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日時：10月1日（木）14:00～
場所：本館E-232D室

[担当]：Quoc Cuong Nguyen
[題目]：Velocity Structure and Seismicity in the Region of Induced Earthquakes of Song Tranh Dam, Vietnam

[要旨]：
Vietnam is located in South East Asia and bounded by the Pacific and Mediterranean-Himalaya seismic belts on its eastern, western and southern sides, respectively. The dynamic tectonic processes in this region cause the territory of Vietnam and adjacent areas to have intensive differential movement, making the regional tectonic structure very complicated. The tectonics have led this territory to have moderate seismic activity and complicated geological structures, such as the Lai Chau-Dien Bien fault zone, Red River fault zone, and others. During the last century, two earthquakes occurred in this region with magnitude large than 6.5 and more than 20 earthquakes with magnitude from 5.0 to 6.0. Most of these earthquakes occurred in northern VietNam. Southern Vietnam was always considered as a region with low seismicity, compared to the North.
However, the sequence earthquakes that occurred at Song Tranh dam during the last several years surprised many scientists because the southern region of Vietnam was not expected to have major tectonic activities.
This region where many induced earthquakes are now occurring is associated with the filling of a new reservoir. There have been four M4 earthquakes (maximum earthquake was 4.7 in November, 2012), so it is one of the most active induced earthquakes examples in the world.
It is important to determine the strong motion attenuation relations for this area since damaging earthquakes may be expected in the near future. Also, the attenuation structure can help us understand the movements of water from the reservoir which are related to the induced earthquakes. We collect and process data from 10 seismic stations around Song Tranh dam, include more than 300 events larger than 1.5 and more than 2000 seismic waveforms to determine arrival times and locate the earthquakes in the Song Tranh dam region. The genetic algorithm is one method to solve non-linear problems. In this study, we have tried to apply 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), more than a thousand earthquakes with magnitude less than 1.5 have not yet been processed. We need to continue to update the seismic analyses with information from smaller earthquakes to improve our results.

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