a detailed seismic anisotropy study during the 2011 2012 unrest period in the santorini volcanic complex abstract the santorini volcanic complex svc is an area in the southern aegean greece which has been characterized by low seismicity rates for the last decades especially in the santorini caldera where they have been very low until 2010 this pattern changed completely in february 2011 when intense microseismic activity was initiated within the caldera during the manual analysis of the events the shear wave splitting phenomenon was observed revealing the existence of an anisotropic upper crust in the svc area a detailed anisotropy study has been conducted using 231 events within the shear wave window that fulfilled the selection criteria the polarization direction of the fast shear wave the time delay between the two split shear waves and the source polarization direction were calculated after visual inspection using both the polarigram and the hodogram representations this procedure applied for eight local stations resulted in the determination of 340 splitting parameters the obtained mean anisotropy directions are not homogeneous revealing a complex regime in the activated area nevertheless these results are explained by the ape model related to the stress sensitive behavior of fluid saturated microcracked rocks a detailed analysis of the temporal evolution of both the time delay and anisotropy direction was carried out the time delays measured in the band 1 window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity as well as to the major m w 5 1 and 5 2 events which took place about 40 km sw of santorini on 26 and 27 january 2012 respectively on the other hand no significant temporal variations or 90 flips of the s fast polarization direction were observed

laboratory study of hydraulic fracturing laboratory tests on teton dam soil were performed by increasing the water pressure in model bore holes and in simulated rock joints the water pressure required to cause hydraulic fracturing was found to depend on soil density water content confining stress and test duration on the basis of the test results it was hypothesized that 1 hydraulic fracturing is a weak link phenomenon in that fracturing will occur in the least resistant soil subjected to increased water pressure and 2 hydraulic fracturing can probably occur only in the presence of a discontinuity within which the water pressure can act to create a wedging mechanism thus creating tensile stresses in the soil

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less

low p wave velocity at the base of the mantle a tool for investigating p wave vp structure at the base of the mantle is presented sks waves at distances around 107 are incident upon the core mantle boundary cmb with a slowness that allows for coupling with diffracted p waves pd at the base of the mantle the p wave diffraction occurs at both the sks entrance and exit locations of the outer core the resulting phase spdks arrives slightly later in time than sks having a wave path through the mantle and core very close to sks the difference time between sks and spdks most strongly depends on vp at the base of the mantle near sks core entrance and exit points digitized long period 5 15s observations from deep focus tonga events recorded by north american wwssn and canadian seismographic network stations and south american events recorded by european and eurasian wwssn stations exhibit anomalously large spdks sks difference times sks and the later arriving spdks are separated by several seconds more than predictions made by the global average prem model models having a pronounced low velocity zone in vp at the base of the mantle best predict the observations these models are perturbations of the prem model whereby the lowermost 50 100 km of the d layer has a negative vp gradient with the mantle side cmb vp reduced from prem by 5 to 13 0 km s at the base of the mantle

critical hydraulic gradients of internal erosion under complex stress states abstractinternal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow which affects the hydraulic and mechanical behavior of the soil in this research extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths isotropic drained triaxial compression and triaxial extension stress paths these tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients the entire erosion process can be divided into four phases stable initiation development and failure accordingly three critical gradients termed as initiation skeleton deformation and failure hydraulic gradients can be defined these critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton the buckling of the strong force chains formed by the coarse particles and the soil failure re

a stress forecast earthquake with hindsight where migration of source earthquakes causes anomalies in shear wave polarisations abstract the stress aligned shear wave splitting observed throughout most of the crust is a three dimensional function of the effective elastic anisotropy of distributions of fluid saturated grain boundary cracks and preferentially oriented pores fluid saturated microcracks are highly compliant and crack geometry responds readily to changes in stress monitoring with shear wave splitting allows the accumulation of stress before earthquakes to be recognised and the time and magnitude of the impending earthquake to be stress forecast such increases in stress have been observed some 15 times with hindsight and one successful real time stress forecast here we report a further with hindsight stress forecast in northern iceland where increases of time delays implying stress accumulation are observed before a 4 9 earthquake at two seismic stations at epicentral distances of 50 km and 92 km there are however two anomalies changes in shear wave polarisations at the nearest station appear to be due to migration of shear wave source earthquakes in a uniform anisotropic symmetry into ray path directions with different anisotropic shear wave polarisations such directional changes in shear wave polarisations are occasionally observed above small earthquakes and may be misinterpreted as structural or temporal changes in anisotropy a further anomaly is that the station at 92 km displays variations in time delays but a station just 4 km farther away on the other side of a major fault appears to be in the stress shadow of the fault and does not display temporal variations despite having adequate source data

