A Study Of Seismic Amplification In Ground Surface Sedisments

Seismic amplification in ground surface sedisments refers to the amplification of seismic waves of specific frequencies due to the effects of physical properties of the media in near-surface geological structures. Different near-surface deposition has different seismic amplification effects. Some devastating disasters, such as Mexico Earthquake in 1985 and Taiwan Earthquake in 2002, verified that surface deposition is one of the main factor to affect the earthquake vibrations. China, a country suffers from frequent earthquakes, has serious earthquake casualties. Therefore, from the perspective of prevention and reconstruction work of earthquake, the study of quantitative relationship between the near-surface deposition of the seismic activity zone and seismic amplification becomes very important.With the support of the research projects funded by the Sichuan Science and Technology Department:"The surface deposition earthquake amplification effect text in the piedmont zone of Longman mountains" and "The influence study of WenChuan earthquake on the fault structure belt in the piedmont zone of Longman mountains", this thesis takes near-surface geological structure-strata physical property parameters-aftershock amplification effect as a breakthrough, makes use of comprehensive engineering geophysical exploration-geologic structure modeling-seismic amplification forward modelling method-seismic amplification effect base on the earthquake data as a technology roadmap, this thesis carry out research into the earthquake amplification effect characteristic (frequency, amplitude) in the surface deposition and explore correlation technique to analyze regional danger level, which, therefore, provides reference and support for post-earthquake reconstruction and earthquake prevention.(1) Through devising a set of high accuracy comprehensive engineering geophysical exploration composite pattern and choosing a higher SNR observation system and a group of high resolution exploration parameters, we can improve parameter information precisely. According to the Geological survey result, we improve transverse resolution by reducing group interval. With the purpose of curtailing maximum offset and seismic journey, reducing interference, improving SNR in reflected longitudinal wave seismic exploration, we adopt intermediate excitation, twelve traces for receiving, Six time coverage observation system. Longitudinal wave velocity which be obtained by refraction longitudinal wave exploration can be used to be a reference of velocity picking. The burial depth of gravel layer and bedrock which be obtained by longitudinal wave exploration and high-density resistivity method exploration can be used to correct profile of refraction longitudinal wave exploration and eliminate interference of reflected wave event in gravel-cobble stratum. According to the comprehensive analysis result, we get the thickness parameter of ground surface sedisments. Though surveying Shear wave velocity and density, we get wave velocity and density of ground surface sedisments. And these parameters are used to modeling.(2) We adopt a new kind of recursive algorithm to model process of SH waves in property parameter model and study of seismic amplification in ground surface sedisments. First, different ground surface sedisments (surface soil layer, gravel layer、strong weathering bedrock、moderately weathered rock、weakly weathered rock) and different parameter(density、shear wave velocity、thickness) have been used to modeling and simulatting, in order to analyze influence rule of seismic amplification. We find the density parameter has smaller influence for the seismic amplification with thin layer model or thick layer model. The thickness parameter has larger impact for the seismic amplification. And it has different effects on the thin layer model or thick layer model. The shear wave velocity parameter has larger for the seismic amplification. And it has a much higher impact on thin layer model than on thick layer model. Second, we summarise the change rule of biggest earthquake magnification and seismic amplification of ground surface sedisments. We find that seismic amplification has big change in root fan, and has steady state in the middle fan and fringe fan where have many frequency band having amplification. Thirdly, we summarize the distribution of the biggest earthquake magnification similar to this researched region (alluvial fan). We find that the biggest earthquake magnification in middle fan where has not be corroded by river is the highest, then the bigger earthquake magnification in channel sedimentary area, and the river bed has the lowest biggest earthquake magnification.(3) we devise a set handling method of earthquake data, and use this methods to study seismic amplification in ground surface sedisments. Base on the result of geological survey and comprehensive engineering geophysical exploration, we install mobile seismograph on the outcrop of bedrock and quaternary sediments to receive earthquake records. We should affirm the parameters(longitude and latitude、depth、 earthquake magnitude、time、toponym) of earthquake wave record,remove low frequency below 0.1 Hz, affirm the separation of longitudinal wave and shear wave, cut shear wave, obtain fourier amplitude spectrum by FFT, smooth curves with trigonometry, calculate observation point’s fourier amplitude spectrum earthquake magnification of EW component and NS component relative to reference point. Disposing earthquake data of twe observation points whose sediment thickness has significant differences, analyzing their characteristics, we find observation point having thicker sediments get lower dominant amplification frequency and frequency band having amplification. What’t more, in the quaternary sediments, every observation point’s earthquake magnification of NS component is bigger than that of EW component, the overlap ratio of frequency range having amplification is higher and the dominant amplification frequency is similar. It reflects that earthquake wave in the researched region may has directional polarization phenomenon.(4) Through the contrast between the result of earthquake data and recurrence algorithm modeling, we analyze the characteristic of seismic amplification in ground surface sedisments. The results show that the simulation result of SOI observation point mainly reflects the seismic amplification characteristic of NS component, overestimate the EW component amplitude amplification characteristic. And the simulation result of S06 observation point between NS component and EW component reflects preferably this observation point’s amplitude amplification characteristic. If we want to make simulation result become more close to the truth, we should reduce the uncertainty of model. As a result, it is feasible to use recurrence algorithm to model SH waves. We can accurately reflect observation point’s magnifying characteristic as long as we have accurate physical property parameter model.