stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

seismotectonics of the coast ranges in the vicinity of lake berryessa northern california tectonic deformation as manifested by seismicity and active faulting in the coast ranges in the vicinity of lake berryessa appears to be concentrated along the green valley cedar roughs fault trend and to a lesser degree the coast ranges sierran block crsb boundary zone the green valley cedar roughs fault trend appears to be a complex discontinuous or segmented shear zone consisting of multiple faults geomorphic evidence of quaternary displacement exists along many portions of the fault trend with holocene features and evidence for fault creep occurring along the green valley fault in contrast no historic earthquakes greater than ml 5 are known to have occurred along the fault trend earthquakes relocated in this study from 1970 to 1988 occur in the vicinity of mapped faults however the seismicity is discontinuous with a strong tendency to occur in clusters also superimposed on the general north northwest fault trend are north south epicentral trends not associated with any mapped faults focal mechanisms suggest predominantly right lateral strike slip faulting on both north and north northwest trending planes the existence of reverse and normal focal mechanisms also attests to the complex faulting occurring along the green valley cedar roughs trend in general the geologic and seismogenic characteristics are consistent with the hypothesis that the green valley cedar roughs fault trend represents young faulting possibly related to a newly developing plate boundary east of the san andreas fault earthquakes are confined to the top 12 km of the upper crust seismicity within the crsb boundary zone since 1970 has been generally confined to a sequence in 1978 near madison with a mainshock of ml 4 2 thrust reverse faulting appears to be the source of this activity consistent with the seismogenic processes observed along other segments of the boundary zone only one earthquake has occurred since 1970 in the epicentral area of the 1892 ml 6 vacaville winters earthquake the state of stress in the lake berryessa region as exhibited by focal mechanisms appears to be characterized by ne directed tectonic compression as observed elsewhere away from the san andreas fault system in northern and central california

three dimensional normal faulting models of the valles marineris mars and geodynamic implications inversion of martian topography using a three dimensional boundary element model permits revised estimates of fault dip angle depth of faulting paleogeothermal gradient and extensional strain in the valles marineris region the major normal faults dip at 40 55 to depths of 60 75 km with comparatively small footwall uplifts 500 m for throws to 10 km implying that paleogeothermal gradients during faulting were 10 k km 1 or less assuming relatively rapid strain rates appropriate to the extending martian crust accumulation of dip slip offsets along the main normal faults likely was associated with anticlinal flexure and deformation of preexisting strata within the troughs the magnitudes of the predicted vertical and horizontal displacements outside the troughs as well as extensional strains are spatially variable depending on both cross and along strike position relative to the troughs the predicted displacements and strains attain maximum values at the troughs and decay to background values at cross strike distances of 250 km corresponding to 3 4 times the depth of faulting with an average province wide strain of 4 15 by implication the aggregate strain field surrounding tharsis is a highly variable spatial and temporal composite of the inhomogeneous strain fields each associated and scaling in size with an individual graben from the smallest structures through the largest valles marineris troughs

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

corrections to heat flow and energetics of the san andreas fault zone and some additional comments on the relation between fault friction and observed heat flow

the kinematic paradox of the san andreas fault the pacific north america plate boundary along the san andreas fault system is notoriously a right lateral transpressive margin where both almost pure thrust and strike slip tectonics take place the pacific plate travels wnw forming an angle of about 25 with the boundary since the pacific is moving wnw faster than north america right lateral transtension should result along the san andreas system north america in turn travels westward obliquely to the boundary and a left lateral transpressive component would be expected along the same margin therefore the right lateral transpression of the san andreas system can be partitioned into i a sinistral transpression along the southwestern margin of the north america plate obliquely overriding ii a faster right lateral transtension occurring along the transfer margin of the pacific plate between the east pacific rise in the california gulf and the gorda ridge to the north west this is due to the oblique trend of the pacific and north america plate margins with respect to their motion in a absolute reference frame the geodynamics of california is marked by a unique setting in which there is a special subduction where in contrast with classic subduction zones the footwall of the subduction plane is obliquely diverging from the hanging wall in an e w section while it is converging at slower rates in a ne sw direction the extensional e w component is absorbed into the basin and range rifting whereas the compressive ne sw component is mainly expressed in the coast ranges and california offshore the compression perpendicular to the san andreas is then not intrinsic in the strike slip movement but it is rather an independent tectonic factor therefore the san andreas system cannot be considered as an archetype of a pure strike slip fault

