Kurt L. Feigl (1) Duong Chi Cong (2) Tran Dinh To (3) Nguyen QuangXuyen (3)

1. Insignificant horizontal strain across the Red River Fault near ThacBa, Vietnam from GPS measurements 1994-2000 Kurt L. Feigl (1) Duong Chi Cong (2) Tran Dinh To (3) Nguyen QuangXuyen (3) (1) University of Wisconsin-Madison, USA [feigl@wisc.edu] (2) Vietnam Institute of Geodesy and Cartography, Ministry of Natural Resources and Environment, Hoang Quoc Viet road, CauGiay District, Hanoi, Vietnam [chicong.duong@gmail.com] (3) Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Gate 84, Chua Lang St., Dong Da District, Hanoi, Vietnam

2. Post-Miocene tectonics Geomorphology Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. DNCV = Day Nui Con Voi metamorphic belt

3. GEODYSSEA GPS Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. (Michel et al. EPSL 2001)

4. GEODYSSEA GPS Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. (Simons et al. GRL 1999)

5. GEODYSSEA GPS Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. Date (Simons et al. GRL 1999) insignificant motion between: South China part of Eurasia (CAM) Sundaland (NON) CAM-NON = 5.0 ± 3.2 mm/yr at N52°W ± 37°

6. SPOT Image Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. Date

7. Topography, faults, and seismicity Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. From field guide courtesy of PhanTrong Trinh and Nguyen Hoang

8. Geography Seismicity Geomorphology Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Optical image from SPOT satellite showing study area. The Red River and the Red River Fault system run NW-SE. Whether or not this fault system is currently active is a subject of some debate. Resolution is 20 meters. From field guide courtesy of PhanTrong Trinh and Nguyen Hoang

9. Earthquake epicenters with magnitude M > 1.0 between 1903 and 1990 Seismicity Earthquake epicenters (open circles) of north Vietnam with magnitude M > 1.0 between 1903 and 1990 with several earthquake focal mechanisms based on "Geodynamic Seismicity of Vietnam", Hanoi Branch - Institute of Oceanography, NCNST, Hànôi, Vietnam, 1994, (in Vietnamese). Heavy lines denote fault traces from Lacassin et al. [1994]. Thin lines denote rivers; dashed lines, international borders. Published in Duong and Feigl [J. Geodesy, 1999] Earthquake locations from Hanoi Branch of the Institute of Oceanography [1994] Heavylinesdenotefault traces fromLacassin et al. [1994].

10. Evidence for active slip on Red River Fault • Offset stream drainages: 3 to 7 mm/yr • “mid-valley” strand in Gasa Valley in Yunnan, China [Allen et al.,1984] • tenuous assumptions on rate of stream incision • Large river offsets: 5 mm/yr • SPOT images in Yunnan, China [Replumaz et al., 2001] • 25 km right-lateral offsets since 5 Ma • long (~1000 year) recurrence • Trenching: 1 to 4 mm/yr • “mid-valley” strand in Gasa Valley in Yunnan, China [Weldon et al.,1994] • 14C dating • most probable slip rate is 2 mm/yr • lower bound • Earthquakes • moderate magnitudes ( M ~ 5) • fewer on Red River Fault in Vietnam than in China • right-lateral focal mechanisms

11. Evidence for active slip on Red River Fault Continental block model quasi-rigid kinematics [Peltzer and Saucier, 1996] South China Block w.r.tSundaland: 10 ± 5 mm/yr at N110° => On Red River Fault: right-lateral strike slip: 9 ± 5 mm/yr extension: 5 ± 2 mm/yr GPS geodesy at Continental Scale insignificant motion between: South China part of Eurasia (CAM) Sundaland (NON) 5.0 ± 3.2 mm/yr GEODYSSEA project [Michel et al., 2001] Surveying at Local Scale Triangulation in 1963 and 1983, GPS in 1994 Maximum rate of shear less than 0.3 microradian/year Slip rate 1 to 5 mm/yr Duong Chi Cong and Kurt Feigl [J. Geodesy, 1999]

12. GPS Surveys near ThacBa 1994 Cong, To, Xuyen, Kurt et al. install and measure stations 1996 To, Cong, Xuyen, Matthias et al. install & measure stations 2000 Re-measure both sets of stations 2001 Re-measure a few stations

