Identification Analysis Of Underground Explosion Based On Focal Mechanism

Screening underground explosion from seismic events is an important part of the seismic verification process.Among the many identification criteria,focal mechanism is the key information to distinguish between underground nuclear explosions and other seismic events,and is the physical basis of most identification criteria.Therefore,the focal mechanism inversion of underground explosion and the interpretation based on focal mechanism to the waveform characteristics of sensitive seismic events are of great significance to the identification of underground explosion.From the point of focal mechanism,this paper analyzes the North Korean nuclear tests and the collapse event after the 2017 nuclear test,and the earthquakes in the eastern Tianshan area of China.There are inherent problems in focal mechanism inversion of underground explosion.Unlike natural earthquakes,the depth of underground explosion is shallow,and under the boundary condition of free surface,it is difficult to distinguish all the elements in the total moment tensor solution in the waveform dominated by long period surface waves.In order to solve this problem,this paper systematically expounds the problem of focal mechanism inversion of underground explosion.Based on this,a linear full moment tensor inversion program is developed to simulate the focal mechanism inversion process of underground explosion.It is found that P wave can be used to obtain more reliable results than long period surface wave inversion.In this paper,the focal mechanisms of six North Korean nuclear explosions and the collapse event after the sixth nuclear explosion in 2017 are systematically inversed using the data collected from the dense regional stations near the nuclear test site of North Korea.The results show that the main component of the optimal solution of the nuclear test is the ISO source,which is near the ISO or Crack on the source type map,and the other solutions with small residual are located in the upper right part of the source type map,while the main component of the optimal of the nuclear test is the CLVD source,and the other solutions with small residual are located in the lower left part of the source type map.From the source type diagram,we can see that the resolution of nuclear test solution is high,but the resolution of collapse event solution is poor.For the focal mechanisms of nuclear explosion and post-collapse events in 2017,we further try to use the surface wave amplitude ratio to restrict the inversion results based on the waveform method.Combined with waveform inversion and amplitude ratio,the resolution of the solution can be further improved and the inversion results can be optimized.The optimal solution of nuclear test is near the Crack source position,and the source type of the collapse event is still uncertain.Based on the source theory of underground explosion,the influence of composition weight and source time function of CLVD and DC source on the Mb:Ms identification criterion is simulated by synthetic seismogram calculations.On this basis,the mechanism of failure of the mb:Ms criterion in the first five nuclear explosions of North Korea is discussed.It is found that the small magnitude may be an important factor besides the lack of CLVD source.The suspected sPL phase appeared in the aftershocks of 2017 North Korean nuclear exploson on the CBT and YNB stations.We used synthetic seismograms to match the time difference between sPL and Pg,and measured the focal depth of these aftershocks to be 2-4 km.Based on the focal depth,we think that the seismicity mechanism of these aftershocks may come from the structural stress adjustment induced by the nuclear explosion.In this paper,the focal mechanisms and waveform characteristics of some seismic events in eastern Tianshan region are studied.It is found that the focal mechanisms of earthquakes in eastern Tianshan region is north-west striking thrust-slip faults occurring in the crust.The focal mechanism can well explain the positive initial P wave motion in IMS Stations,and the larger P/S amplitude ratio on the region station MK31 is also likely to be related to the modulation of the focal mechanism.For small magnitude seismic events,although there is no way to direct inverse their focal mechanism,the source characteristic can be explained by the comparison of waveform,P/S amplitude ratio,P wave polarity and so on with the large magnitude seismic event occuuring nearby.The mb:Ms criterion is effective in identifying earthquakes of large magnitude in this area,but not in identifying earthquakes of small magnitude.