brittle tectonic evolution along the western margin of south africa more than 500 myr of continued reactivation abstract the brittle structural history of western south africa has been investigated by remote sensing and field studies to build a conceptual scheme for its 500 ma long evolution paleostress tensors were computed from a significant fault slip dataset and a relative geochronological succession of brittle deformation events was established this was aided by separating in time faulting events through the usage of cretaceous weathering horizons silicified fluvial deposits paleosols and 77 54 ma olivine melilitite plugs as time markers the oldest features recognized formed during four compressional episodes assigned to the neoproterozoic pan african evolution this history is expressed by sub vertical conjugate fracture sets and fits well the inferences derived from remote sensing the greatest compressive direction rotated from nw se to nne ssw and finally to almost e w a subsequent ene wsw oriented extensional episode is associated with the local effects of the opening of the atlantic ocean and was followed by a second ca e w extensional episode linked to the well acknowledged mid cretaceous 115 90 ma event of margin uplift a late santonian 85 83 ma nw se compressive paleostress deformed the late cretaceous sequences and was in turn followed firstly by a renewed episode of ne sw extension and later by ca nne ssw late maastrichtian 69 65 ma shortening the latter is broadly coeval with the emplacement of the gamoep magmatic suite a phase of wnw ese cenozoic extension is assigned to the extensional phase recorded in the okawango delta interpreted as reflecting propagation of the east african rift system into southern africa no stress tensor was computed for the present day wegener anomaly stress field oriented nw se however in situ stress measurements were used to perform slip tendency analysis which indicates that under the currently existing stress conditions wnw ese and nnw sse striking faults are critically stressed and are the most likely reactivated in agreement with the present seismicity

booming plutons source of microearthquakes in south carolina 1 loud shallow microearthquakes m 3 0 occurring in the vicinity of granitic plutons represent a different category of seismicity compared to other recognized seismic sources in south carolina we demonstrate this difference by comparing the locations of microearthquakes in the vicinity of three granitic plutons in south carolina with the results of two dimensional numerical modeling and analytical studies the less rigid plutons embedded in more rigid country rock were loaded by applying ambient tectonic plate stresses along the direction of maximum horizontal compression the results of modeling showed that regions of computed high stresses lie on the periphery of the plutons and coincide with both the observed locations of seismicity and with lobes of elevated stresses obtained by analytical calculations for a weak pluton subjected to a homogenous stress field the amplitude of the modeled stresses appears to be a function of the shape and size of the pluton

late quaternary slip on the santa cruz island fault california the style timing and pattern of slip on the santa cruz island fault were investigated by trenching the fault and by analysis of offset late quaternary landforms a trench excavated across the fault at christi beach on the western coast of the island exposed deformation of latest pleistocene to holocene sediments and pre quaternary rocks recording repeated large magnitude rupture events the most recent earthquake at this site occurred ca 5 ka coastal terraces preserved on western santa cruz island have been dated using the uranium series technique and by extrapolation using terrace elevations and the eustatic record offset of terraces and other landforms indicates that the santa cruz island fault is predominantly left lateral having a horizontal slip rate of not more than 1 1 mm yr and probably about 0 8 mm yr the fault also has a smaller reverse component slipping at a rate of between 0 1 and 0 2 mm yr combined with measurements of slip per event this information suggests a long term average recurrence interval of at least 2 7 k y and probably 4 5 k y and average earthquake magnitudes of mw 7 2 7 5 sense of slip recurrence interval and earthquake magnitudes calculated here for the santa cruz island fault are very similar to recent results for other faults along the southern margin of the western transverse range including the malibu coast fault the santa monica fault the hollywood fault and the raymond fault supporting the contention that these faults constitute a continuous and linked fault system which is characterized by large but relatively infrequent earthquakes

the 1930 santa monica and the 1979 malibu california earthquakes the malibu earthquake m l 5 0 that occurred 1 january 1979 in santa monica bay was located at 33 56 9 n and 118 41 3 w and at a depth of 10 km the focal mechanism of the main shock as determined from first motions recorded by short period high gain seismic stations in southern california shows reverse faulting with one plane dipping 55 to 60 to the north striking 275 to 290 and the second plane dipping 30 to 38 to the south and striking 100 to 120 more than 400 aftershocks were recorded during 1979 focal mechanisms for 20 aftershocks of m 3 0 show similar reverse mechanisms as the main shock the main shock and aftershocks m 2 5 were relocated using a local velocity model and a set of corresponding station delays the aftershocks define a volumetric hypocentral zone 4 km 6 km west trending with depths ranging from 6 to 11 km this 1979 malibu main shock aftershock sequence probably occurred on the eastern end of the anacapa dume fault the santa monica earthquake m l 5 2 that occurred 31 august 1930 was relocated using station delays calculated from the 1979 date set the relocated 1930 epicenter is located near the western end of the santa monica fault the 1979 and 1930 main shocks can be interpreted to define a 8 to 10 km north south sidestep between the anacapa dume and santa monica faults possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

