the crustal structure of southern baffin bay implications from a seismic refraction experiment baffin bay represents the northern extension of the extinct rift system in the labrador sea while the extent of oceanic crust and magnetic spreading anomalies are well constrained in the labrador sea no magnetic spreading anomalies have yet been identified in baffin bay thus the nature and evolution of the baffin bay crust remain uncertain to clearly characterize the crust in southern baffin bay 42 ocean bottom seismographs were deployed along a 710 km long seismic refraction line from baffin island to greenland multichannel seismic reflection gravity and magnetic anomaly data were recorded along the same transect using forward modelling and inversion of observed traveltimes from dense airgun shots a p wave velocity model was obtained the detailed morphology of the basement was constrained using the seismic reflection data a 2 d density model supports and complements the p wave modelling sediments of up to 6 km in thickness with p wave velocities of 1 8 4 0 km s 1 are imaged in the centre of baffin bay oceanic crust underlies at least 305 km of the profile the oceanic crust is 7 5 km thick on average and is modelled as three layers oceanic layer 2 ranges in p wave velocity from 4 8 6 4 km s 1 and is divided into basalts and dykes oceanic layer 3 displays p wave velocities of 6 4 7 2 km s 1 the greenland continental crust is up to 25 km thick along the line and divided into an upper middle and lower crust with p wave velocities from 5 3 7 0 km s 1 the upper and middle continental crust thin over a 120 km wide continent ocean transi tion zone we classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data the profile did not reach continental crust on the baffin island margin which implies a transition zone of 150 km length at most the new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern baffin bay

the seismic cycle in the area of the 2011 mw9 0 tohoku oki earthquake we model seismic and aseismic slip on the japan megathrust in the area of the m w9 0 2011 tohoku oki earthquake based on daily time series from 400 gps stations of the geonet network and campaign measurements of six sea floor displacements the coseismic and postseismic slip distributions are inverted simultaneously using principal component analysis based inversion method pcaim exploring a wide range of boundary conditions and regularization constraints we found the coseismic slip distribution to be quite compact with a peak slip between 30 and 50 m near the trench our model shows deep afterslip fringing the downdip edge of the coseismic rupture but also a dominant zone of shallow afterslip afterslip over the first 279 days following the main shock represents about 40 of the coseismic moment we compare the coseismic and postseismic models with an interseismic coupling model derived from inland and sea bottom measurements determined in a self consistent manner assuming that seismic and aseismic slip had to match the long term slip rate along the megathrust the recurrence time of m w9 0 earthquakes is estimated to 100 300 years while historical and paleotsunami records suggest a return period more of the order of 1000 years the discrepancy is smaller if the shallower portion of the megathrust is assumed to produce both aseismic slip as the afterslip model suggests and seismic slip during occasional large tsunamigenic earthquakes

amplification of moment and strain energy release due to interaction between different size fault slip zones previous work of rudnicki and kanamori which used collinear crack solutions to examine quantitatively the effects of fault slip zone interaction on moment stress drop and strain energy release is extended by considering the effects of interaction between different size slip zones the calculations demonstrate that the presence of a large preexisting slip zone can substantially amplify the low frequency seismic signal due to slip on a nearby smaller slip zone for example if the length of the larger slip zone is l that of the smaller is l 10 and the distance between the nearest ends of the zones is l 100 the seismic moment due to slip on the larger zone induced by slip on the smaller zone is about 5 times that due to slip on an isolated zone of length l 10 furthermore because of the interaction between slip zones the moment due to slip on the smaller zone is about 2 5 times the value for an isolated zone of the same size and subjected to the same effective stress conversely estimates of stress drop based on the measured value of the moment and the total length on which slip occurs substantially underestimate the actual stress drop although the stress drop on the larger slip zone is zero the induced slip there does contribute to wo the difference between the strain energy change and the frictional work in fact it is demonstrated that wo em 2 where e is the effective stress m is the actual value of the seismic moment and is the shear modulus

extent of oceanic crust in the labrador sea1 this paper examines the mode of formation of the northern margins of the labrador sea observations of faulted and rotated crustal blocks across the northwest margin of greenland had led to the suggestion that this region is underlain by extended continental crust a detailed analysis of all geophysical data from this region presented here suggests on the contrary the possibility that it could have been formed by an excessively slow seafloor spreading process and ridge axis propagation this gave rise to rotated fault blocks at a time when spreading was mainly confined to the greenland side prior to its shift to the west supporting evidence for such a reconfiguration of the spreading axis comes from gravity and magnetic observations of this and neighbouring regions modelling of magnetic anomalies in the area based on the average rate of spreading for the entire labrador sea shows that a gradual shift of the ridge axis to the west started after the formation of anomaly 32 because the earliest seafloor spreading anomalies are highly attenuated in amplitudes other models like extended and intruded continental crust are possible if one only considers the magnetic anomalies however an extended continental crustal model poses problems in accounting for other geophysical observations based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope with a narrow belt of extended continental crust along the west greenland margin while a much wider zone of perhaps attenuated continental crust extends off labrador breakup of the continental landmass occurred much closer to the west greenland coast than to labrador the applicability of a slow spreading model to other passive margins where similar geophysical observations have been reported is discussed

an application of synthetic seismicity in earthquake statistics the middle america trench this paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard compared to forecasts constructed from a handful of earthquake recurrence intervals forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude they can extrapolate seismicity to higher magnitudes than have actually been observed and they are formulated from a catalog which can be extended as long as needed to be statistically significant synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle i find that estimates of fault slip rate are unbiased regardless of sampling duration while estimates of earthquake recurrence time are strongly biased recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short for the middle america trench mat it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to 10 events m 6 have as much as a 60 probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks this probability drops to less than 15 for m 7 events increasing gap time generally increases conditional probability of earthquake occurrence but the effect is weak for the mat the spread parameters of the best fitting lognormal or weibull distributions 0 75 are much larger than the 0 21 intrinsic spread proposed in the nishenko buland hypothesis stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been suggested

lithospheric structure of the labrador sea from constrained 3 d gravity inversion regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the labrador sea and its margins to generate 3 d density anomaly models to investigate broad scale crustal structural variations across the extinct spreading centre benchmarked against independent seismic moho depth constraints a density anomaly isosurface within the inverted volumes was selected as a moho proxy and regional maps of moho structure were developed inversions using two different sources for depth to basement constraints revealed similar moho structures with a depth to moho of 12 km beneath the labrador sea which deepens to 20 km and greater towards davis strait and beneath the offshore extension of the grenville province density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted moho proxy and the seismic moho with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide angle reflection refraction profiling studies the inverted depth to moho estimates were combined with depth to basement constraints to investigate crustal thickness both for the full crust and for individual crustal layers revealing that the crust of the labrador sea is generally 5 10 km thick but thickens to 20 25 km towards davis strait and beneath the offshore extension of the grenville province not taking into account high density underplates or anomalously high density lower crust sediment and crustal thickness variations were investigated to compute stretching factors across the labrador sea and to identify zones which deviate from local isostatic compensation assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model much of the labrador sea has experienced 70 90 per cent thinning the derived values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern labrador sea inboard of known oceanic crust isostatic considerations reveal sediment excess and deficiency on the labrador and greenland margins respectively possibly reflecting fundamental rheological asymmetry with the greenland margin appearing weaker than the labrador margin a strong gradient from sediment deficiency to excess exclusive to the labrador margin may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments

upgrading a synthetic seismicity model for more realistic fault ruptures a quasi static synthetic seismicity model for multiple interacting faults of any orientation is upgraded to include a more realistic rupture model in the original model entire ruptures covering many fault patches occur instantaneously and rupture propagation is ignored now time is introduced into the rupture process via a patch interaction delay in proportion to distance shear wave velocity also to try to mimic observed rise times for real earthquakes a simple time dependent friction law is used comparison of results for the old and new models for a simple case of two strike slip faults show that clustering of large events is somewhat reduced in the new model it remains to be seen if this conclusion will be verified in more complex fault networks the new model takes about twice as much computation time but the results may now be used to calculate synthetic seismograms

seismic rupture and arc segmentation within the shumagin islands seismic gap alaska the locations of teleseismically well recorded earthquakes suggest that up to 80 of the shumagin gap has ruptured seismically during the last 90 years an earthquake in 1917 ms 7 9 nucleated near the gap s eastern edge and appears to have ruptured as much as a 200 km long section of the gap and an event in 1948 ms 7 5 appears to have ruptured the deepest portion of the main thrust zone 30 km in the central section of the gap the rupture extents of these earthquakes suggest that the gap can be divided into two or three smaller arc segments these observations do not preclude the possibility of failure of the entire region in a single great earthquake mw 8 5 in the near future the two historically documented recurrence intervals for the eastern portion of the gap are 59 and 70 years if the average recurrence interval for large earthquakes near the shumagin islands is 65 years the conditional probability of a large earthquake rupturing this portion of the arc in the next 20 years is between 62 and 96

