Variations In Shear-wave Splitting Research Articles

Anisotropy-revealed change in hydration along the Alaska subduction zone

AbstractMegathrust earthquake behavior in subduction zones is controlled by a variety of factors including the hydration state of the subducting slab. Increased hydration reduces the occur-rence of great, damaging earthquakes by diminishing the strength of the material along the interface between tectonic plates. Understanding variations in hydration in subductions zones is necessary for properly assessing the overall hazard posed by each region. Fortunately, seismic anisotropy is strongly dependent upon hydration of the subducting crust and litho-sphere. I present shear-wave splitting measurements that illuminate changes in anisotropy, and therefore hydration, of the subducting Pacific plate beneath the Alaska subduction zone (northern Pacific Ocean). Variations in shear-wave splitting directly correlate to changes in the behavior of great, megathrust earthquakes. My measurements show that the Shumagin seismic gap is characterized by a hydrated subducting slab, explaining the long-term lack of great earthquakes. Observations in the immediately adjacent Semidi segment, which experiences great events regularly, indicate a far less hydrated slab. These results are driven by the preferential alignment of paleo-spreading fabrics of the Pacific plate. Where fabrics are more closely aligned with the orientation of the trench, outer-rise faulting and plate hydration is enhanced. These results highlight the importance of changes in preexisting slab structures and subsequent hydration in the production of great, damaging earthquakes.

Variable seismic anisotropy across the Peruvian flat-slab subduction zone with implications for upper plate deformation

We performed shear wave splitting analyses to investigate seismic anisotropy across the northern extent of the Peruvian flat-slab subduction region. We used core-mantle refracted SKS, SKKS and PKS phases from teleseismic events (88° > Δ < 150°) recorded at 45 broadband seismic stations from the Peruvian permanent and portable seismic networks as well as from, international networks (CTBTO, RSBR-Brazil, and RENSIG-Ecuador). The results reveal a complex anisotropy pattern with distinct variations in shear wave splitting along strike. In the northernmost region, the mean delay times range between 1.0 ± 0.2 s and 1.5 ± 0.2 s with fast directions predominantly oriented ENE-WSW approximately perpendicular to the trench, parallel with the motion of the subducting Nazca plate. In the central region of Peru, the predominant fast directions change to a SE-NW orientation that is oblique with the trench. These fast splitting directions are consistent with the pattern seen previously over the southern extent of the flat-slab and correlate well with the current geodetically derived motion of the overriding forearc, the Peruvian Sliver. These characteristics suggest a fundamental change in anisotropic behavior between the northern and central portions of the Peruvian flat-slab and imply that the upper plate deformation is a controlling factor.

Variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake region

In this paper, variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake sequence were studied. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay time between fast and slow shear waves were derived from seismic recordings at eight stations on the southern segment of the Longmenshan fault zone. In the study region, the fast polarization directions show partition characteristics from south to north. And the systematic changes of the time delays between two split shear waves were also observed. As for spatial distribution, the NE fast polarization directions are consistent with the Longmenshan fault strike in the south of focal region, whereas the NW fast direction is parallel to the direction of regional principal compressive stress in the north of focal region. Stations BAX and TQU are respectively located on the Central and Front-range faults, and because of the direct influence of these faults, the fast directions at both stations show particularity. In time domain, after the main shock, the delay times at stations increased rapidly, and decreased after a period of time. Shear-wave splitting was caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main shock and the aftershocks. The stress on the Lushan Ms7.0 earthquake region increased after the main shock, with the stress release caused by the aftershocks and the stress reduced in the late stage.

Short-scale variations of shear-wave splitting across the Dead Sea basin: Evidence for the effects of sedimentary fill

[1] We examine shear-wave splitting of SKS waveforms collected by a temporary array of 68 stations in the region of the Dead Sea basin. The observed splitting parameters exhibit systematic variations along a dense, EW-trending 60 km profile across the basin. The delay times vary significantly between 1.0 and 2.8 seconds with smaller values in the very center of the profile. The fast polarizations are oriented more-or-less parallel to the strike of the Dead Sea transform fault and vary between −10 and 20 degrees with respect to North. Finite-frequency waveform modeling reveals that the source-region of the small-scale lateral variations is likely located within the crust. The modeling further shows that purely isotropic velocity variations affect shear-wave splitting: To a large degree, the observed variations of splitting parameters can be explained by the sedimentary fill of the basin and its low isotropic seismic velocities, whereas the mantle is uniformly anisotropic. Our study indicates that precaution must be taken when interpreting short-scale lateral variations of shear wave splitting in terms of anisotropic structures in the crust or upper mantle.

