Analysis Of The Crustal Displacement Characteristics In Stages Before Earthquake By Using GPS Data

As the Development of GPS(Global Positioning System) and other space geodetic techniques, the observation precision is improved from traditional measurement 10-6 to 10-8~10-9. The plate movements may be described in a certain Terrestrial Reference Frame, dependent on GPS high precision observation network by GPS stations posed, the purpose of monitoring crustal deformation and movement. With the building of CMONOC(Crustal Movement Observation Network of China) and data sharing, we are able to get GPS data increasingly. GPS data can be used to analysis on the modern movement and deformation characteristics of long-term large-scale active blocks. It has played an important role in the long-term earthquake prediction work. Long-term earthquake prediction concerns the location and intensity of the earthquake may occur in the future, relative fuzzy time of the earthquake; short-term and imminent earthquake prediction is required approach time of the earthquake, so as to achieve the effect of earthquake disaster reduction. GPS continuous observation data are very complicated not only contains crustal movement information, also contains information about the various noise and periodic non-tectonic movement. We extract multi-scale tectonic movements and dynamic deformation information from the rich GPS continuous observation data, and understand the characteristics of crustal deformation before earthquake. This study employs the method in Chen et al.(Chen C H, 2013) and three components, NS, EW and UD, of the daily solutions data are data source. We independently program to achieve GPS data processing and comprehensive analysis of GPS data with a variety of mathematical methods. We obtained the surface displacement characteristics before M7.0 Lushan earthquake in 2013 and M6.4 Menyuan earthquake in 2016 on north-south seismic belt. Rudimental results are showed as below.1. Take M7.0 Lushan earthquake on middle-south section of north-south seismic belt as an example. Continuous observation data from 22 GPS stations are selected in the area(29.0°N~32.0°N, 100.0°E~105.0°E) for this study(the period is from April 20, 2012 to April 19, 2013). The following steps are GPS data analysis: 1) Remove the singular points in records. 2) Filter three components, NS, EW and UD, of the daily solutions by the Hilbert–Huang transform(HHT) with frequency band of 5.787×10-7~ 7.716×10-8Hz(20–150 days in period). 3) Set search radius on the grid, horizontal step length and vertical step length, calculate the regional consistency of the GPS horizontal angle difference, so as to analyze the regional stage characteristics of displacement before Lushan earthquake. The calculation results show that the spatial and temporal evolution characteristics of GPS horizontal displacement before Lushan earthquake can be divided into three stages. The first stage is from 55 days to 21 days before the earthquake. The movements of GPS stations around the epicenter have no specific direction, and the magnitude is smaller. There is no obvious stress disturbance. The second stage is from 20 days to 8 days before the earthquake. Three GPS continuous observation stations(LS05, LS06 and LS07) have significant northward displacement(approximately 0.5cm) in the west of the epicenter. The magnitude on the second stage is 3~5 times of the magnitude on the first stage. The third stage is before the earthquake the 7 days to 2 days for each GPS level again in a random state, and the magnitude of the average level. The movements of GPS stations are disorder and random again, and the magnitude return to average. The spatial and temporal evolution process of GPS horizontal displacement before Lushan earthquake is similar to the process, which described and analyzed by Chen C. H.(2014), before M9.0 earthquake in northeastern Japan.2. Take M6.4 Menyuan earthquake on north section of north-south seismic belt as an example. Continuous observation data from 24 GPS stations are selected in the area(33.0°N~41.0°N, 95.0°E~105.0°E) for this study(the period is from Jan 1, 2015 to Jan 20, 2016). The following steps are GPS data analysis: 1) After removing the serious random drift GPS stations in the study area and singular points in records, Filter three components, NS, EW and UD, of the daily solutions by the Hilbert–Huang transform(HHT) with frequency band of 5.787×10-7~ 7.716×10-8Hz(20–150 days in period). 2) Test the stability of filter results with different Terrestrial Reference Frame, and analyze the dynamic characteristics of micro displacement before Menyuan M6.4 earthquake are studied by using the temporal dependencies and cross spectrum analysis. The analysis results show that from 55 days before the earthquake, the undulatory motions are shown by series data from three GPS stations(QHME, QHMD and QHYS), setting near the profile perpendicular to the seismogenic fault plane on the Qinghai-Tibet Plateau, and their magnitudes are higher than the average level. The focal mechanism(strike141.0°/dip43.0°/slip73.0°) shows that the seismogenic fault is a thrust fault with northwest strike. The increase of northeast micro movement recorded before the earthquake is conducive to the earthquake’s occurrence. The GPS data with different Terrestrial Reference Frame are the input of the calculation. The comparisons show their moving directions and their magnitudes are consistent. Before Menyuan earthquake, the crustal movement recorded by GPS stations does not depend on Terrestrial Reference Frame, so the motion characteristics are objectively and truly. Then, comparative results of different bands display the band of 5.787×10-7~ 7.716×10-8Hz(20–150 days in period) is better for identification of ground surface displacement before earthquake.3. Through the analysis and researches of the two cases above, we clarified the crustal movement information recorded by GPS continuous observation stations. The stations LS01 and LS04 have recorded the motion of the western end of the Southern China plate, and they reflect the stability of the Southern China plate. The three GPS continuous observation stations(QHME, QHMD and QHYS) have recorded information reflect that Qinghai-Tibet Plateau block are pushing Alashan block and Ordos block.4. Lushan earthquake and Menyuan earthquake are located on middle-south section and north section of north-south seismic belt. There are differences on the aspects of magnitude and seismogenic structure. We analyze the data of GPS stations near the epicenter, and have captured stress disturbance process before the earthquake. It shows that from less than a year time scale of GPS continuous observation data can dig out the prediction value of crustal deformation and earthquake information. The information helps us understanding the dynamic deformation process of earthquake inoculation and trigger. It is a vital significance work. In addition, with the development of GPS technology, researchers should continue try to use GPS continuous observations for studying the deformation process of the earthquake generation. Widespread and more GPS stations will help to analysis and determine the crustal movement in detail. So improving the network of GPS continuous stations is much more necessary.