high resolution mapping of two large scale transpressional fault zones in the california continental borderland santa cruz catalina ridge and ferrelo faults new mapping of two active transpressional fault zones in the california continental borderland the santa cruz catalina ridge fault and the ferrelo fault was carried out to characterize their geometries using over 4500 line km of new multibeam bathymetry data collected in 2010 combined with existing data faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single channel and multichannel seismic profiles compiled over the past three decades the two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes the geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures historical and recent focal mechanisms obtained from first motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse oblique mechanisms on adjacent structures transpression on west and northwest trending structures persists as far as 270 km south of the transverse ranges extension persists in the southern borderland a logjam model describes the tectonic evolution of crustal blocks bounded by strike slip and reverse faults which are restrained from northwest displacement by the transverse ranges and the southern san andreas fault big bend because of their potential for dip slip rupture the faults may also be capable of generating local tsunamis that would impact southern california coastlines including populated regions in the channel islands

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

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

spatiotemporal complexity of continental intraplate seismicity insights from geodynamic modeling and implications for seismic hazard estimation continental intraplate seismicity seems often episodic clustered and mi grating the observed seismicity shows both spatial clustering in seismic zones and scattering across large plate interiors temporal clustering followed by long periods of quiescence and migration of seismicity from one seismic zone to another here we explore the complex spatiotemporal patterns of intraplate seismicity using a 3d visco elasto plastic finite element model the model simulates tectonic loading crustal failure in earthquakes and coseismic and postseismic stress evolution for a later ally homogeneous lithosphere with randomly prespecified perturbations of crustal strength the model predicts various spatiotemporal patterns of seismicity at different timescales spatial clustering in narrow belts and scattering across large regions over hundreds of years connected seismic belts over thousands of years and widely scat tered seismicity over tens of thousands of years the orientation of seismic belts coincides with the optimal failure directions associated with the assumed tectonic loading stress triggering and migration cause spatiotemporal clustering of earth quakes when weak zones are included in the model the predicted seismicity initiates within the weak zones but then extends far beyond them if a fault zone is weakened following a large earthquake repeated large earthquakes can occur on the same fault zone even in the absence of strong tectonic loading these complex spatiotemporal patterns of intraplate seismicity predicted in this simple model suggest that assessment of earthquake hazard based on the limited historic record may be biased toward over estimating the hazard in regions of recent large earthquakes and underestimating the hazard where seismicity has been low during the historic record

deposition extension and the shape of downbuilding salt diapirs the authors present a new hypothesis for diapir growth in which deposition and extension interact with coeval salt flow the hypothesis is based on scaled tectonic experiments in which viscous polymers simulated ductile salt and frictional plastic dry sand simulated brittle overburden the model diapirs all grew by downbuilding beneath accumulating overburdens without regional extension sedimentary differential loading causes salt to rise where the overburden is thinnest the diapir shape depends on the overburden aggradation rate relative to the flow rate of salt up the diapir conduit with rapid deposition strata onlap the diapir which narrows upward and eventually becomes buried and inactive with moderate deposition strata uplap against vertical diapir flanks with slow deposition salt spreads by extrusion or shallow intrusion and steeply overrides toplapped strata the diapir widens upward until salt supply is depleted with very slow deposition subhorizontal tongues of salt override toplapped strata thin skinned regional extension initially promotes piercement by tectonic differential loading but it also impedes upward salt flow by horizontally stretching the diapir with rapid extension the diapir sags and is indented by growth faults and overlain by depotroughs with moderate extension rates the diapir widens with time and thus narrows upward only where extensionmore is very slow can upward widening or near vertical diapir walls form changes in rates of aggradation flow or extension during diapirism can explain complex geometries such as inverted teardrop shapes without invoking shrinking stems model results are applied to examples from the gulf of mexico and other salt regions less

