a microearthquake study of the mygdonian graben northern greece during march and april 1984 a temporary network of 29 portable stations was operated in the region of the mygdonian graben near thessaloniki northern greece where a destructive earthquake ms 6 5 had occurred in the summer of 1978 during a period of six weeks we recorded 540 earthquakes with magnitudes ranging from 0 2 to 3 0 from this set of data 254 events are selected which according to us have a precision in epicenter and depth better than 1 5 km a total of 54 single event focal mechanisms have been determined the seismicity and focal mechanisms show a rather complex pattern there are no clear individual faults but the e w and nw se striking zones show n s extension zones striking nne ssw show dextral strike slip motion but nw se zones with sinistral strike slip are also observed in the center of the graben where the 1978 earthquake was located we observe several thrust mechanisms distributed in two groups showing either nw se or e w compression these earthquakes seem to be located 2 km above the earthquakes showing normal mechanisms the mean direction of the t axes found from the focal mechanisms trends n15 and dips sub horizontal we propose a model for the formation and evolution of a complex graben system comprising several stages in the initial stage the deformation occurs along pre existing nw se or nne ssw faults with normal or strike slip movements in the second stage a new e w trending group of normal faults is formed over the ancient fault network these new faults have a direction perpendicular to the mean t axis and accommodate better the actual state of stress at this stage the initial faults adjust to the deformation produced by the e w trending new faults and may constitute geometric barriers to the evolution of the new normal faults

a numerical method for determining the state of stress using focal mechanisms of earthquake populations application to tibetan teleseisms and microseismicity of southern peru the numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms this method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane the proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes this enables one to test the data homogeneity furthermore this restricts the space where the principal stress axes have to be searched then for each principal stress reference whose location is constrained by above confined zones the r value chosen equal to 2 1 3 1 is computed which fits the slip vector on each nodal plane this permits one to select a set of preferred seismic fault planes from a set of auxiliary planes finally a state of stress is computed from the preferred seismic fault plane set using the non linear simplex method already applied to fault populations this algorithm is constructed so as to avoid two major difficulties misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions this calculation does not take into account possible changes of the strike and dip of the nodal planes however we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress tibetan teleseisms and southern peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions this suggests that major seismic events may give a good evaluation of the regional mean state of stress focal mechanisms of the small events are also modelled by a mean stress deviator but this differs significantly from the regional state of stress in southern peru deduced from quaternary faulting this may be due to imprecise location of the small events however some movements are strongly different from those predicted by the computed mean stress deviator this suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

palaeostress determinations from fault kinematics application to the neotectonics of the himalayas tibet and the central andes this paper concerns the determination of the stress deviator from a set of striated faults or of focal mechanisms of earthquakes southern tibet and the cuzco region of the high andes illustrate the applied methodology then the spatial and temporal changes in the tectonic regimes of tibet and central andes are briefly examined the quaternary present day stress patterns in these two high plateaus favour models explaining the spatial changes in the state of stress at a large wavelength as a result of a compensated elevated topography major temporal changes in the tectonic regimes are attributed tentatively to uplift for the tibetan plateau and for the already elevated andean plateau to changes in the magnitude of the boundary forces due to subduction processes

active stress from earthquake focal mechanisms along the padan adriatic side of the northern apennines italy with considerations on stress magnitudes and pore fluid pressures abstract the active tectonic regime along the outer northern apennines padan adriatic area is a matter of debate we analyse the active tectonic regime by systematically inverting earthquake focal mechanisms in terms of their driving stress field comparing two different stress inversion methods earthquakes within the area often deviate from andersonian conditions being characterized by reverse or transpressional slip on high angle faults even if the regime is almost purely thrust faulting e g reggio emilia 1996 and faenza 2000 earthquakes we analyse the stress conditions at faulting for the reggio emilia and faenza earthquakes in order to infer the stress magnitudes and the possible role of fluid pressures the stress analysis defines a consistent pattern of sub horizontal active deviatoric compression arranged nearly perpendicular to the eastern front of the padan adriatic fold and thrust system independent of the stress inversion method used the results are consistent with active compression operating within the padan adriatic belt the stress field is thrust faulting sub vertical 3 except for the cesena forli and ancona areas where a strike slip regime sub vertical or steeply plunging 2 operates the strike slip regimes are interpreted as being caused by the superposition of local tensional stresses due to oroclinal bending i e rotations of the belt about vertical axes on the regional compressional stress field kinematic complexities characterize the 1996 reggio emilia seismic sequence the distribution of these complexities is not random suggesting that they are due to local variations of the regional stress field within the unfaulted rocks surrounding the coseismic rupture the stress conditions at faulting for the reggio emilia 1996 and faenza 2000 earthquakes coupled with the observation that seismicity in the padan adriatic area often occurs in swarms suggest that high pore fluid pressures pf 70 of the lithostatic load operate within the compressed crust the estimated stress difference 1 3 is 460 560 mpa at 15 20 km depth

