Source characteristics of local reservoir-triggered seismicity (1.0 ≤ ML ≤ 3.6) in the Song Tranh 2 Region, Vietnam

Earthquakes have occurred near the Song Tranh 2 hydropower since the reservoir's impoundment in November 2010. These include several thousand earthquakes, with a magnitude Ml 4.7 event on 15 November 2012 being the largest. The Institute of Earth Sciences deployed digital data acquisition systems near the Song Tranh 2 reservoirs to investigate earthquake generation. In the present study, source parameters of 97 local events (1.0 ≤ M_L ≤ 3.6) that occurred around the Song Tranh 2 region from September 2013 to April 2017 have been estimated by using spectral analysis. The source parameters including moment magnitude (M_w) seismic moment (M₀), source radius (r), displacement (s), corner frequency and stress drop (∆σ) are found to vary from 1.4 ÷ 3.7, 1.4 × 10¹¹ ÷ 4.3 × 10¹⁴ Nm, 66.1 ÷ 188 m, 0.16 ÷ 10.4 mm, 6.9 ÷ 19.7 Hz and 0.2 ÷ 58 MPa, respectively. Empirical scaling laws among these parameters are developed for the first time in the region. Stress drops in the Song Tranh 2 region increase with magnitude and concentrate in the northern cluster near the reservoir at a depth of 3 to 4 km, consistent with the higher seismic activity observed in that area and depth range. The intersection of tectonic activity and Song Tranh 2 reservoir overburden pressure likely creates stress heterogeneities, leading to higher energy release and stress drops.