关键词: 玛多地震, 震源破裂过程, 有限断层反演, 震源机制解

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