quantifying damage saturation and anisotropy in cracked rocks by inverting elastic wave velocities crack damage results in a decrease of elastic wave velocities and in the development of anisotropy using non interactive crack effective medium theory as a fundamental tool we calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic properties using this same tool we show that both the anisotropy and shear wave splitting of elastic waves can be derived two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation reaching up to 60 in the dry case comparison with experimental data on two granites a basalt and a marble shows that the range of validity of the non interactive effective medium theory model extends to a total crack density of approximately 0 5 considering symmetries up to orthorhombic in the isotropic case kachanov s 1994 non interactive effective medium model was used in order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions inversions are stable and give coherent results in terms of crack density and aperture evolution crack density variations can be interpreted in terms of crack growth and or changes of the crack surface contact areas as cracks are being closed or opened respectively more importantly the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio and therefore aperture with pressure which has broader geophysical implications in particular on fluid flow the recovered evolution of aspect ratio is also consistent with current mechanical theories of crack closure in the anisotropic cases both transverse isotropic and orthorhombic symmetries were considered anisotropy and saturation patterns were well reproduced by the modelling and mean crack fabric orientations we recovered are consistent with in situ geophysical imaging

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

crack induced anisotropy in crustal rocks predicted dry and fluid saturated thomsen s parameters abstract in upper crustal conditions elastic anisotropy of rocks is mainly due to cracks as long as the anisotropy of the background matrix remains low because in general rock anisotropy remains weak in the sense that hexagonal symmetry may be sufficient to handle it thomsen suggested that a set of three parameters would be convenient to describe it using effective elasticity we derive thomsen s parameters for the case of dry and fluid saturated cracks in a rock of isotropic background in terms of crack density parameters and fluid compressibility these results can be of interest for the interpretation of seismological and seismic data they provide moreover a way to identify the fluid nature gas or liquid and to extract the crack density tensor from elastic wave data frequency effects are discussed when fluid is present so that frequency dependence can be accounted for

isotopic evidence b c o of deep fluid processes in fault rocks from the active woodlark basin detachment zone we report results from boron carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during odp leg 180 in the western woodlark basin off papua new guinea in this area of active continental extension crustal break up and incipient seafloor spreading a shallow dipping seismically active detachment fault accommodates strain defining a zone of mylonites and cataclasites vein formation and fluid infiltration syntectonic microstructures and vein fill mineralogy suggest frictional heating during slip during extension and exhumation of moresby seamount low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids 13cpdb down to 17 18opdb down to 22 formed by both dehydration and decarbonation boron contents are low 7 ppm indicating high grade metamorphic source rock for the fluids some of the 11b signatures 17 35 parent solutions to calcite vein fills are low when compared to deep seated waters in other tectonic environments likely reflecting preferential loss of 11b during low grade metamorphism at depth pervasive devolatilization and flux of co2 rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip multiple rupture and healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and hence seismogenic fault movement

interseismic coupling and seismic potential along the central andes subduction zone we use about two decades of geodetic measurements to characterize interseismic strain build up along the central andes subduction zone from lima peru to antofagasta chile these measurements are modeled assuming a 3 plate model nazca andean sliver and south america craton and spatially varying interseismic coupling isc on the nazca megathrust interface we also determine slip models of the 1996 m w 7 7 nazca the 2001 m w 8 4 arequipa the 2007 m w 8 0 pisco and the m w 7 7 tocopilla earthquakes we find that the data require a highly heterogeneous isc pattern and that overall areas with large seismic slip coincide with areas which remain locked in the interseismic period with high isc offshore lima where the isc is high a m w 8 6 8 8 earthquake occurred in 1746 this area ruptured again in a sequence of four m w 8 0 earthquakes in 1940 1966 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a m w 8 5 earthquake the region where the nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period low isc and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years in southern peru isc is relatively high and the deficit of moment accumulated since the m w 8 8 earthquake of 1868 is equivalent to a magnitude m w 8 4 earthquake two asperities separated by a subtle aseismic creeping patch are revealed there this aseismic patch may arrest some rupture as happened during the 2001 arequipa earthquake but the larger earthquakes of 1604 and 1868 were able to rupture through it in northern chile isc is very high and the rupture of the 2007 tocopilla earthquake has released only 4 of the elastic strain that has accumulated since 1877 the deficit of moment which has accumulated there is equivalent to a magnitude m w 8 7 earthquake this study thus provides elements to assess the location size and magnitude of future large megathurst earthquakes in the central andes subduction zone caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench except offshore lima where sea bottom geodetic measurements have been collected suggesting strong coupling

listric normal faulting stress refraction and the state of stress in the gulf coast basin the close association of overpressured shales and the coincident flattening of listric normal faults has been noted and is usually explained in terms of gravity sliding models however incorporation of overpressure into mohr coulomb frictional faulting theory does not predict normal faults dipping at angles less than 45 even for very low coefficients of friction we suggest that low angle normal faults may form because of a rotation of the maximum principal stress away from vertical with depth such a rotation occurs if a significant contrast in viscosity exists between two adjacent strata such as a normally pressured sandstone and an underlying overpressured shale or evaporite the stress refraction model renders the presence of a preexisting fault plane or an extremely low coefficient of friction unnecessary to explain listric fault geometry and offers a failure mechanism in agreement with frictional faulting theory based on rheological arguments

