determination of present day stress tensor and neotectonic interval in the spanish central system and madrid basin central spain a brittle deformation tectonic analysis was performed in central spain spanish central system and madrid basin in order to decipher and understand the deformation processes that take place in a typical intracontinental zone 1174 fault slickensides obtained in materials with ages between late cretaceous and quaternary have been analyzed by means of fault population analysis methods to reconstruct paleostress tensors nine earthquake focal mechanisms have been determined with magnitudes ranging between 3 and 4 1 with regard to regional structural features and sedimentary record data the characteristics of present day and neotectonic stress fields have been figured out which determine the neotectonic period for this region thus we have established that the intraplate zone represented by central spain has been subjected to a stress field from the middle miocene until the present day with a largest horizontal shortening direction shmax located between n130e and n160e finally three paleostress maps with the main active structures are presented for a middle miocene to late miocene the period when the spanish central system was mainly formed b late miocene to quaternary and c the present day stress field deduced from earthquake focal mechanisms

keys and pitfalls in mesoscale fault analysis and paleostress reconstructions the use of angelier s methods whereas most of the stress inversion methods using fault slip data only minimize the angle between the measured striation and a computed shear stress to find the best fitting reduced stress tensor angelier 1990 proposed an alternative method named invd that also takes into account the relative shear stress magnitude which allows the fault to move using artificial datasets and particular fault geometries we compare this method with one of the classical methods based on the minimization of the shear slip angles r4dt angelier 1984 and we show that in most cases the new method has improved the quality of the results furthermore as proposed by angelier we point out that the quality of the stress inversion primarily depends on the quality of the field data we give advice and warn about some pitfalls concerning determination of sense of slip on fault planes recognition of successive faulting events and their chronology drawer or wedge faults stress permutations faults in vertical bedding we also argue that in case of tilted sequences fault diagrams should not be presented without bedding planes but we show that stress inversions when realized with caution and with the correct method can havemuch more applications than reconstructing stress fields like for determining the paleo horizontal the nature and the sense of motion of large faults the chronology and age of large structures

geometry and distribution of regional joint sets in a non homogeneous stress field case study in the ebro basin spain abstract three regional joint sets striking n s e w and wnw ese affect the tertiary rocks of the central ebro basin from analysis of their chronological relationships and spatial distribution it is concluded that they correspond to two different tectonic events the n s set oldest and the e w set younger are present in the southern and central sectors while the wnw ese joint set predominates in the northern one the n s joints propagated in response to joint normal and fluid loads under an intraplate stress field with shmax oriented near n s related to forces caused by the convergence of africa iberia and europe and rifting at the valencia trough during the sedimentary infilling of the basin these joints are only present in the southern part of the area the e w joint set in the southern central sector records the same fracturing event as the wnw ese set does in the northern one its orientation was modified by the presence of the older n s set in the south which perturbed the regional stress field the younger wnw ese and e w joint sets are interpreted as unloading joints these propagated as a consequence of flexural uplift and exhumation related to isostatic rebound at the pyrenees and the ebro foreland basin a numerical approach is used to explain the inhomogeneous distribution of the n s joint set in terms of their absence being controlled by the depth of the water table at the time of their formation

direction of resolved shear stress a construction and discussion abstract a simple construction facilitates identification of the direction of resolved shear stress for andersonian conditions in which one principal axis is vertical a constructed horizontal rake guide for specific stress and strike will project perpendicularly onto any dipping plane as the direction of resolved shear stress consideration of rake guides for different strikes for each fixed stress state illustrates that tectonic regimes are identifiable by ranges but not by individual values of rake the tectonic regime parameter and the rake guide angle are useful parameters for andersonian stress

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

superposed deformation in the peninsula mitre recess fuegian thrust fold belt southern argentina and implications for orogenic curve kinematics abstract a detailed study of the structure of the peninsula mitre recess in the fuegian thrust fold belt of southern argentina reveals superposed deformation related to contrasting shortening directions one group of structures trend parallel to the recess and verge either towards the foreland or towards the hinterland a second group of structures accommodate contraction superposed on the previous group analysis of fault slip data reveals that the first group is related to 1 perpendicular to the surface traces of thrusts and or fold axes and are therefore compressive structures in contrast the paleostress tensors of the second group of structures show 1 orientations parallel or at a low angle with respect to local structural trends the style of the superposed structures of the second group which mostly indicate contraction subparallel to the trend of the recess is consistent with structures resulting from tangential longitudinal shortening associated with oroclinal bending this scenario implies that the peninsula mitre recess formed as a progressive curve involving rotation due to slip trajectories convergent toward the recess causing the observed tangential contraction this kinematic model supports previous interpretations of buttressing of the thrust fold belt against a basement promontory

quick clay a case study of chemical perspective in southwest sweden quick clay a soil that changes from normal firm ground to a liquid mass when it is disturbed has been involved in most of the large and serious clay slides in sweden norway and canada the location time of occurrence and size of quick clay slides are difficult to predict and large slides may cause great devastation some geochemical studies of swedish quick clay were done in the 1960s and early 1970s but no systematic studies of the interrelationships of pore water chemistry mineralogy geotechnical properties and other parameters on quick clays in sweden have been published such studies are of national and general interest because of the many combinations of rock flour source areas and sedimentation conditions that occurred across central sweden and into the baltic sea area during deglaciation in this study geotechnical properties related to the in situ chemistry at one quick clay site were extensively studied and spot sampling was conducted at two other locations in southwest sweden in this area the clay minerals mainly are non expanding phyllosilicate minerals illite and primary minerals quartz feldspar which is consistent with previous studies of quick clay mineralogy extensive leaching has occurred at all three locations at the extensively studied site surte the lowest salinity was found at the greatest depth inferring that the leaching by fresh water was accomplished by water movement upward and laterally through the sediment from the underlying bedrock this is consistent with the local setting where bedrock hills rise sharply to over 100 m above the marine sediment surface an artesian pressure would also be anticipated at this location there is a correlation negative between sensitivity and salinity but there is an indication that the maximum salinity or electrical conductivity consistent with the quick clay behaviour is higher than reported elsewhere however for high sensitivities the salinity is about the same as reported elsewhere in the deepest part of the borehole there is a higher content of fe and al in the pore water indicating reduced state further work is needed to confirm the difference in salinity and to investigate the possible interplay of salinity and potential dispersing agents such as the role of anoxic conditions in this region further work is especially needed in the locations where the sediment accumulation occurred under lower salinity conditions at all three locations high remoulded shear strength and low sensitivity have been seen near the surface together with a decrease in pore water cation concentrations

late pliocene pleistocene stress field in the teruel and jiloca grabens eastern spain contribution of a new method of stress inversion samples of non striated fracture surfaces within clastic materials of late pliocene pleistocene age from the teruel grabens eastern spain have been analysed using a stress inversion method based on observations of slip sense the results obtained at 21 sites are compared with late miocene early pliocene extensional stress tensors previously inferred from striated faults in the same area the similarity between both sets of stress states suggests that the extensional miocene pliocene stress field essentially continues with minor changes into pliocene pleistocene times the main changes involve a the dominant trend of 3 trajectories which evolve from ese to ene b the waning of the compressional component caused by europe iberia africa convergence and c the progressive trend towards a multidirectional extension regime stress deflection caused by large scale extensional faults as well as switching of 2 and 3 axes induced by fracture development are common within this stress field they produce groups of local stress ellipsoids with 3 axes orthogonal to each other and either orthogonal or parallel to the faults bounding the grabens the regional consistency of the new results gives support to the new inversion method and demonstrates its utility in research on young sedimentary rocks where gaps in palaeostress records may exist due to absence of striated faults

opposite vergence of a kink fold and pressure solution cleavage southeast spain a study of the relation between paleostress and fold kinematics abstract pressure solution cleavages define vergences that are opposite to those of asymmetric kink folds in limestones from the betic foreland southeast spain this cleavage fold relation is explained by an evaluation of paleostress and kinematics during kinking it is concluded that stress orientations deduced from stylolites match with models of kinking in non metamorphic rocks in this case opposite vergence of cleavage and folds is the logical consequence of the deformation mechanisms operating at different scales

the tr method the use of slip preference to separate heterogeneous fault slip data in compressional stress regimes the surface rupture of the 1999 chi chi taiwan earthquake as a case study abstract synthetic contractional fault slip data have been considered in order to examine the validity of widely applied criteria such as the slip preference slip tendency kinematic p and t axes transport orientation and strain compatibility in different andersonian compressional stress regimes radial compression rc radial pure compression rc pc pure compression pc pure compression transpression pc trp and transpression trp are examined with the aid of the win tensor stress inversion software furthermore the validity of the recently proposed graphical tr method which uses the concept of slip preference for the separation of heterogeneous fault slip data is also examined for compressional stress regimes in these regimes only contractional faults can be activated and their slip preferences imply the distinction between real i e rc rc pc and pc and hybrid i e pc trp and trp stress regimes for slip tendency values larger than 0 6 the activated faults dip at angles from 10 to 50 but in the hybrid regimes faults can dip with even higher angles the application of the tr method is here refined by introducing two controlling parameters the coefficient of determination r2 of the final tensor ratio line ftrl and the normal or inverse distribution of the faults plotted within the final tensor ratio belt ftrb the application of the tr method on fault slip data of the 1999 chi chi earthquake taiwan allowed the meaningful separation of complex heterogeneous contractional fault slip data into homogeneous groups in turn this allowed the identification of different compressional stress regimes and the determination of local stress perturbations of the regional or far stress field generated by the 1999 chi chi earthquake this includes clear examples of stress permutation and stress partitioning caused by pre existing fault structures such as the n s trending chelungpu thrust and the ne sw trending shihkang shangchi fault zone

