Activity Features Of The Fangzheng Section Of The Yilan-Yitong Fault Zone In The Late Quatenrary

The Tan-lu fault zone plays a significant role in the tectonics of eastern China.More importantly, it is also the largest seismic zone in East China, where many majordevastating earthquakes have occurred along this zone. These events are unevenlydistributed in both time and space, mostly took place within the Weifang-Jiashansegment and Xialiaohe-Laizhouwan segment. Therefore, many researches onneotectonic movements and active tectonics focus on these two segments. As thenortheastern branch of the Tan-lu fault zone, the Yilan-Yitong fault has no majorearthquakes greater than M6, nor the reports of activity during the late Quaternary. Atpresent, it is generally considered to be active in the early-middle Pleistocene.However, new discovery has revealed that there exist two earthquake surface rupturezones within the Yilan-Yitong graben which may indicate a M7event that might haveoccurred in recent time. These new discoveries contradict with the generalunderstanding of the Yilan-Yitong fault. This thesis focuses on the Fangzhengdepression which is located in the central-northern Yilan-Yitong graben, and makesintensive studies on the latest activity features of this segment of the Yilan-Yitongfault.Through the analysis of previous data, along with interpretations of high-resolution satellite images and detailed field investigations, the geometricaldistribution of the latest active fault is determined. Great efforts have also been madein the measurement and dating of mircotopography offset by the fault to calculate itsslip rate in the late Quaternary. In the meantime, trenches are excavated in an attemptto analyze its rupture history ever since late Quaternary. Besides, based on theanalysis of deep geophysical data and the regional geodynamics, one possible modelfor the formation of the earthquake fault is proposed. Through this thesis, majorconclusions are drawn as follows:(1) The Yilan-Yitong fault zone exhibits three large-scale faults in its Fangzhengsegment, of which two are the boundary faults of the Fangzheng basin and the otherone is the associated fault which develops in the central basin (named "YihantongFault" in petroleum industry). Since the late Pleistocene, no sign of new activity of the boundary faults is found, while the latest activities mainly occur on the fault in thecentral basin.(2) The latest active fault turns out to be a Holocene active fault, which extendsabout70kms in NE direction. It can be divided into two geometrically discontinuoussegments, namely Lianhua pond-Haha pond segment and Haha pond-Xinxingreservoir segment. Along the fault, features of displaced landform appear obviouswith a series of ponds along the fault whose axes extend in the same direction as thefault. The heights of fault scarps are hierarchically distributed and the direction of thescarps got inverted along the northern segment of the fault.(3) Based on geomorphologic mapping in the field,4phases of geomorphicsurface along the fault can be identified. Among them, the latest geomorphic surfaceT0is the flat floodplain which is mainly composed of Q43or Q42alluvial deposit andmodern fluvial deposit. Features of displaced landform appear not conspicuous on T0terrace. The secondary geomorphic surface refers to the T1and T1’ terrace of theSonghua River and T1terrace of the ant river which is one of the first-class tributaryof the Songhua river. T1terrace is mainly composed of Holocene fluvial deposit andcommonly displaced by the fault, creating a~1m high scarp. The third-classgeomorphic surface refers to T2terrace that formed during the late Pleistocene uponwhich are distributed2.4~4.4meters tall scarps. The oldest planation surface isidentified as T3terrace located at the foot of the low hill in the study area and ismainly made up of middle Pleistocene loess-like sand and gravel. The fault avoids theoldest planation surface on the whole.(4) The fault is dominated by a right-lateral strike-slip with a relatively smallvertical motion component and unstable dip-slip. Jointly Influenced by the long-termdextral strike-slip motion of the fault, and the change of the inner dynamic conditionsof the river caused by the motion of the fault, a big elbow-shaped curve emergeswhere the fault intersects the Songhua River. Through analysis and deduction of thedisplaced gully north of the Lingchang village, it is suggested that the dextral slip ratemight be above0.41±0.1mm/a, but less than0.7±0.07mm/a since late Pleistocene.The fault’s dip-slip remains unstable with its northwestern wall thrusting over itssoutheastern wall for the lianhua pond-haha pond segment while a pivotal faultcharacterizes the Haha pond-Xinxing reservoir segment. Through calculations ofwell-preserved geomorphologic scarp, it is considered that thrusting rate of the faultsince the late Pleistocene might be around0.1mm/a which is much less than itsstrike-slip. (5) Excavations of trenches in typically displaced microtopography reveal fivepaleo-earthquakes in total. Based on analysis of the phenomena exposed by10trenches we dug, such a conclusion is made: the latest earthquake event, Event A, issupposed to occur during1730±40yr BP～684±20cal yr BP with quite reliableevidence. Event A represented the latest earthquake which ruptured the wholesegment. Event B took place during28.5±1.2ka B.P.~21.7±0.9ka B.P.. It wasshown in only one trench located in Nanlou village and could hardly be comparedwith events revealed in other trenches. Three earlier earthquake events took placebefore abandonment of T2terrace and were revealed in trenches located on T2terrace.A rough conclusion is drawn that at least one event occurred during35812±329calyr BP~33.0±2.4Ka BP and probably another two occurred before it. Evidence forthese three events could only be found in the northern segment of the fault, bringinguncertainty about whether these events were confined to the northern segment orfarther spread to the southern segment. According to the previous empirical formula,the latest event occurred with the magnitude around7.2and dextral strike-slipdisplacement around3.5meters.(6) The geodynamics of Northeast China is related with the westwardsubduction of the West Pacific plate, while the far field effect by the India-Eurasiacollision only plays a minor role. The compressive force caused by theNWW-trending high-speed oblique subduction of the West Pacific plate acts upon theNE～NEE trending Tan-Lu fault zone, whose shear component and compressioncomponent respectively results in the dextral strike-slip and thrust motion on theTan-lu fault zone. The earthquake fault discussed in this thesis might be formed due tothe reactivation of the pre-existing major fault in the central basin which is associatedwith the bordering faults under the influence of this special tectonic setting and thestress field discussed above. Thus, with the special tectonic setting of the Yilan-Yitongfault zone and previous researches taken into consideration, this work proposes moreattention should be paid to the pre-existing faults developed in the central basinsalong the Yilan-Yitong graben for their possible revivals and enhancements in theiractivities.The contents and results of this work to some extent enrich the basic dataconcerning active faults in northeastern China, and provide a scientific evidence forearthquake-proof and prevention and some referable guidance for future work to becarried out in this area or other areas alike.