Variations of Shear Wave Splitting in the 2010 Yushu Ms7.1 Earthquake Region

AbstractIn this paper, variations of shear wave splitting in the 2010 Yushu Ms7.1 earthquake sequence were studied. By analyzing S‐wave particle motion of local events in the shear wave window, the polarization directions of fast shear waves and the delay times between fast and slow shear waves were derived from seismic recordings at 3 stations near the Garzê‐Yushu fault belt. The shear wave splitting results of 3 stations in the area of the fault belt reveal the NEE polarization directions of fast shear waves, which are consistent with the direction of regional principal compressive stress. Systematic changes of the time delays between two split shear waves were also observed. As for the spatial distribution, in focal region, the delay times at stations L6304 and YUS in the north branch fault of the Garzê‐Yushu fault belt are more than that of station L6303 in the south branch fault, meanwhile, the delay time at station YUS is more than that of station L6304. As for temporal changes in shear wave delay times, after the main shock, the delay times at station YUS increased rapidly, and decreased after a period of time. Seismic shear wave splitting is caused mostly by stress‐aligned microcracks in rock below the stations. The results demonstrate the changes of local stress field in the course of the main‐shock and aftershocks. After the main shock, the stress in the main‐shock region was released by the main‐shock, the crustal stress transferred to the southeastern part of the Garzê‐Yushu fault zone, and caused the delay time at station YUS to be greater than that of station L6304. The north branch fault of the Garzê‐Yushu fault belt is the causative fault of the 2010 Yushu Ms7.1 earthquake, whereas the south branch fault has a small impact on the earthquake. The stress at station YUS increased after the main‐shock, with the stress release caused by the aftershocks the stress reduced in the later stage.

Converted-wave splitting estimation and compensation

Converted-wave (C-wave) splitting estimation and compensation (SEAC) estimates and removes the effects of shear-wave splitting from C-wave data. A locally 1D earth is assumed where a priori rotation of the field data to radial-transverse coordinates is valid. Subsurface fractures (horizontal transverse isotropy [HTI] layers are assumed) polarize C-wave reflection energy onto the transverse component, and introduce azimuth-dependent traveltime variations to the radial component. SEAC estimates the fast principal direction of the fractures, and the amount of traveltime splitting, from input radial and transverse azimuth-sectored stacks. SEAC also produces a splitting-compensated radial component, and a data misfit transverse component. Local fracture variations not accounted for in the coarse-interval inversion may be interpreted in the data misfit. Synthetic data generated by anisotropic reflectivity modeling for a model containing two HTI layers having different principal directions was used to illustrate SEAC. The field data example used was from a large3-C, 3D Vectorseis survey from onshore China. Preprocessed C-wave data (radial and transverse components) were prestack time migrated into offset planes within 10-degree azimuth sectors. These data were then corrected for residual moveout (azimuth-independent correction), and stacked over offset to produce azimuth-sectored stack gathers that were input to SEAC. SEAC estimated the azimuth of the fast principal direction [Formula: see text] and the amount of traveltime splitting [Formula: see text] that describe the overburden anisotropy. Spatially variable parameter estimates for the entire 3D data set, ([Formula: see text] and [Formula: see text]), produce significantly reduced energy on the transverse component at all record times after inversion. Azimuth-dependent traveltime variations on the input radial data were also significantly reduced at all record times, resulting in a postinversion radial full-azimuth stack having improved reflection continuity and temporal bandwidth. The data misfit (transverse component after inversion) potentially revealed local variations in shear-wave splitting not accounted for by the overburden layer-stripping correction.

Variations of shear wave splitting in the 2008 Wenchuan earthquake region

Through the analysis of S-wave particle motion of local events in the shear wave window, the polarization directions of the faster shear wave and the delay times between the faster and the slower shear waves were derived from seismic recordings at the stations near the fault zones. The shear wave splitting results of seven stations in the area of Longmenshan fault zone reveal spatial variation of the polarization directions of the fast shear wave. The directions at stations in the southeastern side of the Longmenshan fault zone (in the Sichuan Basin area) are in the NE direction, whereas the direction at station PWU (in the Plateau), which is in the northwestern side of the faults, is in the EW direction. Systematic changes of the time delays between two split shear waves were also observed. At station L5501 in the southern end of the aftershock zone, the delay times of the slower shear wave decrease systematically after the main shock. After the main shock, the delay times at station PWU were longer than those before the earthquake. Seismic shear wave splitting is caused mostly by stress-aligned microcracks in the rock below the stations. The results demonstrate changes of local stress field during the main-shock and the aftershocks. The stress in the southern part of Wenchuan seismogenic zone was released by the main-shock and the aftershocks. The crustal stresses were transferred to the northeastern part of the zone, resulting in stress increase at station PWU after the main-shock.