a crustal model for northern melville bay baffin bay the interpretation of seismic refraction and gravity data acquired in 2010 gives new insights into the crustal structure of the west greenland coast and the adjacent deep central baffin bay basin underneath melville bay the depth of the moho varies between 26 and 17 km stretched continental crust with a thickness of 25 to 14 km and deep sedimentary basins are present in this area the deep melville bay graben contains an up to 11 km thick infill of consolidated and unconsolidated sediments with velocities of 1 6 to 4 9 km s seaward at the 60 km wide transition between oceanic and stretched continental crust a mount shaped magmatic structure is observed which most likely formed prior to the initial formation of oceanic crust the up to 4 km high magmatic structure is underlain by a 2 km thick and 50 km wide high velocity lower crust more to the west in the oceanic part of the baffin bay basin we identify a two layered 3 5 to 6 km thin igneous oceanic crust with increasing thickness toward the shelf beneath the oceanic crust the depth of the moho ranges between 11 5 and 13 5 km in the western part of the profile oceanic layer 3 is unusually thin 1 5 km a possible explanation for the thin crust is accretion due to slow spreading although the basement is notably smooth compared to the basement of other regions formed by ultra slow spreading the oceanic crust is underlain by partly serpentinized upper mantle with velocities of 7 6 to 7 8 km s

misalignment of the orientation of fractures and the principal axes for p and s waves in rocks containing multiple non orthogonal fracture sets summary azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with diverent polarizations if the source polarization is not aligned with the principal axes of the medium in the presence of two or more non orthogonal sets of vertical fractures the symmetry of the medium may be approximated as monoclinic with a horizontal plane of mirror symmetry if in the absence of fractures the rock is transversely isotropic ti with the symmetry axis perpendicular to the bedding plane for such a medium the fast and slow polarization directions for vertically propagating shear waves are not parallel or perpendicular to any of the fracture planes but lie in directions given by the principal axes of a second rank fracture compliance tensor this tensor is independent of the normal compliance of the fractures it follows that for vertical propagation of shear waves in a vertically fractured ti medium any number of arbitrarily oriented vertical fracture sets is equivalent to two mutually perpendicular fracture sets provided that the seismic wavelength is large compared to the size of the fractures for ovsets typical of surface seismic acquisition the p wave velocity at fixed ovset varies with azimuth as cos 2 w w 0 where w is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave w 0 depends on both the normal and the shear compliance of the fractures and may diver from zero if the ratio of the normal to shear compliance of the fractures varies significantly between fracture sets if this ratio is similar for all fractures w 0 0 and the principal axes of the variation in p wave velocity with azimuth for fixed ovset are determined by the principal axes of the second rank fracture compliance tensor

crust and uppermost mantle structure of greenland and the northwest atlantic from rayleigh wave group velocity tomography the greenland landmass preserves 4 billion years of tectonic history but much of the continent is inaccessible to geological study due to the extensive inland ice cap we map out for the first time the 3 d crustal structure of greenland and the nw atlantic ocean using rayleigh wave anisotropic group velocity tomography in the period range 10 80 s from regional earthquakes and the ongoing glatis glisn seismograph networks 1 d inversion gives a pseudo 3 d model of shear wave velocity structure to depths of 100 km with a horizontal resolution of 200 km crustal thickness across mainland greenland ranges from 25 km to over 50 km and the velocity structure shows considerable heterogeneity the large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper 5 15 km within the upper continental basement velocities are systematically lower in northern greenland than in the south and exhibit a broadly nw se trend the thinning of the crust at the continental margins is also clearly imaged upper mantle velocities show a clear distinction between typical fast cratonic lithosphere vs 4 6 km s 1 beneath greenland and its ne margin and anomalously slow oceanic mantle vs 4 3 4 4 km s 1 beneath the nw atlantic we do not observe any sign of pervasive lithospheric modification across greenland in the regions associated with the presumed iceland hotspot track though the average crustal velocity in this region is higher than that of areas to the north and south crustal anisotropy beneath greenland is strong and complex likely reflecting numerous episodes of tectonic deformation beneath the north atlantic and baffin bay the dominant anisotropy directions are perpendicular to the active and extinct spreading centres anisotropy in the subcontinental lithosphere is weaker than that of the crust but still significant consistent with cratonic lithosphere worldwide

the continental margin off southern greenland along strike transition from an amagmatic to a volcanic margin the southern part of the continental margin off southern west greenland is an amagmatic margin that may have taken at least 30 and possibly more than 60 million years to form during the cretaceous at an average extension rate of between 8 7 and 4 4 mm a 1 to its northwest and east are volcanic continental margins formed in the early tertiary when sea floor spreading started above the hot north atlantic plume head the survival of the amagmatic margin means that plume head material could never have been present under it and therefore the plume head could not have had the circularly symmetric shape commonly depicted in the literature

earthquake precursors activation or quiescence summary we discuss the long standing question of whether the probability for large earthquake occurrence magnitudes m 6 0 is highest during time periods of smaller event activation or highest during time periods of smaller event quiescence the physics of the activation model are based on an idea from the theory of nucleation that a small magnitude earthquake has a finite probability of growing into a large earthquake the physics of the quiescence model is based on the idea that the occurrence of smaller earthquakes here considered as magnitudes m 3 5 may be due to a mechanism such as critical slowing down in which fluctuations in systems with long range interactions tend to be suppressed prior to large nucleation events to illuminate this question we construct two end member forecast models illustrating respectively activation and quiescence the activation model assumes only that activation can occur either via aftershock nucleation or triggering but expresses no choice as to which mechanism is preferred both of these models are in fact a means of filtering the seismicity time series to compute probabilities using 25 yr of data from the california nevada catalogue of earthquakes we show that of the two models activation and quiescence the latter appears to be the better model as judged by backtesting by a slight but not significant margin we then examine simulation data from a topologically realistic earthquake model for california seismicity virtual california this model includes not only earthquakes produced from increases in stress on the fault system but also background and off fault seismicity produced by a bass etas driving mechanism applying the activation and quiescence forecast models to the simulated data we come to the opposite conclusion here the activation forecast model is preferred to the quiescence model presumably due to the fact that the bass component of the model is essentially a model for activated seismicity these results lead to the weak conclusion that california seismicity may be characterized more by quiescence than by activation and that bass etas models may not be robustly applicable to the real data

estimating physical parameters of cracked porous oil reservoirs by inverting shear wave splitting summary we present the problem of determining crack density porosity and permeability of cracked porous carbonate reservoirs by examining the origin of anisotropy considered from a physical model of porous reservoir rock with a system of vertical fluid filled cracks this stabilizes the non linear optimization inversion procedure and determines a set of significant reservoir parameters on a basis of qp qs1 and qs2 velocities measured in a vsp experiment

gravity changes from a stress evolution earthquake simulation of california 1 the gravity signal contains information regarding changes in density at all depths and can be used as a proxy for the strain accumulation in fault networks a stress evolution time dependent model was used to create simulated slip histories over the san andreas fault network in california using a linear sum of the gravity signals from each fault segment in the model via coseismic gravity green s functions a time dependent gravity model was created the steady state gravity from the long term plate motion generates a signal over 5 years with magnitudes of 2 gal the current limit of portable instrument observations moderate to large events generate signal magnitudes in the range of 10 to 80 gal well within the range of ground based observations the complex fault network geometry of california significantly affects the spatial extent of the gravity signal from the three events studied

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

instability model for recurring large and great earthquakes in southern california the locked section of the san andreas fault in southern california has experienced a number of large and great earthquakes in the past and thus is expected to have more in the future to estimate the location time and slip of the next few earthquakes an earthquake instability model is formulated the model is similar to one recently developed for moderate earthquakes on the san andreas fault near parkfield california in both models unstable faulting the earthquake analog is caused by failure of all or part of a patch of brittle strain softening fault zone in the present model the patch extends downward from the ground surface to about 12 km depth and extends 500 km along strike from parkfield to the salton sea the variation of patch strength along strike is adjusted by trial until the computed sequence of instabilities matches the sequence of large and great earthquakes sincea d 1080 reported by sieh and others the last earthquake was them 8 3 ft tejon event in 1857 the resulting strength variation has five contiguous sections of alternately low and high strength from north to south the approximate locations of the sections are 1 parkfield to bitterwater valley 2 bitterwater valley to lake hughes 3 lake hughes to san bernardino 4 san bernardino to palm springs and 5 palm springs to the salton sea sections 1 3 and 5 have strengths between 53 and 88 bars sections 2 and 4 have strengths between 164 and 193 bars patch section ends and unstable rupture ends usually coincide although one or more adjacent patch sections may fail unstably at once the model predicts that the next sections of the fault to slip unstably will be 1 3 and 5 the order and dates depend on the assumed length of an earthquake rupture in about 1700

