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

a reconnaissance ion probe study of hafnium isotopes in zircons a sims technique for the isotopic analysis of hafnium in zircons using the shrimp ion microprobe has been developed and a precision of typically 0 5 2 achieved in the mean reduced 176hf177hf ratio measured at several spots on a single grain unfractionated chondritic initial hf isotopic compositions have been measured on a number of archaean zircon populations these include the oldest known terrestrial minerals the 4 2 ga old mount narryer detrital zircons thereby confirming their antiquity in contrast positive initial hf relative to the chondritic model composition has been found in several post archaean zircon populations reflecting the increasing involvement of isotopically evolved depleted mantle sources in the formation of younger crust the 570 ma old sri lankan zircon standard sl7 yielded an exceptionally low initial hfof 23 implying a metamorphic origin as a reworked product of ancient crust shrimp u pb analyses of zircons from archaean tonalitic gneiss at watersmeet michigan yield a precise crystallization age of 3636 6 ma 2 and show that a previously reported correlation between 176hf177hf and u pb isotopic discordance in bulk zircon samples patchett 1983 was caused by the addition of radiogenic hf in discrete overgrowths of new zircon ca 2 7 ga ago the original 3 64 ga grains show no evidence of disturbance to their original chondritic hf isotopic composition there is presently no evidence for significant isotopic exchange of hf between zircon and other minerals in crustal rocks

multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the tibetan plateau and tectonic implications of the haiyuan fault abstract the haiyuan fault is a large strike slip fault in the northeastern ne margin of the tibetan plateau using new data recorded by a temporary seismic array this study obtains crustal seismic anisotropy measurements in different scales across the haiyuan fault by analyzing splitting of shear waves from local earthquakes parameters of shear wave splitting sws indicate clear zoning characteristics in seismic anisotropy in the upper crust the study area is divided into two different anisotropic subzones by the haiyuan fault the dominant orientation of fast polarizations is nne or ne north of the faults and wnw to ew south of the fault this pattern of spatial variation indicates the controlling impact of haiyuan fault to observed seismic anisotropy with a zone of dozens of kilometers from the haiyuan fault the dominant orientations of fast polarizations are wnw generally coincidental with the strike of the haiyuan fault except for several stations it indicates that the influence of the fault on stress could reach to about 10 km from the fault distance spatial pattern of anisotropy indicates multiple effects by stress faults and local tectonics it suggests different anisotropic mechanisms including stress induced anisotropy and tectonic or structural anisotropy the dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics the normalized time delays are higher around the haiyuan fault than those away from the fault suggesting stronger crustal seismic anisotropy in a narrow belt around the strike slip fault zone the sws parameters suggest that the haiyuan fault is the actual crustal boundary of ne tibetan plateau about 200 km north of the reported boundary of the tibetan plateau block combining with other studies in the crust and the lithosphere this paper infers possible two layer crustal anisotropy beneath the yinchuan graben north of the haiyuan fault the dominant orientation of fast polarizations are nne or ne in the upper crust probably originated from crack induced anisotropy but become wnw or nw in the middle lower crust possibly originated from deformation anisotropy

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

upper crust seismic anisotropy study and temporal variations of shear wave splitting parameters in the western gulf of corinth greece during 2013 abstract during 2013 the western gulf of corinth wgoc central greece experienced a period of increased seismicity with a total of over 4700 earthquakes this fact in combination with the existence of dense seismological networks provided an excellent opportunity for the study of crustal seismic anisotropy of special note is the seismic crisis period of may october during which the main feature was the occurrence of the helike seismic swarm polarigrams and hodograms were employed to analyze local waveforms this method resulted in 659 measurements of shear wave splitting parameters namely the direction of the fast shear wave s fast the time delay t d between the two split shear waves and the source polarization direction a pattern of a general wnw ese anisotropy direction parallel to the goc s fault systems strike is established with the exception of two stations located in adjacent areas at the north this is in agreement with the existence of fluid filled microcracks oriented according to the regional stress field the obtained splitting parameters are compared to the results of other anisotropy studies performed in the wgoc a detailed analysis of the temporal evolution of the normalized time delay t n was performed to associate temporal stress changes to seismicity fluctuations increase in normalized time delays and drop before the occurrence of the first significant event belonging to the july cluster which occurred between the 13th and the 16th of the same month was observed for most of the analyzed stations

