Long Paleoseismic Record On The Wuzunxiaoer-Xorkoli Section Of The Central Altyn Tagh Fault

The 2000 km-long Altyn Tagh Fault(ATF)is one of the longest active strike-slip faults in the world,forming the boundary between the Tibetan Plateau and the Tarim Basin in western China.The ATF accommodates a portion of the relative horizontal motion between the Indian and Eurasian plates and is the key active fault in kinematic models of the Himalayan-Tibetan orogen.Several studies of the ATF have demonstrated a relative displacement of 350-400 km on the eastern and central segments.The slip rate of the ATF along the central segment is 8~10 mm/yr.Along the western segment of the ATF,two earthquakes(Yutian MW7.1 and Mw6.9)produced surface ruptures with 31 and 28 kilometers in 2008 and 2014,respectively.There are nearly 1100 kilometers of documented surface rupture along the ATF,demonstrating that large earthquakes have occurred recently along most,if not all,of the fault.However,earthquakes able of producing surface rupture are not documented in history.Although several paleoseismic studies were carried out along the ATF,not enough temporal continuity to construct a robust rupture history.Thus,the lacking of historic strong earthquake and long series of paleoseismic record limits seismic assess of the ATF.Large plate boundary strike-slip faults usually have their own characteristic recurrence pattern.For example,the Alpines fault in New Zealand has strong quasi-periodic pattern of earthquake recurrence;the North Antonia fault has domino's surface rupture;earthquakes occurred on the Dead Sea fault is clustered.Does the ATF as one of the largest intra-plate strike-slip faults have its own special recurrence pattern of strong earthquakes? Between 85°E and 95°E longitude,the ATF is divided by a series of restraining double bends into five right-stepping segments.Is each segment isolated fault with its own erthquakes rupture? Does bend completely halt earthquake propagation? Do earthquakes occurred on both side of bend have interaction with each other? Are the paleoseismic record able to test the propagation of large earthquakes?In addition,number of earthquakes record can effect recurrece pattern.Long series of earthquake records are usually located on large plate boundary faults,but less on intra-plate faults.What kind of sedimentary environment can reveal more paleoearthquakes? How can we assess the reliability of event evidence more objectively? What's the main factor that affect the coefficient of variation(COV)of earthquakes?In order to anwser these questions,Wuzunxiaoer-Xorkoli segment of the central ATF is chosen to our study area.Here,this segment is separated into two sub-segments,Wuzunxiaoer and Xorkoli,by Aksay,Pingdingshan,and Akato Tagh double bends.Salty lake and fans were faulted by the Wuzunxiaoer segment.The Xorkoli basin is an elongated valley along the central ATF.The valley is an arid region of alluvial fan and playa environments that make the basin an excellent candidate for paleoseismology.By compiled information for worldwide paleoseismic data of strike-slip faults,sedimentary environment and event evidence are discussed to determine where is suitable for revealing long sequence of paleoearthquakes and how to assess the reliability of event indictors.We also recalculated the COVs of long series of earthquakes and want to find factors that are good predictors of COV.Above all,from this study,the following insights are attained:1)Successive,high sedimentary rate,rhythmic strata with high resolutionEighty five paleoseismology paper of strike-slip faults were compiled.Much information of trenching is collected,such as coordinate,altitude,geomorphic location,sedimentary rate,strata character,number of events,dating method,and coseismic slip.Trenches revealing more than five events are mainly located on sag pond,pull-apart basin,small scale lake and basin,alluvium fan,and shutter ridge.Except alluvium fan,all others are easily water agglomerated environments and deposition rate is much higher.Seasonal rainfall and sediments from different provenance easily form rhythmic and successive strata in above environments.Therefore,these geomorphic area should be considered when you decide excavation places to get long series of earthquakes.2)Event indicators and classificationEvent indicators were also picked out,such as vertical offset,upward terminations of faults,growth strata,fissure fills,angular unconformity,liquefaction,colluvium,and folding.Vertical offset,growth strata,fissure fills,angular unconformity and folding are high-quality event indicators depend on which exact event horizons can be identified.Upward terminations of faults and liquefaction are poor-quality indicators.The quality of indicators are usually lowered by erosion,disturbance of animal and plant,and superimposition of the deformation from a younger event.Thus,to objectively assess reliability of events,all kind of cases should be considered,such as indicator rank and total number of event indicators observed.3)Relationship between COV and slip rateWe recalculated the COVs from 35 trenching sites with long records of paleoearthquakes on strike-slip faults.The results suggest that 74.3% of COVs are less than 0.87,indicating that quasi-periodic recurrence is common for strike-slip faults.In addition,faults with higher slip rate have lower COVs relatively.4)Long records of paleoearthquakes at Copper Mine site on Xorkoli segmentTwo large trenches were excavated at the Copper Mine site in 2013 and record eight or nine events.The event indicators are fissure fills,angular unconformity,folding,growth strata and vertical offset of strata.Most of strata have carbon dating materials which are mainly wood pieces,and sporadic dung.The main trench,T1,was dug more wide and deep in 2016.The depth of new trench is nearly 13 m.Evidences revealed in the new trench confirmed the events identified in old trenches.Moreover,one oldest event is identified in the new trench.Thus,totally nine or ten events were identified at the Copper Mine site.They occurred at 1490-1737 A.D.,676-926 A.D.,732-589 B.C.,1206-716 B.C.,1369-1235 B.C.,2233-1987 B.C.,2731-2601 B.C.,2878-2743 B.C.,3522-3204 B.C.,4568-3980 B.C.The average recurrence is 650±390 a,and COV is 0.63±0.03.5)Correlation between different trenching sites and surface rupture propagationThe correlation between the Copper Mine site and Annanba site indicates that some surface ruptures of the earthquakes at the Copper Mine site propagated to the middle part of the Aksay double bend,demonstrating the Multi-cycle rupture modeling by Duan and Oglesby(2005).The model is that occasional ruptures can break through a higher bend angle by taking advantage of residual stress concentrations from prior earthquake rupture terminations.6)Paleoseismic record on Wuzunxiaoer segment and surface rupture propagationThe T4 trench to west of the Wuzunxiaoer salty lake revealed four or five earthquakes.The carbon dating materials are lack in strata,especially in the lower part.Thus,the dates of these events are not well constrained except the most recent event.The most recent earthquake?occurred at 1220-1773 A.D.;Events ?,?,? occurred at 407-215 B.C.;the oldest event ? is little bit earlier than 1608-1462 B.C..By correlation between T4 and paleoseismic data on Xorkoli segment,the age range of event?strongly overlaps with the date of the most recent event on the Xorkoli segment,indicating that both sides of Pingdingshan double bend may rupture together during the most recent event.In addition,other events do not overlap in age,but their ages are nearly close,indicating that earthquakes on both strands may have a response with each other.Highlights in the thesis:1)Nine well-resolved events and one probable additional event are identified in the last 8000 years,which is the longest record on the Altyn Tagh fault.2)Our paleoseismic records demonstrate that occasional ruptures can break through a higher bend angle by taking advantage of residual stress concentrations from prior earthquake rupture terminations.3)The recurrence pattern of paleoearthquakes on long rectilinear,strong isolated fault segment is weakly periodic.4)In China,semi-quantitative method was used to assess reliability of paleoearthquakes.5)The relatively long strike-slip faults,higher slip rate tend to correspond with a lower COV.