the physical basis of mb ms and variable frequency magnitude methods for earthquake explosion discrimination we use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake explosion discrimination we use six three dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials four factors contribute to the success of the mb ms discriminant source spectra focal mechanism near source elastic properties and interference of p spectral differences contribute about 0 5 magnitude unit to the separation between earthquake and explosion populations for large events but this difference vanishes for events with mb 4 0 using a frequency band of 0 5 3 0 hz for small events the vfm discriminant is complementary to mb ms in several ways vfm is improved by p interference while mb ms is degraded vfm is more effective for hard rock explosions while mb ms will preferentially discriminate explosions in low velocity materials and vfm is insensitive to fault orientation vfm is more sensitive to noise and regional attenuation differences than mb ms simultaneous use of both methods should allow improved discrimination of small events

models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

the mechanisms of shallow earthquakes and the monitoring of a comprehensive test ban summary the ability of seismological criteria to identify earthquakes from underground explosions depends partly on the orientation of the earthquake source well determined double couple moment tensor solutions for a large number of earthquakes have been published in the harvard centroid moment tensor cmt and united states geological survey usgs catalogues statistical analyses of these catalogues indicate that the distribution of the orientation of earthquake mechanisms is not random the distribution of the t axes shows significant clustering around the downward vertical indicating that a larger number of earthquake mechanisms radiate compressional p wave energy to teleseismic distances from near the maximum of the radiation pattern than is predicted if earthquake sources are randomly oriented double couples the clustered t axes correspond to compressional dip slip mechanisms and it is this type of mechanism which is believed to cause both the mb kfs the ratio of body wave to surface wave magnitude and first motion criteria to misidentify an earthquake as an explosion

shallow seismic source parameter determination using intermediate period surface wave amplitude spectra summary estimating reliable depths for shallow seismic sources is important in both seismo tectonic studies and in seismic discrimination studies surface wave excitation is sensitive to source depth especially at intermediate and short periods owing to the approximate exponential decay of surface wave displacements with depth a new method is presented here to retrieve earthquake source parameters from regional and teleseismic intermediate period 100 15s fundamental mode surface wave recordings this method makes use of advances in mapping globaldispersiontoallowhigherfrequencysurfacewaverecordingsatregionalandteleseismic distances to be used with more confidence than in previous studies and hence improve the resolution of depth estimates synthetic amplitude spectra are generated using surface wave theory combined with a great circle path approximation and a grid of double couple sources are compared with the data source parameters producing the best fitting amplitude spectra are identified by minimizing the least squares misfit in logarithmic amplitude space the f test is used to search the solution space for statistically acceptable parameters and the ranges of these variables are used to place constraints on the best fitting source estimates of focal mechanism depth and scalar seismic moment are determined for 20 small to moderate sized 4 3 mw 6 4 earthquakes these earthquakes are situated across a wide range of geographic and tectonic locations and describe a range of faulting styles over the depth range 4 29km for the larger earthquakes comparisons with other studies are favourable however existing source determination procedures such as the cmt technique cannot be performed for the smaller events by reducing the magnitude threshold at which robust source parameters can be determined the accuracy especially at shallow depths of seismo tectonic studies seismic hazard assessments and seismic discrimination investigations can be improved by the application of this methodology

seismic discrimination in southern xinjiang the 13 march 2003 lop nor earthquake earthquakes that occur near known nuclear test sites are invaluable for evaluating the capabilities of the international monitoring system ims currently being established to monitor compliance with the comprehensive nuclear test ban treaty ctbt on 13 march 2003 a seismic disturbance with magnitude m b national earthquake information center neic 4 8 occurred near the chinese nuclear test site at lop nor southern xinjiang here we attempt to identify this disturbance as an earthquake by using three types of seismic data 1 teleseismic p waves recorded at ims stations 2 long period surface waves recorded at a network of stations in eurasia that simulates the proposed ims seismic network in this region and 3 long period full waveform p s and surface wave modeling at station wmq which is approximately 250 km from the reported epicenter of the 13 march 2003 disturbance we find that all impulsive teleseismic p wave onsets show compressive first motion and that discrimination using the m b m s criterion requires assumptions that cannot be justified on theoretical grounds however by combining the three data types we conclude that the observations are consistent with a double couple source with strike 125 10 dip 40 10 rake 90 10 and moment m 5 5 1 1015 n m that said consistency of the seismic wave field with a double couple does not immediately rule out an explosion source because tectonic release accompanying an explosion can make it impossible to resolve the isotropic moment using long period data the strongest argument for an earthquake is that both the regional surface waves and waveform modeling at wmq give an estimated focal depth of 6 1 km this result reinforces the importance of focal depth determination to ctbt monitoring in particular for shallow earthquakes with mechanisms that are close to perfect 45 reverse dip slip which are difficult to discriminate by using other methods fortunately depth determination using surface waves is especially favorable for this type of earthquake if a sufficient number of stations can be used we therefore recommend that long period data recorded at auxiliary seismic stations of the ims be utilized by the international data centre to monitor compliance with the ctbt

