陕北塌陷记录的区域特征研究

Study on the regional characteristics of the subsidence seismic records in the northern Shaanxi

  • 摘要: 选取陕西省境内2011—2018年震级为ML2.7—3.1的18个天然地震事件的287条记录和2013—2018年上述震级范围的20个陕北塌陷事件的185条记录,从时间域、频率域及时频域进行分析对比,总结出陕北塌陷记录区别于天然地震事件的特征:① 在时间域,就P波初动而言,塌陷的P波初动弱而平缓,只有极少数台的垂直向P波初动方向清晰,而天然地震的P波初动强而尖锐,垂直向P波初动方向清晰,有象限分布的特征;在体波特征方面,塌陷的体波周期要比天然地震的大;面波发育是塌陷区别于天然地震最显著的特征之一,即使震中距很小,塌陷记录依然有明显的短周期面波Rg波发育;在震中距<50 km时,塌陷的振幅比AS/AP值要远远大于天然地震,且随着震中距的增大,AS/AP值变小并与相同震中距下天然地震的AS/AP值相近;相同震级,塌陷比天然地震尾波持续时间长,能量衰减慢。② 在频率域,塌陷的频率域较窄,主要为低频信号,集中在0—3 Hz内;天然地震的频率域较宽,低频信号较少;定义频带内频谱的方差与均值的比值为频谱变异系数α,塌陷的频谱变异系数α整体高于天然地震的。在震中距200 km范围内,塌陷的拐角频率fc较天然地震小,集中在1—3 Hz,天然地震的拐角频率fc在3—15 Hz均有分布。③ 从时频谱的对比可见,塌陷的主要频率成份为低频面波,而天然地震频率成份最丰富的为横波波段。

     

    Abstract: This paper selects 287 records of 18 natural earthquake events with magnitudes of ML2.7—3.1 in Shaanxi Province from 2011 to 2018 and 185 records of 20 subsidence seismic events in northern Shaanxi with the same magnitude range from 2013 to 2018. Through the analysis and comparison in time and frequency and time-frequency domain, the characteristics of subsidence seismic records in the northern Shaanxi different from natural earthquake events are summarized as follows: ① In the time domain, the P-wave initial motion of subsidence is weak and gentle, while that of natural earthquake is strong and sharp. To the subsidence, only a few vertical direction of P-wave initial motions are clear, while that of natural earthquake is clear, and it has the characteristics of quadrant distribution. The body wave period of the subsidence is larger than the natural earthquake. The development of surface wave is one of the most significant characteristics that could distinguish subsidence from natural earthquake. Even if the epicenter distance is very small, subsidence seismic records still have significant short-period surface wave Rg. When the epicenter distance is less than 50 km, the amplitude ratio value AS/AP of subsidence is larger than that of natural earthquake. With the increase of epicenter distance, the amplitude ratio value AS/AP of subsidence becomes smaller and similar to that of earthquake under the same epicenter distance. With the same magnitude, the subsidence has a longer coda duration and slower energy decay than that of the earthquake. ② In the frequency domain, the frequency domain of subsidence is narrower, mainly low frequency signal, concentrated in 0−3 Hz. The frequency domain of the natural earthquake is wider, but the low frequency signal is less. The ratio of spectral variance to the mean in the frequency band was defined as the spectral difference coefficient α, and the α of the subsidence is generally higher than that of the natural earthquake. In the range of epicenter distance 200 km, corner frequency fc of the subsidence is smaller than that of the natural earthquake. Corner frequency fc of the subsidence is concentrated in 1−3 Hz, while fc of the natural earthquake is distributed in 3−15 Hz. ③ From the comparison of the time-frequency spectrum, it can be seen that the main frequency component of the subsidence is low-frequency surface waves, and the natural earthquake frequency component is most abundant in the shear wave band.

     

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