鄂尔多斯西南缘P波三维Q值层析成像

Three-dimensional Q Tomography of P-wave Data in the Southwestern Ordos Margin

摘要:

利用国家测震台网数据备份中心提供的2009年1月至2015年11月的地震波形数据，选取震级范围为M_L1.8~6.7，震中距小于800 km的1 820个地震事件的16 156条Pg波到时数据，对鄂尔多斯西南缘P波三维Q值分布特征进行了研究。结果表明：鄂尔多斯西南缘一系列呈"L"型分布的断裂带上，Q值呈现出比较低的水平；而其两侧地区，Q值相对较高，且分布存在较大差异。研究区内呈现低Q值状态的海原地区和天水—礼县地区存在明显的低阻—低速层，而高Q值的渭源—定西—通渭—西吉地区则在地壳内缺失低阻层。另外，存在明显高热流的渭河盆地和天水—礼县地区，Q值都明显偏低，而其周围地区则Q值较高。沿临潼—长安—富城—蒲城断裂走向，有一条比较显著的垂直形变梯度带和重力变化等值线，水平形变沿此断裂方向呈现条带状变化特征，而该区低Q值的状态反映了Q值分布与地壳大面积的垂直、水平形变及重力变化等存在一定联系。

Abstract:

In this study, seismic wave data of January 2009 to November 2015 are collected from China National Seismographic Network, and 16 156 Pg wave travel times from 1 820 seismic events (M_L=1.8~6.7 and epicentral distances less than 800 km) are selected to study the 3D Q-value distribution characteristics of P-wave in the southwestern Ordos margin. The results show that Q-values are relatively lower along the fault zones with "L" shape distribution in the southwestern Ordos margin, while on both sides of this zone, the Q-values are higher and vary greatly in their distribution. In the research area, the resistivity and velocity layers are significantly low in the Haiyuan and Tianshui—Lixian regions which have low Q-values, but the Weiyuan—Dingxi—Tongwei—Xiji region, which has higher Q-values, lacks a low resistivity layer. In addition, the Weihe Basin and Tianshui—Lixian region with a high heat flow have low Q-values, but surrounding areas have higher Q-values. There is a significant vertical deformation gradient belt and gravity change isoline along the Lintong—Chang’an—Fucheng—Pucheng fault. Moreover, the distribution of horizontal deformation along the fault is characterized by distinct stripped variations. The low Q-values in this area indicate that Q-value distribution is related to the vertical and horizontal deformations and gravity changes in the crust.