Active Fault Setback Research On Engineering Sites

In modern seismology, strong earthquakes are closely related to active faults, i.e.,strong earthquakes always happen with slipping of active faults. Slipping of active faults, under some conditions, generates surface rupture which can damage the structures in the rupturing zones. Therefore, to avoid active faults during engineering sites selection is a key issue in geotechnical engineering and seismic design. Series of research on evaluating the setback distance becomes an interesting topic in engineering and scientific societies.Analyzing the surface-rupturing site characteristics on active faults, the relationships of active faults and surface rupture characteristics are summarized in this thesis. Through analysis of rupturing characteristics(e.g., rupturing width), analysis of the influence of surface rupture on structural damage, and analysis of surface rupture and ground motion, a method and responding procedures for evaluating setback distance for engineering sites is proposed by means of case studies and theoretical simulations.The main work can be outlined as following.1. Analysis of the relationship of active faults with earthquakes, and the relationships of surface-fault-rupture with earthquakesThe procedures of evaluating engineering active faults under the conditions of surface rupturing are defined. The surface rupture definition and the rupturing conditions are clearly demonstrated. The critical database of setback distance for engineering structures to surface rupture, i.e., situ repeatability of surface-fault-rupture cased by strong earthquakes, are established, which have been demonstrated through typical cases of characteristic earthquakes and surface rupture segments. A technique for precisely locating surface rupture has been systematically presented.2. Characteristic analysis of surface-fault-rupture(rupture width)Summarizing surface rupturing characteristics, horizontal plane characteristics, and sectional profile characteristics, the mechanism of rupturing is discussed and the modes of rupturing is systematically outlined as well, providing fundamental data for numerically simulating surface-fault-rupture. The influence factors on surface rupturing have been investigated, through statistical analysis of the widths of surface-fault-rupture and fault permanent displacement bands, a method of calculating the rupturing width is proposed, considering rupturing modes and the influencing factors.3. Analyzing the relationship of surface rupture and engineering structure damage,i.e., the setback distance of engineering structures to surface-fault-ruptureAnalyzing the structural damage on the surface rupture traces in 5 typical earthquakes(normal, reverse and strike-slip), a setback distance to rupture is proposed.Meanwhile, through damage analysis of individual structures on the surface-fault-rupture traces in the 5 selected earthquakes, a structure-damage databasecaused by similar disaster of surface rupture is established for the first time, which provides guideline for structural design and construction of new buildings. Meanwhile,the database can provide basic data for numerically simulating structural damage caused by surface-fault-rupture.4. Numerical simulation of surface-fault-rupture and ground motionA calculation model and relevant parameters for theoretical simulation are presented, typically in the cases of thin overburden layers and soil-gravel medium, and the influence of surface rupturing repeatability(i.e., re-rupturing during seismic recurrence periods) on ground motion is discussed. Selecting near-field pulse-type and ordinary acceleration time histories, the peak acceleration at different spots of dip-slip faults and strike-slip faults are analyzed under peak acceleration inputs of 0.1g, 0.2g and0.4g. Comparing the response spectra, the reasonable setback distance and the seismic design response parameters near the surface rupture zones are proposed for the first time as well as the relevant explanations.5. Proposal of an operational and applicable design method for calculating setback distanceThrough deterministic analysis of the factors as surface rupturing width, rupturing length, and probabilistic analysis of surface rupturing, and structural collapse density function at different distance to the rupture, a systematic method and its procedures which can bound the rupturing zones or setback zones are given considering the precision results of locating the rupture. By means of statistical equations and diagram of rupturing width, the recommended values of structural setback distance as well the hazard effect near rupture of individual structures, and the analysis of seismic response of peak acceleration and response spectra under input of pulse-type seismic wave,reasonable values of structural setback distance are proposed. The current setback distance evaluationmethods used in China and overseas are systematically outlined herein, which can supplement the regulations for structural setback distance in China.