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

extreme dynamic weakening of faults during dehydration by coseismic shear heating the dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release high velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0 6 to 0 15 as the slip velocity reaches 1 1 m s at normal stresses up to 24 5 mpa the extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding the rapid heating at asperities causes serpentine dehydration in impermeable fault zones in nature serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening this dynamic fault weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the san andreas

the widespread occurrence of low angle normal faults in a rift setting review of examples from thailand and implications for their origin and evolution abstract at least 31 low angle 1 km cenozoic normal faults are identified on seismic reflection data onshore and offshore thailand although some faults have been moderately rotated to lower angles pre rotation dips still indicate formation at low angles about 25 and 35 the dominant east dip of low initial fault dips suggest that pre existing fabrics controlled the fault dip direction in the mergui basin patches of gently dipping basement reflections on seismic reflection data support this observation well data points to a pre rift basement with a widespread component of slaty phyllitic and chlorite schist lithologies that are associated with significant strength anisotropy the subduction zone setting of se asia with a high fluid flux into the crust associated with high levels of co2 has parallels with the setting for lanfs in italy where it is suggested high pore fluid pressures played an important role in lanf reactivation in cross section most lanfs curve from a high angle in sedimentary section to planar or stepped in basement until they reach the brittle ductile transition zone in a few cases lanfs have a convex up geometry some long segmented faults are composed of mixed regions of high and low angle segments the local mix of high and low angle faults indicates that the principal stresses probably were vertical and horizontal during fault formation any rotation of stresses would have to be local not regional lanfs in the mergui basin have very high displacement gradients as low as 2 1 indicating the same fault segments some 20 30 km long have repeatedly slipped while displacements can decline dramatically at key discontinuities the lanf segments are inferred to be very weak and probably slipped mostly aseismically while adjacent lower displacement high angle segments were probably seismically active hence in areas of mixed high and low angle fault segments seismicity may erroneously suggest that high angle normal faults are the dominant fault type the presence of such a high concentration of low and high displacement lanfs in a non metamorphic core complex setting demonstrates that lanfs can form initially at a low angle that isostatic rotation of initially high angle faults to a low angle is not necessary or appropriate to explain the faults and that low angle normal faults are not just features associated with orogenic collapse

fault geometry and mechanics of marly carbonate multilayers an integrated field and laboratory study from the northern apennines italy abstract sealing layers are often represented by sedimentary sequences characterized by alternating strong and weak lithologies when involved in faulting processes these mechanically heterogeneous multilayers develop complex fault geometries here we investigate fault initiation and evolution within a mechanical multilayer by integrating field observations and rock deformation experiments faults initiate with a staircase trajectory that partially reflects the mechanical properties of the involved lithologies as suggested by our deformation experiments however some faults initiating at low angles in calcite rich layers i 5 20 and at high angles in clay rich layers i 45 86 indicate the important role of structural inheritance at the onset of faulting with increasing displacement faults develop well organized fault cores characterized by a marly foliated matrix embedding fragments of limestone the angles of fault reactivation which concentrate between 30 and 60 are consistent with the low friction coefficient measured during our experiments on marls s 0 39 indicating that clay minerals exert a main control on fault mechanics moreover our integrated analysis suggests that fracturing and faulting are the main mechanisms allowing fluid circulation within the low permeability multilayer and that its sealing integrity can be compromised only by the activity of larger faults cutting across its entire thickness

on the rotation and frictional lock up of normal faults explaining the dip distribution of normal fault earthquakes and resolving the low angle normal fault paradox abstract classical rock mechanics predicts that normal faults should form at 60 65 rotating to 30 40 before locking so widespread slip at around 20 is considered paradoxical furthermore the dip distribution of normal fault earthquakes has a distinct unexplained peak at 45 for both problems some combination of low friction high fluid pressures stress rotation and efficiency of low angle slip have been suggested but provide at best a partial solution a simple quantitative model for normal fault rotation iterating between slip on faults and distributed strain predicts that faults spend more time at lower angles combining this result with mohr coulomb analysis of the range where seismogenic normal faults should lock up predicts the dip distribution of seismogenic normal faults and matches both the range and 45 peak observed for normal fault earthquakes as even the lowest dips of normal fault ruptures fall within the limits of fault reactivation slip on seismogenic low angle normal faults is not paradoxical a new mohr griffith solution to the limits of fault reactivation extends the analysis into the top few km where low angle slip is best constrained by field observations and shows that low cohesion normal faults can remain active at