13. Velocities measured by GPS at stations occupied 1994-2000:w.r.t. XUY0

14. Velocities measured by GPS at stations occupied 1994-2000:XUY0 wrt ITRF

15. Velocities measured by GPS at stations occupied 1994-2000:NAM0 wrt ITRF

16. Velocities measured by GPS at stations occupied 1994-2000:NTH0 wrt ITRF

17. Velocities measured by GPS at stations occupied 1994-2000:OAN0 wrt ITRF

18. Velocities measured by GPS at stations occupied 1994-2000:SOC1 wrt ITRF

19. Velocities measured by GPS at stations occupied 1994-2000:TAM1 wrt ITRF

20. 5 Categories:n = number of campaignsΔt = time span • (A) n> 2 AND Δt> 5 years • Uncertainties < 1 mm/yr • LAP1 • (B) n = 2 and Δt> 5 years • Uncertainties < 2 mm/yr • XUY0, OAN0, NAM0, DOI0, NTH0 • VUA0 (only 1 occupation in 1994) • CAM1 (Cam Pha, not shown) • (C) n = 2 and Δt> 5 years • Uncertainties > 2 mm/yr • SOC1, SON1 • (D) Δt< 5 years • Uncertainty > 5 mm/yr • TAM1, HOA1 • (F) issues? • LAN (tie, not shown)

21. Motion w.r.t. XUY0 • Of category A and B stations, only VUA0 is significantly different from zero. • But, only 1 occupation in 1994

22. 3 Hypotheses • 1. The fault is dead. • Deformation in this area ceased sometime after the end of the Miocene (5 Ma). • => Low hazard • 2. The fault is active, but currently locked. • In other words, the present day falls in an interseismic interval. The fault zone is accumulating strain as elastic potential energy to be released in a future earthquake. • => Hazard • 3. The fault is aseismic. • The fault actively releasing tectonic strain aseismically, presumably as creep • => Low hazard

23. Hypothesis 2 2. The fault is active, but currently locked. In other words, the present day falls in an interseismic interval. The fault zone is accumulating strain as elastic potential energy to be released in a future earthquake. => Hazard

24. Recommendation Install a network of continuously-operating GPS stations http://www.earthscope.org http://pboweb.unavco.org

25. Recommendation Monitor deformation of the dam at ThácBà reservoir ThacBa Hydropower Plant is constructed on Chay River in Yen Binh district, Yen Bai province. This is the first hydropower plant of Vietnam which was assisted by the former USSR. The plant was commenced on August 19th 1964 and inaugurated on October 5th 1971. The first unit was commissioned on October 5th 1971.The second and third units were commissioned on March 10th 1972 and May 19th 1972, respectively. Salient parameters of the plant are as follows: Catchment area: 6,430km2. Installed capacity: 120MW.. Maximum height of dam: 48m. Dam length: 657m. Dam volume: 1.33 million m3. Effective storage capacity of the reservoir: 2,160,000,000 m3. Total storage capacity of the reservoir: 2,490,000,000 m3. Reservoir surface area corresponding to normal water level: 235km2. Maximum length of the reservoir: 60km. Normal water level: +58.0. Flood water level 0.01%: +61.0. Flood water level 0.1%: +59.65. Flood water level 1%: +58.8. Dead water level: +46.0 Water level prior to flood: +50.3 Maximum flood release capacity: 3,650m3/s. Average electricity output: 400 million kWh/year. http://www.vncold.vn

26. Pacoima Dam, near Los Angeles, California, USA The GPS antennae are the circled white cones, DAM1 on the left abutment, and DAM2 near the center of the dam arch. A third station, CMP9, located several kilometers away on stable ground, serves as a nearby reference point for monitoring dam displacements. The dam is a 113 meter tall concrete arch structure, completed in 1928, that experienced severe shaking in both the 1971 San Fernando and the 1994 Northridge earthquakes. In September 1995, a system of three continuously operating GPS receivers was deployed to monitor the displacements of Pacoima Dam relative to a stable station nearby at Fire Camp 9 (2.5 km away). This study demonstrates the feasibility of effective and timely monitoring of engineered structures using the Global Positioning System (GPS). [Kenneth W. Hudnut and Jeffrey A. Behr, U. S. Geological Survey, Continuous GPS monitoring of Structural Deformation at Pacoima Dam, California]. http://pasadena.wr.usgs.gov/office/hudnut/SRL/

27. Thank you Merci beaucoup Cảmơnbạn