Seismic Hazard Assessment for Thuong Tan-Tan My Quarries ( Vietnam )

This paper presents the seismic hazard assessment for Thuong Tan-Tan My quarries in Di An commune, Binh Duong province, Vietnam. Combination methods of gravity and magneto-telluric were used to estimate the dip angle and the width of the seismic source. The highest water column of 160 m will cause direct stress on the reservoir bottom with a maximum value of 1535.600 kPa and Coulomb stress of 68.693 kPa (at a depth of 2 km). The typical components of natural earthquake hazard (Mn.max = 5.0, depth of 10 km) in Thuong Tan - Tan My reservoir have the following values: peak ground acceleration PGA = 0.073 g ÷ 0.212 g; peak ground velocity PGV = 2.662 cm/s ÷ 7.984 cm/s; peak ground displacement PGD = 0.706 cm ÷ 1.918 cm at 10% probability of exceedance in 50 years. The typical components of triggered earthquake hazard (Mtr.max = 3.5, depth of 6 km) in Thuong Tan - Tan My reservoir have the following values: peak ground acceleration PGA = 0.024 g ÷ 0.172 g; peak ground velocity PGV = 0 ÷ 5.484 cm/s; peak ground displacement PGD = 0.061 cm ÷ 0.461 cm at 10% probability of exceedance in 50 years.


Introduction
Thuong Tan-Tan My building stone quarries consist of 17 quarries (13 quarries in Thuong Tan commune and four quarries in Tan My commune) located in Bac Tan Uyen district, Binh Duong province, Vietnam. These are open-pit quarries, exploiting Andesite extrusive sedimentary rocks (aged T3-J) under the weathered overburden of approximately 1m, on the terrain of 10-20 m height. Currently, these 17 quarries are exploited at different levels, from -30 m to -100 m (Fig. 1). Due to the great demand for building stones, the Binh Duong Provincial People's Committee has proposed connecting Thuong Tan-Tan My quarries for exploitation to -150 m and then converting this pit into a reservoir. According to the proposed plan, the connection and expansion of Thuong Tan-Tan My quarries will create a pit of 37.0 km2 with a depth of over 160÷170 m (up to the level of -150m and the terrain height of 10-20 m).
One of the important tasks in evaluating the environmental impact on the exploitation method at Thuong Tan-Tan My quarries is the seismic hazard assessment which is assigned to the Institute for Applied Geophysics -VUSTA (Vietnam Union of Science and Technology Association). This paper concisely presents the methodology and results of this task with the following contents: determination of seismic source; assessment of maximum credible earthquake; calculation of incremental stress and Coulomb stress in case of water impoundment; and seismic hazard assessment for Thuong Tan-Tan My quarries.
UAV technology has been used to collect topographic data for related researches (Dieu Tien Bui, et

Determination of seismic source and assessment of maximum credible earthquake
Thuong Tan-Tan My quarries are located in the Sai Gon River fault zone, dividing the Da Lat -Can Tho structural block into two sub-blocks (Hung Cat Nguyen, et al., 2009): Da Lat in the north and Can Tho in the south. The fault acts as a dynamic hinge in Cenozoic between two different tectonic regimes: uplift, denudation during the Cenozoic in Da Lat subblock in the northeast and subsidence, Cenozoic sedimentary fill, with the greatest thickness of 2100 m (Tra Cu basin) in Can Tho sub-block. The depth of influence of this fault zone is through the Earth's crust (over 30 km), and the sphere of influence is 20÷30 km. This fault zone nearly coincides with the photo lineament length-density anomaly strip and the DEM-Lineament length-density anomaly reaches 200÷300 m/km2 (Linh Do Van, et al., 2008). The dextral displacement amplitude of rivers based on results of Landsat image analysis in 2002 is 500÷2000 m (Linh Do Van, et al., 2008). The largest vertical displacement amplitude of the Cenozoic sedimentary basement is 330÷446 m (Linh Do Van, et al., 2008). The latest research results (Linh Do Van, et al., 2008;Hung Cat Nguyen, et al., 2009;Nam Bui Xuan, et al., 2020) show that: -The Sai Gon River fault zone is likely to be active in the modern period, including the Sai Gon River main fault and two accompanying faults: Dong Nai River and Thien Tan -Submission date: 06-03-2020 | Review date: 22-09-2020 Binh Son which are predicted to generate earthquakes with Mn.max of 5.5, and 5.0, respectively.
-The Thien Tan -Binh Son seismic fault, which directly affects to Thuong Tan-Tan My quarries, has Mn.max = 5.0.