using footwall structures to constrain the evolution of low angle normal faults controversy exists over the presence of normal faults that initiate and slip at low angles c 15 east on the island of elba italy indicates that the early stages of low angle faulting were synchronous with development of subsidiary high angle footwall faults these footwall structures controlled fault rock distribution and caused the development of flow folds similar to those produced in analogue studies of synchronous high and low angle faulting the current configuration of footwall structures indicates that the zuccale fault cannot have originally dipped greater than c 20 east and represents a primary low angle normal fault

thermomechanics of an extensional shear zone raft river metamorphic core complex nw utah abstract a detailed structural and microstructural analysis of the miocene raft river detachment shear zone nw utah provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes combined microstructural electron backscattered diffraction strain and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east along the transport direction compatible with observed finite strain markers and a model of necking of the shear zone microstructural evidence quartz microstructures and deformation lamellae suggests that the detachment shear zone evolved at its peak strength close to the dislocation creep exponential creep transition where meteoric fluids played an important role on strain hardening embrittlement and eventually seismic failure empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results indicate that the shear zone developed under stress ranging from 40 mpa to 60 mpa using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10 12 and 10 14 s 1 we suggest that a compressed geothermal gradient across this detachment which was produced by a combination of ductile shearing heat advection and cooling by meteoric fluids may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

quaternary low angle slip on detachment faults in death valley california detachment faults on the west flank of the black mountains nevada and california dip 29 36 and cut subhorizontal layers of the 0 77 ma bishop ash steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0 64 ma lava creek b tephra and the base of 0 12 0 18 ma lake manly gravel these faults sole into and do not cut the low angle detachments therefore the detachments accrued any measurable slip across the kinematically linked hanging wall faults an analysis of the orientations of hundreds of the hanging wall faults shows that extension occurred at modest slip rates 1 mm yr under a steep to vertically oriented maximum principal stress the black mountain detachments are appropriately described as the basal detachments of near critical coulomb wedges we infer that the formation of late pleistocene and holocene range front fault scarps accompanied seismogenic slip on the detachments

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

possible seismogenic shallow dipping normal faults in the woodlark d entrecasteaux extensional province papua new guinea active normal faults dipping less than 30 have not been documented in the seismic record but they are commonly observed in geologic structures one interpretation is that seismogenic low angle faulting is physically impossible and that the geologic structures do not form at shallow dips to better examine this problem teleseismic body waves were inverted to determine source parameters for five large earthquakes in the woodlark d9entrecasteaux region a rare example of active continental extension associated with pliocene quaternary metamorphic core complexes the inversions show that at least two and possibly four events are consistent with normal dip slip on shallow dipping faults the largest earthquake m w 6 8 is located near the mapped core complexes the shallow dipping plane for this event determined to dip between 10 and 25 is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low angle structures although slip on the conjugate high angle plane cannot be ruled out observations of quaternary uplift and the similarity to the adjacent core complex bounding structures favor motion on the low angle plane similar earthquake mechanisms have not yet been documented in most other extensional provinces perhaps because instances of active core complex exhumation are rare

seismic slip on a low angle normal fault in the gulf of corinth evidence from high resolution cluster analysis of microearthquakes the gulf of corinth in western greece is one of the most active extensional zones in the aegean region it is still an open question whether extension can be actively accommodated on low angle faulting or if those faults as seen in geological records have been rotated whilst numerous fault plane solutions obtained from a dense temporary network deployed in the western part of the gulf in july august of 1991 showed one of the nodal planes as a subhorizomal plane slip on the high angle conjugate plane is equally probable from the focal mechanism data rigo et al 1996 since part of the activity occurred in spatial clusters with similar focal mechanisms we used a high resolution cluster analysis to determine the most likely active plane exploiting the waveform similarity of these events relative onset times of p and s waves could be determined at subsample accuracy less than 0 01 s the cluster analyzed here contains 12 evems among these 8 have a well con strained normal faulting fault plane solution with a shal low 12 20 o north dipping plane and a steeply south dipping plane a master evem relocation shows that the relocated 12 hypocenter cemroids are aligned along the low angle plane showing clear evidence for active low angle normal faulting

deformation fabrics of faulted rocks and some syntectonic stress estimates from the active woodlark basin detachment zone ocean drilling program odp leg 180 investigated in the western woodlark basin off papua new guinea the nature and evolution of continental extension eventually leading to crustal break up and sea floor spreading at moresby seamount the rift related extension is localized at a recently active low angle 30 detachment fault partly buried beneath a pliocene pleistocene sedimentary synrift sequence data from three drillsites sample the detachment fault itself secondary faults in its hanging wall and a steep normal fault cutting the footwall the fault plane itself is manifested as a strongly altered fault gouge deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms and accompanied by intensive veining and pervasive diagenetic cementation the metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization syn mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust two palaeo piezometers were applied to calcite filled veins that show evidence of plastic deformation differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall

low angle normal fault earthquakes and triggering geologic evidence for low angle normal faulting is overwhelming and recent seismic geodetic and neotectonic studies have identified probable active examples thus apparent low levels of historical seismicity on such faults is enigmatic however one large mainshock was a low angle normal event and three others may have been one earthquake sequence had significant moment release from a triggered low angle normal fault and three earthquakes may have had normal subevents on low angle planes such sub events are hard to recognize may be underrepresented in catalogs helping to explain the apparent seismic enigma and pose increased seismic hazard due to delayed strong motions triggered sub events may be along strike or down dip from their triggering shock in cases where the triggered slip is downdip of the steep fault and under its hanging wall elevated pore fluid pressure or fault weakness may be required

evidence for fault weakness and fluid flow within an active low angle normal fault determining the composition and physical properties of shallow dipping active normal faults dips 35 with respect to the horizontal is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere seismic reflection images1 and earthquake source parameters2 show that a magnitude 6 2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25 30 located ahead of a propagating spreading centre in the woodlark basin here we present results from a genetic algorithm inversion of seismic reflection data which shows that the fault at 4 5 km depth contains a 33 m thick layer with seismic velocities of about 4 3 km s 1 which we interpret to be composed of serpentinite fault gouge isolated zones exhibit velocities as low as 1 7 km s 1 with high porosities which we suggest are maintained by high fluid pressures we propose that hydrothermal fluid flow possibly driven by a deep magmatic heat source and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low angle normal fault

damage patterns stress rotations and pore fluid pressures in strike slip fault zones 1 active faults unfavorably oriented with respect to the regional maximum compressive stress have been labeled as weak the seismic hazards posed by these faults make understanding this apparent weakness a priority stress rotations in these fault zones together with an increase in mean stress could enable high pore fluid pressures to weaken a fault zone such a model requires a foundation in the physics and mechanics of damage this paper presents a new model for stress rotations in fault zones by combining the effective medium theory with anisotropic poroelasticity this approach enables the quantitative characterization of crack damage and the prediction of progressive changes in the elastic properties of rocks across the fault zone the processes of fault growth and wear will lead to distinct patterns of crack damage with different effects on the elastic properties elevated pore fluid pressures have long been known to change the effective normal and shear stresses of anisotropic rocks and this work incorporates these effects into a multilayer fault zone model it is shown that high pore fluid pressures in the anisotropic rocks of the core zone can generate large stress rotations i e more fault parallel and increases in mean stress sufficient to weaken the fault stress rotations in the damage zones of unfavorably oriented faults tend to be away from the fault i e more fault normal for likely combinations of damage patterns and pore fluid pressure

a low angle normal fault in the umbria region central italy a mechanical model for the related microseismicity abstract in the northern apennines the upper crust is thinned by a set of east dipping low angle normal faults lanfs the easternmost and more recent of these lanfs is the altotiberina fault atf located in northern umbria the geometry of the atf has been reconstructed matching surface geology with seismic reflection profiles and borehole data the fault whose average dip is 20 borders the upper pliocene quaternary tiber basin and has a displacement of about 8 km the deeper portion of the atf is located below the axial zone of the northern apennines where the strongest instrumental and historical seismicity is recorded the microseismicity of the region m 1 to evaluate the boundary conditions for the brittle activity of the atf the fault can be reactivated for low values of differential stress 1 3 t 10 mpa and tensile fluid overpressure p f 3 e g v 0 93 in the peculiar situation of the northern apennines the deep emissions of large amounts of co 2 documented in the area can be entrapped in their ascent by structural seals e g atf favouring localised fluid overpressures the impossibility of sustaining p f 3 for wide fault portions counteracted by hydraulic fracturing increased permeability under low effective stress and load weakening behaviour for normal faulting would prevent the nucleation of moderate ruptures along the fault the short lived attainment of p f 3 along small fault portions can account for the microseismic activity located along the atf which occurs on rupture surfaces in the range of 10 1 10 3 km 2

