Changes In Shear-wave Splitting Research Articles

Changes in shear-wave splitting at the 2014 Bárðarbunga seismic crisis and dyke intrusion in Iceland compared with earthquakes and other eruptions

We use shear-wave splitting (SWS) above small earthquakes to monitor stress-variations before the 2014 Bárðarbunga seismic crisis and dyke intrusion in central Iceland. SWS is sensitive to stress-induced variations in the internal geometry of the distributions of stress-aligned fluid-saturated microcracks pervading almost all crustal rocks. This allows stress-accumulation before both impending earthquakes and impending volcanic eruptions to be monitored and potentially the events stress-forecast. We compare the behaviour of SWS before Bárðarbunga with the behaviour before large earthquakes and other volcanic episodes where SWS has been monitored. On the basis of the duration of stress-changes, we classify volcanic episodes into three broad classes. During analysis of SWS before Bárðarbunga, temporal variation in polarizations and another retrospective stress-forecast earthquake were recognized.

Fluid substitution effects on seismic anisotropy

AbstractWe derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil) on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P‐wave moduli increase as much as 56% and P‐wave velocity increases up to 19% for gas‐to‐brine substitution. For the direction parallel to fractures, P‐wave velocity remains almost constant when porosity is low (5%) but can increase up to 4% if porosity is high (25%). Since P‐waves in two different directions have different sensitivities to fluids and fractures, the Thomsen's parameters (defined for HTI and orthorhombic symmetries), ϵ and δ, are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear‐wave splitting (γ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures, and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation and 6% decrease for high porosity gas sands. We find that the percentage changes in gas‐to‐oil substitution are about half that of the gas‐to‐brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Changes in shear wave splitting before the 2010 Eyjafjallajökull 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.

An effective medium model for the stress-dependence of anisotropic seismic velocities in fractured rock

Abstract Reliable analysis of time-lapse seismic data to detect movements of pore fluids requires consideration of variations in subsurface stress as well as fluid saturations. Achieving this goal requires reliable rock physics models that relate these stress changes to seismic velocity. Although there are many theories for the effective elastic properties of fractured media, only a small number directly include stress-dependence. Therefore, we have developed a new model that provides expressions that are comparatively easy to work with and requires estimation of only a small number of parameters. In this paper, we apply this model for the first time to the inversion of published laboratory measurements of pressure-dependent anisotropic seismic velocities. Specifically, we utilize measured velocities in two orthogonal directions to estimate the model parameters for a transversely isotropic medium. The model is then used to predict directional velocity variation in other directions at ambient pressure, and the errors in all cases are small. Synthetic shear wave seismograms computed for a hypothetical carbonate reservoir model show the changes in shear wave splitting and travel times that are predicted when seismic properties are computed using the theoretical model and help to illustrate the potential applications of the theoretical model to reservoir characterization.

Search for temporal changes in shear-wave splitting associated with the 2012 Te Maari Eruptions at Mount Tongariro, New Zealand

We investigate changes in shear wave splitting and VP/VS ratios of local earthquakes from the GeoNet catalogue during a 16month period beginning a year before the first Te Maari eruption at Mount Tongariro on August 6, 2012, focusing on four permanent seismographs located in proximity to the volcano. We identify four time periods bounded by sharp transitions that comprise the study period, during which moving averages of the shear-wave splitting parameters, Φ (fast direction) and δt (delay time), are fairly constant. At all stations, VP/VS is steady throughout most of the study period at 1.75. Small variations occur during the earthquake swarm at the volcano, which started a month before the first eruption, and for some low magnitude events occurring after a change in earthquake location method. Analysis of data sets in which epicentre location, hypocentre depth and event magnitude are restricted illustrates that observed temporal changes in shear-wave splitting parameters are likely due to the spatial variation of paths. This in turn is governed by the spatial distribution of seismicity and measurement quality. We think the short term variation in VP/VS ratios is due to event origin time uncertainty of low magnitude earthquakes or incorrect S-phase arrival timing for events in the Tongariro swarm. These results suggest that any volcanic processes able to cause changes in shear-wave splitting or VP/VS associated with the two eruptions during our study period were too localised to Te Maari to be observed at the seismographs studied using our methods. Dominant Φ observed during the study period are oriented approximately tangential to the Tongariro/Ngauruhoe massif at all four stations. We suggest that this may result from gravitational loading of Tongariro and Ngauruhoe mountains inducing fracturing or dilatation of tangentially oriented microcracks. There may also be some effect from layered material causing horizontal propagating rays yielding faster speed SH waves than SV at station NGZ. Measurements from shallow earthquakes in immediate proximity to the seismographs indicate the potential presence of a shallow, highly anisotropic body in the volcano.

