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

摘要：

2021年5月22日2时4分, 青海省果洛藏族自治州玛多县发生了M_S7.4地震, 震中位于巴颜喀拉地块北部边界东昆仑断裂带以南约70km处(34.59°N, 98.34°E), 震源深度17km。此次玛多M_S7.4地震是2008年汶川M_S8.0地震之后中国大陆发生的震级最大的一次地震。震后的野外地质考察表明, 此次地震发生在距县城玛查里镇南约30km的野马滩地区。地震地表破裂具有明显的分段性, 初步可分为3、 4段, 自东向西破裂呈左阶展布, 逐步向野马滩盆地中部靠近, 断裂性质以左旋走滑为主。此次地震在国道214线以西—鄂陵湖以南形成规模较大、 连续性好的地表破裂, 长约45km, 走向 N95°~105°E, 地表破裂带由一系列挤压鼓包与右阶雁列式裂缝组成, 形成规模较大的地震鼓包(梁)、 地震裂缝、 左旋位移等地貌特征, 且发育大量喷砂冒水、 软土震陷等地震地质灾害。在214国道以东—血洛东, 破裂带的走向为N100°E, 由不连续的小规模张剪裂缝、 小型鼓包(梁)等组成。在昌马河乡血麻村附近发育一段长约10km, 走向N75°E, 规模较大、 连续性较好的张剪裂缝、 地震鼓包(梁)等。分析认为, 此次地震发生在巴颜喀拉块体内部, 发震构造为江错断裂。根据地震地表破裂的规模、 长度和建筑物破坏情况等综合调查认为, 狼玛加合日地区地表破裂规模大、 破裂带连续性较好且发育多类型的地震地表破裂, 可初步将该地区定为宏观震中, 地理坐标为(34.736°N, 97.794°E)。

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