Characteristics Of The Late Quaternary Activity Of Heihe Fault In Southwestern Region Of Yunnan Province

In this paper, we did a detailed research on late Quaternary activity of the Heihefault and deformation zone of Ms7.6event in Lancang,1988and we surveyed thepaleoearthquake ruputure zone of the western segment of the Heihe fault, combinedwith previous research, we got the following conclusions:(1) Heihe fault, located in the southwestern part of Yunnan province, with thelength of168Km and the trend of280°-310°,is an active fault zone in the latequaternary, which is mainly right-lateral strike slip with dip slip. Through satelliteimage interpretation and field geological investigation，the geological characteristics,dislocation landforms and slip rate of the fault have been studied in this paper. It turnsout that Heihe fault zone can be divided into three the secondary fault segments, i.e.,Cangyuan-Mujia (F1-1), Mujia-Nandai (F1-2), Nandai-Mengwang (F1-3).(2) Heihe fault is an active fault zone in the late quaternary, and a variety ofdislocation landforms have been formed in the new fault activities since the lateQuaternary, such as dextral dislocation of gullies and ridges, fault trench, fault pass,fault scarp and pit. There are some differences among three secondary fault segmentsof slip rate. Overall, the slip rate of northwest segment is greater than that of southeastsegment. This is consistent with the phenomenon of landform. At Xuelidazai village,the age of the displaced stratum is（2530±25）a BP，and on the fault plane，looselyconsolidated gouge is developed in the several cross section of the fault．It means thatthe latest active time of the northwestern segment on the Heihe fault islate-Pleistocene to Holocene. A series of dextral dislocation of gullies and ridges, andscratches on the cross section along the fault show that the northwestern segment onthe Heihe fault has been dominated by right-lateral strike slip. At Baishuijing，according to the differential GPS measurement, the sample dating and the regionalcomparison of the fault slip rate, the right-lateral strike slip rate is calculated to be（3.54±0.78）mm/a since Holocene. In addition, at Huoshaozhai and Baishuijingwest of the middle segment of Heihe fault, there are several terraces or platform inlarger gullies. In the new fault activities, these terraces formed different dextraldislocation.By the ages of terraces in the east and west of Yunnan Procince(JI,2000),the right-lateral strike slip rate is calculated to be（3.7±0.3）mm/a since Holocene.Itaccords with regional characteristics of the fault slip rate.(3) Deformation zone of Ms7.6event in Lancang,1988can be divided into twodistinct zones. The seismic deformation zone is mainly composed of a series ofsurface ruptures and distributed along the Heihe and Lancang faults. There areright-lateral and left-lateral ground fissures. The mechanical property of right-lateralshear of the deformation zone is analysed. Left-lateral shearing movement exhibitedin some segments of the zone is an illusion. The axis of principal stress for the eventis SW-NE direction, so the seismic faulting of the event should represent aright-lateral displacement. The groud fissures which exhibit left-lateral movement areformed by right-lateral displacement of faults. This phenomina may be a X-type fracture. This complex source process is consistent with the surface rupturecharacteristics.(4) Along the west segment of the Heihe fault, we find a seismic deformationzone whose age is unknown. There are a series of dextral dislocation of balks, withaverage1.25m. By the empirical relationship between moment magnitude ofstrike-slip earthquake and coseismic average dislocation, we can estimate themagnitude of the earthquake is about7.3. It is similar with Ms7.6event inLancang,1988. Their coseismic dextral displacements are roughly equal. There isearthquake rupture zone of Ms7.6event in Lancang in the middle of the fault, and theaverage recurrence interal of characteristic earthquake is188-499years. There is anabout49km of seismic gap fron the Nanling to Sandaohe along the eastern section ofthe Heihe fault. By the empirical relationship between moment magnitude ofstrike-slip earthquake and fracture length, we can estimate the maximum magnitudeof the characteristic earthquake is7. According to the seismic gap theory, the seismicrisk of the eastern segment is largest with the western segment followed, while themiddle segment is smallest.