The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam

240 240 Vietnam Journal of Earth Sciences, 39(3), 240-255, DOI: 10.15625/0866-7187/39/3/10269 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam Bui Van Duan, Nguyen Anh Duong* Institute of Geophysics (VAST) Received 23 February 2017. Accepted 5 June 2017 ABSTRACT In this study, the method of Fault Movement Potential (FMP) proposed by Lee et al. (1997) is used to assess the movement potential of some major faults and its relationship with the seismic activities in Northwestern Vietnam. The fault movement potential is assessed by the magnitude of FMP (from 0 to 1) and calculated based on the rela- tionship between kinematic characteristics of the faults and regional tectonic stress field. The maximum compressive stress axis in Northwestern Vietnam has the nearly horizontal trend (7.4±4.0) and the direction of 170.2±6.7. The cal- culated results of FMP at 60 geological survey points along major faults in Northwestern Vietnam show that the movement potential of these faults is mainly from medium to high (40/60 points with FMP = 0.6 - 0.9). In particular, the faults in Dien Bien Phu have the highest movement potential (FMP = 0.8 - 0.9). An earthquake catalog that con- sists of 177 earthquakes with M = 4.0 - 6.8 occurring in the studied area from 1277 to 2016 is used to assess the rela- tionship between seismic activities and fault movement potential. The research results indicate that at the locations on the faults with high movement potential, seismic activities occur frequently. On the segment of Son La fault zone in Tuan Giao, Tua Chua, the movement potential has the highest value (FMP = 0.9), corresponding to the occurrence of Tuan Giao earthquake with M = 6.8 in 1983. On the contrary, weak earthquake or even no earthquake occur at the locations with low movement potential such as the segment of Son La fault in Moc Chau, Yen Chau, the segment of Ma River fault in Lang Chanh and the Than Uyen fault in Than Uyen. With the complete survey dataset on slip sur- face attitude of faults in contemporary times, FMP is significant for assessing the level of seismic activities on each part of the faults, serving the segmentation of faults, establishment of seismogenic regions, earthquake prediction and seismic hazard assessment. Keywords: Northwestern Vietnam, active fault, fault movement potential, tectonic stress field, earthquake, seis- mic activity. ©2017 Vietnam Academy of Science and Technology 1. Introduction Spatial and temporal manifestations of intraplate earthquakes are often different from those of the earthquakes occurring at the *Corresponding author, Email: duongnaigp@yahoo.com boundaries of tectonic plates. At the plate boundaries, after the large earthquakes, the energy is rapidly accumulated in the tectonic displacement to continue causing other earth- quakes. Therefore, the locations of recent earthquakes and the average time intervals be- 241 241 Vietnam Journal of Earth Sciences, 39(3), 240-255 tween them are consistent with the evolution of faults in terms of geology and seismology. Meanwhile, intraplate earthquake activities are often associated with intraplate faulting activities (e.g. Tuttle et al., 2002; Nguyen Dinh Xuyen et al., 2004a; Nguyen Ngoc Thuy et al., 2005a). The occurrence of great earth- quakes in the stable continental regions clear- ly demonstrates that a significant amount of elastic strain energy is accumulated and re- leased within the geological structures far from the plate boundaries. The geological structure in Northwestern Vietnam is complicated with the major fault zones such as Son La fault, Ma River fault, Lai Chau - Dien Bien fault, Da River fault and Red River fault. These fault zones are likely to generate the greatest earthquakes in Vietnam. From 1900 until now, some large earthquakes have occurred in the studied area, particularly the Dien Bien earthquake with M = 6.7 in the Fu May Tun fault zone in 1935, the Tuan Giao earthquake with M = 6.8 in the Son La fault zone in 1983. These two earthquakes produced the strong shakings on a large scale, caused the landslides, destroyed the houses and made several dozen people dead and injured (Nguyen Ngoc Thuy et al., 2005a). In the geodynamic model of Southeast Asia, the Red River fault is considered as the northeast tectonic boundary between the South China block and the Sunda block (Simons et al., 2007). However, the greatest earthquakes have not occurred at this