calculable fluid rock interactions this paper introduces a model of rock deformation anisotropic poro elasticity or ape where the response of fluid saturated rock to changing conditions prior to fracturing can be calculated the driving mechanism for deformation is fluid migration along pressure gradients between neighbouring intergranular microcracks and pores at different orientations to the stress field the parameters that control changes to microcrack geometry also control the splitting birefringence of seismic shear waves so that changes in deformation can be directly monitored by analysing the shear wave splitting which is observed in almost all rocks analysis of shear wave splitting in the earth9s crust and ape modelling show that distributions of mostly intergranular cracks in the crust are always geometrically close to fracturing with the implication that shear wave splitting is sensitive to comparatively minor changes of stress and minor changes of in situ conditions this has important implications for the state of criticality of the rockmass and as a consequence changes in shear wave splitting have been observed before larger earthquakes on those few occasions when suitable source receiver geometry coincides with appropriate seismic activity ape also has implications for monitoring changing conditions in reservoirs during hydrocarbon recovery

a review of retrospective stress forecasts of earthquakes and eruptions changes in shear wave splitting sws monitor stress induced changes to the geometry of the stress aligned fluid saturated microcracks pervading almost all sedimentary igneous and metamorphic rocks in the earth s crust and upper mantle changes in sws implying stress accumulation and stress relaxation suggesting crack coalescence before large earthquakes have been observed retrospectively in the rock mass surrounding large or larger earthquakes in one case the time magnitude and fault plane of a m 5 earthquake in sw iceland was successfully stress forecast 3 days before it occurred similar characteristic behaviour of shear wave splitting has been observed retrospectively before 17 other earthquakes and before three volcanic eruptions these retrospective stress forecasts have been published in different formats in different journals for clarification this paper redraws all observations of stress accumulation and stress relaxation in a consistent normalised format that allows the overall similarities in behaviour to be recognised before earthquakes and volcanic eruptions such behaviour inconsistent with conventional sub critical geophysics confirms the compliance of the new geophysics of a critically microcracked earth where the microcracks are so closely spaced that they verge on failure and hence are critical systems that impose a range of fundamentally new properties on conventional sub critical geophysics one of the implications of new geophysics is that there are similarities in the behaviour of stress before earthquakes and volcanic eruptions the normalised formats show such similarities and include the opportunity to stress forecast both earthquakes and eruptions

seismic anisotropy time variations at mt etna this article has been accepted for publication in geophysical journal international the authors 2020 published by oxford university press on behalf of the royal astronomical society all rights reserved uploaded in accordance with the publisher s self archiving policy

on the structure of the lowermost mantle beneath the southwest pacific southeast asia and australasia abstract the region of the lowermost mantle beneath the southwest pacific australasia and southeast asia 50 s 50 n and 80 190 e has been studied using a wide variety of seismic techniques we complement these studies with results obtained from long period global digital seismograph network gdsn data using a recently developed phase stripping technique that permits the isolation of d reflections from the stronger neighbouring s and scs signals we identify patches with d reflections and areas where we cannot confidently determine the presence or absence of a d reflection a synthesis of our results with other studies suggests that d varies dramatically through the region generally thickening from 100 150 km in a central zone to about 300 km at the eastern and western margins however there are irregularities in this overall pattern including areas where there seems to be little evidence for a d discontinuity inspection of waveform amplitudes shows considerable scatter in not only the d reflected phases but also core mantle boundary reflected phases experiments with synthetic seismograms for a variety of d models and the observed lateral variability through the region suggest that this is to be expected furthermore ray theory calculations for large scale 3d earth models predict significant variations 30 in s wave amplitudes of lowermost mantle turning rays finally we investigate possible correlations between lower mantle velocity flow and d thickness we find some correlation between d thickness and lower mantle velocities obtained from tomographic inversions with a thin d layer in high velocity regions and a thickening of the layer toward slower regions the relationship between predicted lower mantle flow and d thickness is less clear these results are qualitatively consistent with thermal boundary layer predictions for d but do not preclude the possibility of compositionally distinct material in the layer

smaller source earthquakes and improved measuring techniques allow the largest earthquakes in iceland to be stress forecast with hindsight summary a group of three earthquakes in 2000 june in sw iceland included the two largest earthquakes in iceland in the past 30 yr previously temporal changes in shear wave splitting had not been recognized before these earthquakes as there were too few small earthquakes to provide adequate shear wave data and they were not stress forecast even with hindsight these large earthquakes were subject to a special investigation by the european community funded prepared project during which the seismic catalogue was extended to include smaller magnitude earthquakes this more detailed data set together with a semi automatic programme for measuring the parameters of shear wave splitting greatly increased the number of time delay measurements the new measurements displayed the typical temporal variations before larger earthquakes as seen elsewhere a long term increase in time delays interpreted as stress accumulation before the earthquake and a decrease interpreted as crack coalescence immediately prior to the earthquake the logarithms of the durations of both the implied accumulation of stress and the crack coalescence have the same self similar relationships to earthquake magnitude as found elsewhere in iceland this means that in principle the time and magnitude of the larger earthquakes could have been stress forecast in real time had the smaller source earthquakes of the extended catalogue and the improved measuring procedures been available at the time

