Presence Of Seismic Anisotropy Research Articles

Crustal Anisotropy in the Martian Lowlands From Surface Waves

AbstractThe largest seismic event ever recorded on Mars, with a moment magnitude of 4.7 ± 0.2, is the first event to produce both Love and Rayleigh wave signals. We measured their group velocity dispersion between about 15 and 40 s period and found that no isotropic depth‐dependent velocity model could explain the two types of waves wave simultaneously, likely indicating the presence of seismic anisotropy. Inversions of Love and Rayleigh waves yielded velocity models with horizontally polarized shear waves traveling faster than vertically polarized shear waves in the top 10–25 km. We discuss the possible origins of this signal, including the preferred orientation of anisotropic crystals due to shear deformation, alignment of cracks, layered intrusions due to an impact, horizontal layering due to the presence of a large‐scale sediment layer on top of the crust, and alternation of sedimentation and basalt layers deposits due to large volcanic eruptions.

Evidence for crustal seismic anisotropy at the InSight lander site

We analyzed broadband and low-frequency events recorded on Mars and made the first detection of horizontally polarized shear wave reflections, which help constrain the crustal structure at NASA's InSight lander site. Coherent signals from five well-recorded marsquakes appear to be independent of the focal depth and are consistent with SH-wave reflections off the topmost crustal interface (8 ± 2 km). This phase confirms the existence of the ∼8 km interface in the crust and the large wave speed (or impedance) contrast across it. The range of acceptable parameters determined from the detected SH-wave reflections differs from the majority of the vertically polarized shear wave models resulting from a previous receiver function study, indicating that the velocity of the vertically polarized waves is larger than that of horizontally polarized waves. We propose that this inconsistency results from the presence of seismic anisotropy within the top crustal layer at the lander site. Modeling results show that dry- or liquid-filled cracks/fractures and igneous intrusions can reproduce the observed radial anisotropy.

An iterative factored topography-dependent eikonal solver for anisotropic media

Accurate and efficient eikonal solvers for heterogeneous media play an important role in many areas of seismology, such as seismic tomography, migration, and earthquake localization. Incorporating seismic anisotropy and complex topography remain a computational challenge for finite-difference eikonal solvers. In recent years, the topography-dependent eikonal equation (TDEE) has been proposed as an effective way to calculate seismic traveltimes for isotropic and anisotropic media with irregular topography. However, the Lax-Friedrichs sweeping method used in previous studies to approximate the viscosity solution of TDEE for anisotropic media is more dissipative and needs a much higher number of iterations to converge. In addition, the TDEE solution for the initial point source has an upwind source singularity, which makes all TDEE solvers, even the high-order ones, exhibit polluted convergence and relatively large errors that propagate from the point source to the entire computational domain. To solve these problems, we have formulated the factored topography-dependent anisotropic eikonal (FTDAE) equation in tilted transversely isotropic (TTI) media using the factorization principle. Then, the resulting quartic equation can be numerically solved by using a fixed-point iteration technique based on the simpler elliptical anisotropic eikonal (EAE) equation with a high-order source term due to anelliptical anisotropy introduced by TTI media. At each iteration, the unknown traveltime in the EAE equation is factored into two functions: One of the functions is specified analytically to capture the source singularity, such that the unknown factor is differentiable in the source neighborhood and could be solved by the fast sweeping method. Numerical examples indicate that our FTDAE solver can treat source singularity successfully and achieve high accuracy after just a few iterations, independently of the mesh size, which could provide a more efficient and robust tool for traveltime calculation in the presence of seismic anisotropy and complex surfaces.

On the Origin of Radial Anisotropy Near Subducted Slabs in the Midmantle

AbstractRecent seismic studies indicate the presence of seismic anisotropy near subducted slabs in the transition zone and uppermost lower mantle (mid‐mantle). In this study, we investigate the origin of radial anisotropy in the mid‐mantle using 3‐D geodynamic subduction models combined with mantle fabric simulations. These calculations are compared with seismic tomography images to constrain the range of possible causes of the observed anisotropy. We consider three subduction scenarios: (i) slab stagnation at the bottom of the transition zone; (ii) slab trapped in the uppermost lower mantle; and (iii) slab penetration into the deep lower mantle. For each scenario, we consider a range of parameters, including several slip systems of bridgmanite and its grain‐boundary mobility. Modeling of lattice‐preferred orientation shows that the upper transition zone is characterized by fast‐SV radial anisotropy anomalies up to −1.5%. For the stagnating and trapped slab scenarios, the uppermost lower mantle is characterized by two fast‐SH radial anisotropy anomalies of ∼+2% beneath the slab's tip and hinge. On the other hand, the penetrating slab is associated with fast‐SH radial anisotropy anomalies of up to ∼+1.3% down to a depth of 2,000 km. Four possible easy slip systems of bridgmanite lead to a good consistency between the mantle modeling and the seismic tomography images: [100](010), [010](100), [001](100), and . The anisotropy anomalies obtained from shape‐preferred orientation calculations do not fit seismic tomography images in the mid‐mantle as well as lattice‐preferred orientation calculations, especially for slabs penetrating into the deep lower mantle.

