Content of directories

I. INTRODUCTION

Seismic hazard and seismic risk assessment are two consecutive and interdependent steps of a whole procedure. The typical seismic hazard problem essentially lies in the determination of the effects associated with earthquakes, on both regional and local scales. The outputs from seismic hazard assessment, which are usually presented in terms of shaking maps, representing the spatial distribution of strong ground motion parameters, such as peak ground acceleration A_max, particle ground velocity V_max or peak ground displacement V_max, are used as input for the risk assessment procedure.

The use of a single value of a ground motion parameter for seismic risk analysis at a point can lead to incorrect risk estimations, since that parameter itself can not express all the effects associated with the ground shaking. For example, information on the frequency content and the duration of seismic wave-train, which can greatly affect the ground behaviour, is not always included in a single value of PGA. Therefore, a precise estimation of seismic risk can be obtained only when the effects of local ground shaking amplification or site effects are taken into account. These effects are associated with near-surface geological and topographic conditions of the study area and often occur in the regions with strong impedance contrast between near-surface sedimentary deposits and underlying bedrock. In such cases, the trapping of seismic energy at the boundary of soft sediment and bedrock layers and the onset of vertical resonances can lead to the considerable increase of ground motion amplitudes. In fact, the macroseismic observations of the World’s destructive events show the strong influence of site effects under the megacities, since most of the urban areas are settled on sedimentary basins or river valleys.

There are two approaches of reliable definition of the seismic input at a given site. The first approach is based on the analysis of the available strong ground motion data, recorded at seismographic networks, by grouping of the seismograms with similar source, path and site effects. This approach usually can not be widely applied due to the scarce ground motion data, being collected only in developed countries, or where destructive earthquakes occurred. The second approach is based on modelling techniques, developed from knowledge of the seismic source process and the propagation of seismic waves, which can realistically simulate the ground motion associated with a given earthquake scenario.

This paper introduces a method for assessment of seismic local site effects following modelling approach, firstly applied in Việt Nam. A ground motion scenario is proposed for a downtown area of Hà Nội City, with pre-defined source parameters and structural models developed for both regional and local scales. A hybrid calculation technique is used for Hà Nội with an assumption of a laterally heterogeneous anelastic media. The results are presented in the form of synthetic seismograms showing three components of such ground motion parameters as acceleration, velocity and displacement. A complete set of synthetic seismograms obtained provides an excellent choice of the realistic input for seismic risk analysis and loss estimation for Hà Nội City.

II. SEISMOTECTONIC CHARACTERISTICS OF THE HÀ NỘI AREA

The tectonic activity of Hà Nội area is controlled by several seismically active fault systems, crossing through or near the city territory. The most significant fault zones are Red River (Sông Hồng), Chảy River (Sông Chảy), Vĩnh Ninh, Lô River (Sông Lô) and Đông Triều - Uông Bí ones [5, 10].

The Red River fault zone separates the South China microplate with the Northwest Việt Nam multistage folded domain. Starting from the southeast of Himalaya range, this zone stretches along the Red River valley, gone through the Vietnamese territory to the Bắc Bộ Gulf, with a total length of over 1000 km and a narrow, straight width. Being an ancient zone, the Red River fault zone has been activated during a long development history and plays an important role in tectonic framework of the Southeast Asian region. According to the geophysical and geological data, it is formed by a deep-seated main fault breaking through to the Moho’s surface. The fault plunges northeastward, having the dip angle of 78-80^o and movement of normal right-lateral strike slip type and with a speed of 1 mm/year.

The Chảy River fault zone is characterized by a deep-seated fault with rooted throughout the Earth’s crust, which branched off the Red River at Bản Phiệt (Phiệt Village) and stretching in NW-SE direction. The main fault plunges northeastward with a dip angle of 80^o. The Chảy River fault zone is believed to control the occurrence and development of the Red River graben.

The Vĩnh Ninh fault zone crosses the southern part of Hà Nội territory, with a total length of over 30 km and NW-SE direction. The main fault is clearly depicted in the surface as a small-sized rupture zone and has the normal type with dip angle of 70-72⁰plunging northeastward. The dip angle increases with depth until it reaches the depth of 18-20 km. The vertical amplitude of the fault reaches 2-3 km.

The Lô River fault zone stretches in NW-SE direction, being a part of the Red River - Chảy River faults system, with similar dynamic regime and development characteristics. The Lô River main fault appeared since PZ₂ and is known as a normal fault plunging southwestward.

Figure 1. Seismotectonic scheme of the Hà Nội City

Figure 2. Cross-section along the profile showing the local near surface sediment layers
with geological and geotechnical information

Figure 3. A profile adopted for site response assessment in downtown area of the Hà Nội City

Table 1. Structural parameters for the M2 (bedrock) model

Thickness (km)	ρ	V_P	V_S	Q_P	Q_S	Depth (km)	Layer name
2	2.20	2.20	1.27	220.00	100.00	2	Phong Châu
5	2.60	4.60	2.70	220.00	100.00	7	KZ
10	2.80	6.40	3.60	440.00	200.00	17	Cristal Shield
15	2.90	6.50	3.70	440.00	200.00	32	Moho
70	3.15	7.90	4.57	440.00	200.00	102	Upper Mantle

Table 2. Structural parameters for the M3 (bedrock+1 layer) model

Thickness (km)	ρ	V_P	V_S	Q_P	Q_S	Depth (km)	Layer Name
0.1	1.95	0.83	0.60	110.00	50.00	0.1	Phù Cừ
2	2.20	2.20	1.27	220.00	100.00	2.1	Phong Châu
5	2.60	4.60	2.70	220.00	100.00	7.1	KZ
10	2.80	6.40	3.60	440.00	200.00	17.1	Cristal Shield
15	2.90	6.50	3.70	440.00	200.00	32.1	Moho
70	3.15	7.90	4.50	440.00	200.00	102.1	Upper Mantle

Table 3. Structural parameters for the M4 (bedrock+2 layer) model

Thickness (km)	ρ	V_P	V_S	Q_P	Q_S	Depth (km)	Layer Name
0.06	1.80	0.40	0.26	110.00	50.00	0.06
0.1	1.95	0.83	0.60	110.00	50.00	0.16	Phù Cừ
2	2.20	2.20	1.27	220.00	100.00	2.16	Phong Châu
5	2.60	4.60	2.70	220.00	100.00	7.16	KZ
10	2.80	6.40	3.60	440.00	200.00	17.16	Cristal Shield
15	2.90	6.50	3.70	440.00	200.00	32.16	Moho
70	3.15	7.90	4.57	440.00	200.00	102.16	Upper Mantle

Table 4. Structural parameters for the M5 (bedrock+3 layers) model

Thickness (km)	ρ	V_P	V_S	Q_P	Q_S	Depth (km)	Layer name
0.05	1.80	0.40	0.26	110.00	50.00	0.05	-
0.1	1.95	0.83	0.60	110.00	50.00	0.15	Phù Cừ
2	2.20	2.20	1.27	220.00	100.00	2.15	Phong Châu
5	2.60	4.60	2.70	220.00	100.00	7.15	KZ
10	2.80	6.40	3.60	440.00	200.00	17.15	Cristal Shield
15	2.90	6.50	3.70	440.00	200.00	32.15	Moho
70	3.15	7.90	4.50	440.00	200.00	102.15	Upper Mantle

Giang earthquake in 1961 (M = 5.6) and the Yên Thế earthquake in 1987 (I = VI-VII). Another row of events with intensity VII-VIII are believed to be originated from the Chảy River fault in the XIII century (historical earthquakes of 1277, 1278 and 1285). On the territory of Hà Nội City, the only recorded event was the Yên Lạc earthquake in 1958 (M = 5.3). Outside the city, in Lục Yên (Yên Bái Province), two consecutive events with intensity VII occurred in 1953 and 1954. Weaker and less frequent events have been observed along the Red River, Vĩnh Ninh and Lô River fault zones, with intensity not exceeding VI.

Fig. 1 shows a scheme of simplified above described fault systems with seismicity of the Hà Nội area.

III. DEVELOPMENT OF A GROUND MOTION SCENARIO FOR HÀ NỘI CITY

A ground motion scenario is developed for site response assessment in downtown area of Hà Nội City, that combines two models: 1) seismic source model (or scenario earthquake) and 2) a set of structural models for the study area at two scales: regional and local. With this approach, the source, path and site effects are all taken into account, and therefore, it is possibly to carry out a detailed study of the wave field that propagates at large distances from the epicenter.

The definition of an earthquake scenario in Hà Nội is based on the fault-source model developed for Việt Nam, with assumption that earthquakes are originated on seismically active tectonic faults [8, 9]. For this study, an earthquake scenario was defined as originated from the Vĩnh Ninh Fault, with moment magnitude M_W = 5.6 and focal depth H = 10 km. Information on geometry and motion mechanism of the Vĩnh Ninh fault were taken from GIS database of seismically active faults for the territory of Việt Nam and adjacent areas [9].

For the rest part of ground motion scenario, two types of structural models are developed for the Hà Nội area. The local model represents near-surface soft sediment layers lying beneath the downtown area of Hà Nội City, while the regional model represents average structure properties for a much larger area including Hà Nội City.

For defining a local model named as M1, a geological cross-section along a 12 km long profile is adopted. The selected profile is parallel with the Vĩnh Ninh fault, crossing several downtown districs of Hà Nội (Fig. 2). Geotechnical and geophysic data available for each layer of the cross-section are taken from previous studies [9, 10].