a multidisciplinary study of a slow slipping fault for seismic hazard assessment the example of the middle durance fault se france summary assessing seismic hazard in continental interiors is difficult because these regions are characterized by low strain rates and may be struck by infrequent destructive earthquakes in this paper we provide an example showing that interpretations of seismic cross sections combined with other kinds of studies such as analysis of microseismicity allow the whole seismogenic source area to be imaged in this type of region the middle durance fault mdf is an 80 km long fault system located southeastern france that has a moderate but regular seismicity and some palaeoseismic evidence for larger events it behaves as an oblique ramp with a left lateral reverse fault slip and has a low strain rate mdf is one of the rare slow active fault system monitored by a dedicated dense velocimetric short period network this study showed a fault system segmented in map and cross section views which consists of staircase basement faults topped by listric faults ramping off triassic evaporitic beds seismic sections allowed the construction of a 3 d structural model used for accurate location of microseismicity southern part of mdf is mainly active in the sedimentary cover in its northern part and in alpine foreland seismicity deeper than 8 km was also recorded meaning active faults within the crust cannot be excluded seismogenic potential of mdf was roughly assessed resulting source sizes and estimated slip rates imply that the magnitude upper limit ranges from 6 0 to 6 5 with a return period of a few thousand years the present study shows that the coupling between 3 d fault geometry imaging and accurate location of microseismicity provides a robust approach to analyse active fault sources and consequently a more refined seismic hazard assessment

regional state of stress and characteristic fault kinematics instabilities shown by aftershock sequences the aftershock sequences of the 1978 thessaloniki greece and 1980 campania lucania italia earthquakes as examples abstract usual analysis of fault kinematics in terms of stress supposes that the slip occurs in the direction of the shear stress resolved on the fault plane due to an homogeneous state of stress however as a block moves this yields perturbations of the static stress conditions the resulting displacement may produce interactions of blocks and may induce complex fault motions such as those demonstrated by the aftershock sequences of the thessaloniki and campania lucania earthquakes each of which shows a main group of focal mechanisms whose normal fault motions are modelled using a stress deviator tm in agreement both with the motion of the main seismic fault and with the regional stress pattern deduced from quarternary and recent faults it is therefore considered as representative of the mean state of stress acting in the epicentral area but two other families of fault motions are also demonstrated one shows reverse motions explained by a compressional stress deviator tc the other involving normal motions is explained by a tensional stress deviator te these deviators are not representative of the mean regional state of stress the compressional 1 axis of tc and the tensional 3 axis of te are roughly orthogonal to the 3 axis of the mean state of stress tm these tc and te deviators may be deduced from the mean state of stress tm by adding respectively td1 and td2 deviators to this latter this means that on each fault plane having a reverse c or a normal e motion this motion may be deduced from the normal kinematics m it should have occurred in the mean regional state of stress tm by adding local pushes d1 and d2 respectively the deviator td2 being nearly axisymmetric around a vertical 3 axis the local reverse pushes d1 are nearly dip slip and td2 having a high angle dipping 2 axis the local pushes d2 are essentially strike slip approximated solutions of these td1 and td2 deviators have been calculated by taking tm td1 tc and tm td2 te the mean state of stress tm being divided into two parts tv and ts which respectively produce the dip slip and essentially strike slip components of the stress resolved on the fault plane a first approximation of the data shows that td1 kv tv and td2 ks ts with kv and ks equal to 2 this approximation supposes that tm tc and te are coaxial and that td1 and td2 are respectively axisymmetric around the vertical 3 axis and planar in the horizontal plane actually the data give kv 2 and ks in the order of 4 5 it is suggested that the local reverse and strike slip pushes are induced by the reaction of the rocky body surrounding small blocks moving as a consequence of a motion on a major fault particularly as there are incompatibilities of motion of adjacent small blocks however data are not sufficient to demonstrate the mechanical and geometric conditions which control the observed different behaviors of the small seismic faults numerical simulations or experiments could probably precise these conditions whatever under certain conditions the stable solution m is replaced by kinematics instabilities e and c however it is usually observed that faults studied in the field in a simple way often yield a single stress deviator this is probably due to the nature of the measured striations which are often growth fibers and oblique stylolithes resulting from creep fault motions in this case incompatibilities of block motions are probably accommodated by a pressure dissolution phenomena therefore it is suggested that the bifurcation parameter which controls the occurrence of the characteristic instabilities possibly depends on a ratio of the deformation rate versus the pressure dissolution rate of the material a question remains unanswered are these instabilities scale invariant or is there a changeover in the behavior between minor and major faults

