Values for peak ground acceleration and peak ground velocity using in seismic hazard assessment for Song Tranh 2 hydropower region

Abstract

Song Tranh 2 Hydropower Plant was built in 2010 with the seismic design for maximum possible magnitude M= 5.5. On March 2011 the people of Bac Tra My area nearby the Song Tranh 2 dam began to feel slight ground motion accompanying with the sound of explosions. This phenomenon happened with increasing frequency. On November 15, 2012, an earthquake of magnitude M = 4.7 occurred. This earthquake caused some damage to Song Tranh project management office and houses in the Bac Tra My area. An unusual series of frequent earthquakes has alarmed, caused significant worries and stress for both the local people and local authorities. Results of our survey of the area around Song Tranh 2 (December 2012) have shown that recently, the earthquakes occur frequently and caused minor damage on some houses. While the houses had cracks in the walls, but the columns did not experience any damage. The damage is consistent with the shaking of the intensity in the range of VI - VII (MSK scale). However, the maximum value of acceleration (PGA) measured from accelerometer recordings is equivalent to a higher shaking intensity levels of VII - VIII. Value of the maximum velocity (PGV) measured from accelerometer recordings is more in line with the survey results which is equivalent to the shaking intensity levels VI - VII. The reason for this difference is that PGV is less affected by shaking than PGA at high frequencies. The key conclusion from this experience is that we should also consider PGV in earthquake hazard assessment for the design works because so far we have only used the PGA.

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