Abstract:

The northeastern margin of the Qinghai-Tibet Plateau located near the middle and northern section of the north-south seismic belt in China is a place where historical seismicity is extremely high and many strong earthquakes of magnitude above 7 occurred, including the August 8^th 2017 M7.0 Jiuzhaigou earthquake and the May 22^th 2021 M7.4 Maduo earthquake. There are many active faults in this region, most of them are NWW trending, among which the Maqin-Maqu segment on the east verge of the East Kunlun Fault is especially dangerous because a seismic gap exists on it. The 2017 M7.0 Jiuzhaigou earthquake and the 2021 M7.4 Maduo earthquake which occurred near the seismic gap are still not able to remove the danger of a future strong earthquake along the Maqin-Maqu segment. The seismic activity of a certain region is closely related to the variation of regional stress field and the physical properties of underground medium. With the development of the broadband digital seismic networks, source parameters, attenuation characteristics of seismic waves and site responses can be deduced from three-component seismic signals of regional small and medium earthquakes. Study results on the current state of regional stress and underground medium properties can provide basic information for the establishment of strong ground motion attenuation model and the assessment of seismic hazard in the study area.
In this study, the geometric attenuation model, Q value, source parameters of 444 earthquakes and the site responses of 118 stations are obtained near the northeast margin of Qinghai-Tibet Plateau using the joint inversion method based on the three-component S-wave data of small and medium-sized earthquakes occurring during 2010 to 2019. The results shows that the geometric attenuation model in the study area conforms to a form of 3-segment piecewise function, while the inelastic attenuation model meets Q(f)=401.8×f^0.2963 based on this geometric attenuation model. Among the site responses of 118 stations, 88 site responses show characteristics of rock site, 20 site responses show amplification effect, particularly in the high frequency, and 10 site responses show the overall value below 1, which may be affected by the spatial anisotropy of velocity structure and Q value. Through the correlation study of local magnitude, moment magnitude, stress drop and apparent stress, it is found that the moment magnitude has a linear correlation with the local magnitude as a whole, and the local magnitude has a positive correlation with the earthquake stress drop and apparent stress under the same moment magnitude. There is a significant statistical correlation between stress drop and apparent stress and an obvious linear correlation in logarithmic coordinate system. Under the same moment magnitude, the ratio of the apparent stress to the stress drop is higher in the earthquake with lower local magnitude, which means the seismic rupture is more sufficient and the radiation energy is relatively small. While the ratio of apparent stress to stress drop is relatively low in the earthquake with higher local magnitude, indicating the energy loss during fault rupture is relatively small and the radiation energy is relatively high.

Key words: the northeast margin of Qinghai-Tibet Plateau, Q value, stress drop, apparent stress, attenuation model

摘要：

文中基于青藏高原东北缘2010—2019年中小地震三分量S波数据, 采用联合反演方法, 获得了研究区的地震波几何衰减模型、 Q值、 区内444次地震的震源参数以及118个台站的场地响应结果。研究显示, 青藏高原东北缘几何衰减符合三段式分段函数形式, 在该几何衰减模型的基础上, 非弹性衰减模型满足Q(f)=401.8×f^0.2963; 在研究区内的118个台站中, 88个台站场地响应符合基岩场地特征, 20个台站场地响应存在一定的放大效应, 且多数台站的放大效应表现在高频段, 另有10个台站的场地响应计算结果整体略<1, 可能受到速度结构和Q值空间各向异性的影响。通过地方震级、 矩震级、 应力降和视应力的相关性研究, 发现矩震级与地方震级整体呈线性相关, 在矩震级相同的情况下, 地方震级与地震应力降和视应力整体呈正相关。应力降和视应力间存在较为显著的统计相关性, 在对数坐标系下呈明显的线性相关关系。在矩震级相同的情况下, 地方震级较低的地震视应力与应力降的比值较高, 表明地震破裂较充分, 地震波辐射能相对较小; 地方震级较高的地震视应力与应力降的比值较低, 表明断层破裂过程中损耗的能量较小, 地震波辐射能相对较高。

关键词: 青藏高原东北缘, Q值, 应力降, 视应力衰减模型