The Dynamic Amplification Effect of A Site with Earth Fissures: A Case Study in the Taiyuan Basin, China

As a widespread geological hazard, the disaster development process of earth fissures is irreversible and difficult to control, which seriously affects the construction and safe operation of engineering facilities. However, few clear conclusions and special regulations have been given regarding the influence of earth fissures on the dynamic response characteristics of a site and earthquake prevention and disaster reduction measures. Therefore, the microtremor was used instead of earthquake motions to reveal the dynamic response of a site with fissures. The earth fissures in the Taiyuan Basin, which exhibit a large amount of activity, were used as examples. In order to reveal the dynamic response from several aspects, four methods, including the Fourier spectrum, the horizontal-to-vertical spectral ratio (HVSR), the response acceleration, and the Arias intensity, were employed. The results show that the spectrum peaks increase sharply at an earth fissure and return to a stable value approximately 20–25 m away from the fissure, indicating that the earth fissures have an amplification effect on the dynamic response of the site. Additionally, a greater amplification occurs on the hanging wall of the earth fissure. The influence range of the dynamic response of site can be divided into four areas. Suggestions on the seismic fortification intensity and setback distances were also proposed. The amplification mechanism was summarized as the coupling of the changes in the soil properties caused by earth fissure activity, the catadioptric effect of the earth fissure interface, and the multiple amplifications caused by secondary fissures.

A probabilistic method for mapping earth fissure hazards

Earth fissures caused by tectonic forces, human activities, or both seriously threaten the safety of people’s lives and properties. The Taiyuan Basin, a Cenozoic downfaulted basin located in the centre of the Fen-Wei Basin tectonic belt, in northwestern China, presents the ideal study area for a hazard assessment of earth fissures. A total of 104 earth fissures have been observed in the Taiyuan Basin, with a total length of approximately 128 km. In this paper, we proposed a probabilistic method for mapping earth fissure hazards by integrating the analytic hierarchy process (AHP), the area under the curve (AUC), and the certainty factor model (CFM). Geomorphic units, geologic formations, active faults and land subsidence zones of the Taiyuan Basin were mapped in detail. Correlations between these factors and earth fissures were evaluated through spatial modelling in ArcGIS. The AUC was introduced into the AHP to weight each factor and thus, to derive an earth fissure susceptibility map. Finally, the modelled earth fissure susceptibility was compared with a digital inventory of earth fissures to develop a probability function and map the spatial variability in failure probability through the CFM. The study indicates that active faults have the greatest contribution to the generation of earth fissures. Earth fissures are prone to develop in the piedmont alluvial-diluvial clinoplain and the transitional zone near the geomorphic boundary. This mapping procedure can assist in making rational decisions regarding urban planning and infrastructure development in areas susceptible to earth fissures.

Modelling multi-fissure zones above buried rock ridges in subsiding basins

<p>Aseismic earth fissures due to the excessive groundwater exploitation have caused seriously damage in many subsiding sedimentary basins worldwide. Generally, multiple fissures almost parallel to each other with equal distances are prone to develop where a compacting aquifer system overlies impermeable and/or incompressible ridges. Here, an advanced finite-element interface-elements modelling approach is employed to understand this process within unfaulted sedimentary sequences. A simplified geological setting is initially used to investigate the effect of the ridge slope on ruptures behaviors. Then, we reproduce the case of Guangming village, China. In both the proposed scenarios, the model simulates the occurrence of multi-fissures that initiate at land surface and propagate downward, as observed in the sites. The earth fissures are formed as a result of the combination of tensile stress (bending condition) and shear stress (shearing conditions) accumulation around and above the tip and the slopes of the ridge, respectively. The numerical outcomes indicate that the steeper ridge results in higher magnitude stress accumulation above the ridge tip which favors the formation of fissures with significant opening and small or null offset, but at expense of the reduction in stress accumulation area and fissure distribution. In Guangming case, the outcomes show that two ruptures started sliding and only one year later a central fissure opened and propagated down to 15-30 m depth. The simulated maximum opening and sliding of the central and side fissures, respectively, approximate 30 cm, which are almost in agreement with the observations. The numerical results prove that the proposed modeling approach is an effective way to predict and analyze multi-fissure onset and development in subsiding basins.</p>

