Research On The Tectonic Field In Gansu And Its Adjacent Areas

Gansu and its adjacent areas belong to the northeastern margin of theQinghai-Tibet plateau. The study area locates the junction of Alashan, Qilian, Qaidamand Ordos blocks. The tectonic structure is complicated. The research of tectonicstress field in the study areas has very important scientific significance for earthquakeprediction in Gansu area.This dissertation is composed of three parts：In the first part, characteristics of present-day tectonic stress field in Gansu andits adjacent areas is studied based on245ML≥3.5focal mechanism solutions sinceJanuary2001to June2012. The results show that the dip angle of principalcompressive stress axis of the whole region is generally small. And the dip angle ofprincipal tensile stress axis varies widely. It is small in the western part, whilerelatively larger in the east. The maximum principal compressive stress axis strikesnearly north-east direction in the plate margin of Tibet plateau. At the outer edge ofthe Tibet plateau plate, its strikes show a radiated pattern, i.e., NS in west part,towards NNE in the middle section, and NNW in the East. The relative stress valuesare more than0.5in the western areas and less than0.5in the eastern areas. Thatmeans that the maximum principle compressive stress solutions are more reliable inthe western areas, while the maximum principle tensile stress solutions are morereliable in the eastern areas.In the second part, we use the data recorded by the record of the broadbandportable seismic array in the southeast area of Gansu province and the fixed seismicstations in Gansu and surrounding provinces. Based on the near earthquakeobservation data recorded from November1,2010to November30,2011, we relocatethe earthquakes occurred in this area by Zhang’s (2012) Method. Finally,2379earthquakes and12129P wave polarity data are obtained. The directions of principalstress axes of Southeast Gansu are inferred according to the large number of P wavepolarity data in this research. The result shows that the maximum compressive stressaxis strikes performs a radiated pattern, i.e., NEE in north part, towards EW and SEEin the East. Maximum principal compressive stress axes are steep on the South-Western margin of the Ordos block. This is probably because of the materialhere moving to east along the West Qinling fault, therefore the stress environment istensional.In the last part, we use Wan’s (2008) method to determine the fault surfaceparameters. And the results of stress field above are used to calculate the slip angle.We obtained five faults’ strike, dip and slip angles and their deviation.Compared with previous research, this research used data richer and can solvethe stress field on detail. The direction and the relative size of the tectonic stress fieldin1°×1°grid in Gansu and its adjacent areas and0.25°×0.25°grid in southeast areaof Gansu province are presented for the first time. The stress directions show that thesource of the stress field in Gansu and its adjacent areas is driven by the India plate toEurasian plate extrusion. Inversion has similarities with the previous results, reflectsthe correctness of the approach.