【作者】 高永新；

【导师】 胡恒山；

【作者基本信息】 哈尔滨工业大学 ， 固体力学， 2010， 博士

【摘要】 已有观测资料表明,地震发生时地震波经过的地方会产生电磁扰动,但目前对同震电磁信号产生的机理尚不清楚,双电层动电效应可能是引起这种震电耦合现象的一种机制。岩石孔隙固-液界面处存在着双电层,当地震波在岩石中传播时会引起孔隙流体相对骨架的运动,这种相对运动带动孔隙溶液中的净剩带电离子运动产生电磁场。本文基于双电层动电效应,研究(由断层滑动导致的)天然地震诱导的电磁场。当考虑的波长以及观测点到震源的距离远大于断层特征尺度时,将断层滑动视为一个双力偶点源,论文考察了双力偶点源在无限空间中辐射的震电波场。在基于Pride动电耦合方程组推导出了全空间单点力源的位移、电场和磁场的格林函数之后,进一步推导出了地震矩张量类型点源激发的位移、电场和磁场的解析表达式,获得了双力偶点源激发的震电波场的全空间解析解。计算出了天然地震频段双力偶点源脉冲激发的位移、电场和磁场的波形。在这些波形中,能观察到伴随P波的电场信号以及伴随S波的电场和磁场信号,并发现伴随S波的电场幅度小于伴随P波的电场幅度。作者证明了在天然地震频段,在孔隙地层(动力协调或接近动力协调的孔隙地层除外)中S波诱导电场的能力弱于P波。在模拟出的波形中,还观察到了震源激发的辐射电磁波,该电磁波速度比地震波高出至少一个数量级,几乎是在地震发生后“瞬间”就到达了观测点,但是其幅度比伴随地震波的电磁场小很多。研究还发现,震电波场与观测方位有关,在某些方位观测不到P波的位移信号,但是能观测到伴随P波的电场信号;在某些方位能观测到S波的位移信号,却观测不到伴随S波的磁场信号。文中还注意到了孔隙地层弹性参数之间的协调性对伴随P波电场的影响,从动电耦合方程组出发,证明了在弹性参数满足动力协调条件的孔隙地层中,P波不诱导电场。本文还考察了双力偶点震源在分层介质中激发的震电波场。基于动电耦合方程组导出了地震矩张量类型点源对应的位移-应力-电磁间断向量,将前人计算水平分层介质中点源激发震电波场的方法推广至地震矩张量类型点源,从而可模拟空间任意走向、倾向和滑动的断层所激发的震电波场。分别模拟了双力偶点源在半空间地质模型和多层水平分层地层中激发的震电波场。结果表明,由于自由表面的存在,P波和S波到达地表观测点时都会引起同震的电场和磁场扰动,瑞利波会引起电场和水平磁场扰动。研究发现,同震的电场和磁场与相应的地震波场具有明显的相关性,它们在波形上是相似的,但存在一定的相位差。研究还表明,同震电场的幅度与孔隙流体的矿化度和黏滞系数成反比,同震磁场的幅度与黏滞系数成反比。在模拟的波形中,还观察到了早于地震波到达的临界折射电磁波信号,地层中的地震波以及辐射电磁波入射到地表均可激发出此种临界折射电磁波,该电磁波的幅度与矿化度和黏滞系数成反比,比同震电磁场的幅度弱很多。模拟结果还表明从动电效应角度计算出的震电、震磁信号较强,具有可观测性。当考虑的波长以及观测点到震源的距离接近或小于小于断层特征尺度时,断层不能视为点源,此时将断层划分为若干个子断层,每个子断层均满足点源模型假设,整个断层滑动激发的波场由这些具有不同发震时刻的点源激发的波场叠加得到。本文分别对走滑型断层和倾滑型断层激发的震电波场进行了模拟。结果表明,断层滑动引起了显著的位移场、电场和磁场扰动,并且位移场、电场和磁场的扰动都具有方向性和上盘效应,即处于断层破裂传播前方区域上的电磁扰动幅度强于处于断层破裂传播相反方向区域上的电磁扰动,处于倾滑断层上方的接收点的位移场、电场和磁场的幅度强于下盘。文中还分别考察了覆盖层和断层埋藏深度对震电波场的影响。结果表明,地震波在覆盖层中发生了多次折射和反射,使得位移、电场和磁场的扰动幅度都得到了增强;断层的埋藏深度影响地表位移、电场和磁场的幅度,埋藏深度越浅,断层在地表引起的位移、电场和磁场扰动的幅度就越大。模拟结果还表明,无论对于走滑断层还是倾滑断层,在断层破裂终止和地震波扰动过后,断层附近不仅会产生永久位移,还存在残余电场,但不存在残余磁场,其中残余电场随着时间推移缓慢衰减。研究还表明,残余电场对接收深度很敏感,其中水平残余电场的幅度在自由表面处为零,且随深度增加而增大,竖直残余电场在自由表面处最大,且随深度增加而减小。在地表附近竖直电场扰动强度(包括同震的振荡幅度以及地震波过后的残余值)比水平电场高2个数量级。深度增加到一定值,竖直电场和水平电场处于同一数量级。论文基于双力偶点源模型和有限断层滑动模型的模拟研究揭示了因动电效应产生的同震电磁场的特点,对地震电磁观测提出了如下建议:电磁观测台站的布置应避开动力协调或接近动力协调的地区,并选择在流体矿化度和黏滞系数低的地层以便获得更强的同震电磁信号;在监测电场时应重视对其竖直分量的测量。更多还原

