models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

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

broadband modelling along a regional shield path harvard recording of the saguenay earthquake summary the saguenay earthquake 1988 november 25 is one of the first large shield type events recorded by a broadband high dynamic range instrument the streckeisen system installed at harvard station hrv the event is sufficiently large to be well recorded teleseismically and thus the source characteristics can be determined by independent means and considered known this allows a detailed study of the propagational effects along this path at an epicentral distance of 625 km where the strengths of the surface waves can be compared with the crustal body phases broadband modelling using standard analytical techniques and flat layered models works amazingly well over the period range of 0 5 20 s a detailed strategy for modelling broadband regional phases is given in terms of a decomposition of the vertical and radial seismograms into three segments pnl containing pn ppn spn pmp p coupled pl waves snl containing sn ssn sms etc and the fundamental rayleigh waves information about the upper crust is obtained from the fundamental rayleigh waves while crustal thickness and velocity gradients in the mantle are obtained from pnl and snl this particular crustal model has a thickness of 35 km with a sharp moho and a substantial gradient in the top 20 km of the mantle 0 01 km s 1 per km for both p and s velocities the mantle velocities 8 2 and 4 55 km s 1 are slower than expected for a shield environment attenuation is not required for waveform modelling or for absolute amplitude estimation

spatial and temporal variations of fennoscandian seismicity summary we present an evaluation of the seismicity in fennoscandia b values have been assessed for the region between 54 72 n and 4 32 e and for the time period 1967 83 a presented contour map with different b values indicates the relative seismicity between different regions during the time period considered strain release rate has also been mapped and recalculated to give the possible maximummagnitude earthquake at a given point for time intervals of 17 and 90yr the observed time variation of b in epicentral regions of some of the major earthquakes that occurred in fennoscandia reveals that b has the tendency to increase several years prior to the occurrence of the event and decreases again about two years before the event takes place this has been the case for at least two of the major fennoscandian events i e for the solberg 1983 earthquake in sweden and the southwestern norway event in 1989 on the other hand similar changes in the behaviour of b have not been observed for the skovde 1986 earthquake in southern sweden space time variations of b were studied for the region with the highest observed b value i e for the region between 64 68 n and 17 32 e not associated with any particular event as well as in the vicinity of the epicentre of the solberg earthquake we will be discussing temporal and spatial variations of b in relation to the state of stress in the region and we believe that a better understanding of the behaviour of b may well help our efforts towards medium term earthquake

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

p wave crustal structure of the lake vanern area sweden eugeno s profile 6 abstract to investigate the deep structure of the precambrian basement airgun shots in lake vanern southern sweden were recorded on land by two 100 km lines of mars stations to the south two geostore stations east and west of the lake and a 7 km multichannel reflection array on the varmlandsnas in the centre of the lake modelling of two profiles running roughly n s parallel to the major tectonic grain and on either side of the mylonite zone shows variation in crustal seismic velocity structure from east to west comparison of these models with near vertical incidence stacked sections from the varmlandsnas experiment and with an e w profile suggests continuity of reflecting structures in a n s direction this e w profile shows shallowly west dipping strongly reflective structures probably associated with the mylonite zone and also a small scale moho structure tentatively interpreted as the western limit of a wedge of overriden older precambrian crust

uniform slip rate along the kunlun fault implications for seismic behaviour and large scale tectonics a long term slip rate is derived from concordant 10 be 26 al and 14 c dating of cumulativeoffse ts along much of the length of the kunlun fault values at 6 sites indi cateuniform slip 11 5 2 0 mm yr since 40 kyr bp this relatively high slip rate corresponds to a first order discon tinuity in the asian crustal velocity field m 8a nd m 7 5 earthquakes on 2 segments of the fault recur with charac teristic slip 10 2ma nd 4 4 0 4m ev e ry 850 and 420 yrs respectively

seismotectonics of the southern appalachians abstract the southern appalachians of eastern tennessee and western north carolina is a region for which high quality instrumental seismic data have been very limited in this paper we report more than 100 new or relocated hypocenters obtained from the initial 2 1 2 yr of operation of the southern appalachian regional seismic network sarsn a 16 station four state telemetered network of short period vertical instruments these data provide a first order snapshot of southern appalachian seismicity that confirms some previous observations and adds new results for the region the most important new observations are well constrained focal depths and focal mechanisms for southern appalachian earthquakes the level of activity exceeds expectations and concentrates more heavily in the valley and ridge province of eastern tennessee as opposed to the blue ridge or inner piedmont than is evident in historical listings the large majority of these events lie between the new york alabama lineament and the clingman ocoee lineaments magnetic anomalies produced by deep seated basement structures an analysis of the uniqueness and stability of hypocentral depth yields reliable depths for 58 earthquakes and demonstrates that most southern appalachian seismic activity occurs beneath the decollement thus lending quantitative support to the argument that appalachian seismicity is unrelated to surficial geological or tectonic features several new focal mechanisms of a higher quality than previously available indicate that the basement of the valley and ridge is under ne sw compression and the principal mode of faulting is right lateral strike slip on nearly north south faults in both depth and type of faulting the seismogenic model we propose for the southern appalachians closely emulates the giles county zone to the north of our study area

