backazimuthal variations of splitting parameters of teleseismic sks phases observed at the broadband stations in germany sks phases observed at broadband stations in germany show significant shear wave splitting we have analyzed sks and skks phases for shear wave splitting from 13 stations of the german regional seismic network grsn from 3 three component stations of the grafenberg array grf and from one austrian station sqta the data reveal strong differences in the splitting parameters fast direction and delay time t from a single event at various stations as well as variations at the individual stations for events with different backazimuths the backazimuthal variations of the splitting parameters at some stations can be explained by two layer anisotropy models with horizontal symmetry axes the best resolved two layer model is the gra1 model upper layer 40 t 1 15s lower layer 115 t 1 95s the upper layer can be attributed to the lithosphere because of the magnitude of the delay time of the upper layer the lower layer must lie within the asthenosphere at other stations splitting parameters are consistent with an anisotropic one layer model for the upper mantle stations near the bohemian massif show fast directions near ew throughout ne germany the directions are oriented nw se the reason for this direction is probably the nearby tornquist teisseyre line the observed fast axes are subparallel to this prominent transeuropean suture zone at stations in southern germany near the alps we observed ene wsw directions below some stations we also found indications of inclined anisotropic layers

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

effects of seismic anisotropy on p velocity tomography of the baltic shield the upper mantle structure of swedish part of baltic shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves i e p waves and shear waves recorded by the swedish national seismological network snsn nonlinear high resolution p and sv and sh wave isotropic tomographic inversions reveal velocity perturbations of 3 down to at least 470 km below the network separate sv and sv models indicate several consistent major features many of which are also consistent with p wave results a direct cell by cell comparison of sh and sv models reveals velocity differences of up to 4 numerical tests show that differences in the two s wave models can only be partially caused by noise and limited resolution and some features are attributed to the effect of large scale anisotropy shear wave splitting and p travel time residual analyses also detect anisotropic mantle structure distinct back azimuth dependence of sks splitting excludes single layer anisotropy models with horizontal symmetry axes for the whole region joint inversion using both the p and s data reveals 3d self consistent anisotropic models with well defined mantle lithospheric domains these domains of differently oriented anisotropy most probably retain fossil fabric since the domains origin supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the archean the possible disturbing effects of anisotropy on seismic tomography studies are investigated and found to be potentially significant p wave arrival times were adjusted based on the estimates of mantle anisotropy and re inverted the general pattern of the velocity perturbation images was similar but changed significantly in some places including the disappearance of a slab like structure identified in the inversion with the original data thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images and should not be ignored if as we believe our estimates of anisotropy are reasonably correct then the model based on the adjusted data should give a more robust and correct image of the mantle structure

subcrustal lithosphere around the saxothuringian moldanubian suture zone a model derived from anisotropy of seismic wave velocities abstract we have modelled the thickness and large scale fabric of the subcrustal lithosphere from the variations of p wave delay times and the shear wave splitting observed at seismological observatories and four mobile stations in central europe the saxothuringian s lithosphere is characterized by a total thickness of 90 120 km and by a coherent fabric in its mantle part with olivine a c foliation planes dipping to the north northwest the moldanubian m lithosphere is generally thicker 120 140 km on the average and characterized by a c foliations dipping to the south the divergently dipping anisotropic structures may represent remnants of successive paleosubductions of the ancient oceanic lithosphere frozen in the subcrustal lithosphere of both units a long time before their variscan collision the deep s m contact is marked by a lithosphere thinning to about 80 90 km beneath the s m surface boundary within the subcrustal lithosphere a complex structure of the transition of both units extends to about 150 km toward the south as indicated by the anisotropic characteristics of body waves

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

upper mantle seismic anisotropy beneath antarctica and the scotia sea region summary recent investigations on shear wave splitting from recordings of permanent and temporary antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the scotia sea region and the continental margin in the eastern weddell sea in terms of their tectonic evolution the analysis of shear wave splitting from teleseismic core sks skks pks and direct s waves reveals the seismic anisotropy and the strain field of the upper mantle similar to the caribbean anisotropy structures in the antarctic peninsula and scotia sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting nazca plate in general anisotropy polarization directions in the scotia sea do not contradict this hypothesis with polarizations oriented nearly e w and therefore aligning with the suggested mantle flow patterns anisotropy strengths decrease from delay times of dt 1 8 s pmsa palmer station in the west towards the east with delay times of dt 0 3 s beneath hope south georgia and cand candlemas south sandwich islands nevertheless a lithospheric and therefore fossil origin cannot be ruled out only the exceptionally high delay times at pmsa probably originate in part from recent asthenospheric flow around the subduction slab of the former phoenix plate beneath the northwestern margin of the antarctic peninsula the continental margin of western dronning maud and coats land plays a crucial role in understanding the early processes during the break up of gondwana upper mantle seismic anisotropy with delay times well over dt 1 s in this region gives new constraints on ancient deformation processes during break up and former episodes two layer modelling reveals archaean anisotropy in the upper layer corresponding well to polarization directions of the south african kaapvaal craton lower layer anisotropy is assumed to have been created during early gondwana rifting stages

