【摘要】 甚长周期VP型垂直摆倾斜仪对某些地球物理信号有特殊的敏感性,除了在观测小地震、慢地震等方面有优势之外,还可有效应用于低频地球自由振荡信号的探测.利用安置在武汉大学珞珈山的我国自行研制的VP型垂直摆倾斜仪在2011日本Mw9.0级大地震之后不同长度的观测数据,联合EEMD方法、自回归估计(AR)方法和bootstrap法,本文不仅检测到该地震激发的频率小于4.7 mHz的零阶球型振荡(0S2至0S38)和环型振荡(0T4至0T35)几乎所有振型以及15个谐频振型,还检测到5个其频率低于1mHz的低阶球型多线态(0S2、2S1、0S3、0S4和1S2)的部分或全部谱峰分裂现象,并给出了所有检测结果的精度评估.此外,本文分析了某些球型和环型振荡之间的耦合效应,结果表明耦合效应将显著影响地球自由振荡信号的相关参数.更多还原

【Abstract】 Very-long-period Vertical Pendulum Tiltmeter(VPT)can directly detect ground tilt,and has a special sensitivity to some geophysical signals.Besides the advantage of observing small earthquakes,slow earthquakes,etc.,VPT can be also effectively used to detect low-frequency Earth′s free oscillation signals.Using the VTP observations with different lengths after the 2011 Japan Mw9.0event,the low-frequency free oscillation of the Earth at multiple frequencies is analyzed.For the ground tilt records with high noise level and finite amplitude,the ensemble empirical mode decomposition(EEMD)is applied as a dyadic filter bank to enhance the signal-to-noiseratio.The estimation of the center frequencies and quality factors is based on the autoregressive(AR)method proposed by Chao and Gilbert.Moreover,it is widely accepted that the observed results have hardly meaning if the corresponding errors are not given.So the bootstrap method is jointly applied to estimate the errors of observations.Using the three effective data processing approaches,almost all of the spheroidal oscillation modes(0S5to0S38)and toroidal oscillation modes(0T4to0T35)with their frequencies lower than4.7mHz and 15 harmonic modes(including one first-order mode,six two-order modes and eight three-order modes)excited by the earthquake were observed.Parts or all of the spectral splittings of five low-order spheroidal triplets(0S2,2S1,0S3,0S4and1S2)with their frequencies lower than1 mHz were also detected.The corresponding accuracy estimates for all of the observation results are provided.Moreover,we analyze the spectral coupling of some spheroidal oscillation and toroidal oscillation.The results show that the spectral coupling has significant impact on relevant parameters of the Earth′s free oscillation.The results can help us better understanding some geodynamical processes to induce the real coupling effect.We emphasize that it does not mean VTP cannot record the spectra splitting of intrasonic elastic normal modes despite we cannot extract these signals using the direct FFT analysis.Some effective methods can distinguish the weak signals from the mixed strong background noises.In addition,the ground tilt responding to the Earth free oscillations can indirectly reflect the coseismic displacement,which can be used to constrain focal mechanism of the earthquake.All of the observational results with high accuracy can help us to restrain the 3DEarth model.更多还原