利用SH波对盖州震群震源谱参数的稳健反演研究

Robust inversion of seimic-source spectral parameters for Gaizhou swarm based on SH-component

本文在Brune震源模型的基础上引入截止频率fmax,使用高频截止震源模型(High-Cut),采用两步反演拟合的方法稳健获得震源谱参数,并给出其置信区间.首先对观测波形逐步消除仪器响应、几何扩散、传播路径介质衰减等影响后,经过坐标系旋转得到SH分量,其次对SH波进行快速傅里叶变换后,经拟合反演得到盖州震群56个地震事件(ML≥3.0)的震源谱特征参数和震源参数.结果表明:(1)High-Cut模型的理论谱对观测谱有很好的拟合效果,可明显改善拐角频率识别的准确度,表明该理论模型更适用于观测震源谱.(2)56次地震事件的地震矩在1011~1015 N·m之间,地震矩M0与ML存在明显的线性关系:logM0=1.248ML+8.329;拐角频率在0.8~2.9 Hz范围内,拐角频率时域演化基本维持在2倍均方差内;盖州震群应力降σ变化与ISD模式一致,与震级存在正相关的依赖关系:logσ=1.55ML-10.93,但是应力降水平整体偏低(σ≤0.1 MPa).(3)结合速度结构、精定位以及盖州地区地质灾害调查等结果,综合讨论认为盖州附近地区中小地震多发、应力降偏低的原因可能在于隐伏的NWW向断裂为海水入侵创造了条件,从而使多条活动断裂周边破碎介质的孔隙压力增加,摩擦系数减小,这导致大量中小地震发生在比天然构造地震更低的构造应力地区,所以流体在盖州震群的孕育和发生过程中起了重要的诱发作用.

Based on the Brune Model, we propose a High-Cut model by introducing the cut-off frequency fmax. We use a two-step inversion method to fit the observed seismic-source spectra and resolve its characteristic parameters and confidence interval. The seismic-source observation spectrum of 56 events(ML 3.0) of the Gaizhou swarm was obtained by the Fast Fourier Transformation after the influences of instrument, geometry spreading, anelastic attenuation were corrected and then rotated about azimuth to obtained SH-component of ground motion. The results demonstrate that the theoretical spectrum of the High-Cut model has a good fitting effect on the observed spectrum, which can improve the accuracy of spectra parameters significantly, indicating that the theoretical model is more suitable for observing the source spectrum. Seismic moments of events vary from 1011 N·m to 1015 N·m with increase of the magnitude, and the scaling relationship between seismic moment(M0) and magnitude(ML) for the region as: logM0=1.248ML+8.329;The range of corner frequency of the Gaizhou earthquake swarm is 0.8~2.9 Hz, and its Variation basically maintained within double mean square deviation. Moreover, The stress drop increases with magnitude, which is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model. And the scaling relationship between stress drop(σ) and magnitude(ML) for the region as: logσ=1.55ML-10.93, however, the Gaizhou swarm has obviously lower stress drops(σ≤0.1 MPa). Based on the results of velocity structure, precise positioning, and survey of geological hazards in the Gaizhou area, a comprehensive discussion suggests that the reason for the frequent occurrence of small and medium earthquakes and low stress drop in the area near Gaizhou may be that the hidden NWW faults created conditions for seawater intrusion, and the increase of pore pressure and the decreases of friction that lead to the frequent earthquake and lower stress drops in hypocentral region. In general, fluid intrusion may plays an important role in the conception and occurrence of the Gaizhou earthquake swarm.