buckling instabilities of subducted lithosphere beneath the transition zone a sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds recent tomographic images beneath subduction zones together with quantitative fluid mechanical scaling laws suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth beneath subduction zones such as central america and java the fast anomalies in elastic wavespeed are too broad 400 km to be explained by simple compressional thickening of subducted lithosphere that is initially 50 100 km thick by contrast a periodic buckling mechanism can account for both the characteristic wedge shapes of the anomalies and for their widths which agree closely with those predicted by the scaling laws in addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions interpretation of geoid anomalies and modeling of secular changes in earth s moment of inertia

deformation in the lowermost mantle from polycrystal plasticity to seismic anisotropy in the deep earth deformation occurs at many scales large scale convection produces subduction of slabs and upwelling of plumes in the mantle at the high temperature high pressure conditions strain is accommodated through crystal plasticity either by diffusion or the movement of dislocations slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale in this study we use polycrystal plasticity with experimentally derived deformation mechanisms for perovskite post perovskite and magnesiowuestite to predict texture development along streamlines in a 2d geodynamic convection model of the lowermost mantle strong preferred orientation develops during subduction and upwelling while during spreading along the core mantle boundary the orientation pattern is relatively stable from preferred crystal orientation and single crystal elastic properties bulk elastic properties can be calculated and compared with seismic observations post perovskite with predominant 001 slip and magnesiowuestite with 110 and 111 slip produce anisotropy patterns which are consistent with observed anisotropy i e fast s waves polarized parallel to the core mantle boundary and anti correlation between p and s wave anisotropies in contrast perovskite with dominant 001 slip and two post perovskite models with dominant slip on 010 and 100 produce anisotropy patterns which are inconsistent with seismic observations

automatic time domain calculation of source parameters for the analysis of induced seismicity abstract a time domain methodology for the automatic on line calculation of source parameters is introduced the method is based on applying parseval9s theorem to the spectral equations proposed by andrews 1986 the appropriateness of the technique for the analysis of induced seismicity was tested by comparing on line values of seismic moment seismic energy and static stress drop with those obtained in the frequency domain off line for an aftershock sequence of events recorded underground at creighton mine ontario our analysis shows that the calculated on line and off line source parameters are in good agreement with an average slope offset of 4 from the 1 1 line suggesting that the proposed method is reliable additionally by applying empirical on line attenuation corrections coda q similar results are obtained to those calculated using off line corrections spectral decay improving the average slope offset to 1

evidence for shear velocity anisotropy in the lowermost mantle beneath the indian ocean teleseismic recordings 87 of a deep earthquake beneath the banda sea at stations in tanzania show a difference in the arrival time of the radial ssv and transverse component ssh s wave ranging from 1 3 s shear velocity anisotropy in the lowermost mantle beneath the indian ocean is the likely cause of this signal because recordings at the same stations of closer in events 80 in the same source region do not present a comparable differential travel time for the banda sea event the ssh signals are broader than ssv signals suggesting that a discontinuity or strong vertical gradient in primarily vsh marks the sudden onset of transverse isotropy in d with a magnitude of 1 4 2 7 about 350 km above the core mantle boundary skks coda s to p converted phases at the moho and upper mantle heterogeneity beneath the stations obscure the onset of ssv and complicate wave shapes it is therefore difficult to evaluate whether general anisotropy needs to be invoked into a model of shear velocity anisotropy

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

fabric development in mg fe o during large strain shear deformation implications for seismic anisotropy in earth s lower mantle abstract large strain shear deformation experiments were performed on mg 1 x fe x o x 0 25 1 0 one of the important minerals in earth s lower mantle deformation experiments were made on coarse grained 15 20 m grain size hot pressed aggregates at conditions of t t m 0 46 0 65 t temperature t m melting temperature and 0 4 10 3 to 0 9 10 3 differential stress shear modulus up to the shear strain of 7 8 under these conditions deformation occurs by dislocation creep the microstructural development in mg fe o is found to be sluggish and the complete dynamic recrystallization and nearly steady state fabric lattice preferred orientation are achieved only after shear strains of 4 at nearly steady state mg fe o shows strong fabrics characterized by the 1 1 0 axes being parallel to the shear direction and the poles of the 1 0 0 planes and to a lesser extent the poles of the 1 1 1 planes normal to the shear plane the seismic anisotropy corresponding to the deformation fabrics in mg fe o was calculated the nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution anisotropy changes also with depth pressure due to the large variation of elastic anisotropy of mg fe o with depth seismic anisotropy caused by the deformation fabric of mg fe o is very weak in the shallow lower mantle v sh v sv anisotropy assuming that mg fe o occupies 20 volume fraction of the lower mantle and little shear wave splitting of vertically travelling waves a result that is consistent with the seismological observations in the d layer of the circum pacific regions thus the deformation fabric of mg fe o is a vital candidate of the cause of seismic anisotropy in these regions anisotropy caused by the lattice preferred orientation of mg fe o has a distinct azimuthal anisotropy with a strong 4 term azimuth the direction of propagation of the fastest slowest sh or p wave is parallel perpendicular to the flow direction and the slowest fastest sh or p wave is at 45 from these two directions

