Characteristics Of The Late-Quaternary Fault Activity Of The Xianshuihe Fault

Knowledge of the rupture behavior of large seismic events is critical to understand the spatial-temporal variations of strain loading and release on active faults.It is also important to study regional kinematic feature and dynamic mechanisms and assessment of seismic hazards.In 1970-1980,some classical models,e.g.periodic model,time-predicable model,slip-predictable model,quasi-periodic model,clustered model,had been summarized by scientists for fault activity and recurrence behavior of large earthquake.The time-dependent earthquake recurrence model is rooted in the elastic rebound theory that Reid(1910)proposed.This theory states that tectonic strain accumulates gradually between earthquakes and suddenly released during earthquakes.It also implies that the relationship between the slip rate and the recurrence intervals of characteristic earthquakes is linear or approximately linear on active strike-slip faults.In other words,the slip rates of strike-slip faults are consistent during different time scales.However,recent studies have argued that fault behavior does not completely follow the theory and slip rates on strike-slip faults show spatial-temporal variability over time periods.For example,some slip patterns related to strike-slip faults,such as characteristic slip,spatial-temporal slip variability and pulsed slip behavior,have been suggested during the last two decades.This knowledge provides us a new perspective for understanding the kinematic characteristics of large strike-slip faults.The large strike-slip faults are generally considered to be the boundary faults of the active blocks and their active rupture behavior are the key to understanding regional dynamic mechanisms.Unfortunately,few faults have ruptured more than once during the instrumental or historical period,nor it is difficult to identify evidence of different seismic events.As an important left-lateral strike-slip boundary fault along the eastern margin of the Tibetan Plateau,the Xianshuihe fault produced 8 earthquakes with magnitude?7 along the whole fault during the past 300 years,which showed strong activities of large earthquakes.For example,the 1816 M7.5 and the 1973 M7.6 earthquakes have successively occurred on the Luhuo segment during the past 157 years.The Daofu segment has been successively ruptured by the 1904 M7,the 1923 M7.3 and the 1981 M6.9earthquakes during the past 100 years.The high strong earthquake activity resulted the Xianshuihe fault to be one of the most promising sites for studies of earthquake prediction and seismic hazard evaluation.The fault is also selected to be one of the important research objects of China Seismic Experimental Site.This dissertation focused on the Xianshuihe fault and studies its characteristics of the Late-Quaternary fault activity.Based on high-resolution surveying and mapping of the deformed landscape,some good trenching sites were chosen for paleoseismic studies of the fault.We excavated multiple trenches,and analyzed the detailed deformation of the strata and identified paleoseismic events.Based on radiocarbon dating,we estimated the paleoearthquake sequence and recurrence behavior of the fault segments and constrained slip rates through analysis on the coseismic offsets or cumulative displacement related to the large earthquakes.The primary results of the dissertation are summarized as follows.Taking the Huiyuansi basin as a structure boundary,the northwestern and southeastern segments of the Xianshuihe fault show different characteristics.The northwestern segment,consisting of the Luhuo,Daofu and Qianning sections,shows in left-stepping echelon by simple fault strands.However,the southeastern segment(Huiyuansi-Kangding segment)has a complex structure and is divided into three sub-faults:the Yalahe,Selaha and Zheduotang faults.To the south of Kangding County,the Moxi segment of the Xianshuihe fault shows a simple structure.Six seismic events,which are dated at 776 BC,(318–545)AD,(677–833)AD,(992–1457)AD,1816 AD,1973 AD,were identified on the Luhuo segment during the past 3000years.Their recurrence intervals are 1200,324,470,592,157yrs,respectively.Four events were identified on the Daofu segment during the past 2000 years.The ages are300BC-70AD,(51-171)AD,(435-535)AD,1923 AD,respectively.The corresponding recurrence intervals are 226,374,1438 yrs.We suggested five seismic events on the Qianning segment since Holocene,which were dated at(8070-6395)BC,(5445-5125)BC,(4355-4180)BC,(625-726)AD,1893 AD.The recurrence intervals are 1948,1018,4925,960 yrs,respectively.Four seismic events were identified on the Selaha fault of the Huiyuansi-Kangding segment,which were dated at(15599-14938)BP,(11121-10041)BP,(3745-2695)BP,1725 AD,respectively.And their recurrence intervals are 4700,7360,3000 yrs,respectively.We found the surface rupture of the1700 AD earthquake on the Yalahe fault.According to the coseismic offsets of 2.5-3.5m,the magnitude of the 1700 AD earthquake was estimated to be approximately M_W7.2.Through analysis of previous paleoseismic results on the Yalahe fault,we identified three events that were dated at 5105±114 BC,1460±55 BC,1700 AD,respectively.And their recurrence intervals are both more than 3000 years.On the Zheduotang fault near Yajiageng,we found a new seismic scarp produced by the 1955AD earthquake and an older scarp possibly related to the penultimate earthquake.According to the age of the old seismic scarp,the penultimate earthquake approximately occurred at 1263-1056 BC.The recurrence interval between the two earthquakes is also longer than 3000 years.Obviously,the Yalahe fault,Selaha fault and Zheduotang fault can produce a large earthquake and the recurrence intervals of the large earthquakes are much longer than the northwestern segment of the Xianshuihe fault.We also estimate that the slip rates of the northwestern segment is unevenly distributed on the three secondary faults.We identified five events on the Moxi segment during Holocene,which were dated at 310-6308 BC,1717-1609 BC,110BC-37 AD,809-1230 AD,1786 AD respectively.Their recurrence intervals are5646,1627,1056,766 yrs,respectively.We suggest that the rupture behavior of the Xianshuihe fault may not be consistent with the time-predictable or quasi-periodic models.The behavior of large earthquakes is characterized by clustering behavior.Specifically,the Luhuo and Daofu segment showed the characteristics of abnormal strain-accelerated release.Obviously,large earthquakes were very active on the Xianshuihe fault for the past3000 years.However,there was a long seismic quiescence for at least 3000 years during the period of 3500-6500 years ago.The quiescence may balance the process of strain accumulation and release on the Xianshuihe fault,especially the high activity of the large earthquakes during the past 3000 years.The Xianshuihe fault,characterized by strong seismic activity,has been widely considered to be a fast left-lateral strike-slip fault with slip rate of 10-17mm/yr that appears to be consistent with GPS data(10-15mm/yr).However,based on paleoseismic trenching and radiocarbon dating,we used interpretation of surface ruptures to constrain smaller slip rates on the fault.Specifically,the slip rate of the northwestern segment with a simple geometric structure is less than 8.4mm/yr,the Yalahe,Selaha and Zheduotang faults are determined to be 0.7-1.0mm/yr,1.2mm/yr,1.3-3.4mm/yr,respectively.The slip rate of the Moxi segment has a similar geometric structure with that of the northwestern segment shows slip rate of 4.0-5.2mm/yr.The Xianshuihe fault shows temporal variations in slip rates.Especially,we suggested two slip pulsed that are about two to three times faster than the long-term average slip rate of 8.4 mm/yr on the Luhuo segment during the past 3000 years,which might be related to the two different stages,slower-than-average and faster-than-average strain accumulation,or an alternate process on the Xianshuihe fault.The obtained low slip rates on the Xianshuihe fault may be more consistent with“continuous deformation”model rather than the“continental extrusion”model to explain dynamic processes of the eastern margin of the Tibet Plateau.