| Part I:Dabie-Sulu orogenic belt is one of the major tectonic zones in eastern China. In order to obtain the velocity structure of the crust and upper mantle, the continuous three components seismograms about two years time span (2009.05～2011.05) recorded by Shandong, Henan, Anhui, Jiangsu, Hubei provincial seismography networks and China Broadband Digital Seismography Networks were analyzed by seismic ambient noise tomography technique. Seismic ambient noise tomography is a new passive tomography technique developed in recent years. By this method, we don’t need earthquake waves; instead, we get the Green’s function (GF) of every station pairs by stacking the correlation functions of station pairs. The traditional surface wave tomography heavily rely on the azimuths of the earthquakes, however, in seismic ambient noise tomography we can obtain a GF between arbitrary two stations, therefore, we can obtain enough rays and high resolution imaging maps.When we obtained the GFs of Rayleigh waves, we used the traditional method; but we used slightly different method to get the GFs of Love waves:we correlated the N-N, N-E,E-N,E-E components of the station pairs at first and stacking them by phase weighted stacking (PWS) method, then we rotated the four correlation functions (N-N, N-E,E-N,E-E) to RR, RT, TR, TT components. The empirical Green’s function (EGF) was T-T component and integrated it we got the GF. Finally we obtained more than5000dispersion curves of Rayleigh wave and more than4000dispersion curves of Love waves by frequency time analysis (FTAN). We reconstructed the8～32s phase velocity maps of Rayleigh waves and Love waves and6-40s group velocity maps of Love waves by Torantola inversion method. Based on the imaging results and combined them with the existing geological and geophysical research results we can reach the following several conclusions:(1) The phase velocity maps of8s (the6～10s group velocity of Love waves) well-correlated with surface geological features. According the existing research, the high velocity anomaly of Dabie-Sulu orogenic belts is not only because of its shallow semimetal layers but also the distribution of high-pressure metamorphic rocks. Huabei basin, Jianghan basin, Nan-Xiang Basin and Subei-South yellow Sea basin show low velocity anomaly due to deep sediments. (2)Phase velocity maps at16s (Group velocity at15-20s) is similar to the phase velocity at8s. There are three main reasons:at first, the phase velocity is sensitive to the S velocity at depth of one third wavelength (almost20km), but the depth resolution is poor, in another word it is a average result within the scope of certain depth; at second, so this maps is still affected by the surface geological structures. At second, the depth distribution of the high-pressure metamorphic rocks is more than10km, which affected the velocity. At third, compare to the S velocity of the Dabie-Sulu orogenic belt in middle crust is still higher than adjacent regions.(3)Dabie-Sulu orogenic belt exhibits a small scale of high velocity, which may suggest there exist high velocity materials in lower crust and upper mantle.(4)Tanlu fault zone exhibits high velocity at the periods of8-24s. This may be caused by the upwelling of the mantle material and the mass exchange between the lower crust and mantle due to in the state of extension since the Cretaceous.(5) The average dispersion curves of Love waves in Dabie and Sulu orogenic belts are all higher than the theoretical dispersion curves of AK135model, whereas the dispersions of Rayleigh waves are not, possibly suggesting strong radial anisotropy in Dabie and Sulu orogenic belts.(6)Phase velocity maps from8s to24s (Group velocity at6-30s) are consistent well with the distribution of b-value. It may imply that the results of tomography not only show the heterogeneity of the media, but also relate the stress state.(7)It has low velocity anomaly in the north of Wuhe—Shuihouling Fault (WSF), but it exhibits high velocity anomaly in the south of WSF, possibly suggesting the boundry of north Dabie and south Dabie is WSF.Part II:After the Wenchuan earthquake, there are still many small and moderate aftershocks. The seismogenic fault will active for decades. To detect the shape of the fault is one of the most important aspects in Geophysics. In order to cooperate with the Wenchuan fault scientific drilling (WFSD) program and understand the shape of the shallow faults near the boreholes by the micoro earthquakes (ML-2～3) detecting,15mine seismographs produced by Institute of Mine Seismology(IMS) in Africa were deployed in Tianchi township, Mianzhu city and Nanba township, Mianyang city, Sichuan Province, respectively. Because it was the first time for us to use mine seismographs to detect natural microearthquakes, we encountered some problems in layout of stations, data collection and data processing. In this paper we talked about how to solve these problems in some extent.For the property right protecting, IMS didn’t provide the origin format, but they offered a software called JMTS by which we were able to transfer the origin data to ASCII format. To facilitate the data storage, exchange and processing, what we did at first was transfer the ASCII format to SAC binary format. Then we used S transform to compare the data from the micro seismographs to the data from the short-period seismograph near the micro seismographs, we found that the data from the micro seismographs was reliable in the condition that arrival time difference of P wave and S wave was less than1second.The aperture of the microseismic network is too small, so a lot of microearthquakes are outside of the network. It is difficult for us to determine the earthquake location. We used the source scanning algorithm (SSA) with two more constraints:first we used polarization analysis to constrain the earthquakes’azimuths; second we used Wadati method to constrain the origin time of earthquakes. With these two constraints we was able not only constrained the location, but also improved the computational efficiency. The locations are discrete and have no obvious predominant directions, possibly indicating there are no small blind faults in this region, and this conclusion is also supported by a few similar earthquakes. The earthquakes magnitudes range from-1.5to125km2around the stations can be reliably monitored by the IMS micro seismographs. |
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