improving global seismic event locations using source receiver reciprocity the leading source of error in seismic event locations is travel time perturbations caused by three dimensional earth structure the reciprocity of travel times between sources and receivers provides a method for testing the effectiveness of empirical methods for improving event locations that rely on nearby calibration events of known location we apply this approach to travel time residuals obtained by engdahl et al 1998 for almost 100 000 teleseismic events by analyzing the residual patterns at thousands of seismic stations of known location we characterize the spatial coherence of station event mislocation vectors we find that on average calibration events are likely to improve locations only if they are located within 100 150 km of the target events for 84 events of known location we find that applying source receiver reciprocity can significantly reduce location errors by correcting for the teleseismic residual pattern observed at stations close to the target events these results have implications for efforts to improve event locations for nuclear explosion monitoring purposes

k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

geophysical studies of the structure of the appalachian orogen in the atlantic borderlands of canada results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the appalachian orogen in eastern canada wide angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins high velocity lower crust attributed to underplating is found below the former laurentian continental margin in newfoundland and below the magdalen basin potential field data are used to trace the surface zones of the orogen from the northeast newfoundland shelf to cape breton but extrapolation to new brunswick and quebec is unclear because of late paleozoic basin development the central mobile belt of the orogen is only a few tens of kilometres wide in southwest newfoundland and cape breton but broadens substantially to around 200 km elsewhere reflection images show a strong deep crustal fabric that runs along the orogen with a margin that crosses into the avalon zone in southern newfoundland but coincides with the avalon gander zone boundary el

development and hydrocarbon potential of mesozoic sedimentary basins around margins of north atlantic the late triassic early jurassic rift basins of iberia offshore eastern canada and the continental shelf of western europe are fragments of a formerly coherent northeast trending rift system that probably formed as a result of tensional stress between europe africa and north america the separation of europe north america and iberia was preceded by a late jurassic early cretaceous rifting phase that is clearly distinct from the earlier mesozoic rifting episode and was little influenced by it the two periods of rifting are separated by a middle jurassic relatively tectonically quiet period the distribution of hydrocarbon finds within the rift basin suggests that the best prospects are in areas where the two rifting episodes are superimposed lack of mature source r cks in the later mesozoic rift basins and an unfavorable temporal relationship between hydrocarbon generation and tectonic activity in the early mesozoic basins are proposed as explanations for the apparently poor hydrocarbon prospectivity of large areas of the mesozoic basins

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

mesozoic seafloor spreading between iberia europe and north america abstract a new earliest cretaceous palaeogeographic reconstruction for the iberian north american and european plates is proposed which attempts to clarify some of the outstanding reconstruction problems in the region from the time of initial opening to the time of magnetic anomaly 34 the instantaneous motion of iberia can be described by a single rotation about a pole off the southwest coast of france in contrast to many previous proposals the new reconstruction places galicia bank east of flemish cap but satisfies all the constraints imposed by palaeomagnetic studies deep sea drilling project results and magnetic anomaly and ocean continent boundary identifications

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

new seismotectonic data from an intraplate region focal mechanisms in the armorican massif northwestern france summary focal mechanism solutions are determined for 11 small intraplate earthquakes that occurred between 1990 and 1998 in normandy and the channel islands these mechanisms are obtained from the p wave first motion polarities recorded by stations from local and regional seismic networks the accuracy of each hypocentral location is closely examined and the quality of each fault plane solution is discussed by considering the influence of the velocity structure the predominant feature of the computed focal mechanisms is the relatively widespread near horizontal ne sw t axis orientation horizontal p axes strike roughly nw se mechanism solutions for the earthquakes in the avranches region show left lateral strike slip on a nnw sse fault zone for the overall region it seems that nodal planes of normal faulting solutions trend nw se or wnw ese whereas those of thrust faulting solutions trend ne sw this is in agreement with the general regional stress pattern the ne sw normal fault plane solution of the 1990 jersey event is unique because it is not consistent with the regional style of faulting