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

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

the 1982 enola arkansas swarm and scaling of ground motion in the eastern united states abstract the north central arkansas swarm which began in january of 1982 and was still active as of december 1983 has produced over 30 000 events three of these events had duration magnitudes greater than 4 0 fifty three locatable events in the magnitude range 3 8 and below were recorded during the 2 1 2 weeks our 10 digital seismographs were in place the hypocentral locations are confined to a tight cluster with a radius of approximately 1 km depths range between 4 and 7 km focal mechanisms determined for 10 of the events show a combination of strike slip and normal faulting source parameters seismic moment source radius and stress drop were calculated for 48 of the events the seismic moments range from 10 18 to 10 21 dyne cm while the stress drops range from 0 2 to 60 bars acceleration spectra are band limited at the high end at 30 hz the stress drops increase with seismic moment up to 10 20 dyne cm but appear to be constant at greater moment values the arkansas source parameters are similar to other body wave source parameters determined from near source investigations of earthquakes in eastern and central north america e g new brunswick and south carolina and in california e g mammoth lakes oroville and anza but differ substantially from lg wave source parameters estimated for earthquakes in the eastern and central united states

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

indication of high pore fluid pressures in a seismically active fault zone summary polarisations of seismic shear wave splitting observed above small earthquakes in iceland are typically approximately ne to sw parallel to the direction of maximum horizontal stress in contrast the polarisations of shear waves at three new stations sited over the husav ik flatey fault a major seismically active transform fault in northern iceland are approximately nw to se orthogonal to the stress aligned polarisations elsewhere modelling suggests that these 90 flips in polarisations are caused by propagation through cracks containing fluids at high pore fluid pressures within one or two mpa of the critical stress these observations suggest that high pore fluid pressures which play a key role in earthquake source mechanisms can be monitored by analysing shear wave splitting above seismically active fault planes

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

stress induced temporal variations in seismic anisotropy observed in microseismic data summary in situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms volcanic eruptions and changes in hydrocarbon reservoirs shear wave splitting induced by stress controlled cracks in the shallow crust provides a way to infer this stress temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations here we present evidence of temporal variations in shear wave splitting from a continuous time series with a high occurrence of microseismic events recorded in a borehole we interpret these observations in terms of stress controlled cracks and are able to infer changes in stress and via modelling suggest the cause of the anisotropy possible origins of the temporal variation in per cent anisotropy are tidal or oil field production processes our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock

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

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

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

observations of shear wave splitting parameters in the western gulf of corinth focusing on the 2014 mw 5 0 earthquake abstract the western gulf of corinth located in central greece is an active neotectonic graben that features intense seismicity events recorded by dense local networks during 2014 including those belonging to an offshore swarm between nafpaktos and psathopyrgos were used to perform a shear wave splitting study with recordings that fulfill strict criteria the splitting parameters determined by visually inspecting polarigrams and hodograms are the polarization direction of the fast shear wave and the time delay between the two split shear waves over 960 observations were obtained for 13 stations in the area temporal variations of normalized time delays were investigated to extract patterns that indicate stress accumulation and release such patterns were identified in 5 stations where linear trends of increase followed by a decrease a couple of weeks before the mw 5 0 7th november 2014 earthquake were identified these observations highlight similarities with behavior of time delays before the occurrence of earthquakes in other areas nevertheless scattering of measurements poses a significant challenge in interpreting these variations the mean polarization direction of the fast shear wave in each station is consistent with the known regional maximum horizontal stress component i e wnw ese except for three stations located in the northern shore of the gulf which present a general ne sw polarization direction the former is interpreted with the anisotropic poro elasticity model where fluids within microcracks aligned with the regional stress regime affect shear wave splitting the latter are located in an area where faults of similar direction have been mapped which could indicate the presence of secondary local stress states

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

effects of point singularities on shear wave propagation in sedimentary basins summary in most directions of propagation in anisotropic solids seismic shear waves split in regular and predictable ways that in principle can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass in all anisotropic solids however there are directions of propagation known as shear wave singularities where the split shear waves have the same phase velocities for directions of propagation near the commonest type of singularity the point singularity the relationship between the phase and group velocities may undergo rapid variations for small changes in direction this results in shear waves along rays propagating at the grozq velocity behaving anomalously with irregular polarizations and amplitude changes as if they were propagating near cusps although the degree of anisotropy may be too small to cause conventional cusps on the group velocity wave surfaces the effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well established phenomenon of azimuthal isotropy transverse isotropy with a vertical axis of symmetry caused by horizontal lithology or by fine layering ptl anisotropy combining with the more recently recognized azimuthal anisotropy caused by distributions of near parallel near vertical fluid filled inclusions eda anisotropy this paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins if not identified correctly the effects of such point singularities could be mistakenly attributed to structural irregularities and if correctly identified the directions of such singularities can place tight constraints on possible combinations of ptl and eda anisotropy in sedimentary basins

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

preliminary analysis of crustal shear wave splitting in the sanjiang lateral collision zone of the southeast margin of the tibetan plateau and its tectonic implications

paris basin vsps case history establishing combinations of fine layer or lithologic anisotropy and crack anisotropy from modelling shear wavefields near point singularities summary this paper examines shear wave splitting in multi offset vsps at a borehole site in the paris basin and demonstrates that the sedimentary structure must have at least orthorhombic anisotropic symmetry it is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine layer or lithologic anisotropy and crack anisotropy transition zones close to shear wave singularities where there are rapid changes of shear wave polarizations and delays can account for the anomalous observed waveforms observations of large variations in shear wave polarizations in propagation directions close to point singularities are probably the first positive identification of the effects of singularities in sedimentary basins