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

a few earthquake conundrums resolved several fundamental questions conundrums about earthquakes and rocks are inexplicable in terms of conventional sub critical geophysics these questions have become so familiar that they are now generally accepted as the way earthquakes and rocks behave and are not recognised as presenting conceptual difficulties these conundrums are resolved by a new understanding of fluid rock deformation where fluid saturated microcracks in almost all rocks are so closely spaced they verge on failure and hence are highly compliant critical systems which impose a range of new properties on conventional sub critical geophysics this new understanding of fluid rock deformation this new geophysics allows earthquakes to be stress forecast and has implications and applications to many solid earth developments

nonlinear physics of double diffusive convection in geological systems double diffusive convection d d c is a nonlinear fluid dynamical phenomenon driven by thermal and compositional effects on the density we discuss the nature of subcritical finite amplitude double diffusive convection for infinite prandtl number and large lewis number le which have applications in magma chambers and at the d layer at the core mantle boundary cmb numerical solutions by two dimensional finite element method are presented to portray the nature of time dependent d d c in both the diffusive and the finger regimes the compositional heterogeneities at the cmb are far more complex than the local thermal structure because of the largele and may have important implications for the scattering of seismic waves off the cmb the effects of increasing the buoyancy ratior are to suppress time dependent d d c even at highra in narrow slots shear heating instabilities can be triggered in the finger regime by the abrupt overturn of the compositional interface this phenomenon may occur in tall magma chambers with low dissipation numbers of of order 0 0 01

reply to comment on systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory changes by yunfeng liu ta liang teng and yehuda ben zion by stuart crampin and yuan gao we thank crampin and gao 2005 hereinafter referred to as cg for their interest in the analysis and results of liu et al 2004 hereinafter referred to as ltbz we disagree that the lines they draw through some of the data points of figure 14a of ltbz reproduced here as fig 1 carry quantitative scientific information and we maintain our assessment that the results of ltbz do not show systematic temporal evolution of anisotropy parameters that could be used to forecast impending large earthquakes in our study area our response to cg focuses on features involving directly the results of ltbz we do not discuss the broader claims of cg on general scaling relations associated with their figures 2 and 3 other than to note that they have been controversial e g aster et al 1990 1991 seher and main 2004 prompted by the comment of cg we perform a more detailed analysis of spatio temporal variations of the time delays between fast and slow shear waves in our data the detailed examination indicates clearly that the apparent precursory pattern identified by cg is dominated by spatial rather than temporal variations of properties figure 1 observed time delay values for the section deeper than 0 2 km versus time each data point is a time delay measurement with an error given by the vertical line the mean values and standard deviations of the measurements before and after the chi chi mainshock are indicated by solid and dashed lines respectively the numbers near the points mark the sequential ordering of the measurements figure 1 is an enlarged version of figure 14a of ltbz with sequential numbering of the data points before the 1999 chi chi mainshock and several later points the subtle temporal changes referred to by cg are associated with a

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

shear wave splitting changes associated with the 2001 volcanic eruption on mt etna summary the time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 july 17 august 9 2001 flank eruption on mt etna sicily normalized time delays measured by singular value decomposition show a systematic increase starting several days before the onset of the eruption on several occasions before the eruption the polarization directions of the shear waves at station mnt closest to the eruption show 90 flips where the faster and slower split shear waves exchange polarizations the last 90 flip being 5 days before the onset of the eruption the time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a relaxation phenomena due to crack coalescence this behaviour has many similarities to that observed before a number of earthquakes elsewhere

stress forecasting not predicting earthquakes a paradigm shift after 120 years of unsuccessful endeavor a paradigm shift is required before earthquakes can be predicted the most sensitive diagnostic of low level changes of stress in in situ rock variations in microcrack geometry can be monitored by analyzing shear wave splitting the suggested paradigm shift is that instead of investigating the source zone we monitor stress accumulation before earthquakes at possibly substantial distances from the source characteristic temporal variations of shear wave time delays have been observed in retrospect before 14 earthquakes worldwide on one occasion when changes were recognized early enough the time magnitude and fault break of an m 5 earthquake in southwest iceland were successfully stress forecast in a narrow time magnitude window such stress accumulation can be theoretically modeled and is believed to be at least partially understood when sufficient shear wave source earthquakes are available increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs this is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break the new result confirming these ideas and justifying the paradigm shift is that logarithms of the durations of both increases and decreases in time delays are found to be proportional self similar to the magnitudes of impending earthquakes