seismic cycles along the aleutian arc analysis of seismicity from 1957 through 1991 we catalog and relocate aleutian arc seismicity between 1957 and 1991 two great earthquakes ruptured the same 250 km long portion of the central aleutian arc the 1957 aleutian islands earthquake and the 1986 andreanof islands earthquake because accurate estimates of the moment distribution of the 1957 earthquake are not available the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes earthquake relocations are based on p wave arrival times published in the international seismological summary the bureau central international seismologique and the international seismological centre bulletins and include corrections for the near source velocity structure associated with the down going slab magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records our catalog is complete above magnitude 5 5 aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas east of the main shock epicenters aftershock locations are anticorrelated west of the main shock epicenter aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake if we assume aftershocks rim the distribution of seismic moment release associated with each event these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake this suggests that we should use caution in identifying mechanically strong portions of a fault asperities by simply mapping the moment distribution of a single great earthquake a fundamental tenet of the asperity model that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch may in the case of the central aleutian arc not be correct thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake

basement structure and sediment distribution in northwest atlantic ocean we have interpreted all available single channel and multichannel seismic reflection profiles to produce maps of structure contours on basement and isopachs of sediment thickness in the northwestern atlantic ocean including the labrador and irminger basins where appropriate we have modified and incorporated existing charts into the compilation contours are in meters derived by applying velocity regression equations based on sonobuoy results this gives a more accurate representation of geologic relations than is possible in reflection time mapping major structural relations and sediment distribution for the mid atlantic ridge mid labrador sea ridge davis strait and adjacent greenland and eastern canadian continental margins are clearly defined in the maps observ d basement structure is in good agreement with that expected from current plate kinematic models of the region the sediment thickness patterns are controlled by interaction of such factors as age of underlying oceanic crust crustal tectonic history structural trends in basement locations of sediment sources and nature of the sedimentary processes delivering sediment to the basins

tectonic evolution of southern baffin bay and davis strait results from a seismic refraction transect between canada and greenland 1 wide angle reflection refraction seismic data were acquired on a 450 km long transect in southern baffin bay extending from baffin island to greenland dense air gun shots were recorded on 22 ocean bottom seismometers apwave velocity model was developed from forward and inverse modeling of the observed travel times beneath the baffin island shelf a three layered continental crust is observed with velocities of 5 5 to 6 9 km s typical for transform margins there is a sharp transition between continental and oceanic crust off baffin island 7 km thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map beneath the deep baffin bay basin 9 km thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone the thicker than normal oceanic crust indicates an ample magma supply possibly related to melt extracted from a mantle plume seaward of the greenland continental crust 20 km thick igneous crust 6 3 to 7 3 km s is encountered in a 25 km wide zone interpreted as a leaky transform fault that can be correlated southward through davis strait the igneous crust is bounded by a 20 km wide basin to the west underlain by 4 km thick crust of unknown affinity this structure is probably associated with transform movements a high velocity lower crustal layer 7 1 km s of 8 km thickness is indicated beneath the greenland crust and can be correlated into the adjacent thick igneous crust both the thick igneous and greenland crust are covered by up to 4 km of paleogene volcanics 5 2 to 5 7 km s

evolution of the west greenland margin offshore thermostratigraphic data and modelling the origin and age of topography along the west greenland margin is a matter of continued debate evidence for tectonically driven neogene uplift has been argued from interpretations of offshore seismic surveys onshore fission track data and inferred episodes of cooling here analysis of seismic reflection profiles and 1d modelling of exploration wells along the greenland margin of davis strait demonstrate that the data are consistent with a model of ancient continental topography affected by late cretaceous early palaeocene rifting followed by thermal subsidence where offshore neogene tectonic uplift is not required this interpretation for the offshore evolution of the west greenland margin has implications for the adjacent onshore evolution and for other continental margins developed throughout the atlantic arctic rift system

anisotropy survey for reservoir definition summary in the context of franco soviet cooperation a seismic survey was carried out on the romashkino reservoir in the tatar republic ussr this survey involved evaluating azimuthal anisotropy parameters from s wave vsps recorded on three wells and from a surface seismic line the latter in order to monitor any lateral variations in the parameters in this paper we present results which indicate in particular that for the two wells in which oil production has been established the anisotropy rate appears to be proportional to production figures this seems to confirm the hypothesis that variations in production are asociated with the presence of subvertical fractures which cause s wave birefringence by processing the reflected converted p s waves of a surface seismic line it is possible to monitor lateral variations in the anisotropy parameters and well ties effectively confirm differences revealed by the s wave vsp data

structural characteristics of the ocean continent transition along the rifted continental margin offshore central labrador abstract studies of the structural deformation and the nature of the ocean continent transition zone at the rifted continental margin offshore central labrador and its conjugate off west greenland have been conducted over several decades and helped to define our understanding of the controlling factors in the development of magma poor margins globally new two dimensional seismic reflection data are now available across the labrador margin which can help to modify and refine this understanding in this paper we describe some of these new seismic data and the corresponding potential field data in relation to earlier studies including an existing deep seismic crustal velocity profile these data cross the three major structural and compositional zones below the sedimentary wedge defined in earlier deep seismic experiments as 1 extended zone of continental crust including the hyper extended region partly underlain by serpentinized mantle 2 exhumed and serpentinized continental mantle zone and 3 zone of oceanic crust we are able to extend these zones laterally over 200 km along the margin the rifted continental crust is hyper extended over tens of kilometers landward of the breakup location where many varieties of normal faults and possible low angle detachment surfaces are present some of which are near or at the base of the crust further seaward mantle was serpentinized and likely exhumed and possesses a seismic character very different from the continental crust basement in this zone may locally form prominent ridges within the oceanic zone we define a further sub division into a proto oceanic domain with thin variable oceanic crust of about 70 65 ma in age magnetic chrons 27 to 31 and a steady state seafloor spreading region further seaward thus seafloor spreading starts at about chron 31 time potential field data and models are broadly consistent with these zones a strongly reflective post rift volcanic event is present across much of the region especially over the exhumed mantle zone and parts of the oceanic crust these results are discussed and compared to earlier studies of the west greenland conjugate margin and to other well studied magma poor margins given that this rift occurred in thick cold cratonic lithosphere the results are remarkably similar to those off iberia and newfoundland suggesting that a long rift history and or mantle metasomatism may have weakened the lithosphere prior to the main rifting event

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

an inhomogeneous fault model for gaps asperities barriers and seismicity migration we develop a model for a fault in which various areas of the fault plane have different stress slip constitutive laws the model is conceptually simple involving nonlinear algebraic equations which can easily be solved by a graphical method of successive iterations application is made to the problem of explaining seismicity patterns associated with great earthquakes the model quantitatively explains phenomena associated with seismic gaps asperities and barriers

fluid dependent shear wave splitting in fractured media azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium for vertical propagation of shear waves in a horizontally layered medium containing vertical fractures the shear wave splitting depends on the shear compliance of the fractures but is independent of their normal compliance if the fractures are not perfectly vertical the shear wave splitting also depends on the normal compliance of the fractures the normal compliance of a fluid filled fracture decreases with increasing fluid bulk modulus for dipping fractures this results in a decrease in shear wave splitting and an increase in shear wave velocity with increasing fluid bulk modulus the sensitivity of the shear wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network the permeability of the background medium and on whether the fracture is fully or partially saturated

variable rupture mode of the subduction zone along the ecuador colombia coast three large earthquakes occurred within the rupture zone of the 1906 colombia ecuador earthquake m w 8 8 in 1942 m s 7 9 1958 ms 7 8 and 1979 m s 7 7 we compared the size and mechanism of these earthquakes by using long period surface waves tsunami data and macroseismic data the 1979 event is a thrust event with a seismic moment of 2 9 10 28 dyne cm and represents subduction of the nazca plate beneath south america the rupture length and direction are 230 km and n40 e respectively examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction the seismic moment estimated from the tsunami data and the size of the rupture zone is 2 10 29 dyne cm the 1942 and 1958 events are much smaller about 1 5 to 1 10 of the 1979 event in the seismic moment than the 1979 event we conclude that the sum of the seismic moments of the 1942 1958 and 1979 events is only formula of that of the 1906 event despite the fact that the sequence of the 1942 1958 and 1979 events ruptured approximately the same segment as the 1906 event this difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between the weak zone normally behaves aseismically but slips abruptly only when it is driven by failure of the asperities a small earthquake represents failure of one asperity and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small a great earthquake represents failure of more than one asperity and consequently involves much larger width and slip

