Induced seismicity assessment in Salman-Farsi Dam (Ghir), Iran

Nowadays investigating the reservoir induced seismicity phenomenon (RIS) is one of the important subjects because of the extension of dammings in Iran and the potential hazards that they may cause. The phenomenon of dam-triggered earthquakes is known as reservoir-induced seismicity (IRS). It has been expressed through earthquakes with magnitudes ranging from 3 to 6.3 on the Richter scale. For the last 70 years, over 80 cases of RIS have been reported such as Lake Mead, USA, Kremasta, Greece, Oued Fodda in the world and Lar, Latian, Shirin dare, Masjed Soleiman, Karkheh and Karun dams in Iran.
Salman-Farsi (Ghir) dam is a two arched gravity concrete dam that is located at 30 km from northeast of Ghir town in Fars province. Geographically, the dam is located at 28° 30ʹ N latitude and 53°15ʹ E longitude. It is situated in Zagros fold-thrust belt zone. This zone is active in the view of seismology and tectonic activates. The lithology of the site dam is mostly limestone with fracture zones in it. The impounding of Salman-Farsi dam started from April 2009 (1st day of Ordibehesht 1388) which has experienced 9 water years after filling up to now. The maximum and minimum entrance annual water amounts are 347 and 125 million m³ belongs to the 1396 and 1392, respectively.
We used earthquakes data from Iranian seismological centre (ISC) and dam’s data to study the impoundment effects in 9 years period before and after the filling from 2000 to 2018. The studied region is a circle with 30 km radius from the dam where it is in the centre. The area of this circle is 2826 km² include 134 earthquakes from 2000 to 2018. There is one historical earthquake with magnitude 7.1 which belongs to 1440 in this region.
Statistical results present an increase in seismicity rate change, number of earthquakes, and reduction in the depths and magnitudes of them. The Number of earthquakes after the filing period is 4.7 times more than before it. There are earthquakes with magnitudes ranging from 3 to more than 3 in all stages of loading and unloading. The average depth of earthquakes is 15 km before the reservoir impounding that is reduced to 10 km after it. The growth of seismicity rate changes 7 times more than that before. Comparison of seismic data and reservoir water level fluctuations indicates that between the water level and changes of seismic activity of the area, there is a correlation. Some of the earthquakes have occurred a few days after the reservoir impounding and others have occurred after a monthly period from its first filly time. Based on the evaluation of seismicity in the dam site, most earthquakes have occurred in the vicinity of the reservoir dam and seismic response has been rather fast. Furthermore, we suggest that the occurred earthquakes have an induced seismicity nature. In fact, the reservoir impounding has changed the seismic behaviour of the dam site so that the region experiences more earthquakes with smaller magnitudes. This dam does not have the immigrant behaviour in earthquake locations as some other dams in this region such as Masjed Soleiman, have shown.