Paleoearthquake And Large Earhquake Recurrence Interval Of Tazang Fault Luocha Segment Of The East Segment Of The East Kunlun Fault

Paleoearthquake includes the prehistoric earthquakes stored in the geologicalstrata and the earthquakes without clear historical record（Wallace R E,1981）. Forgeologists, the geological record is primary（Lin Jingxing,2008）. Paleoseismologycan improve and extend instrumental records with a relatively short period andhistorical earthquakes, which can help to research the characteristic of the recurrenceof strong earthquakes in a longer period and set up the model of recurrence, to analyzethe seismic risk, and it is significant for the continental interior with a longerrecurrence interval（Deng Qidong,2008）. In addition, paleoseismology is one of thekey components for the research of active fault, which is important in areas likesegmentation of active fault, active intensity contrast, dynamics study（Min Wei,1998）.Tazang Fault is situated in the east segment of the East Kunlun Fault, inside therange of the Maqu seismic gap （Wen Xueze et al,2007）. The coulomb stress onTazang fault was increased caused by the Wenchuan Ms8.0earthquake（Shao Zhiganget al,2010）, and the result from the regional GPS shows the strain field ischaracterized by an increase（Ren Jinwei et al,2005）. However, there is littlequake-proof research work about paleoearthquakes and the seismic risk of the regionhas not been profoundly recognized. Through3months’ field investigation, this paperexplores paleoearthquakes in the aspects of history earthquake-related landforms andtrenches near Tazang Fault, and obtained several viewpoints as follows:1. Earthquake-related landformsBased on field investigation, there are plenty of tectonic landforms distributingalong Tazang fault, especially near Luocha segment, such as: offset gullies, offsetrivers, offset terraces, sag-pond, slop-hanging-ditch, fault-scarps, linear abnormalvegetation, et al, forming a deformation belt with an approximate length of50km. Thedistribution ranges of offsets are:（5.5⁶）m,（18²7）m,（68⁷5）m and（200²20）m, and （5.5⁶）m corresponding with the landforms are most common,relatively new. According to the statistics of DGPS data around the three trenches, the left-slipdisplacement are22m,71m around trench1, and25m,27m,72m near trench2andtrench3, and the distribution characteristic of displacements near the three geomorphicsurfaces is near to the and trenches’. All above displacements are mainly distributedin two ranges:（22²7）m and（71⁷2）m, which might be caused by severalpaleoearthquakes.2. Paleoearthquakes by trenchesWe have excavated5trenches and got33dating results of samples of14C in all.The strata of the5trenches have a significant difference. Because of thenondeterminacy coming from stratigraphic record, the occurrence-time of earthquakesand grading phenomena of earthquakes, there is much nondeterminacy in stratigraphicrecord related paleoearthquakes. Therefore, it is better to carry out the stratigraphiccorrelation in the analysis of earthquakes using trenches to lessen the nondeterminacy,from plural perspectives: the characteristic of sediment, sedimentary environment andtectonic information. Taking the indeterminacy of the source of14C samples inearthquake-wedges into account, the data from upper covered strata must beconsidered to constrain the time of paleoearthquakes.Synthesizing materials of the relation between cut and cover of strata,earthquake-wedges, and dating results, after the stratigraphic correlation and multipletrench constraining method, we can conclude that the occurrence time of the2constrained events are respectively as follows:（320±30）^500±30aBP and（2730±30）^2840±30aBP, and the departure time of last earthquake is（320±30）^500±30aBP. The recurrence interval is about2300a.Taking the attenuation gradient of velocity、velocity and the distance from Maquto Luocha，the estimated velocity of Luocha segment can be gained as3.01mm/a. Bythe estimated velocity and the maximum co-seismic displacement, the recurrenceinterval can be gathered as2000a.Ground on the relationship of rupture length, maximum co-seismic displacementand magnitude, the calculated magnitude can be gathered as: Ms7.5and Mw7.4. 3. Discussion about the recurrence intervalsThe recurrence interval obtained by trenches is close to the interval obtained byvelocity, which may favor relatively complete earthquake sequence.Combining with the paleoearthquake data of the East Kunlun Fault, It can be foundout that, since2800a, the recurrence intervals of all segments from west to east showan rising tendency, which is corresponding with the speed tendency from west to east.