Reply to "Comment on 'Operational Earthquake Forecasting: Status of Knowledge and Guidelines for Implementation by Jordan et al. [2011]' by Stuart Crampin"

In his commentary on the International Commission on Earthquake Forecasting (ICEF) report [Jordan et al. 2011], Crampin [2012] claims that observable changes in shear-wave splitting can predict large earthquakes on short time scales with high reliability and skill, and he challenges a central ICEF finding—that no method has yet demonstrated such a predictive capability. In particular, Crampin asserts that observations of seismic shear-wave splitting monitored at a three-borehole Stress-Monitoring Site (SMS) in central Italy could monitor stress-accumulation and stress-relaxation (crack-coalescence) before all damaging (M ≥ 5) earthquakes within 1000 km of the SMS […] This means that installation of one expensive SMS (preferably two throughout the length of Italy) would stress-forecast the time, magnitude, and in some circumstances fault-break of all potentially damaging on the mainland of Italy. Crampin's assertions are based on prediction methods he has advocated for several decades [e.g., Crampin et al. 1984; Crampin et al. 1990]. As part of the ICEF study, we reviewed the literature on these methods. Nothing contained in the present commentary compels us to alter our previous findings. [...]

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 29months 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 10,500km 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.

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.

Temporal variations of shear-wave splitting in field and laboratory studies in China

Observations of shear-wave splitting at seismic stations above a swarm of small earthquakes on Hainan Island, China, and other examples world-wide, suggest that the time-delays of split shear-waves monitor the build up of stress before earthquakes and the stress release as earthquakes occur. Rock physics experiments on marble specimens also show variations of shear-wave time-delays with uniaxial pressure analogous to the field observations. The rock experiments show an abrupt decrease in time-delays immediately before fracturing occurs. Similar precursory behaviour has been observed before earthquakes elsewhere, and is believed to be important for two reasons. Precursory changes in shear-wave splitting could be used for short-term forecasting, but of greater importance may be the information such behaviour provides about the source processes in earthquake preparation zones.

A four-year study of shear-wave splitting in Iceland: 2. Temporal changes before earthquakes and volcanic eruptions

Abstract This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a build-up of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between M 3.5 and M 5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a M 5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of in situ rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.

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 Earth's 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.

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.

Temporal changes in shear wave splitting after theM=5.2 shimian earthquake of June 9, 1989

Just after the occurrence of anM=5.2 earthquake in Shimian, Sichuan, two three-component seismometers were set up near the epicenter of the earthquake. Field observations were made for the four-year period from 1989 to 1992. Analysis of the data recorded from either aftershocks following theM=5.2 earthquake or from small earthquakes scattered in this region indicates the presence of shear wave splitting. And shear wave splitting varies with time. The mean direction of polarization of the faster shear waves is N18°W during the period of aftershock activity, which is consistent with the strike of the faulting plane of the mainshock; but has turned to N46°W from 1990 to 1992, which is consistent with the regional maximum compressive stress. The time delays between split shear waves measured on records from 1990 to 1992 are about half of that in 1989. The results obtained from observations at two temporary stations are similar. This indicates that the temporal changes may be related to the occurrence of theM=5.2 mainshock.

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 build up 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 our interpretation of temporal changes in shear wave splitting is the well established existence, throughout at least 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. We 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.

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.