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

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

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

seismic shear wave splitting in upper crust characterized by taiwan tectonic convergence summary this study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of taiwan with a large amount of earthquake data the lateral variation of seismic shear wave splitting sws is fully examined in terms of crustal deformation process as the well known vigorous orogeny subjected to the eurasian philippine plate collision tectonic convergence of taiwan is presumably propagating from east to west the acquired sws waveform data cover areas from the slightly deformed western plains to the intermediate to high metamorphic western foothills and central mountain ranges by means of waveform cross correlation the sws parameters the fast wave polarization orientation and delay time infer that the mechanism of lithologic deformation of taiwan convergence can be classified into two domains the convergence parallel laminating west of the deformation front and the convergence perpendicular striking east of the deformation front the convergence parallel sws measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength whereas the convergence perpendicular measurements reveal the lateral accommodation of deformation as the stress strain surpass the yielding strength of rock where the predominant sws polarization is in the ne sw direction similar to the general trend of taiwan s mountain ranges it is remarkable that there is no correlation between metamorphic degrees with sws parameters the geological province which corresponds to higher metamorphism is not consistent with large sws parameters this may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone furthermore comparison of the sws delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers and the fabric lamination causing the anisotropy may be confined only within the shallow crust

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

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

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

natural laboratory nw bohemia comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes comprehensive studies of co2 rich fluids close to the swarm earthquake region nov kostel at the czech german border have been started 15 years ago and have in particular included two extended chemical and isotope monitoring studies lasting for several years each the regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies three degassing centers cheb basin marianske lazn and karlovy vary with high co2 flux and the same level of 13c values but different levels of 3he 4he ratios have been identified the studies have located the co2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations a seismically triggered release of crustal helium was confirmed by both monitoring campaigns finally indications for a presently active magmatic process beneath the cheb basin have been found in contrast to volcanically active regions magma accumulation in the study area takes place at the crust mantle boundary and is not yet accompanied by heat transfer to the surface likewise reactive magma derived components are absent in the degassing fluids the area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes the results of our fluid monitoring including the stunning observation of mantle derived free fluids marked by 3he 4he ratios within the subcontinental mantle range are supported by geophysical findings from seismic studies and geologic indications

distribution of 230th and 231pa in the water column in relation to the ventilation of the deep arctic basins distribution of 230th and 231pa in the water column was measured in the eurasian basins of the arctic ocean i e the makarov amundsen and nansen basins the profiles obtained were compared to one previously published from the alpha ridge bacon et al earth and planetary science letters 95 1989 15 22 the amundsen and nansen basins are similar in their 23oth and 231pa distribution whereas in the makarov basin and at the alpha ridge these radionuclides are comparatively enriched in the deep basin this may be due to low particle flux in conjunction with low water mass renewal rates the 230th concentration in water depth 1000 m in the makarov basin is relatively low compared with the alpha ridge which may be caused by an exchange of water masses between the amundsen and makarov basins in the eurasian basin deep water 230th profiles can be explained both by a scavenging model and by a mixing scavenging model enhanced particle load caused by resuspension may explain the increased scavenging in the eurasian basin bottom water the differences in scavenging between the norwegian greenland sea and the eurasian basins at present are not large enough to cause significant fractionation between 230th and 231pa

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

effects of the january 2018 seismic sequence on shear wave splitting in the upper crust of marathon ne attica greece abstract on the 15th of january 2018 a moderate mw 4 2 earthquake occurred in the area of marathon ne attica this event is of great interest given that it is located in a region characterized by low seismic activity even though there are previously identified faults e g penteli fault the sequence started 7 days before the mainshock and lasted until the end of the january 2018 in the present study waveforms from stations of the hellenic unified seismological network were utilized to explore the seismotectonic regime using data recorded since 2012 while the focal mechanism of the mainshock is in agreement with known local faults relocated hypocenters indicate the existence of blind faults in addition the proximity of two stations dion and ptl to the january 2018 sequence provided a unique opportunity to investigate seismic anisotropy of the upper crust in the study area for the first time analysis of local events between 2012 and 2018 led to the revelation of an anisotropic layer beneath marathon however a complex anisotropy regime is identified the polarization direction of the sfast in dion presents stable characteristics and is oriented according to the regional maximum horizontal stress component however ptl exhibits rotation to a direction perpendicular to the one that conforms to stress orientation possibly associated with local faults of similar direction time delays through the whole period are uncommonly low for such studies in both stations nevertheless this changes for the january 2018 sequence in dion higher values of time delays permit the establishment of trends that could be attributed to stress accumulation and relaxation related to the mw 4 2 event in agreement with the anisotropic poro elasticity model

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