陕甘宁交界地区地壳S波Q值成像特征研究

Crustal QS Tomography in Intersection Area of Shaanxi-Gansu-Ningxia

摘要:

利用陕甘宁交界地区测震台网记录到的2009—2018年4423个近震波形资料，采用衰减结构层析成像方法对该地区地壳Q_S值进行成像分析，获得了陕甘宁交界地区地壳衰减模型，分析讨论了Q_S值分布与活动断裂及强震活动等之间的关系。结果显示，陕甘宁交界地区地壳剪切波衰减横向变化显著，整体上，海原-六盘山构造带和西秦岭构造带地壳Q_S值相对偏低，鄂尔多斯盆地和陇中盆地地壳Q_S值相对较高。活动构造带和活动断裂交汇区呈现低Q_S值，结合前人研究成果，推断这与活动断裂带及交汇区地壳岩石较为破碎且富含流体有关。有历史地震记录以来，研究区6级以上地震大多分布在高、低Q_S值的过渡区，分析认为这与“硬”、“软”地壳介质交界处容易积累地震应变能有关。研究区域低Q_S值分布与低横波速度和低电阻率区的主要分布特征较为一致，但是与大地热流研究结果存在区域性差异。

Abstract:

In this study,we collected the waveform data of 4423 local earthquakes recorded by regional seismic stations in the intersection area of Shaanxi-Gansu-Ningxia during the period from 2009 to 2018. The attenuation model of the crust beneath the study area was constructed by using two-dimensional seismic attenuation tomography,and the relationships between distribution of Q_S values and active faults and strong earthquakes were analyzed and discussed. The results show that there is a significant lateral heterogeneity in Q_S distribution in the study area,namely,the Q_S values in the Haiyuan-Liupanshan tectonic belt and Xiqinling tectonic zone are relatively low,and the Q_S values in the Ordos Basin and Longzhong Basin are relatively high. Furthermore,the Q_S values in the active faults and the intersection area are low,which might be due to the fact that the crustal rocks in the active fault zone and the intersections are relatively broken and rich in fluids. The M≥6.0 earthquakes are basically located in the low-Q_S region or their margins in historical record,which may be due to the accumulation of seismic strain energy on the intersection zone of “hard” and “soft” crust media. The main distribution characteristics of the low Q_S values are consistent with that of the low S-wave velocity and the low resistivity in the study area,but there exist regional differences between the low-Q_S region and the low terrestrial heat flow zone.