neogene to ongoing normal faulting in the inner western alps a major evolution of the late alpine tectonics 1 widespread brittle extension occurs in the internal zones of the southwestern alps forming a dense fault network which overprints the compressional structures the extension has been followed by transcurrent motions in terms of paleostress this extension is radial to the belt close to the crustal penninic front and multidirectional in the eastern part of the internal zones the paleostress field of the strike slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults globally we propose that the internal zones of the southwestern alps underwent a single neogene transtensive tectonic regime to strengthen our structural results the current tectonics has been analyzed using seismotectonic tools this approach allowed the recognition of several active faults moreover the ongoing tectonics is coherent with the observed brittle deformation the fault network controls the present day seismicity the combined seismotectonic and structural approaches give coherency and continuity from the neogene to ongoing extensional tectonics in the southwestern alps in the framework of the whole western alps a synthesis of our results with neotectonic related data allowed us to provide an accurate map of the neogene to present kinematics of the belt extension which appears as a major feature of the internal zones has been coeval with the propagation of thrusts in the outer zones during neogene times the geodynamic processes which rule this major tectonic evolution of the belt remain a matter of debate

an inverse problem in microtectonics for the determination of stress tensors from fault striation analysis abstract a method is proposed to compute some characteristics of a possible stress tensor related to striations measured in a given faulted area if several tectonic phases are responsible for the striations the method separates and determines the successive stress tensors the iterative algorithm consists of alternatively a sorting the data attributed to a tectonic phase and b computing the characteristics of the stress tensor of this phase the method is tested using an application to synthetic and actual data

the roer valley graben earthquake of 13 april 1992 and its seismotectonic setting a moderately sized pure normal dip slip earthquake occurred in the roer valley graben rvg near roermond the netherlands on 13 april 1992 at 1h 20m utc this contribution presents an overview of the locations fault plane solutions and magnitudes obtained for the mainshock and the aftershocks by the different scientific groups involved in their analysis the observed maximum intensity of vii is compared with that of other earthquakes in the region to illustrate the relatively low level of damage caused by the mainshock using sh and lg waves recorded at seven local and regional broadband stations we determine a seismic moment of 1 4 1017 nm a static stress drop of 9 7 mpa and an average displacement of 33 cm over a rupture surface of approximately 11 km2 the seismotectonics of the region extending from the rvg to the city of liege including the western part of the rhenish massif wrm and the eastern part of the brabant massif ebm is analysed based on the roermond earthquake studies and data collected since 1985 by the belgian seismic network the geographical distribution of focal mechanism reveals four different seismotectonic regimes in this area from stress tensor inversion we find that s3 coincides with the minimum horizontal stress component in the rvg the wrm and possibly in the ebm while in the liege region s3 is approximately vertical the minimum horizontal stress component shows a 30 rotation to the north in the wrm and the liege region and possibly 50 in the ebm when compared with the minimum horizontal stress component in the rvg