seismicity in deep gold mines of south africa implications for tectonic earthquakes we have studied induced seismicity associated with five deep gold mines located in the far west rand district republic of south africa focusing on the digital data recorded from january 1994 until february 2000 by in mine arrays of three component geophones the observed seismicity which exceeds 1 000 events per day can be divided into two kinds of events designated as types a and b type a events are tightly clustered in time and space and generally occur within 100 m of an active mining face or development tunnel their spectra are comparatively enriched in high frequencies and they have an upper moment magnitude cutoff at mmax 1 we associate these events with the fracture dominated rupture of com petent rock induced by dynamic stresses during blasting and quasi static stress per turbations from the excavation and closure of individual stopes in contrast type b events are temporally and spatially distributed throughout the active mining region they represent friction dominated slip in existing shear zones such as faults or dikes and have source scaling properties that agree well with extrapolations from tectonic earthquakes in the far west rand region type b events can have large magnitudes m 3 but they show a distinct lower magnitude cutoff at mmin 0 we interpret this cutoff in terms of a critical patch size for nucleation of shear failure and we show that the data are consistent with a rate and state dependent friction model in which the critical slip distance d c 10 4 m both the spectral predictions of this model and accelerograms of type b events agree that f max 200 hz

state of stress and intraplate earthquakes in the united states recently compiled data on the state of stress have been used to define stress provinces in the conterminous united states in which the orientation and relative magnitude of the horizontal principal stresses are fairly uniform the observed pattems of stress constrain mechanisms for generating intraplate lithospheric stresses coupled with new information on geologic structure and tectonism in seismically active areas of the midcontinent and east these data help to define some characteristics common to these areas and to identify key questions regarding why certain faults seem to be seismically active

coastal tectonics on the eastern margin of the pacific rim late quaternary sea level history and uplift rates channel islands national park california usa abstract the pacific rim is a region where tectonic processes play a significant role in coastal landscape evolution coastal california on the eastern margin of the pacific rim is very active tectonically and geomorphic expressions of this include uplifted marine terraces there have been however conflicting estimates of the rate of late quaternary uplift of marine terraces in coastal california particularly for the northern channel islands in the present study the terraces on san miguel island and santa rosa island were mapped and new age estimates were generated using uranium series dating of fossil corals and amino acid geochronology of fossil mollusks results indicate that the 2nd terrace on both islands is 120 ka and the 1st terrace on santa rosa island is 80 ka these ages correspond to two global high sea stands of the last interglacial complex marine isotope stages mis 5 5 and 5 1 respectively the age estimates indicate that san miguel island and santa rosa island have been tectonically uplifted at rates of 0 12 0 20 m ka in the late quaternary similar to uplift rates inferred from previous studies on neighboring santa cruz island the newly estimated uplift rates for the northern channel islands are however an order of magnitude lower than a recent study that generated uplift rates from an offshore terrace dating to the last glacial period the differences between the estimated uplift rates in the present study and the offshore study are explained by the magnitude of glacial isostatic adjustment gia effects that were not known at the time of the earlier study set in the larger context of northeastern pacific rim tectonics channel islands uplift rates are higher than those coastal localities on the margin of the east pacific rise spreading center but slightly lower than those of most localities adjacent to the cascadia subduction zone the uplift rates reported here for the northern channel islands are similar to those reported for most other localities where strike slip tectonics are dominant but lower than localities where restraining bends such as the big bend of the san andreas fault result in crustal shortening

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

a velocity description of constant thickness fault propagation folding abstract the expression of the geometric model of constant thickness fault propagation folding as a velocity description of deformation allows the derivation of rates of displacement uplift and fault propagation the velocity model of fault propagation folding and sedimentation are combined in a finite difference scheme and two examples of growth strata associated with overturned fold forelimbs illustrate the application of the model

the 29 september 1969 ceres south africa earthquake full waveform moment tensor inversion for point source and kinematic source parameters the ceres earthquake of 29 september 1969 is the largest known earthquake in southern africa digitized analog recordings from worldwide standardized seismographic network stations powell and fries 1964 are used to retrieve the point source moment tensor and the most likely centroid depth of the event using full waveform modeling a scalar seismic moment of 2 2 2 4 1018 n m corresponding to a moment magnitude of 6 2 6 3 is found the analysis confirms the pure strike slip mechanism previously determined from onset polarities by green and bloch 1971 overall good agreement with the fault orientation previously estimated from local aftershock recordings is found the centroid depth can be constrained to be less than 15 km in a second analysis step we use a higher order moment tensor based inversion scheme for simple extended rupture models to constrain the lateral fault dimensions we find rupture propagated unilaterally for 4 7 s from east southwest to west northwest for about 17 km average rupture velocity of about 3 1 km s online material waveform and spectral fits