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

hydromechanical modeling of granular soils considering internal erosion this paper attempts to formulate a coupled practical model in the framework of continuum mechanics to evaluate the phenomenon of internal erosion and its consequences on the mechanical behavior of

comment on systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory changes by yungfeng liu ta liang teng and yehuda ben zion liu et al 2004 systematically analyze the parameters of shear wave splitting before and after the 1999 chi chi earthquake in taiwan they claim that they see no evidence for the temporal changes in shear wave time delays which some of us use to monitor the buildup of stress before earthquakes in fact the only diagram where one would expect to see the type of temporal changes observed elsewhere figure 14a in liu et al 2004 does show two types of temporal variations of time delays here we fit least squares lines to these temporal changes and show that the duration earthquake magnitude relationships of these various changes agree very well with those seen elsewhere thus the behavior before the chi chi earthquake lends further support to the use of shear wave splitting to stress forecast the times and magnitudes of impending earthquakes

observations of stress relaxation before earthquakes summary theory and observations suggest that seismic shear wave splitting caused by fluid saturated stress aligned microcracks directly monitors low level deformation before fracturing faulting and earthquakes occur in the past it had been assumed that the accumulation of stress before earthquakes continued until stress was released by faulting at the time of the earthquake however new data and reappraisal of existing data sets now suggests that the stress begins to relax and cracks close from tens of minutes to months before the earthquake actually occurs with the logarithm of the duration of the relaxation proportional to the magnitude of the impending earthquake the duration of the relaxation appears to be directly correlated with earthquake magnitude and may have implications for the earthquake source process and the ability to predict earthquakes

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

the scatter of time delays in shear wave splitting above small earthquakes summary measurements of time delays in seismic shear wave splitting above small earthquakes typically display a scatter of often as much as 80 per cent about the mean changes in the average timedelay appear to be related to changes of stress but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time delays above earthquakes in contrast measurements of shear wave time delays in controlled source exploration seismics are typically well controlled and display little scatter previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes here we show that 90 flips in shear wave polarizations due to fluctuating high pore fluid pressures on seismicallyactive fault planes are the most likely cause of the scatter

gradual entrainment of a chemical layer at the base of the mantle by overlying convection summary a dense chemical layer may exist at the base of the mantle the overlying convection tends to entrain some of this layer reducing its thickness through time the lower more dense fluid is dragged into cusps beneath upwelling limits of the overlying convection and some material is entrained it is obvious that a high density of intrinsically more dense material retards this entrainment an upper limit and probably a reasonable estimate for the amount of entrainment is obtained by considering the carrying capacity of the vertical flow above cusps and ignoring the complications within the cusps themselves flow through conduits formed by the reduced viscosity of hot material is less effective at entrainment than broad scale flow controlled by the background viscosity of the normal mantle cylindrical plumes are less effective at entrainment than 2 d flow intrinsic density contrasts of around 6 per cent are needed for the layer to persist through geological time assuming the thermal expansion coefficients is 2 10 5 in addition the chemical layer may convect internally this causes the layer to entrain less dense material from above and thus aids entrainment from above by reducing the excess density of the layer convection in the chemical layer and stripping of the layer into cusps by the overlying flow also cause much of the temperature contrast to be in the overlying mantle rather than the chemical layer the difficulty of having more than a few 100 k temperature contrast across the layer unless it is internally stratified negates a reason for supposing the layer exists

viscous heating a potential mechanism for the formation of the ultralow velocity zone abstract the detection of several ultralow velocity zones ulvz on the core mantle boundary cmb together with the recent estimates of the lower mantle solidus would suggest that some sort of melting may occur inside the upwellings in the deep mantle we have studied the possibilities of viscous heating as a mechanism for producing favorable conditions leading to melting inside deep mantle upwellings a two dimensional model in an aspect ratio 3 box with temperature and pressure dependent rheology and two major phase transitions has been employed both viscous and adiabatic heating have been included and various amounts of internal heating have been applied viscous heating can heat up the interior of the deep mantle plumes to around 100 degrees above the cmb temperature this amount of temperature rise is sufficient to cause melting inside upwellings in the deep lower mantle we propose here that viscous heating plays an important role in producing conditions leading to the formation of the ulvz massive melting within rising plumes might have been more prevalent in the past