a comparison of strain rates and seismicity for fennoscandia depth dependency of deformation from glacial isostatic adjustment summary we investigate the influence of the glacial isostatic adjustment gia on the deformation at the surface and at seismogenic depths in fennoscandia the surface strain rate field derived from geodetic data is controlled by gia which causes nw se extension of up to 4 10 9 yr 1 in most of mainland fennoscandia surrounded by regions of radial shortening towards the centre of uplift the seismic deformation field derived from a new compilation of focal mechanisms shows consistent nw se compression on the norwegian continental margin and a tendency towards tension in mainland fennoscandia the seismic moment rate is at least two orders of magnitude smaller than the geodetic moment rate we propose that the low level of seismicity and the tendency towards tensional focal mechanisms in mainland fennoscandia may be explained by the destructive interference of the regional stress from ridge push with the flexural stress due to gia

the south scandinavian crust structural complexities from seismic reflection and refraction profiling abstract pioneering work on mapping the scandinavian crust commenced in the early 1960s and since then numerous profiling surveys have been undertaken particularly as part of the on going eugeno s project however the most significant contribution to mapping crustal structural details came from the m v mobil search cruises in the skagerrak and off the west coast of norway 16 s twt reflection profiling all past and present crustal profiling results have been integrated to produce detailed maps of moho depths and crustal thicknesses for south scandinavia the thinnest crust is found in the north sea and skagerrak approximately 20 km while east central sweden features very thick crust approximately 50 km other interesting features are the apparent correlation between crustal thinning and sedimentation subsidence magmatic activity earthquake occurrences and the tectonic age of the crust moho depths and the crustal thicknesses clearly reflect the tectonic evolution and the present structural features of the region investigated

quantification of neotectonic stress orientations and magnitudes from field observations in finnmark northern norway abstract fieldwork was conducted in finnmark northern norway with the purpose of detecting and measuring stress relief features induced by quarrying and road works and to derive from them valuable information on the shallow crustal stress orientations and magnitudes two kinds of stress relief features were considered in this study the first consists of drillhole offsets that were found along blasted road cuts and which were triggered by the sudden rock unloading following the actual blasting vertical axial fractures found in the concave remains of boreholes represent the second kind of stress relief feature the axial fractures are tension fractures produced by gas overpressure inside the drillhole when the blast occurs as such their strike reflects the orientation of the ambient maximum horizontal stress axis the borehole offsets show mostly reverse slip displacements to the e se and the axial fractures trend nw se on average in agreement with nw se compression induced by north atlantic ridge push forces mechanical considerations of the slip planes offsetting some of the drillholes lead to the conclusion that the magnitude of the maximum horizontal stress at the surface is in the range 0 1 1 mpa this range of magnitudes is 1 2 orders less than the horizontal stress magnitudes measured at the surface in other post glacial environments e g canada it is suggested that this difference is related to the marked decline in stress that followed the tremendous post glacial burst of earthquake activity that affected fennoscandia but apparently not the canadian shield

an integrated magnetotelluric and aeromagnetic investigation of the serra da cangalha impact crater brazil abstract the research is aimed at delineating the post impact structural characteristics across the serra da cangalha impact crater in brazil using a combination of magnetotelluric mt and aeromagnetic data the mt survey was carried out along three radial mt profiles trending nw se ene wsw and nne ssw across the crater for mt sites located further away from the centre of the crater isotropic mt responses were observed suggesting a 1d conductivity distribution in the subsurface in the frequency range of 100 10 hz for sites located in the vicinity of the inner ring of the crater anisotropic responses were observed for the same frequency range we believe that this zone probably represents the areas of structural disturbance a 2d resistivity inversion of these data reveals a four layer model representing a thin resistive layer underlain by a conductive layer a weathered basement and a resistive crystalline basement the depth to the top of the basement is estimated to be about 1 2 km this is in good agreement with the estimation of depth to the basement of about 1 1 km calculated using the aeromagnetic data however in view of the circular geometry of the crater we have carried out a 3d forward modeling computation to supplement the derived 2d model the 3d resistivity forward model fitting the mt responses by trial by error revealed a five layer model showing a significant reduction in the basement resistivity this perhaps could be due to the structural disturbances that have been caused by the impact on the crater resulting in brecciation fracturing alteration and shocked zone filled with weak magnetic materials and fluids we have calculated the effect of the impact on the overall structural deformation beneath the serra da cangalha crater following the classical crater scaling relation of holsapple and schmidt holsapple k a schmidt r m 1982 on the scaling of crater dimensions ii impact processes j geophys res 87 1849 1870 and found to be about 2 km