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

tectonics climate and mountain topography 1 by regressing simple independent variables that describe climate and tectonic processes against measures of topography and relief of 69 mountain ranges worldwide we quantify the relative importance of these processes in shaping observed landscapes climate variables include latitude as a surrogate for mean annual temperature and insolation but most importantly for the likelihood of glaciation and mean annual precipitation to quantify tectonics we use shortening rates across each range as a measure of topography we use mean and maximum elevations and relief calculated over different length scales we show that the combination of climate negative correlation and tectonics positive correlation explain substantial fractions 25 but 50 of mean and maximum elevations of mountain ranges but that shortening rates account for smaller portions 25 of the variance in most measures of topography and relief i e with low correlations and large scatter relief is insensitive to mean annual precipitation but does depend on latitude especially for relief calculated over small 1 km length scales which we infer to reflect the importance of glacial erosion larger scale averaged over length scales of 10 km relief however correlates positively with tectonic shortening rate moreover the ratio between small scale and large scale relief as well as the relative relief the relief normalized by the mean elevation of the region varies most strongly with latitude strong positive correlation therefore the location of a mountain range on earth with its corresponding climatic conditions not just tectonic forcing appears to be a key factor in determining its shape and size in any case the combination of tectonics and climate as quantified here can account for approximately half of the variance in these measures of topography the failure of present day shortening rates to account for more than 25 of most measures of relief raises the question is active tectonics overrated in attempts to account for present day relief and exhumation rates of high terrain

state of stress and crustal deformation along weak transform faults the state of stress and style of crustal deformation along weak transform plate boundaries is discussed in the context of available data and simple analytical models appreciable evidence indicates that while the frictional strength of the upper crust is high in general accordance with simple faulting theory and laboratory derived coefficients of friction the frictional resistance to motion along transform plate boundaries is extremely low these conditions require that horizontal principal stresses must be oriented approximately parallel and perpendicular to the transform fault zone to minimize the shear stresses acting parallel to the transform along plate margins that must accommodate relative convergence transpressive margins a pattern of near fault normal compression and fault normal crustal shortening is expected along divergent plate margins transtensional margins extension is expected to occur perpendicular to the transform as the direction of minimum horizontal principal stress is expected to be nearly perpendicular to it these patterns of stress and deformation can be observed along a number of transform faults around the world

length displacement scaling and fault growth abstract following an earthquake in a fault zone commonly the co seismic rupture length and the slip are measured similarly in a structural analysis of major faults the total fault length and displacement are measured when possible it is well known that typical rupture length slip ratios are generally orders of magnitude larger than typical fault length displacement ratios so far however most of the measured co seismic ruptures and faults have been from different areas and commonly hosted by rocks of widely different mechanical properties which have strong effects on these ratios here we present new results on length displacement ratios from 7 fault zones in holocene lava flows on the flanks of the volcano etna italy as well as 10 co seismic rupture length slips and compare them with fault data from iceland the displacement and slip data from etna are mostly from the same fault zones and hosted by rocks with largely the same mechanical properties for the co seismic ruptures the average length is 3657 m the average slip 0 31 m and the average length slip ratio 19 595 for the faults the average length is 6341 m the average displacement 73 m and the average length displacement ratio 130 thus the average rupture slip ratio is about 150 times larger than the average length displacement ratio we propose a model where the differences between the length slip and the length displacement ratios can be partly explained by the dynamic young s modulus of a fault zone being 101 2 times greater than its static modulus in this model the dynamic modulus controls the length slip ratios whereas the static modulus controls the length displacement ratio we suggest that the common aseismic slip in fault zones is partly related to adjustment of the short term seismogenic length slip ratios to the long term length displacement ratios fault displacement is here regarded as analogous to plastic flow in which case the long term displacement can be very large so long as sufficient shear stress concentrates in the fault