models of seismic anisotropy in the deep continental lithosphere seismological observations sks wave polarizations systematic p residual variations azimuthal dependence of pn and surface wave velocities or a dispersion of surface waves are not consistent with isotropic if laterally heterogeneous upper mantle structure therefore an anisotropy should be considered as an a priori aspect of future large scale studies of mantle structure most studies of anisotropy however have assumed horizontal or vertical axes of symmetry but such orientations cannot explain bipolar patterns of spatial variations of p residuals which we have observed at many seismological stations on the basis of the petrophysical properties of real upper mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine orthopyroxene and clinopyroxene rotations of the aggregates about vertical and horizontal axes allow us to find the three dimensional orientations of symmetry axes that fit combinations of both p and s seismological observations in central europe and in western north america the orientations with plunging symmetry axes velocity extremes seem to be consistent across large spatially uniform tectonic units and change abruptly at important suture zones

mapping the lithosphere asthenosphere boundary through changes in surface wave anisotropy abstract the alignment of olivine crystals is considered as the dominant source of seismic anisotropy in the subcrustal lithosphere and asthenosphere different components of large scale anisotropy can be traced in depth distributions of the radial and azimuthal anisotropy of surface waves we propose a global model of the lithosphere asthenosphere boundary lab as a transition between a frozen in anisotropy in the lithosphere to anisotropy in the sublithospheric mantle related to the present day flow due to different orientations of velocity maxima in the anisotropic subcrustal lithosphere and the asthenosphere the velocity contrast related to the lab can increase in particular directions because of their long wavelengths and horizontal propagation surface waves suffer from poor lateral resolution however surface waves with various wavelengths allow us to map gross features of the lab with a good vertical resolution we estimate depths to the lab to be between 200 and 250 km for the precambrian shields and platforms around 100 km for the phanerozoic continental regions and 40 70 km beneath oceans from the world wide depth distribution of the radial and azimuthal anisotropy of surface waves

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

subduction induced strain in the upper mantle east of the mendocino triple junction california we observe splitting of teleseismic shear waves at five stations of the berkeley digital seismic network located east of the mendocino triple junction in northeastern california that is dependent on the arrival direction of the seismic phases the observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis we assume that the anisotropy is caused by the preferred alignment of olivine crystals a grid search over possible orientations of the olivine a axes reveals that south of the mendocino triple junction they dip to the east whereas north of the triple junction they dip to the northeast on the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow p wave velocity anomalies in the upper mantle we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast p wave velocity anomaly assuming that the fast p velocity anomaly represents subducted slab material we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid strong down going plate and the surrounding mantle south of the triple junction the fabric may have developed while subduction of the farallon plate was still ongoing in this region prior to 6 ma however we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the north american lithosphere the flow is modulated by the presence of rigid lithosphere to the north and east

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

gravity modelling of the lithosphere in the eastern alpine western carpathian pannonian basin region abstract gravity models illustrate changes in the degree of continental convergence in the eastern alpine western carpathian region and modifications to the lithosphere due to the plate convergence and subsequent pannonian basin extension analysis of the continental collision zone incorporates a kinematic model of ocean basin closure whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening erosion and isostatic rebound thick crust and high topography in the eastern alps along with a broad bouguer anomaly of 140 mgal amplitude are consistent with about 175 km of crustal shortening followed by 10 km of isostatic rebound eastward crustal thicknesses and gravity anomaly widths and amplitudes are less so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the western carpathians preservation of thick flysch deposits and small isostatic rebound are attributable to the high density shallow mantle of the intact continent ocean transition zone seismic delay time studies have suggested that relative to the average thickness of the region the lithosphere thickens by about 70 km beneath the eastern alps and thins by about 60 km under the pannonian basin in both regions gravity anomalies cannot be explained fully without considering this large relief on the lithosphere asthenosphere boundary the eastern alpine crustal root which extends 15 km below the average depth for the region overcompensates the topography and results in gravity anomalies that are 40 mgal lower than those observed the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mgal difference observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the pannonian basin whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle regional cross sections suggest that shallowing of the lithosphere asthenosphere boundary associated with pannonian basin extension has propagated northward beyond the carpathians to within the european platform crustal thinning however appears to be confined to exotic terranes of the carpathian interior so that crustal structure in the eastern alps and outer carpathians is a remnant of the earlier collision orogen

seismic anisotropy and velocity variations in the mantle beneath the saxothuringicum moldanubicum contact in central europe we report on results of a passive seismic experiment undertaken to study the 3 d velocity structure and anisotropy of the upper mantle around the contact zone of the saxothuringicum and moldanubicum in the western margin of the bohemian massif in central europe spatial variations of p wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere a joint interpretation of p residual spheres and shear wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the moldanubicum