strain localisation and weakening of the lithosphere during extension we explore the sensitivity of extensional systems to the thermal structure of the lithosphere using numerical simulations that fully couple the energy momentum and continuity equations the rheology of the lithosphere is controlled by weakening processes such as shear heating that localises strain into shear zones and faults numerical models show that during extension of an initially unpatterned lithosphere structures develop spontaneously out of basic thermodynamic energy fluxes and without the imposition of ad hoc rules on strain localisation this contrasts with the classical mohr coulomb theory for brittle localisation which prescribes the angles of faults by a mathematical rule our results show that the mode of extension is sensitive to subtle changes in rheology heat flux and geometry of the system this sensitivity lies at the core of the variety and complexity observed in extensional systems localisation processes make the lithosphere weaker than previously estimated from the brace goetze quasi static approach consequently typical estimates for plate tectonic forces are capable of splitting the lithosphere under extension even without the role of active magmatism

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

evidence for seismogenic normal faults at shallow dips in continental rifts several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping 35 dips often considered shallow most of these occur in the woodlark and aegean rifts the woodlark and aegean rifts are found to generate significantly more earthquakes than others and are the most rapidly extending so display the widest variety of fault behavior even within the woodlark rift system extension rates vary along strike with the shallowest dipping faults confined to the most rapidly rifting segment here several events mw 6 0 6 8 feature nodal planes dipping 23 35 these planes are subparallel to shear zones bounding nearby metamorphic core complexes including one imaged to 8 9 km depth by seismic reflection profiling in the western gulf of corinth at least one large event mw 6 4 occurred on a fault dipping 33 as the woodlark example this rift segment exhibits a high opening rate 10 20 mm yr several other cases elsewhere based on older historic data microseismicity or geologic inference suggest seismic slip at similar or shallower dips still no documented large earthquake exhibits seismic slip on sub horizontal surfaces dip 10 15 stress rotation may explain the 23 35 dips but thus far no realistic mechanism has been found more likely these faults represent surfaces somewhat weaker than surrounding rock through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault such weakening may be a consequence of high slip rates which rapidly generate large offsets and mature fault systems

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

boat towed radio magnetotelluric and controlled source audio magnetotelluric study to resolve fracture zones at aspo hard rock laboratory site sweden boat towed radio magnetotelluric rmt measurements using signals between 14 and 250 khz have attracted increasing attention in the near surface applications for shallow water and archipelago areas

reflection seismic imaging and physical properties of base metal and associated iron deposits in the bathurst mining camp new brunswick canada the bathurst mining camp northern new brunswick canada is a major base metal producing region that includes the brunswick no 12 and smaller past producing brunswick no 6 deposit sustaining mining activity in the camp requires exploration of orebodies near and adjacent to existing mines in this paper we recovered re processed modeled and interpreted a 2d high resolution reflection seismic profile in the vicinity of the brunswick no 6 open pit with the aim of providing key information on the geological structures associated with mineral deposits at depth the seismic data quality is good to excellent with numerous strong reflections in raw shot gathers resulting from a careful survey design that included test shots and a priori considerations for the geological environment physical properties of lithological units and mineralized zones including gamma gamma conductivity magnetic susceptibility density and sonic data were studied from borehole geophysical measurements conducted near the mine this helped to correlate seismic data and shows that the brunswick horizon a key ore prospecting horizon and associated lithological contacts are strongly reflective the results demonstrate that reflection seismic imaging has been particularly effective for imaging steeply dipping structures of the brunswick no 6 deposit many of which intersect the surface and thus allow for correlation with surface geology massive sulfides and iron formation of the brunswick horizon are identifiable locally within a distinctive reflective package that can be used as a broad guide in the region for the exploration of deep base metal deposits in several locations a change from highly reflective to semi transparent seismic character is explained by the presence of faults that juxtapose highly reflective lithological units with seismically transparent lithologies