seismotectonics of the norwegian continental margin the norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide indicating a potential earthquake deficit the adjacent oceanic crust is mostly aseismic except for parts of the lofoten and norway basins which have experienced rapid deposition of glacial sediments the excess load has enhanced the local stress field and in turn the seismic activity the marginal highs along the continent ocean transition and the part of the more and voring basins which experienced crustal extension prior to the early tertiary breakup are practically aseismic this region is also underlain by high velocity lower crust emplaced during breakup suggesting crustal strengthening which also may increase the return periods for the largest events farther landward a spatial correlation of seismic activity and the glacial sediment wedge suggests a causal relationship expressed through preferential rejuvenation of late jurassic early cretaceous faults the locally high seismic activity in the coastal region occurs in areas where the continental crust is relatively thick 25 30 km but still thinner than in the shield area farther east 45 50 km although stress relations are complex we suggest that increased stress due to appreciable postglacial rebound gradients in the coastal region may be a contributing factor while the stress field along the margin complies in general with the ridge push force we infer that regional and local stress enhancement factors are not only present in this region but also necessary for explaining the earthquake activity the relative importance of local stress sources is supported by several cases of 90 stress rotations relative to the ridge push direction the superposition of regional and local stress together with the existence of weakness zones and faults yields potentials for earthquakes primarily through structural reactivation

determining source parameters of moderate size earthquakes from regional waveforms abstract waveform modeling of the teleseismic long period body wave phases of shallow earthquakes has proven quite effective in determining source parameters for events of magnitude larger than six unfortunately these teleseismic phases become too weak for smaller events and regional data must be used which are generally much more complicated this is because the crust mantle system acts as a leaky waveguide and a large number of rays are required to model the observations in this report we review a procedure for the systematic determination of source parameters from regional body waves a least squares inversion technique is used which is based on a cross correlation of the data and a synthetic seismogram the synthetic waveforms are a linear combination of those for the three fundamental types of fault a set of profiles of the synthetic waveforms is presented although the synthetic waveforms are generated for a model developed for the western united states this model seems to be adequate for most continental regions the inversion is parameterized in terms of fault strike dip and rake these parameters are relatively insensitive to small changes in crustal structure several examples are presented

recent kattegat earthquakes evidence of active intraplate tectonics in southern scandinavia abstract on june 15 1985 an earthquake with a local magnitude m l upp value of 4 6 occurred in the kattegat area close to the swedish danish border it was one of the largest earthquakes in sweden and denmark during this century two more events occurred in the same area on april 1 1986 m l upp 4 2 and may 24 1990 m l upp 3 3 the derived focal mechanisms have north south trending p axes which deviate by 45 from the nw trending compressive stress field postulated by the ridge push theory the mechanisms can however be explained by local neotectonism both the locations and focal mechanisms strike slip faulting on nw striking planes correlate well with the dominant neotectonic feature of the region the skalderviken depression seismic moments of the 1985 1986 and 1990 events were 3 6 10 14 nm 1 4 10 14 nm and 6 0 10 12 nm respectively the 1985 earthquake had an estimated maximum intensity of vii modified mercalli scale and was felt over an area with a mean radius of 180 km the 1986 earthquake had a maximum estimated intensity of vi and a radius of perceptibility of 100 km despite the recent low seismicity of the area the earthquakes studied here indicate the potential for the occurrence of major events this is supported by the historical seismicity

causality between interplate north atlantic and intraplate fennoscandia seismicities abstract annual seismic energy release patterns have been calculated for the time period 1917 1987 for fennoscandia and the north atlantic ridge nar between 40 n and 80 n a brief visual comparison shows the great similarity between the energy release patterns in fennoscandia and individual segments along the nar to express the observed resemblance quantitatively and to provide an objective test of the phenomenon systematic cross correlations between seismicities along various nar segments and in fennoscandia were carried out the cross correlations obtained are statistically significant and show maximum values for time lags mostly between 0 and 3 years negative time lags mean that the energy release in the nar precedes that in fennoscandia the closest similarity is found between fennoscandia and the ridge segment between iceland and jan mayen 66 71 n periodicities of 8 13 and 38 years are seen in the energy release from both regions the significant cross correlations and consistent periodicities in the temporal patterns of seismic energy release along the nar and in fennoscandia serve as an indication of a tectonic connection between the interplate nar and intraplate fennoscandia seismicities