effect of water on the rheology of experimentally deformed quartzite empirical flow laws have been determined for simpson quartzite samples deformed to mechanical steady state in the quartz stability field using griggs blacic solid medium deformation apparatus experiments were conducted on samples both as received dry and with water added via the dehydration of a talc confining medium wet best fits of the power law type yield a stress exponent of 2 72 0 19 an activation enthalpy of 134 32 kj mol 1 and a preexponential constant of 1 16 1 15 0 58 10 7 mpa 2 72 s 1 for the dry quartzite law and are 2 61 0 15 145 17 kj mol 1 and 5 05 5 00 10 6 mpa 2 61 s 1 for the wet quartzite the enhanced hydrolytic weakening of the wet experiments appears to affect the flow laws mostly in the preexponential constants possibly as a defect concentration term that is higher in the wet than in the dry law over the range of experimental conditions 750 900 c 10 7 to 10 4 s 1 and 1 0 1 25 gpa confining pressure dry specimens are 1 5 2 5 times stronger than wet samples deformed at the same conditions the microstructures produced are analogous to those observed in many natural quartz tectonites recrystallization is well developed in the wet but not the dry specimens at flow stresses below 500 mpa microstructures indicative of recovery are well developed a comparison of our creep activation enthalpies with those determined from diffusion experiments are consistent with deformation of wet simpson quartzite that is limited by oxygen self diffusion inconsistencies between the present study and previous studies are probably the result of differences in sample material sample treatment and sample assemblies chemical environment

seismic anisotropy of the mantle lithosphere beneath the swedish national seismological network snsn body wave analysis shear wave splitting and p travel time residuals detect anisotropic structure of the upper mantle beneath the swedish part of fennoscandia geographic variations of both the

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

azimuthal anisotropy beneath the bohemian massif from broad band seismograms of sks waves abstract shear wave splitting in sks phases recorded by the two broad band stations khc south bohemia and ksp lower silesia is used to determine the anisotropic properties of the lithosphere beneath the marginal regions of the bohemian massif directions of fast velocity of 80 and 120 clockwise from north were obtained for khc and ksp respectively these directions are consistent with the predominantly e w direction of fast velocity found by different authors using similar techniques in some regions of western europe they disagree however with the directions found for the investigated region by other authors on the basis of teleseismic p wave traveltime residuals the value of the time delay between the two splitted shear waves is close to 1 s for both stations implying that the thickness of the anisotropic zone in the upper mantle essentially exceeds 80 km

dynamic recrystallization in semi brittle faults abstract experiments designed to investigate the microstructure and mechanical behavior of shear zones across the cataclastic faulting to dislocation creep transition indicate that some mylonites may be the products of frictional faulting under semi brittle conditions the experiments involve simple shear of thin layers of polycrystalline halite to strains of 5 8 at temperatures to 250 c normal stresses to 90 mpa and at shear strain rates of 7 10 3 s 1 microstructural and mechanical data define a transitional field of semi brittle flow steady state stress in the semi brittle field is a function of the normal stress and the temperature and is significantly less than that predicted by the power law equation used to describe dislocation creep and by the friction law used to describe cataclastic faulting fracture and localized slip are characteristic processes in the fields of cataclastic faulting and semi brittle flow whereas intracrystalline plasticity recovery and dynamic recrystallization are characteristic of semi brittle flow and dislocation creep for both semi brittle flow and dislocation creep migration recrystallization is activated only after a critical strain and temperature stress condition are achieved the recrystallized grain size is consistent with previously established piezometric relations for dislocation creep these experiments indicate that the field of semi brittle flow for halite is characterized by frictional behavior even though the corresponding microstructure primarily reflects microscopic plastic deformation and softening processes

seismic anisotropy a key to resolve fabrics of mantle lithosphere of fennoscandia abstract by merging different regional sets of body wave data we compile a relief of the lithosphere asthenosphere boundary in one of the oldest continental regions on the planet the fennoscandian shield a maximum lithospheric thickness of 200 220 km is observed in the central part of the shield and a significant thinning at its margins particularly in the southwest at the trans european suture zone fabric of the mantle lithosphere is modelled from independent sets of anisotropic parameters relative p residual spheres and shear wave splitting the mantle lithosphere of fennoscandia consists of several domains consistent olivine fabrics of dipping foliations in 3d anisotropic models of the proterozoic lithosphere or predominantly dipping lineations in models of the archean lithosphere the strength of seismic anisotropy agrees with findings from mantle xenoliths we interpret the anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro continents assembled we envisage an early form of plate tectonic processes such as successive subduction episodes stacking of oceanic plates and accretion of magmatic arcs such processes could create the generally observed fossil dipping fabrics of the thick precambrian mantle lithosphere starting as early as the archean

upper mantle anisotropy in victoria land antarctica during the austral summers 1993 94 and 1994 95 we operated two temporary seismic arrays in the victoria land region antarctica the first was located around the mt melbourne volcano near terra nova bay and the second southward along the acrup1 geotraverse the aim of these experiments was to provide additional constraints on the moho geometry and on lithosphere asthenosphere structure in this zone for this reason a number of techniques were applied to recorded teleseismic data in this paper we describe the results of the analysis on sks and s wave splitting which show the presence of seismic anisotropy computed fast polarization directions range on average between 131 and 166 and delay times are on the order of 1 s we presume that anisotropy is mainly located in the upper mantle although we cannot detect at present if a contribution from the crust does exist the fast polarization direction we determine is quite parallel to the opening of the ross sea an active rift system but also to the absolute plate motion direction therefore we assume that the identified anisotropy may be induced by the extension activated by plate motion and is still related to it