propagation of seismic slip from brittle to ductile crust evidence from pseudotachylyte of the woodroffe thrust central australia abstract the woodroffe thrust central australia is a 1 5 km wide mylonitized shear zone marked by large volumes of mm to cm scale pseudotachylyte veins the pseudotachylytes display typical melt origin features including rounded and embayed clasts spherulitic and dentritic microlites and flow structures within a fine grained matrix three types of pseudotachylyte are identified on the basis of deformation texture vein morphology and host rock lithology cataclasite related c pt mylonite related m pt and ultramylonite related um pt the m pt and um pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization these pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine grained vein matrix and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite these observations constrain the timing and environment of m pt and um pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust the m pt and um pt veins as well as the host mylonite are overprinted by cataclasis and multiple generations of late stage c pt veins that were generated in the brittle dominated regime of the upper crust during uplift and exhumation of the shear zone the coexistence of multiple generations of voluminous c pt m pt and um pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the woodroffe thrust the timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle dominated seismogenic zone and propagated down through the brittle ductile transition into the ductile dominated regime of the crust

unusual long period rayleigh wave radiation from a vertical dip slip source the 7 may 2001 north sea earthquake 1 crustal structures with a thick surficial sediment layer with low seismic wave speeds produce a reversal in the polarity of the shear stress eigenfunctions of long period rayleigh waves at shallow depth consequently seismic disturbances with a strong vertical dip slip component that are within or just below the sediment layer should generate rayleigh waves that show a polarity reversal when compared with rayleigh waves from the same source in a more typical crustal structure here the first observation of this unusual behavior is presented by modeling surface waves from the 7 may 2001 north sea earthquake a previous study finds a focal mechanism close to vertical dip slip for this earthquake and suggests that the source is within the 6 km thick sediment layer found in this region an appropriate structural model is used to generate synthetic seismograms and estimate a double couple focal mechanism for the source the orientation of the fault plane determined here is similar to that found by the previous study however the slip direction is opposite demonstrating that the use of an incorrect structural model has a profound effect on focal mechanism determination for this type of seismic source

planning of urban underground infrastructure using a broadband seismic landstreamer tomography results and uncertainty quantifications from a case study in southwestern sweden abstractwe have developed a multicomponent broadband seismic landstreamer system based on digital sensors and particularly suitable for noisy environments and areas in which high resolution images of the subsurface are desired we have evaluated results interpretations and approaches using the streamer in the planning of an approximately 3 km long underground tunnel in the city of varberg in the southwestern sweden prospective targets were imaging of the shallow 20 m bedrock surface and weak zones such as fracture and shear zones over the course of three weeks 25 profiles were acquired with a total length of approximately 7 5 km using a source and receiver spacing of 2 4 m a novel approach of the data acquisition was to integrate the landstreamer with wireless sensors in areas in which the accessibility was restricted by roads and also to increase the source receiver distances offsets although the area was highly noisy the seismic data in conjunction with available boreholes successfully l

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

seismic cone penetration test and seismic tomography in permafrost two high resolution multi offset vertical seismic profile vsp surveys were carried out in a permafrost mound near umiujaq in northern quebec canada while performing seismic cone penetration tes