slip preference on pre existing faults a guide tool for the separation of heterogeneous fault slip data in extensional stress regimes abstract synthetic fault slip data have been considered in the present paper in order to examine through a simple graphical manner the validity and use of the widely mentioned and applied criteria such as the slip preference slip tendency kinematic p and t axes transport orientation and strain compatibility the examination and description concern extensional stress regimes whose greatest principal stress axis 1 always remains in vertical position as in andersonian stress states in particular radial extension re radial pure extension re pe pure extension pe pure extension transtension pe trn and transtension trn are examined with the aid of the win tensor stress inversion software in all of these extensional stress regimes only extensional faults can be activated the lower dip angle of the reactivated faults is about 40 assuming that the coefficient of friction is no smaller than 0 6 the increase of the stress ratio and or the fault dip angle up to 70 results in the increase of the slip deviation from the normal activation based on the present examination of the slip preference and slip tendency in different extensional stress regimes a new simple and practical method is proposed herein in order to separate originally heterogeneous fault slip data into homogeneous fault groups by which different extensional stress regimes could be determined the application of the method on the already published fault slip data of lemnos island supports its validity since over 90 the resulted fault groups and stress regimes coincide to the already published ones

plio quaternary stress states in ne iran kopeh dagh and allah dagh binalud mountain ranges abstract ne iran including the kopeh dagh and allah dagh binalud deformation domains comprises the northeastern boundary of the arabia eurasia collision zone this study focuses on the evolution of the plio quaternary tectonic regimes of northeast iran we present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors the inversions of fault kinematics data reveal distinct temporal changes in states of stress during the plio quaternary since 5 ma the paleostress state is characterized by a regional transpressional tectonic regime with a mean n140 10 e trending horizontal maximum stress axis 1 the youngest modern state of stress shows two distinct strike slip and compressional tectonic regimes with a regional mean of n030 15 e trending horizontal 1 the change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional n trending 1 the inversion analysis of earthquake focal mechanisms reveals a homogeneous transpressional tectonic regime with a regional n023 5 e trending 1 the modern stress state deduced from the youngest fault kinematics data is in close agreement with the present day stress state given by the inversions of earthquake focal mechanisms according to our data and the deduced results in northeast iran the arabia eurasia convergence is taken up by strike slip faulting along ne trending left lateral and nnw trending right lateral faults as well as reverse to oblique slip reverse faulting along nw trending faults such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field this implies that no strain and or stress partitioning or systematic block rotations have occurred in the kopeh dagh and allah dagh binalud deformation domains the plio quaternary stress changes documented in this paper call into question the extrapolation of the present day seismic and gps derived deformation rates over geological time intervals encompassing tens of millions of years

evidence for the permutation of stresses σ1 and σ2 in the alpine foreland the example of the rhine graben abstract the orientation of the principal stresses 1 2 and 3 and the ratio r 2 3 1 3 which characterizes the shape of the stress tensor can be calculated using earthquake focal mechanisms in the rhine graben area two types of stress tensor can be determined one of them is an extensional stress state subvertical 1 horizontal 2 n130 in the northern part of the rhine graben the other is a strike slip stress state horizontal 1 n150 in the southern part these results show that the maximum horizontal stress is constant in an approximately n130 150 direction 1 in the south 2 in the north this permutation of the stresses 1 and 2 may be imputed to the interaction between the decrease in alpine pressure and the asthenospheric uprise under the rhenish massif the superficial in situ stresses measured from the alps to the lower rhine embayment showing a decrease in maximum horizontal stress h max support this hypothesis

the campania lucania southern italy earthquake of 23 november 1980 abstract we present the main seismological results of our study of the campania lucania earthquake of 23 november 1980 a complete set of far field and local data has been analysed from long period body waves data we determine the fault plane solution 1 140 1 60 2 75 2 54 a depth of 15 km and calculate a seismic moment of 6 1025 dyne cm and a source duration of 6 s from data of a local network deployed immediately after the event we determine aftershock locations they are aligned in a direction nw se that fit extremely well with the focal solution determined above we can choose as fault plane the plane striking 140 and dipping at 60 and the event is a normal event with a large component of left lateral strike slip the source area evaluated from this aftershock distribution 14 km 40 km is quite suitable for an earthquake of a seismic moment of 6 1025 dyne cm

