More than 80 layers of seismites were recognized from the Early Cretaceous Dasheng Group in the Mazhan and Tancheng graben basins in the Tanlu Fault Zone, eastern China. The responsible seismic events took place about 110-100 Ma in the Early Cretaceous. The fault zone was affected at the time by strong tectonics, due to tension-related stretching and scattered squeezing by strike-slip faults. These tectonic activities induced a series of strong earthquakes with Richter magnitudes (M) of 5-8.5. The earthquakes affected saturated or semi-consolidated flood and lake sediments, and produced intra-layer deformations by several processes, including liquefaction, thixotropy, drop, faulting, cracking, filling and folding, which resulted in the formation of various soft-sediment deformation structures, such as dikes and veins of liquefied sand, liquefied breccias, liquefied homogeneous layers, load structures, flame structures, ball-and-pillow structures, boudinage, diapirs, fissure infillings, a giant conglomerate wedge, and syn-sedimentary faults. The seismites are new evidence of tectonic and seismic activities in the Tanlu Fault Zone during the Early Cretaceous; the series of strong seismic events that can be deduced from them must be considered as a response to the destruction of the North China Craton.
Three identical model boxes were made from transparent plexiglass and angle iron. Using the method of sinking water and according to the sedimentary rhythm of saturated calcium carbonate (lime‐mud) intercalated with cohesive soil, calcites with particle sizes diameters of ≤ 5 μm, 10–15 μm and 23–30 μm as well as cohesive soil were sunk alternatively in water of three boxes to build three test models, each of which has a specific size of calcite. Pore water pressure gauges were buried in lime‐mud layers at different depths in each model, and connected with a computer system to collect pore water pressures. By means of soil tests, physical property parameters and plasticity indices (Ip) were obtained for various grain‐sized saturated lime‐muds. The lime‐muds with Ip ranging from 6.3 to 8.5 (lower than 10) are similar to liquid saturated silt in the physical nature, indicating that saturated silt can be liquefied once induced by a strong earthquake. One model cart was pushed quickly along the length direction of the model so that its rigid wheels collided violently with the stone stair, thus generating an artificial earthquake with seismic wave magnitude greater than VI degree. When unidirectional cyclic seismic load of horizontal compression‐tension‐shear was imposed on the soil layers in the model, enough great pore water pressure has been accumulated within pores of lime‐mud, resulting in liquefaction of lime‐mud layers. Meanwhile, micro‐fractures formed in each soil layer provided channels for liquefaction dewatering, resulting in formation of macroscopic liquefaction deformation, such as liquefied lime‐mud volcanoes, liquefied diapir structures, vein‐like liquefied structures and liquefied curls, etc. Splendid liquefied lime‐mud eruption lasted for two to three hours, which is similar to the sand volcano eruption induced by strong earthquake. However, under the same artificial seismic conditions, development of macroscopic liquefied structures in three experimental models varied in shape, depth and quantity, indicating that excess pore water pressure ratios at initial liquefaction stage and complete liquefaction varied with depth. With size increasing of calcite particle in lime‐mud, liquefied depth and deformation extent increase accordingly. The simulation test verifies for the first time that strong earthquakes may cause violent liquefaction of saturated lime‐mud composed of micron‐size calcite particles, uncovering the puzzled issue whether seafloor lime‐mud can be liquefied under strong earthquake. This study not only provides the latest simulation data for explaining the earthquake‐induced liquefied deformations of saturated lime‐mud and seismic sedimentary events, but also is of great significance for analysis of foundation stability in marine engineering built on the soft calcium carbonate layers in neritic environment.
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