geometry of eclogite facies structural features implications for production and exhumation of ultrahigh pressure and high pressure rocks western gneiss region norway 1 well preserved eclogite facies structural features in metamorphosed gabbro and diorite gneiss in baltica crust formed at a minimum depth of 60 70 km during the scandian orogeny allow direct inferences to be made regarding the geometry of folds kinematics and original structural orientations related to production and exhumation of high pressure rocks folds associated with eclogite facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation strain estimates and l s or l s fabrics indicate that these structures were formed in a constrictional strain field locally they are well preserved in eclogite facies mylonite zones at least 40 m thick that cut proterozoic gabbro and adjacent gneiss where steeply dipping they show a north side up shear sense across a younger northeast trending shallowly plunging amphibolite facies anticlinorium formed in a constrictional noncoaxial strain field where original structural facing direction can be inferred small circle rotation of the eclogite facies lineation about the anticline axis indicates that the relative motion vector of baltica with respect to overlying imbricated crust and laurentia was oriented 320 in a present day reference frame this result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with late silurian to early devonian oblique sinistral convergence between baltica and laurentia results of this study indicate that coherent baltica crust at 60 70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops southeast thrusting parallel to plate motion high pressure rocks were progressively imbricated and stacked along eclogite facies mylonite zones with the deforming zone separating coherent baltica crust from imbricated basement above the constrictional strains under eclogite facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

joint inversion of lake floor electrical resistivity tomography and boat towed radio magnetotelluric data illustrated on synthetic data and an application from the aspo hard rock laboratory site sweden joint inversion of lake floor electrical resistivity tomography and boat towed radio magnetotelluric data illustrated on synthetic data and an application from the aspo hard rock laboratory site sweden

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

non invasive detection of fractures fracture zones and rock damage in a hard rock excavation experience from the aspo hard rock laboratory in sweden abstract a key requirement for licensing of the construction of underground repositories for nuclear waste is the demonstrated capability to verify design assumptions involving the presence and extent of the excavation damage zone around tunnels shafts emplacement holes and caverns as part of ongoing work to select and refine key technologies and techniques towards this end geophysical surveys were performed at two locations within the aspo hard rock laboratory in sweden earth resistivity res induced polarization ip and ground penetrating radar gpr data were collected using a variety of survey parameters light detection and ranging lidar data were collected as a reference for surface structures surface topography and site geology based on an analysis of the data models for the highly damaged zone hdz and excavation damage zone edz at both sites were developed the hdz was found to be approximately 5 to 10 cm in thickness and the edz was found to extend between 15 and 35 cm below the excavation surface two dimensional 2d res profiling generated the most reliable assessment of the hdz whereas chargeability data and gpr data were more useful in the estimation of the edz dimensions

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

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

which fractures are imaged with ground penetrating radar results from an experiment in the aspo hardrock laboratory sweden abstract identifying fractures in the subsurface is crucial for many geomechanical and hydrogeological applications here we assess the ability of the ground penetrating radar gpr method to image open fractures with sub mm apertures in the context of future deep disposal of radioactive waste gpr experiments were conducted in a tunnel located 410 m below sea level within the aspo hard rock laboratory sweden using 3 d surface based acquisitions 3 4 m 19 m with 160 mhz 450 mhz and 750 mhz antennas the nature of 17 identified gpr reflections was analyzed by means of three new boreholes bh1 bh3 9 9 5 m deep out of 21 injection and outflow tests in packed off 1 m sections only five provided responses above the detection threshold with the maximum transmissivity reaching 7 0 10 10 m2 s most gpr reflections are situated in these permeable regions and their characteristics agree well with core and optical televiewer data a 3 d statistical fracture model deduced from fracture traces on neighboring tunnel walls show that the gpr data mainly identify fractures with dips between 0 and 25 since the gpr data are mostly sensitive to open fractures we deduce that the surface gpr method can identify 80 of open sub horizontal fractures we also find that the scaling of gpr fractures in the range of 1 10 m2 agrees well with the statistical model distribution indicating that fracture lengths are preserved by the gpr imaging no measurement bias our results suggests that surface gpr carries the resolution needed to identify the most permeable sub horizontal fractures even in very low permeability formations thereby suggesting that surface gpr could play an important role in geotechnical workflows for instance for industrial scale siting of waste canisters below tunnel floors in nuclear waste repositories

