科研之友
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摘要
The North-South tectonic belt (NSTB) is located in the eastern margin of the Tibetan Plateau. It is a north-south tectonic boundary between the eastern and western Chinese mainland with a very complex structure, showing a significant change in geology, geomorphology, and geophysical field characteristics on both sides. Meanwhile, the NSTB is a strong earthquake belt, where half of strong earthquakes greater than M8. 0 in Chinese mainland have occurred based on historical records, so also named the North-south seismic belt. Thus, the NSTB provides a unique nature laboratory for understanding continental interiors and lithosphere's deformation. An effective technique for understanding crust and mantle deformation is the analysis of seismic anisotropy. Here we present new shear wave splitting observations from dense temporary seismic array and permanent seismic stations to map out variations in the dynamics of the southern segment of NSTB. Mantle anisotropy is constrained with 440 XKS (SKS, SKKS, and PKS) shear wave splitting observations, 350 from portable deployments in the NSTB (2011-2013, the ChinArray Phase I), and 90 permanent stations from the Chinese National Seismic Network (2003-2012). We determine the XKS fast wave polarization directions and delay times between fast and slow shear waves for 440 new seismic stations in the southern segment of NSTB using both the grid searching method of minimum transverse energy and stacking analysis methods. To obtain a reliable estimate of splitting parameters, the following were used as diagnostics for successful splitting parameter estimations: (1) clear XKS arrivals and distinct tangential component, (2) the horizontal particle motion is elliptical when anisotropy is present, (3) the two horizontal fast- and slow-component waveforms are coherent, (4) the particle motion becomes linear following correction for anisotropy, and (5) successful removal of tangential energy in the case of core phases. The results at most stations are good, the error of azimuth is less than 100, and the error of delay time is less than 0. 2 s. The fast polarization directions and delay times do not depend on back azimuth, thus a single layer of anisotropic fabric is able to sufficiently explain the data without the need for additional layer. Based on polarization analysis from XKS data, we developed an anisotropic image of upper mantle in the southern segment of NSTB. In the study region, the fast polarization directions and delay time show partition characteristics from south to north. The fast polarization directions trend near N-S in the north, while the fast polarization directions rotate to near E-W in the south. The average delay time at stations in the north is 0. 8 s, whereas it is 1. 1 s in the south. Thus, the average delay time at the stations in the north is obviously less than that in the south. By analysis the anisotropic characteristics of the study region, this study has shown lithosphere deformation plays a major role in observed anisotropy in the north, and is undergoing vertically coherent deformation. In the south, observed anisotropy is mainly attributed to asthenospheric mantle flow beneath thin lithosphere. The subduction and rollback/retreat of Burma/Sunda slabs generate a southeastward flow which produces a differential flow between the lithosphere and subasthenospere. The differential shear is sufficient to generate the observed anisotropy in the south.
- 出版日期2015-11
- 单位中国地震局