shear wave splitting and earthquake forecasting seismic shear wave splitting sws monitors the low level deformation of fluid saturated microcracked rock we report evidence of systematic sws changes recorded above small earthquakes monitoring the accumulation of stress before earthquakes that allows the time and magnitude of impending large earthquakes to be stress forecast the effects have been seen with hindsight before some 15 earthquakes ranging in magnitude from an m1 7 seismic swarm event in iceland to the ms7 7 chi chi earthquake in taiwan including a successfully stress forecast of a m5 0 earthquake in sw iceland characteristic increases in sws time delays are observed before large earthquakes which abruptly change to deceases shortly before the earthquake occurs there is a linear relationship between magnitudes and logarithms of durations of both increases and decreases in sws time delays before large impending earthquakes however suitably persistent swarms of small earthquakes are too scarce for routine stress forecasting reliable forecasting requires controlled source cross hole seismics between neighbouring boreholes in stress monitoring sites sms it would be possible to stress forecast damaging earthquakes worldwide by a global network of sms in real time

high pore fluid pressures at bhuj inferred from 90 flips in shear wave polarizations summary shear wave splitting above local earthquakes in the aftershock zone of the 2001 m w7 6 bhuj earthquake display a spatial variation of anisotropy related to crack orientation at most seismic stations the fast polarization directions are approximately nne ssw parallel to the inferred directions of maximum horizontal stress suggesting the dilatancy induced anisotropy resulting from approximately stress aligned parallel vertical microcracks in contrast the fast polarization directions at station samkhyali skl very close to the epicentre of the main shock are fault parallel approximately ew and almost orthogonal to the stress aligned polarizations elsewhere ape modelling and observations elsewhere suggest that these are 90 flips in shearwave polarizations caused by propagation through microcracks containing fluids at critically high pore fluid pressures surrounding the seismically active fault plane the presence of high pore fluid pressures on all seismogenic faults also explains the high scatter in shear wave time delays the average path normalized delay time varies from 2 to 4 ms km 1 except at station skl where higher delays of up to 13 ms km 1 are found the large scatter in measured time delays is caused by cracks saturated with high pressure pore fluids these observations can be explained by the presence of both normal stress aligned and 90 flipped shear wave polarizations at bhuj this suggests that the kutch mainland fault is a major tectonic feature

a successfully stress forecast earthquake summary a m 5 earthquake in iceland has been successfully stress forecast by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress modified microcracking reaches fracture criticality within the stressed volume where strain is released local investigations suggested the approximate location of the forecast earthquake we report the criteria on which this stress forecast was based

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

scale lengths of shear velocity heterogeneity at the base of the mantle from s wave differential travel times resolving the seismic velocity heterogeneity in the lowermost mantle is essential for understanding the chemical and dynamical processes operating in the boundary layer between the core and the mantle several regions of the lowermost mantle appear to have abrupt increases in shear velocity several hundred kilometers above the core mantle boundary at the top of the d sup prime prime layer this structure is revealed by seismic wave energy refracted by the velocity increase resulting in an extra arrival scd in teleseismic s wavetrains at distances from 65 to 95 degree anomalies in differential travel times of the extra arrival relative to direct s which turns at shallower depths in the midmantle or scs which reflects from the core mantle boundary below the velocity increase provide probes of the velocity heterogeneity in the lowermost mantle with better vertical resolution than provided by conventional scs s measurements we explore the spatial patterns in scd s scs scd and scs s differential time residuals for paths through the lower mantle beneath alaska eurasia and india all being regions with coherent scd phases to place constraints on the strength and scale lengths of shear velocity heterogeneity and or variable topography of the lower mantle discontinuity in these regions the observed patterns are poorlymore predicted by existing global tomographic models significant small scale heterogeneity with lateral length scales of 200 endash 500 km or less exists even within regions that display a relatively uniform d sup prime prime discontinuity structure over scale lengths of 1500 endash 2000 km the strongest travel time variations are associated with structure above the d sup prime prime region in contrast to common assumptions rapid lateral fluctuations in scs scd differential times suggest that the anomalies accumulate within a relatively thin zone less than the overall thickness of d sup prime prime abstract truncated less

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

systematic analysis of shear wave splitting in the aftershock zone of the 1999 chi chi taiwan earthquake shallow crustal anisotropy and lack of precursory variations we analyze shear wave splitting sws in a high quality waveform data set recorded at surface and downhole 0 2 km seismometers in a region around the 20 september 1999 m w 7 6 chi chi taiwan earthquake sequence the data set was generated by events in a 5 year period before during and after the mainshock the purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes results from downhole records show a stable polarization direction of the fast shear wave that matches well the local global positioning system gps velocity field a slightly different polarization direction of the fast shear wave is obtained from surface data this suggests a possible anisotropy change between the top 0 2 km structure and the deeper section of the crust measured time delays below the downhole station have an average value of 0 16 sec without systematic changes for sources from about 8 km to 20 km in depth estimates of time delays in the top 0 2 km of the crust based on shear waves reflected from the free surface give a constant 0 04 sec a likely depth distribution inferred from these two types of measurements and an s velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km the measured polarization directions and time delays give essentially constant values over the 2 7 year premainshock and 2 3 year postmainshock periods in the region adjacent to the chi chi rupture and within 10 km from the epicentral region of its two large m 6 0 aftershocks analysis of sws in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the chi chi earthquake sequence the results raise doubts on the general usefulness of sws measurements for earthquake forecasting an apparent coseismic increase in anisotropy time delay of approximately 10 is observed for depths 0 2 km however this value is clearly affected by spatial changes associated with different event locations before and after the chi chi mainshock