fine scale partitioning of contemporary strain in the southern walker lane implications for accommodating divergent strike slip motion treating the crust as a micropolar continuum and inverse modeling a high quality catalog of earthquake focal mechanisms reveals that km scale heterogeneous strain is partitioned at km scales into pairs of plane strains with sub perpendicular principal axes these earthquakes occurred in a shallow crustal volume centered on the western indian wells valley of eastern california the site of divergent right lateral strike slip motion along the boundary between the sierran microplate and the southern basin and range province the seismic events defining the plane strains occur in a diffuse seismic source zone and cannot be additionally subdivided on the basis of the type of strain they accommodate the results indicate that seismic moment release for background earthquakes is partitioned into events associated with either fault zone perpendicular extension by normal faults or horizontal shearing by conjugate strike slip faults the spatial scales over which this partitioning occurs suggest that outcrop scale brittle faults may in some cases record nearly perpendicular plane strains related to a single continuous deformation

paleoenvironmental and geoarchaeologic implications of late quaternary sediments and paleosols north central to southwestern san juan basin new mexico abstract soil sediment relationships from the west fork of gallegos canyon near farmington nm to the manuelito plateau northwest of gallup nm provide insight into the late pleistocene and holocene history of the north central to southwestern san juan basin soil formation suggests six widespread episodes of land surface stability the oldest and strongest paleosol is in eolian alluvial and playa sediments that were buried between 9560 and 7800 14 c yrs b p the second oldest paleosol developed within eolian and alluvial sediments and was buried at ca 4500 14 c yrs b p subsequent eolian alluvial and playa deposits contain a third paleosol that was buried ca 3100 14 c yrs b p in turn these sediments are mantled by eolian alluvial and playa sediments that exhibit remnants of a fourth paleosol that was buried by ca 2100 14 c yrs b p a fifth paleosol within eolian and alluvial sediments is associated with basketmaker iii pueblo iii cultural materials dating from 1300 to 660 14 c yrs b p the sixth soil sediment unit consists of eolian and alluvial deposits that date from 660 14 c yrs b p to the present and exhibits the modern soil in addition a very weak paleosol that was buried at ca 300 14 c yrs b p was encountered at a few locales because of discontinuous burial and preservation not all of these units are present at every locale within the study area several locales across the basin however exhibit five to six of these units in stratigraphic succession the widespread occurrence of these paleosols suggests that six episodes of regional stability and soil formation occurred in the western san juan basin during the holocene

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

regional state of stress fault kinematics and adjustments of blocks in a fractured body of rock application to the microseismicity of the rhine graben a simple mechanical model of a homogeneous state of stress and independent fault motions is generally used to interpret fault kinematics in terms of stress however analysis of focal mechanisms of microearthquakes shows that as a result of incompatibilities of block motions in an assemblage of rigid or elastic blocks deformation of a fractured body of rocks is in general heterogeneous analyses of focal mechanism sets show that they are in general composed of a main group whose fault motions result from a mean state of stress tm in good agreement with the regional state of stress the remaining complex fault motions are explained by a compressional tc and or a tensional deviator te an important observation is that within the computed uncertainties these tc and te deviators are co axial with the mean state of stress tm these deviators statistically model the local fault slips probably due to the incompatibilities of block motions and cannot be considered as other mean states of stress thus if the regional state of stress is known for a fault plane whose strike and dip are given its kinematics cannot be surely predicted two types tc and te of motions different from that tm predicted by the regional state of stress may exist they are the kinematic instabilities as a consequence all the motions shown by focal mechanisms or by striated faults are not significant of the regional tectonic regime this is well illustrated by the microseismicity of the rhine graben where it is shown that the compressional fault motions are kinematic instabilities and the extensional fault motions are representative of a regional state of stress 1 vertical with a n45 trending 3 axis hence the simple mechanical model of homogeneous state of stress and independant fault motions is valid only if there are no incompatibilities of block motions possibly these incompatibilities may be accommodated by a pressure solution crystallization process for creep faulting or by internal cataclastic deformation of large blocks separated by major stick slip faults

