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Abstract:
The latest co-seismic deformation and its long-term history since the late Pleistocene of a seismogenic fault are basic data for constraining its deforming behavior during strong earthquakes. They are also key data for modeling the recurrence of surface-rupturing events and assessing seismic risks. They are of great significance for understanding regional tectonics and dynamics. The 2021 Madoi M7.4 earthquake provides an opportunity to study active tectonics and related strong earthquakes in the Bayan Har block of the Tibetan Plateau. In this paper, we conducted photogrammetry using a DJI M300RTK drone with a P1 camera in 5 sites. We obtained ortho-rectified images and Digital Elevation Models (DEM) with centimeter-level resolution and accuracy. We mapped and measured co-seismic slips of typical ruptures and accumulated displacements of offset landforms since late Quaternary from these aerial images and DEMs. According to the rupture patterns, distribution of co-seismic slips, and rupture geometry, the co-seismic rupture zone was divided into four sections named Langmajiaheri section, Yematan section, Lamucao-Donghu section, and Langma'oer-Changmahe section, respectively. This strong earthquake was a result of cascaded rupturing of these four sections. At the site Langmajiaheri along the Langmajiaheri section the rupture was characterized by left-lateral strike slip with slight normal slip of 0.61±0.03 m. The Yematan section slipped in a manner of left-lateral strike slip with reverse slip. The dip slip was 0.31±0.04 m and the strike slip was 2.83±0.13 m at the site west of Yematan. The left-lateral slip was 1.97±0.08 m at the site west of Jiangcuo almost at the east end of the Yematan section.The co-seismic left-lateral slip reached 3.55±0.24 m at the site Langma'oer and 0.5~1.2 m on the Changmahe riverbed and terraces along the Langma'oer-Changmahe section. Accumulated offsets of late Pleistocene or Holocene landforms were observed at three sites along the western sections. The Jiangcuo fault may control the boundary development between mountains and basins to the west of Huanghe town since the late Pleistocene. Based on the accumulated displacements and the age estimations of terraces, we obtained a recurrence interval of 2100~2500 years for strong earthquakes similar to the 2021 Madoi event. The left-lateral slip rate is about 1.1~1.3 mm·a-1 at the site west of Yematan. However, no accumulated displacement was recorded to the east of the Gebolonggeqia stepover except the co-seismic deformation during the 2021 Madoi M7.4 earthquake. We infer that the Jiangcuo fault experienced an eastward extending from Gebolonggeqia to Changmahe during this earthquake. As a synthesis of surficial fault geometry, deep crustal structures and tectonic dynamics, we proposed a scheme to divide the Bayan Har block into two sub-blocks according to the belt from the A'nyêmaqên restraining bend of the Kunlun fault zone to the Garze pull-apart of the Yushu-Garze-Xianshuihe fault zone. There exists about 2 mm·a-1 of contraction across the boundary between these two sub-blocks. This boundary may be developing in an early stage of destruction of the Bayan Har block, which is still expecting more studies on its dynamics and deep structure. The surface rupture of the 2021 Madoi earthquake gave evidences that the Jiangcuo fault has cut the previously named Madoi-Gande fault. There are paralleled faults developed in the western sub-block of the Baya Har block, such as the Xizangdagou-Changmahe fault, the Madoi fault, the Gandenan fault, the Dari fault and the Kunlun pass-Jiangcuo fault. More quantitative kinematic data and paleo-earthquake study are expected for assessing their seismic risks.
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