teleseismic p residual study in the iberian peninsula abstract teleseismic p wave residuals cleared of the effects of various source regions and deep mantle paths reflect the composition and structure of the lithosphere and asthenosphere beneath seismological stations we use a method with which we compute two components of the relative residuals for each station namely a directionally independent average and azimuth incidence angle dependent terms and also the ach 3 d inversion method the representative averages show consistent variations over the iberian peninsula with a relatively steep gradient in some areas the iberian massif and the betic cordillera these changes attain values around 1 s over a distance of about 200 km the lateral variations of these residual averages are transformed into a tentative model of the lithosphere thickness a thick lithosphere is observed beneath the iberian massif the pyrenees and beneath part of the betic cordillera waves from events at different teleseismic distances and various azimuths provide consistent residual patterns and a systematic directional dependence of the relative residuals especially at stations in the betic cordillera

lithospheric thickness and velocity anisotropy seismological and geothermal aspects abstract we compare inferences of regional variations in the lithospheric thickness in central europe based on three types of observations seismologically determined thicknesses derived from representative p wave residuals agree in most regions both with the magnetotelluric inferences of depths of the top of a layer of increased electrical conductivity and with the geothermally based calculations that allow inferences of the lithospheric thickness from surface heat flow data the seismological definition of the lithosphere as a high velocity layer overlying the low velocity zone in the upper mantle seems to be close to the geothermal concept in which the lithospheric base is defined by temperatures near partial melting decreasing the seismic velocities and increasing the electrical conductivity interpretations of seismic data used for constraining geothermal models of the deep lithosphere assume velocity variations to reflect mainly changes in the mineral composition however recent observations of large scale seismic anisotropy show clearly that in the subcrustal lithosphere velocity variations due to anisotropy caused by preferred orientations of olivine crystals can be much larger than the velocity variations caused by temperature or compositional changes

variations of shear wave splitting in greenland mantle anisotropy and possible impact of the iceland plume abstract seismic anisotropy was investigated by measuring shear wave splitting at 19 broadband stations in greenland we examined mostly sks and skks phases but also some pks and depth phases of sks e g psks ssks for deep events splitting parameters fast polarization and time delay were determined for these phases the fast polarizations at nine sites in southern greenland are quite uniformly oriented about n ne two sites in central northern greenland show a similar geometry to southern greenland similar fast polarizations in southern and central northern greenland suggest continuity of structural fabric beneath large parts of greenland this coherent pattern extends across a number of geological provinces of varying age and suggests a common cause of anisotropy not related to the bitwise formation of the greenland continental block four sites in an east west oriented belt crossing central greenland show varying fast polarizations and suggest a separate process causing the anisotropy there which may indicate that these processes are not currently active the overall pattern of anisotropy in our results with the exception of variations across central greenland is similar to results obtained from rayleigh waves the irregular geometry of splitting across central greenland may be related to the impact of the iceland plume at 60 ma reported splitting time delays range from 0 4 to 1 4 s with an average of 0 8 s which can generally not be explained by crustal anisotropy alone if confined to a lithosphere of thickness on the order of 100 km time delays of up to 1 4 s indicate anisotropy of up to about 6 assuming that the a crystallographic axis of olivine is preferentially contained in the horizontal plane we suggest that the anisotropy beneath greenland is located mainly in the upper mantle but some contributions from the crust and lower mantle may be present

anisotropy across the sorgenfrei tornquist zone from shear wave splitting abstract during the tor 1 passive seismic experiment in 1996 97 a maximum of 139 temporary seismograph stations were operating over the sorgenfrei tornquist zone stz in an area extending from northern germany through denmark to central sweden one of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the trans european suture zone to achieve this goal broad band and intermediate period 5 s data of the tor 1 stations and additional stations of permanent networks grsn geofon were analysed for splitting of sks and skks phases as a result of the relatively dense station spacing the method offers good lateral resolution of anisotropy preliminary results suggest that the directions of the fast horizontal s wave velocity are affected by the stz in central europe and southern sweden far away from the stz fast s wave directions are approximately e w while they turn more northerly closer to the stz where they are approximately parallel to the trend of the stz no significant shear wave splitting was observed north of 57 n and east of 14 e small delay times between 0 2 and 0 5 s observed at the northernmost tor 1 station t40s and t60s may be controlled by anisotropy in a thickened crust the mantle contribution of horizontal anisotropy within the stz is probably constrained to an approximately 60 km thick zone in the depth range between 70 and 300 km the observations are consistent with a model where azimuthal anisotropy is not governed by present day mantle flow in the asthenosphere but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity

observations of teleseismic shear wave splitting in the basin and range from portable and permanent stations observations of shear wave splitting were obtained from temporary and permanent stations in western nevada and southern california measurements of fast polarization azimuth and delay time t were made on both sks and direct s phases for stations in the northern basin and range the results were consistent yielding an average of t 75 8 0 9 0 3 s one station in the southern basin and range gave 40 11 0 4 0 1 s and one in the mojave desert gave 54 3 1 2 0 1 s t 0 9s is consistent with a 100km thick mantle layer characterized by 4 anisotropy for the northern stations the anisotropy appears unrelated to the present day extension or absolute plate motion rather we suggest that it is fossil anisotropy associated with pre miocene extension whose direction is about 68 for the california station is nearly parallel to the strike of the san andreas fault system and is attributed to shear strain associated with relative plate motion