results from a detailed aeromagnetic survey across the northeast newfoundland margin part i spreading anomalies and relationship between magnetic anomalies and the ocean continent boundary abstract a detailed aeromagnetic survey carried out across the northeast newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34 correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time three fracture zones can be identified by dislocations in the magnetic anomalies their positions are confirmed on the depth to basement map of this region an eastward extension of the southernmost fracture zone at latitude 49 n matches well with the faraday fracture zone across the mid atlantic ridge and with a basement ridge known as pastouret ridge mapped off goban spur by combining the present survey data with the previously collected shipborne measurements we have also traced the westward continuation of the charlie gibbs fracture zone under the newfoundland shelf a large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics long wavelength small amplitude anomalies on the landward side and quasi lineated anomalies on the seaward side seismic data compilations show that this large anomaly coincides with the ocean continent boundary at most places north of flemish cap modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust this situation is similar to that observed across the goban spur margin which is a conjugate of the flemish cap margin the presence of highly magnetized oceanic crust landward of anomaly 34 and within the cretaceous magnetic quiet zone is attested to by the presence of similar large amplitude anomalies south of the flemish cap and goban spur regions but these do not mark the ocean continent transition

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

structure across the northeastern margin of flemish cap offshore newfoundland from erable multichannel seismic reflection profiles evidence for a transtensional rifting environment we present the results from processing and interpreting nine multichannel seismic reflection lines collected during the 1992 erable experiment over the northeastern margin of flemish cap offshore newfoundland these lines combined into five cross sections provide increased seismic coverage over this lightly probed section of the margin and reveal tectonically significant along strike variations in the degree and compartmentalization of crustal thinning similar to the southeastern margins of flemish cap and the grand banks a transitional zone of exhumed serpentinized mantle is interpreted between thinned continental and oceanic crust the 25 km wide transitional zone bears similarities to the 120 km wide transitional zone interpreted as exhumed serpentinized mantle on the conjugate irish atlantic margin but the significant width difference is suggestive of an asymmetric conjugate pair a 40 50 km wide zone of inferred strike slip shearing is interpreted and observed to extend along most of the northeastern margin of flemish cap individual shear zones szs may represent extensions of szs and normal faults within the orphan basin providing further evidence for the rotation and displacement of flemish cap out of orphan basin the asymmetry between the flemish cap and irish conjugate pairs is likely due in large part to the rotation and displacement of flemish cap which resulted in the flemish cap margin displaying features of both a strike slip margin and an extensional margin

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

robust inversion of iasp91 travel time residuals for mantle p and s velocity structure earthquake mislocations and station corrections using both p and s arrival time information 41 108 events in the international seismological centre isc catalog for the years 1964 to 1987 are relocated relative to the iasp91 velocity model the mean absolute horizontal relocation is 7 7 km and the mean absolute depth relocation is 9 1 km the mean absolute origin time shift is 1 2 s the relocation procedure increased the p residual standard deviation slightly from 2 3 s to 2 5 s while decreasing the s residual standard deviation from 6 8 s to 6 1 s when plotted as bottoming point averages the resulting iasp91 p and s arrival time residuals show coherent patterns as a function of geographic location an iterative lp residual norm minimization algorithm is used to estimate the set of p and s velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals the procedure is robust in that extremely large travel time residuals which are common in the isc data do not unduly influence the velocity estimates both the p and s models of lateral heterogeneity contain prominent circum pacific low velocities 1 to 2 perturbations underlying the back arc basins between 35 and 200 km depth this ring of negative deviations continues into the depth interval 200 400 km the continental cratons are underlain by high velocity anomalies with maximum amplitudes of 2 iceland and the azores are underlain by low velocity mantle material that extends down to at least 400 km the benioff zones are only intermittently imaged as 1 2 high velocity regions in the uppermost 400 km they are best resolved in the p velocity variations both the p and s velocity models contain a circum pacific ring beneath the benioff zones of 1 2 positive velocity deviations in the depth range 660 870 km coherent high velocity features are seen below south america borneo tonga fiji the marianas and the northern kuriles the anomalies beneath south america and borneo extend into the 870 1070 km depth range below depths of 1270 km for p variations and 1070 km for s variations the amplitude of the heterogeneity has decreased significantly it is only in the lowermost mantle 2670 km to the core mantle boundary that the level of p heterogeneity rises significantly above the estimated noise level in this depth range a partial ring around the pacific basin is observed although this