western davis strait a volcanic transform margin with petroliferous features abstract we present a compilation of the western davis strait region offshore southeastern baffin island nunavut canada with new subsurface geological structural details and observations regarding past hydrocarbon occurrences from scientific and commercial exploration this consists of seismic mapping with archival data correlated with a filtered marine bouguer anomaly gravity compilation and magnetic data sets covering northern saglek basin the western part of the lady franklin basin and the ungava fault zone from south of baffin bay a regional seismic horizon for mapping basin architecture comes from the top of the paleogene volcanic syn magmatic zone where pervasive volcanic flows and intrusions are intermingled with the sedimentary section the seismic depth map to the top of the regional volcanic seismic horizon shows pre rift sedimentary basins having maximum depths of approximately 4 5 km flanking the shallower ungava fault zone correlation of bouguer anomaly gravity and magnetic data interpretations with the seismic mapping indicate that over some areas true crystalline basement is deeper than can be determined by reflection seismic as the base of the syn magmatic section is not resolvable for seismic mapping extensive paleogene mafic intrusives and extrusive basalts dominate the architecture of this volcanic rifted margin as seen by the dominant high amplitude magnetic anomalies associated with many deeper structures the over 250 km long davis strait fault is recognised as the key structural element of the ungava fault zone ufz and valley complex the adjoining davis strait high maps on seismic as a continuous structure running the length of davis strait plunging southwards to a bouguer gravity high feature that terminates the western end of the extinct eocene spreading zone south of davis strait this horn shaped structural feature establishes the presence of an accommodation zone with multiple faults and localised thrust faulting as required to fit the complex mechanics for strike slip motion that occurs along the south end of this transform fault system the revised marine bouguer anomaly gravity data set also defines two new near shore grabens east of cumberland sound the southern tariut basin and the northern imaqpik basin each extending over 100 km in length with sediment thicknesses of at least 4 km these pre rift basins most likely originate from the paleozoic based on seafloor dredge and nearby shallow drill cores from previous studies the hydrocarbon charge of a previously unexplained petroliferous shallow marine drill core that is adjacent the eastern edge of the tariut basin is attributed to paleozoic source rocks that have undergone enhanced thermal maturation from sill intrusion associated with rifting the imaqpik basin shows strong evidence of a hydrocarbon system from the proximity of clustered interpreted sea surface oil slicks mapped from satellite radar images and a local zone of anomalously high dissolved methane measured within the water column that originates from the seafloor immediately east of cape dyer at the northern limit of that basin the overlying cape dyer flood basalt field extends from limited onshore exposures into the offshore and maps with magnetic data over an area of approximately 13 000 km2 this extrusive feature and it s implicit underlying intrusive components are likely the unconventional heat source for the anomalous enhanced thermal maturation of paleozoic to paleogene source rocks in these pre rift basins that host this previously unrecognised petroleum system

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

eocene continental breakup in baffin bay abstract we question the timing of continental breakup and early oceanization in baffin bay north east atlantic north of the ungava fault zone the breakup was syn magmatic and led to the development of conjugate volcanic passive margins vpms we investigated the innermost part of the w greenland vpm where a remarkable inner sdr is fully exposed in the svartenhuk area our new radiometric ages and paleomagnetic data from syn tectonic basaltic lavas indicate that continental stretching and thinning spanned the c26r to at least the c24r period giving an eocene lower boundary age for continental breakup in baffin bay these results contradict the proposed flooring of baffin bay by a paleocene oceanic crust older than c24n and also question the accretion of oceanic crust before c22 we confront our results to the dynamics of the northward oceanic rift propagation across the ungava transform fault system and we suggest that plate breakup in baffin bay occurred 8 m y later than in n labrador sea as a result of the thermal and mechanical barrier effect induced by the ungava transform zone

interpreting data matrix asymmetry in near offset shear wave vsp data poor experimental control in shear wave vsps may contribute to unreliable estimates of shear wave splitting and possible misinterpretation of the medium anisotropy to avoid this the acquisition and processing of multicomponent shear wave data needs special care and attention measurement of asymmetry in the recorded data matrix using singular value decomposition svd provides a useful way of examining possible acquisition inaccuracies and may help guide data conditioning and interpretation to ensure more reliable estimates of shear wave polarization azimuth three examples demonstrate how variations in shear wave polarization and acquisition inaccuracies affect the svd results in different ways in the first example analysis of synthetic seismograms with known depth changes in the polarization azimuth show how these may be detected in the second example a known source re orientation and polarity reversal is detected by applying svd to near offset shear wave vsp data recorded in the romashkino fiel

aligned fractures in carbonate rocks laboratory and in situ measurements of seismic anisotropy by vertical seismic profiling and shear wave analysis we show that a packet of carbonate reservoir rocks found at nearly 3000 m depth in the north german basin is seismically anisotropic for vertical paths of wave propagation the estimated velocity difference of the split shear waves is 10 no shear wave birefrigence is observed within the hangingwall which therefore has to be regarded as isotropic or transversely isotropic additional laboratory investigations of the petrography of drilled carbonate samples and of their seismic velocities show that the anisotropy is most probably caused by subvertical fractures with preferred azimuthal orientation the strike direction of the aligned fractures determined by analysis of split shear waves is approximately n55 e this value agrees with recently published directions of maximum horizontal tectonic stress in pre zechstein sediments in the eastern part of the north german basin but it is in contrast to the world stress map

shear wave splitting and fracture alignments at the northwest geysers california shear wave splitting is commonly observed on three component microearthquake seismograms recorded at the northwest geysers geothermal field california the polarization of leading shear waves around each recording station is predominantly n10 40 e suggesting that the shear wave splitting is caused mainly by pervasive north northeast orientated anisotropy time delays between the leading and second shear waves can be as large as 10ms km to 30ms km we propose that the direction and magnitude of this shear wave splitting is due to the presence of pervasive stress aligned secondary fractures related to several northwest trending right lateral faults in the geysers area

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

distribution of attenuation in the kaoiki hawaii source volume estimated by inversion of p wave spectra a new method to simultaneously invert for q structure and source parameters was used on a set of 635 microearthquakes 0 9 m 2 0 in the kaoiki area of southern hawaii approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km the hypocentral depths ranged from o to 14 km with the bulk of the sources in the 7 5 10 5 km range the hypothesis to be tested was that the source volume of the m 6 6 kaoiki main shock of november 16 1983 may be heterogeneous in attenuation distribution we assumed that the observed p wave displacement spectra could be modelled by a source spectrum with an 2 high frequency decay a single layer resonance filler to account for local site resonances and whole path attenuation along the ray path in a next step the attenuation factor q was constrained by tomographically reconstructing the three dimensional q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency the seismic moment m0 and a new q value this process was iterated until the results changed less than 0 1 and were accepted as final the average q was approximately constant and very low 105 q 115 between o and 7 km depth it increased to approximately 160 between 9 and 11 km in the depth ranges were the spatial coverage was good 3 9 km the q values in the nw part of the 1983 aftershock volume were larger by approximately 10 15 compared to those in the se part this 12 contrast correlates with other evidence for heterogeneity the nw region shows high coda q no surface faulting medium seismicity rate and medium precursory quiescence while the se region shows low coda q pervasive faulting high seismicity rate strong precursory quiescence and possibly low b values we conclude that the kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters the new simultaneous inversion technique is well suited to derive detailed q structure and source parameter information from microeaithquake spectra recorded at close epicentral distances

inversion for crustal structure using reflections from the passcal ouachita experiment an interface inversion has been tested and applied to wide angle reflection data from the 1986 passcal ouachita experiment an interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion velocity depth curves derived for shot points 14 19 using the tail sum method are similar to an average one dimensional velocity model from forward modeling using travel time correlations from southern profiles 14 19 the similarity of these velocity depth curves positioned over the southern half of the passcal experiment suggests that crustal structure in this region is approximately laterally homogeneous lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications a two dimensional velocity structure was derived by using the average one dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces the final inversion model is found to be consistent with previous refraction interpretations south of the ouachita orogenic trend and concurrent interpretations of the passcal data set inversion results for the central and southern portion of the passcal profile indicate a depth of 10 12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km a lower crustal layer with an average thickness of 12 km and a moho depth of approximately 29 5 km are also determined interface depths are in agreement with a normal moveout stack of the passcal data set in particular the shallowing of the moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the paleozoic continental margin geophysical studies of the modern atlantic continental passive margin provide the simplest comparison to the crustal structure derived here the lower crustal layer found south of the shallowing of the moho to 30 km beneath the passcal profile is analogous in thickness and position to rift stage lower crust however other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships

geophysical evidence for the extent of crustal types and the type of margin along a profile in the northeastern baffin bay investigating the crust of northern baffin bay provides valuable indications for the still debated evolution of this area the crust of the southern melville bay is examined based on wide angle seismic and gravity data the resulting p wave velocity density and geological models give insights into the crustal structure a stretched and rifted continental crust underneath southern melville bay is up to 30 km thick with crustal velocities ranging between 5 5 and 6 9 km s the deep melville bay graben contains a 9 km thick infill with velocities of 4 to 5 2 km s in its lowermost part west of the melville bay ridge a 80 km wide and partly only 5 km thick continent ocean transition cot is present west of the cot up to 5 km thick sedimentary layers cover a 4 3 to 7 km thick two layered oceanic crust the upper oceanic layer 2 has velocities of 5 2 to 6 0 km s the oceanic layer 3 has been modeled with rather low velocities of 6 3 to 6 9 km s low velocities of 7 8 km s characterize the probably serpentinized upper mantle underneath the thin crust the serpentinized upper mantle and low thickness of the oceanic crust are another indication for slow or ultraslow spreading during the formation of the oceanic part of the baffin bay by comparing our results on the crustal structure with other wide angle seismic profiles recently published differences in the geometry and structure of the crust and the overlying sedimentary cover are revealed moreover the type of margin and the extent of crustal types in the melville bay area are discussed

seismic anisotropy in granite at the underground research laboratory manitoba the shear wave experiment at atomic energy of canada limited s underground research laboratory was probably the first controlled source shear wave survey in a mine environment taking place in conjunction with the excavation of the mine by test tunnel at 420 m depth the shear wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear wave survey to date inversion of the shear wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023 degree and dip 35 degree the in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering however there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive dilatancy anisotropy and that of the gneissic layering eight raypaths with shear waves propagating wholly or almost wholly through granodiorite rather than granite do not show the expected shear wave splitting and indicate a lower in situ anisotropy which maymore be due to the finer grain size and or the absence of gneissic layering within the granodiorite these results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass this has potential applications in mining and waste monitoring less

seismic study of the transform rifted margin in davis strait between baffin island canada and greenland what happens when a plume meets a transform 1 the davis strait transform margin was studied using a 630 km long wide angle reflection refraction seismic transect extending from se baffin island to greenland dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line a p wave velocity model was developed from forward and inverse modeling of the wide angle data and incorporation of coincident deep multichannel reflection seismic data off baffin island in the saglek basin 7 to 11 km thick two layered continental crust 5 8 6 6 km s is observed off greenland continental crust is divided into three layers 5 4 6 8 km s with a maximum thickness of 20 km farther offshore greenland the crust thins to 7 12 km and the lower crust disappears between the continental blocks a 140 km wide zone with oceanic crust layer 2 is 5 4 6 2 km s and layer 3 is 6 7 7 0 km s is located the western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors the eastern part is associated with the ungava fault zone ufz the major transform fault in davis strait the ufz thus acted as leaky transform fault during phases of transtension southward flow of material from the iceland plume created a 4 to 8 km thick underplated layer 7 4 km s beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust plume related paleogene volcanism is indicated by an up to 4 km thick layer 4 3 5 8 km s with basalts and interbedded sediments that can be traced from se baffin island 400 km toward the east

a case example of near surface correction for multicomponent vsps a deterministic near surface correction procedure is developed for multicomponent vsp data shot using directional sources and recorded using three component receivers the method is capable of removing unwanted effects of acquisition such as unequal source strengths or misorientations but may also remove near surface multiples and anisotropy this is of considerable benefit for obtaining accurate and consistent estimates of subsurface anisotropy from different source combinations application of the technique is illustrated using a dataset from the romashkino reservoir in russia where three or four different directional sources are used at the same source locations the technique corrects for the large discrepancies which exist between the estimates obtained using different source combinations application of the technique to three wells in the survey region reveals a nearly isotropic subsurface except for a few isolated zones of moderate to high 2 to 8 anisotropy which lie close to the expected depth for the reservoir although there is no significant correlation with the production figures for each well the qs1 polarization azimuth within the reservoir does vary at each well location suggesting that this may be a more sensitive indicator of reservoir drocess

new evidence on the structure of the labrador sea greenland continental margin a 275 km section of a regional seismic reflection profile from the greenland side of the labrador sea has been reprocessed and interpreted three zones of different structural style have been identified the innermost of which is block faulted magnetic modelling based on the seismic line shows that sea floor spreading anomalies are confined to the outermost zone and the oldest identified anomaly is 27n all attempts to model the area landward of anomaly 27n as a series of remanent magnetizations of alternating polarity failed however a model in which the innermost zone is interpreted as extended and block faulted continental crust and the intermediate zone as continental crust intruded and overlain by reversely magnetized igneous material fits the magnetic and seismic data it is concluded that at least in the northern labrador sea sea floor spreading started in palaeocene times chron 27 and large areas formerly thought to be underlain by oceanic crust should now be considered to be continental one plate kinematic consequence may be a reduced need to postulate large strike slip movements along nares strait

partial rupture of a locked patch of the sumatra megathrust during the 2007 earthquake sequence the great sumatra andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones 1 2 3 4 in march 2005 the sunda megathrust ruptured again producing an event 5 of moment magnitude mw 8 6 south of the 2004 rupture area which was the site of a similar event in 1861 ref 6 concern was then focused on the mentawai area where large earthquakes had occurred in 1797 mw 8 8 and 1833 mw 9 0 6 7 two earthquakes one of mw 8 4 and twelve hours later one of mw 7 9 indeed occurred there on 12 september 2007 here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period this indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture this variability probably arises from the influence of non permanent barriers zones with locally lower pre stress due to the past earthquakes the stress state of the portion of the sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007 the moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833 the potential for a large megathrust event in the mentawai area thus remains large

seismic evidence for plume derived volcanism during formation of the continental margin in southern davis strait and northern labrador sea the crustal structure in the southern davis strait and the adjacent ocean continent transition zone in ne labrador sea was determined along a 185 km long refraction wide angle reflection seismic transect to study the impact of the iceland mantle plume to this region a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by ocean bottom seismographs a coincident industry multichannel reflection seismic profile was used to guide the modelling as reflectivity could be identified down to moho the model displays a marked lateral change of velocity structure the sedimentary cover velocities 1 8 3 9 km s 1 is up to 4 km thick in the north and thins to 1 km in the south the segment of the line within southern davis strait is interpreted to be of continental character with a two layered 13 km thick crust with p wave velocities of 5 6 5 8 and 6 4 6 7 km s 1 in the upper and lower crust respectively the crust is underlain by a 2 to 4 km thick high velocity layer 7 5 km s 1 this layer we interpret as underplated material related to the iceland plume the southern segment of the line in labrador sea displays a 2 km thick layer with a velocity of 4 5 km s 1 this layer can be correlated to a well about 100 km to the west of the line where palaeocene basalts and interbedded sediments were drilled underneath is a 12 km thick crust with a 2 km thick upper layer 5 8 6 6 km s 1 and a 10 km thick lower layer 6 8 7 2 km s 1 this crust is interpreted to be of oceanic character s wave modelling yields a poisson s ratio of 0 28 for the lower crust compatible with a gabbroic composition the igneous crust is 5 km thicker than normal oceanic crust we suggest that the increased magma production was created by buoyancy driving flow

application of an inhomogeneous stress patch model to complex subduction zone earthquakes a discrete interaction matrix approach in recent years it has been recognized that the level of shear and normal stress along a fault can vary thus the stress is spatially and temporally inhomogeneous moreover it has also been suspected that faults might interact in some way with the result that a variety of earthquake magnitudes might be produced along a given length of fault at varying times in order to explore these ideas we have developed a quantitative formalism which we call the interaction matrix method to express the influence of one fault upon another this matrix is calculated by use of the energy change for a system of interacting cracks or faults and therefore gives energy consistent results specifically the interaction matrix relates the area averaged stress on the fault segment to the area averaged slip state on all the other fault segments in the system since any fault can be subdivided into an arbitrary number of fault segments the interaction matrix can have arbitrary dimension in fact the continuum limit is recovered as the dimension of the matrix approaches infinity we combine this matrix method with a segmentation or patch model for earthquakes in which each discrete segment of a fault has the same coseismic stress change defined as the difference between the driving stress at which healing occurs minus the driving stress at which sliding starts each time it slips we show that slip on a patch during an earthquake can vary substantially depending on how it interacts with other nearby patches in this model it is quite possible for the spatial distribution of stress on the fault following an event to be again in a spatially inhomogeneous state rather than in a uniform state as is often assumed hence the seismic moment produced by an earthquake on a given set of patches can vary substantially depending on the sequence of sliding and healing on the different patches to apply these ideas we devised a means to calculate the interaction matrix elements and used them to quantitatively examine earthquake sequences off the colombia ecuador coast and in the nankai trough near japan

processing of a nine component near offset vsp for seismic anisotropy a convolutional sequence of matrix operators is offered as a convenient deterministic scheme for processing a multicomponent vertical seismic profile vsp this sequence is applied to a nine component near offset vsp recorded at the conoco borehole test facility kay county oklahoma these data are corrected for tool spin and near surface anisotropy together with source coupling or imbalance after wave field separation using a standard f k filter each source and receiver pair for the upgoing waves is adjusted to a common reference depth using a matrix operator based on the downgoing wavefield the up and downgoing waves are then processed for anisotropy by a similarity transformation to separate the qs1 and qs2 waves from which the anisotropic properties are estimated these estimates reveal a strong apparent vertical birefringence in the near surface but weak or moderate values for the majority of the subsurface the target zone consists of a thin sandstone and deeper shale layer both of which possess a strong vertical birefringence the sandstone corresponds to a zone of known fluid flow an observed qs2 attenuation and polarization change in the shale suggest it contains large fractures