paleomagnetism and neogene tectonics of the northern channel islands california geologic observations and previous paleomagnetic studies have suggested that the western transverse ranges arrived at their anomalous east west orientation by clockwise tectonic rotation a paleomagnetic study of the northern channel islands was undertaken in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern california borderland the islands of anacapa santa cruz santa rosa and san miguel form an east west trending chain and are considered part of the transverse range physiographic province oligocene through miocene volcanic and intrusive rocks occur on these islands and were sampled for this paleomagnetic study eocene sandstones were sampled on san miguel island and eocene sandstones and miocene siltstones were sampled on santa cruz island paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69 to 81 from expected directions and inclinations which are too shallow by 10 to 25 normal and reversely magnetized units yield antipodal mean directions the mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests these results suggest that the northern channel islands have been tectonically rotated into place since early miocene time as the outer borderland area translated northwestward within a large shear zone between the pacific and north american plates the data from the northern channel islands when averaged with the data from the santa monica mountains yield a result of i 36 1 5 1 d 72 6 6 3 for the southern part of the western transverse ranges 52 units 405 samples the shallow paleomagnetic inclinations suggest a northward latitude translation of 14 0 3 7 or 3 9 for the western transverse ranges however most plate tectonic reconstructions suggest only 4 3 of northward translation the discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components or motions of small tectonic plates between the north american and pacific plates as the farallan plate broke up and was subducted

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

sea level paleogeography and archeology on california s northern channel islands abstract sea level rise during the late pleistocene and early holocene inundated nearshore areas in many parts of the world producing drastic changes in local ecosystems and obscuring significant portions of the archeological record although global forces are at play the effects of sea level rise are highly localized due to variability in glacial isostatic adjustment gia effects interpretations of coastal paleoecology and archeology require reliable estimates of ancient shorelines that account for gia effects here we build on previous models for california s northern channel islands producing more accurate late pleistocene and holocene paleogeographic reconstructions adjusted for regional gia variability this region has contributed significantly to our understanding of early new world coastal foragers sea level that was about 80 85 m lower than present at the time of the first known human occupation brought about a landscape and ecology substantially different than today during the late pleistocene large tracts of coastal lowlands were exposed while a colder wetter climate and fluctuating marine conditions interacted with rapidly evolving littoral environments at the close of the pleistocene and start of the holocene people in coastal california faced shrinking land intertidal and subtidal zones with important implications for resource availability and distribution

time variable deformation in the new madrid seismic zone new geodetic measurements show that the new madrid is currently deforming too slowly if at all to account for large earthquakes in the region over the past 5000 years this result together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicates that either tectonic loading rates or fault properties vary over a few thousand years

microseismic activity and basement controls on an active intraplate strike slip fault ceres tulbagh south africa earthquakes far from plate boundaries are poorly understood because of few well studied examples and uncertainty of what controls the location of these events in 1969 a damaging local magnitude m l 6 3 strike slip earthquake with no surface expression occurred in the ceres tulbagh region in the south african stable continental interior here we present a microseismic study of the ceres tulbagh area conducted over three months in 2012 in which 172 events recorded on at least three stations follow a gutenberg richter relationship for 1 5 m l 0 5 the events delineate a 5 km wide subvertical zone that is microseismically active to a depth of 15 km this fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress we argue that the microseismically active zone is guided by inherited structures in the basement geology this and similar structures may represent significant earthquake hazard in plate interiors online material tables of station and event parameters and p and s wave arrival times

can deglaciation trigger earthquakes in n america a spherical self gravitating viscoelastic earth model is used to calculate the spatial and temporal evolution of the glacially induced lithospheric stress and fault stability in north america the predicted onset time of a pulse of earthquake activities the mode of failure and the magnitude of instability are investigated for three sites with different distance from the former ice margin it is found that glacial unloading is able to trigger paleo earthquakes within the ice margin near charlevoix 47 5 n 70 1 w and in wabash valley 38 5 n 87 w outside the ice margin however rebound stress decays away from the former ice margin thus glacial unloading is unlikely to have triggered the large m8 earthquakes in new madrid 36 6 n 89 5 w

sycamore knoll a wave planed pop up structure in a sinistral oblique thrust system southern california continental borderland at the boundary between the western transverse ranges province and inner continental borderland of southern california strain is partitioned across the sinistral oblique anacapa dume fault system sycamore knoll a pop up structure 20 km west of point dume in the hanging wall of the north dipping anacapa dume fault stands out as an anomalous bathymetric high along the southern california coast we synthesize new and existing geophysical datasets including multibeam bathymetry backscatter and single multichannel seismic to understand the morphology and tectonic implications of sycamore knoll we identify a complete eustatic low stand submerged marine terrace package encircling sycamore knoll platform that corresponds to the last glacial maximum lgm the marine terrace deposits and wave planed surface of sycamore knoll require a late quaternary uplift rate of 0 55 0 5 0 3 mm yr relative to sea level by combining our uplift rate with previous estimates of basin subsidence and a revised structural model of the anacapa dume fault we determine a maximum slip rate in latest quaternary time of 1 50 0 5 0 3 mm yr and hence a post lgm rate of convergence across the anacapa dume fault of 0 60 0 2 0 1 mm yr