bounda- ry but on the faults such as Fu May Tun and Son La in Northwestern Vietnam (Nguyen Dinh Xuyen et al., 2004a; Phan Trong Trinh et al., 2012). It may be because Northwestern Vietnam is located in the transitional area be- tween the South China block, Sunda block and Baoshan sub-block (Findlay and Phan Trong Trinh, 1997; Nguyen Anh Duong et al., 2013). Northwestern Vietnam is considered as the most seismic active region in Vietnam; conse- quently, many in-depth studies on active faults and earthquakes have been conducted in this region. The fault segmentation was first studied in Vietnam in 1994 (Winter et al., 1994). In 2013, Phan Trong Trinh and his col- leagues conducted the fault segmentation along the Red River and Ca River fault zones in Hoa Binh, facilitating the maximum earth- quake assessment (Phan Trong Trinh et al., 2013). The comprehensive researches on tec- tonic faults and geodynamics in Northwestern Vietnam that use the methods of geomorphol- ogy, geology, remote sensing, tectonophysics and structural lineaments are typically Nguyen Van Hung (2002) and Nguyen Van Hung et al. (2016). In these researchers, the major fault zones in Northwestern Vietnam have been determined along with their charac- teristics such as fault dynamics, geomorphol- ogy, geology, structure, movement mecha- nism, movement velocity, long-term historic evolution, etc. In addition, some detailed stud- ies on the specific fault zone or fault segments in a small area have been carried out. Van Duc Tung (2011) studied the tectonic - geo- dynamic characteristics, segmentation and tectonic evolution of Lai Chau - Dien Bien fault zone. The author pointed out that this fault zone has undergone 5 phases of tectonic deformation since the Early - Middle Jurassic and has 3 segments in Vietnam’s territory. Ngo Van Liem et al. (2016) studied the ge- omorphological processes and tectonic activi- ties in the Red River and Chay River fault zones. As a result, 44 basins in the Con Voi mountain range have been categorized into three typical shapes: straight-shaped, S-shaped and concave. The synthesis of research results of active faults in many studies has allowed updating, supplementing and detailing the catalog of ac- tive fault systems in Northwestern Vietnam. Accordingly, many studies on seismic hazard 242 242 Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) assessment have been conducted. Typically, Nguyen Ngoc Thuy et al. (2005a) carried out the detailed seismic zoning in Northwestern Vietnam, concentrated on the seismic micro- zoning for 7 urban areas in this region. The regional and Vietnam seismic station networks have recorded several large earth- quakes in Northwestern Vietnam along with their aftershocks such as the Dien Bien earth- quake with M = 5.3 on Lai Chau - Dien Bien fault in 2001, the Bac Yen earthquake (Son La) with M = 3.9 on Muong La - Bac Yen fault in 2009, the Quan Son earthquake (Thanh Hoa) with M = 4.2 on Son La fault in 2010, the Sop Cop earthquake (Son La) with M = 4.8 on Ma River fault in 2010. The determination of earthquake focal mecha- nisms has contributed to clarifying the region- al tectonic setting. A number of studies on the earthquake focal mechanisms in the major fault zones in Northwestern Vietnam have been conducted based on the methods of mo- ment tensor inversion according to broadband seismic data (Ha Thi Giang, 2012), direction of P-first motions observed at each station (Le Tu Son, 2000; 2004) and macroseismic field modeling (Nguyen Van Luong, 1996). The study results show that the fault zones in Northwestern Vietnam mostly have the strike- one in studying contemporary tectonic stress field. The centralization and systematism of the orientation in focal mechanisms are the basis for establishing the average stress field in a region. Nguyen Van Luong and Bui Cong Que (1997) used the methods of the direction of P-first motions and macro seismic field modeling to determine the focal mechanisms of 81 earthquakes in Vietnam and adjacent re- gions. The analysis of tectonic stress field in Northwestern Vietnam shows that the com- pressive and tensile stresses are nearly hori- zontal in the north-south and east-west direc- tions, respectively. Phan Trong Trinh (2012) identified the contemporary tectonic stress field in the East Vietnam Sea and adjacent ar- eas through 1291 stress indicators, including the data on borehole breakouts, drilling- induced fractures, focal mechanisms and young geological