ACTIVITY AND EFFECT OF MAIN FAULTS IN NEAR FIELD OF BALA HYDROPOWER STATION IN MAERKANG

The activity of a main fault in the near field of Bala hydropower station in Maerkang is very important to engineering design and dam site selection.With the integrative analysis of investigation,ESR dating,quartz shape scanning in the fault gouge and seismic activity,we find that all faults in the near field region of Bala power station except Songgang fault has no perceptible activities in the earth surface since the Late Pleistocene.So they have no side influence to the engineering construction.In the middle section of Songgang fault,small earthquakes occurred frequently in the past several decades.But hardly any evidence can be found to indicate the fault is a active fault.The south section was still active in Late Pleistocene,because the second level river terrace was cut by 0.3m.The north section has been inactive since the Late Pleistocene.The Ribu fault is not active.It has been as same as the middle section of Songgang fault since the Late Pleistocene.But the seismic activity is distinct in the deep part of subsurface.Engineering must keep away from the faults such as Ribu fault and the middle and sourth sections of Songgang fault.It may induce large surface deformation and fracture.The seismic risk evaluation of the Songgang fault has indicated that the maximal magnitude of earthquake possibly induced by Songgang fault might be Ms=6.95. Considering the factors of inadequate safe distance between Songgang fault and upper site,poor petro-physical properties in upper site and small height of water in lower site,its better to choose the middle site for the hyderpower dam.