Activity Characteristics Since Late Quaternary And Seismic Risk Of Aheqi Segment Of Maidan Fault

The Maidan Fault is the boundary of Kalpin nappe. The geometric distribution and hiatorical seismic distribution revealed the relat ively weak seismic acticity in the foot of Kalpin nappe, so the majority of scholars believe that Maidan Fault has low activities since late Quaternary. however we found some reliable evidences about the activety of Maidan Fault since late-quaternary,which was more than the frontal faults of kalpin nappe in frequency. The activities of the Tianshan root fault migrate to the frontal fault of the prudent nappe, and the root fault activity weakened gradually. But the activity in Kalpin nappe does not accord with this mode. therefore, we believe it is a out-of-sequence thrusting fault. Based on the remote sensing image interpretation and field investigation we get the geometric distribution of Maidan Fault. by detailed geological survey, we make it clear the distribution characteristic of the ridge, water system, river terrace, outwash fan, and we try to find out the evidences of its latest activiy, such as fault scarps, river terraces and alluvial fan Ridge dislocations landforms. System measurements, comparative analysis of the fault scarp height, degree of development, direction, faulted era geomorphic surfaces. Tracking the spatial distribution and geological and geomorphological features fault rupture, and collect and analyze relevant information, revealing Late Quaternary fault activity and fault segmentation study in detail. Select the appropriate location of excavation trenches, exploration vessel for seismic events for analysis, collection of age samples. And apply those parameters to seismic hazard eva luations. The main conclusions obtained are as follows:(1)Maidan Fault is the North boundary of kalpin nappe, and it is located at the sharp gradient zone of the crust – mantle boundary between the South Tianshan and the Tarim basin. It consists of 3 approximately parallel N E faults which form an integrate detachments surface in the deep along the gypsum stratum in Cambrain, formed the progressive deformation in N-S compression, and the latest fault f1 showed the special activity(2)Maidan fault is a non-sequence thrust system in the other aspects.,Maidan Fault has a long development history, with several parallel faults, and the tracks of the surface fracture has a certain randomness. what’s more, there may occurred more than two thrusts for the co-seismic rupture.(3)During the late Quaternary, the fault was still very active. The Fault distribution map of Aheqi Segement show us The fault zone with the length of about 60 km is mainly constituted b y 2-4 approximately parallel North-dipping listric thrust faults,companied with a backthrust fault.In Aheqi segment the main thrust fault controled the Seismic Activity. Footwall strata basically had no distortion, and the main formation is restricted on the hanging wall by the reverse fault.The associated reverse thrust fault rupture widespread development at the shallow part, limited on the surface. based on the field investigation, the scale of the reverse fault was much smaller than that of the main thrust fault, developed with the main thrust fault. the reverse fault has no seismogenic ability.(4) There are several trace of sinistral movement on water system and edge of terrace. Despite its non-geological structure factors, sinistral strike-slip distance was 1.5-3 times of scarp in height. Up to now we didn’t get the age of the terrace surface, therefore we can’t estimate the strike-slip rate.(5) Akqi segment show us the reverse pluvial fanes. The vertical fracture dislocation of co-seismic surface ruptures is 2.9m, the average vertical sliding velocity of the fault is 0.75~0.67mm/a. the crustal shortening rate caused by the fault is 1.15mm/a, the trench across the Maidan Fault in yushanguxi shows that the paleoearthquake occurred before 1.76±0.22 ka.(6) seismic risk analysis. The First, based on the seismic gap theory, we qualitatively obtained Maidan fault located in a strip seismic gap area, this seismic gap should belong to the second kind of seismic gap defind by Mogi. Then According to the experiential relations between earthquake magnitude, rupture scale and displacement, it is estimated that the magnitude of the maximum potential earthquake on Aheqi segment is 7.5; Based on the vertical sliding velocity and the elapsed time we believe that the potential earthquake on Aheqi segment is 7.4; Based on Earthquake Recurrence Sources Repeated Behavior, the average recurrence Intervals of C haracteristic Earthguakes is 3082-3324a; At last, according to Real-time model method which is derived from theory of characteristic earthquake and probability, we got The probabilities of occurrence of strong earthquake in the future 50, 100 and 200 years.