汶川与昆仑山强烈地震激发的地球自由振荡频谱的对比分析

强烈地震激发的地震波信息非常丰富,高频信息能量部分会在小范围短时间内迅速衰减,而低频信息特别是较低频信息能量部分则衰减较慢,会绕地球很多圈后才会消耗殆尽,从而激发地球的自由振荡.虽然地球自由振荡的固有频率不变,但不同震源机制的地震激发的地球自由振荡频率成分会随着震级、震源机制、方位等的不同而有显著差别.本文利用2008年5月12日汶川(M_w7.9)地震与2001年11月14日昆仑山(M_w7.8)地震这两个能量震级相当但震源机制不同的地震,基于地震波传播理论,考虑横向各向同性PREM地球模型并融合Crust2.0地壳模型、S20RTS地幔模型及PREM径向衰减模型,利用谱元法结合高性能并行计算,对这两次强烈地震激发的全球地震波传播进行了数值模拟,计算它们激发的地球自由振荡频谱特征差异.计算结果显示了汶川地震的逆冲-走滑特性及昆仑山地震的水平左旋走滑特征.利用理论计算得到的地震波动记录进行功率谱密度估计对比分析,研究结果显示这两次强烈地震释放的低频信息能量大小相当,但总体上看昆仑山地震释放的能量略小于汶川地震的.对比分析表明,不同地震激发的地球自由振荡模式在同一台站处的发育程度不同;同一地震激发的地球自由振荡在不同台站处记录到的振型频率成分也有很大差异.对于不同地震激发的地球自由振荡,有一些频率成分共同出现,表现为地球自由振荡功率谱能量均较大;也有一些振型由于相长干涉,仅在某些地震中被激发而得以良好发育,表现为功率谱能量明显较大;而一些振型由于相消干涉,在一些地震激发方式下被压制,表现为功率谱能量明显很小,甚至接近于0.计算结果清楚地显示了不同地震激发方式下地球振荡模式丰富而复杂的特性,需要对不同地震激发的地球振荡信息进行综合深入的分析,才能对地球内部结构尤其横向不均匀特性给予全面的科学解释.

Comparative analysis on the characteristics of low-frequency energy released by the Wenchuan earthquake and Kunlun Mountain earthquake

The M_w7.9 Wenchuan earthquake of 12 May 2008 and M_w7.8 Kunlun Mountain earthquake of 12 November 2001 have equivalent energy magnitudes. However, the characteristics of the fault rupture processes and the geographical positions of the two great earthquakes are very different, so the damage and the public attention of the Kunlun Mountain earthquake are far less than Wenchuan earthquake. The seismic wave energy information generated by strong earthquakes is very rich. High-frequency energy will be quickly decayed within short time in the small scope, but low-frequency energy especially very low-frequency energy will be decayed slowly, and can be spread many laps around the Earth before consumed. We employ the spectral element method incorporated with large-scale parallel computing technology to investigate the characteristics of seismic wave propagation excited by the two great earthquakes. The transverse isotropic PREM model is employed as a prototype of the numerical global Earth model. The Crust2.0 and S20RTS models are taken into consideration. These wave propagation processes are simulated by solving three-dimensional elastic wave governing equations. The visualization of the numerical results displays the profile with three components of the seismic wave propagation. Our calculation displays the thrusting and strike-slip of the source rupture process of the Wenchuan earthquake and the left-lateral strike-slip of the source rupture process of the Kunlun Mountain earthquake, respectively. By taking comparison of low-frequency energy information of synthetic seismograms excited by the two earthquakes, the results show that the energy at low-frequency oscillation modes is relatively equivalent, but the energy of the Kunlun Mountain earthquake is slightly smaller than the energy of the Wenchuan earthquake. The results also demonstrate that the frequency components are almost the same at low-frequency displacement amplitude spectra recorded by different stations between the two great earthquakes, but the energy is obviously different. This can further reveal that the characteristics of Earth's oscillation triggered by large earthquakes with different source ruptures are different, and the characteristics of Earth's oscillations recorded by different stations are also different.