滇西南地区地壳S波速度结构反演及强震孕震环境分析

Inversion of Crustal S-wave Velocity Structure and Analysis of Seismogenic Environment of Strong Earthquakes in Southwestern Yunnan

摘要:

基于滇西南地区31个宽频带地震台站记录的远震三分量波形数据,提取径向P波接收函数,以“中国地震科学实验场地区地壳剪切波速模型”为初始模型,进行精细化结构反演(垂向精度达2km级),并采用Bootstrap重采样技术进行统计和评估,获取了各台站下方的S波速度结构,结合地壳厚度、泊松比分布,对滇西南地区的地壳深部结构及强震孕震环境进行研究。结果表明:①研究区地壳S波速度分布在横向和垂向上均存在明显的非均匀特性。在中上地壳0～20km深度范围内存在明显且连续的低速层,平均厚度约15km,最大厚度达20km。在中下地壳20～28km深度范围内局部区域存在明显的低速层(厚度约8km),主要分布在普文断裂北端至阿墨江断裂中部区域(23°N附近),另外,在红河断裂附近和澜沧江断裂北部的局部区域也有分布; ②从孕震环境来看,景谷 M_S6.6 和 M_S5.8、墨江 M_S5.9 地震均发生在中上地壳低速层内,老挝 M_S6.0 地震发生在高、低速度的过渡区域。滇西南地区新生破裂带较为发育,地震波速的减小可能与地壳断裂或微裂隙中有流体的存在有关,研究区NW向新生破裂带的存在以及流体的长期作用,为带内高应变的突然释放提供了条件。另外,4个地震均发生在泊松比变化的高梯度带上,且地震发生处地壳厚度变化均较大。地壳厚度、泊松比变化剧烈的地区,地壳物质组成差异明显,壳内应变易于积累,有利于强震的诱发。

Abstract:

Based on the teleseismic three component waveform data recorded by 31 broadband seismic stations in southwestern Yunnan,the radial P-wave receiver function is extracted,and the “crustal shear wave velocity model of China Seismic Science Experimental Field” is taken as the initial model to carry out refined structural inversion(vertical accuracy up to 2km),and Bootstrap resampling technology is used for statistics and evaluation,so as to obtain the S-wave velocity structure under each station,combined with the crustal thickness and Poisson's ratio distribution,The deep crustal structure and the seismogenic environment of strong earthquakes in southwest Yunnan are studied. The results show that:① The crustal S wave velocity distribution in the study area has obvious non-uniformity in both horizontal and vertical directions. There is an obvious and continuous low velocity layer within the depth range of 0～20km in the middle and upper crust,with an average thickness of about 15km and a maximum thickness of 20km. There is an obvious low velocity layer(about 8km thick)in local areas within the depth range of 20～28km in the middle and lower crust,which is mainly distributed in the area from the north section of Puwen Fault to the middle section of Amo River fault(around 23° N). In addition,such velocity layer is also found in local areas near the Honghe fault and north of Lancang River fault. ② From the perspective of seismogenic environment,Jinggu M_S6.6 and M_S5.8,Mojiang M_S5.9 earthquakes occurred in the low velocity layer of the middle and upper crust,and Laos M_S6.0 earthquake occurred in the transition region of high and low velocities. The Neogenic Fault zones are relatively developed in southwest Yunnan,and the reduction of seismic wave velocity may be related to the presence of fluids in crustal fractures or microcracks. The existence of NW trending Neogenic fault zones and the long-term effect of fluids in the study area prepare conditions for the sudden release of high strain in the zone. In addition,the four earthquakes all occurred in the high gradient zone of Poisson's ratio change,and the crustal thickness at the location where the earthquake occurred changed greatly. In areas with drastic changes in crustal thickness and Poisson's ratio,the composition of crustal materials is significantly different,and the crustal strain is easy to accumulate,which is conducive to the induction of strong earthquakes.