A New Software to Model Earth Fissure Caused by Extensive Aquifer Exploitation and its Application to the Guangming Village Case, China

Earth fissures accompanying anthropogenic land subsidence due to excessive aquifer exploitation create significant geohazards in China. Numerical models represent a unique scientific approach to predict the generation and development of earth fissures. However, the common geomechanical simulators fail to reproduce fissure development because they cannot be effectively applied in discontinuous mechanics. An innovative modelling approach developed recently is applied to develop a software to simulate fissure development. The pressure changes are used as forcing factors in a 3D geomechanical model, which combines Finite Elements and Interface Elements to simulate the deformation of the continuous aquifer system and the generation and sliding/opening of earth fissures. The approach has been applied to simulate the earth fissures at Guangming Village in Wuxi, China with land subsidence of more than 1 m caused by the overexploitation of the second confined aquifer. The modelling results highlight that the earth fissures at Guangming Village have been caused by tension and shear stresses. Based on the developed modelling approach and the application case study, a software platform is developed to provide a fast preliminary evaluation of the risk of fissure occurrence associated to land subsidence. The software allows for the simulation of a simplified 2D conceptual geologic model of earth fissures, which can be used to investigate how the main factors controlling the geomechanical response of the aquifer system, such as pressure changes, geometry of aquifer system, geomechanical properties, and depth of bedrock/fault etc.

Geometrical and geotechnical characterization of the earth fissures appeared in the Guadalentín Valley (southeastern Spain) after the September 2012 flooding

Two earth fissures appeared in Murcia province (southeastern Spain) after the flood occurred because of a heavy downpour on 28 September 2012. In this area, located within the Guadalentín Valley, up to 212 L m−2 was reported that day. More than 200 agricultural exploitations were destroyed by the flooding, with a devastating effect on many infrastructures, such as a main A-7 highway bridge that collapsed in this event. The earth fissures appeared after this flooding in the towns of Puerto Lumbreras and Totana. The first fissure showed a straight-line direction approximately parallel to the main geological structures of the Guadalentín Valley. The total length of the fissure was 400 m and was 2 to 3 m in depth. The soil in the fissure is classified as a low-plasticity silt with some sand and clay, ML, according to the Unified Soil Classification System. From the sieve and hydrometer tests, the percentage of silt in these samples was between 48 % and 68 %, the clay content between 12 % and 30 % and the sand content between 2 % and 40 %. The plasticity index was smaller than 9.2 for all the samples. To evaluate the piping and internal erosion susceptibility of the soil, pinhole, crumb and geochemical tests were done on the collected samples. A result of non-dispersive soil was obtained from crumb and pinhole tests. Nevertheless, the pH, sodium adsorption ratio and exchangeable sodium percentage tests showed that some samples could be affected by the dispersion of the soil. Also, the collapsible potential of the soil was studied, showing negative results for all the samples except for that collected at the southern end of the fissure, which showed a medium to high potential. Concerning the Totana fissure, it appeared with different branches and holes instead of as a rectilinear pattern. The total length of the fissure was 190 m, with the soil characterized as a silty soil. Lastly, INSAR data, GPS, GPR and land subsidence maps were used to study the possible origin of these fissures.

Different disaster characteristics of earth fissures and their influence factors in the Beijing plain

To analyze the generation of different ground fissure disasters, two typical ground fissures were selected. With geological survey and exploration data, the spatial characteristics of the Songzhuang and Gaoliying fissures were investigated. The different occurrence factors for the Songzhuang and Gaoliying fissures were analyzed based on geological structure and groundwater. The conclusions contain are as follows. The affected body of the Songzhuang fissure exhibits obvious tensile deformation, and it is not contact with buried faults. The fracture-affected body of the Gaoliying fissure shows obvious vertical dislocation and shear, and this is compounded with buried faults. The distribution characteristic of the Songzhuang fissure was controlled by the tectonics and the normal fault, while the buried fault not only control the distribution feature of the Gaoliying fissure but also controlled its deformation characteristic. A buried fault is the geological background for the formation of the Gaoliying fissure. The long-term exploitation of groundwater has caused the horizontal deformation of the soil and the rigid rotation of stratum in the subsidence edge. Both of them are the reason for the tensile deformation. Due to the activities of buried faults and differential subsidence in small areas, the affected bodies of the Gaoliying fissure showed vertical dislocation and shear deformation.