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

full waveform based complete moment tensor inversion and source parameter estimation from downhole microseismic data for hydrofracture monitoring downhole microseismic monitoring is a valuable tool in understanding the efficacy of hydraulic fracturing inverting for the moment tensor has gained increasing popularity in recent years as a way to understand the fracturing process previous studies utilize only part of the information in the waveforms such as direct p and s wave amplitudes and make far field assumptions to determine the source mechanisms the method is hindered in downhole monitoring when only limited azimuthal coverage is available in this study we develop an approach to invert for complete moment tensor using full waveform data recorded at a vertical borehole we use the discrete wavenumber integration method to calculate full wavefields in the layered medium by using synthetic data we find that at the near field range a stable complete moment tensor can be retrieved by matching the waveforms without additional constraints at the far field range we discover that the off plane moment tensor component is poorly constrained

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

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

faulting induced by forced fluid injection and fluid flow forced by faulting an interpretation of hydraulic fracture microseismicity carthage cotton valley gas field texas we analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the carthage gas field of east texas the treat ments were conducted in two adjacent boreholes within interbedded sands and shales of the upper cotton valley formation the microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers events throughout all the treatments show strike slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction these events are consistent with the reservoir s prevalent natural fractures known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien tation when this uniform fracture system was activated exclusively the detected shear deformation measured as the moment release per unit volume of fluid injected was constant independent of various fluid viscosities and flow rates used within the base of the upper cotton valley formation anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones the mechanisms are also strike slip but the fault planes are more favorably oriented for failure the dense clusters show location patterns diverging in time suggesting the expulsion of fluids from compressive fault jogs fluid flow forced by the adjacent slip induced loading appears to initially extend the treatments but the loading also tends to lock up and concentrate stress at the jogs as evident by fewer but larger events populating the structures as treatments progress as a result effec tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs these high moment asperities are similar to dense patches of seismicity observed along creeping sections of the san andreas fault where they have been attributed to localized zones of strength or stress con centration surrounded by larger regions undergoing stable aseismic slip this sim ilarity plus large moment deficits in terms of volume injected suggests a large component of aseismic slip is induced by the cotton valley treatments

magnitudes of induced earthquakes and geometric scales of fluid stimulated rock volumes sometimes rock stimulations by fluid injections into geothermal boreholes are able to trigger perceptible or even potentially damaging earthquakes this does not seem to be the case for hydraulic fracturing of hydrocarbon reservoirs reasons for such a difference and factors defining magnitudes of induced earthquakes triggered tectonicly as well as induced artificially remain unclear we analyzed microseismic data obtained by fluid stimulations at different geothermal and hydrocarbon sites this analysis indicates that a rupture corresponding to a fluid induced earthquake seems to be only probable along a surface located mainly inside a stimulated rock volume we approximated the stimulated volume by an ellipsoid and compared the statistics of induced events with the statistics of randomly distributed thin flat disks modeling rupture surfaces we found that one of the main factors limiting the probability to induce a large magnitude event is the minimum principal axis of a fluid stimulated rock volume