large volumes of anhydrous pseudotachylyte in the woodroffe thrust eastern musgrave ranges australia a mylonitic thrust zone at least 1 5 km thick forms a sharp contact between granulite and amphibolite facies gneisses in the eastern musgrave ranges central australia the thrust dips gently to the south and is interpreted as an extension of the woodroffe thrust which was formed about 550 ma ago mylonites at the base of the thrust grade upwards into ultramylonites which pass abruptly into a pseudotachylyte bearing zone approximately 1 km thick containing approximately 4 of pseudotachylyte veining the orientation of the veins appears to be random pseudotachylytes occur only in the granulite facies rocks and their precursors are felsic pyroxene and or garnet granofelses rotated blocks of ultramylonite are present in some of the pseudotachylytes and some pseudotachylyte veins have been plastically deformed suggesting nearly contemporaneous semiductile and brittle behaviour the matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites also present are glass devitrification microstructures spherulites evidence of liquid flow and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material these features confirm that the pseudotachylyte formed by melting in anhydrous conditions the matrix of the pseudotachylyte veins is less siliceous than the host rocks owing to non equilibrium melting of pyroxene garnet and plagioclase the igneous assemblages of the melt notably the crystallization of pigeonite are consistent with rapid cooling from very high temperature 1000 c melting and quenching is probably due to very local short lived rises in temperature accompanied by dilation

large earthquakes initiate by the acceleration of slips on the downward extensions of seismogenic faults abstract in order to examine the relationship between anomalous crustal deformations and subsequent large earthquakes we compiled and examined data on preseismic anomalous crustal deformations for two earthquakes that occurred in and around the japanese islands in recent years we found that the slip on a downward extension of a seismogenic fault accelerates before the main shock and that the average slip velocity is inversely proportional to the time remaining until the main shock occurs

structural evolution of the whipple and south mountains shear zones southwestern united states the whipple and south mountains of the southwestern united states have undergone a strikingly similar sequence of deformations in both ranges gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low angle detachment fault kinematic indicators reveal that the mylonitic rocks brittle structures and detachment faults are kinematically coordinated and were all formed by top to the northeast shear the structural evolution of both areas can be explained in terms of major shallow dipping shear zones that accommodated tertiary crustal extension we suggest that detachment faults and associated zones of brecciation cataclasis and seismic slip were originally continuous downdip along the low angle shear zones into mylonitic gneisses formed below or near the ductile brittle transition as the mylonites were drawn out from beneath the brittlely extending upper plate they were progressively uplifted above the ductile brittle transition and were overprinted by successively more brittle structures

present activity and seismogenic potential of a low angle normal fault system citta di castello italy constraints from surface geology seismic reflection data and seismicity abstract we present new constraints on an active low angle normal fault system in the citta di castello sansepolcro basin csb of the northern apennines of italy new field data from the geological survey of the carta geologica d italia carg project define the surface geometry of the normal fault system and lead to an interpretation of the crop 03 deep crust seismic reflection profile castiglion fiorentino urbania segment with particular attention paid to the geometry of the plio quaternary extensional structures surface and sub surface geological data are integrated with instrumental and historical seismicity in order to define the seismotectonics of the area low angle east dipping reflectors are the seismic expression of the well known altotiberina fault af a regional extensional detachment on which both east and west dipping high angle faults bounding the csb sole out the af breakaway zone is located 10 km west of the csb within the extensional allochthon synthetic east dipping planes prevail displacement along the af is 4 5 km which agrees with the cumulative offset due to its synthetic splays the evolution of the csb has mainly been controlled by the east dipping fault system at least since early pleistocene time this system is still active and responsible for the seismicity of the area a low level of seismic activity was recorded instrumentally within the csb but several damaging earthquakes have occurred in historical times the instrumental seismicity and the intensity data points of the largest historical earthquakes 5 events with maximum mcs intensity of ix to ix x allow us to propose two main seismogenic structures the monte santa maria tiberina mmax 5 9 and citta di castello mmax up to 6 5 normal faults both are synthetic splays of the af detachment dipping to the ne at moderate 45 50 to low 25 30 angles and cutting the upper crust up to the surface this study suggests that low angle normal faults at least with dips of 25 30 may be seismogenic

protolith ages and exhumation histories of ultra high pressure rocks across the western gneiss region norway the timing of protolith formation ultrahigh pressure uhp subduction and subsequent exhumation for the ultrahigh pressure to high pressure units across the eastern part of the western gneiss region norway were assessed using u pb zircon th pb monazite and 40 ar 39 ar white mica ages u pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the baltica basement before the entire mass was subducted to ultra high pressure eclogites within the allochthons across the entire western gneiss region are caledonian and show a degree of zircon re crystallization that increases with peak pressure permitting the interpretation that the entire region underwent synchronous subduction 40 ar 39 ar white mica ages of 399 ma indicate that the eastern part of the western gneiss region had been exhumed to shallow crustal levels while uhp metamorphism was ongoing farther west indicating a westward dip to the slab the 40 ar 39 ar white mica ages also show a clear east to west gradient across the entire western gneiss region indicating that the western gneiss region rose diachronously to crustal levels from east to west between 399 and 390 ma

