关键词: 玛多7.4级地震, 巴颜喀拉块体, 地震地表破裂, 左旋走滑, 发震构造

Abstract:

At 02:04, May 22, 2021, an earthquake with M_S7.4 occurred in Maduo County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province. The epicenter of the earthquake is about 70km(34.59°N, 98.34°E)south of the east Kunlun fault zone on the northern boundary of the Bayan Har block, with a focal depth of 17km. The Maduo M_S7.4 earthquake is the largest in China after the 2008 Wenchuan M_S8.0 earthquake. As of 07:00 on June 12, 2021, 58 aftershocks of M≥3.0 had been recorded, including 0 earthquakes of M7.0~7.9, 0 earthquakes of M6.0~6.9, 1 earthquake of M5.0~5.9, 17 earthquakes of M4.0~4.9 and 40 earthquakes of M3.0~3.9.
Field geological surveys after the earthquake showed that the earthquake occurred in the Yematan area, which is more than 30 kilometers south of the county seat of Machali Town. The seismic surface rupture shows obvious segmentation, which can be initially divided into 3~4 segments. The rupture spreads from east to west in a left step, gradually approaching the middle of the Yematan Basin. The nature of the fault is mainly left-lateral strike-slip.
The earthquake produced a large-scale continuous surface rupture in the area from the west of National Highway 214 to the south of Eling Lake, with a length of about 45km and a strike of N95°~105°E. The surface rupture zone is composed of a series of compressional bulges and right-hand echelon fractures, forming large-scale seismic bulges(ridges), seismic fissures, left-lateral displacement and other geomorphic features, and producing the seismic geological disasters such as sand and water gushing, soft soil seismic subsidence and so on. From the east of National Highway 214 to the east of Xueluodong, the fracture zone strikes N100°E, which is composed of discontinuous, small-scale tension shear cracks and small-scale bulge(ridge). In the vicinity of Xuema village, Changmahe Township, a section of about 10km long, N75°E striking, large-scale tension shear fracture and seismic bulge(ridge) with good continuity is developed.
The earthquake caused left-handed displacement of geological bodies, water system gullies, roads, etc. and formed strike-slip scratches in the strata. Through measurement, the horizontal displacement of this rupture is 1.5m in the Langmajiaheri area, 1.3m in the area of Yematanshangtou, and 1.1m west of Xuema Village. There is an obvious vertical displacement of 1.4~0.8m near Yematanshangtou, and the vertical displacement of other sections is not obvious. Generally speaking, the horizontal displacement is greater than the vertical displacement, and the rupture is dominated by strike-slip.Based on the field geological survey results, it is considered that the seismic rupture of this earthquake is large in scale and has a good continuity at its both ends, while the rupture scale is small and the continuity is poor in the middle. The preliminary inversion results of seismic rupture process, InSAR processing results and small earthquake precise positioning results show that the Maduo earthquake is a bilateral rupture with a rupture length of about 170km. The field geological investigation results are basically consistent with the geophysical inversion results.
The Maduo M_S7.4 earthquake(the instrument epicenter is located at 34.59°N, 98.34°E)occurred inside the Bayan Har block on the south side of the main Arak Lake-Tosuo Lake section of the east Kunlun fault zone. Existing data show that a number of nearly parallel NW-trending strike-slip faults are developed around the earthquake sequence. According to previous studies and this geological survey, the seismogenic structure of this earthquake is determined to be the Jiangcuo Fault. According to a comprehensive survey of the scale and length of the earthquake surface rupture and the damage to the buildings, it is believed that surface rupture zone in the Langmajiaheri area is large in scale with good continuity and multi types of surface ruptures. The area can be preliminarily determined as the macro-epicenter. The geographic coordinates of the macro-epicenter are 34.736°N, 97.794°E, which is nearly 50km away from the micro-epicenter. The difference is mainly due to the sparse seismic stations and weak monitoring capability in the area.
The fact that the Maduo earthquake occurred inside the Bayan Har block on the south side of the east Kunlun main fault demonstrates the possibility of generating earthquakes with magnitude 7 or greater in the interior of this block. Therefore, the seismogenic conditions and mechanism of strong earthquake activity inside the Bayan Har block should be a scientific issue that needs more attention in the future.

Key words: Maduo M_S7.4 earthquake, Bayan Har block, earthquake surface rupture, sinistral strike-slip, seismogenic structure