anatomy of a buried thrust belt activated during hydraulic fracturing abstract tectonically active fault networks are often inter connected but in the case of injection induced seismicity prior knowledge of fault architecture tends to be severely limited in most cases reactivated faults due to fluid injection are inferred after the fact by the spatial distribution of induced seismicity hypocenters such reliance on post injection seismicity impedes any pre operational risk analysis as well as development of a more holistic understanding of fault system models by combining high resolution depth migrated 3 d seismic data with a new focal depth estimation method that reduces spatial uncertainty of hypocenters this study pinpoints microearthquake fault activation within a buried thrust belt in the montney formation in western canada british columbia during hydraulic fracturing operations rupture nucleation occurred on seismically imaged thrust ramps that cut through the debolt formation a massive carbonate layer that underlies the stimulated zone high resolution seismic images reveal transverse structures interpreted as basement controlled fold hinges or tear faults that transferred displacement between thrust faults during late cretaceous paleogene compressional shortening the spatio temporal pattern of induced seismicity suggests that these transverse structures provide permeable pathways for aseismic pore pressure diffusion thus connecting distinct thrust faults and enabling earthquake triggering on a timescale of days and at distances of up to 2 km from the injection wells inferred relationships highlight how the fault system is connected including apparent stress concentrations at the intersections of transverse structures and orogen parallel thrust ramps

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

mantle convection with internal heating and pressure dependent thermal expansivity recent laboratory work 1 suggests that the thermal expansivity of the mantle decreases strongly with pressure determines the buoyancy of plumes and the rate at which plumes lose their thermal signature as they rise and so may be expected to have a strong influence on the temperature and velocity structure of the mantle numerical simulations were conducted on convection in an internally heated compressible mantle including constant and pressure dependent and thermal conductivityk they show that a pressure dependent allows the existence of thermal plumes rising from the core mantle boundary cmb however the plumes are weak and dissipate higher in the mantle

the robustness of seismic moment and magnitudes estimated using spectral analysis we present an assessment of how microseismic moment magnitude mw e stimates vary with the method and parameters used to calculate seismic moment this is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity it is therefore imperative that these parties know and understand what errors exist in given magnitude values something that is poorly reported this study concentrates on spectral analysis techniques and compares mw computed in the time and frequency domains using recordings of mw 1 5e vents at cotton valley east texas the maximum discrepancy between mw estimated using the different methods is 0 6 units a significant variation by adjusting parameters in the mw calculation we find that the radiation pattern correction term can have the most significant effect on m w generally up to 0 8 units following this investigation we make a series of recommendations for estimating microseismic m w using spectral methods noise should be estimated and removed from recordings and an attenuation correction should be applied the spectral level can be measured by spectral fitting or taken from the low frequency level significant factors in obtaining reliable microseismic mw estimates include using at least four receivers recording at 1000 hz and making radiation pattern corrections based on focal mechanism solutions not average values

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

apparent break in earthquake scaling due to path and site effects on deep borehole recordings 1 we reexamine the scaling of stress drop and apparent stress rigidity times the ratio between seismically radiated energy to seismic moment with earthquake size for a set of microearthquakes recorded in a deep borehole in long valley california in the first set of calculations we assume a constant q and solve for the corner frequency and seismic moment in the second set of calculations we model the spectral ratio of nearby events to determine the same quantities we find that the spectral ratio technique which can account for path and site effects or nonconstant q yields higher stress drops particularly for the smaller events in the data set the measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set mw 0 8 our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole just as they do at the earth s surface the scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated

aseismic motions drive a sparse seismicity during fluid injections into a fractured zone in a carbonate reservoir author s duboeuf l de barros l cappa f guglielmi y deschamps a seguy s abstract 2017 american geophysical union all rights reserved an increase in fluid pressure in faults can trigger seismicity and large aseismic motions understanding how fluid and faults interact is an essential goal for seismic hazard and reservoir monitoring but this key relation remains unclear we developed an in situ experiment of fluid injections at a 10 meter scale water was injected at high pressure in different geological structures inside a fault damaged zone in limestone at 280 m depth in the low noise underground laboratory france induced seismicity as well as strains pressure and flow rate was continuously monitored during the injections although nonreversible deformations related to fracture reactivations were observed for all injections only a few tests generated seismicity events are characterized by a 0 5 to 4 khz content and a small magnitude approximately 3 5 they are located within 1 5 m accuracy between 1 and 12 m from the injections comparing strain measurements and seismicity shows that more than 96 of the deformation is aseismic the seismic moment is also small compared to the one expected from the injected volume moreover a dual seismic behavior is observed as 1 the spatiotemporal distribution of some cluster of events is clearly independent from the fluid diffusion 2 while a diffusion type pattern can be observed for some others clusters the seismicity might therefore appear as an indirect effect to the fluid pressure driven by aseismic motion and related stress perturbation transferred through failure