the seismic cycle at subduction thrusts 1 insights from laboratory models 1 subduction megathrust earthquakes occur at the interface between the subducting and overriding plates these hazardous phenomena are only partially understood because of the absence of direct observations the restriction of the instrumental seismic record to the past century and the limited resolution completeness of historical to geological archives to overcome these restrictions modeling has become a key tool to study megathrust earthquakes we present a novel model to investigate the seismic cycle at subduction thrusts using complementary analog paper 1 and numerical paper 2 approaches here we introduce a simple scaled gelatin on sandpaper setup including realistic tectonic loading spontaneous rupture nucleation and viscoelastic response of the lithosphere particle image velocimetry allows to derive model deformation and earthquake source parameters analog earthquakes are characterized by quasi periodic recurrence consistent with elastic theory the interseismic stage shows rearward motion subsidence in the outer wedge and uplift of the coastal area as a response of locked plate interface at shallow depth the coseismic stage exhibits order of magnitude higher velocities and reversal of the interseismic deformation pattern in the seaward direction subsidence of the coastal area and uplift in the outer wedge like natural earthquakes analog earthquakes generally nucleate in the deeper portion of the rupture area and preferentially propagate upward in a crack like fashion scaled rupture width slip proportionality and seismic moment duration scaling verifies dynamic similarities with earthquakes experimental repeatability is statistically verified comparing analog results with natural observations we conclude that this technique is suitable for investigating the parameter space influencing the subduction interplate seismic cycle

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

a theory of intraplate tectonics the theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation a new theory of intraplate tectonics is developed to calculate the first order intraplate deformation induced by horizontal displacement of deformable plate boundaries it is based on simple assumptions that link the well established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries inward outward and tangential displaced boundaries the theory predicts the direction of intraplate displacement displacement rate strain and stress fields in terms of small circles great circles and 45 loxodromes around the pole of rotation of two adjacent plates the 45 loxodromes are two orthogonal sets of directions clockwise and counterclockwise that intersect both small and great circles at 45 the principal axis of the maximum horizontal stress follows small circles for inward displaced boundaries great circles for out ward displaced boundaries and loxodromes for tangential displaced boundaries the theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project zoback 1992 the theory indicates that the first order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid ocean ridges and plate interiors ridge push the simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system

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

numerical modeling of cenozoic stress patterns in the mid norwegian margin and the northern north sea numerical modeling of cenozoic stress patterns in the northern north sea and the mid norwegian margin is presented and the sense of potential slip along major fault planes belonging to the two areas is restored we assume that the main regional source of stresses is the atlantic ridge push as demonstrated by previous studies furthermore we also assume a nearly consistent nw se strike for the far field stress from continental breakup between greenland and norway earliest eocene to present day first we applied the commercial two dimensional distinct element method udec to simulate cenozoic stress and displacement patterns in the study area variations in rheology and major fault zones were introduced into the model the more trondelag fault complex and its inferred continuation into the shetland platform forms the major mechanical discontinuity in the model second we used the sortan method developed at the university of paris vi to predict the sense of potential slip along major fault planes the input for the sortan model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the udec modeling our results show that the more trondelag fault complex and its inferred continuation into the shetland platform act as a weak fault zone this fault zone divides the study area into two different stress provinces the continental margin and the northern north sea this result agrees well with the observed differences in cenozoic structural evolution of the two areas compressive structures are observed along the continental margin whereas relative tectonic quiescence characterizes the northern north sea during the tertiary the restored stress patterns in the northern north sea and the mid norwegian margin also agree well with the observed present day stress configuration our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools udec and sortan as a result it is demonstrated that oblique slip motions are mainly expected in particular strike slip and reverse dip slip faulting are simulated