uncharted seismic risk the potential for earthquakes along the plate boundaries has been mapped with reasonable success our attention should now focus on the threat posed by unanticipated quakes located in the continental interiors

lowstand terraces on tyrrhenian sea steep continental slopes abstract wave built terraces formed during sea level stillstands at depths lower than present are found along margins with fairly steep morphology and no to ill developed continental shelf for instance the margin of volcanic islands terraces deposits are up to 20 30 m thick and are flat topped with well defined edges they extend up to 1 km in dip section the deposits are composed of intrabasinal sediments mainly bioclastic sand and silt and are probably due to deposition below the storm wave base during large scale energy episodes two areas of the tyrrhenian sea have been analysed one in a volcanic context the pontine archipelago and the other in a highly active structural context the calabrian arch similar wave built terraces were found in both areas with a main terrace placed at a depth of 100 150 m which is thought to be wurmian older terraces are also present emplaced during the preceding lowstands in the late pleistocene a possible present day terrace is found at about 20 m depth in the pontine archipelago a mechanism of subsequent formation below wave base level and removal because of sea level lowering has been hypothesised during long lasting periods of late pleistocene sea level falls terraces are thus thought to preserve only at the minimum lowstand position and to remain relict on the slope during sea level rise and highstand the analysis of the distribution and depths of these terraces allows some neotectonic interpretation for example in the volcanic archipelago whose western sector has probably risen at an average rate of 2 5 mm yr over the last 20 000 years

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

neoproterozoic to early cambrian crustal evolution of the pan african saldania belt south africa abstract the saldania belt forms part of a system of neoproterozoic mobile belts that surround and weld older cratons on the african continent it is a poorly exposed low grade orogenic belt composed of a number of inliers unroofed in mega anticlinal hinges of the permo triassic cape fold belt along the southern tip of africa deformed meta volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep rooted sinistral dextral strike slip fault zones which display evidence of prolonged reactivation sedimentary rocks and volcanics of the boland terrane malmesbury group lower kango group and gamtoos group are considered to be distal facies of rift successions overlying the mid proterozoic crystalline basement on the southwestern edge of the kalahari craton these lithologies have been deposited in semi pull apart basins that formed in response to the breakup of rodinia and progressive opening of the proto atlantic adamaster ocean from 780 750 ma fragments of juvenile pan african crust with wpb morb characteristics occur in the bridgetown formation swartland terrane malmesbury inlier and attest to ocean floor spreading in this belt the thick turbidite successions of the swartland and tygerberg terranes malmesbury group reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean continental margin basin marked by increasing sea levels and reduction of continental freeboard reversal of spreading and closing of the adamastor ocean possibly initiated by the opening of iapetus occurred in the period 600 570 ma although this movement vector caused sinistral transpressional reactivation of the gariep and saldanian margins there is no proper collision orogen developed in the saldania belt the bridgetown formation represents the suture between the southern rifted margin succession of the kalahari craton and a poorly developed southern orogenic belt represented in part by the swartland and tygerberg terranes this geosuture is similar to that of the south atlantic which formed in the analogous gariep belt between 575 and 543 ma multiphase s i and a type granites of the cape granite suite intruded the saldania belt in a pervasive transpressive regime between 550 and 510 ma comparing well with the timing of peak metamorphism collision in the gariep belt 545 ma the absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the saldania belt seem to suggest that the cape granite suite was generated in a strike slip regime a discrepancy of approximately 50 ma between the age of the collision granites of the dom feliciano belt and cape granite suite suggests that they are genetically unrelated similarities in structural styles magmatism and dated events between the saldania ross delamarian chain of orogens however indicate a common history the evolution of this chain and that of the gariep damara dom feliciano belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of laurentia from south america and opening of the iapetus ocean loading and depression of the western and southern kalahari craton margin by thrust stacks of the late neoproterozoic orogens led to the formation of syn to post orogenic peripheral foreland basins such as those represented by the nama and vanrhynsdorp groups lowering of sea level and increased continental freeboard deposition of the unconformity based upper kango group and possibly the molassic franschhoek formation occurred in intra orogen pull apart basins which possibly developed between 530 and 510 ma in the saldania belt

