No abstract

In this paper, the authors describe a study examining the characteristics of seismic activity in the ladder zone of Da river hydro-electric plants. The study results show that:- The occurrence of natural earthquakes in the ladder zone of Da river hydro-electric plants is the strongest in Vietnamese territory. An earthquake magnitude of M=6.7 has been observed in the Tuan Giao and Lai Chau areas, with a b coefficient value (Gutenberg-Richter law) of 0.935.- Natural earthquakes with magnitudes in the range of 6.0-6.9 seem to occur in 13 seismic sources, these being: Muong Te, Nam Nhe, Muong Nhe, Lai Chau - Dien Bien, Son La, Tuan Giao, Phong Tho, Muong La - Bac Yen, Song Da (Da River), Nam Tong, Mu Cang Chai, Lao Cai - Ninh Binh (Phan Sipan), and Mai Chau. Of these, an apparent risk of strong earthquakes, with a magnitude of M=6.9, appears in Muong Te, Lai Chau - Dien Bien, Tuan Giao, Yen Chau, and Mai Chau areas.- Triggered earthquakes with a magnitude M>4.0 have occurred in the region of the Hoa Binh and Son La hydro-electric plant reservoirs, which have a maximum depth of over 100 meters, and a geological structure comprising mainly limestone. - No likelihood of triggered earthquakes in the region of the Hoa Binh hydro-electric plant reservoir; however, in the Son La hydro-electric reservoir region, a triggered earthquake is expected in the future with a strong magnitude of M=4.3, or possibly even M=5.0.

The reservoir-triggered seismicity at the Song Tranh 2 reservoir in Vietnam is investigated by using Shannon entropy, a well-known informational method used to analyze complexity in time series in terms of disorder and uncertainty. The application of the time-varying Shannon entropy to the time series of the interevent times of seismicity has evidenced clear links with the temporal fluctuations of the water level of the reservoir, strengthening the belief that the reservoir operational regime is one of the sources of the seismicity occurring in the area. Shannon entropy has also shed light on the tectonic mechanisms of generation of reservoir-triggered seismicity, revealing that the change in stress due to the variation in water level causes the seismic system to be in a state of greater disorder and instability, well depicted by Shannon entropy, which would lead to an increase in seismic activity.

In this study, the visibility graph analysis of seismicity triggered by Song Tranh 2 hydropower (Vietnam) is performed. The relationship between the seismic (the Gutenberg–Richter b-value) and topological (the k–M slope) parameters of seismicity is analysed. Our findings indicate that the relationship between the Gutenberg–Richter b-value and the k–M slope of the investigated seismicity is in agreement with that characterising the tectonic seismicity. The results obtained from analysing the reservoir-triggered seismicity of Song Tranh 2 area could contribute to better characterisation of the relationship between the seismological and topological parameters of seismicity, strengthening the universal character of the relationship between the b-value and the k–M slope.

The M4.7 earthquake that occurred at the end of 2012 confirmed the danger of triggered earthquakes in the Song Tranh 2 hydropower region, where earthquakes have not ever been recorded before the impoundment in 2010. The task of assessing the triggered earthquake hazard and forecasting its impact on the hydroelectric dam is extremely urgent. To improve the accuracy of this impact assessment, it is necessary to access the active faults for triggered earthquakes to be more effective in earthquake seismic hazard assessment. In this paper, the authors presented a combination of analytical methods combining geological - geophysical data in identifying active sources of triggered earthquakes that stimulate Song Tranh 2 hydropower reservoir with a 3-step analysis process. Research results have identified 5 sources of earthquakes related to 4 active faults connected with the Song Tranh 2 hydropower reservoir.