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

temperature and chemistry of the core mantle boundary seismic anomaly structures at the bottom of the lower mantle may be interpreted in terms of large temperature variations based on new laboratory data at very high pressure 1 sound velocity measurements at mantle pressures show an increase in ln ln p with depth this and the strong decrease in the thermal expansion coefficient yield thermal anomalies t of 1000 k using the seismically measured lateral velocity variations 2 high melting gradients in fe and fe o compounds measured at core pressures yield a temperature increase across the core mantle boundary in excess of 1300 k 3 melting temperatures of mg fe sio3 perovskite measured to deep mantle conditions extrapolate to 7500 k at the bottom of the mantle and result in ttm values between 0 3 and 0 4 this may strongly decrease the temperature dependence of viscosity if the average temperature profile of the lower mantle is nearly adiabatic and if the viscosity systematics found at low pressure are applicable at lower mantle conditions and 4 for dry conditions mgsio3 in contrast to mgo does not react with molten iron at pressures of the core mantle boundary this suggests that chemical interactions between the mantle and the core play a minor role

a four year study of shear wave splitting in iceland 1 background and preliminary analysis this is part 1 of a report of a four year study of seismic shear wave splitting in iceland designed to seek temporal variations before earthquakes shearwave splitting is observed routinely in iceland whenever shear waves arrive within the shear wave window of seismic stations and whenever adequate data are available temporal and spatial variations in shear wave splitting are observed before both earthquakes and volcanic eruptions shear wave splitting is caused principally by the stress aligned fluid saturated microcracks and pore throats in almost all in situ rocks fluid saturated microcracks are the most compliant elements of the rock mass and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone such changes were found and are reported in paper 2 this paper presents the background preliminary observations and analysis of shear wave splitting in iceland

mapping the lowermost mantle using core reflected shear waves a map of laterally varying d velocities is obtained for the region from 50 s to 50 n in latitude and 70 e to 190 e in longitude velocities are found using an analysis of the differential travel time residuals from 481 scs s and 266 sscs ss phase pairs the long period data are taken from the global digital seismograph network digital waveform catalog for the time period of january 1980 to march 1987 each differential travel time is found by a cross correlation of the s phase ground displacement corrected to simulate differential attenuation with all following phases travel times are corrected for ellipticity and mantle heterogeneity outside of their d paths and the remaining residuals are interpreted as the result of d heterogeneity ray tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies the resulting map reveals significant long wavelength d structure including a 3 low velocity region beneath northeastern indonesia surrounded by three identified high velocity zones beneath northwestern pacifica 4 southeast asia 3 and australia 3 5 this structure is of continent ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle the low velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature d rock which has been heated by the core has become gravitationally unstable and begun to rise a chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site much the way continents are dragged toward subduction zones the high velocity zones possibly result from the downward convection of cold lower mantle plumes which pond at the core mantle boundary these seismic anomalies may also contain a chemical signature from faster iron poor materials brought down through the lower mantle or the additional presence of sio2 stishovite perhaps in its higher pressure polymorph

spatial variations of shear wave anisotropy near the san jacinto fault zone in southern california we examine crustal anisotropy at several scales along and across the san jacinto fault zone sjfz by systematically measuring shear wave splitting sws parameters the analyzed data are recorded by 86 stations during 2012 2014 including five linear dense arrays crossing the sjfz at different locations and other autonomous stations within 15 km from the main fault trace shear phase arrivals and sws parameters fast directions and delay times are obtained with automated methods the measurement quality is then assessed using multiple criteria resulting in 23 000 high quality measurements we find clear contrast of fast directions between the sw and ne sides of the sjfz stations on the sw side have fast directions consistent overall with the maximum horizontal compression direction shmax while stations on the ne side show mixed patterns likely reflecting lithological topographic variations combined with fault zone damage the fast directions in the anza gap section with relatively simple fault geometry agree with the inferred shmax and the delay times at an array within that section are smaller than those observed at other across fault arrays these indications of less pronounced damage zone in the anza section compared to other segments of the sjfz are correlated generally with geometrical properties of the surface traces significant variations of fast directions on several across fault arrays with station spacing on the orders of a few tens of meters suggest that shallow fault structures and near surface layers play an important role in controlling the sws parameters

