Seismic anisotropy tomography: New insight into subduction dynamics (original) (raw)
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P-wave anisotropic tomography of the crust and upper mantle under Hokkaido, Japan
Tectonophysics, 2009
We determined a 3-D anisotropic velocity structure beneath Hokkaido in northern Japan using a large number of first P-wave arrival times. The travel-time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming P-wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally. The geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific plate are taken into account in the model. Our results show that low-velocity zones exist beneath active arc volcanoes in the crust and in the central portion of mantle wedge above the high-velocity Pacific slab, which are similar to the previous tomographic images. P-wave anisotropy is revealed in the crust, mantle wedge and the subducting Pacific slab. In the upper crust, the anisotropy is possibly caused by microcracks and cracks. In the lower crust, the anisotropic structure is different from that in the upper crust and it may be affected considerably by the plastic flow deformation. In the mantle wedge, the fast-velocity direction (FVD) is generally trench-normal in most of the study area, while it becomes trench-parallel in the Pacific coast areas close to the trench, which agrees with the shearwave splitting results. Beneath the volcanic front, the FVD is also trench-parallel, which may suggest the existence of complex 3-D mantle flow in the mantle wedge. The FVD in the subducting Pacific slab is nearly north-south, which likely keeps the original fossil anisotropy of the Pacific plate formed at the mid-ocean ridge or is affected by the olivine fabric transition from A-type to B-type fabric due to the changes in water content, stress, and temperature.
Seismic Anisotropy Tomography and Mantle Dynamics
Surveys in Geophysics
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D variations of both structural heterogeneity and seismic anisotropy, providing unique constraints on geodynamic processes in the Earth’s crust and mantle. Here we introduce recent advances in the theory and application of seismic anisotropy tomography, thanks to abundant and high-quality data sets recorded by dense seismic networks deployed in many regions in the past decades. Applications of the novel techniques led to new discoveries in the 3-D structure and dynamics of subduction zones and continental regions. The most significant findings are constraints on seismic anisotropy in the subducting slabs. Fast-velocity directions (FVDs) of azimuthal anisotropy in the slabs are generally trench-parallel, reflecting fossil lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation due to transform faults produced at the mid-ocean ridge and intraslab hydra...
2023
Body-wave and surface-wave tomography, receiver-function imaging, and shear-wave splitting measurements have shown that seismic anisotropy and heterogeneity coexist in all parts of subduction zones, providing important constraints on the mantle flow and subduction dynamics. P-wave anisotropy tomography is a new but powerful tool for mapping three-dimensional variations of azimuthal and radial seismic anisotropy in the crust and mantle. P-wave azimuthal-anisotropy tomography has been applied widely to the Circum-Pacific subduction zones, Mainland China and North America, whereas P-wave radial-anisotropy tomography was applied to only a few areas including Northeast Japan, Southwest Japan and North China Craton. These studies have revealed complex anisotropy in the crust and mantle lithosphere associated with the surface geology and tectonics, anisotropy reflecting subduction-driven corner flow in the mantle wedge, frozen-in fossil anisotropy in the subducting slabs formed at the mid-ocean ridge, as well as olivine fabric transitions due to changes in water content, stress and temperature. Shear-wave splitting tomography methods have been also proposed, but their applications are still limited and preliminary. There is a discrepancy between the surface-wave and body-wave tomographic models in radial anisotropy of the mantle wedge beneath Japan, which is a puzzle but an intriguing topic for future studies.
Geophysical Journal International, 2021
SUMMARY We determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle beneath NE China using high-quality traveltime data of local earthquakes and teleseismic events recorded at 334 network and portable stations. In the upper crust, nearly E-W fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the central Songliao basin, which is surrounded by circular-shaped FVDs along the basin edges. The E-W FVDs may reflect microcracks or fractures in the upper crust, which are aligned under the control of regional tectonic stress. In the lower crust, low-velocity (low-V) anomalies with NE-SW FVDs exist along the Tanlu fault zone, which may reflect NE-SW trending ductile deformation or viscous flow along the fault zone. The FVDs are mainly NNW-SSE to N-S in the uppermost mantle beneath most of the study region, which may reflect fossil deformation of the mantle lithosphere caused by the Palaeo-Pacific plate subduction. High-velocity anomalies with NE-SW...
Geophysical Journal International
SUMMARY Measurements of seismic anisotropy provide a lot of information on the deformation and structure as well as flows of the Earth's interior, in particular of the upper mantle. Even though the strong and heterogeneous seismic anisotropic nature of the upper mantle has been demonstrated by a wealth of theoretical and observational approaches , most of standard teleseismic body-wave tomography studies overlook P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. Conventional methods of seismic anisotropy measurement have their limitations regarding lateral and mainly depth resolution. To overcome this problem much effort has been done to develop tomographic methods to invert shear wave splitting data for anisotropic structures, based on finite-frequency sensitivity kernels that relate model perturbations to splitting observations. A promising approach to image the upper mantle anisotropy is the inv...
Deep azimuthal seismic anisotropy in the southern Kurile and Japan subduction zones
Journal of Geophysical Research, 1997
Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earthquakes in the southern Kurile and Japan subduction zones are combined with splitting parameters obtained from SKS and SKKS waves to determine depth variation in shear wave splitting both above and below the earthquake. Local S wave splitting results measured at station MAJO (Matsushiro, Japan) show fast directions, from NNE to NE, parallel to the extension directions measured from Quaternary fault and geodetic data. The inferred finite strain field from shear wave splitting is consistent with the closing of the Sea of Japan. At YSS (Yuzhno Sakhalinsk, Russia) the shear wave splitting parameters obtained from local S waves are approximately oriented N-S and appear to be controlled by deformation of the upper plate rather than the subducted slab. SKS phases recorded at YSS, located on the southern tip of Sakhalin Island, show an approximately N-S oriented fast direction and lag time of 1.3 + 0.3 s. Local S phases recorded at station YSS yield fast directions similar to the SKS results and the magnitude of splitting varies systematically with depth. These results combine to indicate that very little splitting occurs in the asthenosphere below the southern Kurile slab at about 450 km depth. S waves from deep earthquakes beneath MDJ (Mudanjiang, Heilongjiang Province, China) show -1.0 s of lag time, while SKKS results show -• 1.6 s of lag time. One SKS phase, however, shows similar lag times as those observed by the split local S waves. This difference in shear wave splitting lag times indicates significant variation in azimuthal anisotropy beneath a minimum depth of 350 km. This inference is consistent with the source-side splitting of-0.8 s lag time observed from deep teleseismic S waves which traverse the lower parts of the upper mantle and the upper mantle/lower mantle boundary. A plausible explanation for the presence of deep seismic anisotropy is that shear wave splitting is occurring in the metastable olivine in the flattened and broadened southern Japan slab. Another explanation for these observations is the presence of an anisotropic layer composed primarily of highly anisotropic •-spinel at the base of the 410-km discontinuity. 1987; Ribe, 1992]. The seismic anisotropy resulting from mantle flow has generally been observed to be confined to depths above the transition zone [Silver and Chart, 1991; Kaneshima and Silver, 1992; Iadaka and Obara, 1995] and shear wave anisotropy magnitudes are typically 3-6% for upper mantle materials [e.g., Christensen, 1984; Mainprice and Silver, 1993]. A lack of seismic anisotropy in the transition zone can be explained by the existence of diffusion creep rather than dislocation creep as the dominant form of deformation below the Lehmann discontinuity at approximately 200 km depth [Anderson, 1989; Karato, 1992]. Alternatively, Meade et al. [1995] suggest that the lower mantle is made up of silicate perovskite, which is elastically isotropic. However, we might expect to observe azimuthal anisotropy below the 410-km discontinuity since it is widely thought that there is a transition in mineralogy to the minerals [3-spinel, y-spinel, and majorire garnet [Anderson, 1989; Mainprice and Silver, 1993. [3-spinel is thought to be the dominant mineral in the upper 70 km of the transition zone (410-670 km) and is an elastically anisotropic mineral in contrast to y-spinel and majorite garnet which are essentially isotropic minerals [Mainprice and Silver, 1993]. Therefore it is conceivable that seismic anisotropy could exist in the upper portion of the transition zone. There is also the possibility that when slabs are subducted into the mantle they are much colder than the surrounding area [e.g., Ringwood, 1976]; hence anisotropic metastable olivine might be present at depths as deep as 520 km. Rubie and Ross [ 1994] have found experimental 9911
Journal of Geophysical Research: Solid Earth
P wave tomography has been recently used to study 3-D azimuthal and radial anisotropy of subduction zones and continental regions. However, the fundamental issue about the trade-off between the isotropic and anisotropic structures is still unclear. In this study, we investigate this issue systematically with comprehensive synthetic tests. Our results indicate that good ray coverage in the azimuth (for azimuthal anisotropy) and incidence (for radial anisotropy) is required for determining reliable anisotropic models. The isotropic and anisotropic structures are strongly coupled, and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence. We therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis (i.e., the ratio of the short-axis and long-axis lengths) for estimating the ray coverage quantitatively. Applying our novel approach to a large number of high-quality arrival time data of local shallow-and intermediate-depth earthquakes, we obtained new tomographic images of 3-D P wave azimuthal and radial anisotropy in Northeast Japan. Both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region, whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab. Our results show dominant trench-normal and vertical-fast anisotropy in the mantle wedge while trench-parallel and horizontal-fast anisotropy in the subducting slab, which indicates different dynamics in different domains of the subduction zone.
P wave tomography and anisotropy beneath Southeast Asia: Insight into mantle dynamics
Journal of Geophysical Research: Solid Earth, 2015
Southeast Asia is surrounded by subduction zones resulting from the interactions of several lithospheric plates. Its evolution has been also influenced by active tectonics due to the Indo-Asian collision in the Cenozoic. In this study, we use a large number of arrival-time data of local and regional earthquakes to determine 3-D P-wave tomography and azimuthal anisotropy in the mantle beneath SE Asia. High-velocity (high-V) anomalies representing the subducting slabs are clearly visible in the upper mantle and the mantle transition zone (MTZ). Low-velocity (low-V) zones with trench-normal anisotropy are revealed in the uppermost mantle, which indicate back-arc spreading or secondary mantle-wedge flow induced by the slab subduction. In contrast, trench-parallel anisotropy dominates in the deep upper-mantle and reflects structures either in the subducting slab or in the upper mantle surrounding the slab. The trench-parallel anisotropy is also significant in the lower MTZ, which may contribute to shear-wave splitting observations. A low-V body extending down to the lower mantle is visible under the Hainan volcano far away from the plate boundaries, suggesting that Hainan is a hotspot fed by a lower-mantle plume. The low-V body under Hainan is connected with low-V zones in the upper mantle under SE Tibet and Vietnam. Our P-wave anisotropy results reflect significant mantle flow existing in the asthenosphere from SE Tibet to Hainan and further southwestward to Vietnam. The present study, especially the 3-D P-wave anisotropy results, provide important new insight into mantle dynamics in SE Asia.
Reactivation and mantle dynamics of North China Craton: insight from P-wave anisotropy tomography
Geophysical Journal International, 2013
We determined the first 3-D P-wave anisotropic tomography beneath the North China Craton (NCC) using a large number of high-quality arrival-time data from local earthquakes and teleseismic events, which reveals depth-dependent azimuthal anisotropy in the crust and upper mantle down to 600 km depth. In the NCC western block, the fast velocity direction (FVD) varies from east-west in the southern part to northeast-southwest in the northern part, which may reflect either the interaction between the Yangtze block and NCC or fossil lithospheric fabrics in the craton. Under the NCC eastern block, a uniform northwest-southeast FVD is revealed in the lower part of the upper mantle (300-410 km depths) and the mantle transition zone (410-660 km depths), which may reflect horizontal and upwelling flows in the big mantle wedge (BMW) above the stagnant Pacific slab in the mantle transition zone. The NCC central block exhibits a northeast-southwest FVD, consistent with the surface tectonic orientation there, suggesting that the cold and thick (>300 km) cratonic root of the NCC western block may obstruct the northwest-southeast trending mantle flow induced by the Pacific Plate subduction, resulting in a northeast-southwest trending mantle flow under the central block. Our present results indicate that the corner flow in the BMW associated with the deep subduction of the Pacific Plate is the main cause of NCC reactivation and mantle dynamics under East China.
Mantle dynamics of Western Pacific and East Asia: New insights from P‐wave anisotropic tomography
Geochemistry, Geophysics, Geosystems
Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high-quality P wave arrival times from local and regional earthquakes to determine a high-resolution tomographic model of 3-D P wave azimuthal anisotropy down to 1,000-km depth beneath East Asia. Our results show that trench-parallel fast-velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (<80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench-normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE-SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction-parallel trend at depths of 80-400 km to a subduction-normal trend in the MTZ. Large-scale low-velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE-SW. The FVDs along the Izu-Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3-D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at~300-km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North-South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.