Journal of GEOLOGY, Series B, No

I. INTRODUCTION

Up to present, the Hoà Bình hydropower dam with more than 120 m in height is the highest dam in SE Asia. The plant contributes great importance to the economic development of the Việt Nam country. In the Hoà Bình area, two active normal faults are located not far from the dam in the east. In the 1970 year, in the preparation for the construction of the Hoà Bình dam, Vietnamese and Russian workers carried out a series of geological and geophysical investigations. However, they did not pay attention to active faults. For seismic risk assessment, they used only old experimental law and seismic attenuation for far source. So, the implicit seismic risk of destruction of the dam threats directly not only the Hà Nội City, but also all the Red River Delta with more than 20 millions inhabitants. For the prevention of catastrophe and ensure absolute safe of the Hoà Bình hydropower dam, we have to realize the analysis of active tectonics and series of measurement in details of geology, geomorphology, tectonophysics and, especially, the remote sensing analysis in the Hoà Bình hydropower zone to make the seismic hazard assessment and proposals of most sensible operation for the Hòa Bình reservoir. In areas of high density of seismicity with great deformation rate, the probability is less efficient where recording time is too short and carried out in areas with long recurrent interval of earthquakes. In this case, it is necessary to assess seismic hazards by using simultaneously seismotectonic methods and probability analysis for each site or for certain areas. The seismogenic capability of active faults is identified through satellite images, topographical and geological maps, and field survey and earthquake catalogue. The capability at one site is showed by maximum credible earthquake (MCE) and peak ground acceleration (PGA).

II. TECTONIC SETTING

The process of collision between the India and Eurasia plates that took place 50 Ma ago had changed basically the tectonic framework of Asia. Systematic studies along the Red River Fault zone (RRFZ) from Yunnan to Việt Nam [2] proved that almost all gneiss structures situated along the Red River metamorphic zone were formed during Cenozoic. The RRFZ plays an important tectonic role in the formation and development of a series of Cenozoic structures at all scales in the West Bắc Bộ region and a part of the East Bắc Bộ one. Many features of neotectonic deformation and topographical development of the West Bắc Bộ in general, and Hoà Bình hydropower zone in particular, have been clarified thanks to analyses of the deformational history of the RRFZ. Neotectonic activity in the West Bắc Bộ has been expressed not only by the left lateral strike-slip displacement of the RRFZ, but also happening along a series of fault zones in the same NW-SE direction. One can observe a series of structures of the West Bắc Bộ that was formed during Cenozoic. Overthrusting phenomena were widely developed in the West Bắc Bộ as well as East Bắc Bộ, such as Hòa Bình arc and Sa Pa marble.

The Hoà Bình dam zone is the regional boundary between RRFZ and Đà River zone (Fig. 1). Uplifting of the N-S fault system is expressed clearly by a high and average topographical elevation of more than 200 m in comparison with the southwest wing. This fault expresses clearly as an active normal fault. The clearest manifestation is expressed along eastern segments. Triangular facets are expressed clearly by the westward dip. The height of triangular facets reaches 100-120 m. Typical hung valleys prove the fast uplift of the NE wing that goes beyond erosion speed of gravel, sand, soil. The Hòa Bình N-S fault system does not extend continuously, but it is divided into short segments. Strong separation and extensive activity of fault are manifested in this segment; the fault system is divided into 2 discontinuous segments that are called by us as Hoà Bình 1 and Hoà Bình 2 Fault segments.

III. ACTIVE TECTONICS IN HOÀ BÌNH AND ADJACENT AREAS

From the Landsat, Spot satellite images and DEM, we defined three N-S trending segments developed next to the Hoà Bình dam root which generates a nearly vertical-flank graben (Figs. 3 & 4). In some localities, the graben is 2.5 km wide and filled up with alluvial-proluvial deposits of up to 70 m in thickness. The fault segment in the eastern flank is 8.4 km long, and that in the west flank is 4 km long. The distance between the latter and the dam is only 0.3-2.5 km (Fig. 2). Diverging movement of the two fault segments can be clearly observed on Spot images, Landsat images, DEM and in the field. This movement is apparently characterized by triangular facets. Results acquired from tectonophysical methods are completely consistent with the above representation: if the stress field is predominantly compressive in pre-Pliocene, it is predominated by normal extending trend in Pliocene-Quaternary.

