SUMMARY An important element of seismic tomography is the inversion process. In this work, we use... 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...
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D v... 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...
In order to better understand the deep structure and dynamics of the Earth's interior, we have at... more In order to better understand the deep structure and dynamics of the Earth's interior, we have attempted to develop a new model of whole mantle seismic tomography with a novel approach. We adopted a grid parameterization instead of blocks which were used in most of the global tomographic studies. Ray paths and travel times are computed with an efficient 3-D ray tracing scheme. Moreover, the topography of mantle discontinuities at 410 and 660 km depths are taken into account in the tomographic inversions. This new approach was applied to a large data set of ISC travel times (P, PP, PcP, pP) to determine a whole mantle P-wave tomography. For the shallow mantle, our new model contains the general features observed in the previous models : a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0-400 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction zone regions, which suggests that most of the slab materials are stagnant for a long time in the transition zone before finally dropping down to the lower mantle. Plume-like slow anomalies are visible under the hotspot regions in most parts of the mantle. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, which suggests that plumes are not fixed in the mantle but can be deflected by the mantle winds. As a consequence, hotspots are not really fixed but can wander on the Earth's surface, as evidenced by the recent geomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the coremantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, which suggest that most of the mantle plumes under the hotspots may originate from the CMB. However, there may be some small-scaled, weak plumes originating from the transition zone depths. 1.
We conducted checkerboard resolution tests to confirm the reliability of the obtained tomographic... more We conducted checkerboard resolution tests to confirm the reliability of the obtained tomographic images. To make a checkerboard, we assigned alternative positive and negative velocity anomalies of 6% to all the 3-D grid nodes. Random errors with a standard deviation of 0.1 s were added to the synthetic arrival times calculated for the checkerboard model to account for the picking errors existing in the real data.
We relocated the great 2011 Tohoku-oki earthquake (Mw 9.0) and its foreshocks and aftershocks usi... 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.
We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori ea... more We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori earthquake (M 7.3) and the 21 October 2016 Central Tottori earthquake (M 6.6) which occurred near the Daisen volcano in SW Japan. The two large events took place in a high-velocity zone in the upper crust, whereas low-velocity (low-V) and high Poisson's ratio (high-σ) anomalies are revealed in the lower crust and upper mantle. Low-frequency micro-earthquakes (M 0.0-2.1) occur in or around the low-V and high-σ zones, which reflect upward migration of magmatic fluids from the upper mantle to the crust under the Daisen volcano. The nucleation of the Tottori earthquakes may be affected by the ascending fluids. The flat subducting Philippine Sea (PHS) slab has a younger lithosphere age and so a higher temperature beneath the Daisen and Tottori area, facilitating the PHS slab melting. It is also possible that a PHS slab window has formed along the extinct Shikoku Basin spreading ridge beneath SW Japan, and mantle materials below the PHS slab may ascend to the shallow area through the slab window. These results suggest that the Daisen adakite magma was affected by the PHS slab melting and upwelling flow in the upper mantle above the subducting Pacific slab.
* We examined two types of pressure-temperature (P-T) path. In type I experiments, samples were a... more * We examined two types of pressure-temperature (P-T) path. In type I experiments, samples were annealed under the target conditions and then quenched. In type II experiments, the sample was further deformed by increasing the pressure slightly. Hs 1 , perpendicular to the compression direction; | |SD, parallel to the shear direction.
We present seismic tomography and geochemical evidence for the existence of significant lateral h... more We present seismic tomography and geochemical evidence for the existence of significant lateral heterogeneities in the lunar mantle and make a comparison with the Earth's heterogeneity and seismicity. The Procellarum KREEP Terrane (PKT) is a unique province on the nearside of the Moon. It constitutes only about 15% or less of the lunar surface, but appears to owe a large portion of the Moon's radioactive heat-producing elements. We found a correlation between the Thorium (Th) abundance distribution and seismic tomography of the lunar nearside. The area with high Th abundance exhibits a distinct low shear-wave velocity, and the low-velocity anomaly extends down to 300-400 km depth below the PKT, suggesting that the thermal and compositional anomaly has a depth extent of 300-400 km in the lunar mantle. The distribution of deep moonquakes shows a correlation with the seismic-velocity variations in the deep lunar mantle, similar to the earthquakes which are affected or controlled by structural heterogeneities in the terrestrial crust and upper mantle. The presence of deep moonquakes and seismic-velocity heterogeneities in the mantle implies that the interior of the present Moon may be still thermally active.
