Abstract: When P waves from distant earthquakes meet a velocity discontinuity in the earth's crust and upper mantle, they give rise to a series of converted PS waves besides PP refracted waves. It is possible to monitor the variation of the physical properties of the medium in the limited formation space above the transition zone of the seismogenic zone by measuring the time difference between the teleseismic PS converted wave and the first arrival P wave, that is, time-variation Δt_PS=t_PS-t_P. The advantage of this method is that the transition point of the teleseismic source with similar source is relatively stable at the transition interface, and the accuracy of the measured relative time is high, and the change of the medium in a small range of the seismogenic zone above the conversion interface can be monitored.
This paper studies the variation of the travel time difference Δt_PS in focal region before and after Wenchuan M_S8.0 earthquake. We select 2001 to 2012 as the research period, use teleseismic waveforms which occurred in the southern region of Sumatra and Hindu Kush area recorded by Sichuan station YZP and JJS. These teleseisms satisfy 5.0 ≤ M ≤ 6.5, and their waveform signal-to-noise ratio is high with clear initial P-wave motion. The epicentral distance of teleseisms is less than 3 degrees. Then we obtain the variation of the travel time difference Δt_PS between teleseismic PS converted wave and PP transmitted wave recorded during the study period in the two stations. The results show that there is a slow increasing trend of Δt_PS before 2006, and an obvious low value process of Δt_PS appeared in the period about 2 years before the Wenchuan earthquake. The maximum decline was about 0.2~0.3s, more than 4~5 times the measurement error. The low value has a certain degree of return about 2~3 months before the earthquake.
The change of arrival time difference indicates that the medium is in different states in different periods of seismogenic process. The sharp decrease in Δt_PS from 2006 to January 2008 may be due to the strong disturbance caused by the stress accumulation of the medium. At this stage, the velocity of P wave and S wave increases with the increase of stress, and the increase of S wave velocity will result in the decrease of Δt_PS. The change of Δt_PS is greatly affected by S wave velocity, so Δt_PS appears to decrease rapidly. Regarding the low value that has a certain degree of return about 2~3 months before the Wenchuan earthquake, the possible reason is that the release of stress is much higher than the accumulation of stress in meta-instability stage. At this stage, the velocity of S wave decreases and the decrease of S wave causes Δt_PS to increase. Then, the Wenchuan earthquake of magnitude 8.0 occurred. It is shown that the teleseismic converted wave method in this paper can monitor the variation of medium's wave velocity before large earthquakes, and it has a good prospect in seismic monitoring and worth further experimental study.

Key words: travel time difference between teleseismic PS converted wave and P wave, Wenchuan M_S8.0 earthquake, wave velocity variation of crustal medium

摘要： 利用远震初至波穿越地壳、上地幔等速度界面时产生的一系列PS型透射转换波，测定远震PS转换波与初至P波的到时差Δt_PS=t_PS-t_P随时间的变化特征，有可能监测孕震区转换界面以上有限的地层空间内介质物性的变化。选取2001-2012年作为研究时段，通过四川地震台网的YZP和JJS 2个台站记录到的兴都库什、苏门答腊南部地区2组震中距变化<3°的远震数据，得到2个台站在2008年5月12日汶川M_S8.0地震前后记录的PS转换波与初至P波到时差Δt_PS的变化特征。结果表明，在2006年以前Δt_PS有一个缓慢增大的过程；汶川地震前约2a内Δt_PS出现明显的降低过程，最大降幅达0.2~0.3s，超过测量误差4~5倍；震前2~3个月低值有一定程度的回返。文中讨论了可能引起Δt_PS变化的原因，分析表明文中提出的远震转换波法在地震监测中有很好的应用前景，值得进行进一步实验研究。

关键词: PS转换波与P波到时差, 汶川M_S8.0地震, 地壳介质波速变化