stress evolution and structural inheritance controlling an intracontinental extensional basin the central northern sector of the neogene teruel basin abstract the teruel basin is a nne ssw trending intracontinental extensional basin located in central eastern iberia it is asymmetrically bounded to the east by a major fault zone but intrabasinal faults with diverse orientation nne ssw to ne sw e w or nw se also appear offsets of the successive sedimentary units and of two planation surfaces reveal that tectonic activity initiated at the border faults while intrabasinal ones mainly developed in a later stage fractures on a map scale show a prevailing n s strike in neogene synrift rocks while a dense network made of four main fracture sets ne sw e w to ese wnw n s and nnw sse likely inherited from mesozoic rifting stages is observed in pre rift units the results of palaeostress analyses indicate an overall predominance of 3 directions around e w although two stress episodes have been distinguished during the late miocene pleistocene i triaxial extension with 3 e w ii almost radial extension 1 vertical 2 3 with a somehow prevailing 3 ene wsw a scenario in which the evolving extensional stress field was able to gradually activate major basement structures with different orientation inherited from previous tectonic events is proposed as responsible for the evolution and overall pattern of both the eastern active margin and central parts of the central northern sector of the teruel basin

the two stage opening of the western central mediterranean basins a forward modeling test to a new evolutionary model abstract the algero provencal and the tyrrhenian extensional basins developed in two stages in a geodynamic setting characterized by the nearly n s convergence between africa and eurasia the spreading of the provencal basin occurred in the early miocene following a long period of rifting in the western mediterranean area a dramatic eastward shift of the active extensional deformation resulted in the tortonian to quaternary opening of the tyrrhenian basin in a companion paper carminati et al propose that a trench retreat process in a geodynamic setting locked by the continental collisions in the alps and in the betic chain is a viable mechanism for the late oligocene early langhian opening of the western mediterranean a langhian slab detachment episode along the north african margin is likely to have caused the end of the trench retreat along this part of the boundary determining the end of active expansion in the western mediterranean and the beginning of active extension in the tyrrhenian basin the objective of the present paper is to quantitatively test this proposed scenario we calculate by means of a thin shell model the effects of these plate boundary reorganizations on the european stress and strain field we show that the two stage opening of the western and central mediterranean can be explained by the evolution proposed by carminati et al and that in particular the eastward shift of the active extension which produced the termination of the first opening stage and the beginning of the second is likely to have been triggered by the slab detachment episode along the north african margin

recent intraplate stress field in the eastern duero basin n spain palaeostresses inferred from brittle mesostructures in the eastern duero basin show a recent stress field characterized by an extensional regime with local strike slip and compressional stress states orientations of the maximum horizontal stress shmax show a relative scattering with two main modes nne to ne sw and nw se these orientations suggest the existence of two stress sources responsible for the dominant directions of the maximum horizontal stress in northeastern iberia extensional structures within a broad scale compressional stress field can be related to both the decrease in relative stress magnitudes from active margins to intraplate regions and rifting proccesses ocurring in eastern iberia stress states with nw se trending shmax are compatible with the dominant pattern established for western europe ne sw orientations of shmax suggest the occurrence of tectonic forces coming from the pyrenean zone geological and geophysical data indicate the existence of both orientations from the upper miocene to the present day in ne iberia

breddin s graph for tectonic regimes abstract a simple graphical method is proposed to infer the tectonic regime from a fault and slip data set an abacus is overlaid on a plot of the rake versus strike of the data this yields the horizontal principal stress directions and a constraint on the stress tensor aspect ratio in a manner similar to breddin s graph for two dimensional strain analysis the main requirement is that one of the principal stress directions is close to the vertical this method is illustrated on monophase synthetic and natural data but is also expected to help sort out multiphase data sets

subevent analysis for the tabas earthquake of september 16 1978 using near field accelerograms we analyze the strong motion accelerograms of the large magnitude mb 6 4 ms 7 7 mw 7 4 m0 1 32 10 20 tabas earthquake of september 16 1978 the earthquake occurred due to a unilaterally propagating complex rupture process on a thrust fault system the imprints of rupture of four and possibly five asperities sub faults have been identified on the accelerograms using derived sh wave records the locations of these asperities which lie on the main causative fault and the relative times of their occurrence have been determined a foreshock that occurred about 6 s prior to the main shock has also been identified and located further using sh wave spectral data we provide near field estimates of the strike dip and slip of these asperities as well as their seismic moment moment magnitude fault area released shear wave energy and average stress drop the spectra of the sub events conform to the 2 brune spectra the total seismic energy of the sub events is consistent with the radiated energy estimated from teleseismic observations the fault plane solutions show generally wnw ese strike which is sub parallel to the regional strike of the geological fault system existing in the area the stress drops are in the range of 7 30 bar with the exception of the main sub event that showed a stress drop of 100 bar overall the sub events show self similarity the average shear wave quality factor qs which in the near field is quite sensitive to the local site geology has been found to lie between 600 and 1200 the decay of pga with distance is found to be compatible with niazi and bozorgnia s niazi m bozorgnia y 1992 the 1990 manjil iran earthquake geology and seismological overview pga attenuation and observed damage bull seism soc am 82 774 799 attenuation relation for the region 2005 elsevier b v all rights reserved

analysis of a gradual change in stress regime example from the eastern iberian chain spain abstract the compression extension passage in the eastern iberian chain during the miocene is analyzed structural data suggest that it is a gradual change in stress regime which does not involve rotation but interchange of 1 and 2 axes one of them remaining always vertical the gradual stress evolution is interpreted by a numerical graphic method y r diagram which optimizes combinations of y azimuth of the maximum horizontal axis and r stress ratio explaining groups of fault movements the y r diagrams of some fault populations measured in the castellon region provide optimum tensors appearing within a continuous cloud of solutions from the compressional field to the extensional one movement sequences obtained from superimposed striations indicate a systematic stress change between these two regimes

on the tectonic origin of iberian topography the present day topography of the iberian peninsula can be considered as the result of the mesozoiccenozo ic tectonic evolution of the iberian plate including rifting and basin formation during the mesozoic and compression and mountain building processes at the borders and inner part of the plate during the tertiary followed by neogene rifting on the mediterranean side and surface processes acting during the quaternary the northern central part of iberia corresponding to the geological units of the duero basin the iberian chain and the central system shows a mean elevation close to one thousand meters above sea level in average some hundreds of meters higher than the southern half of the iberian plate this elevated area corresponds to i the top of sedimentation in tertiary terrestrial endorheic sedimentary basins paleogene and neogene and ii planation surfaces developed on paleozoic and mesozoic rocks of the mountain chains surrounding the tertiary sedimentary basins both types of surfaces can be found in continuity along the margins of some of the tertiary basins the bouguer anomaly map of the iberian peninsula indicates negative anomalies related to thickening of the continental crust correlations of elevation to crustal thickness and elevation to bouguer anomalies indicate that the dierent landscape units within the iberian plate can be ascribed to dierent patterns 1 the negative bouguer anomaly in the iberian plate shows a rough correlation with elevation the most important gravity anomalies being linked to the iberian chain 2 most part of the so called iberian meseta is linked to intermediate elevation areas with crustal thickening this pattern can be applied to the two main intraplate mountain chains iberian chain and central system 3 the main mountain chains pyrenees and betics show a direct correlation between crustal thickness and elevation with higher elevation crustal thickness ratio for the central systemvs the betics and the pyrenees other features of the iberian topography namely the longitudinal pro le of the main rivers in the iberian peninsula and the distribution of present day endorheic areas are consistent with the tertiary tectonic evolution and the change from an endorheic to an exorheic regime during the late neogene and the quaternary some of the problems involving the timing and development of the iberian meseta can be analysed considering the youngest reference level constituted by the shallow marine upper cretaceous limestones that indicates strong dierences induced by i the overall tertiary and recent compression in the iberian plate responsible for dierences in elevation of the reference level of more than 6 km between the mountain chains and the endorheic basins and ii the eect of neogene extension in the mediterranean margin responsible for lowering several thousands of meters toward the east and uplift of rift shoulders a part of the recent uplift within the iberian plate can be attributed o sostatic uplift in zones of crustal thickening

stress field evolution of the southernmost andean cordillera from paleostress analysis argentine tierra del fuego this work has been funded by projects ctm2011 30241 c02 02 ctm201460451 c2 2 p and ctm2017 89711 c22 p of the spanish r d national plan and by pict funding from anpcytfoncyt argentina

strain analysis from calcite e twins in the cameros basin nw iberian chain spain abstract calcite twins and c axes from sparry grains that fill micro veins were used to unravel the deformational history of the cameros massif an inverted mesozoic sedimentary basin located in the northwestern part of the iberian chain spain calcite c axis and microvein orientations indicate the existence of two extension directions nw se and ne sw during the basin formation probably related to the slip along two syn sedimentary faults calcite e twins were developed only in micro veins from the frontal ramp area i e in the northeast side of the basin where the stresses were higher during the basin inversion twin analysis yields three different orientations nw se e w and ne sw for the minimum horizontal extension direction i e s hmax under coaxial conditions which suggests that the basin inversion was developed through several thrusting episodes during which the frontal and lateral ramps of each one of these episodes are interchanged moreover the perpendicularity between c axes and the minimum extension directions suggests that the basin inversion was produced along the same faults that controlled the basin formation the minimum extension directions deduced from e twins are also consistent with the shortening directions established previously for the tertiary evolution of the iberian chain which suggest that the basin inversion involved several tectonic events during the neogene these results show the advantage of the combined c axis and e twins analysis to unravel deformational and kinematic histories

