Ambient Seismic Noise Tomography In Southeastern Gansu Region

In recent decades, the ambient seismic noise tomography is a novel tomographic method, it is used to extract ambient noise dispersion curve, and then to imaging of underground structure information of the Earth. Due to the method of ambient seismic noise tomography is not rely on natural earthquake and source location, and we can obtain the media velocity results of different depth through different period dispersion curve. So, it has been widely used around the world.The southeastern Gansu region locates in the northeast of Qinghai-Tibet plateau, it in the middle of eastern Kunlun fault and northern edge of West Qinling fault, which are the two regional major fault. Some strong earthquakes have occurred on the southeastern Gansu region, it is the strong of north-south earthquake belt of tectonic activity. The tectonic activity is strong and earthquake activity is frequent in this region. It is one of the tectonic activities strong region in northeastern of Qinghai-Tibet plateau, because of this, it has always been the hot spot of the geophysical research.We choose the vertical component records of 150 broadband seismic stations from January to December 2010 in the southeastern Gansu region, the dense stations are deployed by China Seismological Bureau of Lanzhou institute of seismology and China seismological bureau of Institute of Geology. Through calculating the cross-correlation functions of ambient seismic noise of station pairs, we obtained Rayleigh surface wave signals of period 5s to 40 s. Then used the method of signal-to-noise ratio(SNR) and normalized background energy flow(NBEF) measurement to study the characteristics of ambient noise source of period 5~10s and 10~20s in southeastern Gansu region based on the asymmetrical features of cross-correlation functions, and we extract dispersion curve from Rayleigh surface wave, then we obtain the image of Rayleigh surface group velocity of period 6s to 30 s by the method of surface wave inversion. We make the following aspects of research progress and results:(1) Seasonal variations of ambient noise source for both 5~10s and 10~20s and the both ranges have their respective azimuthal distribution of noise energy in southeastern Gansu region. The azimuth of period of 5~10s is 180°~240° in summer, which means the energy of ambient noise source is mainly affected by the Indian Ocean, and it moves to 100°~150°, which means the energy of ambient noise source is mainly affected by the North Pacific Ocean. In contrast, the characteristics of ambient noise source in period of 10~20s is more complex, the azimuth of ambient noise source is controlled by several oceans. The energy of ambient noise source focuses on the azimuth of 170°~210° in summer, which means it is mainly affected by the Indian Ocean, then it moves to 90°~150° and 310°~355° in winter, which means it is mainly affected by the North Pacific Ocean and the North Atlantic Ocean.(2) The results of the group velocity tomography show that the distribution form of low velocity region are consistent with faults in the surface of period 6~8s. Maybe the velocity structure control the deep extension characteristics of the fault. The period 14~16s represents the middle of the crust, its high velocity region and low velocity region are almost consistent with that in the earth’s crust. And low velocity region are isolated from three high velocity region, which has the characteristics of zonal distribution. The period over 24 s represents the lower curst, its two velocity anomaly region by northern edge of West Qinling fault and "V" segmentation by tectonic system turning parts. Overall, the group velocity is high in western region and is low in eastern region of southeastern Gansu region from upper crust to the middle and lower crust.(3) The velocity structure of upper crust to the middle-lower crust is heterogeneous in southeastern Gansu region, the group velocity is high in western region and is low in eastern region. Due to the obstruction of Ordos block in the direction of north-east, the maximum principal stress axes is clockwise rotation in Gansu southeastern region, the more that the region close to the east, the extrusion more obvious, this may be the reason of causing the group velocity structure is high in west and is low in west of southeastern Gansu region.(4) The spatial distribution of earthquakes from 1970 to 2015 reveals that the southeastern Gansu region is inhomogeneous and shows prominent characteristics of geologic motion. Earthquakes with magnitudes more than 4.3 that mostly occurred at the transition zones between high and low group velocity, and extending to zone of high group velocity. It is of great significance for the further studying of the seismogenic environment and judging the danger zone of seismic activity.