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福建地区脉动资料的处理与分析
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摘要
脉动——这一随时随地都可以观测到的微小振动,因为其特有的性质而正受到越来越多人的关注。本文首先总结了国内外对脉动的研究的现状,然后对2005年5月1日至2006年8月12日期间福建省数字地震监测台网9个宽频带台站所记录到的脉动进行了研究分析,主要工作如下:
     1.利用福建省数字地震监测台网在2005年5月1日至2006年8月12日期间所记录到脉动资料,分析了福建地区脉动的均方根速度和卓越频率随时间和空间的变化规律,分析结果表明,9个宽频带台站脉动记录的均方根速度的变化表现出很好的一致性,沿海台站记录到的脉动的幅值要高于内陆台站的记录,在台风期间各个宽频带台站的均方根速度都有一个明显增大的过程;9个宽频带台站脉动记录的卓越频率一般在0.3Hz之间,而且各个宽频带台站脉动记录的卓越频率的变化也表现出很好的一致性;在台风期间,卓越频率都有一个明显的降低。最后本文研究了脉动均方根速度的变化与福建省内ML≥2.5和周边地区ML≥4.5的地震之间的对应关系,发现两者之间并无十分明显的对应关系。
     2.在台风期间,脉动的幅值明显增大,这种被加强的脉动信号也许能更好的反映出脉动的振动特性,为此,本文特意对2006年8月桑美台风期间福建省内9个宽频带台站的脉动记录进行了分析,结果表明:垂直向脉动与水平向脉动(东西向)之间有很好的相关性,相关系数可以达到甚至超过0.8,而且水平向脉动与垂直向脉动之间相位差为π/2,且垂直向脉动比水平向脉动超前;脉动在传播过程中,引起地表介质的质点作逆椭圆运动,而且椭圆上部质点的运动方向指向海洋一侧;在0.1-0.5Hz频带范围内,水平向脉动(东西向)与垂直向脉动的幅值谱谱比一般在0.5-0.8之间,这和理论上推导得到的自由表面上的瑞利波的幅值谱谱比(0.54-0.79)有一个很好的吻合,而且0.1-0.5Hz频带范围内谱比的均值一般在0.7左右,和理论上得到的泊松介质自由表面的瑞利波水平向与垂直向幅值比(0.68)很接近;因此,本文认为瑞利波是脉动的主要成分。声提取面波格林函数的方法,以5分钟为一间隔,计算25天内不同台站垂直向脉动记录之间相关系数并将其累加,通过累加结果估计了福建地区面波的传播速度,分析中发现,该方法得到的结果比较稳定,台风不会对该方法的结果产生明显的影响,计算得到的福建地区6个宽频带台站之间面波的传播速度在2.7-3.2Km/s之间,和实际观测得到的面波的传播速度比较接近,这说明该方法可用于对地下介质特性变化的近实时监测。
     4.为了搞清台风期间脉动的衰减特性,本文在Peter D.Bromirski等人的研究基础上,假设福建省数字地震监测台网所记录到的脉动主要是由于近海的某一片区域所产生的,,然后在这一假设的基础上,计算了桑美台风期间脉动的衰减规律,结果表明,脉动的振动幅值的衰减和瑞利波的衰减很相似,基本上也是按照1/r~(1/2)规律衰减,这从衰减特性上表明脉动的主要成分为瑞利波。
     5.根据前人所做的研究,本文假设脉动主要由洋面上海浪驻波产生的,海浪驻波高度的变化引起了海底压力的变化,进而产生了脉动。本文在这种假设的基础上,通过理论分析,得到了海浪有效波高与脉动均方根速度之间的关系,并利用福州台的脉动记录估计了福州近海海浪的有效波高,分析发现,理论上推导得到的海浪的有效波高和实际记录到的有效波高相差不大,两者处于同一个数量级,这就从另一个角度为脉动主要是由于海浪的活动引起的这一假设提供了有力的支持。
     最后,作者对全文的研究工作进行了总结,讨论了今后应该进行的研究工作。
At present, the microseisms are paid attention to more people because of its special characteristics despite it is tiny and can be observed everywhere at any moment. In this paper, the current research of the microseisms is briefly summarized and the microseisms recorded by the Seismic Monitoring Network of Fujian province from May 1,2005 to August 12,2006 is mainly researched.
     The main achievements of this study have been summarized as follows:
     1. The microseisms recorded by the Seismic Monitoring Network of Fujian province from May 1, 2005 to August 12, 2006 is utilized to analyse the law of the root-mean-square velocity and the predominate frequency changes with the time and the space. The result shows that the changes of the root-mean-square velocity from the wide-broadband velocity records is in of accord with each other. The microseisms' amplitudes recorded by the inshore station are larger than those recorded by the inland station and get large when the storm comes. The predominate frequency whose changes is in of accord with each other is between 0.2-0.4Hz and get down when the storm comes. Finally the paper studies on the connection between the change of the root-mean-square and the occur of the earthquake. It can be seen that there is no obvious relationship between them.
     2. The strengthen microseism signals, the amplitude of the microseisms are different from the daily noise remarkbly when the storm occurred maybe reflect the relationship of the microseisms characteristics. Therefore the microseism recorded by the Seismic Monitoring Network of Fujian province during August 2006 when the Saomai storm came is investigated. It indicate that there is corking correlativity between the horizontal microseisms and the vertical microseisms and the correlative coefficient can even exceed 0.8 sometimes. Furthermore, the vertical microseisms is anterior to the horizontal microseisms and the phase difference isπ/2. The particle on the surface of the earth removed as a counter-clockwise ellipse when the microseisms transmit. The ratio of the amplitude of the microseisms on the horizontal and on the vertical is about 0.7 in [0.1,0.5]Hz band. Therefore, we think the main component of microseisms is rayleigh waves.
     3. According to the method of extracting the Green's function from the correlation of the microseisms that professor Zheng Sihua in the Institute of Earthquake Science China Earthquake Administration has given us, using the results of the sum of the correlation coefficient between the microseism on the vertical channel that recorded from two site, to estimate the velocities of the surface wave in Fujian, we found the velocities of the surface wave is between in 2.7-3.2Km/s,which is close to the observation.
     4. In order to make the attenuation of the microseisms clearly when the the storm occurred, we use the conclusion of Peter D. Bromirski to estimate the attenuation of the microseisms. Firstly, we suppose the dominant source of the microseisms is near the east of china sea, not in the open ocean where the highest waves occurred. Then base on this suppose to make sure the distance of microseisms transferred and to calculate the attenuation of the microseisms. It is proved that the microseisms attenuate as 1/r~(1/2) and it is similar with the attenuation of the rayleigh waves.
     5. Based on the research of forefathers, we supposed that the nonlinear interaction of opposing wave components having nearly the same wave number results in a pressure excitation spectrum that propagates away from the sea surface at double the ocean wave frequency and couples into seismic waves at the ocean bottom, then we speculate connection between the ocean wave and the microseisms depending on the theory,. According to analysis, the ocean wave height determined from the microseisms recorded by the seismometer in Fuzhou was correspond with height of ocean wave recorded by buoys at Chongwu. It proves that the microseisms is mainly generated by the activity of the ocean wave.
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