late devensian and holocene relative sea level changes on the isle of skye scotland uk detailed litho and biostratigraphical analyses from three coastal sites in contrasting coastal settings on the isle of skye scotland uk reveal evidence for several changes in relative sea level during the late devensian and holocene at the start of the record relative sea level in the area was high at ca 12 500 14c ca 14 800 cal yr bp but then fell reaching a low point during the younger dryas at ca 11 000 10 000 14c ca 13 000 11 600 cal yr bp when a rock platform correlated with the main rock platform was formed in the early middle holocene relative sea level was rising by ca 8000 14c ca 8800 cal yr bp and in northeast skye a lagoonal surface correlated with the main postglacial shoreline was formed at ca 6600 14c ca 7500 cal yr bp by the late holocene relative sea level was again falling but a rise registered at at least two sites began probably before ca 4000 14c ca 4500 cal yr bp and a second lagoonal surface in northeast skye correlated with the blairdrummond shoreline was formed although by ca 3000 14c ca 3200 cal yr bp relative sea level in the area had resumed its downward trend the pattern of relative sea level changes disclosed is compared with evidence elsewhere in scotland

local magnitude discrepancies for near event receivers implications for the u k traffic light scheme local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity such as traffic light schemes tls we demonstrate that significant magnitude discrepancies up to a unit higher occur between seismic events recorded on nearby stations 5 km compared with those at greater distances this is due to the influence of sedimentary layers which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks and requires a change in the attenuation term of the m l scale this has a significant impact on the united kingdom s u k hydraulic fracturing tls whose red light is set at m l 0 5 because the nominal detectability of the u k network is m l 2 this scheme will require the deployment of monitoring stations in close proximity to well sites using data collected from mining events near new ollerton nottinghamshire we illustrate the effects that proximity has on travel path velocities and attenuation then perform a damped least squares inversion to determine appropriate constants within the m l scale we show that the attenuation term needs to increase from 0 00183 to 0 0514 and demonstrate that this higher value is representative of a ray path within a slower more attenuating sedimentary layer compared with the continental crust we therefore recommend that the magnitude scale m l log a 1 17log r 0 0514 r 3 0 should be used when local monitoring networks are within 5 km of the event epicenters

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

impact of geological heterogeneities on induced seismicity in a deep sublevel stoping mine with the aim of understanding the nature of mining induced seismicity microseismic activity in the deep metal mine of garpenberg sweden has been recorded during 2 years of excavations the studied area of the mine is operated using sublevel stoping method with backfilling between depths of around 1000 and 1300 meters spatiotemporal analysis of microseismic activity is presented and correlated with the occurrence of mining blasts a clear dependence is observed between blasts and seismic sequences even if the rock mass response to mining appears to be very variable across space and time two main clusters are observed one located in the major production area central cluster while the second right cluster is located at some distance from the excavations in a zone characterized by a heterogeneous distribution of weak materials and stiff rock masses by analyzing seismic source parameters we demonstrate that the two clusters are characterized by different dynamics in addition we show how right cluster events are mainly controlled by geological heterogeneities which impose high stress concentrations in the stiff rock masses surrounding weak lenses high apparent stresses and corner frequencies associated with the right cluster events agree with our proposed model this suggests elevated stresses in the seismic source region and small source dimensions indeed fractures cannot propagate along great distances due to the presence of weak lenses interbedded with the breaking stiff rock mass

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

source parameters of picoseismicity recorded at mponeng deep gold mine south africa implications for scaling relations abstract we investigate the source parameters of picoseismic and nanoseismic events m w 4 1 recorded with a high sensitivity seismic network at the mponeng gold mine in south africa to gain new insights into the scaling of small seismic events the japanese german underground acoustic emission research in south africa jaguars network composed of one three component 3c accelerometer sensitivity 50 hz to 25 khz and 8 acoustic emission ae sensors sensitivity 1 khz to 180 khz is located at a depth of 3268 m and covers the limited volume of approximately 300 300 300 m the ae sensors are calibrated with respect to the 3c accelerometer in the frequency band 400 hz 17 khz the waveform data of two datasets are analyzed 1 the aftershock sequence of an m w 1 9 event that occurred approximately 30 m from our network and 2 the postblasting activity recorded during working days located at a distance 90 m from the network at the exploitation level for the analysis we applied spectral fitting and spectral ratio methods the calculated values of m w range from 0 8 down to 4 1 with corner frequencies 0 8 khz 13 6 khz source sizes from 8 cm to 1 3 m we observe static stress drops ranging from 1 mpa to 10 0 mpa with apparent stresses of 0 01 mpa 1 00 mpa stress drops are independent of the moment suggesting self similar behavior in the analyzed magnitude range 4 1 m w

