Along with the development of urban and modern life, Hanoi Capital has been constructing more and more sky buildings, big bridges and huge underground constructions. According to historical studies, Hanoi is a region located on the weak soil ground which has a rather thick sedimentary layer, complicated geology structure and shallow ground water aged in the Holocene and Pleistocene. Hanoi city is cut through by a Red River fault which has been considered to be a strong active fault zone. As a consequence, it is able to cause earthquakes with magnitude M = 6.1 corresponding to the 7th ground vibration level in intensity. In order to predict the casualty of this severe earthquake being able to happen, it is surely essential to assess both seismic site response and geological conditions in Hanoi. In the paper, we will analyze the soil ground data and borehole data obtained in this region to find the specific geological factors which are put into the DeepSoil Program to evaluate local site effects.

The estimation of the impact of earthquakes on buildings and mega structures in large urban areas is of extremely importance. That is why it always gets attentions from construction planners and policy makers who are concerned about construction rules. When earthquake occurs, the vibration is transferred to sites. Although the vibration intensity is at first not too strong, the motion probably becomes stronger and lasts longer under special conditions of the local site. Two famous examples for these effects occurred in Mexico City in 1985 and in Taiwan in 1999. There are a number of approaches to this problem, such as evaluations based on seismic field observations, the microtremor method, the method using the weak motion data, the method using the strong motion data, the one-dimensional wave propagation method or the three dimensional wave propagation method with simulation etc. In this paper, we will give an overview and discuss about the advantages and the disadvantages of the methods that have been commonly applied in the world. We also present the application of these methods in studies carried out in Vietnam in general and in particular, in Hanoi city. We found that the studies for Hanoi city were mainly carried out in the western areas of Hanoi and a few positions in the urban districts. In addition, the authors only gave comments about and assessments of the shear wave velocity, and classified the ground type without a detailed map of local site effects for the entire area of Hanoi. In order to obtain a full site effects evaluation for Hanoi city, future studies should focus on the application of 1D analysis for the central area of Hanoi city and combining 1D analysis with 2D or 3D to give a better picture about the impact of local site effects. This hybrid approach is necessary in order to compare and verify the data obtained by the empirical and the analytical methods. On the other hand, many problems need to be addressed, for instance, the construction of a detailed 3D geological model for Hanoi, the calculation of the dominant periods and the amplification of the local soil conditions for the urban areas.

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This paper presents some numerical simulation results on the influence of local site conditions on earthquake ground motions in Hanoi. Numerical simulation method through DEEPSOIL program was used in the study. Geological data of 600 groundwater survey and engineering geology boreholes in Hanoi as well as the acceleration records of Imperial Valley earthquake in 1979 with magnitude M = 6.5, hypocenter of 26.5 km, and PGA of 0.169 g were chosen as input data for the research. By analyzing geological conditions, tectonics, seismic characteristics of different soil types in Hanoi, a geological cross-section cutting through different engineering geology in Hanoi areas have been constructed. Numerical simulations of site effect at typical locations along the cross-section show: (i) The geological conditions on the surface of Hanoi clearly influence the earthquake ground motion, this is confirmed by the increase in the amplitude of the horizontal component of ground acceleration and the change of the strong vibrations from short to longer period. These effects obey a general rule, they become stronger when moving on the cross-section from west to east, corresponding to an increase in the thickness of the weak sedimentary layers covering the hard rock; and (ii) However, at the place where the weak and soft soil layer appear (LK50 borehole), despite of the vibrations that are still converted from short to longer periods, the amplitude of the PGA is strongly attenuated at the surface layer.