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

reliability of earthquake location procedures in heterogeneous areas synthetic tests in the south western alps italy abstract the accuracy of procedures for locating earthquakes and tomographically imaging 3 d velocity structures is influenced by factors such as network geometry errors in picking seismic phases modeling errors and velocity model uncertainties the main purpose of this study is to investigate the importance of different error sources through synthetic experiments the synthetic data sets were generated for a realistic 3 d velocity model of the south western alps adopting a precise raytracing technique the actual station configuration and recorded seismicity during the passive seismological experiment geofrance3d alps conducted between august 1996 and february 1997 were used the station configuration corresponded to an enhancement of the regional networks operating in the area with the addition of temporary stations in addition synthetic data sets were corrupted by applying different error schemes furthermore to assess the locating capabilities of the regional networks synthetic data sets were constructed for synthetic events placed in correspondence to the nodes of a regular three dimensional 3 d grid we mimicked the actual data management scheme for synthetic data processing we re located the earthquakes performed an inversion for a minimum 1 d velocity model with station corrections and then ran a complete 3 d velocity model inversion one of the main results confirming earlier studies was that earthquake locations in strongly heterogeneous areas obtained using a regional 1 d model can have significant errors even a minimum 1 d model with station corrections does not significantly improve the accuracy of parameter locations in the south western alps for high precision locations a 3 d model is needed however a 3 d model cannot be reconstructed from noise contaminated data recorded by a sparse regional network

linear and nonlinear earthquake location approaches in a case study overview abstract several seismic events are calculated by an iterative linear algorithm relocated in a nonlinear grid search with a maximum likelihood hypocenter method the same simultaneous events separately registered by regional and local seismological networks have also been used a verified 1d one dimensional velocity crustal structure model is used in both methods and in all datasets for better comparisons some rock quarry blasts registered by the local receiver stations as ground truths are examined by both methods this study s main objectives were i to carry out a comparative examination of the error estimates in both techniques and ii to generalize location assessment of the permanent low density regional network in central alborz the regional network operates under iranian seismological center and its seismic data are massively applied to reach the research goals the dataset method correlation matrix and the location variables were statistically processed it was shown that the estimated earthquake epicenters by the linear and nonlinear methods are approximately similar for well located events in each network for dense proper distribution of the seismic stations the elliptical error estimates have lower amounts and consequently the seismicity pattern is more reliable the difference between subsurface seismicity patterns resulting from the above mentioned approaches in the studied area is presented some depth calculation errors for the same recorded events are higher in the regional network the correlation comparisons demonstrated that the nonlinear approach and the locally constrained data may be closer to reality and more convincing the nonlinear application is more plausible due to all of the possible solutions and less elliptical error estimates under the gaussian distribution assumption subsequently the combination of the nonlinear method proper location criteria and appropriate crustal velocity structure can effectively improve some of the earthquake location estimates in the regional catalogue as a result this procedure can be useful in seismicity concept evaluations

crustal structure beneath orphan basin and implications for nonvolcanic continental rifting a wide angle seismic profile was acquired across the northeast newfoundland margin seaward to orphan knoll the profile is complemented by existing multichannel reflection data two online deep wells and other deep seismic reflection and refraction lines modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the 400 km wide stretched continental crust on this margin prerift metasedimentary rocks have a topographic relief of up to 5 km with a velocity of 5 2 km s increasing to 5 9 km s unaltered crystalline crust below the metasedimentary section has a velocity range of 6 1 6 5 km s most of the lower crust landward of orphan knoll has a typical continental lower crustal velocity 6 8 7 0 km s and the lower crust shallows significantly beneath orphan knoll our model shows no evidence for the 5 km thick 7 35 km s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating implying nonvolcanic rifting for the ne newfoundland margin we suggest that continental stretching persisted from 180 ma to chron 34n time 101 ma when the final breakup between canada and western europe took place leaving a 400 km wide zone of thinned continental crust underneath the shelf and deep water area of the orphan basin on the landward side of the basin gravity modeling indicates a zone of very thin 6 8 km crust possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between canada and europe a possible sub moho reflector suggests generally uniform stretching of the crust and upper mantle and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the grand banks

