Zhouchuan Huang

Structural heterogeneity in the megathrust zone and mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0)

The great 2011 Tohoku-oki earthquake (Mw 9.0) and its 339 foreshocks and 5,609 aftershocks (9–27 March 2011) were relocated using a three-dimensional seismic velocity model and local P and S wave arrival times. The distribution of relocated hypocenters was compared with a 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 megathrust zone.

Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10–11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15–0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30–0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.

P wave tomography and anisotropy beneath Southeast Asia: Insight into mantle dynamics

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 hot spot 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, provides important new insight into mantle dynamics in SE Asia.

On the trade‐off between seismic anisotropy and heterogeneity: Numerical simulations and application to Northeast Japan

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.

Three‐dimensional P wave azimuthal anisotropy in the lithosphere beneath China

Seismic anisotropy in the upper mantle beneath East Asia has been studied extensively using shear wave (SKS) splitting measurements, which have provided important information on mantle dynamics in this region. However, SKS measurements have poor vertical resolution, and so their interpretations are usually not unique. In this work we use a large number of traveltime data from 34,036 local earthquakes recorded by 1563 seismic stations to determine the first model of 3-D P wave azimuthal anisotropy in the lithosphere beneath China. Our results show that the fast velocity directions (FVDs) are generally correlated with the surface geologic features, such as the strikes of the orogens, active faults, and tectonic boundaries. The FVDs in the upper crust are normal to the maximal horizontal stress (σH) in regions with extensive compression such as the Tibetan Plateau, whereas they are subparallel to σH in strike-slip shear zones such as the western and eastern Himalayan syntax. The comparison of the FVDs of P wave anisotropy with SKS splitting measurements indicates that beneath the Tibetan Plateau the seismic anisotropy in the lithosphere contributes significantly to the SKS splitting observations. In contrast, in east China the P wave FVDs in the lithosphere are different from the SKS splitting measurements, suggesting that the SKS splitting is mainly caused by the anisotropy in the deeper mantle such as the asthenosphere and the mantle transition zone under east China. These novel results provide important new information on the lithospheric deformation and mantle dynamics in East Asia.

Stress field in the 2008 Iwate‐Miyagi earthquake (M7.2) area

We determined the focal mechanism solutions (FMS) of 191 crustal earthquakes as well as the stress tensor in the source area of the 2008 Iwate-Miyagi earthquake (2008 IMEQ, M7.2) that occurred in the central portion of northeast (NE) Japan. The FMS and the stress tensors were determined by using both 1-D and 3-D velocity models, which exhibit almost the same results. The differences caused by the use of 1-D and 3-D models can be neglected when compared with the differences due to the different methods, which indicates that the FMS and the stress tensor determined with a 1-D model are accurate enough to study the crustal stress field in the study region. The obtained P axis (σ1) trends WNW-ESE subhorizontally, and the T axis (σ3) is oriented subvertically in a NNE-SSW belt perpendicular to σ1. The σ1 orientation is consistent with the motion of the Pacific plate relative to NE Japan, which indicates that the plate boundary forces dominate the intraplate stress regime. Both temporal and spatial variations of the stress field in the IMEQ source area are detected, which may be induced by the stress rotation accompanying the main shock and its aftershocks. The seismogenic faults in the study area are estimated to be very weak, which argues against the concept of strong crust. The faults may be weakened by the high-temperature magma and the fluids in the lower crust and uppermost mantle that intrude upward into the shallower crust.

Insight into NE Tibetan Plateau expansion from crustal and upper mantle anisotropy revealed by shear-wave splitting

Abstract The northeastern Tibetan plateau margin is the current expansion border, where growth of the plateau is ongoing. We analyze shear-wave splitting at ChinArray stations in the NE Tibetan Plateau and its margin with the stable North Chine Craton. The measurements provide important information on the seismic anisotropy and deformations patterns in the crust and upper mantle, which can be used to constrain the expansion mechanism of the plateau. Along the margin and within the craton, the dominant NW–SE fast polarization direction (FPD) is NW–SE, subparallel to the boundary between the plateau and the North China Craton. The shear-wave splitting measurements on the NE Tibetan Plateau itself generally reflect two-layer anisotropy. The lower-layer anisotropy (with NW–SE FPDs) is consistent in the whole region and FPDs are the same as those in the North China Craton. The upper-layer FPDs are parallel to crustal motion rather than surface structures within the high plateau. The two-layer anisotropy implies the presence of deformed Tibetan lithosphere above the underthrusting North China Craton. The NE Tibetan shows similar deformation patterns at the surface (inferred from GPS) and within the mantle (inferred from shear-wave splitting), but significant crustal anisotropy (parallel to crustal motion) requires mid-lower crustal channel flow or detachment to drive further tectonic uplift of the plateau.