THE HIGH VELOCITY IMBEDDED LAYER IN THE CRUST OF NORTHWESTERN CHINA
TSENG JUNG-SHENG, TENG CHI-WEN, KAN YUNG-CH AND CHANG CHIA-JU (Institute of Geophysics and Meteorology, Academia Sinica)
During the past several years, an investigation of the earth's crust using seismic reflection method has been carried out in Eastern Chai Dam Basin and Kansu. At the distances of 40—100kms from the shot point, ordinary reflections from the deep boundaries in the earth's crust can be identified. Apart from these ordinary reflections, another kind of phases with different properties have been discovered. Their apparent velocities are noticeably greater than that of the ordinary reflections. Their apparent velocities do not change regularly with distances. Their travel time curves intersect with the ordinary reflections and disappear suddenly at a distance of about 80—90 kms from the shot point. From their properties, it can be shown that a high velocity imbedded layer with continuously increasing velocities must exist in the earth's crust of these regions. The calculated parameters of this high velocity layer in these two regions are: Kansu Eastern Chai Darn Basin Total thickness of overburden layers 18.8 kms 30.5 kms Average velocity of overburden layers 5.5 km/sec 5.3 km/sec Thickness of high velocity layer 6.0kms 3.2kms Velocity of high velocity layer 7.5—8.5km/sec 7.5—8.0km/sec Both the upper and lower boundaries of this high velocity layer are shown to have good reflectivity, and are apt to produce multiples. The parameters of succeeding layers in the earth's crust can be calculated from their corresponding reflection arrivals. The depths of Mohorovii discontinuities from the earth's surface are calculated to be 51 and 52 kms in Kansu and Eastern Chai Dam Basin respectively. It is also argued that if this high velocity layer as discovered in Northwestern China exists extensively in the continental earth's crust, the scatterly distributed velocities of P~* and the mechanism of propagation of Lg waves can be more properly explained.