Determination of the structure of seismic source
A detailed assessment of seismic source includes structure (width, length, depth); dip angle of fault; and fracturing characteristics of rocks (through density and resistivity values). These are important parameters used to assess the magnitude of earthquakes which can possibly occur. In this paper, the authors have proposed using a combination of methods to determine the seismic source as follows: 1/ The length of the seismic source was determined on the basis that (Trieu Cao Dinh, 2010; Trieu Cao Dinh, Vinh Nguyen Duc, 2012) the source segment (fault) is defined as a boundary dividing structural blocks of the Earth's crust with different composition and geophysical characteristics, and dividing gravitational and magnetic fields with the certain contrast. This boundary causes sudden changes in the depth and the altitude of basic boundary surfaces in the Earth's crust and sedimentary layers. They are clearly shown on modern topography, on satellite images or DEM map (Hung Cat Nguyen, et al., 2009), creating special topographic and geomorphologic elements or controlling the formation of Quaternary and modern sedimentary basins, with manifestations of earthquake, landslide, and neotectonic and modern deformations.
2/ The width, dip angle and fracturing characteristics of rocks of the source were determined using highly-detailed gravity method (gravimeter CG3, made in Canada) and magneto-telluric (AGCOS-Advanced Geophysical Operations and Services Inc., made in Canada) (Fig. 1): a-Highly-detailed gravity measurement was carried out along two profiles (at the scale of 1:25000) in Thuong Tan b-Magneto-telluric measurement was conducted on two profiles (at the scale of 1:25000) perpendicular to the Thien Tan -Binh Son active fault zone (nearly coinciding with gravity profiles), serving the study on structural characteristics of this seismic source (Fig. 1). c-Structural model of Thien Tan -Binh Son seismic source to a depth of 12 km is described in Figure 2. Based on the results of this study, we can evaluate the structural characteristics of seismic source, which can affect Thuong Tan

Calculation of incremental stress and Coulomb stress in case of water impoundment Calculation of incremental stress on the reservoir bottom after the impoundment
Both 2D and 3D problems can be used to calculate the incremental stress field and subsidence of the reservoir bottom. In the case of 3D problem, the reservoir was divided into the area elements of a x a km2 by a set of orthogonal straight lines. After determining the water depth (hi) for each area element, we had the vertical force Fi =ρga2hi at the centre of each area that can replace the water column pressure. With Where: v is Poisson's ratio; R=√(x2+y2+z2) -the distance from the origin to the point P(x,y,z) and its projection given by r=√(x2+y2).
To add the distribution of F forces, they were converted to Cartesian coordinate system according to the correlation: (2) Where, the azimuth θ=Artan(y/x) is calculated from the east to the north (counterclockwise).
Stress is considered as the result of total loading on a point collected by taking the total distribution of all F forces for six stress components: σx, σy, σz, σxy, σyz, σzx.
From these parameters, we selected the normal downward stress σ zz and the maximum shear stress τmax= (σ1-σ3).
In addition, under the pressure of reservoir load, the vertical subsidence ∆d (m) due to the effect of total F force was calculated by the formula: Where: E is Young's modulus; R is the distance of point P from the origin. The subsidence is caused by all the point forces and the total subsidence d at point P is the result of reservoir load.
The calculation of incremental stress on the bottom of Thuong Tan-Tan My reservoir after the impoundment was carried out at the depths: 2 km, 4 km, 6 km and 8 km (the hypocenter of a triggered earthquake is usually located at a depth from 2 km to 8 km). Results of stress calculation at different depths with water columns of 130 m and 160 m (corresponding levels of reservoir bottom of -120 m and -150 m, dam height of 15 m at these levels, water rise of 5 m from dam crest) are presented in Fig. 3 & 4