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

recent stress field of the sinai subplate region abstract quaternary and recent tectonic stress data for the sinai subplate region have been compiled including focal mechanism solutions in situ stress measurements fault slip data and alignments of young volcanic feeders the average direction of maximum horizontal stress sh has been determined by means of a linear interpolation method the results indicate that the direction of the average maximum horizontal stress is closely parallel to the direction of the absolute motion of africa analysis of the orientation of principal stress directions suggests that a large part of the sinai subplate region is subjected to a strike slip regime characterized by an average maximum horizontal stress trend of nw 54 w for example out of 50 earthquake focal mechanism solutions 52 are pure strike slip and 10 are either normal or thrust fault with a remarkable strike slip component the remaining are pure normal fault 18 thrust fault 12 or undetermined 8 three different tectonic subprovinces can be delineated northwestern egypt the eastern mediterranean israel lebanon and northern syria are characterized by strike slip often with a remarkable thrust faulting component stress regime the northern red sea the sinai peninsula and a significant part of the arabian plate are characterized by strike slip often with a remarkable normal faulting component stress regime this is also the case for southwestern turkey

late miocene extension in coastal sonora mexico implications for the evolution of dextral shear in the proto gulf of california oblique rift abstract the timing kinematics and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the pacific north america plate boundary in the gulf of california are not well understood well exposed volcanic rocks deposited between 15 and 10 ma in the sierra bacha coastal sonora mexico preserve a record of late miocene deformation on the eastern rifted margin of the gulf of california and offer new insights into the timing and kinematic evolution of oblique rifting detailed geologic mapping fault kinematic analysis u pb and 40ar 39ar geochronology and paleomagnetic data reveal that the 2 km thick composite volcanic section is cut by a series of southwest dipping domino style normal faults and uniformly tilted down to the northeast palinspastic cross section restoration suggests that the region experienced ca 55 60 northeast southwest directed extension between 11 7 and 10 9 ma fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension paleomagnetic results indicating modest clockwise vertical axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline these results support an emerging model in which dextral strain was not ubiquitous across sonora and did not initiate immediately following the 12 5 ma transition from subduction to oblique rifting instead strain east of the baja california microplate at this latitude evolved from extension dominated transtension prior to 8 ma to dextral shear dominated transtension by 7 6 ma the onset of dextral shear in coastal sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at 8 ma the increase in rift obliquity and resultant onset of significant strike slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern gulf of california by 6 ma and continental rupture at 2 1 ma

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

quantifying the energy dissipation of overriding plate deformation in three dimensional subduction models in a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy respectively such deformation might involve extension and back arc basin formation or shortening and mountain building how much of the slab s potential energy is consumed during overriding plate deformation remains unknown in this work we present dynamic three dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force fopd that drives overriding plate deformation and the associated energy dissipation rate opd and we compare them with the negative buoyancy fbu of the subducted slab and its total potential energy release rate bu respectively we varied the viscosity ratio between the plates and the sublithospheric upper mantle with sp um 157 560 and the thickness of the overriding plate with top 0 5 2 5 cm scaling to 25 125 km in nature the results show that fopd fbu has average values of 0 5 2 0 with a maximum of 5 3 and opd bu has average values of 0 05 0 30 with a maximum of 0 41 the results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate our models also suggest that the force required to deform the overriding plate is of comparable magnitude as the ridge push force furthermore we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low 3 15 during steady state subduction

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

tectonic analysis of the dead sea rift region since the late cretaceous based on mesostructures we have measured the orientations of small faults slickensides tectonic stylolites vein with secondary mineralization small folds and dikes in israel and sinai these structures are indicators of paleo stress or strain and provide the pattern of tectonic deformation data were collected at 130 stations most with tens of separate measurements most stations showed consistent deformation stations were located on exposures ranging from precambrian crystalline rocks to pleistocene sediments we have defined two tectonic stress fields each relatively uniform in both time and space one stress field with dominating maximum horizontal compression trending w to wnw in the late cretaceous to eocene rocks in the folds and plateaus west of the dead sea rift the second field with dominating horizontal extension trending e to ene in all rocks inside the rift and proximal thereto the first stress field is called the syrian arc stress and the second is called the dead sea stress a change in style of tectonic deformation which corresponds to the two stress fields is manifested also in the major structures in israel the late cretaceous to neogene deformation is characterized by long wavelength folds and monoclines whereas the neogene to recent deformation is characterized by normal and strike slip faults and volcanic activity

