Seismic fissures, landslides and mudflows are common secondary hazards of earthquakes. Following the occurrence of moderate-strong earthquakes, detailed investigations of landslides and mudflows are usually conducted, but earthquake-induced fissures often receive comparatively less attention, because the characteristics and causes of earthquake-induced fissures are controlled by a combination of factors. The 2023 Jishishan Ms6.2 Earthquake formed an extensive and dense earthquake-induced fissure zone in the Jishishan Mountain front and the interior of the Linxia Basin, but there are large differences in the understanding of the genesis mechanism of earthquake-induced fissures. In order to better understand the characteristics and genesis mechanisms of earthquake-induced fissures in the Jishishan Mountains, 246 fissures were systematically investigated by field observation, statistical analysis, and unmanned aerial vehicle photogrammetry, then the distribution, characteristics and genesis mechanism of the earthquake-induced fissures were analyzed. Combining them with relevant studies of previous researchers, the following new findings are made. Firstly, the 2023 Jishishan Ms6.2 earthquake formed a large number of fissures in the VII-VIII degree zone, which can be generally divided into two categories: tectonic-related fissures and non-tectonic fissures, and the non-tectonic fissures are further divided into ridge gravity fissures, landslide-related fissures, and valley fissure-sand liquefaction combinations on the basis of the topography associated with these fissures. Secondly, the tectonic-related fissures are developed along the eastern margin fault of the Jishishan Mountains, with fissure widths of cm-mm level. The tectonic-related fissures extend through fault planes, joints and other weak parts of the Jishishan fault zone, clearly showing that the over-100 m wide fracture zone (i.e., weak zone in the upper crust) of the eastern margin fault of the Jishishan Mountains dominates the distribution of earthquake-induced fissures. Meanwhile, the tectonic-related fissures show a tendency to develop towards weak parts in manmade structures, such as seams in road and joints in bridges, this “weakening-preferring” behaviour indicates that these fissures are generated by seismic shaking effects on fracture zone along the fault, rather than the direct rupturing of earthquake faulting, so the tectonic micro-fissures are not the surface ruptures mentioned before. Thirdly, the non-tectonic fissures are mainly developed in the loess-covered area around Dahejia-Kexinmin-Ganhetan villages north of the epicentre, the topography is hills and tableland-valley predominantly; ridge (tableland) gravity fissures are large in scale, far extending along the ridge, exhibiting a combination of reticulation, parallelism, or en echelon style; landslide-related fissures on the slope are comparatively smaller in scale, and their distribution are confined by the landslide area; valley fissures are comparable in scale to tectonic micro-fissures, and formed generally in association with sand liquefaction; the widths of fissures decreases from the dm scale on the ridges (tableland) to mm scale in the valley. Fourth, the tectonic-related micro-fissures have healed naturally within approximately half a year, while the healing time of the ridge gravity fissures is much longer than that of the tectonic micro-fissures; some landslide-related fissures may take a longtime to heal naturally and even form permanent displacements, which poses a serious threat to human beings and needs to be repaired. By reviewing and analysing the distribution, types, and genesis mechanisms of earthquake-induced fissures in the 2023 Jishishan Ms6.2 earthquake, and taking into account the magnitude, the depth of epicentre, and minor surface displacements, it is speculated that no surface ruptures were formed in the Jishishan Earthquake. An overview study of earthquake-induced fissures in the Jishishan Earthquake is of great significance for understanding the causal mechanisms of earthquake secondary disasters, analyzing seismotectonics, and proposing strategies for disaster mitigation.
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