a comparison of eastern north american seismic strain rates to glacial rebound strain rates glacial rebound strain rates computed using a simple laurentide glacial loading model are of the order of 10 9 per year within the region of glaciation and extending several hundred kilometers beyond the horizontal strain rates receive approximately equal contributions from horizontal and vertical velocities a consequence of the spherical geometry adopted for the earth model in the eastern united states and southeastern canada the computed strain rates are 1 3 orders of magnitude greater than an estimate of the average seismic strain rate anderson 1986 and 1 order of magnitude greater than predicted erosional strain rates the predicted glacial rebound strain rates are not in general oriented in such a way as to augment the observed state of deviatoric stress possibly explaining why the seismic strain rates are much smaller than the glacial rebound strain rates an exception to this may be seismically active regions in the st lawrence valley

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle

the northeastern ohio earthquake of 31 january 1986 was it induced abstract on 31 january 1986 at 11 46 est an earthquake of m b 5 0 occurred about 40 km east of cleveland ohio and about 17 km south of the perry nuclear power plant the earthquake was felt over a broad area including 11 states the district of columbia and parts of ontario canada caused intensity vi vii at distances of 15 km and generated relatively high accelerations 0 18 g of short duration at the perry plant thirteen aftershocks were detected as of 15 april with six occurring within the first 8 days two of the aftershocks were felt magnitudes for the aftershocks ranged from about 0 5 to 2 5 focal depths for all of the earthquakes ranged from 2 to 6 km except for one small earthquake all of the aftershocks occurred in a very tight cluster with a north northeast orientation focal mechanisms of the aftershocks exhibit predominantly oblique right slip motion on nearly vertical nodal planes oriented n15 to 45 e with a nearly horizontal p axis north of east three deep waste disposal wells are currently operating within 15 km of the epicentral region and have been responsible for the injection of nearly 1 2 billion liters of fluid at pressures reaching 112 bars above ambient at a nominal depth of 1 8 km estimates of stress inferred from commercial hydrofracturing measurements suggest that the state of stress in northeastern ohio is close to the theoretical threshold for failure along favorably oriented preexisting fractures this implies that effective stress conditions near the bottom of the two most active wells may be at or near the critical level for incipient failure two and possibly three earthquakes have occurred within less than 5 km from the wells since 1983 the relative distance to the main shock epicenter and its aftershocks about 12 km the lack of large numbers of small earthquakes typical of many induced sequences the history of small to moderate earthquakes in the region prior to the initiation of injection and the attenuation of the pressure field with distance from the injection wells however all argue for a natural origin for the 1986 earthquakes in contrast the proximity to failure conditions at the bottom of the well and the probable spatial association of at least one earthquake suggest that triggering by well activities cannot be precluded

linking stress field deflection to basement structures in southern ontario results from numerical modelling abstract analysis of stress measurement data from the near surface to crustal depths in southern ontario show a misalignment between the direction of tectonic loading and the orientation of the major horizontal principal stress the compressive stress field instead appears to be oriented sub parallel to the major terrane boundaries such as the grenville front the central metasedimentary belt boundary zone and the elzevir frontenac boundary zone this suggests that the stress field has been modified by these deep crustal scale deformation zones in order to test this hypothesis a geomechanical model was constructed using the three dimensional discontinuum stress analysis code 3dec the model consists of a 45 km thick crust of southern ontario in which the major crustal scale deformation zones are represented as discrete faults lateral velocity boundary conditions were applied to the sides of the model in the direction of tectonic loading in order to generate the horizontal compressive stress field modelling results show that for low strength low friction angle and cohesion fault slip causes the stress field to rotate toward the strike of the faults consistent with the observed direction of misalignment with the tectonic loading direction observed distortions to the regional stress field may be explained by this relatively simple mechanism of slip on deep first order structures in response to the neotectonic driving forces