spectra of the earthquake sequence february march 1981 in south central sweden kulhanek o van eck t john n meyer k and wahlstrom r 1983 spectra of the earthquake sequence february march 1981 in south central sweden in j duda and k aki editors quantification of earthquakes tectonophysics 93 337 350 on february 13 1981 a relatively strong earthquake occurred in the lake v tern region in south central sweden the shock had a magnitude of m 3 3 and was followed within three weeks by three aftershocks with magnitudes 0 5 i m i 1 0 the focal mechanism solution of the main shock indicates reverse faulting with a strike in the n s or ne sw direction and a nearly horizontal compressional stress the aftershocks were too small to yield data for a full mechanism solution but first motions of p waves recorded at two stations are consistent for the aftershocks dynamic source parameters derived from pg and sg wave spectra show similar stress drops for the main shock 2 bar and the aftershocks 1 bar while the differences in seismic moment 1 5 1020 resp 4 10 dyne cm fault length 0 7 resp 0 2 km and relative displacement 0 15 resp 0 03 cm are significant

seismodynamics of sweden deduced from earthquake focal mechanisms summary focal mechanisms from 18 major swedish earthquakes ml upp 3 reveal that 1 several of the studied earthquakes have focal mechanisms with significant extensional stresses which fit well to the idea of postglacial rebound as an important stress generator 2 in areas like south western sweden the north westerly trending compressional axes indicate that ridge push from the north atlantic ridge is also a considerable stress contributor 3 some local tectonic features like the skalderviken depression in kattegat are momentous for seismotectonic interpretations in addition to first p polarities full waveform modelling for frequencies up to 3 hz and epicentral distances up to 225 km provides rather robust focal mechanism determinations focal depths of seven events ranging from 11 to 37 km are well resolved within a few km by waveform modelling

lower mantle viscosity constrained by seismicity around deglaciated regions knowledge of the viscosity structure of the earth s mantle is important for constraining models of mantle convection and isostatic rebound here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle in addition to those previously proposed1 2 calculations using a spherical viscoelastic earth model show that the present day magnitude of the stress fields induced in the lithosphere beneath the now disappeared laurentide and fennoscandian ice sheets is very sensitive to the value of the lower mantle viscosity stresses of 100 bar sufficient to cause seismicity can still remain in the lithosphere for lower mantle viscosities greater than 1022 pa s for lower mantle viscosities of 1021 pa s only a few tens of bars of stress persist in the lithosphere today this influence of lower mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes and hence for earthquake recurrence times

the exceptional earthquakes in kaliningrad district russia on september 21 2004 abstract the earthquakes of magnitudes m w 5 0 and 5 2 in the kaliningrad enclave of russia on september 21 2004 were unexpected in a low seismicity area the earthquakes caused moderate damage in the kaliningrad enclave and smaller damage in northern poland and in southern and western lithuania the largest earthquake was the strongest ever recorded instrumentally in the region and it was felt at distances up to 800 km in directions towards the west and south the perceptibility area is abruptly cut off by the tornquist teisseyre zone the south west margin of the east european craton the earthquakes are instrumentally located at depths 16 20 km under the central northern part of the sambia peninsula in the kaliningrad enclave for these events it is noted that the macroseismic calculations of 10 19 km depths are in reasonable agreement the source mechanism of the largest earthquake was determined to be a right lateral strike slip on a wnw ese near vertical fault of orientation almost parallel to the tornquist teisseyre zone and to the north coast of the sambia peninsula based on available stress information it is interpreted that the underlying cause of the earthquakes is the absolute plate motion historical information is scanty it is searched in an attempt to evaluate past seismic activity in the region and to evaluate vulnerable weakness zones in the geological structures

seismicity and seismotectonics of norway and nearby continental shelf areas the earthquake activity of norway and nearby offshore areas is low to intermediate with few events above magnitude 5 recent significant improvements in instrumental coverage in parallel with a better utilization of older including historical data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the viking graben while from the northern north sea to svalbard the earthquakes in a broad sense follow the continental margin fifty one focal mechanism solutions from these areas about half of them new reveal stress directions that clearly indicate a connection to the plate tectonic ridge push force at least for the areas at a minimum distance from the continental margin along the margin stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects a dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation on a regional level the seismicity in these areas correlate quite well with geologic features such as grabens fault zones fault complexes fracture zones and the margin itself indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field in the northern north sea however an area with quite anomalous stress orientations with strike slip faulting is found in a region transitional between normal and reverse faulting most of the earthquake foci are confined to the presumably brittle parts of the crust but many events are also located quite close to and on both sides of the moho discontinuity

