SUMMARY An important element of seismic tomography is the inversion process. In this work, we use P-wave arrival times of local earthquakes recorded at onshore and offshore seismic stations in East Japan to investigate the influence of... more
SUMMARY An important element of seismic tomography is the inversion process. In this work, we use P-wave arrival times of local earthquakes recorded at onshore and offshore seismic stations in East Japan to investigate the influence of two well-known inversion algorithms (LSQR and L-BFGS-B) on anisotropic tomography. Our synthetic tests show that a large damping parameter in the LSQR algorithm can lead to a stable and fast convergence, but it can result in many small value disturbances. The L-BFGS-B algorithm, which has second-order convergence, could converge fast to the optimal solution without damping regularization, but an inappropriate bound on the unknown parameters makes them hard to be recovered fully and causes strong trade-off between isotropic velocity and azimuthal anisotropy. If appropriate control parameters are adopted, the two inversion algorithms lead to almost the same results, though the L-BFGS-B provides a more efficient convergence and leads to a slightly better...
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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... more
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...
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We relocated the great 2011 Tohoku-oki earthquake (Mw 9.0) and its foreshocks and aftershocks using a three-dimensional seismic velocity model and high-quality P and S wave arrival times recorded by the dense seismic network on the Japan... more
We relocated the great 2011 Tohoku-oki earthquake (Mw 9.0) and its foreshocks and aftershocks using a three-dimensional seismic velocity model and high-quality P and S wave arrival times recorded by the dense seismic network on the Japan Islands. Then we compared the distribution of the relocated hypocenters with a high-resolution tomographic image of the Northeast Japan forearc. The comparison indicates that the rupture nucleation of the largest events in the Tohoku-oki sequence, including the mainshock, was controlled by structural heterogeneities in the interplate megathrust zone.
Research Interests: Earth Sciences, Geology, Seismology, Physical sciences, Upwelling, and 4 moreCrust, Mantle Wedge, Adakite, and Slab
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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... more
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...
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SUMMARY We determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Cape Verde hotspot that is far from plate boundaries. This Vs model is obtained by using a new method of jointly inverting P- and S-wave... more
SUMMARY We determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Cape Verde hotspot that is far from plate boundaries. This Vs model is obtained by using a new method of jointly inverting P- and S-wave receiver functions, Rayleigh-wave phase-velocity data and S-wave arrival times of teleseismic events. Two Vs discontinuities at ∼15 and ∼60 km depths are revealed beneath volcanic islands, which are interpreted as the Moho discontinuity and the Gutenberg (G) discontinuity. Between the north and south islands, obvious high-Vs anomalies exist in the uppermost mantle down to a depth of ∼100–150 km beneath the Atlantic Ocean, whereas obvious low-Vs anomalies exist in the uppermost mantle beneath the volcanic islands including the active Fogo volcano. These low-Vs anomalies merge into a significant column-like low-Vs zone at depths of ∼150–400 km beneath the Cape Verde swell. We propose that these features in the upper mantle reflect a plume-modified oceanic l...
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SUMMARY Cenozoic basalts with ages ranging from 28.5 to < 0.1 Ma are widely distributed in the Indochina block, the South China Sea basin and the Leiqiong area in South China including the Leizhou Peninsula and the northern Hainan... more
SUMMARY Cenozoic basalts with ages ranging from 28.5 to < 0.1 Ma are widely distributed in the Indochina block, the South China Sea basin and the Leiqiong area in South China including the Leizhou Peninsula and the northern Hainan Island, which form the southeastern Asian basalt province (SABP). These Cenozoic basalts share common petrological and geochemical characteristics. However, the origin of the Cenozoic intraplate volcanism in the SABP is still a controversial issue. In this work, we apply a novel technique of multiscale global tomography to study the whole-mantle 3-D P-wave velocity (Vp) structure beneath the SABP. Our results show that low-Vp anomalies prevail in the whole mantle beneath the SABP. Although the strongest low-Vp zones exist beneath Hainan, significant low-Vp anomalies are also visible in the mantle beneath other parts of the SABP. These low-Vp anomalies appear somehow independent, rather than deriving from a single plume. We deem that a cluster of plumes ...
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3-D seismic structures (Vp, Vs and Poisson's ratio) in and around the source zones of the 2018 Anchorage intraslab earthquake (Mw 7.1) and the 1964 Alaska megathrust earthquake (Mw 9.2)
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The mechanism and rupture process of the giant 2011 Tohoku-oki earthquake (Mw 9.0) are still poorly understood due to lack of permanent near-field observations. Using seismic arrival times recorded by dense seismograph networks on land... more
The mechanism and rupture process of the giant 2011 Tohoku-oki earthquake (Mw 9.0) are still poorly understood due to lack of permanent near-field observations. Using seismic arrival times recorded by dense seismograph networks on land and at ocean floor, we determine a detailed seismic tomography model of the megathrust zone beneath the Tohoku forearc. Our results show that the coseismic slip of the 2011 Tohoku-oki earthquake initiated at a boundary between a down-dip high-velocity anomaly and an up-dip low-velocity anomaly. The slow anomaly at shallow depths near the Japan trench may reflect low-rigidity materials that are close to the free surface, resulting in large slip and weak high-frequency radiation. Our new tomographic model can account for not only large slip near the trench but also weak high-frequency radiation from the shallow rupture areas.
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Abstract Seismic tomography is a powerful tool for mapping the three-dimensional structure of the Earth's interior. Tomographic images obtained in the past four decades have greatly improved our understanding of the Earth's... more
Abstract Seismic tomography is a powerful tool for mapping the three-dimensional structure of the Earth's interior. Tomographic images obtained in the past four decades have greatly improved our understanding of the Earth's heterogeneous structure and dynamics, which signify a revolution in Earth sciences. Most of the tomographic models are determined using the first P and S wave data generated by local earthquakes and/or teleseismic events. Five global seismic discontinuities exist in the Earth, including the Moho, the 410 km and 660 km discontinuities, the core-mantle boundary, and the inner-core boundary. In addition, local-scale seismic discontinuities are also revealed, particularly in subduction zones. These sharp discontinuities generate abundant reflected and converted seismic waves, the so-called later phases, which are identified in observed seismograms. In this article, I review the tomographic studies in the past three decades that made use of the later phase data. Because later phases have ray paths different from those of the first P and S waves, they illuminate the Earth's interior structure that is not well sampled by the first waves. Hence, the use of later phases in tomographic imaging has led to new discoveries of anomalous structures and geodynamic processes at different spatial scales, which shed new light on seismotectonics, magmatism and mantle dynamics. These practices indicate that the later phases are very important in seismic tomography, and so they should be collected from seismograms with a greater quantity and quality so as to obtain better tomographic images of the Earth's interior.
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The great 2011 Tohoku-oki earthquake [moment magnitude () 9.0)] is the best-documented megathrust earthquake in the world, but its causal mechanism is still in controversy because of the poor state of knowledge on the nature of the... more
The great 2011 Tohoku-oki earthquake [moment magnitude () 9.0)] is the best-documented megathrust earthquake in the world, but its causal mechanism is still in controversy because of the poor state of knowledge on the nature of the megathrust zone. We constrain the structure of the Tohoku forearc using seismic tomography, residual topography, and gravity data, which reveal a close relationship between structural heterogeneities in and around the megathrust zone and rupture processes of the 2011 Tohoku-oki earthquake. Its mainshock nucleated in an area with high seismic velocity, low seismic attenuation, and strong seismic coupling, probably indicating a large asperity (or a cluster of asperities) in the megathrust zone. Strong coseismic high-frequency radiations also occurred in high-velocity patches, whereas large afterslips took plate in low-velocity areas, differences that may reflect changes in fault friction and lithological variations. These structural heterogeneities in and a...
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It has been proposed that dehydration embrittlement of hydrous materials can trigger intermediate-depth earthquakes and form a double seismic zone in a subducting slab. Seismic anisotropy may provide a possible insight into... more
It has been proposed that dehydration embrittlement of hydrous materials can trigger intermediate-depth earthquakes and form a double seismic zone in a subducting slab. Seismic anisotropy may provide a possible insight into intermediate-depth intraslab seismicity, because anisotropic properties of minerals change with varying water distribution, temperature and pressure. Here we present a high-resolution model of P-wave radial anisotropy tomography of the Japan subduction zone down to ~400 km depth, which is obtained using a large number of arrival-time data of local earthquakes and teleseismic events. Our results reveal a close correlation between the pattern of intermediate-depth seismicity and anisotropic structures. The seismicity occurs in portions of the Pacific and Philippine Sea slabs where positive radial anisotropy (i.e., horizontal velocity being faster than vertical one) dominates due to dehydration, whereas the inferred anhydrous parts of the slabs are found to be aseis...
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ABSTRACT We review recent studies of seismic tomography and earthquake sequences, and discuss their implications for seismotectonics and mantle dynamics beneath eastern Tibet. The crustal structures in the source areas of the 2008 Ms 8.0... more
ABSTRACT We review recent studies of seismic tomography and earthquake sequences, and discuss their implications for seismotectonics and mantle dynamics beneath eastern Tibet. The crustal structures in the source areas of the 2008 Ms 8.0 Wenchuan and the 2013 Ms 7.0 Lushan earthquakes are similar and exhibit prominent low-velocity (low-V) and high-Poisson&#39;s ratio anomalies in the source areas, indicating that in addition to compositional variations, fluid-filled rock matrices exist in the Longmenshan fault zone, which may have influenced the nucleation of the two earthquakes. Significant low-V anomalies are revealed between the Wenchuan and Lushan mainshocks, which may explain why their aftershock zones extended northward and southward, respectively. The relocated aftershocks of the 2011 Ms 5.8 Yingjiang and the 2014 Ms 6.5 Ludian earthquakes show a conjugate-shaped distribution, which may explain why the two moderate-sized earthquakes caused heavy damage. The large earthquakes in eastern Tibet are located at boundaries of low and high velocity anomalies in the upper mantle. The structural heterogeneities in the crust and upper mantle are associated with hot upwelling and corner flows in the big mantle wedge above the subducting Indian slab beneath eastern Tibet, as well as slab dehydration, which affect the seismogenesis in the region.
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Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of... more
Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of rock fracturing and noble gas emission was conducted, but there is no quantitative connection between the laboratory results and observation in field. We report here deep groundwater helium anomalies related to the 2016 Kumamoto earthquake, which is an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas in Southwest Japan. The observed helium isotope changes, soon after the earthquake, are quantitatively coupled with volumetric strain changes estimated from a fault model, which can be explained by experimental studies of helium degassing during compressional loading of rock samples. Groundwater helium is considered as an effective strain gauge. This suggests the first quantitative l...
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A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre... more
A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan P...
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Abstract High-resolution tomographic images of the crust and upper mantle beneath Harrat Al-Madinah, Saudi Arabia, are obtained by inverting high-quality arrival-time data of local earthquakes and teleseismic events recorded by newly... more
Abstract High-resolution tomographic images of the crust and upper mantle beneath Harrat Al-Madinah, Saudi Arabia, are obtained by inverting high-quality arrival-time data of local earthquakes and teleseismic events recorded by newly installed borehole seismic stations to investigate the AD 1256 volcanic eruption and the 1999 seismic swarm in the study region. Our tomographic images show the existence of strong heterogeneities marked with low-velocity zones extending beneath the AD 1256 volcanic center and the 1999 seismic swarm area. The low-velocity zone coinciding with the hypocenters of the 1999 seismic swarm suggests the presence of a shallow magma reservoir that is apparently originated from a deeper source (60–100 km depths) and is possibly connected with another reservoir located further north underneath the NNW-aligned scoria cones of the AD 1256 eruption. We suggest that the 1999 seismic swarm may represent an aborted volcanic eruption and that the magmatism along the western margin of Arabia is largely attributed to the uplifting and thinning of its lithosphere by the Red Sea rifting.
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We investigate the P-wave velocity structure of the crust and upper mantle down to depth 400km beneath the Ordos region and northeastern Qinghai-Xizang plateau using the seismic tomographic technique. The data used are taken from the... more
We investigate the P-wave velocity structure of the crust and upper mantle down to depth 400km beneath the Ordos region and northeastern Qinghai-Xizang plateau using the seismic tomographic technique. The data used are taken from the record by the regional network and a passive seismic observation profile across the investigated area, respectively. Our results show clear lateral variations of the crust and upper mantle velocity structure along the profile, which exist not only between different blocks, but also inside blocks. The average P-wave velocity of the crust and upper mantle beneath the northeastern margin of the Qinghai-Xizang plateau is lower, and that beneath the Ordos block is higher than the global model. Our observations are coincident with the tectonic activities of these two blocks. Between the northeastern Qinghai-Xizang and Ordos block exists a transition zone about 200km wide, which on the surface corresponds to the region between Lanzhou and Haiyuan. This transition zone in the upper mantle appears as a stripe with a slope of about 45- tilting eastward. Its main feature appears as a mixture of high- and low-velocity media. The strike of this stripe in the upper mantle seems to be controlled by the high-velocity body beneath the Ordos block. This implies that the stable Ordos block plays a role in resisting the eastward movement of the Qinghai-Xizang plateau. The upper mantle velocity structure beneath the Qaidam basin in the Qinghai-Xizang plateau has a low average velocity and that beneath the Qilian block has a high average velocity. The difference between both reaches about 8%. We also find the intrusive low-velocity bodies in uppermost mantle beneath Zekog, Lanzhou and Haiyuan areas, respectively. The high seismicity in the north-south seismic zone should be related closely to the medium heterogeneity and very special deep dynamic environment.
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Recent advances in the seismic tomography method are reviewed. These include the refinements in the model parameterization, three-dimensional ray tracing, inversion algorithm, joint use of local, regional and teleseismic data, and the... more
Recent advances in the seismic tomography method are reviewed. These include the refinements in the model parameterization, three-dimensional ray tracing, inversion algorithm, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Then applications of the new generation tomographic methods to the imaging of subduction zones and earthquake fault zones are introduced. These recent studies determined unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge and the crust beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High-resolution tomographic imagings of earthquake fault zones show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic process, such as fluids, in fault zones.