styles of extension offshore mid norway and implications for mechanisms of crustal thinning at passive margins 1 eocene magmatic breakup along the mid norway rifted margin was preceded by extreme jurassic cretaceous crustal thinning in a magma poor environment along the se borders of the rift top basement detachment faults with heaves on the order of 15 40 km evolved in at least two stages to become the boundaries between moderately thinned 20 30 km thick crust and a 100 200 km wide highly extended area with crustal thicknesses generally between 2 and 12 km under the present day more and voring basins in the footwalls of the basin flank detachments lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults under the most highly thinned areas a more distal set of deep seated basin floor detachments incised and extended remnant crust and probably the upper mantle leaving as little as 5 km of continental crust to be preserved under thick synrift and postrift deposits we suggest that basin flank detachments such as the ones described above hold the potential to reduce the crustal thickness down to the critical value required for embrittlement of continental crust thus they prepare the ground for incision by the basin floor detachments which may become responsible for exhumation of deep crust or continental mantle if extension is allowed to proceed

direct calculation of rupture depth for an exhumed paleoseismogenic fault from mylonitic pseudotachylyte multiple generations of pseudotachylyte within granulite facies orthogneiss northern norway caledonides are cut by ultramylonitic shear bands that contain the mineral assemblage garnet clinopyroxene plagioclase quartz mineralogic thermobarometry yields pressure p 950 150 mpa at temperature t 625 675 c for equilibration conditions of the ultramylonite assemblage requiring minimum depths of 35 5 km for coseismic slip that generated pseudotachylyte these depths are well below the base of the standard seismogenic zone 15 20 km ambient temperatures are 200 300 c higher than those commonly cited for the onset of ductile creep in quartzofeldspathic rocks but they are similar to those in modern intraplate settings of higher than average heat flow that have experienced deep crustal earthquakes the occurrence of overprinting mylonites and pseudotachylytes requires tens of kilometers of dynamic downward rupture propagation from the brittle frictional regime into the ductile regime plastic instabilities do not explain the relationship between ultramylonites and mylonitized pseudotachylytes in this setting

earthquake seismograms that show doppler effects due to crack propagation summary examples are presented of earthquake p wave pulses seen on broadband seismograms to show that on such recordings the pulse shapes are more clearly seen than on conventional short period and long period seismograms most of the broadband seismograms have been chosen because they show marked differences between the pulse lengths of p and those of the surface reflections in addition some of the pulses appear to have smooth onsets and abrupt trailing edges so that the onset of the pulse is difficult to observe and the largest amplitude arrivals seen on the seismogram coincide not with the onset of motion but with the termination of motion that is the large arrivals mark stopping phases of motion we assume that the differences in pulse length are due to the effects of a moving source that is a doppler effect and that the pulses with smooth onsets and abrupt trailing edges can be modelled simply by a source propagating on a line with low radiation amplitude at the start of motion a trial and error method guided by a published fault plane solution is then used to obtain a fit between observed and computed seismograms for one of the earthquakes this process leads to an estimate of the crack speed of about 1 4 times the 5 wave speed the errors that may arise in estimating source depths and orientation if stopping phases are not recognized as such is discussed

limited basement involvement in caledonian deformation hinnoy nnorth norway and tectonic implications abstract structural and petrographic relationships on east hinnoy north norway indicate that the caledonian nappes which are exposed directly to the east were emplaced over an older precambrian crystalline terrane lofoten terrane while at amphibolite facies metamorphic conditions caledonian structural and metamorphic effects disappear structurally downward away from the basal thrust of the nappe stack the limited involvement of the basement in caledonian deformation is explained by the limited availability of water which was introduced from external sources the external water source is hypothetically the autochthonous and allochthonous cover and the precambrian austerfjord group metasedimentary rocks within the basement which underwent prograde metamorphic dehydration during caledonian orogenesis the mechanisms by which addition of water concentrated strain in the basement probably include reaction induced ductility and hydrolytic weakening of the constituent silicate minerals there are two main tectonic implications 1 in collisional metamorphic belts the lithosphere of the underthrusted plate may remain rigid below 15 25 km depth and 2 the common phenomenon of detachment of crystalline thrust sheets at mid crustal levels may be controlled in some cases by the limited access of water to pre existing crystalline rocks rather than by thermal structure

