Using data from more than 2000 seismic stations from multiple networks arrayed throughout China (CEArray, China Array, NECESS, PASSCAL, GSN) and surrounding regions (Korean Seismic Network, F-Net, KNET), we perform ambient noise Rayleigh wave tomography across the entire region and earthquake tomography across parts of South China and Northeast China. We produce isotropic Rayleigh wave group and phase speed maps with uncertainty estimates from 8 to 50 s period across the entire region of study, and extend them to 70 s period where earthquake tomography is performed. Maps of azimuthal anisotropy are estimated simultaneously to minimize anisotropic bias in the isotropic maps, but are not discussed here. The 3D model is produced using a Bayesian Monte Carlo formalism covering all of China, extending eastwards through the Korean Peninsula, into the marginal seas, to Japan. We define the final model as the mean and standard deviation of the posterior distribution at each location on a 0.5 • × 0.5 • grid from the surface to 150 km depth. Surface wave dispersion data do not strongly constrain internal interfaces, but shear wave speeds between the discontinuities in the crystalline crust and uppermost mantle are well determined. We design the resulting model as a reference model, which is intended to be useful to other researchers as a starting model, to predict seismic wave fields and observables and to predict other types of data (e.g. topography, gravity). The model and the data on which it is based are available for download. In addition, the model displays a great variety and considerable richness of geological and tectonic features in the crust and in the uppermost mantle deserving of further focus and continued interpretation.
[1] Phase velocities across eastern Tibet and surrounding regions are mapped using Rayleigh (8-65 s) and Love (8-44 s) wave ambient noise tomography based on data from more than 400 Program for Array Seismic Studies of the Continental Lithosphere and Chinese Earthquake Array stations. A Bayesian Monte Carlo inversion method is applied to generate 3-D distributions of Vsh and Vsv in the crust and uppermost mantle from which radial anisotropy and isotropic Vs are estimated. Each distribution is summarized with a mean and standard deviation, but is also used to identify "highly probable" structural attributes, which include (1) positive midcrustal radial anisotropy (Vsh > Vsv) across eastern Tibet (spatial average = 4.8% ± 1.4%) that terminates abruptly near the border of the high plateau, (2) weaker (À1.0% ± 1.4%) negative radial anisotropy (Vsh < Vsv) in the shallow crust mostly in the Songpan-Ganzi terrane, (3) negative midcrustal anisotropy (À2.8% ± 0.9%) in the Longmenshan region, (4) positive midcrustal radial anisotropy (5.4% ± 1.4%) beneath the Sichuan Basin, and (5) low Vs in the middle crust (3.427 ± 0.050 km/s) of eastern Tibet. Midcrustal Vs < 3.4 km/s (perhaps consistent with partial melt) is highly probable only for three distinct regions: the northern Songpan-Ganzi, the northern Chuandian, and part of the Qiangtang terranes. Midcrustal anisotropy provides evidence for sheet silicates (micas) aligned by deformation with a shallowly dipping foliation plane beneath Tibet and the Sichuan Basin and a steeply dipping or subvertical foliation plane in the Longmenshan region. Near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying Tibet.
SUMMARY Two years of continuous recordings of ambient seismic noise observed at 354 stations in South China from 2009 to 2010 are used to estimate Rayleigh wave group and phase velocity maps from 6 to 40 s period. These results are merged with Rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame. Eikonal tomography generates the dispersion maps, which, by Monte–Carlo inversion, are used to estimate a 3‐D Vsv model of the crust and upper mantle down to a depth of 150 km across all of South China with attendant uncertainties. A clear image emerges of the ‘West Yangtze Block’, a region of the western Yangtze Craton characterized by relatively thick crust (∼40 km) overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the Yangtze craton in the Archean and may present a present‐day obstacle to the eastward expansion of Tibet. The West Yangtze Block contrasts with the thinner crust (∼30 km) and mantle lithosphere (∼70–80 km) beneath the eastern Yangtze Craton and South China Foldbelt. These observed differences are consistent with processes associated with flat slab subduction in the Mesozoic that may have eroded the lithosphere of the eastern Yangtze Craton and the South China Foldbelt.
A 3‐D shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the North China Craton (NCC), northeastern China, the Korean Peninsula, and the Sea of Japan. Ambient noise Rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from Chinese provincial networks (CEArray), the Japanese F‐Net, and the IRIS Global Seismic Network. Continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods. The model is motivated to constrain the distributed intraplate volcanism, crustal extension, cratonic rejuvenation, and lithospheric thinning that are hypothesized for the study region. Numerous robust features are observed that impose new constraints on the geometry of these processes, but discussion concentrates only on four. (1) The North‐South Gravity Lineament follows the ∼40 km contour in crustal thickness, and crustal thickness is anticorrelated with water depth beneath the Sea of Japan, consistent with crustal isostasy for a crust with laterally variable composition. (2) The lithosphere is thin (∼70 km) beneath the Songliao‐Bohai Graben but seismically fast. (3) Even thinner more attenuated lithosphere bounds three sides of the eastern NCC (in a horseshoe shape), identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust. (4) Low‐velocity anomalies reach upward (in a Y shape) in the mantle beneath the eastern and western borders of the Sea of Japan, extending well into continental East Asia in the west, and are separated by a ∼60 km thick lithosphere beneath the central Sea of Japan. This anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab.
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