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

a seismic based cross section of the grenville orogen in southern ontario and western quebec a schematic crustal cross section is presented for the southwestern grenville province based on reprocessed lithoprobe near vertical incidence seismic reflection data and compiled seismic refraction wide angle velocity models interpreted with geological constraints the schematic crustal architecture of the southwest grenville province from southeast to northwest comprises allochthonous crustal elements frontenac adirondack belt and composite arc belt that were assembled prior to ca 1160 ma and then deformed and transported northwest over reworked rocks of pre grenvillian laurentia and the laurentian margin primarily between 1120 and 980 ma reworked pre grenvillian laurentia and laurentian margin rocks are interpreted to extend at least 350 km southeast of the grenville front beneath all of the composite arc belt three major structural boundary zones the grenville front and adjacent grenville front tectonic zone the central metasedimentary belt boundary thrust zone and the elzevir frontenac bou

progressive multiple event location pmel abstract this paper introduces a new method of using earthquake arrival times to simultaneously determine earthquake hypocenters and station corrections progressive multiple event location pmel forms a solution in three major steps 1 hypocenters are located by conventional single event methods using current station correction estimates 2 an annulled data set pavlis and booker 1980 is formed and accumulated using householder transformations and 3 station correction perturbations are calculated from a pseudoinverse solution of the annulled data set in practice pmel converges rapidly we analyze the non uniqueness of pmel estimates in terms of resolution and covariance matrices one of the most important features of pmel is the computer algorithm that we describe which requires only a modest amount of computer memory that is independent of the number of earthquakes to be processed consequently this algorithm can process a virtually unlimited number of earthquakes we apply pmel to a set of 8049 arrival times from 652 earthquakes and 4 explosions recorded by a network in the coso range inyo county california pmel converges after four adjustments to the station corrections the final rms misfit of 0 05 sec is remarkably good and the station correction estimates correlate strongly with surface geology

variation of horizontal in situ stress with depth for long term performance evaluation of the deep geological repository project access shaft abstract a site characterization program was carried out for a proposed deep geological repository dgr project for ontario power generation s opg low and intermediate level nuclear waste repository near kincardine ontario the repository is proposed to be constructed at approximately 680 m below ground surface within the competent argillaceous limestone of the cobourg formation the in situ stress state at the project site will have significant impact on both the short and long term performance of repository openings such as emplacement caverns and access shafts as part of the site characterization program an evaluation of the in situ stress state of the project site was conducted which consisted primarily of a review and synthesis of existing stress measurements conducted at various locations throughout ontario and the midwestern u s based on geomechanics data from deep boreholes and stress measurement data a simplified flac3d model of the full stratigraphic profile was developed and used to simulate the influence of regional tectonic strain in the project area in particular this method takes into account the rock properties such as stiffness for discrete units at the dgr site the model was calibrated on the basis of in situ stresses measured at norton mine in a similar geological environment as the dgr site and with site specific borehole televiewer observations i e breakouts the model predicted horizontal in situ stress profile showed general agreement with the observations and also showed the significant influence of discrete rock unit stiffness

linear zones seismicity and the possibility of a major earthquake in the intraplate western lake ontario area of eastern north america seismic hazard assessments conducted in eastern canada rely on seismological data which are essential but alone inadequate that is because the earthquake record is too short to provide a representative picture of where large earthquakes have occurred in the past consequently seismic hazard may be underestimated in areas such as that encompassing western lake ontario that are devoid of documented large earthquakes to attempt to ascertain the likelihood of a major earthquake occurring in that highly populated and industrialized area three regionally extensive geophysically expressed lineaments were investigated and the results were combined with available seismological information the most conspicuous is the niagara pickering linear zone within which there have been at least two if not three periods of brittle faulting including displacements compatible with the current stress field it also appears to represent the same major structure as the akron magnetic boundary in eastern ohio the site of the mb 4 9 leroy earthquake the second is the georgian bay linear zone which extends from georgian bay to new york state it displays evidence of recent outcrop scale faulting an alignment of earthquakes along its southwestern boundary and a possible spatial relationship to other earthquakes including two of m 5 lastly there is the hamilton lake erie lineament which is parallel and proximal to a possible fault and coincides with a linear array of small to moderate earthquakes all three converge on the western lake ontario area which has a higher frequency of seismicity than the adjacent areas thus the western lake ontario area might have a greater potential to experience a major earthquake than heretofore believed resume les evaluations des risques sismiques dans l est du canada sont fondees sur les donnees sismologiques dont

