【作者】 朱伟；

【导师】 倪四道；

【作者基本信息】 中国科学技术大学 ， 地球物理学， 2019， 硕士

【摘要】 近年来,研究表明超临界反射SmS震相会导致地面运动在一定震中距内增强,从而会改变GMPE(地面运动预测方程)模型在该震中距范围内的形态。鉴于此,本文首先回顾了地面运动衰减关系的发展过程,并且介绍了考虑SmS对GMPE影响的研究现状。而目前在国内,基于2008年汶川Mw7.9级地震的一些强地面运动研究表明,在震中距100到300 km范围内,水平分量峰值地面加速度(PGA)值显著增强。为了解释这一观测现象,本文通过观测汶川余震单个地震多个台站及单个台站多个地震的记录波形图,确认了该地区临界震中距内(约100～300 km)有大振幅SmS震相的存在,其振幅是直达S波的3～5倍。根据余震的地面运动衰减关系,本文发现相同震中距范围内水平分量平均峰值地面速度(PGV)和PGA值也有明显增大的趋势。为了进一步确认该地区大振幅SmS震相的存在及其对地面运动衰减关系的影响,本文利用当地的地壳速度模型进行理论测试,结果发现,在相同震中距范围内仍然可以观测到大振幅SmS,并且会使得地面运动幅值增大,这也说明了汶川主震在临界震中距内PGA值增大的原因。同时,根据盆地台与山区台的观测特征,本文发现SmS不一定会对地面运动产生显著影响。因此,本文还测试了SmS强弱的影响因素(地壳结构、震源参数和频率范围)。结果表明,地壳结构越简单越有利于SmS的发育;震源深度会改变SmS发生超临界反射的震中距范围,震源深度越大,SmS发生全反射的临界震中距越小;而震源机制解对SmS的影响较为复杂,没有明显规律,这可能与地震的辐射花样有关;SmS(1～2 Hz)是比直达S(约1 Hz)更加高频的震相。此外,本文还以龙门山地区为例,定量分析了SmS对地面运动的影响,结合汪素云的模型,采用线性拟合的方法,提出了符合汶川主震PGA衰减趋势的三段式衰减模型。综上所述,本文认为SmS对地面运动的影响不可忽视,尤其是对于地壳结构相对简单的地区。更多还原

【Abstract】 In recent years,studies have shown that the post-critical Moho reflections SmS will cause the ground motion to increase within a certain epicentral range,which will change the shape of the GMPE(Ground motion prediction equation)model within the epicentral range.In view of this phenomenon,we first review the development process of ground motion attenuation relationship,and introduce the research status of consider-ing the impact of SmS on GMPE.At present,in China,studies of strong ground motion of the 2008 great Mw 7.9 Wenchuan earthquake indicate that the horizontal compo-nent peak ground acceleration(PGA)is significantly enhanced within the fault distance range of 100 to 300 km.In order to explain this observed phenomenon,we confirm the existence of large-amplitude SmS within the critical epicentral region(about 100 to 300 km)in this area,with amplitude consistently 3 to 5 times stronger than direct S,through analysis of waveform recordings of Wenchuan aftershocks of single earth-quakes at multiple stations and multiple earthquakes at single stations.According to the ground motion attenuation relationship of aftershocks,it is found that the average peak ground velocity(PGV)and PGA values of horizontal components in the same distance range also increase significantly.In order to further confirm the existence of the large amplitude SmS in the area and its influence on the ground motion attenuation relation-ship,we use the local crustal velocity model to conduct theoretical tests.It is found that the post-critical SmS arrivals clearly enlarge the amplitude,which also explains the increased PGA values of the Wenchuan mainshock within the critical distance range.At the same time,according to the observation characteristics of basin and Plateau stations,we find that SmS may not have a significant impact on ground motion.There-fore,we also test the influencing factors of SmS strength(crust structure,source pa-rameters and frequency range).The results show that the simpler the crustal structure is more conducive to the development of SmS;the focal depth will change the epicentral distance range of post-critical SmS,the greater the focal depth,the smaller the epicen-tral distance of post-critical SmS;and the influence of focal mechanism solution is more complicated for SmS,there is no obvious law,which may be related to the radiation pat-tern of earthquake;SmS(1 to 2 Hz)is a higher frequency phase than direct S(about 1 Hz).In addition,we take an example of the Longmenshan area to quantify the influ-ence of SmS on the ground motion amplitudes,and modify an existing model to obtain a new hinged-trilinear attenuation model of this area,which provides a better fit to the recorded PGA amplitudes of the Wenchuan mainshock.In summary,we infer that the impact of SmS on ground motion cannot be ignored,especially for areas with relatively simple crustal structure.更多还原