fennoscandian dynamics present day earthquake activity abstract the fennoscandian earthquake catalogue is constantly updated and revised and we present the best available map of earthquake activity it exhibits significant differences between areas of high and low seismicity the focal mechanisms have been determined for many of the earthquakes the total set of these focal mechanisms shows that the stress pattern is very consistent the horizontal deviatoric stresses are characterized by a grouping in the nw se directions for the maximum compression this stress field is not distorted even by the significant crustal inhomogeneity in the fennoscandian border zone this orientation of the maximum horizontal compression is precisely that expected from ridge push forces from the north atlantic ridge it is not consistent with the stress field expected from postglacial uplift after the last ice age however recently large neotectonic faults have been identified and studied in northern scandinavia and these are taken as indicators of large scale displacements shortly after the end of the last ice age seen in parallel with similar studies in scotland this shows that the stress field has changed drastically with time the present day uniform stress field and the relatively scattered distribution of observed earthquakes lead to the conclusion that the earthquakes concentrate in specific zones of brittle weak crust we claim that arguments based on tectonics must be dependent on the specific tectonic characteristics of each area separately and this remains the outstanding problem of fennoscandian earthquake activity as observed presently

focal mechanisms of earthquakes in southern quebec southeastern ontario and northeastern new york with implications for regional seismotectonics and stress field characteristics focal mechanisms have been derived for 30 earthquakes in the adirondack mountains western quebec seismic zone from a modified version of the computer program focmec using first motions of pn and pg waves and sv p amplitude ratios measured from sg and pg phases reverse faulting on medium to steeply dipping planes is prevalent focal depth and velocity structure can vary within realistic limits without much influence on the focal mechanism solution deviatoric compressive stress axes have a strong horizontal component but the trend varies most align approximately northeast southwest in accordance with both the orientation predicted by models based on plate tectonics and that derived from most other studied earthquakes in the region however for several earthquakes mechanisms have been obtained that deviate from this pattern notably for events near 45 5 n to 46 n 75 w and the north gower main shock aftershock series of october 1983 possible explanations for these anomalies in terms of local structural disturbances or reactivation of preexisting zones of weakness with different structural orientation than those created during the current tectonic regime are presented different hypotheses for the seismotectonic evolution and processes of the area are presented and discussed in addition to orientations of nodal planes and deviatoric stress axes there are other indicators such as the seismicity distribution and patterns of earthquake sequences of inhomogeneity in the proposed adirondack mountains western quebec seismic zone

postglacial rebound and fault instability in fennoscandia summary the best available rebound model is used to investigate the role that postglacial rebound plays in triggering seismicity in fennoscandia the salient features of the model include tectonic stress due to spreading at the north atlantic ridge overburden pressure gravitationally self consistent ocean loading and the realistic deglaciation history and compressible earth model which best fits the sea level and ice data in fennoscandia the model predicts the spatio temporal evolution of the state of stress the magnitude of fault instability the timing of the onset of this instability and the mode of failure of lateglacial and postglacial seismicity the consistency of the predictions with the observations suggests that postglacial rebound is probably the cause of the large postglacial thrust faults observed in fennoscandia the model also predicts a uniform stress field and instability in central fennoscandia for the present with thrust faulting as the predicted mode of failure however the lack of spatial correlation of the present seismicity with the region of uplift and the existence of strike slip and normal modes of current seismicity are inconsistent with this model further unmodelled factors such as the presence of high angle faults in the central region of uplift along the baltic coast would be required in order to explain the pattern of seismicity today in terms of postglacial rebound stress the sensitivity of the model predictions to the evects of compressibility tectonic stress viscosity and ice model is also investigated for sites outside the ice margin it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility for sites within the ice margin the evect of earth rheology is shown to be small however ice load history is shown to have larger evects on the onset time of earthquakes and the magnitude of fault instability

tomographic evidence for localized lithospheric shear along the altyn tagh fault seismic tomography across the altyn tagh fault at the north edge of the tibetan plateau reveals a low p wave velocity anomaly below the fault down to 140 kilometers this anomaly probably reflects strike slip shear in the lithosphere slip partitioning may also induce a wedge of crust from the tarim basin to plunge into the mantle