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

modelling the evolution of asymmetrical basins bounded by high angle reverse faults with application to foreland basins abstract the evolution of asymmetrical basins bounded by high angle reverse faults in a 10 and 20 km thick strong upper crust with a byerlee type strength envelope has been modelled by finite element analysis the substratum is assumed to be inviscid and the basin is filled to datum by sediments the results are compared with those of comparable extensional half grabens as previously studied as the layer is shortened by progressive increments zones of plastic failure extend until throughgoing failure occurs with the basin progressively narrowing as it deepens at this stage further subsidence becomes negligible placing a limit on the depth which depends on the layer thickness and sediment density during the earlier stages of evolution the flexure profiles are very similar to those of the otherwise equivalent extensional half graben but during later stages substantially deeper and narrower compressional basins can form because of the greater crustal strength under compression beyond throughgoing failure further shortening causes plastic compressional folding which develops on top of the earlier gentle flexural undulations eventually producing quite strong anticlines and synclines of progressively decreasing wavelength which decay away from the fault on both sides compressional basins as modelled range from under 25 km to over 150 km in width and they can be up to nearly 8 km deep with greater widths and depths for a thicker layer the most important application of the results is to the origin of foreland basins and foredeeps providing an alternative and possibly complementary mechanism to the supracrustal loading hypothesis for the formation of such basins in particular the asymmetrical thrust basin hypothesis explains how the narrow and deep foreland basins characteristic of european mountain belts can form

modeling the formation of a half graben using realistic upper crustal rheology the evolution of a half graben in a strong upper crust with a byerlee strength envelope underlain by an inviscid substratum has been modeled using elasto viscoplastic finite element analysis the nonlithostatic stresses regions of failure and flexure profiles have been computed as the layer is extended by increments the weaker upper part of the layer fails on stretching and more extensive regions of tensional failure extend progressively downward beneath the downbending half graben and upward from the bottom of the layer beneath the footwall uplift compressional failure occurs locally due to bending or unbending plastic yielding progressively reduces the widths of the half graben and footwall uplift as extension proceeds as the outer hinge line of the half graben migrates toward the fault the plastic deformation reduces the effective elastic thickness of the layer by up to 67 helping to explain the anomalously low observed values associated with normal faulting throughgoing failure in tension eventually occurs beneath the footwall uplift near the fault when subsidence slows down and ceases the strength of the upper crust thus limits the amount of subsidence that can occur for an 8 km thick layer and sediment infill 400 kg m3 less dense than the basement the half graben width decreases from over 80 km at the onset of extension to 42 km when the subsidence has reached its limit of 2 3 km the equivalent elastic layer thickness is then reduced to 3 3 km thicker layers have correspondingly greater widths and limiting depths less dense sediments result in a narrower half graben and smaller limiting depth

localization and fault growth in layered brittle ductile systems implications for deformations of the continental lithosphere we present experiments and numerical simulations dealing with the growth of faults in thin brittle ductile systems to understand deformation modes in the continental lithosphere experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances for large strength ratios between the brittle and ductile layers r 5 10 the deformation localizes into two shear bands the fault pattern is created before reaching 10 shortening and has fractal dimensions varying between 1 6 and 1 8 for small strength ratios r 5 10 deformation never localizes the fault pattern is homogeneous with a trivial dimension of 2 and grows continuously during deformation the transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5 10 times larger than the resistance of ductile layers this transition was also investigated by means of electrical analog simulations a fuse network which represents an elasto brittle layer is coupled with a capacitor layer which models strain rate dependent fluids an ac potential is applied and the fuses progressively burned out until they form a connected network the ac potential frequency is a tuning parameter similar to the applied strain rate in experiments a critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases and a delocalization mode where the density of burned fuses remains constant with increasing system size the scaling dependency of the fracture process as well as the critical frequency are consistent with experimental results available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle to ductile strength ratios