a new look at belgian aeromagnetic and gravity data through image based display and integrated modelling techniques digital processing and image based display techniques have been used to generate contour and shaded relief maps of belgian aeromagnetic data at a scale of 1 300000 for the whole of belgium these highlight the important anomalies and structural trends particularly over the brabant massif north and vertically illuminated shaded relief plots enhanced structural belts trending west east to northwest southeast in the brabant massif and west east to southwest northeast in the core of the ardennes the principal magnetic lineaments have been identified from the shaded relief plots and tentatively correlated to basement structures most short lineaments are correlated with individual folds while the more extensive lineaments are correlated with large scale fault structures magnetic highs within the brabant massif are attributed to folded sediments of the tubize group the magnetic basement in the east of belgium is sinistrally displaced to the north by an inferred deep nnw sse crustal fracture the bouguer anomaly map of belgium identifies the ardennes as a negative area and the brabant massif as a positive area with the exception of a wnw trending gravity low in its western part the southern margin of the brabant massif is defined by a steep gravity gradient coincident with the faille bordiere border fault trial modelling of the gravity and magnetic data carried out along profiles across the brabant and stavelot massifs has identified probable acid igneous intrusions in the western part of the brabant massif and a deep magnetic lower density body underlying the whole ardennes region which is thought to be a distinctive precambrian crustal block

crustal thickness and vp vs variations in the grenville orogen ontario canada from analysis of teleseismic receiver functions abstract we have developed a simple semblance weighted stacking technique to estimate crustal thickness and average vp vs ratio using teleseismic receiver functions we have applied our method to data from 32 broadband seismograph stations that cover a 700 400 km2 region of the grenville orogen a 1 2 0 98 ga himalayan scale collisional belt in eastern north america our seismograph network partly overlaps with l ithoprobe and other crustal refraction surveys in 8 out of 9 cases where a crustal refraction profile passes within 30 km of a seismograph station the two independent crustal thickness estimates agree to within 7 our regional crustal thickness model constructed using both teleseismic and refraction observations ranges between 34 0 and 52 4 km crustal thickness trends show a strong correlation with geological belts but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium the thickest crust 52 4 1 7 km was found at a station located within the 1 1 ga mid continent failed rift the central gneiss belt which contains rocks exhumed from deep levels of the crust is characterized by vp vs ranging from 1 78 to 1 85 in other parts of the grenville orogen vp vs is found to be generally less than 1 80 the thinnest crust 34 5 37 0 km occurs northeast of the 0 7 ga ottawa bonnechere graben and correlates with areas of high intraplate seismicity

modern and early middle holocene shells of the freshwater mollusc unio from catalhoyuk in the konya basin turkey preliminary palaeoclimatic implications from molluscan isotope data carbon and oxygen isotope ratios in the shells of the freshwater unio mollusc yield information on the isotopic composition of the water in which the shell was formed which in turn relates to climatic conditions prevailing during the bivalves life span here we analysed shells from one modern unio from a modern lake shore in anatolia and 4 subfossil unio shells from catalhoyuk dated between 7200 bc and 5000 bc neolithic and chalcolithic periods sequential carbon and oxygen isotope analysis along the surface of the shells provides information on seasonal or shorter term variability of lake waters during the lifetime of the organisms 18o values of the modern shell are consistent with it being collected from a dammed portion of the seyhan river that does not undergo intensive summer evaporation this is in contrast to many of the surface water bodies in anatolia which suffer extensive evaporation in the arid summers 18o values of the subfossil shells from the catalhoyuk middens indicate that the bivalves came from lakes which evaporated extensively during the summer months but were replenished by high winter rainfall suggesting that the subfossil shells lived in relatively small lakes or ponds stable isotope analysis along the growth of freshwater bivalves is one of the few methods for investigating seasonal water fluctuations in the past these results may alter current interpretations on the environment of catalhoyuk during the neolithic although more work is needed to confirm these initial findings