conjugate margins of canada and europe results from deep reflection profiling deep seismic reflection profiles have been collected across the conjugate margins of the north atlantic ocean the eastern north atlantic margin is traversed in the goban spur region and the western north atlantic margin is crossed in the vicinity of flemish cap in the grand banks region these seismic profiles allow us to examine the deep structures and mode of extension in crust that was once contiguous the flemish cap and goban spur margins have different structural styles thick and relatively unfaulted crust is present on the flemish cap margin west of the continent ocean boundary whereas the goban spur margin exhibits a zone of extensional faulting and thinned continental crust the restored rift zone displays overall symmetry in which a bridge of thin crust 15 km thick about 60 km wide joins the two crustal blocks of normal continental thickness 28 km the configuration of the moho the geometry and distribution of the lower crustal reflections and the overall symmetry of the rift zone favor pure shear stretching in the lower lithosphere although either pure or simple shear could accommodate crustal extension

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

epicentral confidence regions of nuclear test events at teleseismic distances the accurate location of seismic events is a basic discriminant for underground nuclear test monitoring bolt 1976 dahlman and israelson 1977 blandford 1982 of particular interest are determining epicentral confidence regions and providing constraints on estimated focal depths in this study only routine teleseismic p travel time data are used as provided by worldwide stations reporting to the international seismological centre isc this lessens the need to model the effects of crustal and shallow mantle velocity variations as is necessary with seismographic networks operating at regional distances blandford 1981 evernden et al 1986

a volcanic province near the western termination of the charlie gibbs fracture zone at the rifted margin offshore northeast newfoundland a mid cretaceous to late cretaceous volcanic province named here the charlie gibbs volcanic province is described near the western termination of the charlie gibbs fracture zone against the rifted continental margin northeast of newfoundland we used seismic data to map 14 volcanic seamounts now buried below younger sediments they rise 0 7 to 2 s two way time twt above the surrounding basement level and are about 8 30 km wide some are conical while others are more flat topped underlying igneous units resembling flows and sills are also observed based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts the total thickness of igneous rocks can locally reach about 8 km this magmatism occurred in the vicinity of the charlie gibbs fracture zone and extends about 150 km to the north along the rifted continental margin the volcanic province also forms the northern boundary of the jurassic early cretaceous orphan basin along a major transform margin there truncation of rift related structures which extend to deep crustal levels is observed at the transform along trends similar to those of prerift appalachian terrane boundaries on the adjacent shelf this suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike slip fault system developed and may have controlled the location of final continental breakup between the rockall and north american plates in the late cretaceous

analysis procedures at the international seismological centre abstract analysis at the international seismological centre isc falls into three main categories association location and quantification difficulties of association on a global scale are not always appreciated the isc currently analyses 40 50 events a day and as the first arrivals from any particular earthquake may span a time interval of up to 20 min readings from events occurring in different parts of the world may overlap considerably in time the present association algorithm depends on time only and results in many chance mis associations or even the synthesis of fictitious events at present these mis associations are rectified by seismologists intervention but full use of amplitude and period information could help to detect these errors automatically revision of locations follows standard least squares procedures based on jeffrey s method of uniform reduction and jeffreys bullen travel times locations are made from p phases only including crustal phases but other first arrivals and secondary phases are identified and residuals calculated if depth cannot be determined by geometric means a search is made for depth phases or failing this the depth is restrained to that given by another agency or to a conventional value no provision is made for local variations in travel time body wave magnitudes are allocated within the distance range 21 100 from reported readings of a and t or their ratio using gutenberg richter calibration curves surface wave magnitudes are calculated from the prague formula using reports of long period a and t in the distance range 5 160 but only values from stations at distances of 20 or more are used to determine an average ms for a particular event there is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies improvements in these procedures could be made through the automatic association of station readings the introduction of local travel times and better determination of earthquake size particularly for local events