the davis strait crust a transform margin between two oceanic basins the davis strait is located between canada and greenland and connects the labrador sea and the baffin bay basins both basins formed in cretaceous to eocene time and were connected by a transform fault system in the davis strait whether the crust in the central davis strait is oceanic or continental has been disputed this information is needed to understand the evolution of this transform margin during the separation of the north american plate and greenland we here present a 315 km long east west oriented profile that crosses the davis strait and two major transform fault systems the ungava fault complex and the hudson fracture zone by forward modelling of data from 12 ocean bottom seismographs we develop a p wave velocity model we compare thismodel with a density model from ship borne gravity data seismic reflection and magnetic anomaly data support and complement the interpretation most of the crust is covered by basalt flows that indicate extensive volcanism in the davis strait while the upper crust is uniform the middle and lower crust are characterized by higher p wave velocities and densities at the location of the ungava fault complex here p wave velocities of the middle crust are 6 6 km s 1 and of the lower crust are 7 1 km s 1 compared to 6 3 and 6 8 km s 1 outside this area densities are 2850 and 3050 kg m 3 compared to 2800 and 2900 kg m 3 we here interpret a 45 km long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern davis strait a high velocity lower crust 6 9 7 3 km s 1 indicates a high content of mafic material this mantle derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the iceland mantle plume with plate kinematic modelling we can demonstrate the importance of two transform fault systems in the davis strait the ungava fault complex with transpression and the hudson fracture zone with pure strike slip motion we show that with recent poles of rotation most of the relative motion between the north american plate and greenland took place along the hudson fracture zone

rupture process of the february 4 1965 rat islands earthquake the great rat islands underthrusting earthquake mw 8 7 of february 4 1965 represents subduction of the pacific plate beneath the north american plate along a 600 km segment of the western end of the aleutian islands body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the rat islands earthquake we have deconvolved world wide standard seismograph network long period teleseismic p wave seismograms to obtain source time functions directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s unilateral rupture in the direction 300 fault length of 420 km and average rupture velocity of 2 5 km s the three pulses of moment release are located along the fault and these regions of high moment release are interpreted as asperities the first asperity extends from the epicenter to 100 km to the wnw this is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s the second pulse of moment release is very jagged is less coherent between stations and is centered 200 km wnw of the epicenter the third pulse of moment release extends from 360 to 420 km wnw of the epicenter although the aftershock area is 600 km in length we can not resolve any moment release from the p waves beyond 420 km wnw of the epicenter the rat islands event was closely followed by a large tensional outer rise event on march 30 1965 ms 7 5 which is located oceanward of the largest moment release associated with the rat islands mainshock rupture detailed analysis of the p waves for this large outer rise event confine the depth extent to the upper 30 5 km of the crust the spatial and temporal association between the february 4 mainshock and the march 30 tensional outer rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter the overriding plate along the western aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins geist et al 1988 we suggest that the central undeformed parts of the blocks have the strongest coupling with the down going plate and hence are the sites of the largest moment release during an underthrusting earthquake the three asperities determined from the p waves correspond to the rat buldir and near tectonic blocks respectively hence the p wave seismic moment release of the rat islands earthquake is controlled by the lateral segmentation of the overriding plate

anisotropy at the lacq hydrocarbon field france from shear wave splitting three component seismograms from local earthquakes located beneath the reservoir of the lacq hydrocarbon field have been analysed for shear wave splitting shear waves recorded at the station usi located on top of the hydrocarbon reservoir show a clear splitting in an average polarization direction n30 40 e which is different from the present day local and regional stress direction this direction seems approximately parallel to the tertiary period stress field in this area which corresponds to the measured main crack directions in the gas reservoir at station ldb located outside of the anticlinal structure no splitting was detected but due to a poor signal to noise ratio only nine events were available and anisotropy could be absent or obscured for a 250 m thick oil or gas field the velocity anisotropy is estimated at about 6 10 which is in the same range as values obtained from vsps in other hydrocarbon fields monitoring induced seismicity by three component stations could be a useful tool to characterize hydrocarbon fields in term of the rock s average properties in a large volume

the volcanic margins of the northern labrador sea insights to the rifting process 1 a new interpretation of seismic reflection data on the continental margin of northern labrador shows basement structures similar to those observed on other volcanic rifted margins seaward dipping reflections inner flows volcanic plateaus and lava deltas are observed on these data magnetic chron 27n 61 ma is coincident with the volcanic plateaus connecting these features to the paleocene volcanism farther north in davis strait therefore we are able to extend the region of volcanism about 500 km south along the margin similar structures are also observed on the conjugate west greenland margin below the volcanic plateaus gravity and wide angle seismic data show thick igneous crust which is fairly symmetrically distributed across the conjugate margins however the geometry of the thinned continental crust is not symmetrical a narrower zone of thinning is observed off northern labrador the thick igneous crust lies seaward of thinned continental crust which exhibits little or no underplating by the magmatic event this observation is compatible with recent models for the formation of nonvolcanic margins followed by a magmatic event late in the rift evolution in mesozoic time rifting of the cold thick cratonic lithosphere occurred between greenland and north america with minimal volcanic activity then in late cretaceous time the zone of rifting narrowed and localized seaward of the shelf creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the paleocene magmas were eventually channeled this scenario is supported by studies of mesozoic igneous rocks on land in coastal west greenland and labrador

synthetic seismicity models of multiple interacting faults a synthetic seismicity model for multiple interacting faults of any strike and dip in a three dimensional elastic half space has been developed the ultimate purpose is to determine how hazard is modified by elastic interactions of events in complex fault systems and to examine the synthetic seismicity for unusual behavior before large events that might be observable in the real world each fault is subdivided into some number of equal sized patches and a coefficient of static friction random within a specified range is assigned to each the patch static strength is then the product of friction and differential normal stress stress both shear and normal accumulates due to some predefined tectonic driving force until one or more patches fail and slip instantaneously the slip can be in any direction within the fault plane and its magnitude is determined by a specified fractional stress drop this initial slip induces shear stress and strength changes on all other patches and faults and perhaps results in other patches failing patches can slip more than once once having failed their strength is reduced resulting in an overshoot that approximates the effect of dynamic stress enhancement an event is over when all patches are stable although present computational constraints do not allow models with more than about 1500 patches or a large number of faults experiments with simple models of two parallel strike slip faults 25 km length 15 km depth 694 m x 714 m patch size driven at different rates and separated by 10 or 15 km give some indicative results a range of friction between 0 1 and 0 2 with a stress drop of 10 about 3 mpa or 30 bars produces a b value 500 000 events close to 1 the models generate a distinct class of large characteristic events that rupture the full fault plane the distribution of interevent times for the background seismicity is very similar to that for a poisson process while the characteristic events are quasi periodic nearly half the larger events directly induce some activity on the other fault the effect of a characteristic event on the occurrence of another characteristic event on the opposite fault is small and depends on the separation at 10 km distance other events are retarded while at 15 km distance they are advanced at 10 km separation the probability of a pair of characteristic events within 1 year is reduced to 0 3 from 1 7 for the case with no interaction consideration of interactions between slip on the faults and the driving mechanism as well as higher stress drops and more overshoot would increase the magnitude of the interaction effects