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

ABSTRACTThe collision of the Indian and Eurasian plates, to the east of the eastern Himalayan syntaxes, forms the Sanjiang lateral collision zone in the southeast margin of the Tibetan Plateau, where there are intense crustal deformation, active faults, earthquakes, as well as a metallogenic belt. Given the lack of adequate seismic data, shear‐wave splitting in this area has not been studied. With seismic data from a temporary seismic linear array, as well as permanent seismic stations, this paper adopts the identification on microseismic event to pick more events and obtains shear‐wave splitting parameters from local earthquakes. From the west to the east, the study area can be divided into three subzones. The “fast” polarization (i.e. the polarization of the fast shear wave) varies gradually from NNW to NS to NNE in these three subzones. The time delay of the slow shear wave (i.e. the time difference between the two split shear waves) also increases in the same direction, indicating the presence of seismic anisotropy above 25 km in the crust. Both shear‐wave splitting parameters are closely related to stress, faults and tectonics. The scatter and the “dual” (i.e. two) dominant orientations of the fast polarizations at several stations indicate strong distortions caused by nearby faults or deep tectonics. The anisotropic parameters are found to be related to some degree to the metallogenic belt. It is worth to further analyse the link between the anisotropic pattern and the metallogenic area, which suggests that shear‐wave splitting could be applied to study metallogeny. This paper demonstrates that the identification on microseismic event is a useful tool in detecting shear‐wave splitting details and exploring its tectonic implications.

Widespread seismic anisotropy in Earth’s lowermost mantle beneath the Atlantic and Siberia

Deep inside the Earth, just above the core-mantle boundary at around 2,700 km depth, large-scale mantle structures are assumed to play a key role for global geodynamic processes. While unusual hot regions are attributed to feed rising mantle plumes and volcanic hotspots, the accumulation of subducted lithospheric plates is associated with colder than average features. In both environments the appearance of dynamic-driven processes such as deformation and mantle flow can directly be inferred by the presence of seismic anisotropy. However, the geometries as well as the interactions of these massive anomalous structures with the surrounding mantle material are still under debate. Based on new seismic data from a dense and large-aperture recording network in Scandinavia we characterize the anisotropic signatures of two so far unexplored regions in the lowermost mantle by using observations of clearly discrepant SKS-SKKS shear wave splitting measurements. Thereby we can demonstrate that anisotropy is located along the northern edges of the Large Low Shear Velocity Province beneath Africa. Furthermore, we recover an anisotropic structure in a region of fast seismic velocity underneath Siberia which provides additional evidence for widespread deformation caused by a deeply subducted slab.

Seismic velocities, anisotropy, and shear‐wave splitting of antigorite serpentinites and tectonic implications for subduction zones

Antigorite, the high‐temperature (HT) form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P‐ and S‐wave velocities (Vp and Vs), anisotropy and shear‐wave splitting of 17 serpentinite samples containing >90% antigorite at pressures up to 650 MPa. The new results, combined with data for low‐temperature (LT) lizardite and/or chrysolite, reveal distinct effects of LT and HT serpentinization on the seismic properties of mantle rocks. At 600 MPa, Vp = 5.10 and 6.68 km/s, Vs = 2.32 and 3.67 km/s, and Vp/Vs = 2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack‐closure pressure (~150 MPa), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is >300°C should be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high‐pressure fluids, or partial melt is also needed to interpret HT serpentinized mantle with Vp < 6.68 km/s, Vs < 3.67 km/s, and Vp/Vs > 1.81. The intrinsic anisotropy of the serpentinites (3.8–16.9% with an average value of 10.5% for Vp, and 3.6–18.3% with an average value of 10.4% for Vs) is caused by dislocation creep‐induced lattice‐preferred orientation of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S‐, L‐, and LS‐tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.

Receiver function study of the Cascadia megathrust: Evidence for localized serpentinization

We characterize the crustal and upper mantle structure of the Cascadian fore arc using receiver function analysis of data from long‐running observatories located along the North American margin in Oregon and Washington. We identify coherent SV‐polarized phases consistent with P‐S conversion from the top of the subducting Juan de Fuca plate throughout the area west of the volcanic arc. In many instances we also find evidence for deeper features, likely associated with the subducted oceanic crust. At many locations SH‐polarized energy in receiver functions is associated with the surface of the subducting plate. These SH‐polarized phases display significant directional variation and show clear polarity reversals, consistent with the presence of seismic anisotropy. Previously, a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic, low‐velocity, high Poisson's ratio layer identified by receiver function analysis at Corvallis, Oregon (Global Seismic Network station COR). Our new results suggest that this feature is widespread in Cascadia, but the specific receiver function signature varies. This anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which, similar to COR, are ∼100 km from the coast and ∼40 km above the slab surface. However, the orientation of rock fabric implied by the anisotropy varies greatly from station to station. Modeling with synthetic seismograms supports the existence of serpentinite beneath station GNW (Green Mountain, Washington). Closer to the coast, where the slab is shallower, an anisotropic layer is likely beneath some stations, but the evidence is less conclusive. We propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths, best explained by the presence of the polymorph antigorite. Hypothetically suctioned upward by slab rollback, a deformed sliver of serpentinite‐rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer, consistent with our observations.

Complex seismic anisotropy in the top of the Earth's inner core beneath Africa

We report complex seismic anisotropy in the top 80 km of the Earth's inner core beneath Africa. The anisotropy in the top 80 km of the inner core is constrained using differential travel times, amplitude ratios, and waveforms of the PKiKP‐PKIKP phases sampling Africa along various directions. The differential PKiKP‐PKIKP time residuals (relative to the Preliminary Reference Earth Model [PREM]) along the polar paths are larger than those along the equatorial paths by 0–1.4 s, indicating the presence of seismic anisotropy in the top 80 km of the inner core. Furthermore, the observations along the polar paths show complex regional variations beneath Africa: the differential PKiKP‐PKIKP travel time residuals vary from 1.2 s beneath eastern Africa, to −0.1 s beneath central Africa, and to −0.2 to 0.8 s beneath western Africa. A correlation between small PKIKP/PKiKP amplitude ratios and large differential PKiKP‐PKIKP travel time residuals is observed. The waveform data are spatially binned into six groups to constrain the regional dependence of velocity and attenuation anisotropy in the top 80 km of the inner core. Overall, the seismic data can be explained by an isotropic upper inner core (UIC) overlying an anisotropic lower inner core (LIC) in the top 80 km of the inner core across Africa. The thickness of the isotropic UIC varies from 0 to 50 km, and the P velocity transition from the isotropic UIC to the anisotropic LIC is sharp, with velocity increases laterally varying from 1.6% to 2.2%. The attenuation structure along the polar paths has a Q value of 600 for the isotropic UIC and Q values varying from 150 to 400 for the anisotropic LIC. The complex seismic anisotropy in the top of the inner core is found in a region where a rapid change of the inner core boundary (ICB) between 1993 and 2003 was discovered (Wen, 2006) and may be explained by complex alignments of iron crystals, resulting from a localized anomalous solidification of the inner core.

On Presence of Seismic Anisotropy in the Asthenosphere beneath Continents and its Dependence on Plate Velocity: Significance of Reference Frame Selection

On Presence of Seismic Anisotropy in the Asthenosphere beneath Continents and its Dependence on Plate Velocity: Significance of Reference Frame Selection

A method to determine microearthquake focal mechanisms in the presence of seismic anisotropy

We use three-component S waves together with P waves from microearthquakes in an isolated intraplate sequence in Arkansas, USA, to determine individual focal mechanisms, taking due account of shear-wave splitting if this is present. The method uses an extension of the relative amplitude method for microearthquakes, in which the polarities and relative amplitudes of two- or three- component direct S waves are measured in addition to P-wave polarities. For each earthquake, the identification of any apparent shear-wave splitting is confirmed by comparing the mutual compatibility of observations at all stations under different interpretations of the supposed slow S wave. This provides evidence of the correct identification of shear-wave splitting which does not rely simply on the analysis of the seismic waveforms themselves. In this example, it is concluded that the supposed shear-wave splitting is real, and hence that anisotropy is present. Well-constrained oblique strike slip focal mechanisms are determined for six events, and these agree with the composite focal mechanisms of other workers who used only P-wave polarities. This success demonstrates the feasibility of determining focal mechanisms for individual microearthquakes using this method. This is important as it removes the need to synthesise composite focal mechanisms under the assumption that all events have the same source orientation, and makes it realistic to examine the distribution of focal mechanisms within a population of microearthquakes.

An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy

Shear wave splitting has been detected in three‐component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the Shikoku area, Japan. The faster shear waves are polarized nearly in E‐W at the stations in the central part of Shikoku, and in NEN‐SWS at the southern stations. The arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations. The faster shear wave polarizations at the stations in the central part of Shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress‐induced microcracks. The difference in polarization directions suggests that the tectonic stress state beneath the southern Shikoku region is significantly different from that beneath the central part of Shikoku. A quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust. Nevertheless, the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out. The average aspect ratio of crack might be greater than of the order of 10−2, or the cracks may not be purely vertical but have some fluctuations in the dip angle.

The inclusion of anisotropy in Maxwell's equations

Summary The presence of seismic anisotropy in the oceanic crust and upper mantle has been detected. Associated electrical anisotropy may be present. Anisotropy is introduced into Maxwell's equations through the conductivity and permittivity. The resulting partial differential equations can be decoupled into separate transverse magnetic (TM) and poloidal magnetic (PM) equations only if the conductivity and permittivity are axially symmetric, i.e. are transversely isotropic. The TM and PM modes do not decouple for transverse anisotropy. The electromagnetic response of a transversely anisotropic earth can be computed by using, for example, an integral equation approach. The integral equation method is ***easily modified to include anisotropy.