PALEOSEISMOLOGY OF THE NORTHERN PIEDMONT OF TIANSHAN-MOUNTAINS, NORTHWESTERN CHINA

The northern piedmont of the Tianshan Mountains consists of three rows of Cenozoic EW-striking fold and reverse fault zones, with en echelon right-lateral steps. The southernmost row involves sediments as young as lower Pleistocene, but there is no evidence of activity along this row during the last 30,000 years. The central row is composed of thre e linear anticlines (Houerguos, Manas, and Tugulu) and associated reve rse faults. The northernmost row includes the Dushanzi, Halaande, and Anjihai anticlines and their associated reverse faults. Abundant fault scarps and folds of late Pleistocene to Holocene river terraces acros s the anticlines within the central and the northernmost rows indicate recent folding and reverse faulting. We divide the northern piedmont into the Dushanzi and Manas fold and reverse fault zone. In the Dushan zi zone, we excavated 15 trenches across scarps controlled by reverse faults and back thrusts. By comparing 11 trench logs among the 15 tren ches, we identify three paleoearthquakes since 13,000 years B.P. The f irst event occurred between 11,300 and 13,300 years B.P., and the seco nd and the third events occurred 6300-8400 and 3000-5000 years B.P., r espectively. Considering the uncertainties of the data, the average re currence interval for large earthquakes in the Dushanzi zone is about 4000 years. A large earthquake along this zone is expected in the near future because the elapsed time since the last surface-rupturing even t is already 3000-4000 years. Three large trenches and several small t renches excavated across the fault scarps along the Manas fold and rev erse fault zone reveal four events. The first, second, and third event s occurred at 18,000-13,000 years B.P., 11,300-10,500 years B.P., and 6900-3600 years B.P., respectively. The fourth, the latest one, is the 1906 M=7.7 Manas earthquake. Field investigation suggests that the 19 06 Manas earthquake occurred along a blind thrust fault. This earthqua ke formed three discontinuous zones of fresh surface ruptures, the lon gest of which is only 8 km long along the eastern segment of the Tugul u reverse fault, and was associated with a zone of uplift 130 km long related to the Manas earthquake. The average recurrence interval along this zone is probably 5000-6000 years. Therefore it is unlikely that a large earthquake will occure along this zone in the near future beca use the Manas earthquake occurred only 89 years ago.