地震层析技术在探测火山地区三维速度结构的可能应用

POSSIBLE APPLICATION OF SEISMIC TOMOGRAPHY IN DETERMINATION OF 3-D VELOCITY STRUCTURE IN VALCANO REGION

本文介绍了地震层析技术的基本原理和方法。在云南地区的应用实例,用刘福田等人发展的地震层析技术;成功的重建了10个不同深度的速度剖面图象。这些速度剖面和6条深地震测深的速度剖面一致。小于20km的上地壳的速度图象与已知的地质结构十分相似,所有的强震几乎都发生在高速与低速的过渡带上。在大于20km深的中地壳云南省的东部和北部地区普遍存在低速层,V=5.6-6.0km/s,但是在西南部却没有低速层。V=6.3-6.4km/s。上地幔顶部的速度在滇北是7.6-7.8km/s,而在滇南是80.5km/s。在200km深的上地幔有明显的低速层。在450km深处仍然有明显的侧向非均匀性。腾冲地区从地壳至上地幔几乎全部低速,也许有某些热物质从上地幔上升至地表。本文用的重建方法有两大优点:(1)速度模式是通过指定节点速度和通过已知基函数的线性插值而确定。这样就建立了一个横向连续而垂向不连续的速度场。这个模式显然比常速度块模式优越。垂向的不连续可以用来模拟存在于地壳和上地幔中的速度间断。(2)速度参数和震源参数通过正交投影算子使之分离。采用顺序正交三角化和修正的奇异值分解,这些措施使内存的需要量减小一个量级,计算量减少一半。在火山地区亦可用CT技术研究它的三维速度和三维Q值结构。但要求有高质量的地震台网。

The principle and method of seismic tomography has been introduced in this paper, and its application, as an example. to Yun-nan province has been made. Images of the velocity cross- section at 10 different depths have been successfully reconstructed, using the lechniques of seismic tomography developed by Liu Futian. These velocity cross-sections are consistent with the results of six velocity profiles of deep seismic sounding. The image of the velocity in the upper crust above 20km depth is very similar to the well-known geological structure and ali the strong earthquakes nearly occurred in the transition zones between high and low velocity regions. There are low velocity layers in most regions of eastern and northern Yun-nan province in the mediate crust beneath 20km. V=5.6--6.0km/s. but no low velocity layers in the south-western part V=6.3--6.4 km/s. The velocity in the top upper mantle is nearly 7.6--7.8km in northern Yun-nan, but 8.05km/s in the south. There are significant low velocity layers in the upper mantle of 200km depth in most regions studied. There is still obvious lateral inhomogeity at 450km depth. The Teng-chong area is an entirely low velocity region from crust to the upper mantle, perhaps some thermal matter rises from the upper mantle to the surface. The reconstruction method of ST used in this paper has two advantages: 1) The velocity model is determined by specifying node-velocities of hexahedron and linear interpolation through known basic function. A velocity field with lateral continuous velocity. value and vertical discontinuities is established. This model is obviously better than the block model of constant velocity. The ver- tical discontinuities allow the model to simulate velocity discontinuities in the crust and upper mantle. 2) The velocity and hypocenter parameters are separated by introducing an orthogonal pro- jeetion operator. Sequential orthogonal triangulariza tion and modified singular decomposition are adopted. These measures reduce the RAM requirement by one order of magnitude and reduce the amount of conputation to a half. The technique of seismic tomography can also be used to determine 3-D velocity and Q- value structure in valcano region. But high quality or seismic network is necessary.