tectonic subsidence v erosional lowering in a controversial intramontane depression the jiloca basin iberian chain spain the jiloca basin is a large intramontane nnw sse trending topographical depression in which the relative role of tectonic subsidence and erosional lowering is currently a matter of discussion geometry and facies of the sedimentary infill at its central sector have now been characterized from compiled borehole data which allows discussions of how the evolutionary model is constrained the central jiloca depression contains a late pliocene to pleistocene sedimentary sequence made up of alluvial fan pediment mantle and episodic palustrine deposits overlying a carbonate unit that could represent an early lacustrine stage of late miocene early pliocene age the geometry of these units is partially controlled by nw se striking normal faults both the morphological depression and the sedimentary basin truncate previous folds whose traces beneath the neogene quaternary infill have been interpreted from the geology of the basin margins borehole data and hydrogeological criteria the northern and southern sectors of the jiloca depression are bounded by faults showing measurable hectometric scale throws calamocha and concud faults moreover in the central sector the 350 400 m tectonic uplift of sierra palomera has been interpreted from a morphostructural reconstruction of the tilted block which separates the teruel and jiloca graben being similar to the height of the sierra palomera mountain front all these features are consistent with a tectonic basin developed within the framework of the neogene quaternary extensional evolution of eastern spain in contrast they are hardly compatible with genetic models based on erosional deepening either topographic lowering by numerous nested tertiary erosion pediplains or sub alluvial pliocene quaternary karstic corrosion

a stress inversion procedure for polyphase calcite twin and fault slip data sets in rocks that are polydeformed an approach which separates faults prior to stress inversion is more appropriate the traditional stress inversion approach involving the concept of the best fit stress tensor e g a tensor which minimises the misfit between calculated and measured fault striae data often risks computing artificial stress tensors that are some form of average of mixed sets of real stress tensors a new approach is proposed in which fault data are pre processed to group the faults on the basis of their response to all possible orientations and magnitudes of applied stress a computer method is described which utilises cluster analysis based on the right dihedra method to divide dynamically mixed fault populations to monophase subsets this division is based on the ranked similarity coefficients of each fault pair from the raw data set the data clusters form dynamically homogeneous subsets which are used for the composite right dihedra solution this solution is re computed for the reduced stress tensor defined by the orientation of principal stress axes and the ratio of their magnitudes

oblique strain partitioning and transpression on an inverted rift the castilian branch of the iberian chain the iberian chain is a wide intraplate deformation zone formed by the tectonic inversion during the pyrenean orogeny of a permian mesozoic basin developed in the eastern part of the iberian massif the n s convergence between iberia and eurasia from the late cretaceous to the lower miocene times produced significant intraplate deformation the nw se oriented castilian branch of the iberian chain can be considered as a key zone where the proposed models for the cenozoic tectonic evolution of the iberian chain can be tested structural style of basin inversion suggests mainly strike slip displacements along previous nw se normal faults developed mostly during the mesozoic to confirm this hypothesis structural and basin evolution analysis macrostructural bouguer gravity anomaly analysis detailed mapping and paleostress inversions have been used to prove the important role of strike slip deformation in addition we demonstrate that two main folding trends almost perpendicular ne sw to e w and nw se were simultaneously active in a wide transpressive zone the two fold trends were generated by different mechanical behaviour including buckling and bending under constrictive strain conditions we propose that strain partitioning occurred with oblique compression and transpression during the cenozoic 2008 elsevier b v all rights reserved

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 guatemala earthquake of 1816 on the chixoy polochic fault abstract forty historical documents have been found which describe a previously unknown very large earthquake on 22 july 1816 this event occurred along the strike slip boundary between the caribbean and north american plates in guatemala and southern mexico modified mercalli intensities are estimated from these accounts and a rough isoseismal map is constructed the damage pattern indicates that the causative fault was the left lateral chixoy polochic fault for which no damaging earthquake has previously been reported damage of modified mercalli intensity vii or greater covered an area of at least 13 000 km 2 extending over 340 km from alta verapaz province in guatemala westward to san cristobal ias casas in chiapas mexico the area of intensity vii indicates a moment of 1 10 28 dyne cm while the estimated length of the intensity vii isoseismal indicates a more likely figure of 3 5 10 27 dyne cm or an equivalent magnitude m w of 7 1 2 to 7 3 4 the reported aftershock sequence is compatible with an earthquake in this magnitude range these data demonstrate that the chixoy polochic fault is currently an active part of the caribbean north american plate boundary and is capable of producing very large earthquakes

paleostress inversion a multi parametric geomechanical evaluation of the wallace bott assumptions wallace 1951 and bott 1959 were the first to introduce the idea that the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface however this simplified hypothesis is questionable since fault mechanical interactions may induce slip reorientations earlier numerical geomechanical models confirmed that the slickenlines slip vectors are not necessarily parallel to the maximum resolved shear stress but are consistent with local stress perturbations this leads us to ask as to what extent the wallace and bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data here a geomechanical multi parametric study using a 3d boundary element method covering i fault geometries such as intersected faults or corrugated fault surfaces ii the full range of andersonian state of stress iii fault friction iv fault fluid pressure v half space effect and vi rock properties is performed in order to understand the effect of each parameter on the misfit angle between geomechanical slip vectors and the resolved shear stresses it is shown that significant misfit angles can be found under specific configurations invalidating the wallace and bott assumptions even though fault friction tends to minimize the misfit we therefore conclude that in such cases stress inversions based on fault slip data should be interpreted with care

carbonate cemented stylolites and fractures in the upper jurassic limestones of the eastern iberian range spain a record of palaeofluids composition and thermal history abstract dolomite and calcite cements fill open stylolites fractures and pipe shaped structures related to faulting in tithonian berriasian limestones of the maestrat basin in the iberian range spain due to the grater susceptibility of the dolomitised limestones to brittle fracturing during the alpine tectonism their location and distribution may have important implications for hydrocarbon prospectively within the studied region iberian range three generations of structures were recognised open stylolites extensional fractures and faults based on field observations cross cutting relationships and cement mineralogy petrographic cathodoluminesence and geochemical analyses electron microprobe fluid inclusion oxygen carbon and strontium isotopes of the carbonates helped unravel the origin and evolution of the fluids from which these carbonates have been precipitated these cements occur in the following structures a the first generation nne trending fractures formed during the late eocene miocene compressional stage were filled by calcite cement 18 o vpdb 8 8 and 13 c vpdb 0 8 b the second generation represented by subvertical extensional fractures and the third generation by pipe shaped structures which are considered to be formed during the miocene pliocene extensional stage contain four carbonate cement phases 1 isolated rhombic dolomites 2 saddle dolomite with fluid inclusions characterised by high salinity 21 5 to 23 5 wt eq nacl radiogenic sr enriched 0 70796 to 0 70857 in relation to the marine standard high temperatures t h 110 155 c and low 18 o values vpdb 11 5 to 11 3 3 calcitized saddle and rhombic dolomite with variable 18 o vpdb 12 2 to 6 8 and 13 c vpdb 4 4 to 0 2 and 4 two phases of calcite cements with moderate to high temperatures t h 110 125 c 15 to 19 7 wt eq nacl and 160 198 c 5 5 to 9 5 wt eq nacl low 18 o vpdb values 13 4 to 10 and positive carbon values 13 c vpdb 0 1 to 1 8 the diagenetic fluid is interpreted to be initially hot saline hydrothermal temperature higher than the estimate of the ambient temperature and sr enriched 0 70800 the dolomitisation event was followed by an input of meteoric water which was related to the extensional stage finally late calcite cement precipitated from fluids which had initially moderate salinity and moderate to high temperature and later evolved to a lower temperature and higher salinity fluid both sr enriched these fluids were probably derived from the progressive mixing of late triassic evaporitic brines with descending meteoric water that migrated via fractures during the miocene pliocene alpine extensional stage major vug filling calcite cement is probably late in timing and related to hydrothermal karst associated with the pipe shaped structure c bed parallel stylolites and subvertical extensional fractures containing idiomorphic quartz crystals for which make it difficult to deduce the timing of its precipitation idiomorphic quartz has homogenisation temperatures of 140 180 c and salinities of 13 6 wt eq nacl the high homogenisation temperature of this quartz and its association with dickite suggest precipitation from hydrothermal fluid

favoured states of palaeostress in the earth s crust evidence from fault slip data abstract in a major survey of published palaeostress estimates obtained by the use of fault slip data over 2000 stress tensors from shallow crustal levels have been compiled the results are derived from regions where deformation is dominated by brittle processes and consist of incomplete stress tensors in which the orientations of the principal stresses 1 2 3 are known together with the stress ratio 2 3 1 3 the orientations of the stress axes display a preferred orientation with a tendency for one of the axes to be vertical this accords with anderson s assumed standard state stress configuration near to the earth s free surface although the tendency is strong there are frequent deviations from the arrangement in 25 of cases the steepest stress axis deviates by more than 25 from the vertical in undertaking studies of palaeostress it would be therefore unwise to use methods that assume a priori one vertical stress axis unless previous local results indicate this to be the case normal strike slip and thrust stress arrangements with 1 2 and 3 as the steepest stress respectively occur in the database with frequencies approximately in the ratio 2 2 1 in the compiled results the raw stress ratios which have a theoretical range from 0 to 1 show a lack of very high values and to a lesser extent very low values this suggests that triaxial stress states are more common than axial compression deviatoric stress and axial tension deviatoric stress however this relative abundance of triaxial stress states is considered to be a natural feature of a uniform distribution of stress tensors in addition there is a marked bias in the collated ratios towards values less than 0 5 and this is expressed by an overall mean of 0 39 several possible explanations for the distribution of stress ratios are discussed these explanations are related to the tendency for tectonic deformation to be of plane strain type coupled with volume change and to the fact that gravitational loading makes an important contribution to the state of stress in the crust