simulation of seismic events induced by co2 injection at in salah algeria a b s t r a c t article history carbon capture and storage technology has the potential to reduce anthropogenic co2 emissions however the geomechanical response of the reservoir and sealing caprocks must be modelled and monitored to ensure that injected co2 is safely stored to ensure confidence in model results there is a clear need to develop ways of comparing model predictions with observations from the field in this paper we develop an approach to simulate microseismic activity induced by injection which allows us to compare geomechanical model predictions with observed microseismic activity we apply this method to the in salah ccs project algeria a geomechanical reconstruction is used to simulate the locations orientations and sizes of pre existing fractures in the in salah reservoir the initial stress conditions in combination with a history matched reservoir flow model are used to determine when and where these fractures exceed mohr coulomb limits triggering failure the sizes and orientations of fractures and the stress conditions thereon are used to determine the resulting micro earthquake focal mechanisms and magnitudes we compare our simulated event population with observations made at in salah finding good agreement between model and observations in terms of event locations rates of seismicity and event magnitudes 2015 the authors published by elsevier b v this is an open access article under the cc by license

frequency dependent attenuation in the crust beneath central france from lg waves data analysis and numerical modeling abstract the quality factor of shear waves in the crust beneath central france is evaluated by the combined use of data analysis and theoretical modeling the measurement is made from lg wave records of 18 earthquakes at 22 short period vertical seismic stations of the french seismic network the geometrical attenuation of lg waves has been calculated through a numerical simulation in a perfectly elastic model the station responses including site effects have been evaluated from lg waves and from records of the seismic noise q has been calculated for a set of 11 frequencies between 0 5 and 10 hz and has been found to be in the form q 290 f 0 52 where f is the frequency the numerical simulation leads to the interpretation of q of lg waves as the mean value of q within the crust theoretical and experimental decay of energy with distance are in very good agreement the lg waves present a different sensitivity to a low q zone whether this zone is associated with the sediments or with scattering in the lower crust the lower crust beneath central france appears from our experimental results to be associated with high values of q

development of finite strain in the convecting lower mantle and its implications for seismic anisotropy 1 seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle a combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics specifically it is investigated whether lattice preferred orientation lpo can explain the anisotropy observed in regions of paleosubduction since lpo is caused by dislocation creep and is destroyed by diffusion creep we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric strain is then calculated in these regions and is related to mineral physics experiments combined with high pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range the effect of variations in transition stress activations parameters and strength of slabs on fabric development is investigated it is shown that lpo is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction

bilinearity in the gutenberg richter relation based on m l for magnitudes above and below 2 from systematic magnitude assessments in parkfield california several studies have shown that local magnitude m l and moment magnitude m scale differently for small earthquakes m 2 than for moderate to large earthquakes consequently frequency magnitude relations based on one or the other magnitude type cannot obey a power law with a single exponent over the entire magnitude range since this has serious consequences for seismic hazard assessments it is important to establish for which magnitude type the assumption of a constant exponent is valid and for which it is not based on independently determined m m l and duration magnitude m d estimates for 5 304 events near parkfield we confirm the theoretically expected difference in scaling between the magnitude types and we show that the frequency magnitude distribution based on m and m d follows a gutenberg richter relation with a constant slope whereas for m l it is bilinear thus seismic hazard estimates based on m l of small earthquakes are likely to overestimate the occurrence probability of large earthquakes

