Seismic Velocity Models In The Transition Zone Beneath Tibet And North-east Pacific Subduction Zone By Triplicate Waveform

The upper mantle transition zone is the boundary between upper and lower mantle, where extensive mass transfer through it. Accurate constraints on properties of the transition zone are necessary for understanding thermal, dynamical and compositional models of the mantle. In this thesis, I start with an introduction of what the transition zone are, and new progress of refining the nature of transition zone with the experimentally development under the high temperature and high pressure.In triplicate waveform modeling, a method of focal depth move-out stacking is firstly present. In this way, we can not only make triplicate phase are more identifiable, but also can test the data's consistency, very useful to analyze same features of triplication for a great deal events. Moreover, for different focal depths of earthquakes, it can be more easily to measured travel time difference to detect abnormity among triplicate arrivals. This method is basically simple, but can be used widely.Triplicate waveform modeling is used to resolve velocity structures in the upper mantle transition zone beneath north-western Tibet and northeast China and its surroundings. And these two regions are characterized as the continental collision and subduction of Pacific slab. Results are summarized in the following.6) Tibet plateau, the world's largest and highest plateau, is a key for understanding super-continental collision. In the 2003, two broadband seismic experiments of Namche Barwa Tibet array (XE) and 2003MIT-China array (YA) are all deployed in southeastern Tibet. Using these seismic profiles comprising of high-resolution, we constrain the P and SH wave velocity structures in the transition zone beneath north-western Tibet, using by triplicate phases recorded in the regional epicentral distance (10～30°). We found SH and P velocity models are decorrelated, a large Vs jump across the 660-km discontinuity and a small velocity gradient above it, while P velocity characterized by a relative small contrast across the discontinuity with a high velocity gradient above it. There is no significant depression of the 660-km discontinuity in both models. In comparison to that of India, Vs near the 660-km discontinuity is normal, while Vp is little high, and thus a little high ratio of Vp/Vs in our study area. This result is in general agreement with previous observation beneath central Tibet and Lhasa terrains, and these similarities cannot be explained by a remnant of detached/subducted mantle lithosphere. Combined with mineral studies, we suggest the decorrelation between Vp and Vs velocity structures can be attributed to a chemical heterogeneity, such as an increased Al-content. An oceanic chemical composition remnant is inferred in the transition zone, possibly associated with subduction or detachment of Tethys oceanic slab in dynamic process of Tibet.7) The northwest Pacific subduction zone provides a suitable region to study the interaction of slab and the upper mantle discontinuities. In this study, we used the waveform data collected by the national and regional seismic networks in China, portable seismic array deployed in north-east China from 2006 to 2009, and Taiwan network download form IRIS. Using these data, we constrain SH velocity model in northwest Pacific subduction zone. Our results show that there is high velocity abnormity, about 2%, at the depth of 430-560km, with the top interface not deeper than 490km, beneath Kuril Islands. However, there is a larger velocity jump across the 660-km discontinuity beneath Kuril Insands and eastern part of the Norh China Craton (NCC). Compared with these observation, there are high velocity abnormity (-2%) at the depth of 580km to 660-km, with a relative small jump size across the 660-km discontinuity, and a deeper-than-normal 660-km discontinuity with a depression by 15-20km beneath Korea. Velocity structure in the transition zone beneath North-East China is in the between eastern part of the Norh China Craton (NCC) and Korea.8) A corresponding anomaly of high SH velocity in the lower transition zone is observed where an anomaly of depression 660-km discontinuity in our regions. This link can be interpreted as thermal variations of olivine phase transformations under the upper mantle pressure and temperature conditions. It is the lateral variation of velocity structure in the transition zone between Kuril Island, North-East China, eastern part of the Norh China Craton (NCC) and Korea, explaining the inconsistency in the previous studies by triplicate waveforms. This lateral variation is connected to the subduction of northwest Pacific slab. We speculate that the northwest Pacific slab subduct across the middle transition zone beneath Kuril Islands, then just arrive at the lower transition zone beneath North-East China. However, the subducted north Pacific slab is sharply deflected to horizontal when it hits the bottom of the lower transition zone, resting immediately above the lower mantle beneath Korea. The differences velocity structure in the transition zone between the eastern part of the Norh China Craton (NCC) and Korea reflect that the northwest Pacific slab don't subduct across all the eastern Sino-Korean craton.