Vietnam Journal of Marine Science and Technology

Among the tsunami source zones defined in the East Vietnam sea region, the Manila trench, west of the Philippines is considered as the most dangerous for the Vietnamese coast. The recent research results show that the maximum expected earthquake magnitude for the Manila Trench source zone may reach to the value of Mw = 8.7, and it takes approximately 2 hours for a tsunami from this source zone to hit the Vietnamese coast at the earliest. In this study, we create a worst-case scenario of tsunami earthquake excited by Manila Trench megathrust and assess the impact to the Vietnamese coast. The source parameters are defined based on the models proposed by Wu T-R. et al (2009) and Megawati K. et al (2009). The earthquake magnitude, Mw, is assumed to be 9.3 generated on the Manila Trench. The tsunami propagation is numerically computed by using the COMCOT open source code. We focus the discussion in Central Vietnam coastal regions, and carefully describe the maximum tsunami wave heights around two coastal cities, Da Nang and Nha Trang. In Central Vietnam coast, the maximum tsunami wave high of 18m is observed at the Quang Ngai province coast. The maximum wave height is 10.2m recorded at the coast of Da Nang city and is 8.8m at the coast of Nha Trang city. It might be concluded that the tsunami hazards from Manila Trench source are devastating to Vietnamese coast, especially to the Central Vietnam coast.

In this study, the mechanism of sea level rise along the Northern coast of Vietnam after the landfall of the Typhoon Kalmaegi (September/2014) at Quang Ninh province was analyzed based on the observation data and the results of a coupled model of surge, wave and tide (called SuWAT), by using asymmetric and symmetric wind and pressure fields. For the asymmetric wind and pressure field, the Weather Research and Forecasting (WRF) model was used, while for the symmetric wind and pressure field, a parametric wind and pressure model was used. In the case using wind fields from the WRF model, the case that did not consider the effect of tail wind field after the typhoon landfall was also conducted in order to assess the role of the wind field before and after the typhoon landfall on the surge. The results showed that the case using wind and pressure field from the WRF model showed better agreement with observation data, because the WRF model well simulated the wind and pressure field before and after the typhoon landfall. The strong tail wind mainly caused the high surge in the area. This research result will be useful in warning and forecasting storm surges in the area.

In the framework of project "Study and appli-cation methods to evaluate the natural resource economy for some typical ecosystems in Haiphong coastal area, and propose the solutions for the sustainable use", the study of evaluation on the state of mangrove ecosystems in Phu Long (Cat Hai district, Hai Phong City) were implemented to provide the scientific basis for identifying and quantifying the ecological values monetized into currency. This article contributes to provide scientific data on the state of the mangrove ecosystem in Phu Long, which has been investigated by scientific collective of the Institute of Marine Environment and Resources from 2008 - 2009. Research results showed that mangrove eco-system in Phu Long is high in biodiversity, has decreased greatly  in area, and declined strongly in the ecosystem’s  quality. The aquatic resources have been in signs of depletion, that shown by changes in sources of the shrimp and fish larvae, and catching production. Recently, the local government has been implimented promptly and efficiently management policies for mangrove forest.

Nghiên cứu hiệu quả kinh tế của mô hình ương cua giống trong bể lót bạt ở huyện Năm Căn, tỉnh Cà Mau được thực hiện từ tháng 8 - 12/2014, thông qua việc phỏng vấn trực tiếp 35 hộ ương cua từ giai đoạn megalop lên cua giống tại xã Hàm Rồng, huyện Năm Căn, tỉnh Cà Mau. Kết quả phân tích cho thấy các nông hộ ương cua trong bể đất lót bạt với diện tích bể ương trung bình 9,6 m2/bể, độ sâu mực nước 0,3 m và giá thể 100% là lưới. Cua được ương với mật độ trung bình 256,7 con/m2, cho ăn bằng thức ăn ruốc sống chiếm 40% số hộ ương và 60% cho ăn kết hợp giữa ruốc sống và thức ăn viên. Tỷ lệ sống đạt trung bình 84,9%, năng suất 237,4 con/m2 và sản lượng đạt 986.000 con/hộ/năm. Mô hình ương có chi phí đầu tư trung bình 18,8 triệu đồng/đợt và lợi nhuận 5,1 triệu đồng/đợt (121,0 triệu đồng/hộ/năm), tương ứng với tỷ suất lợi nhuận là 0,3. Trong đó, có 94,3% hộ có lợi nhuận và 5,7% số hộ lỗ. Ngoài ra, nghiên cứu đã xác định được 5 yếu tố ảnh hưởng đến hiệu quả của mô hình nuôi gồm: (i) Diện tích bể ương; (i) Giá con giống; (iii) Mật độ ương; (iv) Tỷ lệ sống và (v) Giá bán.