the micromechanics of friction in a granular layer a grain bridge model is used to provide a physical interpretation of the rate and state dependent friction parameters for the simple shear of a granular layer this model differs from the simpler asperity model in that it recognizes the difference between the fracture of a grain and the fracture of an adhesion between grains and it explicitly accounts for dilation in the granular layer the model provides an explanation for the observed differences in the friction of granular layers deformed between rough surfaces and those deformed between smooth surfaces and for the evolution of the friction parameters with displacement the observed evolution from velocity strengthening to velocity weakening with displacement is interpreted as being due to the change in the micromechanics of strain accommodation from grain crushing to slip between adjacent grains this change is associated with the observed evolution of a fractal grain structure

the deep lithosphere in the alps a model inferred from p residuals abstract the tomographic studies of the alps and their surroundings based on the relative p residuals indicated two lithosphere roots attaining depths over 200 km separated by an upwelling of the lithosphere asthenosphere transition beneath the central part of this mountain range the directional dependence of relative p velocities indicates an anisotropic wave propagation in the subcrustal lithosphere beneath the alps and their surroundings the presented model of the deep lithosphere structure suggests several phases of the tectonic development of this region collision of a tip of the adriatic plate with europe in the eastern alps and dextral strike slip movement of the adriatic ending with underthrusting of europe beneath the western alps and counterclockwise rotation the latter contributes to a steep plunge of both plates beneath the eastern alps and to stress accumulation in the friuli region a model with delamination of the rigid upper crust and its rotation resolves some of the inconsistencies between the near surface and deep tectonics and also explains the distribution of seismicity

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

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

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

joint interpretation of upper mantle anisotropy based on teleseismic p travel time delays and inversion of shear wave splitting parameters abstract we inverted shear wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self consistent three dimensional 3 d image of the anisotropic upper mantle beneath the continents efficiency of the simultaneous 3 d analyses of p residual spheres and shear wave polarizations is demonstrated on data from two regions southern sweden and the western usa the anisotropic inferences of the subcrustal lithosphere are to a first approximation represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes

european mantle lithosphere assembled from rigid microplates with inherited seismic anisotropy abstract we have modelled three dimensional seismic anisotropy of the mantle lithosphere from anisotropic parameters of teleseismic body waves we invert jointly shear wave splitting parameters and p residual spheres based on data from dense networks of temporary and permanent stations in four european regions ranging from the variscan belt to the baltic shield changes in orientation of the large scale anisotropy caused by systematic preferred orientation of olivine identify boundaries of domains of mantle lithosphere individual domains are characterized by a consistent large scale orientation of anisotropy approximated by hexagonal or orthorhombic symmetry with generally inclined symmetry axes the domains are separated by mapped tectonic boundaries sutures which cut the entire lithosphere besides the change of anisotropy orientation at domain boundaries we often observe a change of the lithosphere and or crust thicknesses we do not detect any fabric of the mantle lithosphere which could have been produced by a collision of microcontinents in a volume detectable by large scale seismic anisotropy the observations of consistent anisotropy within individual blocks of the mantle lithosphere reflect frozen in olivine preferred orientation most probably formed prior to the assembly of microcontinents that created the modern european landmass therefore our findings support a plate tectonic view of the continental lithosphere as a mosaic of rigid blocks of the mantle lithosphere with complicated but relatively sharp contact zones these contacts are blurred by the easily deformed overlying crust terranes

seismic anisotropy of the french massif central and predisposition of cenozoic rifting and volcanism by variscan suture hidden in the mantle lithosphere 1 we model the structure of the lithosphere its thickness and seismic anisotropy caused mainly by systematic orientation of olivine crystals from spatial variations of p wave delay times and the shear wave splitting observed at a dense network of mobile and permanent stations within the french massif central mc three major lithosphere domains with different seismic anisotropy are distinguished a suture in the deep lithosphere limits the thick lithosphere of the limousin western mc characterized by a consistent anisotropic pattern with high velocities dipping to the west the eastern mc comprising two different domains with a weakened mantle fabric in the south shows the high velocities dipping systematically to the east the suture between the western and eastern mc parallels the major crustal boundary the late variscan sillon houiller transfer fault in the south and the tauve aigueperse fault in the north with an offset of 10 20 km to the east the offset of the crustal and mantle parts of the same suture indicates that the rigid upper crust might be detached from the mantle lithosphere we suggest that the mantle suture hidden beneath an allochthonous crust was reactivated during the cenozoic extension of the weakened and thinned lithosphere in the south of the eastern mc and predestined a space for the major volcanism mont dore cantal the rigid northern domain characterized by a thicker lithosphere with a well preserved mantle fabric reacted to the extension predominantly by rifting e g limagne and roanne grabens

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

magnetotelluric and seismological determination of the lithosphere asthenosphere transition in central europe abstract we assembled all available magnetotelluric mt sounding curves that cover characteristic provinces of europe the depths of layers with increased electrical conductivities estimated from mt sounding curves by the respective authors were compared with a model of the lithosphere thickness derived from seismic p residuals regions of a relatively thin and thick lithosphere are characterized by positive and negative p residuals respectively we have found a good correspondence with both sets of results in the central part of the pannonian basin the depth of the lithosphere asthenosphere transition is estimated at 60 80 km and 100 120 km at the flanks the results for the bohemian massif show deeper levels of this transition between 100 and 200 km these depths seem to be typical of the lithosphere base beneath the variscan domain of central europe

