5:15 PM - 6:30 PM
[SSS26-P09] Focal Mechanisms and Seismicity in the Region of Induced Earthquakes of Song Tranh Dam, Vietnam
#Quoc Cuong Nguyen, James Mori (DPRI, Kyoto Univ.)
Keywords:Song Tranh Dam, VietNam, Focal Mechanism , Induced Earthquake, Velocity structure
We also compare our results with mechanisms for tectonic earthquakes in the region (Hung Nhuong Tavi and Tra Bong faults). The results show a difference in focal mechanism between tectonic earthquakes and induced earthquakes which may be related to the increased fluid pressure from filling of the reservoir. To confirm this result, we will need to process the many smaller events with magnitude less than 3.0, which have occurred around Song Tranh Dam.
We used a genetic algorithm method to estimate the local velocity structure. We applied this method to determine a layered model for the Song Tranh dam region. Our results obtained a new 1D model of 7-8 layers. The shallow P wave velocity of 4.6 km/s is slower than 5.9 km/s for previous studies in northern VietNam. For a deeper layers from 6 to 12 km, P wave velocity becomes larger, 5.4 km/s – 5.9 km/s. The Vp/Vs shows relatively higher values of 1.75-1.77 for the depth around 12 km. When layer thickness changes from 21 km to 28 km, the P wave velocity increases and changes from 6.5 km/s to 7.3 km/s, however, Vp/Vs ratio decreases from 1.77 to 1.67. Finally, the depth of the Moho surface changes from 28 to 35 km and the P wave velocity changes from 7.8 to 8.2 km/s, with Vp/Vs value of about 1.78. Earthquakes still occur at Song Tranh dam (a recent M3.3 occurred on August,26th 2015), and more than a thousand earthquakes with magnitude less than 1.5 have not yet been processed. We continue to update the seismic analyses with information from smaller earthquakes to improve our results.