Regional Characteristics Of Seismic Anisotropy In Crust

Seismic anisotropy is one of the main specialties of the Earth, which can be well understoodby shear-wave splitting. In anisotropic rocks, shear-wave splits into two approximately orthogonalpolarizations that travel at different velocities and write characteristic easily-identifiable signaturesinto three component seismic wave trains. The wave at faster velocity is fast shear-wave, while thewave at slower velocity is slow shear-wave. The main parameters of shear-wave splitting arerespectively polarization of fast shear-wave, and time delay of slow shear-wave.The following two aspects are the main work on seismic anisotropy in crust in the thesis.A Seismic anisotropy in crust in Capital and Fujian areasThe requirements of shear-wave splitting analysis are very strict, which means that the waveshould be within shear-wave window with high signal-noise ratio. Thus, seismic anisotropyresearch in regional areas is not so abundant before regional seismic networks are widely built inChina. Since the end of 20st century, many regional seismic networks are built, and a lot ofwaveform data are available, which are quite good for the development of research about seismicanisotropy.Capital area is in the north of North China, including three main geologic structures (TaihangUplift, Yanshan Uplift, and North China Basin). Furthermore, Zhangjiakou—Penglai fault is themain seismic structure in the area. As for Fujian area, it is in the southeast border area of Eurasiaplate, and is near the crossing area of Eurasia plate, Pacific plate, Philippine plate. The geologicstructures in the Capital area and the Fujian area are complicated, which are always the interestedresearch areas in China.With SAM technology (Gao et al., 2004), the seismic waveform data in Capital area (Jan.2002-Aug. 2005) and in Fujian area (Jan. 1999～Dec. 2003) are analyzed. Seismic anisotropy incrust in the Capital and Fujian areas are obtained, and some meaningful conclusions are available.(1) The average polarization of all the records in Capital area is 79.9°±44.3°, whichcorresponds to the direction of maximum horizontal principal compressive stress in North China.Similarly, the average polarization of all the records in Fujian area is 109.4°±42.6°, which isrelated to the direction of maximum horizontal principal compressive stress in South China. Theresults show that the average of polarization of fast shear-wave parallels to the maximumhorizontal principal compressive stress, and is an effective tool to depict the regional stress field.(2) At different local structure areas, the seismic anisotropy specialties in crust are different.(3) Locally complicated stress structures influence on the polarizations of fast shear-waves.For example, the predominant polarization is usually related to the strike of the fault, if the stationlocates on (or near) an active fault; Polarizations are very complicated, or multi-predominant polarizations are shown, if the station locates in the crossing area between two faults.B The specialties of shear-wave splitting before M＞6.0 in 2000 in IcelandThe temporal change of seismic anisotropy in crust is an important research item. However,the requirement of the research is much stricter, which needs the successive small events recordswithin shear-wave window.Iceland, in the west border of the Europe, is passed through by Mid-Atlantic Ridge runningonshore in Southwest and North Central Iceland. Both the seismic and volcanic activities arefrequent in Iceland, which provides a lot of waveform data.In Jun. 2000, a series of M＞6.0 earthquakes occurred in the transform fault in SouthwestIceland, which is the strongest event since recent 30 years. With SWAS technology, theshear-wave splitting parameters of nearly 6 months' small earthquakes before the main shock areobtained. The results show that the time delays of station SAU and BJA show some interestingphenomenon before the main shock. The time delays of slow shear-wave increase with the stressaccumulation before the main shock, while they decrease quickly with the stress relaxationimminently before the main shock.Integrating other events in Iceland, several events in USA, events in Chinese mainland, andChi-Chi event in Taiwan, the relation between seismic magnitude and the duration of time delayincreasing, and the relation between seismic magnitude and the duration of time delay decreasingare calculated. It is found that the relations all show linear direct proportion.Considering seismic anisotropy in crust, seismic activity, small earthquakes relocation, smallearthquakes focal mechanism, seismic anisotropy in upper mantle, seismic tomography, GPS andso on, some preliminary conclusions are available.The principal stress direction is related to the polarization of seismic anisotropy in crust;Multi-predominant polarizations (or complicated polarizations) seem to be connected to thevelocity structure; The relation between seismic anisotropy in crust and in upper mantle can showthe geodynamic specialties of shallow and deep structure, and can hint information aboutgeodynamic mechanism; GPS results show the movement velocity of surface, which can testifythe complicated stress field, just like seismic anisotropy in crust can do; Seismic activity andseismic relocation can show the regional structure information, which just correspond to thestructure information shown by seismic anisotropy in crust.Both the spatial and temporal changes in seismic anisotropy in crust are discussed in thethesis. In addition, the relation between seismic anisotropy in crust and other geophysicalphenomenon are discussed too. Some meaningful conclusions are obtained for the research aboutseismic anisotropy and stress specialties.