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

regional waveform constraints on the source parameters of the xunwu china earthquake of 2 august 1987 with implications for mid plate seismotectonics abstract the xunwu china earthquake of 2 august 1987 m s 5 3 was a damaging intraplate event in a stable continental region in this paper long period p and sh waves recorded by the world wide standardized seismograph network and the chinese standard seismic network stations within 30 epicentral distance have been modeled to determine the focal mechanism and source parameters of the mainshock using body wave waveform synthesis and p wave first motions the focal parameters are determined as follows strike 338 6 dip 53 4 rake 39 7 focal depth 7 4 k moment 3 63 1 57 10 23 dyn cm and stress drop 2 6 1 2 bars analysis of the local tectonic setting geometry of isoseismals and aftershock distribution indicates that the earthquake probably ruptured along a nw se striking secondary fault near its intersection with a nne ssw trending major fault the fault motion was normal oblique slip with a left lateral strike slip component based on the p and t axes the epicentral area is subject to nw se compression with a relatively large plunge and a nearly horizontal ne sw extension the observed large plunge of the p axis is probably due to an anomalous local stress field in comparison with other intraplate events the stress drop of the xunwu earthquake is very low suggesting a weak and fractured crustal medium in the epicentral region

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

effects of temperature and sliding rate on frictional strength of granite layers of artificial granite gouge have been deformed on saw cut granite surfaces inclined 30 to the sample axes samples were deformed at a constant confining pressure of 250 mpa and temperatures of 22 to 845 c the velocity dependence of the steady state coefficient of friction ss was determined by comparing sliding strengths at different sliding rates the results of these measurements are consistent with those reported bysolberg andbyerlee 1984 at room temperature andstesky 1975 between 300 and 400 c stesky found that the slip rate dependence of ss increased above 400 c in the present study however the velocity dependence of ss was nearly independent of temperature

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

azimuthal shear wave velocity anisotropy in the basin and range province using moho ps converted phases an eight station large aperture seismometer array was deployed in the basin and range province of west central nevada for the 1988 1989 program for array seismic studies of the continental lithosphere passcal passive source seismic experiment during the 10 months of data collection a total of 100 teleseismic events were recorded 30 source equalized p wave receiver functions show waveform variations in timing amplitude and polarity in the moho ps phase that is diagnostic of shear wave velocity anisotropy information regarding the crustal anisotropic component can be recovered through the ps phase since it is generated at the moho and is sensitive to shear wave splitting in the crust only we utilize single event stacked and array beam formed receiver functions to minimize the influence of structural variations and scattering on the observed converted phases analysis of shear wave splitting of the ps conversion from the crust mantle boundary indicates a fast azimuth of anisotropy oriented approximately nw se with observed time delays of 0 20 s this strongly suggests coherent crustal anisotropy with a fast direction perpendicular to the strike of fault block mountain ranges this fabric is most likely due to the preferred crystallographic orientation of seismically anisotropic minerals in the middle to lower crust rather than the distribution of basin and range upper crustal fracture systems

seismic anisotropy of the lithosphere around the trans european suture zone tesz based on teleseismic body wave data of the tor experiment abstract a passive teleseismic experiment tor traversing the northern part of the trans european suture zone tesz in germany denmark and sweden recorded data for tomography of the upper mantle with a lateral resolution of few tens of kilometers as well as for a detailed study of seismic anisotropy a joint inversion of teleseismic p residual spheres and shear wave splitting parameters allows us to retrieve the 3d orientation of dipping anisotropic structures in different domains of the sub crustal lithosphere we distinguish three major domains of different large scale fabric divided by first order sutures cutting the whole lithosphere thickness the baltic shield north of the sorgenfrei tornquist zone stz is characterised by lithosphere thickness around 175 km and the anisotropy is modelled by olivine aggregate of hexagonal symmetry with the high velocity ac foliation plane striking nw se and dipping to ne southward of the stz beneath the norwegian danish basin the lithosphere thins abruptly to about 75 km in this domain between the stz and the so called caledonian deformation front cdf the anisotropic structures strike ne sw and the high velocity ac foliation dips to nw to the south of the cdf beneath northern germany we observe a heterogeneous lithosphere with variable thickness and anisotropic structures with high velocity dipping predominantly to sw most of the anisotropy observed at tor stations can be explained by a preferred olivine orientation frozen in the sub crustal lithosphere beneath northern germany a part of the shear wave splitting is probably caused by a present day flow in the asthenosphere

seismic anisotropy and mantle flow beneath the baikal rift zone seismic studies have shown that continental rifts such as lake baikal and the great rift valley of east africa are like mid ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1 4 the direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow induced orientation of mantle olivine crystals5 8 seismic studies of the mid atlantic ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8 10 here we present results from an array of seismic stations across the baikal rift zone in southern siberia the splitting in arrival times of sks seismic waves indicates that the upper mantle beneath the rift zone is anisotropic with the fast direction which reflects the direction of mantle flow being horizontal and normal to the rift axis this suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid ocean rifts this may help to elucidate the processes involved in continental rifting

an array study of lithospheric structure across the protogine zone varmland south central sweden signs of a paleocontinental collision abstract a small seismological array was installed on both sides of the protogine zone pz in varmland south central sweden to study the structure of the mantle lithosphere and lateral variations of its anisotropy no distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the pz the observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture the best solution of a joint analysis of anisotropic inferences from teleseismic p residual spheres and an inversion of shear wave splitting parameters resulted in 3d self consistent anisotropic models of blocks of the subcrustal lithosphere the anisotropic structures within the lithosphere are approximated by hexagonal models k p 5 with low velocity symmetry axes the high velocity planes dip to the e in a region westward of the pz and to the nw eastward of the pz the pz can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin

the brittle ductile transition in a deformation mechanism map for halite abstract the brittle ductile transition in rock deformation has been studied extensively but is still poorly quantified progress in our understanding of this and other transitions in mechanical behavior and deformation mechanisms is facilitated by use of deformation mechanism maps a preliminary deformation mechanism map for halite is presented based on shear experiments at room temperature 22 c over the range of normal stress up to 350 mpa and average shear strain rates between 10 to 10 6 s 1 the map illustrates that transitions in mode of failure mechanical behavior and dominant deformation mechanism which often are referred to collectively as the brittle ductile transition do not necessarily coincide the analysis also suggests that the transition from dominantly cataclastic to crystal plastic deformation mechanism fields is adequately described by the distinct and independent mechanism field of semi brittle flow

variation of shear wave residuals and splitting parameters from array observations in southern tibet a tight array of seismographs spanning a 500 km traverse of southern tibet resolved anisotropy from sks with a spatial variation of its direction and an increase northward of the splitting delay as well as of its first arrival residual both waves split by velocity anisotropy are slow relative to p and their waveform analysis may be interpreted to suggest attenuation anisotropy the array here provides examples of residuals and splitting of other s waves which do not tightly conform to the anisotropy assumed in the simplest model of olivine of transverse isotropy with horizontal symmetry axis s waves are also split with parameters which vary along the array and hence are relevant to near receiver structure like those of sks their splitting delay for non vertical incidence and polarization appears larger than that for sks residuals of s wave first arrivals and splitting delays increase less northwards for s than sks anisotropy with a slow vertical axis may account for these observations its origin may be related to horizontal shear or flow in low velocity layers

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

automated sks splitting and upper mantle anisotropy beneath canadian seismic stations summary we have developed an automated method for measuring shear wave splitting and applied it to sks phases recorded at permanent broad band stations in canada our method performs two measurements one seeking to minimize the energy on the transverse component the other minimizing the smaller of two eigenvalues calculated from the covariance matrix of particle motion a short term average long term average sta lta phase picker is used to identify sks arrivals and successive splitting measurements are made on incrementally larger windows around the sks arrival the final result is derived from the longest series of windows over which the splitting parameters remain constant in this study we have processed over 20 000 station event combinations from 34 stations and 1540 events roughly 2 per cent of the data produce reliable estimates of splitting due to the stringent quality controls applied we find good correlation between our results and previously published results from the same stations at some stations we find significant differences in results between the two methods an observation that has been verified using manual measurements in each case eigenvalue derived measurements are consistent with a single layer of anisotropy but transverse energy derived results require a more complex interpretation this discrepancy can be explained by misalignment of horizontal components tests with synthetic seismograms show that component misalignment can lead to false interpretations of multilayer or dipping anisotropy using the transverse energy method the quality of results at individual stations is variable being influenced both by the volume of available data determined by station deployment date and the distribution of natural seismicity and the performance of the sta lta picker used to define the start of the sks window we compare the orientation of the inferred anisotropy at each station with local tectonic features and directions of absolute plate motion apm there is no consistent correlation many stations give results that agree with both while some agree better with local geology and others agree better with apm directions results are available through a dedicated website and provide a potential aid to other studies of anisotropy in the deep earth

anisotropic measurements in the rhinegraben area and the french massif central geodynamic implications as part of an integrated seismic study polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband semi permanent long and short period and temporary short period seismological stations located in two geodynamically important areas in western europe namely the rhinegraben urach area and the french massif central volcanic field

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

upper mantle anisotropy at the transition zone of the saxothuringicum and moldanubicum in southeast germany revealed by shear wave splitting summary the main structures of the mid european lithosphere crossing europe from the iberian peninsula to the bohemian massif are predominantly formed by the variscan orogeny to investigate the anisotropy of the mantle at the transition zone between the two variscan units the saxothuringicum and the moldanubicum we carried out a field experiment in se germany in 1995 1996 23 mobile broad band stations were installed for 6 months in the vogtland oberpfalz bavarian forest area the station profile crossed the suture zone of the saxothuringicum and moldanubicum near the ktb borehole german continental deep drilling program with a mean station spacing of about 10 km we intended to obtain a high lateral resolution of the anisotropy parameters and to resolve possible changes when passing the suture zone the analysis of the observed birefringence of sks phases shows e w directions for the fast polarization therefore the directions deviate only slightly from the strike of the hercynian mountain belt and from the direction of the absolute plate motion in that region indications for the transition zone come from a rotation of the fast polarization direction from 86 13 in the northern part of the profile saxothuringicum to 110 15 in the southern part moldanubicum as well as from strong variations of the splitting parameters with respect to the azimuths of the incoming waves in the middle of the profile we interpret these variations as an expression of a complex mantle structure formed either by several anisotropic layers with inclined symmetry axes in at least one layer or by a model consisting of inhomogeneous anisotropic layers a comparison of the azimuthal variations of the splitting parameters in the middle of the profile with those observed at the grafenberg station gra1 situated in the central part of the transition zone approximately 100 km to the west shows remarkable differences which may reflect lateral variations in the direction and inclination of the symmetry axes in the transition zone even on a small scale both observations the change in the fast polarization direction from the northern to the southern part as well as the variations with respect to different azimuths in the middle of the profile suggest that the transition zone between the saxothuringicum and the moldanubicum continues down into the upper mantle

