Seismic Phase Characteristics Based Imaging Methods And Their Applications

The knowledge of the earth structure from1D earth to2D or3D, benefits a lot from studies of the seismic phase that can shed light on the earth interior. Significant progresses have been made for seismic tomography, thanks to the technique of observation, computer resources and numerical methods. The methods of tomography may be classified into three categories, ray theory based on traveltime tomography of body waves and surface wave dispersion curve, semi-analytical method based classical waveform tomography, and the numerical simulation based waveform tomography. One of the differences of these methods is how the seismic phases are employed in the imaging process. Traveltimes of body waves are used in the ray theory traveltime tomography, the classical waveform tomography trys to fit mainly the phase and part of the amplitude of seismic phases, while the fully numerical simulation based tomography can make use of the traveltime, phase and amplitude of the seismic phase or even every wiggle of the seismic waveforms. Nowadays it becomes an important research direction to find a suitable way of making use of characteristics of the seismic phase for the purpose of imaging the earth interior. Here we developed imaging methods using the borehole seismic records, Rayleigh wave dispersion and ellipticity curves, and body wave receiver function, we also use the long period waveform tomographic method to image the upper mantle structure of East Asia.We observed the direct phase and its surface reflected phase, also the S-P wave converted from the basement of sediment on the near field borehole seismic records. These phases can help to image the detailed structure of the sediment. The observation and forward modeling of SH component direct phase S1and its surface reflected phase S2shows that these two phases have very small incident angles, and they are less affected by the epicentral distance and focal depth, their relative amplitude and traveltime are sensitive to the structure above the borehole. Based on plane wave approximation, we take the S1and source time function to simulate the waveform of the S2phase, and invert for the Vs structure and Qs above the receiver. The theoretical inversion tests and application to the real data shows that our inversion method is able to obtain fine layered Vs velocity and Qs factor of the near surface structure which are useful data for the studies of strong ground motion and estimating seismic hazards.Surface wave dispersion can help to image the earth interior on both regional and global scales. However dispersion curve only contains the traveltime of surface waves at different periods, and its constrain on the velocity structure is thus limited. The measuring and inversion of Rayleigh wave ellipticity or particle motion start to become popular. We here developed a tool to simulate and invert the surface wave dispersion curve and ellipticity, based on the fast generalized R/T coefficient method. The forward modeling of a series of crustal model indicates that dispersion and ellipticity curves have different sensitivity to the earth structure, dispersion curve can constrain the average velocity very well, while the ellipticity curve is sensitive to the velocity gradient. The joint inversion of these two types of data has been proved to be able to provide us better constrains on the earth structure, however the deeper part such as Moho interface is not well resolved.Body wave receiver function is a popular technique for studying structure of crust and upper mantle. The receiver function is measured from the deconvolution of vertical component body wave from horizontal components which results in a time series that is sensitive to the structure beneath the receiver. Seismic phases on receiver functions are direct P wave and other conversions and reverberations from interfaces in the crust and upper mantle, thus the receiver function is only sensitive to the relatively large velocity jumps, such as Moho et al., while not sensitive to smooth gradients and average velocity. We developed a new tool for jointly inverting the body wave receiver function, Rayleigh wave dispersion and ellipticity. Theoretical inversion tests shows that the joint inversion of body wave and surface wave data can constrains the absolute velocity better, and deep structure such as Moho can be well resolved.East Asia is a seismically active region featuring active tectonic belts, such as the Himalaya collision zone, western Pacific subduction zones and the Tianshan-Baikal tectonic belt. In this study we collected high quality regional and teleseismic waveform data that can be publicly available in this region to image the3D mantle structure using long period waveform (>60sec) inversion method based on non-linear asymptotic coupling theory. The3D velocity model shows strong lateral heterogeneities in the target region, which correlates well with the surface geology in East Asia. The stable blocks in target region have relatively thick lithosphere, such as Siberia platform, Kazakhstan block, Tarim block, Yangtze craton, Indian plate, most area of Tibet except for Chiangtang and Kunlun, are all shown as high velocity anomalies. While lower velocity anomalies are found in Xing-Meng block, Baikal rift, North China craton, South China block, Indo-China block, Sea of Okhotsk—Japan Sea—East China Sea—Taiwan Island, Philippine plate and South China Sea. Our new model suggests that Indian plate has subducted with different north reach from the west to east beneath the Tibetan Plateau, and the Pacific plate has subducted down to the depth of transition zone and stagnates in the transition zone in East China. The dominant fast and slow velocity boundaries in the study region are well correlated with tectonic belts, such as the central Asian orogenic belt and Alty/Qilian-Qinling/Dabie orogenic belt. Our radially anisotropic model shows Vsh> Vsv in oceanic regions and at larger depths(>300km), and Vsv> Vsh in some orogenic zones.