| China is one of several most representative countries which suffer from seismic activities and disasters worldwide,and therefore the study on seismogenic structure of earthquakes is of great scientific and engineering importance.Since the end of 1970 s when there came a breakthrough of active structures in terms of study,theory and method,an important progress of the fundamental research on the relation between earthquake and seismogenic structure has been achieved.Nevertheless,due to the limitation of various aspects such as geological conditions,geographical conditions,historical conditions and natural conditions,many studies on seismogenic structure of earthquakes are questionable.As historical difficult seismic regions,remote difficult seismic regions of inconvenient transportation and difficult seismic regions without surface rupture are formed,the traditional method to study active strictures is not applicable to all above-mentioned difficult seismic regions.The Southeast of Gansu Province,located in the northeastern margin of Tibetan Plateau,is one of the areas with strong structure activities and frequently-occurred strong earthquakes in China.The earthquake of magnitude 8.0 occurred in Tianshui in1654 and the earthquake of magnitude 8.0 occurred in Wudu District in 1879 are recorded as the two largest destructive earthquakes in history in this area.Due to long elapsed time of earthquakes,complicated landform,development of landslide disasters,strong ground surface modification in later period,and low research degree,there is no definite conclusion and cognition on seismogenic structure and activity property about the Tianshui earthquake of magnitude 8.0 and the Wudu earthquake of magnitude 8.0,which have become two typical doubted earthquakes in the history.This paper is based on the previous investigation achievements of seismogenic structure about the doubted earthquake zone and the author's self-investigation achievements in this field,and by analyzing the work content and the application condition of earthquake geology,physical geography,and measurement technology for terrain deformation,meanwhile considering the geologic structure background in Gansu's southeastern areas,and adapting non-traditional technological methods in the investigation of seismogenic structure about two historical doubted earthquakes in Gansu's southeastern areas,the new discoveries are listed as following:1.The detailed field seismological findings showed that Hanan-Qingshanwan-Daoqizi fault zone was the most reliable active fault zone found in the meizo seismal area of Wudu earthquake of magnitude 8.0 in 1879.The overall strike direction of the fault zone was 60°-70°.It was composed of north branch of Hanan-Qingshanwan-Mobali fault(F1)and south branch of Gaolou Shan-Yuelian gba fault(F2),with the length of 80 km and 60 km respectively.The new activi ty is strong in north branch and weak in south branch.2.Hanan-Qingshanwan-Daoqizi fault was a thrust fault with high dip angle and right-lateral strike-slip component.The fault dislocated the Holocene loess,which suggested that it was the primary seismogenic fault of the earthquake of magnitude8.0 occurred in Wudu District.Due to lack of consistent twist evidence of gully level in seismological survey,the determination of the fault strike-slip activity and sliding speed need to combine other technical methods and data.3.Observational data of regional three-dimensional crustal movement velocity field showed that northeastern margin of Tibetan Plateau took place horizontal escape movement along Weihe River Basin and south of Erdos when moved towards the north direction.The southern boundary of horizontal escape included Longmenshan fault zone,Wenxian-Wudu-Chengxian-Taibai fault zone and Lixian-Luojiabao fault.4.According to qualitative judgment of regional GPS horizontal earth movement velocity field,Hanan-Qinshanwan-Daoqizi fault zone was the seismogenic structure of Wudu earthquake of magnitude 8.0,while Lixian-Luojiabao fault was the seismogenic structure of Tianshui earthquake of magnitude8.0.Nowadays,both of them showed dextral strike-slip movement activity.5.According to the rough estimation of dextral diastrophism features in the Yangtang River and Longba River in meizoseismal area of Wudu District,the dextral strike-slip speed of Hanan-Qingshanwan-Daoqizi fault zone was 0.8-1.0mm/a.By adopting non-traditional investigation methods,this paper provides new perspective and comprehension for the investigation of seismogenic structure about historical doubted earthquakes in Gansu's southeastern areas,and the results have great importance in terms of reference and application for constructing regional activity model,seismic regionalization and mid-and-long term prediction in strongshock. |
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