analyzing the variation characteristics of stress field in hetao seismic belt using focal mechanism data

doi:10.3969/j.issn.0253-4967.2015.04.008

Abstract

Abstract:

Based on analysis of background of geological tectonic movement and strong earthquake activity, we first obtained the focal mechanism solutions using amplitude ratio and CAP method, then determined the characteristic of average stress field of the study area by inversion of the stress field. On this basis, we selected the source mechanism consistency parameter as the inspection index to obtain the latest changes of stress field in Hetao seismic zone based on its temporal and spatial analysis. Two methods were used in the stress field inversion for comparison and analysis, which are average stress axis tensor and LSIB(Linear stress inversion bootstrap, LSIB). According to the geological tectonic movement and focal mechanism solutions of M_S≥4.0 earthquakes from 1970, we judge that the stress field evolution process of Hetao seismic belt is controlled jointly by vertical difference movement and horizontal shear movement, resulting in that the normal fault and strike-slip fault mechanisms are dominating.Taking into account the station layout of the study area, and in order to ensure the accuracy of calculation, we calculated 224 earthquakes focal mechanism solutions by using amplitude ratio and CAP method, including 164 earthquakes with 2.8≤M_L<3.5, 42 earthquakes with 3.5≤M_L<4.0, and 18 earthquakes with M_L≥4.0; The statistical results on type of focal mechanisms show that, there are 142 strike-slip earthquakes(63.4%), 50 normal fault earthquakes(22.3%)and 32 thrust fault earthquakes(14.3%). In this study period(from 2001 to 2012), most earthquakes had a strike-slip mechanism in Hetao seismic belt, this is one of the inherent characteristics of the stress field.The result of average stress axis tensor and LSIB shows that, the azimuth of maximum compressional stress is 47°～52°, direction is NE-SW; The azimuth of minimum compressional stress is 313°～322°, direction is NW-SE; This indicates that, the stress field characteristics of Hetao seismic belt and its sub-block are not completely consistent. Linhe Basin exhibits coordinated stress field characteristics with Hetao seismic belt, but Hubao Basin exhibits regional differences, direction of compressive stress has clockwise deflection in Baotou area, and the compressive stress direction is NEE. This heteropical character of stress field is also confirmed by horizontal projection distribution of stress axis of historical strong earthquakes and recent moderate and small earthquakes. Since 2003, the temporal sequence curve of consistency parameter of Hetao seismic belt had a downward trend, this change was caused by focal mechanism consistency parameter of Linhe to Wuhai area, which indicates that this structural position is possible to be a priority area for stress accumulation and accelerated release in future.

Key words: Hetao seismic belt, focal mechanism solutions, stress field inversion, consistency parameter

摘要：

从地质构造运动和1970年以来M_S≥4.0地震的震源机制资料分析认为, 垂直差异性和水平剪切运动共同控制河套地震带的应力场, 使其表现出左旋剪切并具有较多的正走滑型断层机制解。根据平均力轴张量计算和自助线性反演结果, 河套地震带的最大主压应力方位为47°～52°, 作用方向NE-SW; 最小主压应力方位313°～322°, 作用方向NW-SE。平均力轴张量计算结果显示, 所辖次级块体的应力场特征并不完全一致, 临河盆地的应力场与河套地震带相协调, 呼包盆地的应力场表现出区域性差异, 压应力方向在包头地区出现顺时针偏转, 作用方向NEE; 这种非均匀性的应力场分布特征可通过历史中强地震和近期中小地震的应力轴水平投影分布图像得到证实。2003年以来, 河套地震带的震源机制一致性参数的时序曲线趋势下降, 这种有序变化更多地是由临河至乌海地区的震源机制一致性引起的, 表明该构造部位有可能成为未来应力高度积累和加速释放的优先场所。

关键词: 河套地震带, 震源机制解, 应力场反演, 一致性参数

CLC Number:

P315.72⁺7

HAN Xiao-ming, LIU Fang, ZHANG Wen-tao, LI Juan, HOU Di. analyzing the variation characteristics of stress field in hetao seismic belt using focal mechanism data[J]. SEISMOLOGY AND GEOLOGY, 2015, 37(4): 1030-1042.

韩晓明, 刘芳, 张文韬, 李娟, 侯迪. 基于震源机制资料分析河套地震带的应力场变化特征[J]. 地震地质, 2015, 37(4): 1030-1042.

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