strain distribution within a km scale mid crustal shear zone the kuckaus mylonite zone namibia the subvertical kuckaus mylonite zone kmz is a km wide crustal scale proterozoic dextral strike slip shear zone in the aus granulite terrain sw namibia the kmz was active under retrograde amphibolite to greenschist facies conditions and deformed felsic and minor mafic gneisses which had previously experienced granulite facies metamorphism during the namaqua orogeny lenses of pre to syn tectonic leucogranite bodies are also deformed in the shear zone pre kmz deformation d1 is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering shear strain within the kmz is heterogeneous and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges strain is localized along the edge of leucogranites and between gneissic lozenges preserving d1 migmatitic foliations strain localization appears controlled by pre existing foliations grain size and compositional anisotropy between leucogranite and granulite the local presence of retrograde minerals indicate that fluid infiltration occurred in places but most ultramylonite in the kmz is free of retrograde minerals in particular rock composition and d1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space with deformation localization along planes of rheological contrast and along pre existing foliations therefore the spatial distribution of strain in crustal scale ductile shear zones may be highly dependent on lithology and the orientation of pre existing fabric elements in addition foliation development and grain size reduction in high strain zones further localizes strain during progressive shear maintaining the anastomosing shear zone network established by the pre existing heterogeneity

the effects of fine scale surface roughness and grain size on 300 khz multibeam backscatter intensity in sandy marine sedimentary environments abstract high frequency acoustic backscatter intensity that is returned from sediment is affected by a number of seabed characteristics including sediment grain size distribution the nature and magnitude of seabed surface roughness and volume scattering by subsurface scatterers in order to develop quantitative remote sediment classification techniques it is important to determine the relative and combined effects of the dominant variables to which the acoustic signal is sensitive data used in this study were collected at two sandy sites off the south shore of long island new york and include 300 khz multibeam simrad em 3000 backscatter intensity data as well as grab samples and stereo bottom photographs multivariate statistical analyses were used to investigate the relationships between grain size properties seabed roughness and backscatter intensity at a number of sample locations at each study site encompassing a moderate range of backscatter intensities from 32 5 to 24 db the results of this study demonstrate that the relative importance of variables that affect backscatter intensity varies at different sites suggesting that backscatter intensity alone cannot be used to quantitatively predict seabed characteristics however our study indicates that in sandy sediments the median grain size the standardized magnitude of seabed roughness and the sorting of sediment can together be used to predict 300 khz backscatter intensity with high significance and that the relative importance of these variables varies from site to site we present an empirical model derived from a combined dataset that includes two sites of similar sedimentary and hydrodynamic conditions to predict the magnitude of seabed roughness based on backscatter intensity and sediment grain size characteristics the spatially coherent patterns in predicted seabed surface roughness revealed through the use of this kind of model greatly improve our understanding of variations in backscatter intensity in sandy marine sedimentary environments as a result the multivariate statistical approach that we employ improves our overall ability to remotely assess the details of the sedimentary environment that are significant with respect to benthic habitats and physical processes and are applicable to other geophysical datasets including sidescan sonar data

on stress and strain in a continuous discontinuous shear zone undergoing simple shear and volume loss i summarise observations within a continuous discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and deformation approximating simple shear the chrystalls beach complex new zealand comprises phacoids formed from dismembered beds by layer parallel extension enclosed within a relatively incompetent matrix local extension is generally subparallel to the regional direction of shortening and overall it appears that layer parallel extension is a geometrical necessity in low angle shear zones where significant flattening occurs in response to simple shear accompanied by volume loss preferential stress loading of phacoids is predicted by fibre loading theory and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix the principal stress orientations in a phacoid are likely rotated relative to the matrix and either parallel or perpendicular to the phacoid matrix interface as preferential loading of phacoids decreases the stress level in the matrix an increased volume fraction of phacoids increases the strength of the shear zone as a whole however only small matrix volume fractions are required for the composite to act nearly as weak as the matrix 2012 elsevier ltd all rights reserved

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

offshore mud volcanoes and onland faulting in southwestern africa neotectonic implications and constraints on the regional stress field abstract recently discovered mud volcanoes in the orange basin offshore southwestern africa denote the existence of neotectonic faults in the submerged continental shelf interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures diagnostic of strike slip faulting along a n nnw direction analysis at the regional scale of onland neotectonic features in southwestern africa shows that recent faulting occurred both in central namibia and namaqualand south africa and that it created both n nnw and nw trending lineaments it is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field these structures form a set of conjugate transtensive faults which constrain the regional horizontal greatest compressive stress in a nw nnw direction such stress orientation also supported by in situ stress measurements defines the so called wegener stress anomaly the predominant present day stress field of southwest africa the wegener anomaly is incompatible with the stress orientation required by plate scale tectonic constraints mainly in the form of recently published gps motion values for the african plate