interpretation of seismic and potential field data from western new york state and lake ontario abstract lithoprobe and industry seismic profiles have furnished evidence of major zones of easterly dipping grenville deformed crust extending southwest from exposed grenville rocks north of lake ontario additional constraints on subsurface structure limited to the postulated clarendon linden fault system south of lake ontario are provided by five east west reflection lines recorded in 1976 spatial correlations between seismic structure and magnetic anomalies are described from both lake ontario and the newly reprocessed new york lines in the paleozoic to precambrian upper crust the new york seismic sections show 1 an easterly thickening wedge of subhorizontal paleozoic strata unconformably overlying a precambrian basement whose surface has an apparent regional easterly dip of 1 2 minor apparent normal offsets possibly on the order of tens of meters occur within the paleozoic section the generally poorly reflective unconformity may be locally characterized by topographic relief on the order of 100 m 2 apparent local displacement on the order of 90 m at the level of the black river group diminishes upward to little or no apparent offset of queenston shale 3 within the limited seismic sections there appears to be no evidence that the complete upper crustal section is vertically or subvertically offset 4 dipping structure in the paleozoic strata 15 to 35 resembles some underlying precambrian basement elements 5 the surface continuity of inferred faults constituting the clarendon linden system is not strongly supported by the seismic data beneath the paleozoic strata the seismic sections show both linear and arcuate reflector geometry with easterly apparent dips of 15 to 35 similar to the deep structures imaged on seismic lines from nearby lake ontario and on lithoprobe lines to the north the similarity supports an extension of easterly dipping central metasedimentary belt structures of the grenville orogen from southern ontario to beneath western new york state from a comparison of the magnetic and gravity fields with the new york seismic sections we suggest 1 the largely nonmagnetic paleozoic strata appear to contribute negligibly to magnetic anomalies seismically imaged fractures in the new york paleozoic strata appear to lie mainly west of a positive gravity anomaly the relationship between magnetic and gravity anomalies and the changes in the geometry of interpreted precambrian structures remains enigmatic 2 north to northeast trending curvilinear magnetic and gravity anomalies parallel but are not restricted to the principal trend of the postulated clarendon linden fault system paleozoic fractures of the clarendon linden system may partly overlie a southward extension of the composite arc belt boundary zone

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

analysis and reinterpretation of deformation features in the rouge river valley scarborough ontario geometry and timing of deformation affecting ordovician bedrock and overlying pleistocene sediments in the rouge river valley near scarborough ontario are analysed to evaluate whether or not the structures are a result of glacial action or neotectonic activity extensive data on local and regional ice flow directions are used to evaluate the kinematic compatibility between the observed faults and folds and the local ice flow directions jointing and multiple episodes of faulting affect both the ordovician bedrock and the overlying pleistocene sediments at one site the bedrock is displaced by a normal fault by a minimum of 1 2 m crosscutting relationships constrain the majority of the faulting in the rouge river valley as being coeval with deposition of the lower bowmanville till during the nissouri phase ca 23 15 ka and possibly younger at one locality the youngest regional ice flow direction is northwestward however local ice flow directions are highly variable this can be explained by local perturbation enhanced by the presence of drumlinoid features in the area most deformation features are compatible with local and regional ice flow directions glaciotectonic ice push and ice thrust deformation affected the thorncliffe formation after about 23 ka although some faults appear to be kinematically incompatible with ice flow directions six boreholes drilled to 52 m depth revealed only minor vertical offsets of bedrock strata in the uppermost 20 m and an absence of obvious fault offsets deeper precluding the possibility that the faults observed in the surface exposures were caused by deep seated neotectonic stresses

timing of magmatism foreland basin development metamorphism and inversion in the anglo brabant fold belt new data implying crustal activation of eastern avalonia along the anglo brabant fold belt are presented late ordovician subduction related magmatism in east anglia and the brabant massif coupled with accelerated subsidence in the anglia basin and in the brabant massif during silurian time indicate a foreland basin development final collision resulted in folding cleavage development and thrusting during the mid lochkovian to mid eifelian in the southeast of the anglo brabant fold belt acadian deformation produced basin inversion and the regional antiformal structure of the brabant massif the uplift inferred from the sedimentology petrography and reworked palynomorphs in the lower devonian of the dinant synclinorium is confirmed by illite crystallinity studies the tectonic model discussed implies the presence of two subduction zones in the eastern part of eastern avalonia one along the anglo brabant fold belt and another under the north sea in the prolongation of the north german polish caledonides

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

bedrock jointing and geomorphology in southwestern ontario canada an example of tectonic predesign in southwestern ontario canada palaeozoic sedimentary strata are covered by pleistocene glacial sediments the topography of the buried bedrock surface has been mapped over an area of some 45 000 km2 using new computerised depth to bedrock data from more than 200 000 waterwells this information shows that arching of the underlying proterozoic basement canadian shield and differential erosion of carbonate and shale strata are first order regional controls on bedrock geomorphology these data also define a network of buried bedrock channels quantitative analysis of the trends of these channels reveals a statistical correspondence between channel orientation and regional bedrock joint sets formed in response to the mid continent stress field the orientations of bedrock channels and joints correspond regionally with mean trends at 153 and 164 respectively in addition two dominant orientations of postglacial river valleys and bedrock joints correspond with mean trends at 104 vs 104 and 54 vs 52 respectively our findings show that the erosional channelled topography cut on bedrock together with the postglacial drainage pattern cut into younger glacial sediments are related to the trend of bedrock joints and consequently can be described as tectonically predesigned in addition this study yields new insights into the form of the preglacial tertiary drainage system in southern ontario prior to pleistocene glaciation regardless of the precise age of this preglacial system and degree of subsequent glacial modification an important factor in its geomorphic development has been the mid continent stress field