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

recent horizontal stress directions in basement rocks of southern sweden deduced from open microcracks abstract the strike direction of open intragranular microcracks in quartz and feldspar host grains was determined using optical transmission and reflection microscopy on eight oriented samples taken in two study areas in precambrian basement rocks of southern and south central sweden for an area of about 160 km 2 sw of vastervik and two sample locations w of uppsala the vast majority of open microcracks displays a strong preferred nw se strike direction according to the common assumptions that natural cracks in crystalline rocks are predominantly extensional mode i and that open cracks belong to the latest microcrack generation these strike directions should reflect the sub recent main horizontal stress direction h of the recent tectonic stress field this conclusion is supported by corresponding directions known from in situ stress measurements and focal plane solutions in the vicinity of the study areas it is remarkable that even in samples taken close i e a few hundred metres to recently active large scale faults the orientation of microcracks does not deviate from this common direction this may point to slip on already softened faults very local stress reorientations e g m scale or that local stress relief was accomplished by other processes at micro scale e g mechanical twinning in favourably oriented feldspar crystals or slip on grain boundaries

a gelation analogy of crustal formation derived from fractal conductive structures 1 the statistical evaluation of numerical random media models and of electromagnetic array data provides evidence of fractal conductive structures in the middle and lower crust during the search for an electrical conduction mechanism that is compatible with the geophysical anomalies in the middle and lower crust random resistor networks were developed these networks contain two types of resistors representing the rock matrix and the conductive phase here we demonstrate that random resistor network models can also explain the statistical properties of the electric field distortion measured in large scale electromagnetic array experiments correspondence between measured and modeled distortion statistics is obtained if the structures formed by the two resistor types in the networks have a fractal geometry this can indicate that the natural conductive networks also have a fractal geometry and stay close to a percolation threshold a simple scale model of crust formation mechanics is considered in order to find an analog medium and an analog process that creates such a geometry we suggest gelation as a rheological analogy that allows for a coexistence of ductile and brittle behavior of material in the lower crust this process forms fractal structures in the host matrix and fractal conductive networks provided that conductive material is available at that time of crustal evolution the statistical evaluation of the field data provides evidence for a fractal structure with an upper bound of the size of fennoscandia

source inversion of seismic waveforms the koyna india earthquakes of 13 september 1967 abstract the treatment of the seismic source inverse problem when diverse forms of waveform data are available is simple and elegant using a moment tensor formalism if earth structure is known and its effects predictable in terms of vertically inhomogeneous elastic layered models then all types of wave phenomena e g surface waves body waves leaky modes etc for a purely deviatoric moment tensor point source may be represented by at most a sum of three green9s functions for an arbitrary symmetric moment tensor point source one additional green9s function is needed for the p sv system however elegant this formalism may be for posing the linear inverse problem the major difficulties lie in earth structure unknowns and resultant nonlinearities in the green9s functions which can cause significant trade offs with source parameters a hybrid inversion procedure is set up to gain insight into the probable unknowns in particular problems by incorporating both a linearized least squares gradient method for the moment tensor or double couple and smoothed time function parameters and a nonlinear systematic trial and error search for moment tensor or double couple parameters for several assumptions of green9s function the inversion technique is applied to near regional waveform data from a small earthquake associated with the koyna reservoir which occurred 13 september 1967 as the second in a group of three events with very similar waveshapes but differing amplitudes the magnitudes of the second and third events are smaller by 0 2 and 0 8 units respectively compared to the first the absolute magnitude for the first event is poorly constrained but is estimated to be 4 0 to 4 5 rather than the previously published value of 5 5 to 6 0 from the similarity of waveshapes all three events are inferred to have the same mechanism and occurred within about 2 km of the same hypocenter the results from moment tensor and double couple inversions for event 2 data indicate that source depth was 5 km and that left lateral faulting occurred on a plane with a strike of n20 e 5 dip of 90 15 and a rake of 0 35 the inferred far field time function is approximately 3 sec in duration unusually long for the seismic moment of 9 10 22 dyne cm yielding a possible stress drop of about 0 05 bars a fault map was constructed from landsat image interpretation and shows a predominance of nnw to nne striking faults in the koyna area which is used to infer the appropriate nodal plane in the inversion results these faults tend to define a broad en echelon zone which parallels the western ghats in this area

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

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

magnetotellurics along the fennoscandian long range profile summary estimates of magnetotelluric transfer functions along the fennoscandian long range fennolora profile are presented and discussed in relation to the major lithotectonic subdivisions of the swedish part of the baltic shield none of the transfer functions are found to be consistent with a one dimensional earth structure thereby making a quantitative interpretation difficult however the geological units are found to display characteristic differences in the obtained transfer functions and in some areas a quantitative interpretation can be justified the most interesting structure indicated is a low resistive 4 am crustal structure of lateral extension more than 150 km centred around the skellefte ore district a minimum thickness for this structure of 15 km is inferred

