【摘要】 本文利用球对称、非自转、弹性和各向同性地球模型(SNREI)理论模拟计算了地球内核平动振荡的地震激发.以2004年12月26日苏门答腊大地震为例,讨论震源机制解(标量地震矩、走向、倾角、滑动角和深度)对内核平动振荡振幅激发的影响;基于全球21个Mw8.0级以上的大地震,分别计算10个数据资料较好的超导重力台站理论上可以接收到的内核平动振荡信号的频率域振幅.结果表明标量地震矩对内核平动振荡振幅的影响最大,走向、倾角、滑动角和深度对内核平动振荡振幅也有一定影响,但是影响相对较小;不同区域获得的由大地震引起的内核平动振荡信号的幅度存在显著差异,此结果为频率域多台站加权迭积提供了计算基础.另外,只有武汉台站接收到的2011年日本Tohoku Mw9.1地震激发的内核平动振荡的振幅值达到了地球表面高精度、高灵敏度的超导重力仪检测水平,振幅值为0.0103nm·s-2.结果说明地震激发的内核平动振荡的信号极其微弱,信号几乎淹没在背景噪声中,必须利用多台站迭积法才有可能将信号提取出来.更多还原

【Abstract】 The translational oscillation of the solid inner core,which is also called Slichter mode(SM),has an eigenperiod of several hours that is closely related to the Earth′s interior structure,and is one of the fundamental normal modes of the Earth.The SM may be excited by large deep earthquakes,asymmetric crystallization of the fluid material in the outer core,or other unknown excitation sources.The excitation mechanism of them is still unclear.It is supposed that a large earthquake can excite the translational oscillation of the inner core,and amplitudes excited by large earthquakes and received by different stations are calculated in order to verify whether the superconducting gravimeter(SG)can detect such weak signal.Based on the spherical,non-rotating,elastic and isotropic(SNREI)Earth model,the earthquake excitation of translational oscillation in the Earth′s inner core was calculated.Taking the 2004-12-26 Sumatra earthquake as an example,the influence of inner core translational oscillation′s earthquake excitation caused by the focal mechanism solution(scalar seismic moment,strike,dip,slip and depth)was analyzed.Based on 21 earthquakes the magnitude of which were larger than Mw8.0since 1980,10 superconducting gravimeters stations with high-quality observations were chosen,and amplitudes of the inner core translational oscillation received by those stations were calculated theoretically.In the reference focal mechanism solutions,the hypocenter was equivalent to a double couple point source in the focal mechanism model.Six independent seismic moment tensors were determined based on the theory of double couple centroid moment tensors.Based on the Preliminary Reference Earth Model(PREM)combined with the focal mechanism solution,moment tensors of the Green′s function were determined by the weighted sum,and free oscillation displacements were obtained that simulate measured free oscillation signals.It is found that the scalar seismic moment M0 has a major influence on the amplitudes of inner core translational oscillation;additionally,the strike,dip,rake and depth of the hypocenter have minor influences.When one of the M0,strike,dip,rake and depth changes while other keep constant,the difference of amplitudes is 0.223,0.095,0.081,0.139 and 0.187nm·s-2·Hz-1,respectively.And it is indicated that the amplitudes of the inner core translational oscillation in different areas excited by great earthquakes have significant differences,and the results are the basis of weighting stacking the SGs′observation in the frequency domain to detect the signal.In addition,only the Wuhan station received the amplitude excited by 2010 Tohoku Mw9.1earthquake that reached the observational level of the high accuracy and sensitivity superconducting gravimeter with maximum amplitude 0.0103nm·s-2.It implies that the signal is extremely weak and it may be buried in the background noise.It is a good choice that increases the weight of the observations with larger amplitudes after large earthquakes.Weighting stack of the observations to suppress the noise and enlarging the signal should be used to detect the signal.In fact,owing to the ellipticity and rotation of the Earth,the SM splits to form a triplet,including the movement at the Earth′s rotation axis and the equatorial prograde and retrograde translational movements.The earthquake excitation of the triplet based on a rotating,slightly elliptical Earth model would be calculated during the following work.According to the calculation results in this paper,the amplitudes excited by earthquakes are very small,and other possible excitation mechanisms will be researched in the future.更多还原