Since the formation of the earth at about 4.6 billion years ago, its interior has been under constant movement. Such movement was responsible for the geological features observed on the surface. Thus the study of the structure and dynamics of the deep interior of the earth is essential for the understanding of the surficial geological features. It is commonly believed that the most efficient way to explore the interior of the earth is to use elastic wave created by earthquakes or artificial explosions. In this study we utilize all the available seismic data recorded by permanent seismic stations in East Asia to address two important problems: (1) the interaction between the subducting Pacific slab and the base of the mantle transition zone at the depth of about 660 km; and (2) velocity anomalies associated with the collision between India and Eurasia. One of the important data sets to address those problems is the body-wave travel-times for seismic rays from distant earthquakes. To accurately determine the on-set time of each seismogram, we process the seismograms so that they start exactly 20 s before the theoretical arrival time calculated based on a standard earth model, and manually pick the on-set time in small time windows. The accuracy of the picks is about 0.1-0.2 s, depending on the sharpness of the arrival. We then calculate relative residuals between all the possible pairs of stations, and study the variation of the relative residuals with the azimuth of the arriving rays. We have found systematic azimuthal variations for some of the stations, indicating velocity heterogeneities in the vicinity of the stations. The travel-time data can be combined with other types of observations, such as depths of velocity discontinuities from P-to-S converted phases, mantle anisotropy as revealed by shear-wave splitting, and mantle inelasticity from attenuation analysis, for a comprehensive study of mantle structure and dynamics beneath the study area.