seismicity of the iranian plateau and bordering regions abstract seismicity of the iranian plateau and bordering regions during the past 22 1 2 yr from 1 july 1957 through december 1979 is investigated in four successive time intervals by analyzing close to 4000 earthquakes in the region this tectonically active part of the alp himalayan belt is defined by a broadband of diffuse seismicity and contains several mobile belts surrounding small relatively stable blocks the major zones of mobility in decreasing order of activity are hindukush zagros elborz chaman fault system east central iran and the caucasus and eastern turkey the most conspicuous aseismic block is that of western afghanistan but smaller blocks in central iran azarbayejan and the southern caspian sea also show noticeable stability the a and b parameters of the frequency magnitude recurrence curve during successive time intervals in three of the mobile zones which are located in iran show an increasing trend to stabilized levels at the end of the period the annual average of seismic moment rates in zagros during the past 22 1 2 yr requires nearly a two order of magnitude increase to account for the relative plate motion between the arabian and central iran plates

an application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations abstract the slip model of brittle deformation reches 1983 imposes a set of constraints on the angular relationships between faults their striations on slickensides and the type and orientation of the deformation ellipsoid in focal mechanisms of earthquakes these relationships can be used to determine which of the two nodal planes is the fault plane and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity in this approach it is convenient to use pitch dip diagrams de vicente 1988 and p t diagrams angelier and mechler 1977 both developed for fault population analysis in this paper three regions in different plate tectonic situations are analysed using these methods these regions are a zone of intracontinental seismicity in the eastern peruvian andes a zone of continental compressional deformation in the iranian plateau and the fenwei graben of china nodal planes of focal mechanisms published in different papers about seismicity in these regions have been used the following conclusions being reached in the analysed zone of the peruvian andes three regional deformation types appear causing fault movements that range from slightly reverse strike slip to reverse strike slip in addition exclusively at depths in excess of 18 km there is a deformation of an uniaxial shortening type in all of these deformations the horizontal shortening axis is oriented from n87 to n93 in the iranian plateau two horizontal shortening directions appear n151 and n61 even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at n61 in the fenwei graben the ellipsoid of regional deformation has a stretching principal axis at n145 150 and a shortening axis at n55 60 the strike slip movement along faults being as important as the normal one

forty years of paleostress analysis has it attained maturity abstract powerful methods of paleostress analysis from fault slip data have been used since the 1970 s and have demonstrated their capacity for reconstructing stress fields with tectonic significance however in some cases these methods have been applied to samples that do not fulfill basic requirements or developed on the basis of unsound conceptual models most methods of fault slip are based on the wallace bott principle dynamic paradigm while others invoke the necessity of remaining within the alternative kinematic paradigm therefore inferring only finite strain or strain rate axes the scientific community shares neither standard procedures for applying these methods nor standard parameters for assessing the quality of results this indicates that paleostress analysis has not attained maturity it is necessary to see back to the conceptual and methological bases following accurate procedures of data collection using additional field observations and critically assessing the results within the tectonic setting spatial and temporal heterogeneity of stress fields over all scales makes the mean stress tensor obtained from site analysis just an abstract notion though indispensable for understanding lithosphere dynamics

structural analysis of faults related to a heterogeneous stress history reconstruction of a dismembered gold deposit stawell western lachlan fold belt australia in the internationally significant victorian goldfields a complex system of faults dismembers the 5 million ounce magdala gold deposit these faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio the fault geometry is strongly controlled by the pre existing rheology faults have propagated around the flanks of an antiformal basalt dome along earlier ductile cleavages and the margins of porphyry dykes many of the faults do not have andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions much of the faulting is associated with the emplacement of porphyry dykes additional gold mineralisation related to plutonism and late stage deformation post dating the intrusion of the stawell pluton systematic mapping of extension veins associated with faults striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation this successfully predicted the offset direction of the currently mined magdala ore body beneath the studied system of faults the use of extension veins was a critical aspect of the analysis if striations on the fault surfaces had solely been used the offset direction of the new golden gift orebody would not have been correctly ascertained the palaeostress history was delineated via use of compression and tension dihedra stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped the calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing 1 and 3

finite element modelling of tertiary paleostress fields in the eastern part of the tajo basin central spain three subsequent tertiary paleostress fields that are deduced from fault slip data for the eastern part of the tajo basin are analyzed by finite element studies the modelling results show that maximum horizontal stresses shmax are mainly controlled by the geometry of the model limits and the boundary conditions applied the models are used to test two hypotheses on the origin of the altomira range a local stress field responsible for its formation altomira can be modelled successfully by superposition in time and place of two major paleostress fields iberian and guadarrama stress trajectories have been modelled with respect to a homogeneous cover and heterogeneous basement to investigate the role of rheological contrasts between different basement blocks on the orientation of the stress field results of this kind of modelling suggest a mechanical decoupling between the cover and the basement especially for the altomira paleostress field

a stress inversion procedure for polyphase fault slip data sets palaeostress estimation from striated fault data is frequently frustrated by the fact that natural fault data are heterogeneous in the sense that they cannot satisfactorily be explained by reactivation brought about by a single stress tensor in many instances there is clear evidence to show that striation data can record the effects of multiple stress events any attempt to find the tensor which best fits or explains such dynamically mixed data sets risks determining a spurious stress state which is some form of compromise between the real stress states corresponding to different tectonic phases to avoid this problem a strategy is proposed here which involves an initial separation of the raw data into coherent sub sets prior to formal stress inversion this separation is performed by assigning attributes to each fault which describe the fault s compatibility with trial stress tensors using these attributes faults can be grouped into dynamically homogeneous families using the statistical techniques of cluster analysis

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

mesozoic to recent evolution of intraplate stress fields under multiple remote stresses the case of signy island south orkney microcontinent antarctica abstract palaeostresses inferred from approximately 3200 brittle mesostructures measured at 43 sites in signy island show a stress field characterized by compressional strike slip and extensional stress states there is a dominant nw se horizontal compression direction as well as three nnw sse wnw ese and ene wsw secondary 1 horizontal stress directions orientation of 3 shows a main mode trending nw se together with secondary north south and ne sw extensional stress directions the nnw sse horizontal compression is consistent with the stretching shearing direction defined from the fold axis trend this is owing to a subduction related tectonic regime during early to middle jurassic times in middle jurassic early cretaceous times the tectonic regime changes to extensional with north south extensional stress directions the early processes of subduction of the northern sector of the weddell sea oceanic crust below the south orkney microcontinent from the paleocene until the middle miocene is related to the 1 direction wnw ese to nw se the spreading of powell basin took place from the late eocene to the early miocene and is the cause of east west and ne sw extensional processes in the southern south orkney microcontinent finally the nw se 3 direction is related to the current left lateral strike slip movement between the scotia and antarctic plates

palaeostress evolution of the iberian peninsula late carboniferous to present day abstract three main stages can be differentiated in iberia from the upper carboniferous to the present the end of the variscan orogeny late carboniferous the permian early cretaceous rifting and the alpine orogeny late cretaceous to present in this contribution the main features of the stress fields determined in each of these stages are compiled and revised from the analysis of the brittle deformations mainly microfault population stresses are related to the development of the principal structures of each stage in the late variscan fracturing stage sinistral strike slip faults prevailed in a stress field with ne sw subhorizontal compression in the permian to early cretaceous rifting stage the extensional trends varied in each sector during the alpine orogeny primarily the margins of the iberian massif the betic and pyrenee mountain chains were deformed however numerous deformations are also identified in its interior where large mountain chains such as the central system and the iberian chain developed these deformations occurred in a complex stress field where zones of nw se to nne ssw horizontal compression prevailed albeit predominantly extensional zones existed as well at present the stress field in most of iberia has a nw se compressive trend associated with active deformations in the betic cordillera where a continental subduction developed however simultaneously there are zones in which extensional settings predominate such as the internal zones of the betic cordillera and the gulf of valencia region making for quite a complex current stress field

co seismic geomorphic and geologic fold growth associated with the 1978 tabas e golshan earthquake fault in eastern iran we describe the seismicity and late cenozoic deformation associated with a blind thrust fault at tabas e golshan hereafter referred to as tabas eastern iran which generated a devastating mw 7 3 earthquake on the 16th september 1978 measurements from a structural transect through the sardar anticline segment indicate fault propagation folding above a gently 20 eastward dipping blind thrust fault the thrust flattens into a horizontal detachment at a depth of only 2 km tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes with many of the slip surfaces showing fresh striations with a large component of right lateral strike slip a steeply dipping fault zone showing almost pure strike slip is also developed within the forelimb of the fold our field observations are consistent with the source parameters of the 1978 tabas earthquake and additional events in 1979 and 1980 which all involved slip on a shallowly dipping thrust with a significant component of right lateral slip the surface of an alluvial fan which is likely to have been abandoned at 8 10 ka has been folded as it crosses the sardar anticline the age constraints combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth provide an approximate rate of horizontal shortening of 1 5 mm yr shortening at tabas appears to result from transpressional bending at the north end of the nayband strike slip fault a northward continuation of the nayband fault which may be slipping at rates of 2 mm yr is identified along the base of the shotori mountains 10 20 km east of the tabas thrust the range front fault did not move in 1978 and constitutes an additional threat to local populations

