The Amojiang fault zone and Mojiang M5.9 earthquake in 2018 in southern Yunnan Province

Field investigation shows that fault mouths are developed on the planation surface Ⅲ and river terrace 4 in the Amojiang fault zone, and below them there is no indication of fault landforms.Structural rocks dated by SEM are Early Pleistocene in age and are cemented compactly or half consolidated, and a few fault gouges dated by ESR as 549±54ka and developed on the fault plains and Middle-Upper Pleistocene deposits covering the faults have not been displaced or deformed tectonically. All these phenomena suggest that the fault zone was mainly active in Early-Middle Pleistocene. The 2018 Mojaing M5.9 earthquake occurred on the western branch of the Amojiang fault zone, and the long azimuth of isoseismal lines of the quake was in NW direction, consistent with the strike of the fault, suggesting that the seismogenic structure of the quake should be the western branch of the fault zone. Despite of no signs of activeness in Late Quaternary along the fault zone, the Amojiang fault zone itself became a broad relative weak belt. Affected by the SE extrusion of the Tibetan Plateau matters, the Simao block where the epicenter is located moved southward. Although the Amojiang fault zone was in absence of evident ground activity during Late Quaternary, the zone itself formed broad weak structural belt. Under the circumstance of the NS-trending tectonic compression stresses, stresses concentration in local area could also give rise to new break and induce the middle-large earthquakes like the Amojiang M5.9 earthquake.Regionally, there are similar fault zones in structural conditions, such as the Mile-Shizong fault zone in southeastern Yunnan, along which there historically occurred 11 earthquakes of magnitude over M5.0. Therefore, the researchers should pay attention to the regional large faults which were not sub-surficially active in Late Quaternary, but still have potential seismic risk in the regional seismic risk assessment.