| At present, the problem of earthquake prediction is still a worldwide scientific problem. How can more accurately determine the three elements of earthquake prediction(time, place, and magnitude), is a direction in earthquake research need to continue to explore. Existing many earthquake prediction methods, seismic wave velocity change research is a key research direction. Many scientists study results have shown that the physical condition of the source area medium can produce a series of changes in the seismogenic process, the increase of the regional tectonic stress before earthquake focal region and its surrounding areas can lead to change the physical parameters of underground medium, thus cause the change of seismic wave velocity. Therefore, the change of physical state of medium can be reflected through the wave velocity changes, so an early idea of modern seismology is the hope through to the earth’s crust velocity change monitoring and analyzing the change of the inner regions of the crustal stress state, further realize earthquake prediction research.Wave velocity ratio(spVV) is used to study changes before and after the earthquake wave velocity method, because the data is dependent on the distribution of earthquakes and stations, and the natural earthquake occurs mostly in the edge of the continental tectonic plate area or near the fault zone and the uneven distribution of stations, so the results are not very reliable. The use of natural earthquake event data to study the wave velocity changes has such problems as uneven distribution of the source location and ray, imaging result resolution is not high, so the more used in the study of static wave velocity. Using artificial repeat source method can detect the change wave velocity of underground media, available dynamic wave velocity changes of different times, but this cost is too high. While the receiver function method is more sensitive for speed interface, can only be given stations at the bottom of the crust upper mantle structure. Different times are obtained by background noise tomography method, the speed of different depth imaging map, to explore the analysis before and after the earthquake occurred near the epicenter of the earthquake wave velocity change in the future. Compared with the previous imaging method, background noise method does not depend on the earthquake, ray uniform distribution, short-period surface wave can be studied, and the advantages of small cost. Background noise method help people to understand the earth’s interior structure, and detection of underground medium state change provides a new train of thought.On the basis of previous studies, this article using the method of noise imaging studies of north-south seismic belt region before and after the earthquake wave velocity changes, and carries on the comprehensive analysis to the present seismicity, the results for the prediction of earthquakes in the area(32。~40。N,99。~108。E) and provide a basis for forecasting work. By noise cross-correlation calculation to extract the experience of green’s function, and finally get the velocity structure of the image before and after the earthquake to contrast analysis of seismic velocity structure dynamic change, in order to analysis the state changes before the earthquake underground medium, and applied to earthquake prediction research. Based the north section of north-south seismic belt(32。~40。N,99。~108。E),there are94 fixed seismic stations on January 1, 2013 to January 1, 2016, three years of continuous recording seismic network ride to waveform data, using the method of background noise and tomographic imaging inversion in the region of different times, three months the same time window length of Rayleigh surface wave phase velocity structure and evolution of wave velocity. Noise by cross-correlation method to extract the experience of the Rayleigh surface wave green’s function, the use of matched filtering time-frequency analysis technique measurement of Rayleigh surface wave phase velocity dispersion curve, the study area is divided into a grid, using the method of surface wave tomography inversion for the region on July 22, 2013 in MinxianZhangxian Ms6.6 earthquake different periods before and after the earthquake, the same two months time window length of Rayleigh surface wave in the 6s, 8s, 10 s, 12 s, 14 s, 16 s, 18 s, 20 s, 25 s, 30 s, 35 s, 40 s different period, such as phase velocity imaging map, from ray density and the resolution of the inversion result and testing experiment two aspects to discuss the resolution of the inversion results and the reliability of the tomographic imaging results.In this paper, we study preliminary conclusions are the following:(1) North section of north-south seismic belt Rayleigh surface wave phase velocity tomography results show that the most of north-south seismic belt in low speed condition form a dividing line between the west Qin-ling tectonic belt. The study area is divided into two parts, the northern area except the east section of Qilian mountain wave velocity is low, other area most wave velocity range in high speed,especially in the west Qin-lian tectonic belt in eastern areas appear high-speed anomaly area. Wave velocity is its southern region almost all appear the phenomenon of low speed, especially in east Kun-Lun tectonic belt and west Qin-ling tectonic belt junction appear abnormal phenomenon at low speed. A-La-shan block revealed a large area of abnormal low speed, the possible reason is A-La-shan block special thick sediments caused by low speed, and relatively high speed than other areas of south China block, may be the reason for the more shallow sediments. At the same time, the pooldown chang fault belt in the attachment and the surrounding area in high speed and low speed area border or parts of high speed low speed changes. This result and Yi Gui-xi of the north-south seismic belt is always in a relatively low speed, and is divided into east and west two different speed characteristics of the result of the natural boundary.(2) Based on the method of noise tomography method, using continuous waveform records of 94 fixed seismic stations in the north segment of the NorthSouth Seismic Belt region network from January to September,2013. Then, before and after the earthquake, surface velocity distribution of different times in the same window length of two months of Rayleigh surface wave velocity structure and evolution of wave velocity as the stack days is obtained by tomography method in July 22, 2013 Ms6.6 Gansu Minxian-Zhangxian earthquake. The phase velocity imaging results show that before the 2013 earthquake in Minxian-Zhangxian Earthquake,5~6 months relative to 3~4 months, the Lintan-Tanchang fault zone and the surrounding areas appeared the phenomenon of wave velocity to reduce, 8~9 months relative to 5~6 months, after the earthquake Lintan-Tanchang fault zone and the surrounding areas appeared the phenomenon of wave velocity gradually restore to rise. the results show that in Minxian-Zhangxian Earthquake in low velocity anomaly appeared for the first two months of this earthquake.In conclusion, the Ms6.6 Gansu Minxian-Zhangxian Earthquakehave happened the boundary of the region in lowspeed anomaly area.(3) Before the larger earthquake, due to the increase of regional tectonic stress, may cause the source and its surrounding areas dielectric properties change, and prompting seismic wave velocity change. According to the results of this paper, using noise imaging to study the analysis before the earthquake seismic parameter changes of this method is still in exploring study, and combining with the seismicity and related results analysis found that in recent years of north-south seismic belt regional seismicity enhanced obviously. In critical condition, the future in the region still has the possibility of more than 5, and fracture area exists in Mao-Mao hill above level 6 or greater magnitude earthquake zone, speculated that the fracture and its nearby area has the danger of earthquake. To sum up, this area needs to pay close attention. |
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