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

deep crustal earthquakes in the southern baltic shield summary on 1986 july 14 one of the largest earthquakes in the baltic shield during this century occurred near skovde in the province of vastergotland sweden with a magnitude of ml upp 4 5 it was followed by the so far largest number of recorded aftershocks more than 20 from any swedish earthquake the strongest aftershock with ml upp 3 4 occurred about one hour after the main shock a few months later on november 2 an ml upp 3 6 event took place near mariestad some 30 km northwest of the skovde series all these shocks were located in the lower crust with foci at depths between 20 and 35 km indicating active movements in the shield at depths where mainly ductile deformation is usually assumed waveform modelling seems to be an excellent tool to obtain information about the focal depth which is otherwise hard to determine the calculated seismic moment and stress drop for the skovde main shock are 5 9 1014 n m and 2 8 mpa respectively and for the mariestad earthquake 2 3 1014 n m and 7 3 mpa respectively focal mechanisms for the skovde main shock and largest aftershock have been obtained from p wave polarities and synthetic modelling and for the mariestad earthquake from p wave polarities the synthetics show good agreement with short period seismograph records for frequencies up to at least 2 hz and distances up to at least 200 km a comparison of the mechanisms indicates that the faulting of the area is complex with styles ranging from strike slip to normal suggesting that other factors than the push from the north atlantic ridge e g post glacial rebound also contribute to the lithospheric stress pattern this explanation is corroborated with findings from nearby southern norway

the kaliningrad west russia earthquakes on the 21st of september 2004 surprise events in a very low seismicity area abstract the earthquakes in kaliningrad west russia on the 21st of september 2004 were unexpected in a seismically quiet area the main shock of magnitude m b 5 was widely felt around the baltic sea a comparison with some historic earthquakes in northern europe shows that its perceptibility area is smaller than that of the 1904 oslo graben earthquake of an estimated magnitude m l 5 4 but larger than those of the 1759 kattegat and 1819 luroy earthquakes the latter are claimed to have had magnitudes m s in the range of 5 7 6 0 an analysis of the lg phase of the kaliningrad earthquakes as recorded at a number of european stations accords only weakly with the macroseismic intensity pattern that shows fast attenuation towards west northwest and southeast the strike slip focal mechanism of the main shock is discussed in the context of remnant glacial rebound stresses in generating present day seismicity in n europe

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

crustal stress regime in fennoscandia from focal mechanisms an extensive data set of earthquake focal mechanisms is now available for all of northern europe and especially for fennoscandia these mechanisms are considered to provide representative coverage of the stress field the maximum horizontal compressional stress orientations are internally very consistent over the area of northern europe the dominating nw se compressional stresses appear tied to relative plate motion with mid atlantic ridge spreading and european african collision in southern europe for fennoscandia some exceptions to the regional stress pattern exist the causes of these local anomalies have been investigated no correlations with geological provinces or province boundaries were found in addition there does not appear to be any clear correlation between anomalous stress directions and postglacial uplift in the present earthquake activity this lack of correlation is in sharp contrast to geological evidence in the form of large faults indicative of large postglacial earthquakes occurring right after the end of the latest ice age 9000 years ago taken together this evidence suggests a tremendous change of stress field in holocene time from one dominated by the postglacial unloading right after the ice age to one dominated by the present plate motion today

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

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

the determination of source parameters for small earthquakes from a single very broadband seismic station the installation of very broadband seismic stations makes it possible to recover the source parameters of small earthquakes 2 5 ml 5 0 which occur at local and regional distances if the gross crustal structure along the travel path is known it is possible to use the p sv and sh displacement waveforms from a single station to determine the seismic moment tensor although the details of the crustal structure strongly affect the body waveforms at regional distances the signature of the seismic source orientation on the waveform is robust at frequencies less than 1 3 hz we explore the trade offs between crustal model hypocentral depth and filtering for a linear moment tensor inversion procedure the procedure is tested on two small earthquakes which occurred in the rio grande rift and were recorded at the iris usgs station anmo the agreement between the single station moment tensor inversion fault plane parameters and those determined from local first motions is excellent

seismo and neotectonics in finnmark kola and the southern barents sea part 2 seismological analysis and seismotectonics abstract a detailed review and analysis of seismological data acquired since 1979 from finnmark kola and the southern barents sea has provided a much improved delineation of the earthquake activity in that region in the barents sea the seismicity is confined only to the western areas while the finnmark kola coastal areas show some seismicity only north and east of murmansk in the rybachi cape tereberski region further inland there are several clear seismicity lineations that to some extent can be correlated with postglacial faults including the ne sw striking stuoragurra fault in finnmark the spatial correlation between seismicity and mapped faults is interpreted in general as a reflection of broad zones of weakness that respond to the present day stress field a detailed analysis of the earthquakes in this area has also shown a dynamic consistency in that they reveal a reverse faulting pattern that is consistent with a detailed geological study that includes a core drilling the dominant horizontal stress direction in this region is nnw sse consistent with the ridge push effect