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

recent and present day stresses in the granada basin betic cordilleras example of a late miocene present day extensional basin in a convergent plate boundary the diffuse convergent boundary between the eurasian and african plates in the western mediterranean is associated with a seismicity zone more than 300 km wide although the two plates are converging nw se the betic and rif cordilleras contain extensional structures that have been active since the miocene the extensional tectonics in the region which occurred simultaneously with the uplift of the cordillera have been analyzed in the southeastern sector of the late miocene to recent granada basin using earthquake focal mechanisms the determination of paleostresses from the study of the orientation and kinematics of microfaults and the study of the major structures both the geological surface data and the focal mechanisms indicate present day regional conditions of ne sw extension with triaxial to prolate stress ellipsoids however the stress field is heterogeneous with local variations in stress over time with different stresses sometimes even acting simultaneously in adjacent areas the most frequent changes consist of pluridirectional or ne sw extension favored by the prolate character of the stress ellipsoids and nw se subhorizontal compression favored by the regional tectonic setting strike slip faults are scarce even though they are the most likely structure to be expected in a region with sw ne extension and nw se compression seismicity is concentrated in the upper crust and may correspond to the activity of low to high angle normal faults similar to the surface faults although they can not be correlated with them the lower cutoff of this seismicity probably coincides with the 300 c isotherm and suggests a low thermal gradient for the region present day regional stresses have 1 vertical at the surface but in depth plunge toward the sw

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

analyses of the stress field in southeastern france from earthquake focal mechanisms summary owing to the apparent deformation field heterogeneity the stress regimes around the provence block from the fronts of the massif central and alpine range up to the ligurian sea have not been well defined to improve the understanding of the se france stress field we determine new earthquake focal mechanisms and compute the presentday stress states by inversion of the 89 available focal mechanisms around the provence domain including 17 new ones calculated in the current study this study provides evidence of six distinct deformation domains around the provence block with different tectonic regimes on a regional scale we identify three zones characterized by significantly different stress regimes a western one affected by an extensional stress normal faulting regime a southeastern one characterized by a compressional stress reverse to strike slip faulting regime with nnw to wnw trending s1 and a northeastern one namely the digne nappe front marked by a ne trending compression note that the digne nappe back domain is controlled by an extensional regime that is deforming the western alpine core this extensional regime could be a response to buoyancy forces related to the alpine high topography the stress regimes in the southeast of the argentera massif and around the durance fault are consistent with a coherent nnw trending s1 implying a left lateral component of the active reverse oblique slip of the moyenne durance fault in the rhone valley an e trending extension characterizes the tectonic regime implying a normal component of the present day no mes fault displacement this study provides evidence for short scale variation of the stress states which arises from abrupt changes in the boundary force influences on upper crustal fragments blocks these spatial stress changes around the provence block result from the coeval influence of forces applied at both its extremities namely in the northeast the alpine front push and in the southeast the northward african plate drift in addition to these boundary forces the mantle plume under the massif central influences the western block boundary

the recent upper miocene to quaternary and present tectonic stress distributions in the iberian peninsula a general synthesis of the recent and present stress situation and evolution in the iberian peninsula was obtained from microstructural and seismological analysis the stress evolution was deduced from 1 fault population analysis fpa from 409 sites distributed throughout the iberian peninsula 2 paleostress indicators given by 324 stations taken from the bibliography and 3 seismic data corresponding to 161 focal mechanisms evenly spread in the studied region the application of fpa together with the determination of stress tensors and focal mechanisms for the whole iberian microplate has provided two main results 1 the iberian peninsula is undergoing a nw se oriented compression except for the northeastern part pyrenees ebro basin and iberian chain where it is n s to ne sw and the gulf of cadiz where it seems to be e w and 2 the main trends of the stress field have remained almost constant since the upper miocene the analysis performed by zones suggests the presence of local heterogeneities in the stress field