is increased by up to 1 1 5 mpa respectively promoting failure primarily through a reduction in normal stress these stress perturbations may perceptibly alter the seismic cycle of major plate boundary faults but such effects are more likely to be observed on nearby secondary faults with a lower tectonic stress accumulation rate in the specific instance of rapid sea level rise at the black sea the seismic cycle of the nearby north anatolian fault was likely significantly advanced

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

effect of ice sheet growth and melting on the slip evolution of thrust faults abstract field investigations suggest that postglacial unloading and rebound led to the formation or re activation of reverse faults even in continental shields like scandinavia here we use finite element models including a thrust fault embedded in a rheologically layered lithosphere to investigate its slip evolution during glacial loading and subsequent postglacial unloading the model results show that the rate of thrusting decreases during the presence of an ice sheet and strongly increases during deglaciation the magnitude of the slip acceleration is primarily controlled by the thickness of the ice sheet the viscosity of the lithospheric layers and the long term shortening rate in contrast the width of the ice sheet the rate of deglaciation or the fault dip have an only minor influence on the slip evolution in all experiments the slip rate variations are caused by changes in the differential stress the modelled deglaciation induced slip acceleration agrees well with the occurrence of large earthquakes soon after the melting of the fennoscandian ice sheet which led to the formation of spectacular fault scarps in particular in the lapland fault province furthermore our model results support the idea that the low level of seismicity in currently glaciated regions like greenland and antarctica is caused by the presence of the ice sheets based on our models we expect that the decay of the greenland and antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions

applying the community ice sheet model to evaluate pmip3 lgm climatologies over the north american ice sheets we apply the community ice sheet model cism2 to determine the extent to which the last glacial maximum lgm temperature and precipitation climatologies from the paleoclimate modelling intercomparison project 3 pmip3 simulations support the large north american ice sheets that were prescribed as a boundary condition we force cism2 with eight pmip3 general circulation models gcms and an additional model genmom seven gcms simulate lgm climatologies that support positive surface mass balances of the laurentide and cordilleran ice sheets lis cis consistent with where ice was prescribed in the gcms two gcms simulate july temperatures that are too warm to support the ice sheets four of the nine cism2 simulations support ice sheets in beringia in absence of prescribed ice in the driving gcms and in disagreement with geologic evidence that indicates the area remained ice free during the lgm we test the sensitivity of our results to a range of snow and ice positive degree day factors daily monthly and climatological temperature and precipitation inputs and we evaluate the role of albedo and snow in the simulations areas with perennial snow in the gcm simulations correspond well to the presence of ice in the cism2 simulation gcms with unrealistically low surface albedos over the lis yield simulations that fail to simulate realistic ice sheets