滇西地区地壳结构的爆破地震研究

EXPLOSION INVESTIGATION OF THE CRUSTAL STRUCTURE IN WESTERN YUNNAN PROVINCE

本文描述我国滇西地区洱源—江川和遮放—宾川二条剖面的地壳结构爆破地震的研究结果。 资料分析解释的结果说明,该地区的地壳内存在四个界面:Pg、P20、P30与P40面。Pg面为结晶基底面,深度在0.3—3.5km之间,界面速度约5.90km/s。P02面为地壳上部反射面,深度在12至24km间,其界面速度约6.30—6.50km/s。该界面的下方,在大部分地段是一厚梯度层至莫霍界面。在30—35km的深处,局部地区存在弱反射界面P30。而莫霍面（即P40面）的深度在37—46km。遮放—宾川剖面的地壳平均速度约6.40km/s,莫霍界面速度为8.06km/s。但洱源—江川剖面的这两种参数分别为6.19km/s与7.75km/s,其下侧50km的深处还存在P50反射面。 遮放—宾川剖面上的怒江断裂在P40及P20面上有2.5km的深度跳跃。澜沧江断裂经过的地方,P40震相追踪中断,波形畸变。这类现象在洱源—江川剖面的几个地方也有明显反应,在元谋—绿汁江断裂处莫霍面的深度跳跃达3km,断裂还使江川炮的P40震相追踪中断。 在洱源—江川剖面的中段,发现了上地幔低速度异常带,速度值为7.75km/s。

In this paper are presented the results of explosion investigation of the crustal structure of Eryuan to Jiangchuan and Zhefang to Binchuan profiles in Western Yunnan Province.There are four interfaces in the crust of this region. As the results of interpretation show, these are Pg, P2°, P3° and P4° interfaces. The Pg phase corresponds to refraction of the top boundary of the crystalline basement. Its depth is 0.3 to 3.5 km with wave velocity 5.90 km/s. The P2° is reflection from the interface in the upper crust. Great changes of its depth from 11 to 25 km occur here, and the velocity of its head wave is from 6.30 to 6.50 km/s. At the eastern part of Eryuan-Jiangchuan profile there is probably a thin layer with high velocity. There are thick gradient layers in most part of this region under P2° interface. They are continuously extending to Moho boundary. Here is a weak reflection P3° in parts of the region at the depths 30 to 35 km. The depth of Moho discontinuity (P4° boundary) is 37 to 45 km. It generally slopes from south to north in Zhefang-Binchuan profile. The average velocity of crust is about 6.40 km/s with velocity of Pn 8.06 km/s. But the Moho discontinuity does not evidently slope in Eryuan-Jiangchuan prifile. The average velocity of crust here is 6.19 km/s, and the velocity of Pn is 7.75 km/s in the middle part of this profile. Another reflection interface P5° exists under the Moho boundary at the depth 50 km.There is a vertical depth displacement of 2.5 km of Moho and P2° interface near the Nu Jiang fault in Zhefang-Binchuan profile and its northern side is deeper than the southern one. The correlation of seismic phase P4° is interrupted and its waveform is distorted at that place where the Lancang Jiang fault passed nearby. Perhaps here is some kind of broken zone in the lower crustal interior. These pictures also occur clearly at three places in the Eryuan-Jiangchuan profile. Especially, it is quite evident at the Yuanmou-Luzhijiang fault. The vertical displacement of Moho interface here is 3 km, and the fault causes correlation of P4° phase from shot point in Jiangchuan interrupted.At the middle part of Eryuan-Jiangchuan profile the anomalous low velocity region with value 7.75 km/s at the top of upper mantle is revealed.