用钻孔应变数据探测2011年日本Tohoku大地震激发的低频地球环型自由振荡及其谱峰分裂

Probing low-frequency toroidal free oscillations and their splitting excited by the 2011 Tohoku,Japan earthquake using observations of borehole strainmeters

地震激发的地球自由振荡中,环型自由振荡(特别是低频环型自由振荡)的精确探测相比于球型自由振荡更加困难.环型自由振荡对SH波速度结构更加敏感,能更好地监测走滑断层运动,因此精确估算环型自由振荡的谱峰参数可为地球内部大尺度SH波速度结构和大地震的震源机制提供更强约束.本研究利用2011年日本Tohoku地震后四分量钻孔应变仪的观测资料,提取了该地震激发的1.3 mHz以下低频环型和球型自由振荡信号,与STS-1宽频带地震仪的结果相比较,发现四分量钻孔应变仪对1 mHz以下的环型自由振荡具有更强的探测能力.本研究还采用频率域自回归方法精确估计了0T2—0T6的简并频率,结果与PREM模型理论值相差非常小,平均相对误差仅为2.52×10^(-4).此外还用钻孔应变仪观测资料探测到了0T2谱峰分裂现象.研究结果表明,钻孔应变仪能有效探测到大地震激发的低频地球环型自由振荡信号,可为地幔大尺度各向异性结构研究提供高质量的基础数据.

Toroidal free oscillations(especially the low-frequency toroidal modes)excited by earthquakes are much more difficult to be detected comparing with the spheroidal modes.However,toroidal modes are sensitive to SH waves and can be used to monitor the lateral movement of faults.Therefore,the accurate estimation of toroidal modes provides more constraints on the large-scale SH-wave velocity structure of the earth's interior and the focal mechanism of large earthquakes.Using observations of 4-component borehole strainmeters,low-frequency toroidal modes and spheroidal modes below 1.3 mHz excited by the 2011 Tohoku,Japan earthquake are extracted in this study.The results indicate that the 4-component borehole strainmeters have better detectability for toroidal modes below 1 mHz compared to the STS-1 broadband seismometers.The degenerate frequencies of the low-frequency toroidal modes 0T2—0T6 are accurately estimated with the autoregressive method in frequency domain.The estimated values are close to the PREM theoretical values with an average relative error of 2.52×10^(-4).In addition,the mode splitting of 0T2 is detected using the borehole strainmeter data.Our results demonstrate that the borehole strainmeters can effectively detect signals of low-frequency toroidal modes excited by large earthquakes,and then provide high-quality basic data for the investigation of the large-scale anisotropic structure of the mantle.