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

microcracking during triaxial deformation of porous rocks monitored by changes in rock physical properties i elastic wave propagation measurements on dry rocks we present results from two series of triaxial deformation experiments performed on dry samples of two sandstones darley dale and gosford carried out at confining pressures from 25 mpa to 200 mpa over this pressure range the mode of failure in both these sandstones passes from localized brittle failure with a clear through going fault to distributed cataclastic flow during these experiments stress strain compressional wave velocity vp and shear wave velocity vs measurements were made simultaneously in the direction of the maximum principal compressive loading axis initial application of the hydrostatic confining pressure causes both vp and vs to increase and upon raising the axial stress above the confining pressure both velocities increase further at first generally by only a few percent but then decrease as dilatant crack growth commences during dilatancy vs decreases proportionately more than vp and this decrease is generally of the order of 10 15 these velocity measurements allow changes in rock physical properties to be calculated along with the axial and transverse crack volume density parameters x and z the results from two selected tests are analysed in detail these tests were chosen because they exhibit a typical brittle shear failure and b typical ductile cataclastic flow respectively the full interrogating elastic waveforms were also recorded during testing and these have been used to calculate the seismic quality factors qp and qs to our knowledge this is the first time this has been reported for rock samples undergoing triaxial deformation the changes in q values generally exhibit similar trends to those observed in the velocity measurements but the percentage changes in q are an order of magnitude greater suggesting that this parameter is a more sensitive measure of dilatant crack damage the measurements on dry rock samples reported here provide the basis for comparison with measurments of changes in complementary physical properties made on water saturated rock samples under the same experimental conditions and reported in a companion paper in this issue read et al 1995

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

misoriented faults in exhumed metamorphic complexes rule or exception abstract low angle normal faults and other weak faults are common in the metamorphic core of collisional orogens they frequently show a phyllosilicate rich mylonitic foliation that was reactivated under brittle conditions experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on shear failure and frictional sliding eventually inhibiting the nucleation and propagation of new andersonian shear fractures and favoring the localization of brittle failure along the pre existing foliations metamorphic phyllosilicate rich rocks may show a friction coefficient varying from 0 6 at high angles to the foliation to 0 2 0 4 for shear along the inherited foliation to test the influence of mechanical anisotropies on the development of non andersonian faults we have applied a modified slip tendency analysis to three misoriented phyllosilicate rich faults of the european alps the analysis accounts for anisotropy in friction coefficients and has been named anisotropic slip tendency analysis here we show that brittle deformation along misoriented phyllosilicate rich foliations is more probable than the development of new andersonian faults the presence of a well developed network of weak phyllosilicate rich faults may influence the overall structural style and mechanical properties of the brittle lithosphere in collisional orogens

fault zone weakening and character of slip along low angle normal faults insights from the zuccale fault elba italy a seismically active low angle normal fault is recognized at depth in the northern apennines italy where recent exhumation has also exposed ancient examples at the surface notably the zuccale fault on elba field based and microstructural studies of the zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability promoting influx of co 2 rich hydrous fluids this triggered low grade alteration and the onset of stress induced dissolution precipitation processes e g pressure solution as the dominant grain scale deformation process in the pre existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine grained hydrous mineral phases these rocks exhibit ductile deformation textures very similar to those formed during pressure solution accommodated frictional viscous creep in experimental fault rock analogues the presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core which significantly enhanced the sealing capacity of the fault zone a slip model for low angle normal faults in the apennines is proposed in which aseismic frictional viscous creep occurs on a weak slow moving slip rate 1 fault interspersed with small seismic ruptures caused by cyclic hydrofracturing events our findings are potentially applicable to other examples of low angle normal faults in many tectonic settings

pore fluid pressure effects in anisotropic rocks mechanisms of induced seismicity and weak faults many observations and studies indicate that pore fluid pressure in the crustal rocks plays an important role in deformation faulting and earthquake processes conventional models of pore pressure effects often assume isotropic porous rocks and yield the nondeviatoric pressure effects which seem insufficient to explain diverse phenomena related to pore pressure variation such as fluid extraction induced seismicity and crustal weak faults we derive the anisotropic effective stress law especially for transversely isotropic and orthotropic rocks and propose that the deviatoric effects of pore fluid pressure in anisotropic rocks not only affect rock effective strength but also cause variation of shear stresses such shear stress variations induced by either pore pressure buildup or pore pressure decline may lead to faulting instability and trigger earthquakes and provide mechanisms for the failure of crustal weak faults with low level of shear stresses we believe that the deviatoric effects of pore fluid pressure in anisotropic rocks are of wide application in studies of earthquake precursors and aftershocks oil and gas reservoir characterization enhanced oil recovery and hydraulic fracturing

crustal and mantle structure beneath the southern payenia volcanic province using gravity and magnetic data the auca mahuida volcanic field lies on the southernmost payenia volcanic province one of the broadest retroarc volcanic plateaux in the southern central andes 38 s this voluminous basaltic flooding of quaternary age was originated from a deep asthenospheric source interpreted as a mantle plume product of changing slab dynamics the geometry of this source is deduced from magnetotelluric data but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly in order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies we used multiple data sources we combined magnetic and bouguer anomalies curie isotherm depth t c elastic thickness t e and moho depth derived from the global earth magnetic anomaly grid emag2 and terrestrial gravity measurements all together in a holistic geophysical analysis the magnetic data depict a nearly 200 km in diameter circular anomaly that would correspond to a dense body according to the bouguer anomaly geoid data from the gravity field model eigen 6c4 have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data moho inversion yields a crustal attenuation at 36 to 32 km depth coinciding with t e below 20 km depth and a shallow t c 15 km depth at the site where geoid positive undulation was calculated finally surface analysis allowed defining a topographic swell compatible with the dimensions of the identified magnetic anomaly where the main rivers deviated potentially due to a recent base level change 2018 american geophysical union all rights reserved

