Impoundment of the Song Tranh 2 reservoir in Vietnam was started in late 2010, and earthquakes started occurring in its vicinity soon after that. Frequency of earthquakes increased after the reservoir water level was brought down in June 2012. We computed the influence of the reservoir water load and pore pressure and used the concept of fault stability. The reservoir water load alone destabilized existing predominantly dextral strike-slip faults of the region and hence led to triggering of earthquakes soon after its impoundment. Pore pressure further destabilized these faults and brought additional faults under destabilization. Time delay between the maximum reservoir water level and pore pressure buildup due to diffusion leads us to conclude that the increase in seismicity at low water stands in the reservoir is primarily due to the increase in pore pressure during that period. We propose that this is an unambiguous case of rapid and delayed earthquake triggering by a reservoir impoundment.