earthquake source parameters and state of stress for the northeastern united states and southeastern canada from analysis of regional seismograms we examine 16 moderate size earthquakes that have occurred since 1990 in the northeastern united states and southeastern canada source parameters such as focal mechanism focal depth and seismic moment are determined using a regional waveform inversion technique in which the best fitting double couple mechanism is obtained through a grid search over strike dip and rake angles most of the 16 events and 10 previous ones with well determined focal mechanisms show horizontal compression with near horizontal p axes striking northeasterly which is consistent with the maximum horizontal stress orientation for midplate north america four shocks along the st lawrence river however have p axes trending southeast 100 130 which is not compatible with the regional stress field and requires a local stress perturbation close to the st lawrence river we examine several generating mechanisms for this localized stress variation including that a rift pillow structure may exist in the lower crust below the st lawrence river all of the 11 shocks in the united states occurred at shallow depths from 2 to 8 km the 15 canadian events are systematically deeper with foci between 5 and 28 km this geographic variation in focal depth is well correlated with the difference in surface heat flow values between the older grenville and younger appalachian provinces in the study area we also examine apparent discrepancies between m w and m b lg magnitude for 17 events five earthquakes of very similar m w show variations in m b lg as large as 0 6 which implies that for seismic events with similar seismic moment in the study area high frequency seismic radiation behavior differs considerably manuscript received 23 october 2002

glacio seismotectonics ice sheets crustal deformation and seismicity abstract the last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity this growth has been driven largely by the emergence of postglacial rebound models pgm constrained by new field observations that incorporate increasingly realistic rheological mechanical and glacial parameters in this paper we highlight some of these recent field based investigations and new pgms and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation the emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern canada and fennoscandia however many of these models neglect important parameters such as topography lateral variations in lithospheric strength and tectonic strain built up during glaciation in glaciated mountainous terrains glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields in general the short term response to ice fluctuations is similar to the earth s response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre existing stress state when the ice fluctuations occur on the spatial scale and magnitude of the late pleistocene glaciation and deglaciation however the viscoelastic response of the earth especially the mantle causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex the regions of greatest ice thickness and the regions marginal to the late pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting the mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused and continues to cause measureable crustal deformation at hundreds of kilometres from the ice margins for both tectonically active and cratonic regions palaeoseismic investigations of late pleistocene and holocene faults are an important tool in evaluating earthquake hazard the predicted response of the earth over this time however is very dependent on the model assumed for most regions far more carefully designed field observations are needed to constrain the existing rheological and ice models

miogeoclines and suspect terranes of the caledonian appalachian orogen tectonic patterns in the north atlantic region the caledonian appalachian orogen was formed by the closing of a paleozoic iapetus ocean the continental margins of iapetus are identified in the deformed early paleozoic miogeoclines of the caledonian appalachian orogen ophiolitic vestiges of iapetus its oceanic plateaus microcontinents and volcanic arcs are caledonian appalachian suspect terranes these were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines the modern north atlantic ocean opened along an axis that traversed the paleozoic orogen longitudinally its opening dispersed the elements of the paleozoic orogen and led to the present arrangement of disjunct paleozoic miogeoclines and suspect terranes throughout the north atlantic borderlands the western or north american margin of iapetus is represented by the miogeoclines along the west flank of the north american appalachians and caledonides of east greenland a small north american miogeoclinal segment occurs in the britis