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

effect of a large outer rise earthquake on seismic cycles of interplate earthquakes a model study a numerical simulation of the sliding process of a plate interface is performed to investigate the effect of stress perturbation due to a large outer rise earthquake on seismic cycles of large interplate earthquakes in a subduction zone a two dimensional model of uniform elastic half space is set up for repeatedly occurring large interplate earthquakes by assuming that the frictional stress on the plate interface obeys a rate and state dependent friction law derived from laboratory experiments an increase in shear stress on a shallow part of the plate interface due to a tensional outer rise earthquake promotes aseismic sliding on the shallower aseismic zone of the plate interface this aseismic sliding advances the occurrence time of the next interplate earthquake on the other hand a compressional outer rise earthquake tends to delay the occurrence time of the next interplate earthquake the magnitude of time advance or delay of the next interplate earthquake depends on the time of the outer rise earthquake in a seismic cycle due mainly to the complex behavior in propagation of aseismic sliding governed by the rate and state dependent friction law

contrasting rifted margin styles south of greenland implications for mantle plume dynamics abstract we present new and reprocessed seismic reflection data from the area where the southeast and southwest greenland margins intersected to form a triple junction south of greenland in the early tertiary during breakup at 56 ma thick igneous crust was accreted along the entire 1300 km long southeast greenland margin from the greenland iceland ridge to and possibly 100 km beyond the triple junction into the labrador sea however highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest greenland margin at the same time thus a transition from a volcanic to a non volcanic margin over only 100 200 km is observed magmatism related to the impact of the iceland plume below the north atlantic around 61 ma is known from central west and southeast greenland the new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction the extent of initial plume related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere plume mantle is channelled to great distances provided that significant melting does not take place melting causes cooling and dehydration of the plume mantle the associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift we further suggest that thick archaean lithosphere blocked direct flow of plume material into the magma starved southwest greenland margin while the plume was free to flow into the central west and east greenland margins the model is consistent with a plume layer that is only moderately hotter 100 200 c than ambient mantle temperature and has a thickness comparable to lithospheric thickness variations 50 100 km lithospheric architecture the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

spatial distribution of turn of the century seismicity along the alaska aleutian arc abstract epicenters determined for instrumentally observed earthquakes occurring around the turn of the century have figured prominently in estimates of seismic hazard along the alaska aleutian arc unfortunately because of poor timing control and a sparse station distribution the locations quoted in the older catalogs can be very unreliable more reliable epicenter estimates and quantitative assessments of the errors associated with these estimates are needed to address issues related to seismic hazard seventeen earthquakes with surface wave magnitudes above 6 8 that were recorded between 1898 and 1917 are relocated using a modified least squares criterion epicentral estimates are determined using p s and s p arrival times with the a priori constraint that the final location must be near the geographically defined arc although epicenters determined using a standard least squares criterion can be located up to 800 km from the arc the arc proximity constraint provides tectonically more meaningful locations which to a high degree of confidence fit the travel time observations as well as the unconstrained locations epicentral shifts of up to 1 200 km are observed between the relocations and those reported in the earlier catalogs seventy five per cent confidence bounds on the along arc location of each event vary from 30 to 400 km several large events m s 6 9 occurred within or near the rupture zone of the 1965 rat islands earthquake and may indicate that this zone failed in a sequence of events around the turn of the century of the four events previously suggested as being possible candidates for rupturing either the shumagin or unalaska seismic gaps none relocate near these arc segments the relocation of a large m s 7 9 earthquake which occurred in 1917 within the shumagin islands region however provides the first strong evidence of largescale seismic moment release within this intensely studied seismic gap

regional anomalies of sediment thickness basement depth and isostatic crustal thickness in the north atlantic ocean abstract we calculate the anomalous basement topography for the north atlantic ocean from 30 to 70 n latitude and from 0 to 70 w longitude at a resolution of roughly 6 6 km using grids of total sediment thickness and observed and predicted sea floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age plotting this residual topography for various plate reconstructions during opening of the north atlantic we delineate consistent patterns of basement highs related to variations in hotspot related volcanism in addition to iceland and the azores we recognize three centers of excess volcanism at the mid atlantic ridge the milne seamounts and azores biscay rise 75 40 ma the southeast newfoundland ridge and madeira tore rise 130 110 ma and the east and west thulean rises 60 50 ma the duration of volcanic activity ranges from 8 to 10 m y thulean rises to 60 m y iceland and thus it appears that both long and short lived hotspots coexist even in relatively close proximity in contrast during the period 110 60 ma we observe little excess volcanism during either continental breakup or seafloor spreading we estimate isostatic crustal thickness from the anomalous basement depths after first removing dynamic effects created by mantle flow maximum thicknesses of volcanic features from 30 km beneath the greenland iceland faeroe ridge to 15 km beneath the azores biscay rise are broadly consistent with seismic data and predictions of decompression melting widths of volcanic features indicate that thickening primarily occurs within 100 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity i e east greenland and the hatton rockall bank we observe conjugate structures south of greenland and edoras bank where excess volcanism appears to have extended beyond the margin proper and into oceanic crust similar conjugate features appear in the labrador sea south of davis strait finally we identify anomalous oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non isostatic effects within the mantle

the rupture process of the great 1979 colombia earthquake evidence for the asperity model we have determined the asperity distribution in the segment of the ecuador colombia subduction zone that was ruptured by the 1979 mw 8 2 and 1958 mw 7 7 earthquakes for the 1979 earthquake we are able to document directivity in the p wave source time functions and consequently have spatially located the moment release by combining results from both p wave and surface wave directivity we have defined three regions 1 from 0 to 56 km northeast of the epicenter with low moment release and a small seismic displacement 2 the asperity from 56 to 116 km northeast of the epicenter with most of the moment release and a displacement of 6 m and 3 a small displacement region from 116 to 180 240 km northeast of the epicenter thus the 1979 rupture zone is composed of an asperity 60 km in length bounded by two weak small displacement regions the displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 the year of the previous great earthquake in this segment the 1958 earthquake is characterized by the rupture of a small asperity length scale approximately 25 km in the epicentral region the 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event this detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence

potential earthquake triggering in a complex fault network the northern south island new zealand summary a synthetic seismicity computer model of multiple interacting faults has been used to investigate potential earthquake triggering in the alpine marlborough buller fault network northern south island new zealand of particular interest is whether large characteristic earthquakes on the central 300 km long segment of the alpine fault average recurrence time of 240 yr might trigger events to the north and northeast possible temporal clustering of other large events and stress shadows the synthetic seismicity model generates long homogeneous catalogues that sample a wide range of possible stress interactions and stress states allowing statistical analysis the catalogues used here include 1500 central alpine fault events of m 7 8 and approximately 2 000 000 other events of magnitudes down to 4 8 these other events occur on four additional segments of the alpine fault on 30 segments of the major marlborough and buller faults and on 4650 randomly distributed smaller faults the synthetic seismicity model is of the quasi static type governed by the coulomb failure criterion with extensions to better account for rupture propagation and fault healing true rate and state friction and viscoelastic relaxation are not yet included mechanical properties are adjusted so that most events rupture an entire fault segment i e the faults behave characteristically the regional b value of 1 thus arises mainly from the fault size distribution the driving mechanism and fault properties are iteratively adjusted so that the resulting long term slip rates single event displacements and recurrence times match the observed real world values the major results are as follows i large alpine fault earthquakes m 7 8 on the central segment have recurrence times with a broad distribution from 81 to 601 yr average 234 yr ii the regional moment release rate is constant over periods of 1000 yr but is quite variable on scales of a few hundred years iii large events on the central alpine fault will usually propagate at least part way along strike to the northeast past the junctures with the marlborough faults that splay off to the east iv thirty seven per cent of such large alpine fault events lafes will also instantaneously trigger significant activity m 6 5 on one or another of the other major faults here instantaneous means with a 0 to approximately 3 yr delay v characteristic events initiating on other major faults also have a high chance of triggering activity elsewhere vi following the short period of increased risk of triggered events a stress shadow effect predominates probably reflecting the mutually inhibitory nature of parallel strike slip faults and the need for a constant long term moment release rate

asperity distributions and large earthquake occurrence in subduction zones abstract ruff l j 1992 asperity distributions and large earthquake occurrence in subduction zones in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 61 83 plate tectonics and the seismic gap hypothesis provide the framework for long term earthquake forecasting of plate boundary earthquakes unfortunately detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone furthermore larger coseismic slip is commonly associated with larger rupture length hence large earthquake occurrence in subduction zones is characterized by variability in 1 recurrence times 2 rupture length and 3 coseismic slip these facts plus many other observations indicate that there are significant spatial variations in the strength of the plate interface one simple description of these variations and their role in the earthquake cycle is the asperity model where the large strong regions of the plate interface are called asperities and the large earthquakes occur when the large asperities break the asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes to go beyond general qualitative notions i pose the following scientific test are the observed asperity distributions and a simple model of their interaction self consistent with the above three observed features of large earthquake occurrence the distribution of the major asperities along plate boundary segments has now been determined for several subduction zones rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along strike asperity lengths and separations for several adjacent asperities in the kurile islands colombia and peru subduction zones the simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate by a coupling spring to each other and maintain frictional contact with a conveyer belt the lower plate that moves with a constant velocity an earthquake occurs when the net force on the asperity frictional slider reaches some specified level the failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction two different macroscopic failure criteria are used this simple mechanical model displays a remarkable range of behavior from simple to complex when the two asperities are identical in all their properties sequences of identical earthquakes are produced for the more realistic case of non identical asperities earthquake sequences show great variety using system variables from the observed asperity distributions the earthquake sequences typically display 1 variable recurrence times 2 variable rupture length i e a combination of single asperity and double asperity failures and for one of the failure criteria 3 larger coseismic slip for double asperity failures statistical summaries of thousands of simulated earthquake sequences for asperity pairs in the kuriles colombia and peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones the main conclusion is that the asperity model provides a self consistent explanation for fault zone heterogeneity the rupture process and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction in addition a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

