Monitoring Dynamic Variations Of Underground Media By Repeating Earthquakes And Coda Wave Interferometry

Observing temporal variations of material properties in the seismogenic zone has been a long-sought goal of the geophysics community. Some former research showed clearly temporal changes of elastic properties of underground media were detected before and after earthquakes such as coda Q and seismic anisotropy. However, these were not convincing due to uncertainties associated with earthquake locations and station error. Recently, results from artificial controlled source research have shown considerably smaller temporal variations of elastic properties associated with earthquakes. Seismic coda wave is the product of multiple scattering waves in underground media. As a result of more repeated samples, it is sensitive to subtle variations of media which direct waves may not detect. Coda Wave Interferometry(CWI) can be used to extract small changes of source location and monitor subtle variations of underground media due to the characteristics of multiple scattering coda waves.Variations of crustal properties are related to the physical character as well as stress and strain of media, thus dynamic variations of underground media associated with earthquakes may help us understand the preparation and development process better. Compared with active source and ambient noise green’s function, repeating earthquakes do not need any material and financial resources and have significant penetration depth, are more adaptive for CWI in monitor variations of crustal media in seismogenic depth.This thesis collected earthquakes in Gansu area from 2009 to 2014 and in Yingjiang area from 2013 to 2014. Among which Gansu lies in the Northeast margin of the Tibetan Plateau and Yingjiang lies in the Southeast margin of the Tibetan Plateau. First, we relocate all the earhquakes by using hypo DD programs to get more accuracy source location. Then we apply waveform cross correlation method combined with the results of hypoDD to pick out repeating events in the research region. At last variations of crustal media before and after several main shocks as well as seasonal variations of seismic velocity detected by using CWI are being discussed. The research results are as follows.First, our research shows focal depth are increased after hypoDD and the number of repeating events are mainly depended on the identification condition. Repeating events in Northeast and Southeast margin of the Tibetan Plateau have different types mainly due to the different fault properties and tectonic environment in two areas.Second, we found clearly velocity variations before and after a ML 4.5 earthquake in the Northeast margin of the Tibetan Plateau, and these variations are most prominent in S wave as well as its coda waves. Meanwhile, variations similar to scatters moving after this earthquake and seasonal variations are also found in this area.Third, repeating events found in Yingjiang are all repeating aftershocks(Doublets) of 2014 Yingjiang double earthquake, and has very short interval. We also found clearly velocity variations in S wave and its coda waves after the main shock in the station which is close to the main shock. However, no clear variations were observed in the distant station.