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日時：6月2日（木）14:00〜
場所：宇治キャンパス本館E棟2階E-232D
　　　（地震予知研究センター本館セミナー室）

[担当：Presenter]：Quoc Cuong Nguyen
[題目:Title]：Second phase of: Understanding relation of tectonic and induced earthquakes in Song Tranh Dam, Viet Nam (Theoretical study of attenuation process, mechanisms of induced earthquakes and data preparation )
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Focal Mechanisms and Seismicity in the Region of Induced Earthquakes of Song Tranh Dam, Vietnam

[要旨:Abstract]
     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. Southern Vietnam was considered to be a region with low seismicity, compared to the North. However, the sequence of 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 activity. This region where many induced earthquakes are now occurring is associated with the filling of a new reservoir. There have been six 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. We collect and process data during 5 years from 5 seismic stations around Song Tranh dam, include more than 700 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. In the first phase of this study, we used a genetic algorithm method to estimate the local velocity structure for the Song Tranh dam region.  In this study we use time domain analyses to determine focal mechanisms with the result of the velocity structure. We use software of Dreger and Ford (2011) modified for the Song Tranh Dam region. Induced earthquakes processed by this software include events with magnitudes larger than 3 and recorded on 3 or more stations. The earthquakes catalogue show that the hypocenters distribution not only concentrate around the reservoir but also concentrate about 40 km to the south of the reservoir. Because of stations distribution problem we only calculate for the events around the reservoir in this study.
     It is very difficult to compare our results with mechanisms for tectonic earthquakes in the region (Hung Nhuong Tavi and Tra Bong faults) because the lack of data. Our future plan is try to separate in some earthquake catalogues to find 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 also will need to process the many smaller events with magnitude less than 3.0, which have occurred around  Song Tranh Dam.

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