We identified the fault systems which are potential for generating earthquakes and could cause direct damages to the Hoà Bình dam that is the sublongitudinal fault system which is well developed in this area. They also consist of segments focusing on in the central part of the eastern and the western flanks of the Quaternary Hoà Bình - Bất Bạt Graben, on the Kim Bôi Dome and on the Tu Lý arc-uplifting massif. Excepting faults controlling the Hoà Bình - Bất Bạt Graben have the length of more than 10 km, other faults often are less than 10 km long (Figs. 3 & 4).

Figure 1. Sketch of active faults in North Việt Nam: I. Red River Fault zone; II. Điện Biên - Lai Châu Fault zone.

Figure 2. Active fault system in Hoà Bình and adjacent areas based on DEM made from topographic map at 1/50,000 scale.

Figure 3. Stereo model of Landsat image showing the active fault system occurring near the Hoà Bình hydropower dam and adjacent areas.

Figure 4. Stereo model of Spot panchromatic image showing the active fault system occurring near the Hoà Bình hydropower dam and adjacent areas.

Figure 5. Active fault systems in Hoà Bình and adjacent areas on Landsat satellite image.

Figure 6. A: Active shear surface system cutting across the bedding plane system in Triassic siltstones in the Ông Tượng Hill area, Hoà Bình City (shear surface, slickensides and bedding plane of E-W trend and nearly vertical slope angles) filled up with black-grey or yellow-brown or red-brown clay materials; B: Triangular facets of a normal fault in east Bái Yên, Hoà Bình.

Figure 7. Sketch map of neotectonics - active geodynamics in Hoà Bình at 1/25,000 scale.

In the contemporaneous tectonic framework, this is the youngest fault cutting and varying all older structures and former framework. On each flank of the Hoà Bình - Bất Bạt Graben, the normal fault planes are nearly vertical, dipped to the graben centre, and each fault coincides with some landforms and controls the distribution of river terraces and alluvial flat (Fig. 5).

In general view, all of the regions belonging to the SW part of the RRFZ, Quaternary grabens and tectonic breccia zones developed close to the Red River Fault with acute angle exhibit right lateral movement regime of the RRFZ itself during Late Cenozoic.

The sublongitudinal fault system is distributed along the two flanks and controls the structure of the Hoà Bình graben. The western fault branch running across the Ông Tượng Hill area is more than 4 km long, with the fault plane dipping eastward. Along this segment, it can be recognized a number of normal active shear zones which could cause surface cracks in this area. The eastern fault branch consists of one segment whose length is about 8.4 km (Fig. 2). Along this segment, it is strongly developed triangular facets characterizing for normal faulting. In addition, active shear zone system in the Dốc Cun area is also well identified. The fault controls the river flow and stream system according to different base levels.

Fault systems can be seen clearly on satellite images and actual topography through fault scarps, relief and facets. Remarkably, along some segments in the east of the Hoà Bình depression, we have been identifying a series of triangular facets in Làng Ngòi and Làng Sú areas (Fig. 6B). These facets have a height of 70-120 m and the width of foot side of more than 500 m. The age of facets must be in range of Pliocene-Quaternary that was formed by the destruction of the early Miocene peneplain. Along some segments in the west of the Hoà Bình depression, such as in the Ông Tượng Hill area, in mafic effusive formation exposed in the west of Hoà Bình Dam or Dốc Cun area, Hoà Bình City, the active shear zones are of normal slip form, and its shear surfaces are concordant with the zone trend. Rolling materials consist of clay, debris of ferro-gel in fine-soft-porous state, and in some sites they can be considered as slickensides. Clearest in Ông Tượng hill area, the active shear zones cut across 20-50 m-wide bedding plane system (bedding plane of E-W trend, slope angle of 75-80^o) filled up with black grey clay materials (Fig. 6A).

Faulting of branches in the east of the Hoà Bình Depression caused landslide, fissure crack in 1996 in the Ông Tượng hill area damaging severely buildings and water supply plan of this locality. In addition, in the Dốc Cun area or in low hill range situated in the east of the Hoà Bình dam similar normal shear zones also appear with the scale sometimes reaching up to 60-80 cm.

Geochemical anomalies: Radon and mercury investigations show that these gas anomalies are relatively high characterizing for activity of the active sublongitudinal fault system. The Hoà Bình Graben cuts across the Hoà Bình City, stretching from Hoà Bình to Bất Bạt. This structure could generate seismic risk for the hydropower plan. This is a young structure developed between the Phan Si Pan and Ninh Bình Zones.

It is widened in 2.5 km and extends 40 km in length, according to the sublongitudinal trend from Hoà Bình to Bất Bạt (Fig. 7). The Hoà Bình graben is filled up with formations of alluvio-colluvial pebble, gravel, sand and a little of marshy facies in the south of Hoà Bình City. The thickness of these sedimentary layers varies according to each locality, especially 50-60 m in Hoà Bình City.

IV. SEISMOTECTONIC ASSESSMENT IN HOÀ BÌNH AND ADJACENT AREAS

In order to assess seismic hazards for Hoà Bình hydropower dam area, we focus on the fault systems of significant size. The sublongitudinal fault system is distributed along two flanks. The western fault branch is more than 4 km long with nearly vertical fault plane dipping to the east. The shortest distance from the section Hoà Bình 1 to Hoà Bình dam is of 0.3 km, the fault dip is determined as 75⁰. The eastern fault branch is one segment of 8.4 km in length. The shortest distance from the section Hoà Bình 2 to the Hoà Bình dam is of 2.5 km, the fault dip is determined as 70⁰. For seismic assessment, we estimate firstly the Maximum Credible Earthquake (MCE). From MCE and distance from the fault to the hydropower dam, we estimate the Peak Ground Acceleration. We use different methods to estimate MCE basing on fault length, fault area and seismic moment. We take the coefficient 1 for fault length approach, coefficient 2 for rupture area approach, and coefficient 3 for earthquake moment approach. For estimating PGA, we use the attenuation models 1, 2, 3, 4 of Campbell and formulas of Idriss, Xiang and Gao, Woodward-Clyde, Ambraseys, Cornell, Mc Guire, Estena & Rosenblueth. Formula of Cornell, Mc Guire, Estena & Rosenblueth has value for reference only. Taking weight average to summarize the peak ground acceleration by different methods, 3 above methods cannot be used. The model 4 of Campbell is based on global data of strong ground motion near the source, so it has high reliability in case of assessing earthquakes within 50 km or less. The above formulas can use the coefficient 3 for calculating the average weight. The formula of Xiang and Gao can also use the coefficient 2, because it is set up from data of earthquake in Yunnan close to geological conditions and structures of Việt Nam. The estimation of Maximum Credible Earthquake of fault segments and Maximum Peak Ground Acceleration in the Hoà Bình dam is presented in the Table 1.

Table 1. MCE and maximum PGA at Hoà Bình dam provoked by the segments Hoà Bình 1 and Hoà Bình 2.

Fault segment

Length

(km)

Depth

(km)

Dip

Character

Magnitude (MCE)

Maximum

PGA(g)

HB1

4.0

6.0

75^o

Normal

5.6

0.30

HB2

8.4

6.0

70^o

Normal

6.1

0.40

V. CONCLUSIONS

Basing on the satellite interpretation and geomorphologic observation, we recognize that the N-S active fault systems are potential for generating earthquakes and could cause damages to the Hoà Bình hydropower dam. These fault system consist of two fault segments: the first segment is east-dipping, 4 km long, with the dip angle of 75-80^o, and the second is 8.4 km long, dipping westward with an angle of 70-75^o. The fault segment HB1 could produce the Maximum Credible Earthquake of 5.6 and Peak Ground Acceleration at Hoà Bình hydropower dam of 0.3 g. The fault segment HB2 could produce the Maximum Credible Earthquake of 6.1 and PGA at Hoà Bình dam of 0.4 g.

Despite all changes the magnitude is always less than the maximum design earthquake that has been assessed (>7.5 Richter degree). For more detail, we need realize geophysical investigations and do trenching along fault segments for having geological evidences. The stress modeling of Coulomb stress change is necessary to forecast the displacement and stress distribution in depth and on the surface.

REFERENCES

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