At regional scale, four bedrock models are created for the Hà Nội area using geophysic data obtained from previous studies [2]. The simplest model includes only bedrock base of 4 layers. The other ones are created with one or two more thin layers added on the top of the bedrock base. The regional models are named as M2, M3, M4 and M5, respectively. The parameters for each layer of the regional models of Hà Nội area are listed in Tables from 1 to 4.

IV. SITE RESPONSE ASSESSMENT USING SYNTHETIC SEISMOGRAMS

Fäh et al., 1993 developed a hybrid method that combines the modal summation technique, valid for laterally homogeneous anelastic media, with finite difference, and optimized the advantages of both methods [3, 4]. Wave propagation is treated by means of the modal summation technique from the source to the vicinity of the local, heterogeneous anelastic structure that is modelled in detail. A laterally homogeneous anelastic structural model is adopted, which represent the average crustal properties of the area. The generated wavefield is then introduced in the grid that defines the heterogeneous area and it is propagated according to the finite-difference scheme. Schematic representation of the hybrid technique is given in Fig. 4. The hybrid method has been successfully applied for several megacities of the world [8].

For estimating the local site effects in downtown area of Hà Nội City, the hybrid method is applied using the ground motion scenario described in the previous section. It is assumed that seismic waves originated from the source (earthquake scenario) is propagated through a layered media (of regional and local structure models) and recorded at receivers, located along profile at the sites with equal distances from each other, and the synthetic seismograms are computed for each site.

The calculation procedure consists of the following two steps:

1. Computation of synthetic seismograms for 1D layered regional structural models, namely M2, M3, M4 and M5 bedrock models, with the modal summation technique;

2. Computation of synthetic seismograms for 2D laterally heterogeneous anelastic media with the hybrid technique, using a combination of the local model M1 with one of the regional model M2, M3, M4 or M5 at a time.

Calculated synthetic seismograms represent three components of strong ground motion parameters, namely acceleration (PGA), velocity (PGV) and displacement (PGD). For the 2D model, the method allows to calculate synthetic seismograms for frequencies up to 10 Hz. Results obtained for different structural models are compared to study the structural dependence of ground motion. Fig. 5 shows the synthetic three component accelograms calculated for 2D free surface model at sites along selected profile. As can be seen from the seismogram, the value of maximum acceleration reaches 53.28 cm/s² for vertical component and 50.11 cm/s² for transverse component, which are about 2 times higher than the PGA value calculated for the same area by probabilistic method (which is equal to 22,5 cm/s²) [5, 6].

To evaluate the local amplification, the response spectra ratio (RSR), i.e. the response spectra computed (at 5 % damping) from the signals synthesized along the laterally varying (2D) model normalized by the response spectra computed from corresponding signals, synthesized for the bedrock (1D) regional model is calculated at each site. Fig. 7 shows the variation of the RSR (also called spectral amplification) along the selected profile, calculated for three components of spectral acceleration, in relation to epicentral distance and frequency. As can be seen from the figure, at low frequencies (less than 1 Hz) the ratios are approximately 2 along all profile, but at higher frequencies, where the wavelengths are comparable to the dimensions of lateral heterogeneities, the ratios increase. For transverse component, the amplification reaches maximum value of 3 at frequency range from 1.5 to 2 Hz. But for vertical and radial components, the amplification reaches the largest values of 4 and 5 at the frequency range from 2 to 3 Hz. All this indicates that, due to local effects, one may expect much higher shaking intensity comparing with the average values estimated for the area [5, 6].

V. CONCLUSIONS

The method presented in this paper, though firstly applied in Việt Nam, shows a number of advantages. The seismic hazard and risk assessment for Hà Nội City, due to the limitation of earthquake and strong ground motion records, requires to resort to broad band synthetic seismograms, which allow to perform realistic waveform modelling for varying structural environments, taking into account all associated effects of source, path and site properties. Each synthetic strong ground motion scenario, characterized as a function of its damage potential will improve the experts choices in selecting acceleration histories for various seismic risk analyses. Development of databases of modelled near-field and soft soil strong motion seismograms can contribute a great deal to the enhancement of knowledge in damage potential evaluation.

The results obtained for downtown area of Hà Nội City show that the local site effects might considerably exceed the average ground motion value estimated for the area. The determination of the spectral amplification at a certain site plays significant practical role in seismic engineering and antiseismic design, as the amplification factors indicated in building code are usually limited for a fixed range of ground motion values and for a certain soil types [1].

Acknowledgement

The results presented in this paper have been obtained during the 2nd meeting/workshop of network Project “Seismic Hazard in Asia”, co-organized by ICTP and C-MMACS, March 31 to April 11, 2008. We sincerely thank Prof. Panza G.F., ICTP and Dr. Imtyiaz Ahmed Parvez, C-MMACS, for organizing the project and providing us with all facilities to make this work done.

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