the determination of source mechanisms of small earthquakes and revised models of local crustal structure by moment tensor inversion summary we describe a procedure for retrieving the source mechanism of small earthquakes and a revised model of crustal structure in heterogeneous media based on interactive inversion of high frequency seismograms and demonstrate its application to three local earthquakes the earthquakes are located along the rukwa rift south western tanzania and were recorded by a local network in the rift due to strong lateral crustal heterogeneity in the area the procedure involves a single station interactive inversion for moment tensor solution and modelling of the crustal structure along the propagation path between the source and the station based on this source solution and the a priori crustal structure synthetic seismograms are generated and compared with observed displacement seismograms at other stations where the event has been recorded deviations between the calculated and observed seismograms are indicated by time shifts and amplitude mismatches on prominent phases these are employed to correct the phase arrivals used to invert for the partial multistation source solution the moment tensor inversion of high frequency seismogram data in this study is based on a point source approach the process includes generation of synthetic seismograms using the frequency wavenumber integration technique for an elastic horizontally layered medium moment tensor inversion is carried out by minimizing the l2 norm of the difference between synthetic and observed displacement data under a deviatoric condition results for the three small earthquakes show good agreement between single station and multistation source solutions the inversions for these events indicate dominantly dip slip mechanisms with seismic moment magnitudes in the range 2 0 mw 3 3

postglacial crustal doming stresses and fracture formation with application to norway abstract postglacial doming of fennoscandia is estimated to have reached a maximum uplift value of 850 m vertical uplift of this magnitude generates stresses that affect the tectonic activity in the domed area using a simple model of a circular elastic plate subject to deflection it is shown that the maximum tensile stresses generated by this uplift occur in the centre of the dome and at the surface reach a numerical value of nearly 30 mpa this tensile stress is an order of magnitude greater than the typical in situ tensile strength of the uppermost part of the crust so that tension fractures are easily generated by this stress tension fractures can under certain conditions reach crustal depths of several hundred metres but if they propagate to greater depths they change into normal faults the tensile stresses at the surface decrease in magnitude with distance from the centre of the dome and become compressive in the marginal parts of the uplifted crustal plate at the surface of these parts the maximum shear stress exceeds 10 mpa because the inferred driving stresses stress drops in earthquakes are commonly only several mpa it follows that the driving shear stress in these marginal parts is large enough to initiate or reactivate seismogenic faults these model results are in broad agreement with data on the current seismotectonics of fennoscandia the formation and reactivation of tensile and shear fractures in fennoscandia as a result of postglacial uplift is likely to increase the hydraulic conductivity of the solid rocks that constitute the uppermost part of the crust this conclusion is supported by reported observations which indicate that the production of groundwater in wells in many areas in norway varies positively with the maximum postglacial uplift

application of acoustic noise and self potential localization techniques to a buried hydrothermal vent waimangu old geyser site new zealand a seismo acoustic and self potential survey has been performed in the hydrothermal area of the old waimangu geyser new zealand which was violently erupting a century ago nowadays no surface activity is visible there we set up an array of 16 geophones and recorded a high and steady acoustic ambient noise we applied the matched field processing mfp approach to the acoustic data to locate the sources responsible for the ambient noise the white noise constraint processor reveals the presence of a unique and well focused acoustic source at a depth of 1 5 m below the seismic array for this very shallow source the application of mfp enabled the determination of both the source location and the dispersion curve of seismic velocity the study was completed by self potential sp measurements on several profiles around the acoustic noise source which displayed a large positive anomaly above it the results of the sp inversion gave an electric streaming current density source very close to the acoustic one both sources likely belong to a shallow hydrothermal structure interpreted as a small convective cell of boiling water beneath an impermeable layer the joint application of these methods is a promising technique to recognize hydrothermal structures and to study their dynamics

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

modelling short period crustal phases at regional distances for the seismic source parameter inversion abstract waveforms of short period crustal phases observed at regional distances are modelled in an attempt to retrieve the source parameters of small earthquakes for a simple shield type crustal structure the synthetic seismograms calculated for elementary source mechanisms show that the waveforms of crustal phases are quite distinct for different source mechanisms and suggest that these features can be used to constrain the source mechanisms waveforms of the crustal phases are also strongly affected even by small variations in the crustal thickness as well as in the focal depth a generalized linearized waveform inversion technique is used to invert the regional crustal phases for a deviatoric moment tensor source the inversion technique is applied to regional short period records from a small earthquake in central sweden for determining its source parameters the waveform inversion which mainly used lg waves produced a satisfactory fit between the observed and synthetic records suggesting that the source mechanism and focal depth estimates were reasonable the focal depth of 40 km obtained for the event studied suggests the occurrence of a deep crustal earthquake and demonstrates the usefulness of crustal phases in studying small intraplate earthquakes