Liquefaction structures induced by the M5.7 earthquake on May 28, 2018 in Songyuan, Jilin Province, NE China and research implication

An earthquake of magnitude M5.7 occurred in Yamutu village, Songyuan City, Jilin Province, NE China (45°16′12″N/124°42′35″E) on May 28, 2018, with a focal depth of 13 km. The epicenter is located at the intersection of the Fuyu/Songyuan-Zhaodong Fault, Second Songhua River Fault and Fuyu North Fault which lies northwest of Tancheng-Lujiang Fault (Tan-Lu Fault). The earthquake-induced widespread liquefaction structures and ground surface fissures within 3 km from the epicenter, caused serious disasters to the local surroundings. The visible liquefied structures include sand volcanoes, liquefied sand mounds, sand dikes and sand sills. Sand volcanoes can be divided into sand volcano with a crater, sand volcano without a crater and water volcano (no sand). Other soft-sediment deformation structures (SSDS) induced by the earthquake include deformation lamination, load and flame structures, deformation folds, dish structures, convolute bedding and water-escape structures. The formation process of the sand volcanoes comprises three stages: (1) building up excess pore-fluid pressure in the liquefied layer, (2) cracking of the low-permeable overlying layer, and (3) mixture of sand-water venting out of the ground surface. During the upward movement, the liquefied sand is injected into the low-permeable layer to form sand veins, sand sills and various types of deformation structures. Vertical distribution of seismic liquefaction structure can be divided into four zones: the thoroughly liquefied zone, the lower liquefied zone with SSDS, the upper liquefied zone with SSDS, and the ground surface liquefied zone. The liquefaction occurred at a burial depth of 2–5 m, and the thickness of liquefied sand is 2 m. NE-SW (35°–215°) trending compressive stress is possibly the seismogenic trigger of the Songyuan M5.7 earthquake that caused the fault (Fuyu/Songyuan-Zhaodong Fault) to reactivate. The study of the Songyuan seismic liquefaction structures gives insight into the prediction of modern earthquakes and disaster-prone areas. Meanwhile it provides abundant basic material for studying earthquake-induced SSDS in both ancient and modern sediments. The research is obviously of great significance to reveal that the northern Tan-Lu Fault has entered a stage of active seismic activity since the twenty-first century.