Shear-Wave VSP'S: A Powerful New Tool for Fracture and Reservoir Description

Summary Seismic shear waves contain much more information than P waves about the internal structure of the rock along the ray path, which can be interpreted in terms of the crack and stress geometries within the rock mass. Recent field data sets show that shear-wave vertical seismic profiles (VSP's), where a surface source is recorded on geophones downwell, are powerful new tools for detecting fracture orientation and reservoir description. They can provide detailed estimates of the internal crack and stress structure of the reservoir rock. Introduction Several major oil companies recently presented substantial shearwave data sets from reflection surveys gathered for hydrocarbon E and P purposes that reveal crack alignment in the rock mass. Similar oil company data from VSP surveys in the Paris basin, France, and the Silo field, WY, have been reported, Earthquake seismologists have seen the same type of phenomenon, called shearwave splitting, above small earthquakes in many parts of the world (see Ref. 10). There is a now a substantial body of observational evidence that shear-wave splitting is caused by propagation through stress-aligned fluid-filled inclusions. We know that there are fluid-filled fractures, cracks, microcracks, and pores within most rocks in the crust. These fluid-filled inclusions are the most compliant parts of the rock mass and will be preferentially oriented by the current stress field acting on the rock mass. These aligned inclusions are effectively anisotropic to seismic waves, so shear waves that are most sensitive to anisotropy display shear-wave splitting. Confirmation of this interpretation has been provided by recent observations of temporal changes in shear-wave splitting before and after the North Palm Springs earthquake in California and before and after hydraulic fracturing, as reported in this paper. No other explanation for the observed anisotropy displays changes with time in response to comparatively small changes of low-level stress in the comparatively cool upper crust. Shear-wave splitting indicates that the "cracks" are typically aligned by stress into nearly parallel, nearly vertical orientations, striking parallel to the horizontal direction of the current maximum compressional stress (or, more correctly, striking perpendicular to the horizontal direction of current minimum compressional stress). These results are particularly important for reservoir and production engineering. Analysis of shear-wave VSP's can provide information about the internal structure of the reservoir. In particular, analysis of shear-wave splitting yields estimates of the in-situ crack and stress geometries within the reservoir. This improved understanding of the physics of the rock mass is expected to result in improved hydrocarbon recovery. Shear-Wave Splitting In the past, most seismic investigations used seismic P waves to examine the rock mass. P waves arrive before the shear waves because shear waves travel about half as fast as P waves. P waves have longitudinal polarization parallel to the ray path, as illustrated schematically in Fig. 1a. The arrival time, amplitude, and frequency spectrum are the only information in the P wave train. In contrast, shear waves have transverse polarizations (transverse particle motion, Fig. 1b), which carry much more information. In rock without any internal structure (isotropic, uncracked rock, where the properties are the same in all directions), any shear-wave polarization radiated by the source will propagate without significant change of waveform (Fig. 1b). Any two orthogonal polarizations radiated from the same source position will propagate at the same velocity (in isotropic uncracked rock) and the initial polarization of the shear wave will be essentially preserved.

Shear-wave polarizations near the North Anatolian Fault - II. Interpretation in terms of crack-induced anisotropy

Summary. The polarizations of shear waves recorded by networks of digital thxee-component seismometers immediately above small earthquakes near the North Anatolian Fault in Turkey display shear-wave splitting on almost all shear-wave seismograms recorded within the shear-wave window. This splitting is incompatible with source radiation-patterns propagating through simple isotropic structures but is compatible with effective anisotropy of the internal structure of the rock along the ray paths. This paper interprets the phenomena in terms of widespread crack-induced anisotropy. Distributions of stress-induced cracks model many features of the observations, and synthetic polarization diagrams calculated for propagation through simulated cracked rock are similar to the observed patterns. This evidence for widespread crack-induced anisotropy lends strong support to the hypothesis of extensive-dilatancy anisotropy (EDA) suggested by laboratory experiments in subcritical crack-growth. The crucial evidence confirming some form of EDA would be observations of temporal changes in shear-wave splitting as the stress field alters the crack density and crack geometry. There is some weak evidence for such temporal changes at one site, but further analysis of suitable digital three-component seismometer networks in seismic areas is required to confirm EDA.