Application Research Of Observation Sequence Spectrum And Source Parameters

This thesis is divided into two parts. The first part is that using wavelet transform and Hilbert-Huang transform to do spectrum analysis for seismic observation data, the second part is to determine and analyze the focal mechanisms of Wenchuan Ms8.0 aftershock with Ms≥5.0, and we also study regional stress field characteristics of the Longmenshan fault zone and source parameters of small earthquakes in Zipingpu Dam before Wenchuan Ms8.0 earthquake. The obtained results are as follows.Taking the September 26th, 2003 Hokkaido MS 8.0 earthquake in Japan, the September 27th, 2003 MS 7.9 earthquake on the borders of Russia, Mongolia and China and the December 26th, 2004 Sumatra MS 8.7 earthquake in Indonesia as research data, we select the oscillatory wave data of these three great far-field earthquakes observed by digital water level meter and deformation instrument in Sichuan, using the wavelet analysis method to research spectral structure characteristic of oscillatory wave from different instrument record. We extract the characteristics value of different frequency ranges decomposed from water level vibration and deformation wave. For example the center frequency f 0, bandwidth B , the maximum amplitude value A jfin frequency domain, the frequency f Awith respected to the maximum amplitude value and so on. The oscillatory waves excited by three great far-field earthquakes all contain the higher frequency component than the usual observation data, these components may be earth's free oscillations excited by great earthquakes. Simultaneity, we obtain partial vibration mode of earth's free oscillations from water level vibration and deformation wave. Subsequently, author adopts Hilbert-Huang transform to analyze the vertical seismic waveform of the February 23th, 2001 Yajiang earthquake recorded by GDS station, the original signal is decomposed into 11 intrinsic mode functions, their amplitudes are gradually decreased, the frequency gradually become low, and the wavelength increased gradually, the important information of time-frequency-amplitude can be visually obtained from seismic signal. The energy of the vertical seismic waveform of Yajiang earthquake is mainly focused under 5Hz, between 20 and 40s.The focal mechanism solutions of 52 Ms≥5.0 aftershocks of Wenchuan Ms8.0 earthquake sequence are determined by P wave onset method. Based on seismic and geological background, taking Huya-Beichuan-Anxian as boundary, the aftershock area of Wenchuan earthquake is divided into southern segment and northern segment, according to the focal mechanism solutions of aftershocks, we consider that the nodal plane of NE direction is identical to the strike of the Longmenshan fault zone, we seek out and analyze the NE direction nodal plane parameters from focal mechanism solutions at southern segment and northern segment, respectively. Results show that at southern segment of aftershock area, the NE direction nodal planes with strike between 40°and 70°, dip angle between 40°and 90°, the dip of NE direction nodal planes present mainly high dip angle. Rake angle between 90°and 150°is in the majority, most of earthquakes are dominantly characterized by thrust with the component of right-lateral strike-slip. At northern segment, the NE direction nodal planes with strike between 70°and 80°, dip angle between 60°and 70°, rake angle between 90°and 130°, most of earthquakes are also dominantly characterized by dip-slip reverse and right-lateral dip-slip reverse faulting. The calculating results of average stress tensor are 112.0°and 96.7°respectively at southern segment and northern segment of aftershock area. The maximum principal compressive stress axes are all WNW direction; these results show that the calculating results of segment regional stress field are consistent with stress field azimuth of whole aftershock area.We select Longmenshan fault zone and water area of Zipingpu Dam as our study area. the focal mechanism solutions of small earthquakes before the 12 May 2008 Wenchuan MS8.0 earthquake are determined on Longmenshan fault zone. The focal mechanism solutions of small earthquakes show that the direction of principal compressive stress P-axis is WNW-ESE. The hypocenter fault planes are two strikes with NE and NW directions. The plane of NE direction is among N50o⁷0oE, the dip angles of fault planes are 60o 70oand it is very steep. The faults of most earthquakes are dominantly characterized by dip-slip reverse and small part of faults present strike-slip. The results of average principal stress axis are 132.6°and 116°in Zipingpu Dam and Longmenshan fault zone respectively. The Wenchuan MS8.0 earthquake has a principal compressive stress azimuth between 103°and 122°. Source parameters calculated from both Longmenshan fault zone and the initial rupture region of the Wenchuan MS8.0 earthquake show that the principal compressive stress directions are identical with the results of the main shock.According to digital seismic waveform data from Zipingpu reservoir network, we calculate the source parameters of 70 ML≥1.0 earthquakes from August, 2004 to May 11th, 2008. the range of seismic moment is 1012～1014 N·m, the source rupture radius is 371～1045 m, the corner frequency is 1.26～3.88 Hz, the seismic stress drop is 0.04～4.88 bar. The seismic moment and energy of small earthquakes have good linear relation with magnitude in the Zipingpu Reservior area, simultaneity, we also research the spatio-temporal distribution and change of apparent stress differenceΔσ_appin source region before great earthquake, and the result shows that the apparent stress values of small earthquakes are generally higher than fitted values of apparent stress in reservoir area during near three months before the Wenchuan Ms8.0 earthquake. It can be seen from spatial distribution of apparent stress differenceΔσ_app, the epicenter of Wenchuan Ms8.0 earthquake located at low stress distribution area, and the relatively high apparent stress difference is in the east area of epicenter.