Abstract:

On May 22, 2021, an M_S7.4 earthquake occurred in Maduo County, Guoluo Prefecture, Qinghai Province, which is the biggest earthquake in mainland China since the 2008 Wenchuan M_S8.0 earthquake. It occurred in the Bayan Har block in the northern part of the Qinghai-Tibet Plateau, indicating that the Bayan Har block is still the main area for strong earthquakes activity in mainland China. In order to study the source characteristics and seismogenic structure of the Maduo earthquake, we used the double-difference location method to analyze the spatial distribution of earthquake sequences within 15 days after the mainshock. At the same time, the focal mechanism solutions of 15 aftershocks with M_S≥4.0 are also obtained by full-waveform moment tensor inversion. We hope to provide seismological evidences with reference value for the study of the dynamic process of the Madao M_S7.4 earthquake and the geological tectonic activities on the northern side of the Bayan Hala block.

The results of moment tensor inversion show that the moment magnitude of the Maduo earthquake is about 7.24, the centroid depth is 13km, and the best double-couple solution is strike 283°, dip 59° and slip -4° for the nodal plane I, and strike 15°, dip 86° and slip -149° for the nodal plane Ⅱ, which indicates a strike-slip earthquake event. According to the strike of the fault and the distribution of aftershocks in the source area, we infer that the nodal plane I, which strikes NWW, is the seismogenic fault plane. The focal mechanism results of 15 aftershocks show that the aftershock sequence is mainly strike-slip type, which is consistent with the main shock. Meanwhile, there are also some other types reflecting the local complex structure. The differences in the direction and type of focal mechanism may reveal changes in the direction and characteristic of the fault from north to south. The azimuth of the P-axis is NE-SWW, and the azimuth of the T-axis is NNW-SSE. Both plunge angles are within 30° and close to horizontal, which shows that the activities of the Maduo earthquake sequence are mainly controlled by the horizontal compression stress field in the northeast-southwest direction. From NWW to SEE, the dip angle of fault plane increases gradually from 77° to 88°, and the northern segment dips to SW.

Based on the results of relocation, moment tensor inversion and geological structure, preliminary conclusion can be drawn that the seismogenic fault of the Maduo earthquake may be the Kunlun Mountain Pass-Jiangcu Fault, which is a left-handed strike-slip fault. At the same time, there are certain segmental differences along the fault. The strike of the northern section is mainly NW, that of the middle section is NWW, and the southern section is near E-W, and the fault plane dips to the southwest with the dip angle increasing gradually from NWW to SEE.

Key words: Maduo M_S7.4 earthquake, focal mechanism solutions, double-difference relocation, moment tensor inversion

摘要：

2021年5月22日青海果洛藏族自治州玛多县发生7.4级地震, 这是继汶川地震后中国大陆发生的震级最大的一次地震。 为了深入研究玛多7.4级地震序列的时空分布、 震源特性以及发震构造等内容, 文中采用双差定位法对玛多地震序列进行了重新定位, 并采用近震全波形矩张量反演方法反演了玛多7.4级主震和15次M≥4余震的震源机制解, 结果显示: 玛多7.4级地震是一次走滑型地震事件, 矩心深度约为13km; 余震序列以走滑型为主, 也存在部分其他类型, 反映局部存在复杂的构造形态; 余震序列整体呈NWW向线性分布, 长度约150km; 由NWW端向SEE端, 序列的深度逐渐变浅, 断层面的倾角逐渐增大; 序列震源机制的走向和类型存在段落差异性, 可能揭示断层的走向和性质从北向南存在变化。 综合精定位和矩张量反演结果推测, 玛多地震的发震断层为走滑性质, 倾向SW, 走向和断层面倾角具有段落差异性。

关键词: 玛多7.4级地震, 震源机制解, 双差定位, 矩张量反演