长白山天池火山研究区地壳三维界面与速度分布联合成像

迅速发展并得到广泛应用的空间深地震测深技术通过采用三维数据采集的观测系统，利用三维层析技术，以求获得区域地壳三维分层结构和三维速度分布图像。它与传统的二维宽角反射－折射剖面技术相结合，可以有效地研究区域性地壳结构，特别是壳内深断裂和低速层的空间展布特征。Kanasewich等(1985)给出了利用空间深地震测深资料重建地壳三维界面的方法，我们在其方法的基础之上，进一步提出地壳三维界面和速度分布联合反演的方法。正问题的计算是在Chander(1977)关于三维平界面的快速两点追踪算法上的改进，在获得界面三维反演结果的基础之上，利用剩余走时残差，采用模型不分块反演技术(Tarantola，Nercession 1986)重建地壳三维速度图像。 1998年国家地震局地球物理勘探中心在长白山火山区实施了三维深地震测深观测，目的是研究天池火山的岩浆系统。利用本次实验所获得的780余个PmP波走时数据，采用上述的方法重建了研究区莫霍界面和地壳三维速度分布图像。研究结果表明，本区莫霍界面由北东方向向南逐渐加深，在天池火山口下达最深，并且被一些可能存在的地壳厚度陡变带(或深断裂)所切割。在东西方向莫霍面由西向东缓缓加深，其变化较南北方向缓和。特别值得注意的是，存在着一条近北东方向的莫霍面深度陡变带(或深断裂带)从天池火山口西部穿过，相应位置与马鞍山—三道白河地堑型断裂相一致，该断裂带可能对天山火山喷发时岩浆的运移起到重要作用。深度为15km和25km的P波速度图像表明，在天池火山口下分布着近南北走向明显的低P波速度分布，其南北方向延伸的范围约为80－90公里左右。比较这两个深度上的低P波速度体的分布特点，可以看出这个低P波速度体尺度随深度逐渐变小，但在25km深度处仍清晰可见，这表明该区岩浆自上地幔侵入地壳的“痕迹”，这也意味着，长白山天池火山的岩浆系统极有可能延伸到上地幔或更深一些。

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

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.