关键词: 小江断裂带, P波接收函数, 两步反演法, S波速度, 中下地壳低速层

Abstract:

In the past few decades, a large number of geophysical explorations were carried out in the Xiaojiang fault zone and adjacent areas, mainly including GPS, seismic geology, fluid geochemistry, seismicity, historical earthquakes and coseismic displacement of large earthquakes, etc. The results of these studies helped us have a better understanding of the fault structure characteristics, movement attributes, seismogenic environment and dynamic mechanism of the Xiaojiang fault zone. In terms of deep structure, the existing researches are limited by factors such as the density of observation stations, and most studies focused on the structural background on the regional scale, and few are specifically on this fault zone. The implementation of Phase I of the China Earthquake Science Array(ChinArray)detection project provides a good data basis for the study of the fault structure in Yunnan. It is of great practical significance for earthquake prevention and disaster mitigation to carry out deep structural detection of the Xiaojiang fault zone and clarify the fine crustal structure of the fault and its adjacent areas.

S-wave velocity is an important parameter to determine the crustal structure, physical state difference and tectonic evolution process. Extracting the P-wave receiver function from teleseismic body-wave waveform data and inverting it is one of the important methods to obtain the crustal S-wave velocity structure at present. The traditional receiver function S-wave velocity structure inversion relies heavily on the selection of the initial model, which results in strong non-uniqueness inversion results. The two-step inversion method, which takes into account the low and high frequency receiver functions at the same time, effectively suppresses the dependence of the inversion process on the initial model, and improves the reliability of the inversion results.

Based on the three-component waveform data of 238 teleseismic events with epicentral distances ranging from 30°~90° and magnitude M≥5.8 recorded by 48 broadband seismic stations in the Xiaojiang fault zone and adjacent areas from September 2, 2011 to January 16, 2014, this paper calculates the low-frequency(α=1.0)and high-frequency(α=2.5)radial P receiver functions, respectively. Then, on this basis, the S-wave velocity structure below each station is inverted using the two-step inversion method and Bootstrap resampling technique and the deep crustal structure of the Xiaojiang fault zone and its adjacent areas is studied. The following conclusions are drawn:=

(1)The crustal S-wave velocity in the study area is obviously non-uniform in both lateral and vertical directions. The overall distribution is as follows: In the near surface, there is a low-velocity layer about 2~4km thick, which may be related to the distribution of shallow sedimentary rocks or Cenozoic soft overburden; The S-wave velocity in the middle and upper crust is alternately distributed with high and low velocity; There is an obvious low-velocity layer in the depth range of 20~35km, mainly intermittently distributed in the Sichuan-Yunnan diamond block west of the Xiaojiang Fault and the Indosinian block south of the Honghe Fault; Besides, there is also local distribution near the Shizong-Mile Fault.

(2)The low-velocity layer in the middle and north segments of the Xiaojiang fault zone are relatively developed, and it is most prominent in the middle segment, with a maximum thickness of about 28km. There is an obvious high-velocity zone in the depth range of 15~25km in the southern segment.

(3)The Poisson’s ratio in the study area is generally low(average 0.24), unevenly distributed, and has drastic lateral changes. The Poisson’s ratio in the Xiaojiang fault zone generally has a segmental feature of higher in the northern segment, the southern segment coming second, and lower in the middle segment. The corresponding relationship between the distribution of low velocity in the crust and Poisson’s ratio in the study area is not obvious, and most of the low velocity layers seem to lack the conditions for partial melting. The differences and inconsistencies in the geophysical results indicate that the deformation evolution mechanism and physical properties of the low velocity layers in the crust are relatively complex.

Key words: Xiaojiang fault zone, P-wave receiver functions, two-step inversion, S-wave velocity, low velocity in the middle-lower crust