hydrocarbon generation induced microcracking of source rocks laboratory measurements of ultrasonic velocity and anisotropy in kerogen rich black shales of varying maturity suggest that extensive bedding parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts this subhorizontal microcrack alignment is consistent with 1 a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation and with 2 a strong mechanical strength anisotropy of kerogen rich shales caused by bedding parallel alignment of kerogen microlayers microcracks originate within kerogen or at kerogen illite interfaces when pore pressure exceeds the bedding normal total stress by only a few mpa due to the extremely low fractu

fabric related velocity anisotropy and shear wave splitting in rocks from the santa rosa mylonite zone california the directional dependence of p and s wave velocities have been measured at pressures up to 600 mpa and temperatures up to 700 c in rocks from the santa rosa mylonite zone southern california during tectonism these were progressively deformed from granodiorite protolith to mylonite and ultimately phyllonite the mineralogical and chemical composition of protolith and mylonite is nearly identical thus these rocks provide excellent material for documenting the effect of microstructural and textural changes on rock anisotropy velocity anisotropy increases significantly with the degree of deformation whereas average velocities and densities do not change at low pressure 50 mpa the velocity anisotropy ranges from 1 7 in granodiorite up to 19 in phyllonite and is due to both oriented microfractures and crystallographic preferred orientation at high pressure 600 mpa the residual anisotropy up to 12 is mainly due to preferred mineral orientation in particular of biotite significant shear wave splitting is measured parallel to the foliation plane and shows a good correlation with the biotite texture these observations confirm that oriented microcracks and preferred orientation of minerals should be taken into account in the interpretation of seismic reflection and refraction data in terranes with deformed rocks

recognizing the seismic cycle along ancient faults co2 induced fluidization of breccias in the footwall of a sealing low angle normal fault abstract the zuccale low angle normal fault exposed on the island of elba italy is a crustal scale structure which contains a strongly foliated fault core in the immediate footwall of the zuccale fault cohesive fault related breccias which were initially deformed by typical frictional deformation mechanisms experienced fluidization over areas of at least 10 2 10 3 km 2 three internal variants of fluidized breccia are recognized with each related to a separate fluidization event characteristics of the fluidized breccias include 1 a highly irregular intrusive boundary with the overlying fault core 2 no grain scale evidence for typical frictional deformation mechanisms 3 an association with carbonate cements indicating that fluids contained co 2 and 4 a clast preferred orientation suggesting that fluids were moving vertically and spreading laterally as they encountered the foliated fault core our observations suggest that the fluidized breccias are representative of the interseismic period along the zuccale low angle fault and developed across small fault patches during build ups in fluid overpressure attainment of a critical fluid overpressure triggered embrittlement and the formation of low angle slip surfaces and sub vertical tensile veins within the overlying fault core which may account for the presence and the dimensions 10 1 10 3 km 2 of rupture surfaces which produce microseismicity along active low angle normal faults in central italy

seismic velocity and anisotropy in mylonites and the reflectivity of deep crystal fault zones compressional wave velocity anisotropy has been measured to 1 kb confining pressure in several mylonites in one cataclasite and in their protoliths no significant difference in the velocity or density was found between the protolith and the tectonically altered rock at 1 kb strong seismic anisotropy greater than 10 was present in only two of nine mylonites measured the anisotropy was due to their aphanitic texture high content of layered silicates and pervasive foliation most mylonitic rocks studied were not strongly anisotropic despite a well developed macroscopic foliation where anisotropy was present velocities usually were slowest perpendicular to the foliation and fastest parallel to both foliation and lineation these preliminary results suggest that ductile fault zones in the crust are not strong seismic reflectors unless mylonites in the fault zone are aphanitic and contain large amounts of foliated phyllosilicates it is also possible that anomalous environmental conditions such as high pore pressure in the fault zone in situ may explain observed seismic reflectivity