morphostructural analysis and cenozoic evolution of elephant island southern scotia arc antarctica elephant island located in the vicinity of the present day active boundary of the south shetland block and the antarctic and scotia plates is a region of particular interest for understanding the past and present geodynamic evolution of the southern scotia arc lineament from different data sources field measured fractures and geomorphological evidences have been analysed in this context the lineaments extracted from aerial photographs 1 624 from a dem 348 and from radarsat 2 satellite data 1 365 indicate four dominant lineament sets with ne sw nw se n s and w e strikes all data sources identified similar lineament families but differences in the frequency distributions and subsequently on the dominant orientations were observed the measurements direct of fractures were obtained from 23 sites in the field at which 278 planes were measured fracture planes indicate main modes trending in the nne ssw and nnw sse directions and a secondary mode in the e w the major trends of the fracture measurements and the lineaments display a good correlation in the e w direction however there is an angular variation in the azimuth values of the nne sse and nnw sse fractures with respect to the n s ne sw and nw se orientations of the lineaments of approximately 20 this trend deviation may be due to the fact that mapped lineaments are composed of small fracture sets that may be related to shear fractures that cannot be distinguished at the aerial photograph or radar satellite data scales submerged sea floor morphological feature orientations match the studied morphostructures on the island and the main tectonic structures in this part of the scotia arc a linkage of the main lineament families to the tectonic stages from the oligocene to the present has been proposed taking into account the information of the orientation and sense of movement of the fractures and stresses in the elephant island region

interaction between perpendicularly trending thrust wedges and its impact on foreland deformation insights from analogue modelling abstract in this work we used analogue models in order to better understand how two orthogonal fold and thrust belts may interact in the foreland and influence the geometry and kinematics of the resulting structures our experimental device has two vertical mobile backstops oriented perpendicular with respect to each other independent and moving coevally with variable or equal velocities there are strong contrasts between our models results and thrust wedges generated using a unique indenter and simple convergence thrust wedge deformation in front of two perpendicular indenters overlying a basal viscous decollement progressed following two major stages i independent growth of each tectonic wedges then ii formation of major oblique and continuous structures that nucleated in the foreland of both indenters in some models the two indenters moving at different velocities favoured the interference and superimposition of structures strain partitioning structure curvature and vertical axis rotation comparison with the friulian venetian basin southern alps and the altomira loranca thrust belt iberian chain central spain supports the hypothesis that foreland deformation can strongly vary if a basal viscous decollement is present during the interaction of two mutually indenters

stress fields associated with the growth of a large shield volcano la palma canary islands 1 measurements of dyke orientations and fault slip data have been taken in 39 stations located in the northern part of the island of la palma canary archipelago these structures affect the materials belonging to the submarine growth stage and to the lower units of the subaerial shield volcano stage four sets of dykes have been distinguished groups i ii iii 1 and iii 2 the two younger dyke groups iii 1 and iii 2 with highly variable strikes are clearly related to the subaerial growth stage most of the measured faults are normal field relationships and the use of stress inversion methods allowed us to separate three different stress tensors tensor 2 is associated with the dykes of group iii 1 given the spatial distribution of the stations with faults of tensor 2 and dykes of its coeval group iii 1 it has been possible to obtain a map of trajectories of the maximum sh and minimum sh compressive stress axes in the horizontal comparison between this map and the results of theoretical models suggests that the stress field active during at least the early stages of growth of the shield volcano in la palma from near 1 7 myr to at least 1 2 myr ago includes a compressive radial volcanic component plus a regional tensile stress of a tectonic origin this second component might be the imprint in the atlantic oceanic lithosphere of the cenozoic convergence between the african and eurasian plates

fault movements and tectonics of eastern iran boundaries of the lut plate summary from 1977 march 21 to 1981 july 28 about 15 earthquakes with ms 6 0 and many earthquakes with ms 4 5 have occurred in iran the upsurge of seismic activity started following the khorqu earthquake of 1977 march 21 ms 7 0 south east of the fars folded series of zagros this shock had a thrust focal mechanism solution indicating the general northward movement of the arabian plate with respect to the iranian landmass it was followed by six major damaging earthquakes in eastern iran the earthquakes are associated with extensive faulting which surrounds the lut plate 1 the zarand earthquake of 1977 december ms 5 8 was associated with about 20 km of fault trace severe mass wasting and about 20 cm of right lateral movement 2 the tabas earthquake of 1978 september ms 7 7 with about 75 km of multiple thrust faulting and a maximum vertical displacement of 35 cm 3 the kurizan earthquake of 1979 november ms 6 0 with more than 17 km of strike slip fault trace and a maximum right lateral displacement of 90 cm and a vertical displacement of 60 cm 4 the koli earthquake of 1979 november ms 7 1 with at least 65 km of fault trace and a maximum left lateral displacement of 255 cm and a vertical displacement of 380 cm 5 golbaf earthquakes of 1981 june ms 6 0 with at least 16 km of observed fault trace and a maximum vertical displacement of 15 cm 6 the chaharfarsang sirch earthquake of 1981 july ms 7 1 with about 70 km of discontinuous fault trace and 20 cm of right lateral motion and 15 cm of vertical motion portable networks of seismographic stations were deployed following each event results of aftershock studies are compared with field observations the observed faults and aftershock zones appear to mark the broad deformational boundaries of the lut plate

influence of measurement errors on stress estimated from single phase fault slip data abstract the influence of measurement errors on estimated stress is investigated in reduced sigma space on the basis of numerous sets of homogeneous fault slip data generated under different extensional stress and then modified by varying measurement errors for the striation pitch for a given measurement error error propagation to a datum vector depends upon the controlling stress and fault orientation with respect to the controlling stress for a given stress ratio both the dispersion of datum vectors i e root mean square cosine and the misfit between estimated and controlling stresses i e similarity and stress difference increase with the increase in measurement error the two measures appear generally smaller at small measurement error than at large error and have a strong tendency to increase with stress ratio for a given measurement error the measurement error has an apparent effect in estimated stress only if it is greater than 10 implying a limited effect on homogeneous fault slip data that is carefully measured at outcrop in which a smaller error can be guaranteed in a majority of cases

the seismicity of kohistan pakistan source studies of the hamran 1972 9 3 darel 1981 9 12 and patan 1974 12 28 earthquakes abstract long period body waves are examined to show that the hamran 1972 9 3 darel 1981 9 12 and patan 1974 12 28 earthquakes in kohistan had focal depths of about 8 10 km all involved high angle reverse faulting thrusting and had seismic moments of about 2 2 to 2 7 1025 dyne cm these shallow depths contrast with the deeper hypocentres found in the hindu kush and northeast karakoram to the north and in hazara to the south the hamran and patan shocks were assigned depths of 45 km by the isc indicating that even well recorded events in this region may have focal depths in error by 30 km

paleostress analysis of the yesilyurt elazig fault zone and its importance for the tectonic evolution east turkey abstract the yesilyurt elazig fault zone yefz located in eastern anatolia is a morphologically distinct ne sw trending left lateral strike slip fault about 150 km long in this study inversion of fault slip data collected from the yefz and tectonic locations of the stratigraphic units are used in order to reveal the tectonic evolution and chronology of faulting from early paleocene to today based on the inversion of about 250 fault slip data four successive deformation phases are defined in the study area since early paleocene the first deformation phase is characterized by n s compression during the early paleocene the second deformation phase is nw se extension related to slab steepening and breakoff in late paleocene early miocene the third deformation phase is a nne ssw compressional one in the middle miocene early pliocene a strike slip phase with nne ssw compression is the fourth and youngest deformation phase in the study area dip slip striations are overprinted by strike slip striations we hence interpret that these faults were reactivated under the later stress field our results are consistent with other published regional studies

the 1978 tabas iran earthquake an interpretation of the strong motion records the 1978 tabas iran earthquake ms 7 4 was registered on 11 accelerograph stations with source distances ranging from 3 to 350 km and corresponding recorded peak accelerations ranging from 0 95 down to 0 01 g respectively at least four subevents are recognized at dayhook the station nearest the epicenter at the southern limit of the fault the rupture propagated to the northwest with a rupture velocity of 2 7 km sec the subevents occurred along the fault as the rupture proceeded curves which describe the distance dependence of peak values of acceleration velocity and band passed acceleration are constructed for this earthquake although attenuation of seismic waves is similar in iran and the western united states the prediction curves based on peak strong motion acceleration data from the western united states systematically underestimate the peak values of the tabas earthquake and the similar 1952 kern county earthquake prediction curves based on peak velocity in the western united states overestimate peak velocity for distances less than about 60 km site effects are recognized at some recording stations the various estimates for stress operating during the earthquake range from 25 to 90 bars the radiated energy of the main event estimated from accelerograms by two methods is about 2 3 1023 ergs larger than the energy calculated by gutenberg and richter energy formula with ms 7 4 each of the four subevents released approximately the same amount of energy corresponding to ms 7 teleseismic energy estimates for this earthquake are one to two orders of magnitude smaller indicating large systematic errors exist in one or both of the energy estimation procedures

reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings abstract in this work we compare the results of anisotropy of magnetic susceptibility ams with palaeostress analysis at local scale the ams data refer to 14 sites from weakly deformed mudrocks lower and middle miocene lacustrine sediments from two areas of the internal part of the ebro foreland basin the bardenas reales and monegros areas nine sites display a magnetic fabric related to a subtle tectonic overprint showing a roughly e w magnetic lineation whereas the rest of sites most of them located at the monegros area show a sedimentary fabric without a well defined magnetic lineation palaeostress data result from fault populations and joint sets exposed in the limestone beds interbedded with the mudrocks at the nearest possible outcrop from the ams site the origin of the magnetic lineation is related to a n s compression coeval with the earliest diagenesis of the studied sediments prior to consolidation the comparison between magnetic fabric and palaeostress data demonstrates the coaxiality between the ams and palaeostress ellipsoids both at local and regional scales thereby validating the reliability of ams as palaeostress indicator in compressive settings

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

analysis of solution lineations in pebbles kinematical vs dynamical approaches abstract solution lineations in conglomerates resulting from indentation of non soluble grains of the matrix into the surface of soluble pebbles make up a morphological and genetic continuum with gradual transition between orthogonal stylolites oblique slickolites and parallel striations the distributions of incidence angles of matrix grains have been analyzed in individual pebbles in order to discern their kinematical or dynamical meaning as a general rule they fit theoretical models of flow trajectories determined by the bulk strain kinematical hypothesis in contrast they are not consistent with dynamical hypotheses based upon relationships with stress vectors in particular they do not fit the model of frictional sliding which would give rise to a sharp discontinuity between slickolites parallel to the maximum principal stress 1 and true striations parallel to the resolved shear stress therefore solution lineations all around a pebble cannot be considered as an analogue of multiple fault slip data and they should not be generally analysed by methods of stress inversion based upon bott s principle under certain conditions high pebble solubility active pressure solution processes able to accommodate the strain rate earlier cementation the solution lineations tend to be parallel to each other and to the maximum shortening compression axis they therefore assume a double kinematical and dynanical meaning and the deformation involves maximum volume reduction

a geometric test for possible andersonian stress using striated faults a graphical compilation of the rake guides of fault striations of known sense facilitates a simple test for their compatibility with andersonian stress conditions a compilation of negative double strike angles on tracing paper is superimposed on the rake guide plot if the two register the candidate generating stress is given by the relative position and orientation of the two plots

calcite twins from microveins as indicators of deformation history abstract in order to investigate whether the evidence of the deformation history is preserved in the microvein calcite twinning microstructures several families of calcite veins have been studied from weakly deformed mesozoic limestones of the western iberian chain eastern iberian peninsula the maximum shortening directions established with this method correspond to the previously established paleostress evolution of this chain the reconstructed stress regimes evolve from predominantly compressional four compressional episodes with the maximum shortening direction oriented successively nw se n s ne sw and e w to extensional the established orientations of maximum shortening axes are consistent with the directions of paleostrains that have been independently deduced from stylolites and fault striations in the same region moreover the results clearly show that the e twins record some paleostress orientations that are not registered in the studied area by fault population analysis in this sense the e twins record older paleostress orientations than the small faults probably because the faults were reactivated several times and the twins were not modified since their formation the calculated values of twin strains 1 differential stress magnitudes 62 mpa and the existence of type i twins formed below 200 c confirm that this region was slightly deformed under low p t conditions

palaeostresses associated with thrust sheet emplacement and related folding in the southern central pyrenees huesca spain brittle mesostructures have been studied in the detached mesozoic and lower tertiary cover rocks of the south central pyrenees huesca province spain in order to reconstruct the state of stress within the thrust sheets during tertiary compression the structure of the area studied consists of nw se to e w striking south verging thrust sheets located west of the south pyrenean central unit the palaeostresses obtained show dominant n s to ene wsw compression directions for stress tensors with horizontal 1 axes and e w to nw se extension directions for stress tensors with vertical l axes the values of the stress ellipsoid ratios for stress tensors with horizontal 1 are predominantly between 0 and 0 5 indicating uniaxial compression to wrench regime regarding their relationship with macro structures the palaeostresses show transport parallel compression in most sites although thrust parallel compression is also found there are no differences between palaeostresses found in the hanging walls and the footwalls of thrusts stress tensors with vertical 1 do not show systematic relations with macro structures but transport parallel extension and thrust parallel extension are found in some sites the analysis of palaeostresses in areas of fold and thrust belts shows a high degree of complexity and the results are not easy to interpret the continuous variation from n s to ene wsw in the orientation of the compression axes is here interpreted in two ways 1 as the result of the clockwise rotation of the mesozoic and tertiary cover which was pushed in the east by the south pyrenean central unit with a constant n to nne regional compression and 2 due to ne sw compression resulting from a sw directed local transport of minor thrusts palaeostress analysis can therefore reveal important information about thrust kinematics at high structural levels where ductile deformation is absent

comparison of long period surface wave amplitude and phase anomalies for two models of global lateral heterogeneity surface wave raytracing calculations for periods of 150 to 250 s were performed using the two models of global phase velocity heterogeneity produced by harvard and cit pro viding a comparison of predicted amplitude and phase anomalies theoretical r3 r and g3 g amplitude asymmetries resulting from focussing and defocussing have similar azimuthal distribu tions of the more pronounced anomalies for the two structures the azimuthal patterns of long period rayleigh and love wave phase anomalies about a given source region also resemble one another for the two models travel time errors as large as 12 s are accumulated for 200 s period rarrivals by neglecting the raypath deviations from great circles such errors may be significant when inverting for earth structure or when applying corrections for lateral heterogeneity in source studies

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

local stress fields and intraplate deformation of iberia variations in spatial and temporal interplay of regional stress sources tertiary to present deformation in the interior of the iberian peninsula reflects spatial and temporal variations of the activity of the plate boundaries local deformation patterns observed in many of the numerous intraplate tertiary basins and their borders such as the madrid basin and the sierra de altomira are at first sight incompatible with the regional stress field under which they were formed we demonstrate however that they can be explained as the effect of several stress fields that acted on the iberian peninsula from earliest tertiary onward data on local deformation can constrain both magnitudes and directions of forces acting on the plate boundaries of iberia enabling us to estimate the relative importance of the different plate driving and deforming mechanisms providing further constraints on the tectonic evolution of iberia

source geometry and mechanism of 1978 tabas iran earthquake from well located aftershocks abstract aftershocks of the september 16 1978 tabas earthquake located from close in observations made during a four week fielding of temporary stations have been analyzed for the purpose of delineating detailed source geometry of the 1978 earthquake spatial distribution of aftershocks and their composite focal mechanism suggest that the geometry of faulting is far from planar aftershocks define two prominent alignment the southern alignment strikes e w to wnw ese whereas the northern alignment strikes in a n s to nnw sse direction with an abrupt change of nearly 55 60 degrees near 33 4 n latitude both field observations of surface faulting pattern and systematic variation of principal directions of stress axes computed from aftershock focal mechanisms are consistent with the upthrusting and imbrication of a wedge shaped crustal block with the wedge angle of about 120 degrees both geological and seismological evidence suggest that the deformed zone is truncated at the southern edge by preexisting e w fault structures new observations may provide a partial answer to the unexplained farfield asymmetry of the long period rayleigh wave radiation pattern recently observed for the mainshock across ida network

a stress inversion method requiring only fault slip sense in numerous cases the use of standard methods of palaeostress inversion from fault slip data is prevented by the absence of structures that indicate the direction of slip on the fault in such instances however the offset of planar markers such as beds matched in the footwall and hanging wall often allows the sense of dip separation normal or reverse to be observed where such displaced markers are horizontal or at least where their cut off lines are horizontal it can be readily demonstrated that the sense of the dip slip component of net slip is the same as the sense of dip separation ways are explored for characterizing the stress tensor from data consisting of the orientations of a collection of faults each with known dip slip sense normal or reverse on the basis of properties of the stress quadric the relationship between dip slip sense fault plane orientation and the stress tensor is discovered this relationship is used as the basis of a grid search method of stress inversion in which a large number of stress tensors differing in orientation and shape are systematically considered those tensors that successfully explain the senses of offset on the observed faults are considered potential solutions to the palaeostress problem examples of different applications of the new stress inversion method are described including the analysis of fault data from boreholes these examples illustrate the way the new method potentially broadens the scope of palaeostress analysis

understanding the mesozoic kinematic evolution in the cameros basin iberian range ne spain from magnetic subfabrics and mesostructures abstract analysis of anisotropy of magnetic susceptibility ams and brittle mesostructures hydroplastic synsedimentary faults and tension gashes is applied in this study in order to characterize the mesozoic tectonic events in the cameros basin nw iberian range formed between tithonian and albian times low field ams at room and low temperature lf ams at rt and lf ams at lt respectively together with high field ams hf ams measurements allow separating ferro and paramagnetic fabrics the combination of lf ams at lt and hf ams torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the ams magnetic fabric results combined with the analyses of synsedimentary faults indicate a nw se extension direction during aptian and probably barremian times this extension direction is perpendicular to the main extension direction ne sw prevailing during early and late stages of basin evolution it is also consistent with extension direction deduced from large scale bending folds and tension gashes developed after partial lithification cleavage development during albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression

stress field and thrust kinematics a model for the tectonic inversion of the cameros massif spain abstract alpine tectonics of the cameros thrust has been studied from several points of view geometry surface and sub surface structure three dimensional outline of the thrust kinematics transport direction and dynamics characterization of the regional compressional stress field all three aspects are related by means of a model based on bott s equation under a n to nne horizontal compression with an average stress ratio r z x y x 0 2 the eastern ramp of the cameros thrust oriented 155 30 w underwent a strike slip right lateral movement with a small reverse component this gave rise to rigid displacement of the cameros block towards the nnw and produced a guided movement on the western frontal ramp oriented 060 13 s

tectonic regime and slip orientation of reactivated faults the slip orientation of reactivated faults is predicted by assuming that one principal stress direction is vertical that faulting occurs on pre existing planes of weakness with a slip direction along the applied shear stress and by extending the original concept of tectonic regime so as to include the stress tensor aspect ratio the predicted rake of slip is then analysed so as to reveal the respective influence of the fault plane orientation strike and dip and of the tectonic regime the analysis of this direct problem allows one to describe the geometry of the slip directions within any given tectonic regime its evolution as the tectonic regime varies and the constraints provided by slip direction data on the tectonic regime however the question of what fault orientations would satisfy a failure criterion and thus be preferentially reactivated is not addressed in any tectonic regime pure strike slip occurs on vertical planes and only on them except in wrench regimes where it also occurs on all fault planes along four strikes that are determined by the tectonic regime pure dip slip occurs on planes striking along one of the horizontal principal stress directions and only on them except in radial compression and radial extension regimes where it occurs on all planes except vertical ones at fixed strike but varying dip the slip direction is closest to dip slip for zero dip and pure strike slip for vertical dip at fixed dip and varying strike the slip direction is pure dip slip for strikes along one of the horizontal principal stress directions and closest to strike slip for four strikes that are determined by the tectonic regime the horizontal principal stress directions separate four quadrants two where the slip direction is dextral and two where it is sinistral the slip directions of all fault planes of the same dip can be easily deduced from those of subhorizontal planes and can be constructed geometrically as the tectonic regime varies continuously from radial compression to radial extension the domain of fault plane orientations where steep reverse slip directions are possible shrinks and disappears when the extensional regime is entered conversely the domain where steep normal slip directions are possible appears when the wrench regime is entered and grows thereafter the strike slip domain first grows towards shallow dip until the wrench regime is entered and then shrinks after the extensional regime is entered whereas in the model of failure in isotropic rocks dip slip or strike slip data fully determine the original tectonic regime in the model of failure on pre existing planes of weakness constraints on the extended tectonic regime require additional information the sense of shallow slip direction data which are to be found on steeply dipping planes constrains the horizontal principal stress directions and the steepness of slip direction data on shallow dipping planes striking away from the principal stress directions constraints the extended tectonic regime in the case of wrench regimes an extra constraint arises from the fact that the extended tectonic regime also controls the strike at which the dip component of slip changes from normal to reverse these constraints can be quantified by superposing a simple plot of the data rake versus strike with an abacus that is called breddin s graph for tectonic regimes while this graph does not replace inverse techniques it may help in detecting tectonic phases in complex data sets without using this graph or resorting to inverse techniques the transition from wrench to either compressional or extensional regime is difficult to infer from data that display a limited range of fault plane strikes and the same sense of dip component of the slip direction

palaeogene evolution of deformation in the northeastern iberian peninsula during the palaeogene alpine compression three units were differentiated in the studied area south of the ebro basin the catalan coastal range dominated by ne sw major basement faults with a sinistral movement the iberian range where the important basement faults have a nw se direction and a reverse movement often with a dextral component the linking zone between these two ranges where an e w dominant structural direction is marked by an array of folds and thrusts with a northward vergence in the mesozoic cover from the analysis of both these major structures and the small scale structures it can be deduced that the compression in the studied area has evolved from a nw se direction lower middle eocene to a n s direction and to a ne sw one uppermost oligocene the major structures are due to the n s compression later the stress field progressively changed to a distensive regime we suggest a relation between the compressive phases and the displacement direction of the iberian plate with reference to the european plate

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

stress deflection in a tectonic compressional field a model for the northwestern iberian chain spain by means of fault population analysis using the right dihedra y r diagram and etchecopar s methods the states of stress in a number of sites of the cameras massif northwestern iberian chain spain and nearby areas of the ebro basin have been determined these states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the alpine compression oligocene lower miocene the three main compression directions obtained nw se nne ne sw can be explained by a single compression field with horizontal 1 trending nne and vertical 2 whose primary trajectories are deflected due to 1 large scale strike slip faults or lateral ramps of thrusts or 2 frontal ramps of thrusts oblique to the main compressive axis the reliability of this scheme has been checked by means of a two dimensional mathematical model based upon the finite element method and applied on a sheet whose rheological properties are similar to those of a standard limestone this model simulates the main features of the regional stress field and it was constructed after determining the average parameters of the possible external or primary field and simulating the major faults that could deflect the stress trajectories

cosmic impact versus terrestrial origin of the azuara structure spain a review the azuara structure is the largest one proposed so far in spain as possibly related to a cosmic impact event a review of the evidence set forward in favor of and against its cosmic origin indicates that the discussion is not yet finished some megascopic features inverted stratigraphy megabreccia negative gravity anomalies and shock metamorphic effects planar deformational features have been described in relation with the structure although their real significance has been questioned and is still being debated comparison with other similar sized verified impacts suggests that unequivocal impactogenic features are yet to be found before the azuara structure can be related to a cosmic impact until then the azuara structure should be considered as an unverified impact structure and should not be included in global comprehensive maps of terrestrial impact structures

comparison of source scaling relations of eastern and western north american earthquakes source scaling relations have been obtained for earthquakes in eastern north america and other continental interiors and compared with a relation obtained for earthquakes in western north america the scaling relation for eastern north american earthquakes was constructed from measurements of seismic moment and source duration obtained by the waveform modeling of seismic body waves the events used include nine events of m blg magnitude 4 7 to 5 8 that occurred after 1960 and four earlier events with magnitudes between 5 5 and 6 6 the scaling relation for events in other continental interiors was used for comparative purposes and to provide constraints for large magnitudes detailed analysis of the uncertainties in the scaling relations has allowed the resolution of two important issues concerning the source scaling of earthquakes in eastern north america first the source characteristics of earthquakes in eastern north america and other continental interiors are consistent with constant stress drop scaling and are inconsistent with nonconstant scaling models such as that of nuttli 1983 second the stress drops of earthquakes in eastern north america and other continental interiors are not significantly different from those of earthquakes in western north america and have median values of approximately 100 bars the source parameters of earthquakes in eastern north america are consistent with a single constant stress drop scaling relation whereas the source parameters of earthquakes in western north america are much more variable and show significant departures from an average scaling relation in which stress drop decreases slightly with seismic moment

fold trends and stress deviation in the alicante fold belt southeastern spain abstract palaeostress orientations derived from analysis of stylolites and small scale faults can be correlated with plate movements however stress deflections up to the kilometre scale occur at large scale faults the regional fold trend in the eastern pre betic alicante region is east northeast which is characteristic of the betic cordilleras in the southern part of the alicante region an anomalous n s fold trend dominates biostratigraphic data and regional structural analysis show no clear difference in the age of the two fold trends although dome and basin structures suggest overprinting the n s folds are interpreted as en echelon folds in a right lateral wrench zone palaeostress analysis supports this interpretation the e w compression which caused the n s folds is a local fault related deviation of the regional betic nw se compression

large scale distributed deformation controlled topography along the western africa eurasia limit tectonic constraints in the interior of the iberian peninsula the main geomorphic features mountain ranges and basins seems to be arranged in several directions whose origin can be related to the n s plate convergence which occurred along the cantabro pyrenean border during the eocene lower miocene time span the iberian variscan basement accommodated part of this plate convergence in three e w trending crustal folds as well as in the reactivation of two left lateral nne ssw strike slip belts the rest of the convergence was assumed through the inversion of the iberian mesozoic rift to form the iberian chain this inversion gave rise to a process of oblique crustal shortening involving the development of two right lateral nw se shear zones crustal folds strike slip corridors and one inverted rift compose a tectonic mechanism of pure shear in which the shortening is solved vertically by the development of mountain ranges and related sedimentary basins this model can be expanded to nw africa up to the atlasic system where n s plate convergence seems also to be accommodated in several basement uplifts anti atlas and meseta and through the inversion of two mesozoic rifts high and middle atlas in this tectonic situation the microcontinent iberia used to be firmly attached to africa during most part of the tertiary in such a way that n s compressive stresses could be transmitted from the collision of the pyrenean boundary this tectonic scenario implies that most part of the tertiary eurasia africa convergence was not accommodated along the iberia africa interface but in the pyrenean plateboundary a broad zone of distributed deformation resulted from the transmission of compressive stresses from the collision at the pyrenean border this distributed intraplate deformation can be easily related to the topographic pattern of the africa eurasia interface at the longitude of the iberian peninsula shortening in the rif betics external zones and their related topographic features must be conversely related to more local driven mechanisms the westward displacement of the exotic alboran domain other than n s convergence the remaining nnw sse to nw se latest miocene up to present convergence is also being accommodated in this zone straddling iberia and morocco at the same time as a new ill defined plate boundary that is being developed between europe and africa

analogue modelling of a reactivated basement controlled strike slip zone sierra de albarracin spain application of sandbox modelling to polyphase deformation this paper presents the results of an analogue modelling study on the reactivation of riedel shears generated by basement induced sinistral strike slip faulting it is based on a natural example in the sierra de albarracin iberian range spain the area has a polyphase deformation history defined by the variscan and alpine orogenies late variscan deformation was concentrated in a wide nw se shear zone with accompanying kilometre scale e w riedel shears which divided the palaeozoic basement into large fault blocks alpine reactivation resulted in differential movements on the riedel shears as evidenced by a nw se chain of palaeozoic inliers surrounded by a mesozoic cover that generally shows minor deformations except near the e w riedel shears where strata locally appear in near vertical to overturned position sandbox analogue modelling was applied to improve insight into the structural history it focused on the kinematics of spontaneously developed en echelon riedel shears reactivated in a rotated stress field sand with a controlled added strength was used to form riedel shears in a first deformation phase to act as weak zones for a second phase the modelling showed that in the first deformation phase large pop up structures developed between the riedel shears in a basement induced sinistral strike slip zone later reactivation in the n060 e and n135 e shortening directions was taken up respectively by sinistral reverse and dextral reverse shear along the pre existing riedel shears but only if the sand on one side of the fault zone was allowed to move freely along the other scissor faulting along the riedel shears with their complex 3d geometry increased the height of the up squeezed blocks for experiments with fixed boundaries and no oil water emulsion layer between the base plate and sand pack thrusting at the backstop occurred rather than reactivation of the riedel shears this approach provided robust insights on the 4d development of the sierra de albarracin area the geological society of london 2006

