Seismic wave velocity models (Vp, Vs, and Vp/Vs) in the area of Da River ladder of hydroeletric dams

Abstract

In this paper, the authors had used the seismic tomography method for determining the P-wave velocity (Vp), S-wave velocity (Vs), and the Vp/Vs ratio in the Da River ladder of hydroelectric dams. The study results have shown that: - The Vp and Vs are increasing in compliance with the depth: from Vp = 5.3-5.9 km/s and Vs = 2.9-3.4 km/s at the depth of 2 km to Vp = 5.8-6.3 km/s and Vs = 3.4-3.9 km/s at the depth of 15 km. Meantime, the value of the Vp/Vs ratio changes in the range from 1.64 to 1.74 and remains unchanged with the depths. - The triggered earthquakes in the Son La dam region occurred in the area where has the value of Vp/Vs ratio from 1.67 to 1.69, within the scope of the Muong La - Bac Yen seismic source and the part of Da River source, where have been connected with the reservoir. - The triggered earthquake occurred at 20th August, 2014 in the Son La dam region had the magnitude of 4.0 (M4.0). This earthquake is not the largest one because it is forecasted that the maximum magnitude of triggered evident occurrence at this place can reach the value of 5.0-5.1.