seismic anisotropy of the crystalline crust what does it tell us the study of the directional dependence of seismic velocities seismic anisotropy promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of p and s wave velocities the alignment of specific minerals by ductile rock deformation for instance causes specific types of seismic anisotropy which can be identified by appropriate field measurements vice versa the determination of anisotropy can help to discriminate between different rock candidates in the deep crust seismic field measurements at the continental deep drilling site ktb s germany are shown as an example that anisotropy has to be considered in crustal studies at the ktb the dependence of seismic velocity on the direction of wave propagation in situ was found to be compatible with the texture composition and fracture density of drilled crustal rocks

crustal accretion at the reykjanes ridge 61 62 n we report results of a seismic gravity and magnetic survey of the reykjanes ridge spreading center at 61 62 n about 600 km from the center of the iceland mantle plume anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2 4 km hydrophone streamer the velocity within layer 2a is 2 4 0 3 km s 1 and its mean thickness is 400 100 m the velocity at the base of layer 2a is 3 3 0 3 km s 1 on the ridge axis increasing with crustal age to 4 0 km s 1 at 1 5 ma and 4 5 km s 1 at 5 ma assuming that seismic layer 2a on the ridge axis is also the extrusive layer i e the magnetic source layer we have successfully modeled the variations in amplitude of the magnetic field the best magnetic model includes enhanced magnetization within layer 2a at the sites of recent volcanic activity as independently recognized in side scan sonar data we also present a full crustal seismic model based on wide angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys the seismic model is complemented by gravity modeling which further suggests that the ridge crest is in isostatic equilibrium the zero age crust is 10 0 km thick while crust of age 5 ma is 7 8 km thick these crustal thicknesses are greater than those of normal oceanic crust which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center we suggest that the variation in thickness between 0 ma and 5 ma crust is caused by temporal variation in the plume fed asthenospheric temperature beneath the reykjanes ridge

space and time dependent probabilities for earthquake fault systems from numerical simulations feasibility study and first results in weather forecasting current and past observational data are routinely assimilated into numerical simulations to produce ensemble forecasts of future events in a process termed model steering here we describe a similar approach that is motivated by analyses of previous forecasts of the working group on california earthquake probabilities wgcep our approach is adapted to the problem of earthquake forecasting using topologically realistic numerical simulations for the strike slip fault system in california by systematically comparing simulation data to observed paleoseismic data a series of spatial probability density functions pdfs can be computed that describe the probable locations of future large earthquakes we develop this approach and show examples of pdfs associated with magnitude m 6 5 and m 7 0 earthquakes in california

spatially heterogeneous strength in thrust fault zones deformation in the hanging wall of the lewis thrust fault at marias pass northwest montana is heterogeneous in space with regions dominated by either large scale contraction or extension faults the distribution of mesoscopic structures within the two regions also differs the density of fractures is highest in the extensional region whereas the density of solution seams is highest in the contractional regime this spatial heterogeneity of structures at the base of the lewis thrust sheet may result from spatially heterogeneous fault zone strength in analytical models of an elastic thrust sheet overlying a fault represented by spatial heterogeneity in shear traction or displacement stress magnitudes and principal directions vary with position within the model thrust sheet with the application of a mohr coulomb criterion contraction and extension faults initiate within localized regions at the base of the model thrust sheet spatial variations in mean stress at the base of the thrust sheet predicted by the models may explain both the greater density of solution seams in the contractional region than in the extensional region and the greater density of mesoscopic faults and veins in the extensional region than in the contractional region of the marias pass exposure the spatial heterogeneity of pressure solution and cataclasis in response to variable mean stress within a fault zone of heterogeneous strength may facilitate thrust sheet movement relative to movement over a fault zone of uniform strength because a decrease in mean stress enhances cataclasis and an increase in mean stress maintains a constant shear stress during pressure solution

complex earthquake rupture and local tsunamis 1 in contrast to far field tsunami amplitudes that are fairly well predicted by the seismic moment of subduction zone earthquakes there exists significant variation in the scaling of local tsunami amplitude with respect to seismic moment from a global catalog of tsunami runup observations this variability is greatest for the most frequently occurring tsunamigenic subduction zone earthquakes in the magnitude range of 7 mw 8 5 variability in local tsunami runup scaling can be ascribed to tsunami source parameters that are independent of seismic moment variations in the water depth in the source region the combination of higher slip and lower shear modulus at shallow depth and rupture complexity in the form of heterogeneous slip distribution patterns the focus of this study is on the effect that rupture complexity has on the local tsunami wave field a wide range of slip distribution patterns are generated using a stochastic self affine source model that is consistent with the falloff of far field seismic displacement spectra at high frequencies the synthetic slip distributions generated by the stochastic source model are discretized and the vertical displacement fields from point source elastic dislocation expressions are superimposed to compute the coseismic vertical displacement field for shallow subduction zone earthquakes it is demonstrated that self affine irregularities of the slip distribution result in significant variations in local tsunami amplitude the effects of rupture complexity are less pronounced for earthquakes at greater depth or along faults with steep dip angles for a test region along the pacific coast of central mexico peak nearshore tsunami amplitude is calculated for a large number n 100 of synthetic slip distribution patterns all with identical seismic moment mw 8 1 analysis of the results indicates that for earthquakes of a fixed location geometry and seismic moment peak nearshore tsunami amplitude can vary by a factor of 3 or more these results indicate that there is substantially more variation in the local tsunami wave field derived from the inherent complexity subduction zone earthquakes than predicted by a simple elastic dislocation model probabilistic methods that take into account variability in earthquake rupture processes are likely to yield more accurate assessments of tsunami hazards

tectonostratigraphic evolution of the labrador margin atlantic canada abstract the labrador continental margin provides a rich source of data with which to study the relationships between stratigraphy tectonics and paleoenvironment we have completed a regional seismic interpretation and integrated this with new biostratigraphic data based on analyses of palynomorphs from wells in the hopedale and saglek basins which occur on this margin our results are summarized in a tectonostratigraphic chart which displays new and consistent age control for the major lithostratigraphic units and provides more precise evaluation of their depositional and paleoenvironmental history we have identified and dated six regional unconformities in the wells and we can recognize several others on the seismic data the older unconformities cretaceous are related to the tectonics of rifting and seafloor spreading and may delineate the onset of different stages of the rift process in the paleocene early eocene unconformity development was influenced by episodic volcanism due to the passage of the proto iceland hotspot to the north and to a major change in spreading direction in the labrador sea many of these unconformities are also identified in offshore southwest greenland and the grand banks suggesting widespread controlling mechanisms during the post seafloor spreading stage the effects of mass wasting and slumping and of paleoenvironmental controls on the stratigraphy were more pronounced we discuss the petroleum potential of the hopedale basin in terms of the structures we see on the seismic data and highlight the bjarni formation which likely contains the most prospective source and reservoir rocks in this basin

earthquakes in a model of seismic zone with embedded pre existing faults we construct a simple quasi static model of seismic zone with embedded pre existing faults and present a self consistent method for incorporating both the stick slip behavior at the fault surfaces and the long ranged stress redistribution following a rupture or a slip event driven by a slow tectonic plate relative motion the model evolves to a critical state exhibiting gutenberg richter power law for the frequency size distributions results on spatial and statistical features of earthquakes in the model are also presented

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

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

a physical model for earthquakes 2 application to southern california the purpose of this paper is to apply ideas developed in a previous paper to the construction of a detailed model for earthquake dynamics in southern california the basis upon which the approach is formulated is that earthquakes are perturbations on or more specifically fluctuations about the long term motions of the plates this concept is made mathematically precise by means of a fluctuation hypothesis which states that all physical quantities associated with earthquakes can be expressed as integral expansions in a fluctuating quantity called the offset phase while in general the frictional stick slip properties of the complex interacting faults should properly come out of the underlying physics a simplification is made here and a simple spatially varying friction law is assumed together with the complex geometry of the major active faults an assumed spatially varying earth rheology the average rates of long term offsets on all the major faults and the friction coefficients one can generate synthetic earthquake histories for comparison to the real data the result is a set of slip time histories for all of the major faults which are similar to data obtained by geologic trenching studies however the patterns of earthquake occurrence are evidently chaotic in the technical sense meaning that although there is an element of periodicity to the events the patterns shift change and evolve with time time scales for pattern evolution seem to be of the order of a thousand years for average recurrance intervals of a hundred years or so in the synthetic slip histories for the model faults a series of events on the big bend is identified which resembles the pattern of events leading up to the 1857 fort tejon earthquake one of the events is similar enough to the 1857 event that it is taken as a model of that earthquake the horizontal surface deformation 130 years after the model event is calculated for comparison to the data observed by various methods during the past decade in southern california for example horizontal surface rates of strain and velocities are calculated and compared to u s geological survey trilateration data to very long baseline interferometry line length changes and to laser ranging data on the san andreas fault experiment baseline with a few exceptions agreement is good