SUMMARY

Manifestations of lithosphere activities in the West Bắc Bộ region

Lê Văn Dũng, Cao Đình Triều

In this article the authors present some new results about the structure and characteristics of activities of lithosphere in the West Bắc Bộ region, North Việt Nam. Based on the obtained results, the following conclusions can be made:

1. Seismic P-wave velocities (V_P) of the main layers of lithosphere have been defined as follows: the sedimentary crust has the average value of V_P changing from 2.1 to 4.96 km/s (V_P = 2.1-4.96 km/s ); the crystallized crust, V_P = 5.92-7.60 km/s; and the uppermost top of mantle V_P = 7.85-8.00 km/s.

2. The thickness of the sedimentary crust changes in the range from 0 to 7-9 km, where: Hà Nội basin (7-9 km); Sông Đà and Ninh Bình basins (4-6 km); Sông Hồng uplift, Sông Mã and Mường Tè structures (less than 2 km). Thickness of the crystallized crust changes in the range from 18 to 34 km, where in the Hà Nội basin it is less than 24 km; Sông Đà, Tú Lệ and Sông Mã structures: 26-28 km. The thickness of the earth crust in Sông Đà, Tú Lệ, Ninh Bình and Sông Hồng basins, Sông Mã uplift and Thanh Hóa - Nghệ An coastal zone is less than 26 km. The thickness of lithosphere varies from 84 km to 100 km, in which the Sông Hồng uplift, Hà Nội basin and Thanh Hóa - Nghệ An coastal zone it is less than 94 km.

3. The lithosphere in the West Bắc Bộ region is characterized by different vertical movements in the neotectonic period. The areas most affected by the vertical movement include: Tú Lệ Rift (5.0-8.5 mm/yr); Phan Si Pan structure (5.0-8.0 mm/yr and Pu Si Lung (5.0-8.0 mm/yr). The average depth of the seismogenic layer is about 14 km.

4. The following 6 seismic zones of the lithosphere in West Bắc Bộ are: Mường Tè with the maximum of the happened earthquake having the magnitude of M = 5.3; Lai Châu - Điện Biên (M = 5.6); Sông Mã (M = 6.8); Sơn La - Sông Đà (M = 6.7); Sông Hồng (M = 5.3) and Sông Cả (M = 5.2).

Người biên tập: TS. Phan Thiên Hương.