Seismic traveltime tomography is commonly discretized by a truncated expansion of the pursued mod... more Seismic traveltime tomography is commonly discretized by a truncated expansion of the pursued model in terms of chosen basis functions. Whether parametrization affects the actual resolving power of a given data set as well as the robustness of the resulting earth model has long been seriously debated. From the perspective of the model resolution, however, there is one important aspect of the parametrization issue of seismic tomography that has yet to be systematically explored, that is, the space-frequency localization of a chosen parametrization. In fact, the two most common parametrizations tend to enforce resolution in each of their own particular domains. Namely, parametrization in terms of spherical harmonics with global support tends to emphasize spectral resolution while sacrificing the spatial resolution, whereas the compactly supported pixels tend to behave in the opposite manner. Some of the significant discrepancies among tomographic models are very likely to be manifestations of this effect, when dealing with data sets with nonuniform sampling. With an example of the tomographic inversion for the lateral shear wave heterogeneity of the Da layer using S-SKS traveltimes, we demonstrate an alternative parametrization in terms of the multiresolution representation of the pursued model function. Unlike previous attempts of multiscale inversion that invoke pixels with variable sizes, or overlay several layers of tessellation with different grid intervals, our formulation invokes biorthogonal generalized Harr wavelets on a sphere. We show that multiresolution representation can be constructed very easily from an existing block-based discretization. A natural scale hierarchy of the pursued model structure constrained by the resolving power of the given sampling is embedded within the solution obtained. It provides a natural regularization scheme based on the actual raypath sampling and is thus free from a priori prejudices intrinsic to most regularization schemes. Unlike solutions obtained through spherical harmonics or spherical blocks that tend to collapse structures onto ray paths, our parametrization imposes regionally varying Nyquist limits, that is, robustly resolvable local wavelength bands within the obtained solution.
The density of the Apollo 14 black glass melt, which has the highest TiO 2 content of pristine ma... more The density of the Apollo 14 black glass melt, which has the highest TiO 2 content of pristine mare glasses, was measured to 4.8 GPa and 2100 K using an X-ray absorption method. A fit of the pressure-densitytemperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus K T0 = 9.0 ± 1.2 GPa, its pressure derivative K 0 ′ = 16.0 ± 3.4, and the temperature derivative of the bulk modulus (∂K T /∂T) P = −0.0030 ± 0.0008 GPa/K at 1700 K. The high-Ti basalt magma is less dense than the lunar mantle below about 1.0 GPa. Therefore, the high-Ti basalt magma produced in the hybridized source (100-200 km) can ascend to the lunar surface. The basalt formed at the higher pressure could not ascend but move downwards, and solidify in the lunar mantle. The solidified high-Ti basalt components can create chemical heterogeneities in the lunar mantle and can cause the low-velocity anomalies observed seismologically.
High-resolution tomographic images of the crust and upper mantle in and around the area of the 20... more High-resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent lowvelocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future.
SUMMARY We determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle benea... 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...
SUMMARY We determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Ca... 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...
SUMMARY Cenozoic basalts with ages ranging from 28.5 to < 0.1 Ma are widely distributed in the... 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 ...
SUMMARY An important element of seismic tomography is the inversion process. In this work, we use... 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...
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D v... 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...
In order to better understand the deep structure and dynamics of the Earth's interior, we have at... more In order to better understand the deep structure and dynamics of the Earth's interior, we have attempted to develop a new model of whole mantle seismic tomography with a novel approach. We adopted a grid parameterization instead of blocks which were used in most of the global tomographic studies. Ray paths and travel times are computed with an efficient 3-D ray tracing scheme. Moreover, the topography of mantle discontinuities at 410 and 660 km depths are taken into account in the tomographic inversions. This new approach was applied to a large data set of ISC travel times (P, PP, PcP, pP) to determine a whole mantle P-wave tomography. For the shallow mantle, our new model contains the general features observed in the previous models : a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0-400 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction zone regions, which suggests that most of the slab materials are stagnant for a long time in the transition zone before finally dropping down to the lower mantle. Plume-like slow anomalies are visible under the hotspot regions in most parts of the mantle. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, which suggests that plumes are not fixed in the mantle but can be deflected by the mantle winds. As a consequence, hotspots are not really fixed but can wander on the Earth's surface, as evidenced by the recent geomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the coremantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, which suggest that most of the mantle plumes under the hotspots may originate from the CMB. However, there may be some small-scaled, weak plumes originating from the transition zone depths. 1.
We conducted checkerboard resolution tests to confirm the reliability of the obtained tomographic... more We conducted checkerboard resolution tests to confirm the reliability of the obtained tomographic images. To make a checkerboard, we assigned alternative positive and negative velocity anomalies of 6% to all the 3-D grid nodes. Random errors with a standard deviation of 0.1 s were added to the synthetic arrival times calculated for the checkerboard model to account for the picking errors existing in the real data.
We relocated the great 2011 Tohoku-oki earthquake (Mw 9.0) and its foreshocks and aftershocks usi... 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.
We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori ea... more We investigate the 3-D seismic structure of source areas of the 6 October 2000 Western Tottori earthquake (M 7.3) and the 21 October 2016 Central Tottori earthquake (M 6.6) which occurred near the Daisen volcano in SW Japan. The two large events took place in a high-velocity zone in the upper crust, whereas low-velocity (low-V) and high Poisson's ratio (high-σ) anomalies are revealed in the lower crust and upper mantle. Low-frequency micro-earthquakes (M 0.0-2.1) occur in or around the low-V and high-σ zones, which reflect upward migration of magmatic fluids from the upper mantle to the crust under the Daisen volcano. The nucleation of the Tottori earthquakes may be affected by the ascending fluids. The flat subducting Philippine Sea (PHS) slab has a younger lithosphere age and so a higher temperature beneath the Daisen and Tottori area, facilitating the PHS slab melting. It is also possible that a PHS slab window has formed along the extinct Shikoku Basin spreading ridge beneath SW Japan, and mantle materials below the PHS slab may ascend to the shallow area through the slab window. These results suggest that the Daisen adakite magma was affected by the PHS slab melting and upwelling flow in the upper mantle above the subducting Pacific slab.
* We examined two types of pressure-temperature (P-T) path. In type I experiments, samples were a... more * We examined two types of pressure-temperature (P-T) path. In type I experiments, samples were annealed under the target conditions and then quenched. In type II experiments, the sample was further deformed by increasing the pressure slightly. Hs 1 , perpendicular to the compression direction; | |SD, parallel to the shear direction.
We present seismic tomography and geochemical evidence for the existence of significant lateral h... more We present seismic tomography and geochemical evidence for the existence of significant lateral heterogeneities in the lunar mantle and make a comparison with the Earth's heterogeneity and seismicity. The Procellarum KREEP Terrane (PKT) is a unique province on the nearside of the Moon. It constitutes only about 15% or less of the lunar surface, but appears to owe a large portion of the Moon's radioactive heat-producing elements. We found a correlation between the Thorium (Th) abundance distribution and seismic tomography of the lunar nearside. The area with high Th abundance exhibits a distinct low shear-wave velocity, and the low-velocity anomaly extends down to 300-400 km depth below the PKT, suggesting that the thermal and compositional anomaly has a depth extent of 300-400 km in the lunar mantle. The distribution of deep moonquakes shows a correlation with the seismic-velocity variations in the deep lunar mantle, similar to the earthquakes which are affected or controlled by structural heterogeneities in the terrestrial crust and upper mantle. The presence of deep moonquakes and seismic-velocity heterogeneities in the mantle implies that the interior of the present Moon may be still thermally active.
Seismic traveltime tomography is commonly discretized by a truncated expansion of the pursued mod... more Seismic traveltime tomography is commonly discretized by a truncated expansion of the pursued model in terms of chosen basis functions. Whether parametrization affects the actual resolving power of a given data set as well as the robustness of the resulting earth model has long been seriously debated. From the perspective of the model resolution, however, there is one important aspect of the parametrization issue of seismic tomography that has yet to be systematically explored, that is, the space-frequency localization of a chosen parametrization. In fact, the two most common parametrizations tend to enforce resolution in each of their own particular domains. Namely, parametrization in terms of spherical harmonics with global support tends to emphasize spectral resolution while sacrificing the spatial resolution, whereas the compactly supported pixels tend to behave in the opposite manner. Some of the significant discrepancies among tomographic models are very likely to be manifestations of this effect, when dealing with data sets with nonuniform sampling. With an example of the tomographic inversion for the lateral shear wave heterogeneity of the Da layer using S-SKS traveltimes, we demonstrate an alternative parametrization in terms of the multiresolution representation of the pursued model function. Unlike previous attempts of multiscale inversion that invoke pixels with variable sizes, or overlay several layers of tessellation with different grid intervals, our formulation invokes biorthogonal generalized Harr wavelets on a sphere. We show that multiresolution representation can be constructed very easily from an existing block-based discretization. A natural scale hierarchy of the pursued model structure constrained by the resolving power of the given sampling is embedded within the solution obtained. It provides a natural regularization scheme based on the actual raypath sampling and is thus free from a priori prejudices intrinsic to most regularization schemes. Unlike solutions obtained through spherical harmonics or spherical blocks that tend to collapse structures onto ray paths, our parametrization imposes regionally varying Nyquist limits, that is, robustly resolvable local wavelength bands within the obtained solution.
The density of the Apollo 14 black glass melt, which has the highest TiO 2 content of pristine ma... more The density of the Apollo 14 black glass melt, which has the highest TiO 2 content of pristine mare glasses, was measured to 4.8 GPa and 2100 K using an X-ray absorption method. A fit of the pressure-densitytemperature data to the high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus K T0 = 9.0 ± 1.2 GPa, its pressure derivative K 0 ′ = 16.0 ± 3.4, and the temperature derivative of the bulk modulus (∂K T /∂T) P = −0.0030 ± 0.0008 GPa/K at 1700 K. The high-Ti basalt magma is less dense than the lunar mantle below about 1.0 GPa. Therefore, the high-Ti basalt magma produced in the hybridized source (100-200 km) can ascend to the lunar surface. The basalt formed at the higher pressure could not ascend but move downwards, and solidify in the lunar mantle. The solidified high-Ti basalt components can create chemical heterogeneities in the lunar mantle and can cause the low-velocity anomalies observed seismologically.
High-resolution tomographic images of the crust and upper mantle in and around the area of the 20... more High-resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent lowvelocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future.
SUMMARY We determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle benea... 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...
SUMMARY We determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Ca... 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...
SUMMARY Cenozoic basalts with ages ranging from 28.5 to < 0.1 Ma are widely distributed in the... 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 ...
Uploads
Papers by Dapeng Zhao