Abstract:

At 02:04a.m. on May 22th, 2021, a M_S7.4 earthquake struck Madoi County, Qinghai Province, China. The depth of this earthquake is 17km. The epicenter locates at 34.59° north latitude and 98.34° east longitude. It is another major earthquake occurring on a secondary fault within the Bayan Har block in the northern central Tibet Plateau during the past 30 years. Fast finite fault inversion and detailed focal mechanism inversion of the Madoi earthquake can help us better understand the seismogenic environment and its relationship with the faults and thus provide the scientific basis for post-earthquake emergency management and disaster assessment.

In this study, firstly, we use W-phase method to determine focal mechanism of the main shock within 30 minutes after the origin time. The W-phase solution indicates that the main shock is a high-dip strike-slip event and the estimated centriod depth is 11km, the strike/dip/rake of two nodal planes of the optimum double couple model are 102°/81°/11° and 194°/79°/171°. Secondly, as of June 10^th, the China Earthquake Networks Center has reported 57 aftershocks with magnitude larger than 3.0, the distribution of aftershocks indicates a mainly NWW direction. We obtained focal mechanisms of moderate aftershocks with M_S≥4.0 inverted from regional stations in Qinghai, Tibet, Sichuan and Gansu Provinces with the method of full waveform fitting, 12 out of 15 aftershocks are of strike-slip which is consistent with the background tectonics, and the existence of two thrust and one normal type events probably indicates that the rupture process of the main shock was affected by structure in the crust. Finally, combined with the geological background and solution of focal mechanism, we select the nodal plane with strike 102°/dip 81°/ rake 11° as the real fault plane. We use finite fault inversion method to invert the rupture process of Madoi earthquake with teleseismic waveform data. The source time function shows that the total scalar moment M₀ is 1.73×10²⁰N·m(or moment magnitude M_W7.45 ), which is consistent with the result of GCMT. The rupture process has lasted 45 seconds, the energy releasing was slow in the primary 5 seconds, the majority energy released during 10~30s after the main shock, then, the rupture was weakening and the fault was healing gradually. The slip and aftershock distribution of Madoi earthquake indicate an asymmetry bilateral rupture mode. The average rake is~3°, indicating a mainly left-lateral slip. The rupture area is estimated as 140km in length and 15km in depth, the slip distribution on SE and NW of epicenter shows obvious segmentation characteristics. The peak coseismic slip is estimated to be 400cm at 0~20km along strike in SE direction at shallow depth. The rupture of the earthquake did break through the ground surface which possibly causes seismic disaster. On the SE side of the main shock, the slip distribution shows a development into deep crust, while on the NW side, the slip distribution shows a more complicate mode. Over all, our results suggest that the Madoi main shock ruptured on a left-lateral strike-slip fault with high-dip along NWW direction in the Bayan Har block. The rupture length along strike is approximately 140km, slightly less than the length of aftershock distribution and field investigation due to the clear bifurcation geometry at both ends. Focal mechanism result of aftershocks shows that most of them are strike-slip but with variety in strike and dip, indicating the complex seismogenic environment in the fault zone. The slip distribution along strike and depth is highly heterogeneous, indicating that the rupture model has more complicated geometry in the lower crust than the shallow crust which controls the variability of slip distribution.

Key words: Madoi earthquake, source process, finite-fault inversion, focal mechanism

摘要：

对大地震的震源机制和震源破裂过程进行快速反演, 可为震后应急救援、 灾害评估和孕震机理研究提供科学依据。文中采用W-phase矩张量反演方法, 在震后快速反演了青海玛多7.4级地震的震源机制。 区域构造背景和震源机制结果表明, 此次地震断层面解的走向为102°、 倾角为81°、 滑动角为11°, 是一次高倾角的走滑型事件。文中利用区域台网的地震波形资料, 采用近震全波形拟合方法得到玛多7.4级地震15次M_S≥4.0余震的震源机制解, 包含12次走滑型地震、 2次逆冲型地震和1次正断型地震, 暗示主断层破裂受到局部地壳结构的影响。最后, 采用有限断层反演方法获得青海玛多7.4级地震的震源破裂过程模型。断层滑动模型结果表明: 震源破裂具有不对称的双侧破裂特征, 破裂持续时间近45s, 破裂长度约为140km, 地震主体破裂发生在0~15km深度范围内, 最大滑动量为400cm, 玛多地震的最大滑动量出现在近地表处, 推测可能存在地表破裂。综合上述结果及震源区的地质构造背景分析认为, 玛多地震的发震断层为巴颜喀拉板块内部的一条NWW向高倾角左旋走滑断层, 主震的滑动分布沿走向及深度方向表现出较强的不均匀性, 表明地震破裂面在深部比浅部具有更复杂的几何形态。

关键词: 玛多地震, 震源破裂过程, 有限断层反演, 震源机制解