temporal changes in shear wave splitting at an isolated swarm of small earthquakes in 1992 near dongfang hainan island southern china summary an isolated swarm of small earthquakes occurred in 1992 near dongfang on hainan island southern china the institute of geophysics state seismological bureau of china monitored the swarm with five dcs 302 digital accelerometers for three months from 1992 june 1 18 earthquakes with magnitudes m l ranging from 1 8 to 3 6 were well located by five stations and shear wave splitting varying azimuthally was analysed on 27 seismic records from these events the mean polarization azimuth of the faster shear wave was wnw time delays between the split shear waves at two stations varied with time and space the time delays at one station fell abruptly after earthquakes of magnitudes 3 1 and 3 6 but did not change significantly at the second station this behaviour is consistent with the delay time changes being caused by changes in the aspect ratio of vertical liquid filled eda cracks thus the variation in shear wavesplitting time delay could be due to changes in crustal stress related to nearby smallmagnitude earthquake activity the connection between earthquake activity and crustal stress variation measured by shear wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake however our data cannot be considered as definite evidence for such precursors

hazard and seismic reinforcement analysis for typical large dams following the wenchuan earthquake abstract the primary aim of this research is to analyze the hazards and seismic performance of reservoirs and typical large dams based on a field investigation following the wenchuan 8 0 earthquake the current seismic performance standards being achieved are discussed and further suggestions are given for super high dams based on field investigations statistics are also given on damaged small reservoirs and damage severities due to the many various types of earthquake threat it is of interest to note that the major structures of the medium sized hydropower stations on the minjiang river did withstand the wenchuan earthquake despite earthquake intensity exceeding engineering design expectations all the dams remained overall stable the shapai roller compacted concrete rcc arch dam and the zipingpu concrete faced rockfill cfr dam suffered damage the main earthquake induced hazards to the dam structures and to both abutments are detailed case study analyses of the hazards and reinforcement provision revealed that 1 the wenchuan earthquake proved very dangerous for small reservoirs with destroyed reservoirs widely distributed the risk of failure of small dams can be classified into three levels based on field investigation 2 the damage to and behavior of the large dams in the area affected by the earthquake prove that the chinese codes relating to the seismic design of large dams are appropriate 3 appropriate reinforcement can increase the overall stiffness of abutments riverbed foundation and the adjacent slopes further improving the seismic performance of dams 4 the design and management of emergency responses to cope with extreme conditions should be improved and a unified response platform covering all hydropower engineering projects in each valley should be established the effects to be expected and the seismic design measures to be taken still pose great challenges for engineers in respect of large dams

growing the earth s d layer effect of density variations at the core mantle boundary the d region of the mantle has been interpreted as a variable thickness layer of hot chemically heterogeneous material at the base of the mantle this paper reports the results of numerical simulations testing whether growth of a chemically distinct layer at the base of the mantle is possible by influx of molten iron alloy from the core to the mantle a finite element model of thermo chemical mantle convection was used to simulate exchange of material between the mantle and core growth of a significant layer at the base of the mantle depends on several factors we focus on the effect of density of the material in the chemical boundary layer relatively low density material was swept away from the core mantle boundary by mantle upwellings very high density material in contrast was not swept away but flowed into the mantle too slowly to form a layer as thick as d for moderate density contrasts between material in a chemical boundary layer and the overlying mantle a laterally heterogeneous layer formed the stable layer results in an elevated temperature contrast between the core and the mantle high temperatures in this layer are associated with broad regions of elevated temperature in the overlying mantle

shear wave splitting detected by using downhole triaxial seismic detector during dilation of artificial subsurface fracture summary a small change in traveltime and shear wave splitting in elastic waves transmitted through a pressurized single fracture has been detected through downhole triaxial cross hole seismic measurements a field experiment was carried out at higashi hachimantai field of tohoku university japan where an artificial single fracture had been created at 369 m by hydraulic fracturing in intact welded tuff p and s waves transmitted through the pressurized single fracture were measured by using a downhole three component seismic detector installed in a borehole near the fracture in our previous studies a small change in traveltime of about 0 1 ms was detected by using the cross spectral moving window technique in this recent study the arrival times of split shear waves and the polarization directions were detected by using the wavelet transform of a three component signal the traveltime difference between the first and second shear waves was 0 05 0 12 ms when the wellhead pressure was increased up to 3 4 mpa and the incident angle of waves into the main fracture was changed from 12 to 20 a fracture model of a single fracture and a microcrack zone in the vicinity of the main fracture was used to explain the shear wave splitting phenomenon the width of the reopening microcrack zone and the crack density were quantitatively evaluated by using the fracture model and shear wave splitting analysis the best fit theoretical curve to the observed data suggested that the width of the reopening zone was 0 8 m and the crack density was 0 39

spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999 m7 4 izmit and m7 1 duzce turkey earthquake sequences summary we analyse spatiotemporal variations of crustal anisotropy along the karadere duzce branch of the north anatolian fault from similar earthquakes in the aftershock regions of the 1999 mw7 4izmit and mw7 1 duzce earthquakes the similar earthquake clusters are identified by performing cross correlation on waveforms generated by 18 000 earthquakes depending on the applied similarity criterion about 4 60 per cent of the events belong to similar event clusters splitting parameters averaged within each cluster show significant variations for slightly different ray paths indicating strong spatial variations of crustal anisotropy in this area we also find clear changes in the spatiotemporal seismicity patterns following the duzce main shock large apparent co seismic changes up to 30 per cent of shear wave splitting delay times are observed across the time of the duzce main shock at stations near the epicentral region however the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity rather than changes in the properties of the anisotropic medium splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the duzce main shock the results do not show systematic precursory changes before the duzce main shock

time lapse change in anisotropy in japan s near surface after the 2011 tohoku oki earthquake 1 we apply seismic interferometry to strong motion records to detect the near surface i e an upper few hundred meters deep change in anisotropy caused by the mw9 0 tohoku oki earthquake on 11 march 2011 we show that the earthquake increased the difference between fast and slow shear wave velocities arising from shear wave splitting in most parts of northeastern japan but it did not significantly change fast shear wave polarization directions in the near surface through monitoring of anisotropy and shear wave velocity we find that the changes in anisotropy and velocity partially recover with time they are however still different from the pre event values after nine months the comparison of the spatial distribution between changes in anisotropy and velocity indicates the changes in anisotropy and velocity are generally correlated especially in the northeastern honshu the main island in japan the change in the largest principal stress direction weakly correlates with the change in anisotropy

plate wide deformation before the sumatra andaman earthquake rock is weak to shear stress and the energy released by the 26th december 2004 m 9 2 sumatra andaman earthquake the largest earthquake for four decades must have accumulated over enormous volumes of crust and mantle certainly plate wide possibly world wide here we report evidence for plate wide stress accumulation changes in seismic shear wave splitting monitor stress induced changes in the geometry of the microcrack distributions in almost all rocks in the earth s crust such changes observed in iceland show stress accumulation beginning at least four years before the sumatra andaman earthquake these changes were recognised as monitoring stress accumulation before an impending large earthquake and 10 stress forecasts were emailed to iceland meteorological office for some 29 months forecasting an impending large earthquake the remarkable sensitivity of critical systems of microcrack geometry to miniscule changes of stress had not been recognised at that time and the stress accumulation was expected to lead to a m 7 earthquake somewhere in iceland only now is it recognised that the changes in shear wave splitting were monitoring stress accumulation which would eventually lead to the sumatra andaman earthquake at a distance of some 10 500 km on the opposite side of the eurasian plate this extreme sensitivity confirms the critical nature of fluid saturated stress aligned microcracks in the earth s crust

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

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

effects of abutment slopes on the performance of high rockfill dams une methode par elements finis tridimensionnelle non lineaire a ete utilisee pour analyser le comportement general contrainte deformation de barrages en enrochements de grande hauteur dans des vallees etroites l influence de la pente des appuis sur l effet de voute qui prend place dans le sol des barrages est largement discutee de meme que le potentiel de fracturation hydraulique dans le noyau et la fissuration dans les zones de contact du noyau on a aussi analyse et compare le comportement de barrages de differentes hauteurs on suggere des ameliorations pour la preparation des fondations des noyaux et pour la protection vis a vis de la fissuration

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

changes in shear wave splitting before the 2010 eyjafjallajokull eruption in iceland summary we use shear wave splitting sws above microearthquakes to monitor stress variations before the 2010 march and april flank and summit eruptions of eyjafjallaj okull volcano in iceland sws time delays before eyjafjallaj okull show characteristic variations similar to those seen before earthquakes the time delays display a nearly linear increase before the eruption an abrupt change of slope and a rapid nearly linear decrease until the flank eruption begins similar variations before earthquakes are interpreted as stress accumulation increases and stress relaxation decreases as microcracks coalesce onto the eventual fault plane the changes in sws before eyjafjallaj okull are interpreted as a similar stress accumulation increase as magma penetrates the crust and stress relaxation decrease as microcracks coalesce onto the magma conduit prior to magma release we suggest that the remarkable similarity between stress changes before eruption and earthquake is strong evidence for the new geophysics of a critically microcracked crust