## 2 Seismotectonic Settings and Data  
The central part of the Vietnamese continental margin forms the transition from the continental Indochina Block to the East Sea (or South China Sea), which makes the margin a key area for understanding the complex Cenozoic development of Indochina and the East Sea [19; 20; 21]. The Song Tranh 2 reservoir is located on the metamorphic rocks of the Tra Bong and Kham Duc complex formations [22; 23]. The original rock in the study region is primarily metamorphic amphibolite, formed during the orogenic movement of Indochina, which occurred about 245-250 million years ago. Research shows that the study region and its vicinity have undergone several stages of tectonic movements with different distortional characteristics. There were two main ductile deformational phases with the general movement mechanism slipping, including: early phase axis, compressive strain axis, and sub-latitude. Under this stress field, the NW-SE fracture activity slid to the left; by contrast, distortion in the later phase featured local compressive axial sub-meridian and enjoyed the strain axes of the sub-latitude. Under the action of this stress field, the NW-SE fracture slid right. The Cenozoic tectonic motion of the later phase had close ties with volcanic activities in central and southern Vietnam during the Neogene-Quaternary. The Song Tranh 2 reservoir is located within a series of zones with split-snowboarding by NW-SE direction, which tended to raise the compression zone sub-meridian direction and which also creates an E-W spreading trend. The sub-latitude and the NE-SW directions faults are underdeveloped, while the sub-meridian direction fault is hardly absent. There are three main faults: Tra Bui-Tra Nu (F1), Phuoc Gia-Tra Kot (F8), and Tra Leng-Tra Khe (F12); all the remaining faults are branches of the main faults [22; 23]. All the faults are very steep or vertical, striking NW-SE and exhibiting right-lateral strike slip motion (Figure 1).  
The Song Tranh 2 region is a site of reservoir-triggered seismicity, where the seismic activity is controlled by the pore pressure and water level change due to the water impoundment [24]. The Song Tranh 2 hydropower reservoir was impounded in November 2010 and seismicity increased in 2011, with earthquakes continuing to occur. Every year several thousands of earthquakes occur, and there are no signs of decline. The largest earthquake, of ML 4.7, occurred on 15 November 2012, causing minor damage to houses in the Tra My district.  
Digital data acquisition systems were deployed by the Institute of Geophysics (VAST) in the vicinity of the Song Tranh 2 reservoir to get a large amount of data and to help in understanding the physical processes responsible for generating earthquakes in the area.  
A seismic network of 10 stations was established by the Institute of Geophysics-Vietnam Academy of Science and Technology (IGP-VAST). The stations are installed on the hard rock site and the distance from the epicenters to 10 stations ranges from 4 to 23 km. These stations were equipped with Guralp CMG-6 TD seismometers. Guralp seismometers have on-board digitizers with a dynamic range of 130 dB. During the observation period of September 2012 to March 2020, about 8.000 earthquakes were reported by IGP-VAST. These earthquake epicenters are shown in Figure 1.