体波走时层析成像方法及其在中国壳幔结构研究中的应用评述

文中对地震体波走时层析成像技术近30年的发展进行回顾和评述,并对该方法所存在的问题进行了分析和探讨以及给出应对策略。在壳幔速度结构的横向不均匀性、强震发生的深部构造环境、活火山区的深部结构和起源,以及造山带、板块碰撞带区域深部结构等4个方面对应用地震层析成像技术所取得的主要成果做了分类总结和探讨。研究证明,利用地震层析成像技术所获得的高分辨率地球内部结构为探索岩石圈的演化和板块运动规律及地震、火山活动发生的深部构造环境等提供了十分重要的科学依据。     

体波谱振幅相关系数法在肥东地震序列震后判定中的应用

地震直达体波记录的观测谱包含震源、传播路径和台站信息,而发生在同一震源区的不同地震,如:序列地震,在同一地震台站的记录,受传播路径影响基本相同,地震台站的场地响应、仪器响应也完全相同。因此,序列地震体波观测零频谱的相关系数直接反映震源辐射图型因子的相似性,即震源机制解的相似性。将该方法用于2009年安徽肥东地震序列,与传统方法计算的震源机制解作比较,结果显示,谱振幅相关系数法能反映序列地震震源机制解的变化过程。     

comprehensive study on the changing process of focal mechanisms of the 2014 huoshan ms4.3 earthquake sequence

In this paper, the focal mechanisms of Huoshan M_S4.3 earthquake sequence since October 2013 are calculated with the method developed recently by Snoke, which combines the use of first motion of P, SV and SH waves and their amplitude ratios(referred to as Snoke method). We estimate the difference(also referred to consistency parameter θ)between the stress axis direction of focal mechanism solution and the mean stress tensor on one hand, and get the relative changes of focal mechanisms rather than the focal mechanisms themselves on the other hand, by using methods of correlation coefficient of body-wave spectral amplitudes, first motions and amplitude ratios of vertical P wave of single station, as limited by the network layout and density of observation points, focal mechanism solutions of small earthquakes are not easy to obtain. We studied the changing process of focal mechanisms of Huoshan M_S4.3 earthquake sequence since October 2013 with many methods such as consistency parameter, correlation coefficient of body-wave spectral amplitudes, first motion and amplitude ratio of vertical P wave of single station, to determine the type of the sequence. The result shows that: before Huoshan M_S4.3 earthquake of April 20, 2014, the consistency parameter of the earthquake sequence was lower than the mean for a long period, correlation coefficient of body-wave spectral amplitudes was always at high values(about 0.9), odds ratio of first motions of vertical P wave of single station near the epicenter, defined as the ratio of predominant number to contrast number of first motions, was up to 13.8 at station SJH, 3.8 at BZY and 3.6 at LNA, respectively; first motions of vertical P wave of single station showed an obvious predominant distribution as a whole, and the amplitude ratios of vertical P were consistent, which suggests that before Huoshan M_S4.3 earthquake, focal mechanisms were consistent, so the earthquake sequence before Huoshan M_S4.3 earthquake is the foreshock sequence. After Huoshan M_S4.3 earthquake, correlation coefficient of body-wave spectral amplitudes was lower(about 0.6), first motions and amplitude ratios of vertical P wave of single station were in disorder, which suggest that the focal mechanisms are inconsistent after the Huoshan M_S4.3 earthquake, so the earthquake sequence after Huoshan M_S4.3 earthquake is the aftershock sequence, and subsequent larger earthquakes are not likely to occur. The focal mechanisms in Huoshan region during the period of Huoshan M_S4.3 earthquake experienced a process from scattering to convergency and to scattering again.     

Station threshold bias in short-period amplitude distance and station terms used to compute body-wave magnitudes mb

A cause of bias in estimates of distance and station terms used to compute seismic magnitudes ( m _b) is demonstrated in a comparative study on the world amplitude–distance curve. This bias results from the presence of station thresholds in the measurement and reporting of seismic amplitudes and is analogous to that known to be present in routinely computed magnitudes. Distance terms corresponding to low amplitudes such as in the core shadow will tend to be positively biased as are station terms for stations with high thresholds.     

多震相在近震体波层析成像中的应用

近震体波层析成像方法利用地震波走时来反演地下结构,这需要观测到时准确。在实际情况中,观测到时或多或少包含一定的误差,包括人为误差、仪器误差（如钟差）等。本文提出了一种新的近震体波层析成像方法,来消除随时间积累的误差对反演结果的影响,并验证了该方法的正确性和可行性。