time corrections to teleseismic p delays derived from sks splitting parameters and implications for western u s p wave tomography 1 upper mantle anisotropy will affect teleseismic p wave arrival times and create artifacts in isotropic travel time tomography compiled sks splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western u s we use sks splitting parameters and a hexagonally anisotropic elastic tensor fast symmetry axis to estimate azimuthal anisotropy contributions to p wave delay times and evaluate the effects on isotropic p wave tomography estimated anisotropy correction times have a root mean square rms value of 0 16 s which is 37 of p wave delay time rms the magnitude of azimuthal anisotropy rather than its azimuth has the strongest effect on p delay times however if the anisotropy symmetry axis has a plunge p wave travel times may be strongly affected by both magnitude and azimuth applying azimuthal anisotropy corrections significantly increases tomographic p wave velocity beneath the high lava plains and central california coast and decreases velocity beneath the great basin

where can seismic anisotropy be detected in the earth s mantle in boundary layers during the last 30 years considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales but not in all depth ranges we detail which conditions are necessary to detect large scale seismic anisotropy firstly minerals must display a strong anisotropy at the microscopic scale and or the medium must be finely layered secondly the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral and there must be efficient mechanisms of orientation of minerals and aggregates finally the strain field must be coherent at large scale in order to preserve long wavelength anisotropy part of shallow anisotropy can be related to the past strain field frozen in anisotropy however the deep anisotropy is due to the present strain field all these conditions are fulfilled only in boundary layers of convective mantle we review in this paper the seismic data sets which provide insight into the location at depth of large scale anisotropy from the d layer up to the lithosphere in addition to the well documented seismic anisotropy in the lithosphere and asthenosphere there is new evidence of seismic anisotropy in the upper 400 660 km and lower 660 900 km transition zones and in the d layer nonetheless the bulk of the lower mantle seems close to isotropy if we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult however this general statement does not rule out flow circulation between the upper and lower mantles finally the geophysical mineral physics and geological applications are briefly reviewed an intercomparison between surface wave anisotropy and body wave anisotropy data sets is presented we discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root deformation and geodynamics processes

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

inversion of teleseismic s particle motion for azimuthal anisotropy in the upper mantle a feasibility study summary we have developed a technique for the inversion of teleseismic s waveforms in terms of azimuthal anisotropy in the upper mantle we test different models of the earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component the optimum model is found by minimizing the difference between the synthetic vertical component and the observed one using this method we explore the possibility of constraining the distribution of azimuthal anisotropy with depth we present examples of seismic observations where the data are clearly in favour of an anisotropic model these observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes

the lithosphere asthenosphere system in fennoscandia abstract we present a summary of the available information on rayleigh wave dispersion data for the fennoscandian region the observations have been combined to produce regional dispersion relations which have then been subjected to the hedgehog inversion procedure the results are presented on a map outlining the thickness of the lid and the shear velocities in both the lid and the asthenosphere channel lid thickness up to around 135 km is found in the bothnia north central finland area with if any weak shear velocity contrast to the underlying layer the surrounding areas are characterized by lid thickness up to around 75 km a stronger low velocity zone to lid contrast may be found in the caledonian and baltic sea area 0 25 0 45 km s taking into account moho depth data and the aforementioned results a map of the lithosphere asthenosphere system was derived

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

shear wave splitting in the records of the german regional seismic network estimates of the parameters of shear wave splitting in the records of sks and skks of the new german regional seismograph network grsn for one layer model show large azimuthal variations at some stations of the network it is found that the variations are compatible with the presence of two anisotropic mantle layers in the region of the south german triangle sgt the fast direction in the upper layer is between n s and ne sw close to the previously reported estimates of the fast direction in the sub crustal lithosphere of the sgt the fast direction in the lower presumably asthenospheric layer is close to e w the estimates of the effective fast direction for one layer model at the stations of the grsn confirm the previously noted tendency of the fast direction of mantle anisotropy in central europe to change from 50 70 in the western part of the region to 100 120 in the eastern part a correlation of this change with a bending of the present day direction of maximum horizontal stress in the crust suggests that anisotropy in the asthenosphere of central europe can be related to the alpine orogeny

the lithosphere in central europe seismological and petrological aspects abstract the lithosphere thickness in the variscan belt of central europe varies between about 60 and 150 km with typical values of 100 120 km our estimates derived from directionally independent representative p wave residuals are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle the large scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative p residuals vary between 6 5 and 15 2 for p velocities the s wave anisotropies determined from sks polarizations vary between 2 2 and 6 7 these values are in reasonable agreement with the anisotropies of peridotites determined in laboratory systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere the residual patterns at most stations in the saxothuringicum rhenohercynicum and in the massif central imply northwesterly orientated dips of the anisotropic structures while stations in the moldanubicum the alpine foredeep and most of the alps north of the insubric line suggest southeasterly orientated dips in our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere the variscides of central europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the moldanubicum and the saxothuringicum

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust