Joint Tomographic Determination of 3-D Crustal Interfaces and Velocity Distribution in ChangBai Volcano Region

YANG Zhuoxin; Zhang Xiankang; Zhao Jinren; Wang Fuyun; Zhang Zhengke

doi:10.3969/j.issn.1004-4140.2000.z1.016

Abstract：Spatial deep seismic sounding is being developed rapidly and applied widely. It uses three-dimensional observation system and tomographic technique to obtain regional crustal 3-D layer sttucture and 3-D velocity distribution image. In combination with the traditional 2-D wide-angle reflection-refraction profiling technique, it can effectively determine regional crustal structure, especially the spatial distribution feature of deep faults and low velocity layers. Kanasewich et al (1985) presented the method to reconstruet 3-D interface with spatial deep seismic sounding data. On this basis, we further proposed the joint inversion method of 3-D crustal interfaces and velocity distribution. An improved method based on chander's rapid two-point my tracing algorithm for plane interface (1977) is used for forwardprocess, After obtaining 3-D interface result, we could use the remainder residuals of traveltimes to reconstruct 3-D crustal velocity distribution using the inversion method without blocks (Tarantola,Nercessian 1986). In 1998, Research Center of Exploration Geophysics of China Seismological Bureau carried out a 3-D deep seismic sounding observation in ChangBai-Tianchi Volcano region to study magma system in this region. More than 780 traveltimes of Pm P were collected in this experiment. Using the above method, we reconstructed Moho interface and 3-D crustal velocity distribution image for this region. The inversion result shows that the depth of Moho in creases gradually from NE to S in the region and it reaches the deepest value under Tianchi, it is cut by some possible crustal thickness variation belts. In EW direction, Moho depth increases slowly from W to E. The depth variation along EW direction is slower than it is in NS. We should especially notice that there is a near NE trending crustal thickness abrupt variation belt (deep fault )at the depth of Moho in the region, which cuts across the west of Tianchi and corresponds to Ma'anshan-Sandaobaihe graben fault. The fault belt may have played an important role in the migration of magma as the volcano erupted. The P wave velocity image at the depth of 15kmn and 25kmn shows that an obvious and near NS trending low velocity distribution of P wave exists fight under Tianchi. It extends about 80－90kin in NS direction. Comparing low velocity distribution features of P wave at the two depths,we find that it becomes smaller with depth gradually, but it is still visible at the depth of 25km, It shows the “mark” of magma intrusion into the crust from upper mantle. This also indicates that magma system in Tianchi Volcano region has probably stretched into the upper mantle or even deeper.