seismic characterization of fracturing at the aspo hard rock laboratory sweden from the kilometer scale to the meter scale abstract in 1986 the swedish nuclear fuel and waste management co skb decided to build an underground research laboratory the aspo hard rock laboratory hrl the aspo hrl offers the possibility to test and demonstrate a full site investigation sequence as part of this work various seismic techniques were tested and demonstrated from the kilometer to the meter scale the preliminary investigation stage comprised surface reflection profiling and vsp the vsp measurements though limited to one borehole played an important role in defining the site model shallow reflection surveys in crystalline rock have been judged as basically feasible but further development is required to reduce costs during the construction of the laboratory the objective of the seismic investigations was to check and refine the site model within the zone of excavation disturbance experiment project seismic tomographic analyses were made for p and s wave velocity and attenuation at 5 15 m scale crosshole reflection imaging was also performed the tracer retention understanding experiments project also included transmission and reflection tomography but on a 100 300 m scale for the experiments where both transmission and reflection measurements were processed the seismic surveys also produced information regarding the intrinsic mechanical properties of the rock besides inferring a geometrical model of the rock structures

imaging the high temperature geothermal field at krafla using vertical seismic profiling abstract geophysical exploration and in particular active source seismic imaging of geothermal fields is important to assess and optimize the exploitation of natural heat sources for energy production and direct use the first multi offset moving source vertical seismic profiling vsp experiment over the high temperature geothermal field in krafla iceland was carried out in spring 2014 with the aim to test whether vsp is a suitable method to map volcanic stratigraphy fractures dykes steam zones and magmatic bodies at this site and for volcanic environments in general in this study we present a workflow for processing the sparse krafla vsp dataset recorded with receivers in either of two boreholes the analysis involved first arrival traveltime inversion and seismic reflection processing the seismic velocity model obtained by traveltime tomography reveals structural information between the two boreholes and can be linked to an existing geological model showing that the seismic velocities are mainly controlled by lithology the zero offset seismic reflection data were processed into two corridor stacks walk away vsp reflection data were migrated with a novel multicomponent kirchhoff migration algorithm that includes p and s wave isolation to obtain separate pp ps and ss migrated images the reflections imaged in the corridor stacks can be linked to the main lithological units known from borehole logging information migrated images from the walk away data reveal reflectors below and to the sides of the two boreholes considering a priori information such as hypocenter locations from earthquake seismology studies the reflectors can be related to changes in lithology fault zones dykes and possibly the top of the krafla magma chamber we found that vsp is potentially a useful method to image the key lithological boundaries and volcanic stratigraphy in the complex magmatic environment at krafla but a priori information proved to be essential to constrain the processing and interpretation of the sparse array dataset

an enigmatic earthquake in the continental mantle lithosphere of stable north america abstract the existence of earthquakes within continental lithospheric mantle remains a highly controversial topic here we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the wind river range of central wyoming combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically observed depth phases we demonstrate that the 2013 wind river earthquake a m w 4 7 highly oblique thrust faulting event occurred at 75 8 km well beneath the base of the crust the magnitude mechanism and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere as a result of tectonic rather than magmatic processes