sequential faulting explains the asymmetry and extension discrepancy of conjugate margins rifted continental margins are the sites that record the thinning and break up of continents to form new oceans cesar ranero and marta perez gussinye use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this to crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy in which crustal thinning appears to be greater than the extension caused by brittle faulting by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins these authors use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this with crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy where crustal thinning seems to be greater than the extension caused by brittle faulting during early extension cold continental lithosphere thins and subsides creating rift basins if extension continues to final break up the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins although basins and margins are ubiquitous structures the deformation processes leading from moderately extended basins to highly stretched margins are unclear as studies consistently report that crustal thinning is greater than extension caused by brittle faulting1 2 3 4 this extension discrepancy might arise from differential stretching of brittle and ductile crustal layers2 but that does not readily explain the typical asymmetric structure of conjugate margins5 6 in cross section one margin displays gradual thinning accompanied by large faults and the conjugate margin displays abrupt thinning but smaller scale faulting5 whole crust detachments active from early in the rifting could in theory create both thinning and asymmetry1 but are mechanically problematical furthermore the extension discrepancy occurs at both conjugate margins leading to the apparent contradiction that both seem to be upper plates to a detachment fault7 8 alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic resolution faulting9 invisible deformation along top basement 100 km scale detachments8 or the structural complexity of cross cutting arrays of faults3 here we use depth migrated seismic images to accurately measure fault extension and compare it with crustal thinning the observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins contrary to current wisdom the observations support the idea that thinning is to a first degree explained by simple andersonian faulting that is unambiguously visible in seismic data

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

crustal phases in earthquake location for crustal earthquakes of moderate or large size the first reported phases at distant stations are usually the first predicted by the crustal model used for smaller events however or for stations at larger distances the first phase detected is often a later crustal phase of larger amplitude this may be eitherpg which travels entirely in the upper crustal layer with a velocity of about 5 6 km s orp with a path mainly in the lower crustal layer at a velocity of about 6 7 km s many earthquake location programs do not take account of these phases and treat their arrivals as if they were the earlier phasepn at the international seismological centre we re identify up to 200 crustal phases each month this often results in significant improvement in position the direct determination of depth or even the obtaining of a formal solution where none was possible before we find that these re identifications are needed in most continental areas of the world and that the original crustal model of jeffreys and bullen still remains a useful standard

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

estimation of hypocentral parameters of regional earthquakes using a fuzzy logic approach this study presents improvements on the hypocentral location method using the fuzzy logic approach proposed by lin and sanford bulletin of the seismological society of america 91 82 93 2001 in resolving regional earthquakes location in addition to trying to determine epicentral parameters of the earthquake focal depths are also attempted utilizing this method in a three dimensional 3d space in this study instead of p and s wave models a trapezoidal membership function constructed by the direct use of travel time difference between the two stations for p and s arrival times was utilized in this procedure it is not necessary to precisely know the velocity structure of the studied area in order to map the uncertainties in arrival times into the fuzzy logic space the location determination process was performed with the defuzzification of only the grid points having the membership value of 1 obtained by normalizing all the maximum fuzzy output values of the highest values which gives more reliable epicentral locations for both real and synthetic earthquakes the results show that this method for epicentral estimation works well if the depth of the earthquake is sufficiently shallow and according to the distance between the event and seismic network in this method of hypocenter determination firstly epicenter coordinates are determined then the hypocenter location is found by considering only the epicentral area in 3d in the case of very shallow events e g h 10 km the estimation of the focal depth using this method seems to fail causing faulty results in the hypocenter coordinates however the estimation of epicentral coordinates is successful in giving a more reasonable epicentral location otherwise the obtained results would be reversed consequently the deeper the focal depth the better the hypocenter location but a worse epicenter determination this method is only valid in the hypocentral location of the earthquake occurring in the upper crust because only primarily the arrivals of pg and sg on the seismograms and a half space velocity model are used the hypocenter locations of some selected shallow earthquakes that occurred in the south east of the aegean sea have been determined by this method with results that are highly comparable with those obtained by other seismic agencies

