Structural Assemblages And Response To Strong Earthquake Of Several Basins Around The Ordos Block

China is one of counties with many earthquakes, where intracontinental activebasins are widespread. A series of major earthquakes occurred in theseintracontinental basins. Study the relationship between earthquakes and basins is ofgreat significance for earthquake prevention and disaster reduction. In recent years,the globe is in a seismicly active stage. The earthquakes such as the2008WenchuanMs8.0,2009Yushu Ms7.1and2013Lushan Ms7.0all caused heavy casualties andproperty losses. So at present, it is a highly concerned issue what places would be hitby strong earthquakes and need to be monitored. The basin system around the Ordosblock is an important zone with seismic hazard. The historical records show that since1000AD, more than15large earthquakes (M≥7) occurred in this region, amongwhich the magnitude of5events exceed M8. The basins around the Ordos block arecharacterized by superior geographical environments, abundant natural resources,large population and city agglomeration. Therefore, the great potential dangers ofearthquake hazards force people to make a real effort on seismic risk analysis in thisregion, and the cognition degree about the features of causative structure is thefoundation of all things.It is possible to acquire key information of seismogenic structures fromearthquake history. However, the history of instrumental records of quakes is too shortto provide enough information about the long-term law of seismic activity. Thoughthe historic records can provide information of a longer-term (perhaps hundreds tothousands of years), these records have some defects in completeness and richness,and cannot meet the requirements of seismic risk analysis. Paleoseismology canreveal the information of earthquakes preserved in geological records, and providefoundation for study of causative structures and seismic risk analysis.Lots of efforts have been made in the areas around the Ordos block, yielding acomprehensive cognition to single active faults in this region. In addition, many active basins in the region host2or more active faults, and earthquake cases show that largeearthquakes can often rupture more than two faults (or fault segments). Hence,determining whether multi-faults distributed in these basins take the form of causativestructure assemblages is vital to the seismic risk analysis of the region, and canprovide study cases for researching the interaction among neighboring faults andearthquake triggering.In this thesis, by analyzing and summarizing the rupture features of historicalearthquakes, four basins (Ganyanchi basin, Yinchuan basin, Hetao basin and Lingqiubasin) around the Ordos block, in which2or more active faults that (might) haveruptured in the same historical earthquake, were selected for detail research. Based onprevious data and image interpretation, tectonic landform, distribution of active faultsand structural assemblages of the four basins were analyzed. Furthermore,comparative researches of paleoearthquakes on the active faults in these basins weremade. Next, the features and distribution of earthquake ruptures related to historicalearthquakes and paleoearthquakes were explored. Then, the possibility and conditionsof the structural assemblages of these basins rupturing at the same time werediscussed. Finally, the behaviors in response to major earthquakes of these basins andthe relationship between the assemblage styles and seismic intensity were discussed.The following conclusions were drawn:1) The Ganyanchi basin, which is located in the Haiyuan fault in thesouthwestern edge of the Ordos block, is a typical structural assemblage of pull-apartbasins is with boundary strike-slip faults, boundary normal faults and intrabasinnewborn faults. By comparative research of the paleoearthquake on the boundarystrike-slip faults and intrabasin newborn faults, combining the data of residual surfacerupture associated with the1920AD M8.5earthquake,3response types of theGanyanchi basin were suggested.(i) When large earthquake similar to the1920ADevent occurred, two boundary strike-slip faults, boundary normal fault andintrabasinal newborn fault would rupture at the same time.(ii) When earthquake slightly smaller than the1920AD event occurred, only one boundary strike-slip faultwould rupture. Another boundary strike-slip fault would not rupture or rupture aftershort intervals and the intrabasin newborn fault might not rupture, or at least notrupture obviously at surface.(iii) In the basin area, earthquakes smaller than the aboveones could occur and rupture only at local segments along the intrabasin newbornfault, and boundary faults did not response.2) In the process of the extinction of the Ganyanchi pull-apart basin, a series ofpaleoearthquakes occurred, and larger earthquake similar to the1920AD eventamong these earthquakes might play a positive effect on this process. The growth ofthe intrabasin newborn fault may cause some smaller (moderate or strong)earthquakes and its model may be a progressive process propagating from thesouthern piedmont fault of Mount Huangjiawa to the northern piedmont fault of theNanhua and Xihua Mountains.3) The structural assemblage of the Yinchuan basin is made up of the boundarynormal faults controlling basin (the eastern piedmont fault of Mount Helan along thewestern edge and the Huang River fault along the eastern edge) and the intrabasinburied faults (Yinchuan fault and Luhuatai fault) are subparallel to each other.Geophysical exploration data reveal that at depth in the earth, the Yinchuan fault andLuhuatai fault converge with the eastern piedmont fault of the Mount Helan whichconverges with the Huang River fault at depth. Comparative research of main activefaults in the Yinchuan basin shows that the north segment of the eastern piedmontfault of the Mount Helan and the Yinchuan fault ruptured in the same earthquakesimilar to1739AD event. The recurrence interval of the southern segments of thesefaults is longer than the northern region, and the seismicity is characterized byjumping back and forth from the southern segment of Huang River fault and easternpiedmont fault of the Mount Helan.4) The main active faults in the Hetao basin include the Langshan-Seertengshanpiedmont fault, Wulashan piedmont fault and Daqingshan piedmont fault. These three right-stepped, oblique-arranged active faults on the front of the Yinshan Mountainsare boundary faults of three sub-basins respectively. Comparative research of the mainactive faults and the previous studies show that (i) the Daqingshan piedmont fault isthe causative fault for the849AD earthquake, and the7BC earthquake might occuron the Langshan piedmont fault.(ii) These active faults have relatively independentseismic activity, but the possibility that the Daqingshan piedmont fault and Wulashanpiedmont fault rupture in one same earthquake can never be completely dismissed.5) The Lingqiu basin is located in the south of the Jinbei extensional structurezone in the Shanxi fault-depression zone. In the basin, NE-NEE and NW-NNW faultsdeveloped. Combing the exploration of the main active faults, paleoseismic study, andthe case of1626AD M7earthquake, this work suggested that the Xianandi fault andHuashanhe fault constitute a conjugate causative structure assemblage, both willrupture when an earthquake occurs.6) Based on the above conclusions, the relationship between structuralassemblages and the seismic activity of basins:(i) The conjoining section of theintrabasin newborn fault and one boundary strike-slip fault with larger strike-slipdisplacement may be associated with the seismogenic location of large earthquakes,and the leading edge of the propagating intrabasin newborn fault may be thedeveloping location of moderately strong earthquakes.(ii) Structural assemblagedistributing in sub-parallel and intersecting in depth can produce large earthquake.(iii)The surface rupture scale of oblique-arranged active faults controlling two or morebasins (or sub-basins) respectively may be changeable.(iv) The seismicity of thebasin with intersected structural assemblage is obviously higher than the basindeveloping structures with single direction.