multi objective analysis of body and surface waves from the market rasen uk earthquake summary the 2008 february 27 market rasen uk earthquake mb 4 5 is used to show how earthquake source parameters can be estimated using a multi objective optimization approach to the joint inversion of teleseismic body wave and regional surface wave observations to estimate the source depth short period teleseismic seismograms are interpreted in terms of p and the depth phases pp and sp using the f statistic and its associated probability theory results of this analysis are inconclusive as two possible source depths are identified to identify our preferred depth a method of estimating source depth and mechanism by modelling surface wave amplitude spectra is applied to data recorded at regional distances a source depth of 22 km is found to be consistent with the observed body and surface waves upper and lower amplitude bounds and polarities are then placed on the phases interpreted as p pp and sp for a source depth of 22 km the gaussian relative amplitude method is used to calculate a likelihood for all possible orientations of the double couple source a multi objective optimization approach can then used to compare these likelihoods with calculated misfits between observed and synthetic surface wave amplitude spectra for all orientations of the double couple source our preferred focal mechanism s 205 50 and 15 is found by normalizing and summing both misfits to find the focal mechanism with the lowest combined misfit synthetic body wave seismograms surface wave amplitude spectra and seismograms are in good agreement with the observed data for our preferred focal mechanism and depth

chemical heterogeneities of caledonian pseudotachylites in the eidsfjord anorthosite north norway the 1 8 1 7 ga eidsfjord anorthosite complex on langoy vesteralen north norway is thrust over monzonitic gneisses in a 100 m wide fault zone the anorthosite contains abundant pseudotachylite which also overprint ductilely formed ultra mylonites in the monzonitic gneisses 40ar 39ar dating of the pseudotachylites indicates that this deformational event took place during the end of the caledonian orogeny pseudotachylite formation by in situ melting is corroborated by a detailed petrological investigation of chemical and textural criteria identity of host rock mineral compositions and modes with pseudotachylite clasts and calculated modes confirm the common notion that transport over more than a few cm did not occur during pseudotachylite formation because the characteristics of the host rocks are inherited by the pseudotachylite pseudotachylite compositions lie in the range of xrf whole rock analyses of deformed and undeformed host rocks demonstrating that no material was added or lost during pseudotachylitisation the pseudotachylites show strong chemical heterogeneity on a scale of tens of micrometers this compositional variation can be described by an approximately binary mixture of plagioclase with a homogenised mafic component comprising orthopyroxene clinopyroxene and fe ti oxides although the pseudotachylite matrix is not homogenised and locally shows schlieren textures the mafic phase in the anorthositic rocks must have homogenised very well before incorporation into the pseudotachylite different viscosities and mixing properties for the al and the fe rich parts of the pseudotachylite melt are inferred

bias in the centroid moment tensor for central asian earthquakes evidence from regional surface wave data surface waves recorded on the chinese digital seismic network cdsn station urumqi wmq are analyzed for 26 centroid moment tensor cmt earthquakes located within an epicentral distance of 1300 km applying a simple grid search method for source parameter retrieval i obtain revised estimates of the focal depth fault strike and seismic moment m sub 0 from amplitude spectra of love and rayleigh waves and compare these estimates with the cmt best double couple solutions the comparisons show that residuals are unbiased for focal depth and fault strike where population spreads are plus minus 13 km and plus minus 17 degree respectively on the other hand m sub 0 estimates are found to be biased with cmt m sub 0 larger than regional estimates by an average of 0 27 plus minus 0 04 log units the results for focal depth and strike are consistent with previous global and regional studies comparing independent estimates with cmt results recent full waveform modeling studies for central asian earthquakes support the findings of bias in m sub 0 estimates i suggest that causes for m sub 0 bias may be related to great thicknesses of continental crust in hindu kush and tien shan regions and to data censoring practices at small magnitudes m sub 0 m sub b scaling relationships for central asianmore earthquakes show better agreement with western u s scaling when m sub 0 estimates determined in this study are used copyright 1998 american geophysical union less

receiver functions for the tien shan analog broadband network contrasts in the evolution of structures across the talasso fergana fault we determined the structure of the crust beneath the central and western tien shan by analyzing broadband 1 20 s analog seismograms of converted p sv phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations the one dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the talasso fergana fault specifically the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault also velocities at depths between 10 and 35 km are about 10 lower in the east although the depth range of these lower velocities is not well resolved the talasso fergana fault is an important boundary for other observables in addition to the structural discontinuities observed at the surface the areas east of the talasso fergana fault are associated with abnormally low mantle velocities outcrops of basalt low q and short wavelength variations in anisotropy integrating these observations we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust likewise the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids the gross contrast in structure east and west of the talasso fergana fault could reflect a contrast in the dynamics of mountain building in these two regions we postulate that while crustal shortening is the dominant mechanism controlling topography west of the talasso fergana fault vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process