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

structure and mechanics of the hayward rodgers creek fault step over san francisco bay california a dilatational step over between the right lateral hayward and rodgers creek faults lies beneath san pablo bay in the san francisco bay area a key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step over enhancing its maximum possible magnitude if ruptures are terminated at the step over then another important issue is how strain transfers through the step we developed a combined seismic reflection and refraction cross section across south san pablo bay and found that the hayward and rodgers creek faults converge to within 4 km of one another near the surface about 2 km closer than previously thought interpretation of potential field data from san pablo bay indicated a low likelihood of strike slip transfer faults connecting the hayward and rodgers creek faults numerical simulations suggest that it is possible for a rupture to jump across a 4 km fault gap although special stressing conditions are probably required e g harris and day 1993 1999 slip on the hayward and rodgers creek faults is building an extensional pull apart basin that could contain hazardous normal faults we investigated strain in the pull apart using a finite element model and calculated a 0 02 mpa yr differential stressing rate in the step over on a least principal stress orientation nearly parallel to the strike slip faults where they overlap a 1 to 10 mpa stress drop extensional earthquake is expected on normal faults oriented per pendicular to the strike slip faults every 50 500 years the last such earthquake might have been the 1898 m 6 0 6 5 shock in san pablo bay that apparently pro duced a small tsunami historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step over possibly related to the earthquake we used a hydrodynamic model to show that a dip slip mechanism in north san pablo bay is the most likely 1898 rupture scenario to have caused the tsunami while we find no strike slip transfer fault between the hayward and rodgers creek faults a normal fault link could enable through going segmented rupture of both strike slip faults and may pose an independent hazard of m 6 earthquakes like the 1898 event

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

status of the crustal stress in egypt as inferred from earthquake focal mechanisms and borehole breakouts abstract the recent stress field has been investigated by the analysis of the directions of maximum horizontal stress h as derived from earthquake focal mechanisms and borehole breakouts in egypt the results indicate that in the southern part of the gulf of suez the shallow borehole breakouts stress directions are not consistent with the deep earthquakes focal mechanisms stress directions analysis of 30 borehole breakouts measurements indicate wnw ese alignment of the maximum horizontal stress with a vector mean of 110 14 p e however the earthquake studies including orientation of p axes reflect ene wsw alignment of h with a vector mean of 66 30 thus the stress directions are not constant throughout the crust along the gulf of aqaba the southern part of the dead sea fault the focal mechanism solutions indicate that the maximum horizontal stress is presently nw se oriented corresponding to strike slip mechanism in concert with the geological evidence in northern egypt the nine earthquake focal mechanisms for the nile delta cairo bitter lakes region scatter in orientation varying from ne sw to nw se for h about 45 from these mechanisms indicate nw se 22 wnw ese 22 ene wsw and 11 ne sw alignment of the maximum horizontal stress while in southern egypt it is relatively uniform and aligned to nearly e w direction with a vector mean of 95 3

finite element modelling of stress patterns along the mid norwegian continental margin 62 to 68 n the inferred nature of the regional intraplate stress field off the coast of norway is based primarily on borehole breakout information and earthquake focal mechanisms the stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress sh to the northwest with respect to the dominant e w orientation observed in the northern north sea the stress regime is predominantly strike slip and thrust fault although two normal faulting events have occurred on the shelf and shelf edge within the mid norwegian margin segment 62 to 68 n the tr0ndelag platform exhibits large variations of observed sn orientations this may be due to an almost isotropic horizontal stress state sh sh resulting from the superposition of local and regional stress sources this paper presents the predicted tectonic stresses along the mid norwegian margin based on a 3 d finite element analysis the two principal modelling objectives are to elaborate the effects of 1 changes in the regional stress magnitude sh and 2 variations in rock properties i e young s modulus on the horizontal stress anisotropy hsa of the v0ring basin and the tr0ndelag platform the model incorporates simplified mainland topography the tr0ndelag platform the voring basin the v0ring plateau and the lofoten basin our modelling results indicate that horizontal stress anisotropy hsa and the regional sh magnitude within the various structural units is dependent on their strike with respect to the orientation of the present day regional stress field specifically we find 1 a near isotropic horizontal stress magnitude configuration for the southern part of the v0ring basin where the gradient of topography across the mid norwegian margin is oriented oblique to the regional trend of sh 2 a maximum horizontal stress anisotropy hsa in the southern part of the tr0ndelag platform this local hsa maximum bordering the hsa minimum of the voting basin 3 a narrow band of high hsa at the eastern edge of the tr0ndelag platform imposed by compressive gravitational stresses related to the elevated topography of mainland norway 4 a low hsa at the western margin of the tr0ndelag platform related to spreading stresses associated with the topographic lowering towards the v0ring basin and 5 in case of a far field stress of sh 1 1sh the modelling results predict exchanged principal horizontal stresses sh and sh at the western margin of the tr0ndelag platform