source parameters of 1978 tabas and 1979 qainat iran earthquakes from long period surface waves the teleseismic records of three strong earthquakes which inflicted heavy damage and loss of many thousands of lives in northeastern iran during 1978 and 1979 are analyzed the observed r 2 and r 3 phases of the 16 september 1978 tabas earthquake across the ida network are inverted to derive the gross source parameters of the main shock the synthetic seismograms computed for the inferred source model are compared with observations the agreement is generally good at most of the stations however unexpected asymmetrical observations at esk cmo kip and pfo stations cannot be explained by directivity alone possible factors responsible for these observations may include regional heterogeneities or the departure of fault plane from simple planar geometry the resulting focal mechanism agrees with nearly pure reverse mechanism with a nw se strike the seismic moment estimate for this earthquake is about 1 5 10 27 dyne cm m w 7 4 the seismic moment of the same earthquake obtained from long period p wave observations at six wwssn stations is smaller than the above estimate by a factor of 2 starting with the geologic field data of the qainat earthquakes of 14 and 27 november 1979 the r 2 and g 2 phases on the ultra long period instruments at pasadena and berkeley are analyzed to estimate their seismic source moments the estimated values are 1 0 10 26 dyne cm for the 14 november and 8 0 10 26 dyne cm for the 27 november 1979 events their corresponding moment magnitudes are 6 6 and 7 2 respectively

deflection of a compressional stress field by large scale basement faults a case study from the tertiary almazan basin spain abstract palaeostresses deduced from brittle mesostructures more than 50 sites and 1500 structures measured in the mesozoic and tertiary rocks of the almazan basin located in the northwestern iberian chain indicate a regional nne sse directed compression that lasted during all the tertiary shortening stage the stress regime corresponding to this compression ranges from thrust to wrench n s directed compression was the responsible for most macrostructures folds and thrusts forming a s vergent system with ne sw to e w and nw se orientations and mesostructures mainly faults stylolytes and impressed pebbles we propose that the stress trajectories are deflected following anderson s model in the western part of the almazan basin by the ne sw striking left lateral strike slip soria fault this basement fault was inherited from the late variscan fracturing stage and controlled the deposition in the mesozoic basins as well as the location of tertiary compressional macrostructures in the eastern border of the almazan basin palaeostress trajectories were deflected both by right lateral strike slip faults and nw se trending thrusts near which the regional n s directed compression becomes perpendicular to them three minor compression directions have also been recorded in several sites ne sw e w and nw se the ne sw directed compression is interpreted as a deviation of the main n s directed compression at a regional scale whereas the nw se directed stress field corresponds to the regional field responsible for the main tertiary shortening stage in the spanish central system the age of compression ranges from eocene to miocene

inferring stress from faulting from early concepts to inverse methods abstract we review the evolution of concepts on and methods of estimating the state of stress from fault movements theories of failure in isotropic materials suggested a simple geometrical construction of optimal principal stress directions from a fault plane and its associated slip these optimal directions align shear stress and slip directions and maximize the difference between shear stress and frictional resistance on the fault plane optimal stress directions for calcite twinning are obtained by a similar construction with the difference that they maximize shear stress force representation of seismic sources independently introduced pressure p and tension t axes at positions that also maximize shear stress on both nodal planes frictional slip theory and the constraint that slip and shear stress directions be parallel allowed to address reactivation of pre existing faults this suggested that stress could also be inverted from reactivated fault and slip data or earthquake focal mechanisms early methods relied on geometrical constructions as a substitute for calculations whereas later methods relied on software as these calculations became tractable with the help of computers similar methods were developed for the inversion of stress from crystal twin gliding with non optimal geometry with a different criterion that relies on a threshold of the component of shear stress along the gliding line even though these methods seek a common stress tensor compatible with fault and slip data their main use is to separate polyphase data into homogeneous subsets and help deciphering complex tectonic histories fault and slip data can also be analyzed to constrain the strain rather than the stress tensor in most cases this involves a summation and yields an average strain for the considered rock volume stress inversion thus appears better suited for differentiating heterogeneous data whereas strain analysis appears better suited for homogenizing them

estimation of the potential type of faulting in relation to the stress field the north aegean case study the relation between the angle of slip during fault reactivation and the applied stress field is investigated through a plot of the deviatoric stress ratio r v h h h vs the fault strike direction measured from h assuming that one of the principal stresses is vertical this approach is applied to the north aegean trough fault zone on the basis of neotectonic data presented previously by other authors and conclusions are drawn as to the potential type of faulting along the trough since late miocene the same reasoning is also applied to earthquake focal mechanisms of the northern aegean region

stress ratio determinations from striated faults a spherical plot for cases of near vertical principal stress for stress regimes with one principal axis z vertical the stress ratio is best represented by an angular function f arctan 3 z x1 x2 z 3 x1 x2 or if the larger of the horizontal principal stresses x1 and x2 is designated y by f arctan 3 z x y z 3 y x traditional palaeostress regimes normal wrench reverse represent equal angular sectors of f or f and may be subdivided on a spherical projection of f against 2y where y is the bearing of maximum horizontal stress the locus of all combinations of stress orientation and stress ratio capable of generating one datum known slip direction on a known fault plane is a great circle since tanf cos 2y 2s b 3 cosb where tanb tan cosd and s d and are the fault strike dip and striation pitch respectively stereographic construction is simple using pencil and paper and gives a visual appreciation of the definition of palaeostress states which could have generated the fault motions it facilitates better than previous methods 1 the identification of radially symmetrical stress states 2 recognition of suspect or incompatible data and 3 delimiting the palaeostress state according to sense of shear where this is known great circle and pole representations are suited to different purposes and data types for which examples are given

late cenozoic stress field and fracturing in the iberian chain and ebro basin spain detailed study of mesoscopic faults and joints affecting neogene and pleistocene deposits in the iberian chain and the ebro basin involving statistical study of fracture directions and palaeostress analysis from striated fault planes together with analysis of macrostructures suggest a complex late cenozoic stress field in which both primary and secondary stress systems are superposed 1 regional n s compression was caused by convergence between the iberian and african plates 2 multidirectional extension was caused by crustal doming in the iberian chain 3 stress trajectories were deflected near major faults 4 small scale swapping of the horizontal tension axes 2 and 3 was caused by development of fractures at right angles to either the primary or the deflected 3 axis

separation of polyphase fault slip data an objective function algorithm based on hard division the objective function algorithm ofa on the basis of hard division is presented in the paper to separate polyphase fault slip data the separation is made by detection of linear structures existing in the data in fry s 1999 sigma space different from other exhaustive search methods the ofa is direct and robust in theory without any arbitrary assumptions polyphase fault slip data are simulated under prescribed tensors in order to validate the method the results show its efficiency in stress estimation the accuracy of stress estimation is controlled by random errors in the orientation of fault striations and by similarity between prescribed stress vectors related to different tectonic phases the similarity between controlling stress vectors has an obvious effect on the estimation either when random errors are sufficiently large or when some similarity coefficients between the vectors are large enough the accuracy of stress inversion tends to decrease as the range in errors increases the ofa makes a very good approach to recognition of similar controlling stress vectors from polyphase fault slip data which are often associated with spatial and temporal variation of the tectonic stress field in a region thus critical to understanding of the formation of geological structures

superposed buckle folding in the eastern iberian chain spain abstract the aliaga area eastern iberian chain shows large scale examples of buckle superposition developed during tertiary folding in most cases ene trending folds overprint earlier nnw sse trending ones the resulting structures are mapped analysed and genetically classified by comparison with analogue models described by several authors the following types are found standard type 1 1a dome and basin structure 1b unequal wavelength overprinted folds modified type 1 1c t shaped joined folds 1d t shaped abutting folds 1e l shaped folds 1f snake like folds standard type 2 2a non cylindrical buckling of earlier axial surfaces involving hinge replacement different superposed sets of flexural slip striations record successive folding episodes in snake like folds and hinge replacement in the case of type 2a superpositions types 1 and 2 apparently develop where the earlier folds have interlimb angles over and below 90 respectively which fits the results of analogue modelling and theoretical analysis by previous authors types 1b and 1d are associated with higher w1 w2 wavelength ratios than types 1a and 1c other controlling factors are viscosity contrast and erosion processes specifically erosion of competent limestone beds in the hinge zone of a nnw sse trending anticline allowed the near vertical eastern limb to be refolded into snake like folds

surface expression of thrust faulting in eastern iran source parameters and surface deformation of the 1978 tabas and 1968 ferdows earthquake sequences summary previously unrecognised thrust faults in eastern iran were responsible for a destructive earthquake at tabas 1978 september 16 which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range front analysis of long period body wave seismograms shows a simple rupture on a gently dipping 16 thrust with a slight right lateral component this is compatible with the locally recorded aftershock distribution body wave analysis of two later smaller events show similar source orientations several indicators of long term active folding at tabas can be recognised in the geomorphology and surface ruptures from 1978 are consistent with co seismic fold growth drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds body wave seismograms for two earthquakes near ferdows 150 km east of tabas in september 1968 also show thrust faulting at depths of 10 km again the surface geomorphology indicates a region of folding above an eastward dipping thrust fault which is 10 km west of a fault bounded range front in both tabas and ferdows the active faulting appears to show quaternary migration away from the range front possibly in response to stresses produced by the elevated topography the identification of zones of active fault related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics we conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology had we known what to look for at that time