Variations of shear-wave splitting in Greenland: Mantle anisotropy and possible impact of the Iceland plume

Seismic anisotropy was investigated by measuring shear-wave splitting at 19 broadband stations in Greenland. We examined mostly SKS and SKKS phases, but also some PKS and depth phases of SKS (e.g. pSKS, sSKS) for deep events. Splitting parameters (fast polarization and time delay) were determined for these phases. The fast polarizations at nine sites in southern Greenland are quite uniformly oriented about N–NE. Two sites in central northern Greenland show a similar geometry to southern Greenland. Similar fast polarizations in southern and central northern Greenland suggest continuity of structural fabric beneath large parts of Greenland. This coherent pattern extends across a number of geological provinces of varying age and suggests a common cause of anisotropy not related to the bitwise formation of the Greenland continental block. Four sites in an east–west oriented belt crossing central Greenland show varying fast polarizations and suggest a separate process causing the anisotropy there, which may indicate that these processes are not currently active. The overall pattern of anisotropy in our results, with the exception of variations across central Greenland, is similar to results obtained from Rayleigh waves. The irregular geometry of splitting across central Greenland may be related to the impact of the Iceland plume at ∼ 60 Ma. Reported splitting time delays range from 0.4 to 1.4 s with an average of 0.8 s, which can generally not be explained by crustal anisotropy alone. If confined to a lithosphere of thickness on the order of 100 km, time delays of up to 1.4 s indicate anisotropy of up to about 6%, assuming that the a crystallographic axis of olivine is preferentially contained in the horizontal plane. We suggest that the anisotropy beneath Greenland is located mainly in the upper mantle but some contributions from the crust and lower mantle may be present.

A four-year study of shear-wave splitting in Iceland: 1. Background and preliminary analysis

Abstract A four-year study of seismic shear-wave splitting in Iceland was designed to seek temporal variations before earthquakes. Shear-wave 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 Part 2 of this study. This chapter presents the background, preliminary observations, and analysis of shear-wave splitting in Iceland.

Evidence for temporal variation of seismic velocity within the upper continental crust

Observations of systematic temporal variations of seismic anisotropy are presented for an induced‐seismicity experiment at 9 km depth. These observations were made under particularly well‐controlled conditions in the German Continental Deep Drilling Program (KTB) borehole, using shear wave splitting from similar events recorded at a three‐component instrument located at 4 km depth from a hydraulic fracturing experiment at 9 km depth. In a large set of seismic events recorded during the experiment, many can be associated with multiplets exhibiting essentially identical waveforms. Since they must have approximately the same source location and source radiation pattern, these events are particularly useful for testing the hypothesis of time‐dependent anisotropy. Anisotropy itself is clearly a very prominent feature in the data. A simple approach for waveform matching of split shear waves allows unprecedented resolution of variations in shear wave splitting. Importantly, the variation of shear wave splitting with time is a relative measurement, which can be performed with higher accuracy than the associated absolute measurement. In particular, the relative measurement is not affected by timing errors nor by event distance variations. During the experiment the difference between shear wave velocities decreases by ∼2% within ∼12 hours. After that, the medium apparently approaches a state which is stable for at least 5 hours. We suggest that the temporal variation is due to the tectonic stress release from seismic events caused by the fluid injection. This model requires the presence of fluid‐filled cracks at depths larger than 4 km.

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.

Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

Shear wave splitting has been measured from analyzing the three-component digital seismograms recorded at Guiquan station after the 1985M s6. 1 Luquan earthquake in Yunnan Province. The variations in parameters of shear wave splitting with time for over 100 aftershocks have two periods, the local stress period and the regional stress period. In the local stress period, there exist two vertical, paralell crack sets intersecting at about (50–60°), both affect on the propagation of S-waves, and the local stress is slightly stronger than the regional stress. With the activity of aftershock going down and the local stress dying away, it is returned to the state of the regional stress in the focal area. The polarizations of the fast split S-wave and their period variations are identical with the azimuths and changes of the principal compressive stress axis of focal stress field inferred independently from earthquake mechanisms, hense, it is interpreted that the shear wave splitting is the effects of anisotropy of EDA cracks controlled by stress field. The time delay of the slow split S-wave, except the difference between the two periods shows in some examples that it increases in a few hours before an event and decreases in a few days after an event on the individual background of period.

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.