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

earthquake slip weakening and asperities explained by thermal pressurization for the earth s crust to release accumulated stress in the form of an earthquake a fault must weaken and give way otherwise the movement would quickly grind to a halt due to the surrounding pressure this weakening is not really understood laboratory experiments simply don t match seismologists estimates of the distance a fault must slip before it becomes very weak a new study concludes that the missing ingredient is water geological studies and measurements show the slip zones to be narrow impermeable structures water already in the fault zone can quickly become pressurized by frictional heating as the rocks start to move catalysing large earthquakes from minor movements an earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1 2 this slip weakening occurs over a critical distance dc understanding the controls on dc in nature is severely limited however because the physical mechanism of weakening is unconstrained conventional friction experiments typically conducted at slow slip rates and small displacements have obtained dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3 4 5 6 here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the median tectonic line fault zone japan we show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7 8 9 10 is the slip weakening mechanism our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

microfracturing damage and failure of brittle granites 1 the evolution of stress induced damage and the eventual brittle failure are experimentally analyzed for mount scott granite of oklahoma we quantify the damage intensity in two methods and directly compare model predictions and actual damage the 14 samples of the medium grain size granite were loaded triaxially at dry conditions room temperature and under 41 mpa confining pressure microfractures were mapped in five samples and the majority of them 80 belong to two groups tensile microfractures trending subparallel to the loading axis and shear microfractures trending 11 40 off the loading axis the tensile microfractures dominate the low stress stage and they remain intragranular with a stress increase the relative density of shear microfractures increases with increasing stresses and they formed elongated intergranular zones of coalescing microfractures we compared two independent values of damage intensity 1 the macroscopic experimentally measured reduction of the deformation modulus and 2 the expected reduction of this modulus calculated with several damage models for the density of the mapped microfractures our fracture density data best fit the model of noninteracting cracks of kachanov 1992

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

active low angle normal faulting in the woodlark extensional province papua new guinea a physical model 1 in eastern papua new guinea the seismicity geomorphology and results of seismic reflection profiling drilling and structural geological analysis reveal a zone of active low angle normal faulting the only clear known active example of this process worldwide this woodlark extensional province is 250 km long in the east west direction located within the continental crust to the south of the solomon sea ocean basin it adjoins the young oceanic spreading center farther east in the woodlark basin and since 4 ma has accommodated northward extension at rates of in excess of 10 mm yr 1 on low angle normal faults that are rooted to the north this geometry and the region s earlier history are assessed in the light of modern ideas for the mechanics of low angle normal faulting to derive a working hypothesis for the existence of this process in this region the extension in the woodlark extensional province and oceanic spreading in the woodlark basin are thus interpreted as consequences of the slab pull force due to northward subduction of the solomon sea floor which has caused the small solomon sea plate to separate from the australian plate it is suggested that this plate geometry developed by 6 5 ma following the ending of southward subduction of the solomon sea floor beneath what is now the woodlark extensional province the double thickness of lithosphere that formerly existed locally cooled the continental crust making it brittle to the moho following the end of this subduction the base of the continental lithosphere became progressively exposed to the asthenosphere causing dramatic heating of the overlying continental crust this resulted in transient thermal disequilibrium inducing transient horizontal lower crustal flow the shear tractions imparted on the base of the brittle layer by such flow combined with the stress components that develop after some of the lower crust has flowed from beneath a locality requiring it to be supported laterally rather than from below will rotate the principal axes of the stress tensor sufficiently to enable low angle normal faults to form and it is estimated that this sense of deformation was initiated around 4 ma the geometry of this study region means that this process will create north dipping low angle normal faults as are observed this scheme can also account for a great deal of other observational evidence including the available thermochronologic evidence and the observed vertical crustal motions to explain this instance of active low angle normal faulting thus requires knowledge of its relationship to earlier subduction and of effects of temperature dependent changes to continental crustal rheology in crust that was initially cold

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

velocity measurements and crack density determination during wet triaxial experiments on oshima and toki granites a set of experiments on four samples of oshima granite at 15 40 and 60 mpa confining pressure have been performed in order to investigate the damage behavior of granite submitted to deviatoric stress in addition an experiment on one sample of toki granite at 40 mpa confining pressure was performed in order to compare and elucidate the structural effects using acoustic emission data strain measurements and elastic wave velocities allow to define consistently a damage domain in the stress space in this domain microcracking develops the microcracking process is in a first stage homogeneous and close to failure localized elastic wave velocities decrease in the damage domain and elastic anisotropy develops using kachanov s model 1993 elastic wave velocities have been inverted to derive the full second order crack density tensor and characterize the fluid saturation state from the fourth order crack density tensor crack density is strongly anisotropic and the total crack density close to failure slightly above one the results indicate that the rock is saturated in agreement with the experimental conditions the model is thus shown to be very appropriate to infer from elastic wave velocities a complete quantitative characterization of the damaged rock

rate of diffuse carbon dioxide earth degassing estimated from carbon balance of regional aquifers the case of central apennine italy central italy is characterized by an anomalous flux of deeply derived co2 in the western tyrrhenian sector of central italy co2 degassing occurs mainly from focused emissions vents and strong diffuse degassing and thermal springs whereas in the eastern apennine area deep co2 is dissolved in cold groundwaters of regional aquifers hosted by mesozoic carbonate evaporite formations influx of deep co2 into 12 carbonate aquifers 12 500 km2 of the central apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data mass balance calculations estimate that 6 5 1010 mol yr 1 of inorganic carbon are dissolved in the studied aquifers approximately 23 of this amount derives from biological sources active during the infiltration of the recharge waters 36 comes from carbonate dissolution while 41 is representative of deep carbon sources characterized by a common isotopic signature 13c 3 the calculated deep co2 influx rate ranges from 105 to 107 mol yr 1 km 2 increasing regionally from east to west in the study area