the southern cape conductive belt south africa its composition origin and tectonic significance abstract magnetometer array studies have led to the discovery and mapping of the southern cape conductive belt sccb crossing the southern tip of africa from west to southeast coasts the sccb lies just south of the namaqua natal belt of cratonic rocks remobilized about 1000 m y b p it is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m y relationships between the sccb and the basement geochronology geology and tectonics are considered in detail these relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a proterozoic subduction which stopped about 1000 to 800 m y b p associated with this subduction we propose a proterozoic range of andean mountains whose roots are now exposed in the namaqua natal belt later subduction further south near the present south coast is proposed to account for the intrusion between the south coast and the sccb of the cape granites in the time interval 600 500 m y b p there is some evidence for a third yet more distant subduction episode off permian gondwanaland after outlining this tectonic history the paper turns to a closer examination of the hypothesis that the southern cape conductive belt consists of partly serpentinized basalt accumulated at the top of a proterozoic subduction a large static magnetic anomaly which correlates with the sccb over most of its length is well fitted by a model which strongly supports this hypothesis bouguer gravity anomalies along western and central profiles likewise support the hypothesis a discussion follows of the process of formation of the proposed block of serpentinized marine rocks beginning with serpentinization of the crust near oceanic ridges by reaction of warm porous newly extruded basalt with seawater convecting through it the serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction where it accumulates because of its low density examples of such accumulations are cited finally it is shown that any iron in the olivine and pyroxene in the original basalt precipitates when these silicates are hydrated to serpentine as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites in particular the beattie static magnetic anomaly requires in our model calculation a very high susceptibility readily attainable in basalt at 15 20 serpentinization the authors know of no other rock able to provide this high susceptibility a similar percentage of serpentinization provides the density required to model the gravity anomalies

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

southern california earthquake center geologic vertical motion database the southern california earthquake center geologic vertical motion database vmdb integrates disparate sources of geologic uplift and subsidence data at 104 to 106 year time scales into a single resource for investigations of crustal deformation in southern california over 1800 vertical deformation rate data points in southern california and northern baja california populate the database four mature data sets are now represented marine terraces incised river terraces thermochronologic ages and stratigraphic surfaces an innovative architecture and interface of the vmdb exposes distinct data sets and reference frames permitting user exploration of this complex data set and allowing user control over the assumptions applied to convert geologic and geochronologic information into absolute uplift rates online exploration and download tools are available through all common web browsers allowing the distribution of vertical motion results as html tables tab delimited gis compatible text files or via a map interface through the google maps web service the vmdb represents a mature product for research of fault activity and elastic deformation of southern california

the earthquake potential of the new madrid seismic zone the fault system responsible for new madrid seismicity has generated temporally clustered very large earthquakes in a d 900 100 years and a d 1450 150 years as well as in 1811 1812 given the uncertainties in dating liquefaction features the time between the past three new madrid events may be as short as 200 years and as long as 800 years with an average of 500 years this advance in understanding the late holocene history of the new madrid seismic zone and thus the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake induced liquefaction features at more than 250 sites across the new madrid region we have found evidence that prehistoric sand blows like those that formed during the 1811 1812 earthquakes are probably com pound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences from the spatial distribution and size of sand blows and their sedimentary units we infer the source zones and estimate the magnitudes of earth quakes within each sequence and thereby characterize the detailed behavior of the fault system it appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the a d 900 and a d 1450 events as well as in 1811 1812 on the basis of a minimum recurrence rate of 200 years we are now entering the period during which the next 1811 1812 type event could occur

paleoseismicity of two historically quiescent faults in australia implications for fault behavior in stable continental regions paleoseismic studies of two historically aseismic quaternary faults in australia confirm that cratonic faults in stable continental regions scr typically have a long term behavior characterized by episodes of activity separated by quiescent intervals of at least 10 000 and commonly 100 000 years or more studies of the approximately 30 km long roopena fault in south australia and the approximately 30 km long hyden fault in western australia document multiple quaternary surface faulting events that are unevenly spaced in time the episodic clustering of events on cratonic scr faults may be related to temporal fluctuations of fault zone fluid pore pressures in a volume of strained crust the long term slip rate on cratonic scr faults is extremely low so the geomorphic expression of many cratonic scr faults is subtle and scarps may be difficult to detect because they are poorly preserved both the roopena and hyden faults are in areas of limited or no significant seismicity these and other faults that we have studied indicate that many potentially hazardous scr faults cannot be recognized solely on the basis of instrumental data or historical earthquakes although cratonic scr faults may appear to be nonhazardous because they have been historically aseismic those that are favorably oriented for movement in the current stress field can and have produced unexpected damaging earthquakes paleoseismic studies of modern and prehistoric scr faulting events provide the basis for understanding of the long term behavior of these faults and ultimately contribute to better seismic hazard assessments