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

sources of recent tectonic stress in the pannonian region inferences from finite element modelling summary we present the results of finite element modelling of the recent stress field in the pannonian basin and surrounding alpine orogenic belt our results show that the recent predominantly compressive stress regime in the alpine pannonian carpathian dinaric system is governed by distinct tectonic factors of great importance is the deformation of crustal blocks with diverent geometries and rigidities in an overall convergent setting associated with the africa europe collision the most important stress source appears to be the counterclockwise rotation of the adriatic microplate at the southwest boundary of the pannonian basin this plate tectonic unit has been interpreted as moving independently of both the european plate and the african plate additional boundary conditions active shortening and compression in the vrancea zone and the bohemian massif and the evect of the immobile moesian platform also significantly influence the modelling results the incorporation of additional stress sources such as crustal thickness variation and the presence of two main fault zones separating the primary tectonic units in the study area have only locally important evects but improve the fit between the calculated results and the observed stress pattern

scales of structure in the lithosphere images of processes advances in deep seismic sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust mantle transition in this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near vertical reflection and wide angle refraction experiments the abrupt termination of near vertical reflections at the moho and the coincident presence of a strong coda accompanying the supercritical pmp reflection phase in record sections of continental seismic wide angle refraction experiments reveal a new property of the crust mantle boundary the moho forms a sandwiched mix of crust mantle material between lower crust and topmost mantle in combination with a step in mean velocity a significant change in scale of the structural dimensions and of velocity variance this view is strongly supported by the recognition of the subcrustal lithosphere as a high frequency 5 10 hz scattering wave guide for long distance propagation of the pn phase over more than 3000 km detected during seismic experiments in russia using peaceful nuclear explosions evidence is brought forward that this high frequency wave guide is likely to be present also in the phanerozoic upper mantle of western europe it is furthermore hypothesized that the global propagation of teleseismic sn or corresponding pn waves observed on continents and oceans combined with the recording of strong pmp codas for wave paths on continents points to a world wide change of scale of structure and velocities near the moho a change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the moho the global presence of the scattering pn wave guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle locations of present day tectonic activity where horizontal decoupling between crust and mantle are likely to occur e g subduction at continent continent collision zones change of tectonic regimes and continental transform faults are discussed

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

on palaeozoic mesozoic brittle normal faults along the sw barents sea margin fault processes and implications for basement permeability and margin evolution palaeozoic mesozoic brittle normal faults onshore along the sw barents sea passive margin off northern norway give valuable insight into fault and fluid flow processes from the lower brittle crust microstructural evidence suggests that late permian early triassic faulting took place during multiple phases with initial fault movement at minimum p t conditions of c 300 c and c 240 mpa c 10 km depth followed by later fault movement at minimum p t conditions of c 275 c and c 220 mpa c 8 5 km depth the study shows that pore pressures locally reached lithostatic levels 240 mpa during faulting and that faulting came to a halt during early deep stages of rifting along the margin fault permeability has been controlled by healing and precipitation processes through time which have sealed off the core zone and eventually the damage zones after faulting a minimum average exhumation rate of c 40 m ma 1 since the late permian is estimated it implies that the debated late cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation which caused subsequent isostatic recalibration and uplift of the marginal crust the studied faults may be used as analogues of basement involved fault complexes offshore revealing details about the offshore nature of faulting including past and present basement and fault zone permeability

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77