Present-day stress filed in the Hoang Sa archipelago and adjacent areas is determined through the earthquake’s focal mechanism parameters that have been recorded during a time period of more than 100 years. The geometric parameters of the faults (such as the location, strike and dip angles as well as depth, length…) are determined by satellite-derived gravity, seismic data and stress fields. In this study, predictive determination of the magnitude and tendency of the relative displacement of the Earth’s crust is carried out by calculating and assessing the relationship between the stress fields and fault’s geometric parameters. On the basis of the Earth’s crust relative displacement, the geodynamic mechanism of the Hoang Sa archipelago and adjacent areas through the different geological periods can be rebuilt. Magnitude and tendency of the relative displacement of the Earth’s crust are represented by the color spectrum and the vector’s magnitude. Although the displacement appears in the whole region, its intensities are different in particular areas that have the faults systems with different grades.

The  storm  wave  is  a  marine  hydro  dynamical  parameter  which is  very serious,  especially for the marine constructions which often withstand all the severe weathers in  the sea.  The  East Sea is suffered from  one of five storm generation regions in  the world that explain why the storm wave modeling is an important task of the researchers.  in the paper,  the storm wave modeling has been presented.  The  main focus has been paid in the calibration and verification procedure using the modern types of wave measurement, among them the satellite wave  measurements  are  the  most optimistic.  The  results  of validation  show  that  the  storm wave modeling is sufficient for the marine construction design and activities.

This study preliminarily applies the Regional Ocean Modeling System (ROMS) in the two major monsoon seasons (Northeast and Southwest monsoons) for the South Central Vietnam sea (9–14.5oN, 105–112oE), in which the hydrodynamic and ecological modules are coupled. The results show that the plankton only develop in 200 m water on the top, concentrated mainly in the 0–70 m layer and in maximum biomass of 15–40 m layer. In the Northeast monsoon season, the plankton are concentrated mainly in the northern part and open seas of the area, while in the Southwest monsoon season, they are concentrated in the upwelling and adjacent southern areas. These results correctly reflect the basic law of the development of plankton communities in the sea area.

The results of synchronous detection of the emission of various components of the electromagnetic and acoustic fields obtained under conditions of low industrial interference and a high level of microseismicity are investigated. To compensate electromagnetic radiation of lithospheric origin, a compensation method was used. The signal loop antenna, recording a mixture of signals of lithospheric and atmospheric-thunderstorm origins, was used as a magnetic core of a steel casing of the borehole. The compensating signal was recorded using an antenna, the receiving coil of which was identical to the coil of the signal antenna. The measurements were carried out in the seismically active region of Russia (the Kamchatka peninsula, the Paratun hydrothermal deposit) at the intersection of the sublatitudinal and submeridional fault zones. Methods for analyzing registration data of non-stationary processes are considered using the example of acoustic-electromagnetic emission of the lithosphere, the electromagnetic component of which is a component of the natural electromagnetic field of the Earth. Their characteristics are estimated and a statistical model is proposed that uses nested semi-Markov processes and finite atomic functions. Observations of geophysical electromagnetic fields, in addition to useful information about the emission processes, contain interference, mainly of technogenic origin. They are especially strong in downhole measurements when a well casing is used as the receiving antenna. These interferences significantly impede the allocation of useful information. The possibility of using bleaching filters to increase the contrast of observation of useful signals of lithospheric origin against their background is considered.

This study focuses on the use of remotely sensed data for monitoring coastline changes in the Red River Delta during the 1998 to 2008 period. For the satellite image data processing, the shoreline was defined as the mean sea level on the muddy coast where the tide is the dominant dynamic factor and as the mean high sea level on the sandy coast where the ocean waves are the dominant dynamic factor. A GIS approach was used for the quantitative analysis of coastline change. It was observed that the coastline change in the Red River Delta underwent complicated changes during this 10 year period. In this period, the accretion rate in Red River Delta coastal area was about 10,256ha and the erosion rate was about 542ha. In the Hai Hau and Hau Loc coastal areas, erosion occurred in a large scale and was very intense. Along other coastlines of the Red River Delta, sediment accretion dominated at rates of over 30m/year. These zones of varying erosion and accretion have important implications for coastal zone management in the Red River Delta region.

Mối quan hệ giữa động vật đáy (ĐVĐ) có giá trị kinh tế chủ yếu với đặc trưng sinh thái của đầm Thị Nại được xác định thông qua 3 đợt điều tra khảo sát từ năm 2014-2015. Kết quả đã xác định được 11 loài ĐVĐ (5 loài Bivalvia, 5 loài Crustacea và 1 loài Gastropoda) có giá trị kinh tế chủ yếu trong đầm Thị Nại, trong đó nhóm hai mảnh vỏ chiếm trên 91% tổng sản lượng thương phẩm ĐVĐ (7.456,9 tấn) và tập trung chủ yếu vào hai loài Glauconome chinensis và Gari elongata (chiếm trên 90% tổng sản lượng hai mảnh vỏ: 6.817 tấn/năm). Nhóm hai mảnh vỏ và chân bụng có quan hệ chặt chẽ với trầm tích đáy cát và RNM, nhưng nhóm giáp xác có quan hệ chặt chẽ với ở trầm tích đáy cát bùn và TCB. Kết quả nghiên cứu này cung cấp cơ sở khoa học cho việc nghiên cứu về chuỗi thức ăn và cung cấp dữ liệu cho phục hồi, bảo tồn sinh cư; quy hoạch, phân vùng và khai thác thủy sản hợp lý.