New revelation from seismic subsidence of soft soils and earthquake-induced soil-layer deformation structures in Anqiu area, Shandong Province

By means of seism investigation, soil test and ¹⁴C dating, disaster phenomena about seismic subsidence of soft soils and some earthquake-induced association structures of soil layers are identified in soft soils of limnetic facies during the Mid-late Holocene in the Anqiu area of Yishu fault-seismic zone, including subsidence synform folds of soft soils, small scale co-seismic faults, pinch-and-swell structures and seismic fissures, in which subsidence synforms are vertical deformations that directly reflect the seismic subsidence disaster. Small scale co-seismic faults are some little faults caused by earthquakes, with vertical lengths of 0.5~1.8 m under the subsidence synform. A subsidence synform comes at first along with producing of co-seismic faults and developes downward treading on the heels of these faults extending to the even deeper place. The earthquake vibration causes the thixotropic deformation of saturated soft soils, and soil particles flow from the hinge zone to two wings of the synform, in which the seismic additional stresses act continually, resulting in formation of the narrow and deep-seated subsidence synform. The deepest seismic subsidence of soft soils is 1.08 m, and the average depth is 0.68 m. It is estimated that the seismic intensity is VIII degree, and the earthquake magnitude is over M6.0. The formation age of the mucky soils for 3901±33a B.P. is obtained by the¹⁴C dating, so the seismic event occurred in about 19th Century B.C.. The co-seismic fault is an important factor for forming the seismic subsidence of soft soils, which provides the new information for understanding the formation mechanism of the seismic subsidence of soft soils and the new revelation for preventing or mitigating the harm of the seismic subsidence. Therefore, attention should be paid to possible hidden troubles of co-seismic faults to be produced in the underlying layer of soft soils.