basement controlled faulting of paleozoic strata in southern ontario canada new evidence from geophysical lineament mapping abstract basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins in southern ontario the basement consists of complexly structured mid proterozoic ca 1 2 ga crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of paleozoic sedimentary strata reactivation of basement structures is suspected to control the location of paleozoic fault and fracture systems but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement new aeromagnetic and gravimetric lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of paleozoic fault and fracture systems lineament mapping was facilitated by reprocessing and digital image enhancement micro levelling regional residual separation derivative filtering of existing regional gravity and aeromagnetic datasets reprocessed images identify new details of the structural fabric of the basement below southern ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones linear zones parallel the projected trends of mid proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area and are interpreted as zones of shearing and basement faulting mapped aeromagnetic and gravity lineaments show similar trends to paleozoic faults and fracture networks and broad zones of seismicity in southern ontario these new data support an inheritance model for paleozoic faulting involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata

lake sediments as natural seismographs earthquake related deformations seismites in central canadian lakes abstract central canada experiences numerous intraplate earthquakes but their recurrence and source areas remain obscure due to shortness of the instrumental and historic records unconsolidated fine grained sediments in lake basins are natural seismographs with the potential to record ancient earthquakes during the last 10 000 years since the retreat of the laurentide ice sheet many lake basins are cut into bedrock and are structurally controlled by the same precambrian basement structures shear zones terrane boundaries and other lineaments implicated as the source of ongoing mid plate earthquake activity a regional seismic sub bottom profiling of lakes gull muskoka joseph rousseau ontario wanapitei fairbanks vermilion nipissing georgian bay mazinaw simcoe timiskaming kipawa parry sound and lake of bays encompassing a total of more than 2000 kilometres of high resolution track line data supplemented by multibeam and sidescan sonar survey records show a consistent sub bottom stratigraphy of relatively thick lowermost lateglacial facies composed of interbedded semi transparent mass flow facies debrites slumps and rhythmically laminated silty clays mass flows together with cratered kettled lake floors and associated deformations reflect a dynamic ice contact glaciolacustrine environment exceptionally thick mass flow successions in lake timiskaming along the floor of the timiskaming graben within the seismically active western quebec seismic zone wqsz point to a higher frequency of earthquakes and slope failure during deglaciation and rapid glacio isostatic rebound though faulting continues into the postglacial lateglacial faulting diapiric deformation and slumping of coeval lateglacial sediments is observed in parry sound lake muskoka and lake joseph which are all located above prominent precambrian terrane boundaries lateglacial sediments are sharply overlain by relatively thin rhythmically laminated and often semi transparent postglacial silty clay laminations postglacial neotectonic activity is recorded by co seismic water escape structures faults slumps and debris flows in lake simcoe above a terrane boundary and especially within lakes timiskaming and kipawa recording ongoing deformation within the wqsz high resolution seismo stratigraphic data presented here support the model that ongoing mid plate earthquake activity is a consequence of brittle deformation of the upper crust of the north american plate such activity appears to have been greatest during deglaciation but continues today

seismicity of the southern great lakes revised earthquake hypocenters and possible tectonic controls using data from 27 seismograph stations for the period 1990 2001 we have relocated 106 hypocenters of earthquakes with magnitudes from 0 9 to 5 4 in the region of the southern great lakes two complementary methods were used for relocation a conventional least squares approach lienert and havskov 1995 and joint hypocentral determination pujol 2000 these two methods yielded mutually consistent spatial patterns of seismicity with an average difference of 3 7 km in epicentral locations and 1 1 km in focal depths we show that the hypocenter locations are not very sensitive to realistic uncertainties in 1d crustal velocity our sharpened definition of zones of seismicity delineates several clusters beneath lake ontario around niagara falls and near the south shore of lake erie these seismicity zones appear to correlate with areas where the regional magnetic data exhibit prominent short wavelength 5 km linear anomalies the magnetic anomalies are associated with basement structures that formed during the precambrian mesoproterozoic grenville orogen both the seismicity and magnetic anomalies exhibit statistically significant preferred orientations at n40 e n45 e but the correlation of the earthquake clusters with specific aeromagnetic lineaments remains uncertain three preliminary focal mechanisms of earthquakes with magnitudes m n 3 1 to 3 8 show unusual normal faulting with nodal planes in almost the same direction as the magnetic trends n42 e n52 e proximity of the earthquake clusters to large bodies of water coupled with colinearity with magnetic anomaly trends suggests that both surface water and pre existing basement structures may play significant roles in controlling intraplate seismicity in the southern great lakes region manuscript received 3 january 2003