empirical green s function analysis taking the next step an extension of the empirical green s function egf method is presented that involves determination of source parameters using standard egf deconvolution followed by inversion for a common attenuation parameter for a set of colocated events recordings of three or more colocated events can thus be used to constrain a single path attenuation estimate i apply this method to recordings from the 1995 1996 ridgecrest california earthquake sequence i analyze four clusters consisting of 13 total events with magnitudes between 2 6 and 4 9 i first obtain corner frequencies which are used to infer brune stress drop estimates i obtain stress drop values of 0 3 53 mpa with all but one between 0 3 and 11 mpa with no resolved increase of stress drop with moment with the corner frequencies constrained the inferred attenuation parameters are very consistent they imply an average shear wave quality factor of approximately 20 25 for alluvial sediments within the indian wells valley although the resultant spectral fitting using corner frequency and is good the residuals are consistent among the clusters analyzed their spectral shape is similar to the the theoretical one dimensional response of a layered low velocity structure in the valley an absolute site response cannot be determined by this method because of an ambiguity between absolute response and source spectral amplitudes i show that even this subtle site response can significantly bias estimates of corner frequency and if it is ignored in an inversion for only source and path effects the multiple egf method presented in this paper is analogous to a joint inversion for source path and site effects the use of colocated sets of earthquakes appears to offer significant advantages in improving resolution of all three estimates especially if data are from a single site or sites with similar site response

an evaluation of the scsn moment tensor solutions robustness of the mw magnitude scale style of faulting and automation of the method we have generated moment tensor solutions and moment magnitudes m w for 1700 earthquakes of local magnitude m l 3 0 that occurred from september 1999 to november 2005 in southern california the method is running as an automated real time component of the southern california seismic network scsn with solutions available within 12 min of event nucleation for local events the method can reliably obtain good quality solutions for m w with m l 3 5 and for the moment tensor for events with m l 4 0 the method uses the 1d time domain inverse code tdmt invc software package dreger 2003 the green s functions have been predetermined for various velocity profiles in southern california which are used in the inversion of observed three component broadband waveforms 10 100 sec using data from at least four stations moment tensor solutions have an assigned quality factor dependent on the number of stations in the inversion and the goodness of fit between synthetic and observed data if a minimum quality factor is attained the m l or m w is 5 0 or greater and if the event is in the southern california reporting regions the m w will be the official scsn cisn magnitude the m w from the high quality solutions determined from our method generally correlate very well with reviewed m l except in regions at the perimeter of the network the m w reported here indicates the scsn m l systematically underestimates the magnitude in the mexicali region of baja california mono lakes area coso region and the brawley seismic zone and overestimates the magnitude in the coastal ranges comparisons of the moment tensors determined using this model are made with harvard centroid moment tensors generated for larger earthquakes in the california region and recent 3d models for events in the los angeles region with excellent correlation most of the earthquakes with good quality solutions exhibit strike slip faulting in particular along the major late quaternary strike slip faults thrust faulting on east west striking planes is observed along the southern edge of the transverse ranges while northwest striking thrust faulting is observed in the coastal ranges normal faulting is most common in baja california and southern sierra nevada including the western basin and range region poor quality solutions with unreliable m w are caused by excessive background noise in the waveforms small events m l 4 0 can be affected by ambient noise or teleseisms but larger events can also have unreliable solutions if they follow a recent large regional event

parameters and a magnitude moment relationship from small earthquakes observed during hydraulic fracturing experiments in crystalline rocks using source parameters estimated from seismic spectra and magnitudes estimated from coda lengths we demonstrate that the log linear relationship between moment and magnitude holds for events with magnitudes as low as 6 using as a data set events induced by hydraulic fracturing experiments at the fenton hill new mexico hot dry rock hdr geothermal site we find that the relationship between magnitude m and seismic moment mo is log mo 17 27 0 77m moreover the linear relationship between seismic moment and source radius r holds for the fenton hill microearthquakes analyses of the fenton hill data yield the following relationship log r 2 28 0 19log mo

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

breaking the ice identifying hydraulically forced crevassing hydraulically forced crevassing is thought to reduce the stability of ice shelves and ice sheets affecting structural integrity and providing pathways for surface meltwater to the bed it can caus

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

it s a matter of size magnitude and moment estimates for microseismic data microseismic mapping is a valuable tool for assessing hydraulic fracture stimulations during a stimulation treatment borehole near surface or surface geophones record passive seismic data the microseismic events are thought to be shear failures that occur around the opening of a tensile hydraulic fracture as it grows over the past several years there has been a large growth in the number of hydraulic fracture monitoring surveys along with the number of vendors who acquire and process microseismic data here we discuss two attributes of microseismic data which are associated with the size of the passive event seismic moment and magnitude