Study On Travel Time And Attenuation Characteristics Of Seismeic Wave In Qilianshan Area By Using Active Source

The occurrence of earthquake is often accompanied by the slow accumulation and sudden release of stress.A large number of rock physics experiments have confirmed that when the stress state of rock mass medium changes,the characteristics of seismic wave passing through the medium will also change.Large capacity air gun source has the advantages of environmental protection,small near-field damage and high repeatability,which can be used to detect the physical property changes of underground media in a large range.The active source detection system of Qilian Mountain is located in the middle part of Qilian Mountain on the northern margin of Qinghai Tibet Plateau.Strong earthquake activities are very frequent in this area.It is of great significance to study the physical properties of underground media in this area.Based on the observation data of active source in Qilian Mountain,this paper discusses the extraction and analysis method of travel time variation of active source seismic wave in detail,designs the extraction method of attenuation change characteristics of active source seismic wave,describes the travel time and attenuation variation characteristics of each station since the operation of Qilian Mountain active source,explores the abnormal change of travel time and attenuation of active source seismic wave before and after the earthquake,and obtains the following main results:Based on the original processing flow of travel time variation of active source seismic wave,the waveform data with original sampling rate of 100 points/s is interpolated into 1000 points/s,and travel time variation is extracted on the basis of this data.The travel time variation range of Pg phase of each station is basically within±5ms.2.After the travel time variation of active source seismic wave is obtained,each seismic phase is intercepted by travel time migration,and the ratio of the amplitude square mean of the intercepted phase and the amplitude square mean value of the reference station received signal is calculated,and the energy attenuation change of each seismic phase at each station is obtained.The variation range of Pg phase attenuation is basically between±20%.3.The annual variation of travel time and attenuation of some seismic phases at some stations of Qilian Mountain active source is obvious,especially for S-wave phase with long propagation path and deep penetration depth.At the same time,there is a correlation between the travel time of each seismic phase and the attenuation annual variation,showing the same or opposite change.4.Since the completion of the autonomous source,abnormal phenomena of travel time and attenuation have been observed at stations near Menyuan M_S6.4earthquake in 2016 and Zhangye M5.0 earthquake in 2019.The specific performance is that before the earthquake,the continuous decline to the low value,and the rapid rise after the earthquake.Among them,zdy27 station near Zhangye earthquake is the most severe.The travel time of the first arrival wave picked up on its tangential component decreases by about 8ms,which is about twice the minimum value observed in previous years.There is a sudden change of 10ms before and after the earthquake,and the travel time increases gradually after the earthquake.Some phase attenuation of zdy27 station has similar changes with travel time.Before the earthquake,it decreased to the lowest value since the observation,and gradually increased after the earthquake.