Using Double-difference Location Method To Research The Active Tectonic Significance Of Yunnan Earthquakes Distribution

Earthquake location is one of the basic work of seismography, is of great significance for the study of tectonic activity. Double-difference earthquake location algorithm can give precise temporal and spatial distribution of the hypocenters, it is an important method in studying relationship between seismic activity and tectonic activity. In this paper, we used natural earthquake observation report and digital waveform data recorded by Yunnan digital seismic network and used waveform cross-correlation technique to calculate the waveform correlation time difference data. We relocated earthquakes of M?1.0 occurred between 2009 and 2014 years in Yunnan region with double-difference location algorithm. Eventually we obtained the detailed seismicity in whole Yunnan region.This paper emphasizes the usage and calculation of waveform cross-correlation data. The main aim of this study was to obtain stable and reliable high precision waveform cross-correlation time difference data quickly and to get relocation results as much as possible by data combination and to divide the large study area. The final goal was to analyze the relationship between the spatial distribution of the hypocenters and active tectonics in Yunnan which can reflect the detailed features of active tectonics.In this paper we proposed time domain multi-channel correlation detection function which can improve computational efficiency and obtain stable cross-correlation time difference data. To guarantee the reliability of the travel-time difference data we also used some principles to filter the catalog time difference data and cross-correlation time difference data. As catalog data and cross-correlation data both have advantages and disadvantages, we combined this two kinds of data to get relocated earthquakes as much as possible while guarantee relocation accuracy. We separated our study area into 13 regions and relocated earthquakes respectively. We obtained 33328 relocated earthquakes and estimated relocation error and correlation between hypocenter parameters by statistical method. The results showed that cross-correlation data can improve relocation accuracy and weaken correlation between hypocenter parameters. The cross-correlation data is the key of double-difference algorithm.We analyzed the temporal and spatial distribution of earthquakes occurred between 2009 and 2014 in Yunnan and some areas which have a large number of earthquakes. We discussed the relationship between earthquake distribution and active tectonic with previous research and obtained some significant conclusions.The distribution features of earthquake sequence were shown after relocation and had a good correspondence with nearby faults and regional stress field. By analyzing the temporal and spatial distribution of earthquake sequence, we found the in most cases there is a period of time which seismic activity calmed before main earthquake occurred. It was show that regional stress is accumulating in order to provide energy for the occurrence of large earthquakes. There would be peaks of seismic activity when main earthquake occurred, then the seismic activity gradually decay ups and downs overtime. Strong aftershocks could also trigger a higher order of aftershock sequences and made seismic activity appear small peaks. We also found the strength distribution direction of earthquake sequence was related with the nearby faults and regional stress field. Meanwhile great earthquakes can destroy regional stress field and geologic structure nearby and lead the earthquake sequence to a new strength distribution direction. So the distribution of earthquake sequence were also controlled by regional stress field caused by main shock or great aftershocks.By analyzing the hypocenter distribution of earthquakes occurred in Yunnan we found that not only medium-strong earthquakes of M?4.0 distributed along the nearby active faults, but also most of medium-small earthquakes of 4.0M?1.0 distributed along active faults. There were a few of medium-small earthquakes inside the Sichuan-Yunnan rhombic block, and we speculate that the internal stress is not uniform when Sichuan-Yunnan rhombic block moved and nonuniform stress lead strata appeared rupture, formed faults and generated earthquakes. There may be several concealed faults inside the Sichuan-Yunnan rhombic block. The depth distribution of hypocenters was obviously related to active faults. The earthquakes were deeper in the deep fracture. A few of small earthquakes were occurred in lower crust even upper mantle. Most of medium-strong earthquakes were occurred in upper crust and the average depth of the hypocenters became deeper while the range of depth distribution became narrower with the increasing magnitude. The characteristics of earthquake depth distribution were accord with the feather of crust in Yunnan which is "North deep and south shallow"Our research showed that the temporal and spatial distribution of seismic activity can reflect the characteristic of active tectonic and have close relationship with active tectonic.