The Studies On Lateral Heterogeneity Of Q_Lg In Sichuan-Yunnan Region

Q value is one of the main parameters in describing the medium characteristics of the Earth's interior. By the way of studying on the attenuation characteristics from natural earthquakes to understand the earth interior material characteristics and the thermodynamic state, and then further reveal the earth interior constitutions and its variations, is one of the most important research fields of seismology. The way of doing research on crustal Q value by natural seismic wave mainly includes single station ?source, multi-stations and single source, multi-stations and multi-sources method, and due to that Q value dependent on the frequency, studying on attenuation using observed seismic wave in frequency domain is the main method at present.The seismic data to be used include free oscillation data, body wave, surface wave, coda wave, Lg wave and seismic intensity data, etc. Because that Lg wave is the most prominent phase and carry much more energy than other local seismic phases, and that it can transfer for a long distance in the continental crust interior and its'amplitude is very sensitive to the variation of crust structure and the crust physical parameters, therefore QLg is an important parameter revealing the characteristics of crust structure, an important parameter in studying on seismic wave propagation at regional distances, as well as in doing strong ground motion simulation.With the accumulation of observed digital seismic waveform data and the development of tomography technology, the research method on QLg has been developed from getting the average value of a single station-source path to 2D tomography that could describe the heterogeneous feature of the crustal medium more exactly.Sichuan-Yunnan region locates on the southeast boundary of Tibet plateau, where Eurasia plate converges and co-forces with India plate. Special tectonic location and strong crust movement make it to be one of the most remarkable seismic active regions with high frequent and strong seismic activities in China mainland. Since so many faults with different strikes and scales intercept with each other, there are remarkable heterogeneities existing in the crust, especially the variance between the western area and eastern area. The crust depth in Sichuan-Yunnan region also varies rapidly, especially on the edge of Tibet plateau. The crust depth is 35km in the southeast area, but it turns to be 70km in the northwest area. With the operation of the Sichuan and Yunnan seismic station network, a great lot of digital broad brand seismic data have been accumulated through these years. Special location and sufficient data are the main reasons that Sichuan-Yunnan region is selected as the objective region on the research of heterogeneity distribution of QLg in this thesis.The longitude of our research region extends from 97。E to 108。E and the latitude extends from 21。N to 35。N. 22 broadband stations in Yunnan seismic station network, 14 broadband stations and 5 short period seismographs in Sichuan seismic station network are selected. To ensure that every data used in the inversion has clear Lg wave, the spectrum ratio of Lg wave to Pn wave at every frequency point was calculated. Through investigating the propagation characteristic of Lg wave, a velocity range of 2.8km/s-3.6km/s was defined as the Lg wave window to be used, and a velocity range of 6.5km/s-8.0km/s was used as Pn wave window. Only records with the spectrum of Lg wave greater than Pn wave was chose to join the inversion. Finally, we got 535 events with 5146 paths recorded by 22 broad brand stations of Yunnan and 5 broad brand stations of Sichuan to invert QLg below 1.5Hz, while 644 events with 7468 paths recorded by 22 broad brand stations of Yunnan and 14 stations of Sichuan above 2Hz. A lag-window spectral technique was applied to obtain stable estimates of the Fourier spectrum, and geometric spreading model was dealt with 2-segments model. The average QLg at all frequency points were obtained by singular value decomposition algorithm prior to the tomography inversion. According to the resolution test first, the studied region was divided into evenly spaced grid-cells with the dimension of 0.5°×0.5°, then the average QLg value was used as the initial value, and finally the QLg at each grid-cell was inversed for all frequency points. The spatial resolution of our result is less than 100km. Compared with previous work, the spatial resolution and precision of this work are relatively higher. The site effects of 36 seismic stations in Sichuan-Yunnan region and the source spectral parameters of all events were estimated simultaneously. In addition, considering the average QLg value is important to the results, the average QLg were calculated by using the method of Atkinson in the thesis, and the results show fairly small difference between them.The tomography patterns at each frequency are almost consistent, and the variation of QL?g1 in some regions is very remarkable. The spatial resolution results indicate that the resolution get better with the frequency increasing. The Q_Lg tomography results demonstrate that there exist strong lateral Q_Lg^-1 heterogeneities with large difference between the highest and the lowest Q value. Q_Lg is in the range of 150 to 650 at 2Hz. The prominently high attenuation area includes the southeast boundary of Sichuan-Yunnan rhombic block, which extends from Xianshuihe fault zone to Anninghe fault zone southward, it connected with the high attenuation area which goes through the southern segments of the Lancangjiang faults and Honghe faults in NW direction (Lancang—Kunming–Yuxi), and forming a arc like high attenuation zone on the whole. Along Xiangshuihe faults to Xiaojiang faults is the most seismic active region, one Ms8 earthquake occurred on the southern part of Xiaojiang faults. In the Simao-Pu'er region, there are also abound with regional ruptures striking to NE and NW which intercepts together, and faults striking to NW are cut into segments by the faults striking to NE-NNE and form the crisscross structure. This region is the magnitude of Ms6 seismic active zone.Another high attenuation region is Dianxi block, which consists of Jinpingshan–Yulong overriding structure belt, near NS of Chenghai rupture, Lijiang-jianchuan rupture, as well as the southern part of Jinshajiang faults and the northern part of Honghe faults. These regions are also the main regions with strong seismic activities. The other high attenuation regions include area nearby Songpan, west of Longmenshan. There is also relatively high attenuation near the Batang strong earthquake rupture region.Obvious low attenuation areas of Lg wave include the stable eastern Sichuan basin, southeastern Yunnan and Middle of Jinshajiang and Nujiang with the maximum value of Q_Lg reaching 650 at 2.0Hz. There are scarcely earthquakes above 6 occurred on the Middle part of Jinshajiang and Nujiang faults in history. Seismic activities in Sichuan east basin and the southeastern block are also quite rare and they are very stable blocks. Besides, the interior of Dianzhong block locating on the center of the two obvious low Q_Lg values shows relatively low attenuation feature.The high attenuation distribution of Lg wave in Sichuan-Yunnan region correlated with seismic activities, distribution of rupture zones and high terrestrial heat activities. It reveals that strong tectonic activity, broken medium caused by large earthquakes, and heat flow upwelling along active faults may be the main reasons of low Q_Lg in Sichuan-Yunnan region, whereas low attenuation regions correspond to the characteristic of stable tectonic block, weak tectonic activity and high velocity.From 1 to 5Hz, the site effects of Yunnan stations are quite flat and below 2, whereas at sites between the boundary of Sichuan basin and the eastern Tibet plateau, it reaches to 14. This phenomenon may be due to the strong variance of the tomography and terrain and relief of basin's edge.