【Abstract】 Electromagnetic (EM) signals have been observed in earlier researches during earthquakes. These signals accompany seismic waves. However, the reason for the existence of such coseismic EM signals is unclear. The electrokinetic effect corresponding to the electric double layer is one of the mechanisms for the coupling between the seismic and EM fields. When a seismic wave propagates in a rock, a relative fluid-solid flow is induced. Such relative flow drives the excess ions in the pore fluid to cause EM field. In this paper, the EM fields generated by an earthquake due to the electrokinetic effect is studied. The earthquake is modeled by a fault slip.When the wavelength considered and the receiver-to-source distance are larger than the characteristic length of the fault, the fault can be taken as a double couple point source. The properties of the seismoelectromagnetic wave fields induced by a double couple source in an infinite porous medium is investigated. Based on the Pride’s equations coupling the seismic and EM waves, the author derive the Green’s functions of the solid displacement, the electric field and the magnetic field due to a single point source. Furthermore, these Green’s functions are extended to cater for the moment tensor sources. Then the analytical expressions of the solid displacement, the electric and magnetic fields in the frequency-space domain excited by a double couple source are derived. The waveforms of the displacement, the electric and magnetic fields due to a double couple source are then calculated at earthquake frequencies. In these waveforms, there is an electric field accompanying a P wave as well as electric and magnetic fields accompanying an S wave. It is found that the electric field accompanying the S wave is smaller than that accompanying the P wave. It is proved that the S wave has a weaker capacity than the P wave in inducing an electric field at earthquake frequencies in a porous medium which is not dynamically compatible or nearly dynamically compatible. In the waveforms, there is also an independently propagating EM wave, which has a much higher speed than the seismic waves, and reaches the observation point immediately after the source originates. However, it has a weaker amplitude than the EM field accompanying a seismic wave. It is also found that the seismoelectromagnetic field depends on the observation orientation. There are orientations, in which the displacement induced by the P wave is not seeable while the electric field accompanying the P wave is apparent. And there are orientations, in which the displacement induced by the S wave is apparent while the magnetic field accompanying the S wave is not observable. It is noticed that the compatibility of the elastic parameters of the porous medium affects the electric field accompanying the P wave. And on the basis of the seismoelectric coupling equations, it is proved that when the parameters satisfy the dynamically compatible condition, the P wave does not induce any electric field.The seismoelectromagnetic wave fields generated by a double couple in a horizontally-layered geological model are also studied. On the basis of the seismoelectric coupling equations, the stress-displacement-EM discontinuity vectors for the moment tensor sources are derived. And then the author extend the algorithm which is used by previous researchers to calculate the seismoelectromagnetic wave fields due to a point source in layered media to cater for moment tensor sources. Therefore, a simulation of the seismoelectromagnetic wave fields generated by a fault with arbitrary strike, dip and slip rake is allowable. As examples, the wave fields generated by a double couple source in a half-space and multi-layered formations are calculated, respectively. Due to the existence of the free surface, both the P and S waves cause coseismic electric and magnetic disturbances when they arrive at the receivers near the surface. There are also electric field and horizontal magnetic field accompanying a Rayleigh wave. It is found that the coseismic electric and magnetic fields have a close connection to the corresponding seismic fields. The waveforms of the coseismic electromagnetic fields and those of the seismic fields are similar, but of different phases. It is also found that the amplitude of the coseismic electric field decreases when either the salinity or the viscosity increase, and the coseismic magnetic field decreases when the viscosity increases. In the waveforms, there is also a kind of critically-refracted EM wave which reaches the receiving point earlier than the seismic wave. Such an EM wave can be converted from a seismic or EM wave incident on the free surface. Its amplitude is much smaller than the coseismic EM field, and decreases when either the salinity or the viscosity increases. The result shows that the seismoelectric and seismomagnetic signals due to the electrokinetic effect are measurable.When the wavelength considered and the receiver-to-source distance are close to or smaller than the characteristic length of the fault, the fault can not be taken as a point source. In this situation, the fault is discretized to a series of subfaults, each of which can be treated as a point source. The wave fields generated by the whole fault are a stacking of those generated by these subfaults. The displacements, the electric and magnetic fields generated by a strike-slip and a dip-slip fault are simulated, respectively. The result shows that the fault generates notable displacement and EM disturbances in the vicinity of the fault. The seismic and EM fields have clear directivity. The seismic, electric and magnetic fields in front of the fault rupturing direction are much larger than those in the opposite direction. For a dip-slip fault, the seismic and EM signals on the hanging wall are stronger than those on the foot wall. The influences of the cover layer and the source burial depth on the seismoelectromagnetic wave fields are investigated in the present paper. It is shown that when a cover layer is present, the seismic, electric and magnetic fields are amplified because of the multi-reflections and -refractions of the seismic waves in the cover layer. The source burial depth also exerts an impact on the amplitudes of the displacement, and the electromagnetic fields. Thinner burial depth makes stronger seismoelectromagnetic disturbance on the ground.The result also shows that for either a strike-slip fault or a dip-slip fault, there are permanent displacements and remnant electric fields but no remnant magnetic field near the fault after the fault rupture stops and the seismic wave propagates away. Such a remnant electric field damps with time and it is sensitive to the receiving depth. When the receiving depth increases the horizontal component of the remnant electric field increases, while the vertical component decreases. There is no horizontal remnant electric field right at the free surface. Both the coseismic oscillatory component and the postseismic decaying component of the vertical electric field are 2 orders of magnitude larger than those of the horizontal electric field. However, the horizontal and vertical electric fields become on the same order at a certain depth.The coseismic EM fields calculated from a point source model and a finite fault model have been investigated in this paper. Some suggestions are made for the measurement of the EM fields during earthquakes, i.e. avoiding the dynamically compatible or nearly dynamically compatible stratum, choosing a stratum with low fluid salinity and low fluid viscosity, and taking the vertical electric field into account.更多还原