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

very low grade metamorphic sr isotopic resettings of magmatic rocks and minerals evidence for a late givetian strike slip division of the brabant massif belgium rb sr isochron ages recorded by the ashgillian ignimbrites from deerlijk and harelbeke and the caradocian quartz diorites from quenast and bierghes significantly post date their emplacement at quenast two mineral isochrons biotite wr plagioclase 372 13 ma k feldspar wr epidote 375 14 ma reveal that the brabantian branch of the caledonian belts was affected by a post siegenian pre tournaisian event this overprint turns out to be contemporaneous with the development of the oudenaarde bierghes fault zone which is dated to 373 11 ma and 379 13 ma on foliated rocks from bierghes deerlijk and harelbeke this c 375 ma cluster of events is interpreted as representing late givetian stresses developed along the oudenaarde bierghes fault zone for which the balance of geological data favours strike slip movements since this compressive phase of deformation is posterior to the givetian opening of the borain trough the brabant massif appears to have been affected by successive crustal extension and shortening events in the middle devonian petrological evidence demonstrates the very low grade character of the metamorphism in the metasediments on both sides of the oudenaarde bierghes fault zone from this we deduce that sr isotopic homogenization of whole rock ignimbrite and the loss of radiogenic sr from biotite and k feldspar could be operative even at temperatures below 250 c

seismic images of a tectonic subdivision of the grenville orogen beneath lakes ontario and erie new seismic data from marine air gun and vibroseis profiles in lake ontario and lake erie provide images of subhorizontal phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust these reflections are interpreted as structural features of crustal scale subdivisions within the grenville orogen broadly deformed imbricated and overlapping thrust sheets within the western central metasedimentary belt are succeeded to the west by a complex zone of easterly dipping apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the central metasedimentary belt and the central gneiss belt the interpreted central metasedimentary belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes ontario and erie to structures exposed 150 km to the north less reflective west dipping events are interpreted as structures within the

structural analysis of the adjacent acadian and variscan fold belts in belgium and northern france from geophysical and geological evidence abstract two palaeozoic fold belts are juxtaposed in belgium and northern france the northern belt of the brabant massif traditionally considered a part of the european caledonides was folded during the acadian orogeny in the early devonian although it is largely concealed gravity and magnetic maps give a general picture of the existing structures a curved gravity trend is interpreted as an arc shaped succession of granitic batholiths structures north and south of this arc are completely different the northern subdomain containing magnetic lower cambrian metasediments shows multiple thrust faults towards the south over the granites whereas the southern subdomain without magnetic rocks is moulded around the granitic core the southern variscan belt is characterized in general by thrusting with a north nw and ne vergence during the variscan orogeny the brabant massif acted as an obstacle to the general transport of the ardennes massif to the n nw the oblique convergence with the brabant massif caused differential displacement towards the northwest the western areas having travelled farther as seen from a pattern of wnw oriented strike slip faults

a study of the microearthquakes of the gobles oil field area of southwestern ontario abstract from july 1980 to august 1984 the university of western ontario operated a small seismic telemetry network over the gobles oil field in southwestern ontario a total of 478 small earth tremors mainly in the 1 to 2 magnitude range were detected by this array there were three events greater than magnitude 3 felt by residents up to 15 km from the oil field approximately 10 events month were recorded on average although the actual number was rather erratic and tended to have a winter maximum event locations did not correlate with the actual positions of the oil and gas wells in the field but rather are confined to two main areas one lying 1 5 km northwest of the town of gobles and the other just south of the town the depths of the well recorded events were found to be near the depth of the producing horizon the observed first motion patterns indicated that at least two active faults almost perpendicular to each other are present in the area the results are consistent with the hypothesis that the events are induced or triggered by fluids which are pumped in and out of wells during secondary recovery activities

arcuate fold and cleavage patterns in the southeastern part of the anglo brabant fold belt belgium tectonic implications abstract the brabant massif belgium is situated at the southeastern extremity of the anglo brabant fold belt the predominantly concealed eastern branch of the british belgian caledonides in contrast to the other parts of this caledonian slate belt the deformation history in the brabant massif has to date never been constrained properly in this paper a synthesis is presented of the structural observations in the limited outcrop area of the brabant massif in particular with respect to cleavage and fold trajectories and mutual cleavage fold relationships arcuate fold and cleavage patterns as well as the presence of cleavage transected folds are demonstrated similar to the other parts of the belt moreover the arcuate trajectories are considered to reflect the trace of the caledonian deformation front once obscured by the variscan ardenne allochthon combined with interpretations of the aeromagnetic and bouguer anomaly maps an overall kinematic model is proposed for the acadian deformation event in the brabant massif with the proposed model calling upon processes of indentation by promontories and lateral escape of lithostructural domains as well as on the control exerted by rigid granitoid basement blocks the strong similarities with the other parts of the british belgian caledonides become apparent by defining an overall shortening direction and proposing a trace for the caledonian deformation front in this southeastern extremity of the anglo brabant fold belt an integrated tectonic image is obtained for the entire british belgian caledonides