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

assessment of earthquake location accuracy and confidence region estimates using known nuclear tests i have used the known epicenters and depths of 307 nuclear tests at the nevada test site nts and 75 tests at the east kazakhstan test site to compare the distances between actual and calculated hypocenters mislocations with calculated location confidence limits errors each test was located using its isc compiled p first arrivals a starting depth of 33 km and the iasp91 velocity model i also applied azimuth and incident angle dependent station corrections to the arrival times the most accurately located tests are those at the kazakhstan test site where all of the mislocations in latitude and longitude are less than 10 km and 90 of the depth mislocations are less than 20 km bayesian confidence limits calculated from the least squares covariance matrix satisfactorily account for these mislocations at the nevada test site 90 of the mislocations are less than 9 km in longitude 15 km in latitude and 33 km in depth the overestimates in depth are compensated for by origin times that are 2 to 5 sec late the larger mislocations at nts occur in spite of the fact that the closest recording sites are less than 200 km away compared to 1800 km away at the kazakhstan test site a significant fraction of the depth mislocations at nts are much larger than their calculated errors i interpret this failure of the calculated errors to adequately account for the nts mislocations as being due to systematically late arrivals at northwestern american stations relative to the predicted iasp91 values by using a locally determined three layer velocity model for the travel times of phases recorded at distances of less than 1500 km i was able to reduce 90 of the depth mislocations to less than 11 km as well as significantly reducing the latitude longitude and origin time mislocations i conclude that differences between the iasp91 model and velocities in the vicinity of the explosions may introduce a significant bias into the determined depths of nuclear tests a similar bias would therefore be expected in the similarly determined depths of shallow earthquakes

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

a revisit to p wave travel time statics at teleseismic stations to account for local deviations from the one dimensional reference velocity model the station correction formulation of dziewonski and anderson 1983 is extended to include the travel time variation with respect to the ray takeoff angle the station statics are obtained through an iterative retrieval of the lower degree spherical harmonics normalized over the data takeoff angle range on station residual spheres and similar event statics are introduced to decouple the effects of near station and near event heterogeneities except the zero degree constant term all higher degree static terms are obtained on a regional rather than an individual station event location the p wave static corrections up to degree and order 2 are derived for 1343 stations using nearly 3 million international seismological center travel time residuals with respect to the iasp91 model and with event relocation and summary ray sorting the resulting station statics account for nearly 27 of the variance of the p wave travel time residuals patterns of the zero degree statics correlate well with surface tectonic features and with previous results at most stations over subduction zones the slowest direction of the secondorder statics or the second degree in azimuth is perpendicular to the trench

an investigation of seismicity clustered near the cordel field west central alberta and its relation to a nearby disposal well historically seismicity documented in the western canada sedimentary basin has been relatively quiescent and earthquakes are usually restricted to the foreland belt of the rocky mountains however exceptional clusters of events which have remained active for decades are recognized in alberta in this study we investigate the seismicity in this region using data obtained from recently established regional arrays emphasizing the relationship between a disposal well in the cordel field and a nearby cluster of previously reported earthquakes we explore temporal correlations of wastewater pumping rates and local seismic activity dating back to 1960 we find that the first statistically significant increase in seismicity lags the onset of wastewater injection october 1991 by 3 33 years in particular the waveform similarity of 32 events are analyzed from continuous data recorded at nor a nearby 30 km station operated by the university of alberta starting in september of 2006 results from this analysis suggest that many events are well correlated in the characteristics of the waveforms and thus are likely to share a similar origin and source mechanism the most prolific of these multiplets repeats more than 10 times sporadically throughout the entire duration of recorded data from october 2006 to march 2012 despite the limited availability of nearby stations which adversely affects the resolution of our analysis hypocenter depths could be relatively accurately determined from waveform synthesis and double difference methods the results of our analysis provide first order evidence that the seismicity is consistent with fluid injection induced events

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

nuclear explosion locations at the balapan kazakhstan nuclear test site the effects of high precision arrival times and three dimensional structure abstract we have investigated the potential contributions of improved arrival times using waveform cross correlation and the use of three dimensional 3 d velocity models for seismic event location capability our analyses are applied to a dataset of nuclear explosions at balapan kazakhstan for which ground truth locations and some absolute origin times are available this ground truth information allows us to determine excellent origin time estimates for the remaining explosions the combination of excellent ground truth location information and high quality origin time estimates permits us to 1 carry out a detailed examination of the quality of isc picks 2 identify probable timing errors in the digital data 3 evaluate relative and absolute location capability using data from a sparse network 4 assess the influence of event signal to noise ratio snr on relative location accuracy 5 utilize the balapan events as a source array for 3 d tomography beneath the test site and 6 test the influence of 3 d structure local and global on relative location accuracy and precision in a controlled situation our principal finding is that improved arrival times are the primary contributor to improved locations joint and individual relocations of balapan events using the full digital dataset result in average mislocations of less than 1 km and 95 confidence regions of a compatible size to mimic a ctbt scenario more realistically we also carry out relocations using very few stations 4 10 observations location accuracy degrades somewhat but the high quality picks generally result in mislocations less than 10 km even for events with very large azimuthal gaps uncertainty is generally underestimated in these cases tests with artificially degraded snr show that mislocation increases slowly as snr decreases 3 d velocity structure makes a smaller contribution to relative location accuracy than accurate time picks travel time variations due to global 3 d structure vary little across the source region so that location scatter is not reduced when travel time corrections for global 3 d structure are applied travel time variations due to the local 3 d structure estimated using source region tomography are also modest applying travel time corrections that account for the local structure does yield slight location improvement

crustal structure of the flemish cap continental margin eastern canada an analysis of a seismic refraction profile summary the crustal structure of the ne flemish cap margin was determined along a 460 km long refraction wide angle reflection seismic transect flame line to define the thickness structure and composition of the crust and uppermost mantle along the line a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers a coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to moho the model displays a sediment cover of up to 3 6 km thick subdivided into three layers with velocities of 1 8 1 9 km s 1 2 8 3 1 km s 1 and 4 7 4 8 km s 1 for the western part of the flame line over flemish cap the p wave velocity model displays an up to 32 km thick three layer continental crust the continental crust has velocities of 5 8 6 1 km s 1 6 3 6 45 km s 1 and 6 65 6 85 km s 1 and thicknesses of about 5 km 7 km and 20 km in the upper middle and lower layers respectively the thick continental crust thins to a two layer 6 km thick crust upper layer is 5 55 6 0 km s 1 and the layer below is 6 65 6 8 km s 1 over a distance of 45 km s wave velocities are determined in the upper layer of the thick continental crust over flemish cap and the transition zone by assigning poisson s ratios in the p wave velocity model comparison of calculated to observed arrival times gives a poisson s ratio of 0 27 in the upper layer and 0 28 in the layer below which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone the thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7 5 7 9 km s 1 we interpret this layer as partially serpentinized mantle which is consistent with observationsfromthenewfoundlandmargintothesouth theserpentinizedmantleterminates 30 km seaward of the thick continental crust at the seaward most end of the thin continental crust a prominent ridge feature is observed the seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust the reflection seismic data show a high relief basement from the ridge feature and seaward the flame line crosses magnetic anomaly 34 and extends another 50 km seaward well into oceanic crust the ridge is flanked seaward by a two layer oceanic crust the upper layer layer 2 has velocities of 4 8 5 0 km s 1 for the landward most 35 km of oceanic crust and 4 8 6 2 km s 1 for the seaward 60 km the average thickness of layer 2 is 2 km the lower layer layer 3 has velocities of 6 7 7 2 km s 1 and a thickness of 3 5 km the velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature

thinned and subsided continental crust on the rifted margin of eastern canada crustal structure thermal evolution and subsidence history summary seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in orphan basin and flemish pass on the continental margin north east of newfoundland tau p travel time inversion synthetic seismogram analysis and conventional layered model calculations were applied to the data the results show that these outer regions of the margin are underlain by thinned continental crust with a total depth to the m discontinuity of about 22 km there are two main crustal layers with p velocities of about 6 1 and 7 0 km s 1 which appear to be homogenous and separated by sharp interfaces these are overlain by a layer in which p velocities are about 5 5 km s 1 this is interpreted to be precambrian or palaeozoic basement mesozoic and cenozoic sediments cover the basement rocks and are over 4 km thick the results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred the gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km from the ocean continent boundary near orphan knoll landward to the outer continental shelf the subsidence history and thermal evolution of the region was computed assuming that the observed thinning is produced by horizontal extension of the lithosphere it is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere the computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar however the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence assuming cooling began when final continental breakup occurred in the late cretaceous the temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin therefore it is suggested that the response of the lithosphere to sediment loading and the large amplitude of the gravity shelf edge anomalies are directly related to the thermal history of the region