influence of poroelastic behavior on the magnitude of minimum horizontal stress sh in overpressured parts of sedimentary basins in many sedimentary basins of the world the minimum hori zontal stress s h is greater in overpressured zones than in normally pressured zones at equivalent depths a common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress s v and s h within certain bounds and the difference is smaller under lower effective stress i e higher pore pressure p p however in the overpressured parts of the central north sea graben united kingdom and the sable subbasin of the scotian shelf canada conventional friction envelopes underestimate the magnitude of s h these data instead indicate that s h increases at a rate proportional to but less than the rate of increase of p p a condition consistent with a p p induced deformation of the rock called poroelastic behavior this paper argues that whereas friction may govern s h in normally pressured basins poroelastic behavior is responsible for the unusually high s h in the overpres sured parts of these same basins data on the p p and s h gradients from these basins suggest that s h p p 0 7

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

the complexity of 3d stress state changes during compressional tectonic inversion at the onset of orogeny compressional tectonic inversions are classically represented in 2d brittle failure mode bfm plots that illustrate the change in differential stress s1 2 s3 versus the pore fluid pressure during orogenic shortening in these bfm plots the tectonic switch between extension and compression occurs at a differential stress state of zero however mostly anisotropic conditions are present in the earth s crust making isotropic stress conditions highly questionable in this study theoretical 3d stress state reconstructions are proposed to illustrate the complexity of triaxial stress transitions during compressional inversion of andersonian stress regimes these reconstructions are based on successive late burial and early tectonic quartz veins which reflect early variscan tectonic inversion in the rhenohercynian foreland fold and thrust belt high ardenne slate belt belgium germany this theoretical exercise predicts that no matter the geometry of the basin or the orientation of shortening a transitional wrench tectonic regime should always occur between extension and compression to date this intermediate regime has never been observed in structures in a shortened basin affected by tectonic inversion our study implies that stress transitions are therefore more complex than classically represented in 2d ideally a transitional wrench regime should be implemented in bfm plots at the switch between the extensional and compressional regimes extension mode i fractures are classically interpreted to form at low differential stresses less than four times the tensile strength t of rock e g secor 1965 they open perpendicular to the minimum principal stress s3 and propagate in the plane of the maximum s1 and intermediate s2 principal stresses the alignment of fractures has therefore often been used in orogenic belts to determine the orientation of the palaeostress field at the time of fracturing cox et al 2001 gillespie et al 2001 laubach et al 2004 if veins which initiated as extension fractures are uniform in trend over a large area then they can also be used as such a palaeostress indicator e g boullier robert 1992 cosgrove 2001 gillespie et al 2001 mazzarini et al 2010 van noten sintubin 2010 different cross cutting or successive vein generations are even more interesting because they have the potential 1 to serve as a tool to decipher changes in the overall stress regime during multiple phases of fracturing and 2 to define the p2t conditions at which these stress changes occurred on the one hand different crosscutting fracture and or vein generations have been used in many studies to deduce local stress field rotation in a consistent remote stress regime jackson 1991 crespi chan 1996 stowell et al 1999 laubach diaz tushman 2009 wiltschko et al 2009 on the other hand successive cross cutting veins i e vein sets that are oriented differently with respect to bedding have often been used to determine a transition in regional stress regime from extension to compression or reversely manning bird 1991 boullier robert 1992 teixell et al 2000 hilgers et al 2006b van noten et al 2008 2011 laubach diaz tushman 2009 alternatively cross cutting vein sets are observed in close relationship to faults in which they represent fluctuating fluid pressures during fault valve activity and are related to short lived local stress transitions in a consistent remote stress regime boullier robert 1992 sibson 1995 nielsen et al 1998 muchez et al 2000 collettini et al 2006 local mutually cross cutting vein sets may also develop in the presence of mechanical anisotropy around fault planes in a consistent remote stress field healy 2009 fagereng et al 2010 in all these studies transitions between regional stress regimes are defined as tectonic inversions a positive compressional tectonic inversion corresponds to a transition from an extensional to a compressional stress regime while a negative extensional tectonic inversion reflects the transition from compression to extension from healy d butler r w h shipton z k sibson r h eds 2012 faulting fracturing and igneous intrusion in the earth s crust geological society london special publications 367 51 69 http dx doi org 10 1144 sp367 5 the geological society of london 2012 publishing disclaimer www geolsoc org uk pub ethics by guest on august 9 2012 http sp lyellcollection org downloaded from

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust