collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

the greenland earthquake of 11 july 1987 and postglacial fault reactivation along a passive margin abstract intraplate earthquakes in many cases occur near continental margins this observation applies very well to the case of greenland a stable continental region with low seismicity the seismicity of greenland is nearly totally concentrated along the coastlines of the island where it is relatively free of ice cover on 11 july 1987 the largest greenland earthquake of instrumental record m b 5 5 m w 5 4 occurred in continental crust off the northeastern coast in the wandel sea waveforms of long and short period teleseismic p and long period teleseismic sh waves have been modeled to estimate the source parameters for this earthquake the focal mechanism determined is a normal faulting with a strike slip component strike 148 dip 59 and rake 131 the strike direction of one of the nodal planes was found to be consistent with the direction of the dominant structural trend observed in the area a previous normal faulting mechanism with very different strike directions for this event was found to have inferior waveform fit and is not consistent with the fault strike directions from the local geological setting the seismic moment of the july 1987 earthquake is 1 75 10 24 dyne cm the focal depth and stress drop are 5 km and 389 bars respectively the high stress drop may relate to the relatively long earthquake repeat time in the region the observed normal faulting of this earthquake can be interpreted as a rupture due to an extensional stress field at the forebulge outside of the ice sheet because of deglaciation flexure in the region another major earthquake 26 november 1971 m b 5 1 311 km to the south of the 11 july 1987 event was likewise determined to be normal faulting and can be explained as another event triggered by deglaciation in addition we find that the sparse general seismicity of greenland can also be related to postglacial rebound

focal mechanisms of three earthquakes in finland and their relation to surface faults abstract focal mechanisms for three recent earthquakes in finland are determined using p wave polarities together with sv p and sh p phase amplitude ratios the events occurred on may 11 2000 in toivakka central finland ml 2 4 on september 15 2000 in kuusamo northeastern finland ml 3 5 and on may 2 2001 in kolari western finnish lapland ml 2 9 in order to obtain reliable estimates of the source parameters one dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep seismic sounding results the starting models are modified by one dimensional ray tracing using the earthquake observations the events are relocated by employing p and s phase arrival times from the nearest seismic stations and the final velocity models synthetic waveforms calculated with the reflectivity method are used to further constrain and verify the source and structural parameters the toivakka earthquake indicates thrust or reverse faulting mechanism at a depth of 5 km after comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane 358 42 was the fault plane the best fitting fault plane solution of the kolari earthquake suggests pure thrust faulting at a depth of 5 km the nodal plane striking 035 30 correlates well with surface observations of the postglacial possibly listric fault systems in the source area the kuusamo earthquake focal depth 14 km has a normal faulting mechanism with the nodal planes trending 133 47 or 284 47 preference is given to the sw dipping nodal plane as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age the three earthquakes have occurred in old precambrian faults and shear zones which have been reactivated the reactivated faults are favourably oriented in the local stress field

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

observation of glacial isostatic adjustment in stable north america with gps 1 motions of three hundred and sixty global positioning system gps sites in canada and the united states yield a detailed image of the vertical and horizontal velocity fields within the nominally stable interior of the north american plate by far the strongest signal is the effect of glacial isostatic adjustment gia due to ice mass unloading during deglaciation vertical velocities show present day uplift 10 mm yr near hudson bay the site of thickest ice at the last glacial maximum the uplift rates generally decrease with distance from hudson bay and change to subsidence 1 2 mm yr south of the great lakes the hinge line separating uplift from subsidence is consistent with data from water level gauges along the great lakes showing uplift along the northern shores and subsidence along the southern ones horizontal motions show outward motion from hudson bay with complex local variations especially in the far field although the vertical motions are generally consistent with the predictions of gia models the horizontal data illustrate the need and opportunity to improve the models via more accurate descriptions of the ice load and laterally variable mantle viscosity

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

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

crustal structure beneath hudson bay from ambient noise tomography implications for basin formation summary the hudson bay basin is the least studied of four major phanerozoic intracratonic basins in north america and the mechanism by which it formed remains ambiguous we investigate the crustal structure of hudson bay based on ambient noise tomography using 21 months of continuous recordings from 37 broad band seismograph stations that encircle the bay green s functions that emerge from the cross correlation of these ambient noise recordings are dominated by fundamental mode rayleigh waves in the microseismic period band 5 20 s these signals are most prominently expressed in certain preferred azimuths indicative of stationary coastal source regions in southern alaska and labrador seasonal variations are subtle but consistent with more energetic noise sources during winter months when wave heights in the pacific and north atlantic are larger than in the summer noise emanating from hudson bay does not appear to contribute significantly to the cross correlograms group velocity dispersion curves are obtained by time frequency analysis of cross correlation functions we test and implement a signal to noise ratio snr selection method for producing one sided cross correlograms which yields better defined dispersion ridges than the standard two sided averaging approach tomographic maps and cross sections obtained in the 5 40 s period range reveal significantly lower crustal velocities beneath hudson bay than in the bounding archean superior craton the lowest mid crustal velocities correspond to a previously determined region of maximum lithospheric stretching near the centre of the basin pseudosections extracted from the tomographic inversions along profiles across hudson bay provide the first compelling direct evidence for crustal thinning beneath the basin our results are consistent with a recent estimate of 3 km of crustal thinning but not consistent with a proposed model for basin subsidence triggered by eclogitization of a remnant crustal root

stress orientations obtained from earthquake focal mechanisms what are appropriate uncertainty estimates crustal stress orientations provide important information about the mechanics of regional deformation numerous methods exist for inverting earthquake focal mechanisms for stress orientation and the more widely used methods usually obtain similar results for similar data sets however error estimates are highly variable complicating the interpretation of results the southern california stress field for example contains much statistically significant spatial and temporal variability according to the error estimates of one method michael 1984 1987b but very little according to those of another gephart and forsyth 1984 to resolve whether the southern california stress field is generally homogeneous or heterogeneous we must determine which of the error estimates best reflects the true inversion uncertainty to do this we tested both methods on a suite of synthetic focal mechanism data sets containing random errors the method of gephart and forsyth 1984 usually provides more accurate estimates of stress orientation especially for high quality data sets but its confidence regions are in most cases too large the method of michael 1984 1987b is more accurate for very noisy data sets and provides a more appropriate estimate of uncertainty implying that the stress field in southern california is probably heterogeneous

optimal locations for gps measurements in north america and northern europe for constraining glacial isostatic adjustment summary we determine the optimal location in north america and fennoscandia for uplift rate or tangential velocity data that will be useful for addressing ice sheet thickness lithospheric thickness lateral viscosity variation and background viscosity profile in the lower mantle an optimal location is defined by where sensitivity lies above the current accuracy of gps measurements the approach here is different from previous studies that compute sensitivity kernels for viscosity perturbations within a small volume of the mantle the advantage of the current approach is that the total effect of 3 d lateral heterogeneity in the mantle related to seismic tomography of the whole mantle can be studied the sensitivity of ice sheet models and lateral lithospheric thickness variations are also studied our results show that in north america more permanent gps stations are needed in northern canada especially in a region west of the hudson bay until the rocky mountains in fennoscandia the gps network is almost adequate but it should be extended to the last known gia affected areas in the russian part of east europe and to central europe in addition we show locations of prospective gps sites that are sensitive to all four parameters ice sheet thickness lithospheric thickness lateral viscosity variation and background viscosity profile in the lower mantle and locations that are sensitive to only one two or three parameters thus the results are useful for the inversion of one individual parameter or for the separation of the effects of two or more parameters in inversions

intraplate seismicity of a recently deglaciated shield terrane a case study from northern ontario canada the economy of northern ontario canada is heavily dependent on mining so accurate knowledge of seismicity is important for the safe design and op eration of mines and other critical facilities including a proposed underground repos itory for nuclear waste in this study we analyzed 537 cataloged earthquakes that occurred from 1980 to 2006 seismicity is mainly concentrated in topographically elevated archean terranes northwest of lake superior and in the james bay and ka puskasing regions we analyzed waveforms to determine the focal depth for 331 re corded events using the regional depth phase modeling rdpm method coupled with surface wave relative amplitude analysis the majority of events are shallow 350 m although in the east ern part this pattern breaks down and some deeper earthquakes 12 km are ob served based on a moving window event counting technique we show that distinct spatial clusters of seismicity can be delineated that are statistically significant relative to background seismicity levels a particularly active cluster is located within james bay where focal depths range from a few kilometers to more than 20 km another cluster near kapuskasing contains deep focus events and may occur along a hot spot track that runs through western quebec near dryden a shallow 1 km earthquake swarm concentrated in a 1 1 km region commenced in may 2002 faded and then started up again in february 2003 shallow mining induced events are also common around sudbury a major world center for nickel mining the overall pattern of seis micity appears to correlate with upper mantle p wave velocity anomalies suggesting that lateral variations in mantle rheology may play a significant role in controlling intraplate seismicity of shield areas it is also likely that crustal stresses caused by glacial isostatic adjustment are an important factor although the correlation of seis micity with uplift rate is not as clear online material focal depth solutions for earthquakes in northern ontario canada

possible triggering of heinrich events by ice load induced earthquakes north atlantic sediments dating from the last ice age contain layers of rock fragments from northeastern canada so called heinrich layers 1 like modern iceberg borne sediments from greenland these layers have been attributed to ice rafting episodes1 2 3 six heinrich layers have been documented and correlated with climate changes4 5 6 7 8 the layers which are several centimetres thick contain negligible amounts of foraminifera which accumulate at a few millimetres per century implying that they were deposited over just a few years these ice rafting heinrich events are separated by progressively shorter intervals from about 40 to 6 kyr ref 9 and it has been suggested10 that they are related to the milankovitch cycles in the earth s orbital parameters11 alternatively they may be generated by forcing mechanisms arising from the internal dynamics of the laurentide ice sheet12 here we suggest the possibility that the heinrich events were precipitated by ice load induced earthquakes analogous to those produced by reservoir water loads13 we suggest that near its edge the laurentide ice sheet sheared the earth s crust inducing repeated failure that released the ice rafts this region along canada s northeastern seaboard shows evidence of both current14 15 and past seismic activity owing to postglacial rebound our model accounts for the intervals between both the heinrich events and the evidence of palaeoseismicity and can be tested by studying local sedimentary relationships

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

upper crustal stresses and vertical stress migration in eastern canada diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern canada both in situ stress measurements to 2100 m and earthquake fault plane solutions in the upper crust 5 20 km indicate that the maximum horizontal stress is greater than the vertical absence of earthquakes in the lower crust 20 km implies a comparatively lower deviatoric horizontal stress in this layer stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ene octant which correlates with the direction predicted by modeling of plate tectonic stresses thus spreading mid atlantic ridge stress is considered to be one of the important contributors to the stress field in eastern canada viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust while simultaneously reducing that in the lower crust the crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure i e earthquake stress drop can be replenished from the lower crust moreover the directionality predicted by modeling of spreading mid atlantic ridge stress is maintained incomplete postglacial rebound also contributes to deviatoric compression other possible contributors to the upper crustal stress field are basal drag and membrane stress but the relative contributions from these sources are not properly understood

the vardar ophiolite a continuous belt under the axios basin sediments summary the ophiolite belt of this study is located at the eastern side of the vardar zone magnetic and gravity data are interpreted with the aid of seismic reflection data and borehole information results show that the guevgheli oreokastro and chalkidhiki ophiolite bodies probably belong to the same belt this belt continues under the sediments of the axios basin it has a limited thickness of 1 0 3 0 km and width of 15 20 km its flat lower surface and its thickness contradicts the hypothesis of an in situ ophiolite it is more likely that this tethyan mid jurassic ophiolite belt has been thrust to its present position

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

slip rate variations on normal faults during glacial interglacial changes in surface loads geologic and palaeoseismological data document a marked increase in the slip rates of the wasatch fault and three adjacent normal faults in the basin and range province during the late pleistocene early holocene epochs the cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the holocene has remained enigmatic although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of lake bonneville after the last glacial maximum may indicate a causal relationship here we use finite element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip postglacial unloading the removal of mass which decreases the slip rate and lithospheric rebound which promotes faster slip we show that lithospheric rebound caused by regression of lake bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high holocene rates of faulting in the wasatch region our analysis implies that climate controlled changes in loads applied to earth s surface may exert a fundamental control on the slip history of individual normal faults

western quebec seismic zone canada clustered midcrustal seismicity along a mesozoic hot spot track 1 the western quebec seismic zone wqsz is a 160 km wide band of intraplate seismicity extending 500 km from the adirondack highlands united states to the laurentian uplands canada previous authors have proposed that the wqsz is localized over the mesozoic track of the great meteor hot spot here we explore this hypothesis further by investigating regional seismicity characteristics focal mechanisms for wqsz earthquakes including a new mechanism for a moderate mn 4 5 earthquake reveal a pattern of reverse sense faulting with sw trending p axes changing to e w in the southern part of the zone we introduce a simple box counting method to delineate spatial clusters based on exceedance of random seismicity density combining this approach with focal depths from regional depth phase analysis we find that seismicity with shallow focus 0 7 km is characterized by a random spatial distribution whereas earthquakes with an intermediate focal depth 8 18 km are strongly clustered along a diffuse linear band trending n50 w earthquakes deeper than 18 km are confined to a few distinct clusters these clusters are characterized by differing b values and for at least one cluster repeating events projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of precambrian structural features may have occurred however the great meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long lived aftershocks of large past events

some seismological and geometric features of intraplate earthquakes to understand the nature and cause of intraplate earthquakes we carried out a systematic search for common features associated with them various geological geophysical and seismic data were analyzed and geometric and seismic parameters were compared for 29 intraplate events m 4 5 occurring worldwide the results suggest that intraplate earthquakes occur by the reactivation of pre existing zones of weakness in a compressional stress regime which is generally oriented parallel to the absolute direction of plate motion two styles of faulting were observed for type a earthquakes observed in the eastern united states eastern china western europe and west africa deformation occurs by strike slip motion on steeply dipping faults type b events are usually associated with thrust or normal faulting and were observed in eastern canada fennoscandia australia and peninsular india focal depths did not explain the differences between type a and type b earthquakes rather the differences are due to the perturbation of a regional stress field by a local stress field the regional stress field for type a events is primarily due to ridge push forces the method of stress perturbation varied from region to region the earthquakes in the type b category were further divided into three subcategories type b1 occurring in eastern canada and fennoscandia type b2 occurring in australia peninsular india and central brazil and type b3 occurring in the gulf coast of the u s the inferred causes of the perturbing stresses are deglaciation intracontinental resistive forces resulting from the collision of plates and the deposition of a large thickness of sediments respectively type a events were found to be associated with intersecting faults and generally supported the intersection model of talwani 1988 and had analogs in plate boundary events

chronological and geochemical constraints on the pre variscan tectonic history of the erzgebirge saxothuringian zone abstract a combined u pb zircon geochronological and whole rock isotopic and geochemical study has been carried out on high grade orthogneiss meta basite and meta sediments from the erzgebirge the results indicate multiple pulses of ediacaran ordovician magmatism in a transitional volcanic arc to rift basin setting orthogneiss from high pressure nappes exhibit a step like pattern of inherited zircon ages and emplacement ages of 500 475 ma in contrast granite gneiss from the medium pressure core of the erzgebirge is characterised by three pulses of magmatism in the early cambrian late cambrian and early ordovician a trend of decreasing th u ratios in zircon is observed to c 500 ma after which significant increases in the trend and variability of the data is inferred to mark the transition from arc related to rift related magmatism sediments deposited in the early cambrian have continental island arc affinity major detrital peaks in the ediacaran and subordinate tonian palaeoproterozoic and neoarchaean data are consistent with an avalonian cadomian arc and west african craton derivation the early cambrian sediments were locally reworked by a thermal event in the ordovician resulting in leucocratic banding and recorded in ordovician zircon rims characterised by systematically lower th u ratios ptygmatically folded leucocratic bands containing ordovician zircon rims associated with low th u ratios are further observed in the granite gneiss core of the erzgebirge variscan ages are rare except in a fine grained high pressure micaschist which contains exclusively small structure less zircon with a weighted mean age of 350 2 ma these data along with a re evaluation of previously published data have been interpreted as the product of flattening subduction during the early cambrian followed by the opening of slab windows in the late cambrian and finally delamination in the early ordovician delamination of the orphaned slab led to asthenospheric upwellings triggering extension bimodal magmatic pulses recycling of fertile crust high temperature metamorphism and cratonisation of relatively young crust

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

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

earthquakes along the passive margin of greenland evidence for postglacial rebound control an intriguing observation in greenland is a clear spatial correlation between seismicity and deglaciated areas along passive continental margins a piece of evidence for earthquake triggering due to postglacial rebound another piece of evidence for induced seismicity due to deglaciation derives from earthquake source mechanisms sparse low magnitude seismicity has made it difficult to determine focal mechanisms from greenland earthquakes on the basis of two normal faulting events along deglaciated margins and from the spatial distribution of epicenters earlier investigators suggested that the earthquakes of greenland are due to postglacial rebound this interpretation is tested here by using more recent data broadband waveforms of teleseismic p waves from the august 10 1993 mb 5 4 and october 14 1998 mb 5 1 earthquakes have been inverted for moment tensors and source parameters both mechanisms indicate normal faulting with small strike slip components the 1993 event strike 348 9 dip 41 0 rake 56 3 focal depth 11 km seismic moment 1 03 1024 dyne cm and mw 5 3 the 1998 event strike 61 6 dip 58 0 rake 95 5 focal depth 5 km seismic moment 5 72 1023 dyne cm and mw 5 1 these and the two prior events support the theory that the shallow part of the lithosphere beneath the deglaciated margins is under horizontal extension the observed stress field can be explained as flexural stresses due to removal of ice loads and surface loads by glacial erosion these local extensional stresses are further enhanced by the spreading stress of continental crust and reactivate preexisting faults earthquake characteristics observed from greenland suggest that the dominant seismogenic stresses are from postglacial rebound and spreading of the continental lithosphere

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

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

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

paleoseismology in canada a dozen years of progress canadian studies in paleoseismology have led to the publication of over 40 papers in the last 12 years 80 of which have appeared since 1989 considerable progress has been achieved by confirming that the last great earthquake on the cascadia subduction zone happened about 300 years ago and by working toward a chronology of prior earthquakes from their tsunami and turbidite records as yet sparse evidence suggests that the geological effects of large crustal earthquakes are also recorded in southwestern british columbia but there is as yet no chronology in the stable craton of eastern canada studies of silt layers in lakes that are floored by organic sediments have shown considerable potential to provide local earthquake chronologies that can be tested against the more sporadically preserved evidence from liquefaction landslides and other possible indicators of paleoshaking by contrast research using more traditional techniques involving direct examination of faults through geomorphological studies or trenching has been hampered by the difficulty of identifying active faults in the first place on the basis of the past dozen years an accelerating pace of discovery should be expected in the next decade

the first surface faulting from a historical intraplate earthquake in north america although many fault ruptures are known from the boundaries of the earth s lithospheric plates the stable interiors of continents are much less active1 worldwide only ten historical intraplate earthquakes are known to have produced surface faulting2 and none of these was in eastern north america despite the confirmed prehistoric rupture of the meers fault3 some have wondered if intraplate earthquakes in the stable north american craton might somehow be different from plate boundary earthquakes and hence fail to rupture the surface4 this is an important consideration for seismic hazard assessments when the ungava earthquake with magnitude ms 6 3 and apparent shallow depth occurred on 25 december 1989 in northern canada we and independently a johnston of memphis state university concluded that it might have produced surface faulting here we report that there was indeed a surface rupture 8 5 km long with up to 1 8 m of reverse faulting on a steeply dipping arcuate fault plane the faulting confirms the seismological process and indicates that there is a hazard from ground rupturing earthquakes in eastern north america but the poor surface expression shows that finding prehistoric fault ruptures in order to constrain seismic hazard estimates will be difficult

calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor

an estimate of the stress tensor in sweden using earthquake focal mechanisms abstract an estimate of the state of stress in sweden is made by inversion of the fault plane solutions of naturally occurring earthquakes it is shown that the stress state in southern sweden is significantly different from that in northern sweden in southern sweden the principal compression is in a roughly nw se direction similar to that prevailing in northwest europe as a whole this stress direction is is consistent with the stresses expected on the basis of motions of the major tectonic plates in northern sweden however there is no strongly dominant stress direction this latter observation is consistent with a situation in which processes other than first order plate motions are important the most likely explanation in this case is the effect of the deglaciation of fennoscandia which is producing rates of rebound as high as 9 mm year in northern sweden

postglacial faulting in eastern canada nature origin and seismic hazard implications abstract at more than 70 locations in southeastern canada and the adjacent united states tiny bedrock faults displace glacial striations that are about 10 000 years old these reverse faults mostly of small displacement occur in a broad arc from western ontario to newfoundland the faults lie near the margin of the laurentian ice sheet strike generally tangential to it and imply compression nearly normal to the ene directed contemporary regional stress these observations and some age relationships suggest that the faults formed very soon after deglaciation perhaps in the first few thousand years after ice melting in each place and probably represent the release of stresses caused by flexural deformation of the upper crust although the individual offsets are small the faults are believed to be pervasive and individual outcrops reflect amounts of stress relief consistent with stresses implied by the postglacial tilt of nearby lake shorelines

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

am improved source mechanism for the 1935 timiskaming quebec earthquake from regional waveforms the timiskaming earthquake which occurred near the quebec ontario border at the northwest end of the western quebec seismic zone in 1935 is one of the five largest instrumentally recorded southeastern canadian earthquakes previous studies of this earthquake concentrated on modeling teismograms recorded at regional distances a better constrained focal mechanism is obtained the waveforms indicate thrust faulting on a moderately dipping northwest striking plane at a depth of 10 km them w of 6 1 determined in this study is in good agreement with previous magnitude estimates m b 6 1 m s 6 0 andm blg 6 2 6 3 the focal mechanism is similar to those of many recent small to moderate earthquakes in the region and the inferred from thep axis acting stress of northeast compression is consistent with the overall eastern north american stress field the lake timiskaming rift valley in which the earthquake occurred comprises several northwest striking faults consistent with the strike of the 1935 event thus the 1935 earthquake appears to be a result of faulting on the reactivated timiskaming graben

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