modelling spring discharge and solute transport in conduits by coupling cfpv2 to an epikarst reservoir for a karst aquifer abstract karst aquifers differ from unconsolidated granular aquifers with respect to their groundwater flow behavior because of the dual permeability characteristics discrete conduit network coupled to a fissured matrix the duality of recharge point vs diffuse recharge as well as the duality of flow laminar vs turbulent state in many karst aquifers the epikarst frequently plays an important role in redistributing precipitation as diffuse recharge into the fissured matrix and as point recharge into the conduit network as well as for the temporal distribution of recharge input in this paper a coupled groundwater model ce cfpv2 erch is developed consisting of a combination of a simple reservoir model generating the recharge source function assuming the concept of an epikarst horizon erch with cfpv2 a discrete conduit continuum groundwater flow model of the phreatic karst aquifer the ce model is applied to simulate spring discharge and a tracer breakthrough in a karst aquifer near guilin city in southwest china in order to examine the effect of an added epikarst reservoir model simulations are carried out with cfpv2 but without including erch simulation results of both spring hydrograph and tracer breakthrough indicate that cfpv2 with erch performs much better a sensitivity analysis of the ce reveals the relative importance of parameters characterizing the epikarst reservoir point recharge is a main factor determining the large variation in spring discharge and its proportion to total recharge can reach 58 these simulation results highlight the importance of the temporal and spatial distribution of recharge to karst spring discharge response the coupled model provides an effective approach to include the effect of an epikarst zone in cfpv2 and it can easily be applied for the simulation of karst aquifers with similar characteristics as the study site

evidence for lower crustal ductile strain localization in southern new york historic triangulation data have been analysed to determine whether intraplate seismicity is associated with ongoing ductile deformation in the lower crust the model we have attempted to test is basically analogous to strain accumulation and release along plate boundary strike slip faults like the san andreas fault in california that is beneath an elastic seismogenic upper crust 20 km thick strain is preferentially localized within ductile shear zones in the lower crust due to broad scale plate driving forces the localized lower crustal ductile strain causes stress and strain to accumulate elastically in the brittle crust which is eventually released in crustal earthquakes at greater depths this localized shear deformation probably develops into pervasive ductile flow numerous geodetic measurements along the san andreas fault confirm that earthquakes in the brittle upper crust are produced by the release of elastic strain that results from ongoing ductile shear or slip in the lower crust1 2 we have found evidence of high rates of crustal deformation in southern new york which suggest that localized ductile shear is occurring in the lower crust

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

frequency magnitude characteristics down to magnitude 4 4 for induced seismicity recorded at mponeng gold mine south africa this article aims to investigate the frequency magnitude characteristics and lower magnitude limits of the microseismic catalog recorded with a seismic network sensitive to high frequencies at mponeng mine south africa the network composed of one three component accelerometer and eight acoustic emission sensors is located at a depth of 3 5 km below the surface and covers the limited volume of approximately 300 300 300 m the three component accelerometer was used to estimate the moment magnitude for the limited number of 135 events m w ranged from 4 1 to 0 3 well recorded by the network we use the relation between the moment magnitude estimated from accelerometer data and radiated energy moment magnitude estimated from acoustic emission sensors to extend the catalog to lower magnitudes the magnitude of completeness of selected spatiotemporal subsets of the catalog was estimated for 1 an aftershock sequence of an m w 1 9 event that occurred approximately 30 m from the network and 2 postblasting activity during working days located more than 80 m from the network the data follow the gutenberg richter gr frequency magnitude relationship with no visible deviation from self similar behavior of seismicity between m w 4 4 and 1 9 for the aftershock sequence and between 3 5 and 1 5 for the postblasting dataset we estimated the magnitude of completeness of selected subset as low as 4 3 b 1 26 for the aftershock sequence and 3 4 b 1 17 for the postblasting activity differences in magnitude of completeness are attributed to location of recorded activity and site effects

reconciling short recurrence intervals with minor deformation in the new madrid seismic zone at least three great earthquakes occurred in the new madrid seismic zone in 1811 and 1812 estimates of present day strain rates suggest that such events may have a repeat time of 1000 years or less paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years however pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed this suggests that the seismic zone is a young feature possibly as young as several tens of thousands of years old and no more than a few million years old