3 d crust and mantle structure in southern ontario canada via receiver function imaging abstract a teleseismic data set from the polaris project is used to obtain 3 d images of southern ontario using two imaging techniques scattering tomography and common conversion point stacking the resulting images reveal a layered crust the layering being interrupted by discontinuities associated with major crustal scale faulting breaks in crustal continuity and moho deflections associated with the ottawa bonnechere graben indicate that the graben is associated with faulting on a whole crust scale we also detect similar discontinuities across the mississauga domain supporting the previous interpretation that the domain is bounded by crustal scale faults we locate discontinuous sub lithospheric negative polarity arrivals which indicate complex three dimensional structures within the lithosphere and may be associated with subduction remnants or a mid lithosphere discontinuity

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

the october 2005 georgian bay canada earthquake sequence mafic dykes and their role in the mechanical heterogeneity of precambrian crust on 20 october 2005 at 21 16 utc a moderate earthquake mn 4 3 occurred in an area of low seismicity within georgian bay approximately 12 km north of thornbury ontario 44 67 n 80 46 w despite its moderate magnitude it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long term storage facility for low and medium level nuclear waste no damage was reported but ground shaking was felt to a distance of 100 km within 24 hours after the mainshock four portable seismograph systems were in stalled in the epicentral region in total eight events were recorded over a 4 day period including a foreshock and six aftershocks the unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral pa rameters including well constrained focal depths for most events for the mainshock we estimated a seismic moment of m0 4 5 10 14 n m and corner frequency of 3 7 hz based on a spectral fit using brune s source model least squares waveform inversion of p and s phases yielded a double couple focal mechanism with a reverse sense of slip and northwest striking nodal planes the reverse mechanism and mid crustal focal depths 10 12 km are characteristic in general of more abundant seismicity located 200 km northeast of this event in the western quebec seismic zone these parameters differ however from shallow 2 6 km earthquakes with predominantly strike slip mechanisms observed near lake erie 200 km to the south we attribute this north south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound aeromagnetic data in and around the epicentral region reveal prominent northwest striking lineations caused by precam brian mafic dykes under midcrustal conditions the dyke material is mechanically stronger than generally more felsic upper crustal host rocks we propose that where large dykes are favorably oriented with respect to the stress field they may strongly influence the locations of intraplate earthquake rupture in shield regions

north american plate dynamics deformation within the north american plate in response to various tectonic processes is modeled using an elastic finite element analysis a coarse grid contains 328 elements and 190 nodes while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2 3 abundant information about the present day state of intraplate stress constrains the modeling the dominant pattern that must be fit by all acceptable models is an ene trend for the maximum compressive stress for most of the plate east of the rocky mountains the tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere shear and normal stresses transmitted across transforms normal stresses transmitted across convergent boundaries stresses due to horizontal density contrasts within the continent and shear tractions applied along the base of the plate model stresses are calculated with respect to a lithostatic reference stress state distributed ridge forces of magnitude 2 1012 n m predict deviatoric stresses of the order of 20 40 mpa that are capable of accounting for the dominant observed ene stress trend driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate assuming that ridge forces account for the dominant ene stress trend bounds may be placed on other possible forces for example shear stresses transmitted across transform boundaries along the san andreas and caribbean are small of the order of 5 10 mpa also compressive stresses of the order of 5 10 mpa transmitted across the major transforms improve the fit to the data compressive stresses across convergent margins along the aleutians and the middle america trench are important

seismites within ordovician silurian carbonates and clastics of southern ontario canada and implications for intraplate seismicity abstract a regional survey of lower paleozoic sedimentary strata ordovician silurian exposed across some 7000 km2 of southern ontario canada has identified the sedimentary record of paleo seismicity at a time of known faulting and rapid subsidence of appalachian foreland basins shallow marine shoreface sandstones and shales of the lower silurian thorold formation show laterally extensive beds of large