长岛震群震源谱参数反演和震源参数特征

doi:10.12196/j.issn.1000-3274.2022.01.006

摘要/Abstract

摘要： 采用高频截止(High-Cut)震源模型, 以均方根误差最小原则稳健地求解震源谱参数, 并由此推算震源尺度和静态应力降。实际应用显示, 该模型的理论谱对观测谱有很好的拟合, 可明显改善拐角频率识别准确度。计算了长岛震群内71次M_L≥2.5地震事件的震源参数, 结果表明: ① 拐角频率处于2~10 Hz范围, 与震级大小存在一定的相关性, 截止频率范围处于10~30 Hz之间, 与地震大小的相关性不明显; ② 地震矩M₀分布在10¹²~10¹⁴ N·m, 与震级M_L存在正相关关系: logM₀=0.977M_L+10.186; M_L与矩震级M_W之间的关系为: M_W=0.651M_L+0.766; ③ 根据相对应力降时域演化发现, 自2017年3月3日M_L4.5地震之后应力快速释放, 应力降水平在均值附近波动, 而且多数M_L≥3.5地震发生于应力降下降之后的回升过程中; ④ 应力降范围在0.01~1 MPa之间, 应力降与震级的统计关系为: logσ=1.158M_L-6.591。长岛震群应力降水平明显偏低, 反映了震中区域构造应力水平较低, 但是中小地震活跃, 推测主要有两方面的原因: 一方面, 1548年发生的渤海7级地震引起区域性地震矩释放, 导致长岛震源区内介质相对破碎; 另一方面, 长岛海域断层易受到海水渗入, 引起介质孔隙压力增大, 同时内摩擦系数降低, 从而导致断层内剪切应力降低。综合而言, 长岛地区虽然处于较低的构造应力背景之中, 但是流体入侵造成的断层内剪应力降低可能是造成长岛地区发生大量中小地震的主要原因。

关键词: 拐角频率, 震源参数, 应力降, 长岛震群

Abstract: 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. The seismic-source observation spectrum of 71 M_L≥2.5 events of the Changdao swarm was obtained by the Fast Fourier Transformation. The results demonstrate that ① the method can improve the accuracy of spectra parameters significantly. The range of corner frequency of the Changdao swarm is 2~10 Hz, and there is a certain correlation between the stress drop and the moment magnitude. The range of high-frequency is 10~30 Hz, and the relationship between high-frequency and the magnitude of the earthquake is not very obvious. ② Seismic moments of events vary from 10¹²N·m to 10¹⁴ N·m with increase of the magnitude, and the scaling relationship between seismic moment (M₀) and magnitude (M_L) for the region is logM₀=0.977M_L+10.186. Besides, the scaling relationship between moment magnitude (M_W) and magnitude (M_L) for the region is M_W=0.651M_L+0.766. ③ According to the time-domain evolution of relative stress drop, it is found that stress has been released rapidly after the M_L4.5 earthquake on March 3, 2017, and the level of stress drop has fluctuated around the mean value, and most M_L≥3.5 earthquakes occur in the process of recovery after the decrease of stress drop. ④ The range of stress drop is between 0.01 and 1 Mpa. The statistical relationship between stress drop and magnitude is: logσ =1.158M_L-6.591. Moreover, the stress drop increases with magnitude, which is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model. The stress drop level of the Changdao swarm is obviously low, which reflects the low level oftectonic seress in the epicenter region. But it can be seen that the small and medium earthquakes are active in this region, it is speculated that there are two reasons. Firstly, the Bohai M7 earthquake in 1548 caused the weakening of medillm strength surmoueling focal area in Changdao region. Secondly, the faults in Bohai sea are easy to be peretrated by seawater, which resulting in the increase of pore pressure and decrease of fault strevgth, which leads to a decrease in the shear stress within the fault, and finally causes the frequent earthquake activity at Changdao region with relative lower tectonic stress.

Key words: Corner frequency, Source parameters, Stress drop, Changdao swarm

中图分类号:

P315.7

张正帅, 周晨, 郑建常, 刘承雨. 长岛震群震源谱参数反演和震源参数特征[J]. 地震, 2022, 42(1): 85-98.

ZHANG Zheng-shuai, ZHOU Chen, ZHENG Jian-chang, LIU Cheng-yu. Inversion of Seismic-Source Spectral Parameters and Characteristics of the Source Parameters for Changdao Swarm[J]. EARTHQUAKE, 2022, 42(1): 85-98.

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