| Geometric complexities of strike-slip faults,e.g.stepovers,bends and branches,are commonly considered to be persistent segment boundaries.They produce stress heterogeneities along active fault as barriers to fault slip,which impact the initiationor propagation of a seismic rupture,and potentially limiting the length of a fault rupture and the magnitude of the resulting earthquake.Results from numberical model and field observations have shown that discontinuous stepovers >4 km wide could impede rupture propagation.However,there are still big earthquakes can breach stepovers with width larger than 4 km,and result in longer ruptures along a fault.Until now,how these barriers affect,or control rupture is not well understood.How is this process related to local stress accumulattion? Another question is that will stepovers still be a barrier to slip when they are breached during one big event.Numerical simulations of dynamic rupture show quasi-periodic recurrence of ruptures that terminate at predictable locations,resulting from a combination of fault geometry and residual stress concentrations from prior rupture terminations.After multiple seismic cycles,ruptures may propagate further or breach barriers because of high residual stresses left at the ends of prior ruptures.Testing such physics-based models of the spatial-temporal sequencing of earthquake ruptures,and the link to slip barriers,requires multi-cycle earthquake archives from key natural examples.However,modern and historic earthquake records,often on the order of decades to a few thousand years,are short compared to earthquake recurrence intervals on continental active faults,thus not long enough to capture the pattern of rupture repetition.We conducted paleoseismic investigations on the northern and southern strands of the Altyn Tagh fault(ATF)within the Aksay restraining double bend.Logging of trench walls and compilation of event indicators showed four distinct paleo-earthquakes on the SATF and at least five on the NATF.The most four credible SATF events occurred at 765-1347 A.D.,787-590 B.C.,2411-2296 B.C.,3282-3132 B.C.with average recurrence interval of ~1433 ± 57 years.The four young events on the NATF with robust evidence occurred at 468 B.C.-1305 A.D.,1191-678 B.C.,2019-1905 B.C.,3652-3473 B.C.,with average recurrence interval of ~1326 ± 179 years.The occurrence age ranges of the two most recent earthquakes on these two strands overlap.The most recent events AS and AN at each site are interpreted to not correlate based on the evidence of distinctive,youthful offsets only on the SATF.The penultimate event at each site is consistent with events recorded in trench sites both to the east and west of the Aksay bend and is thus a strong candidate for a single large earthquake rupture that breached the bend.This could have ruptured at least ~310 km of the Altyn Tagh fault,from the Pingding Shan to the west,through the Aksay restraining bend to at least the Subei county in the east.This was preceded by at least four thousand years and three or more events per strand that did not breach the Aksay bend,suggesting a throughgoing rupture requires multiple prior earthquakes that terminate within the bent portion of the NATF,as well as a favorable stress condition on the SATF.This provides geological support to findings from multi-cycle dynamic rupture simulations through bent faults and suggests that prior earthquake history within areas of fault complexity,combined with such modeling,may be used to better assess the likelihood of large,multi-segment earthquake ruptures.We conducted a paleoseismic study at the Salt Lake(Ganyanchi)site in a shortcut pull-apart basin on the Haiyuan fault,within the section that broke during the 1920 Mw~8 Haiyuan earthquake.3D excavations at the site exposed fine-grained and layered stratigraphy and ample evidence of multiple paleoseismic events.Charcoal fragments were abundant in the trenches.AMS dating of charcoal fragments showed that multiple events occurred during the past 3600 years.Of these,the youngest three and possibly four events were recorded in the top 2.5 m section of distinctive thinly-layered stratigraphy.A comparison of paleoseismic with historical earthquake records suggests that these three events could be correlative to the historical AD 1920,1760(or 1709)and 1638 earthquakes.Historical accounts of earthquake damage suggest that earthquakes exposed in the trenches were markedly different in magnitude.With the exception of the most recent M~8 earthquake in AD 1920,two earlier events were considerably smaller,with magnitude M<7,and more likely M6 or less.Thus,the Haiyuan fault could produce surface-rupturing earthquakes with a variety of magnitudes,not just characteristic earthquakes.This study indicates that paleoearthquakes exposed in trenches are not necessarily similar in size,and moderate magnitude events might produce surface ruptures,which can be preserved in stratigraphy.There are three other older events,identified at lower part of trenches,occurred at A.D.350-986,B.C.1325-2031 and B.C.1985-2351 respcetively.These events were broardly correlative to events at the Gaowanzi site,to the west of our site and outside the Salt lake pull-apart basin,suggesting that probably the Salt Lake pullapart basin had been breached at least 4000 years ago.The Northern Danghe Nanshan Thrust(NDNT),arguably situated at the regionalscale stepover between the Altyn Tagh fault and the Haiyuan fault,play an important role in strain transforming from one fault to the other.The rupture history along the NDNT is thus key to understand this strain transformation.The most recent rupture along the eastern NDNT might be correlated with a historical earthquake recorded in nearby city of Dunhuang.We used methods of drone-based topographic mapping and paleoseismology to determine the age and magnitude of this rupture.High resolution topography data,produced using the structure from motion(SfM)technique,demonstrated that the vertical offset of the most recent rupture is 0.7 ± 0.2 m on average.Many measurements of higher scarps cluster at 1.3±0.2 m,2.0±0.3 m,2.9±0.4 m,3.7±0.4 m ? 5.4±0.6 m.Trench exposures across small,apparently youthful fault scarps also illustrate the similar vertical displacement with ~1.3 m horizontal shortening.According to the empirical relationship between vertical slip on reverese-type rupture and earthquake magnitude,we estimate that the most recent surface-rupturing earthquake on the NDNT was Mw 6.6±0.5.Combining geochronological data and historical earthquake documents,we argue that this earthquake occurred in A.D.1289,resulting in the destruction of the city walls at Shazhou(modern Dunhuang).Paleoseismic investigation on 3 higher scarps shows more events in the the big trenches,the number of events is basically proportional to the height of scarps.Sample dating constrains the last four events along NDNT occurred since 6700 B.P.The recurrence interval is thus 1400 years on average,and the elapsed time of last earthquake is 728 years,which is about half of the average interval.The uplift rate and shortening rate of the NDNT are 0.3 ± 0.1 mm/yr and 0.8 ± 0.2 mm/yr,respectively constrained by the heights of scarps and sample ages.The distribution of cumulative slip and paleoseismic event sequence show that the NDNT can be described as a characteristic slip reccurrence behavior =.Slip repetition suggests similar earthquake magnitude of ~ 7.Future work on the comparison of paleoseismic sequencesbetween the NDNT and the Altyn Tagh fault may shed light onthe interaction between kinematicallylinked faults of different slip types in a complex fault system. |
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