The 2021 Maduo earthquake, also known as the 5.22 earthquake struck Madoi County in Qinghai Province, China on 22 May at 02:04 local time. The earthquake had a moment magnitude and surface-wave magnitude of 7.4. Highway bridges, roads and walls collapsed as a result of the earthquake. According to an anonymous source, at least 20 people were killed, 300 were injured, and 13 were missing. Officials stated that there were no deaths but 19 people sustained minor injuries. It was the strongest in China since 2008. It was assigned a maximum intensity of X in Machali, Maduo County on the China seismic intensity scale and Modified Mercalli intensity scale. This earthquake was preceded by another unrelated earthquake that occurred 5 hours earlier in Yunnan.
Tectonic setting
Western Sichuan is situated at the edge of the Tibetan Plateau in a vast zone of complex continental deformation caused by the collision of the Indian Plate with the Eurasian Plate. As the thrusting of the Indian Plate beneath the Eurasian Plate along the Himalayas continues, the continental crust within the Eurasian Plate is actively uplifted and thickened, forming the Tibetan Plateau. As there are no active thrust structures within the plateau, compression is accommodated by strike-slip motion along large structures including the Altyn Tagh Fault, Kunlun Fault, Haiyuan Fault and Xianshuihe fault system. Left-lateral strike-slip motion squeezes the crustal blocks of the Tibetan Plateau outwards, forcing it to move eastwards. Meanwhile, the strike-slip motion also results in east-west extension of the plateau, causing normal faults to break within the thickened crust.
One of these blocks is the Bayan Har block; bounded to the east by the Longmenshan Fault, Kunlun Fault to the north, and Xianshuihe fault system in the south.
Earthquake
According to the United States Geological Survey (USGS), the earthquake occurred as a result of dominantly left-lateral strike-slip faulting with a component of normal dip-slip. It had an estimated focal depth of 10–17 km. In a matter of days after the event, expert seismologists in China confirmed that the event occurred on the Kunlunshankou–Jiangcuo Fault; a left-lateral fault located within the Bayan Har block. This fault is located approximately 70–80 km south of the much larger Kunlun Fault.
Analysis of the USGS preliminary finite fault model of the slip distribution suggest the rupture was about 180 km long and the greatest slip was at 3.3 meters. A research paper soon to be published in Science Bulletin found that the largest surface displacements measured 2.4 meters, while the use of satellite imagery to model the earthquake rupture found a maximum slip of 6 meters near the hypocenter. In another earthquake model, up to 7 meters of coseismic slip occurred on the shallow 12 km of the rupture. Meanwhile, another study published in Geophysical Research Letters inferred a maximum slip of 9.3 meters at a depth of 7 km. Field analysis show the quake occurred on the Jiangcuo Fault. Parallel to the Jiangcuo Fault are the Maduo-Grande, South Gande, Dari, and Xizangdagou-Changmahe faults, which did not rupture. Based on inferring the locations of the aftershock zone, which extended for approximately 170 km, and the location of the mainshock epicenter, the event involved a bilateral rupture on the strike-slip fault. At the eastern and western termination of the earthquake rupture, a secondary segment also ruptured. The secondary rupture in the west measured longer than that of the east. Further analysis of the event by seismologists indicate it was a supershear earthquake.
Scientific reactions
Experts from the China Earthquake Networks Center (CENC) said that it is unlikely that another similar-sized earthquake would strike the same area again in the near future. They added that this earthquake likely occurred within the Bayan Har block, a piece of continental crust in the Tibetan Plateau. Its main boundaries are the Xianshuihe Fault to the south and the Kunlun Fault to the north. The largest earthquakes in the vicinity of the May 22 event are the 2001 Kunlun earthquake and the 1937 Tuosuo Lake earthquake, both measuring 7.8 on the moment magnitude scale and involving a rupture on the Kunlun Fault. The earthquake was unexpected and a surprise to seismologists who had previously thought the Bayan Har block was not capable of producing large earthquakes, and that its interior was tectonically stable.