Calculation of Coulomb stress due to the effect of the water column on the bottom of Thuong Tan -Tan My reservoir
According to Bell and Nur (1978), the change of Coulomb stress (ΔS) caused by reservoir impoundment was determined as follows: ∆S=∆τ-μ(∆σ n -∆P), where ∆τ and ∆σ n correspond to the changes of shear stress and normal stress which are caused by reservoir loading on fault surface, ΔP is the change of pore pressure, and µ is the coefficient of friction. The increase of ∆τ and the decrease of ∆σ n mean that ∆S has a positive value, which will stimulate the fault activity and vice Tab. 1. The maximum value of incremental stress with different scenarios of reservoir depth versa. The role of pore pressure always promotes the fault activity due to the lubrication on the fault surface and decreases the shear stress component Δτ Based on the above theoretical basis, the research team has written a program in Matlab language to calculate stress components and Coulomb stress caused by reservoir loading on the study area.
The reservoir was divided into small blocks, the parameters of length, width and depth at each block were determined. The fault parameters consisting of strike angle, dip angle and rake angle were taken in to account the change of stress field. In the study area, Thien Tan -Binh Son River right-lateral strike slip fault near the reservoir is considered as an active fault, and the parameters of this fault (strike angle = 140o, dip angle = 75°, rake angle = 180°) were included in the calculation of Coulomb stress (Hung C. N, et al., 2009). The study area is divided into grid of 0.0018o x 0.0018o; with Poisson's ratio ν = 0.25; Skempton's coefficient B = 0.7; coefficient of friction µ = 0.65 (Tuan T. A, et al., 2017).
From the above input parameters, the calculations are based on the scenario in which the reservoir is fully impounded, and the tectonic stress field in the area is unchanged at the calculation time. Components of Coulomb stress field were calculated with reservoir depths of 130 m and 160 m at 2km, 4km, 6km and 8km depth, respectively (Tab. 2, Fig. 5 & 6). The results show that the areas with a positive value of Coulomb stress ΔS are at risk of a triggered earthquake when the reservoir is fully impounded. These results allow us to delineate the areas at risk of a reservoir-triggered earthquake.
From the above input parameters, the calculations are based on the scenario in which the reservoir is fully impound-ed, and the tectonic stress field in the area is unchanged at the calculation time. Components of Coulomb stress field were calculated with reservoir depths of 130 m and 160 m at 2 km, 4 km, 6 km and 8 km depth, respectively (Tab. 2, Fig. 5 & 6). The results show that the areas with a positive value of Coulomb stress ΔS are at risk of a triggered earthquake when the reservoir is fully impounded. These results allow us to delineate the areas at risk of a reservoir-triggered earthquake.

Seismic hazard assessment for Thuong Tan-Tan My quarries
OQEngine software (using the function of ground motion attenuation from Campbell-Bozorgnia 2008 and seismic source model-SSM) was applied in seismic hazard assessment for Thuong Tan The 10% probability of exceedance in 50 years was used in seismic hazard assessment (natural earthquake, Mn.max = 5.0; triggered earthquake, Mtr.max = 3.5) for Thuong Tan-Tan My quarries, which is shown in Figures 7, 8 and 9: -Natural earthquake with Mn.max = 5.0 will result in PSHA (10% probability of exceedance in 50 years) with the highest values at Thuong Tan-Tan My quarries as follows: peak ground acceleration PGA = 0.073 g ÷ 0.212 g; peak ground velocity PGV = 2.662 cm/s ÷ 7.984 cm/s; peak ground displacement PGD = 0.706 cm ÷ 1.918 cm.
Tab. 2. The maximum value of Coulomb stress caused by reservoir loading with different scenarios of reservoir depth -Triggered earthquake with Mtr.max = 3.5 will lead to PSHA (10% probability of exceedance in 50 years) with the highest values at Thuong Tan-Tan My quarries as follows: peak ground acceleration PGA = 0.024 g ÷ 0.172 g; peak ground velocity PGV = 0 ÷ 5.484 cm/s; peak ground displacement PGD = 0.061 cm ÷ 0.461 cm.

Conclusion
1. Within the Thuong Tan-Tan My quarries, there exist two seismic sources with the magnitude of natural earthquake Mn.max = 5.0, namely Dong Nai River and Binh Long -Binh Chau. The source segment that is likely to generate the triggered earthquake (directly connected to Thuong Tan-Tan My reservoir) has the following structural characteristics: 4.8 km length; 2.8 km width; rake angle = 180o; dip angle = 75o and Mtr.max = 3.5.
2. The highest water column of 160m will cause direct stress on the reservoir bottom with a maximum value of 1535.600 kPa and Coulomb stress of 68.693 kPa (at a depth of 2km). Compared to the breaking stress of rock in the earthquake, the calculated value is very small, only about 1%. It acts as the promoting mechanism and only matters when the natural stress reaches its limit.
3. The typical components of natural earthquake hazard Mn.max = 5.0 occurring at a depth of 10 km (10% probability of exceedance in 50 years) in Thuong Tan-Tan My reservoir have the following values: peak ground acceleration PGA = 0.073 g ÷ 0.212 g; peak ground velocity PGV = 2.662 cm/s ÷ 7.984 cm/s; peak ground displacement PGD = 0.706 cm ÷ 1.918 cm.
4. The typical components of triggered earthquake hazard Mtr.max = 3.5 occurring at a depth of 6 km (10% probability of exceedance in 50 years) in Thuong Tan-Tan My reservoir have the following values: peak ground acceleration PGA = 0.024 g ÷ 0.172 g; peak ground velocity PGV = 0 ÷ 5.484 cm/s; peak ground displacement PGD = 0.061 cm ÷ 0.461 cm.