p wave travel time and amplitude in western north america a study of teleseismic short period 1 s p wave travel time and amplitude variations in western north america reported here reveals a high level of correlation early arrivals relate to low amplitudes and late arrivals to higher amplitudes this trend is opposite to observations noted for the usa and southern canada as a whole1 with some exceptions seismic stations east of the rocky mountain front generally record faster travel times2 8 and higher amplitudes9 15 than those to the west these differences have been attributed to higher attenuation and lower velocities in the upper mantle beneath the tectonically active western region11 16 the correlation in the west between p wave travel time residuals and relative amplitudes is consistent with elastic focusing and defocusing effects and is found for both world wide standardized seismograph network wwssn and long range seismic measurement lrsm stations these results are relevant to the estimation of yields of nuclear explosions because by reciprocity similar elastic focusing and defocusing effects can alter the amplitudes and travel times of p waves from explosions

traveltime tomography of a dense wide angle profile across orphan basin abstractwe used first arrivals and moho reflections from the 500 km long orphan basin wide angle velocity experiment obwave profile with 3 5 km instrument spacing to construct a traveltime tomography section and to delineate the moho discontinuity across the orphan basin the orphan basin is a failed rift located offshore newfoundland canada showing thinned continental crust over an unusually wide region we observed 1 a zone of extreme crustal thinning 7 km thick crust with no evidence for mantle serpentinization 2 basement morphology exhibiting tilted blocks linked to the crustal thinning and 3 a thicker central crustal segment that is probably related to prerift structural inheritance comparison with the adjacent jeanne d arc basin to the southeast suggested the presence of a decoupling zone between the two basins accommodating the difference in extension rates there was a good correlation between the tomographic velocities and the reflection structure derived from a coincident seismic

improving sparse network seismic location with bayesian kriging and teleseismically constrained calibration events monitoring the comprehensive nuclear test ban treaty will require improved seismic location capability for small magnitude events the international monitoring system ims is well suited to locate events that are large enough to be recorded at teleseismic distances however small events are likely to be recorded on a sparse subset of ims stations at regional to upper mantle distances less than 30 and sparse network locations can be strongly effected by travel time errors that result from path specific velocity model inaccuracies in an effort to improve sparse network location capability we outline a procedure that applies empirical corrections to travel times determined with an appropriate velocity model more specifically bayesian kriging and calibration events constrained with a global network are used to estimate epicenter specific travel time corrections for a test sparse network of stations we calculate travel time residuals for the calibration events relative to the ak135 velocity model travel time residuals are assigned to the respective calibration epicenter forming a set of spatially varying travel time correction points the spatial set of correction points is declustered to reduce the dimension of the observations with minimal reduction in accuracy of the travel time corrections we then use the declustered set of calibration points and bayesian kriging to form continuous travel time correction surfaces for each station of the test network the effectiveness of travel time correction surfaces is evaluated by locating with and without corrections a subset of the 1991 racha earthquake sequence caucasus mountains for which we have accurate locations that were independently determined with a dense local network when no travel time correction is applied the mean horizontal distance between the local and test network locations is 42 km and there is a distinct bias in sparse network locations toward the north northwest the mean difference between local and sparse network locations is cut to 13 km when corrections are applied and the bias in location is significantly reduced when calibration events in the racha vicinity are not used to make the correction surfaces there is still a significant improvement in location with mean mislocations of 15 km when corrections are not applied only one of the locally determined locations lies within the associated 90 coverage ellipse determined with the test sparse network however by using travel time corrections and estimates of model uncertainty determined using kriging representative error ellipses are obtained this study demonstrates that kriging correction surfaces based on global network constrained calibration events can improve the ability to accurately locate lower magnitude events while providing representative coverage ellipses

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

repeating events as estimator of location precision the china national seismograph network repeating earthquakes identified by waveform cross correlation with inter event separation of no more than 1 km can be used for assessment of location precision assuming that the network measured apparent inter epicenter distance x of the repeating doublets indicates the location precision we estimated the regionalized location quality of the china national seismograph network by comparing the repeating events in and around china by schaff and richards science 303 1176 1178 2004 j geophys res 116 b03309 2011 and the monthly catalogue of the china earthquake networks center the comparison shows that the average x value of the china national seismograph network is approximately 10 km the mis location is larger for the tibetan plateau west and north of xinjiang and east of inner mongolia as indicated by larger x values mis location is correlated with the completeness magnitude of the earthquake catalogue using the data from the beijing capital circle region the dependence of the mis location on the distribution of seismic stations can be further confirmed