features. Also, the relation- ships between the distribution of contempo- rary tectonic stress field and the forces at tec- tonic boundaries as well as the intraplate forc- es induced by topography, geomorphology of sedimentary basins and local structures were analyzed. The results indicate that the maxi- mum horizontal compression axis of the re- gional stress field is in the north-northwest - south-southeast to northwest - southeast directions. Although many in-depth studies on active faults and earthquakes in Northwestern Vietnam have been carried out, they have only dealt with the qualitative relationship between seismic activities and active faults. In this article, the method of Lee et al. (1997) is used to evaluate the movement potential along the major fault zones in Northwestern Vietnam and its relationship with the seismic activities based on the correlation between kinematic characteristics of faults and contemporary tec- tonic stress field. 2. Method and Data 2.1. Method According to Lee et al. (1997), the fault movement potential (FMP) is considered to be strongly correlated with the tectonic stress field (), the geometry characteristics of faults (G) and the physical properties of the medium inside and on the faults (P). FMP is a function of these parameters and expressed as follows: FMP = f (, G, P) (1) 243 243 Vietnam Journal of Earth Sciences, 39(3), 240-255 The geological medium is generally heter- ogeneous and complicated; however, in terms of statistics, in the theory of Lee et al. (1997), he considered the medium within the faults to be homogeneous, isotropic and elastic. There- fore, the formula (1) is simplified as follows: FMP = f (, G) (2) To investigate the effect of tectonic stress field (in terms of direction) and geometric features of faults on seismic activities, Lo- kajicek et al. (1988) conducted the experi- ments using block models. The results show that the changes in dip angle of faults lead to the significant changes in the time interval between the earthquakes and seismic energy. Additionally, the seismic activities of faults do not occur at some certain values of dip angle. Subsequently, He (1989) performed the theoretical analyses to demonstrate the laboratory results and indicated that the faults can slip when the angle between the maximum principal compressive stress axis and the strike of fault is from 20° to 70°. However, these experimental models were implemented in two-dimensional space, re- sulting in a great difference from the actual fault system. To overcome this limitation, Lee et al. (1997) studied the problem in three-dimensional space by using the theories of Mohr’s stress circle and normalized FMP to quantify the fault movement potential. FMP is expressed as follows: o o ۓ0 θ∈[0 , 30 ሿ θ-60o o o FMP = ۔ 30 o θ∈(30 , 60 ሿ θ-60o ሺ3ሻ o o ە1- 30o θ∈(60 , 90 ሿ where θ is the angle between the direction of maximum principal compressive stress axis (1) and the normal of fault plane. θ is deter- mined by the following expression: cosθ = cosβ1 cosβ2 cos(γ1 - γ2) + sinβ1 sinβ2 (4) γ1 - Direction of the normal of fault plane; β1- Dip angle of the normal of fault plane; γ2 - Direction of maximum principal com- pressive stress axis; β2 - Dip angle of maxi- mum principal compressive stress axis. The maximum principal compressive stress axis (1) of regional tectonic stress field is usually determined by the analysis result of focal mechanism, while the normal of fault plane is determined according to the attitude of fault plane measured in the field. The faults have the highest movement potential when FMP = 1 and no movement potential when FMP = 0. 2.2. Data 2.2.1. The normal of fault plane Northwestern Vietnam has the differentiat- ed topography with high mountains and is dis- sected by many tectonic faults (Figure 1). These faults have been studied in detail in the previous studies and assessed to be active in the neotectonic and contemporary tectonic stages (e.g. Nguyen Van Hung, 2002; Nguyen Van Hung and Hoang Quang Vinh, 2004; Nguyen Van Vuong et al., 2004; Nguyen Ngoc Thuy et al., 2005a, b; Nguyen Anh Duong et al., 2011; Van Duc Tung, 2011; Phan Trong Trinh et al., 2013; Ngo Van Liem et al., 2016). These faults have developed on the basis of ancient faults and acted as the boundaries between tectonic-structural units with different movement mechanisms. The majority of major faults in the studied area 244 244 Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) extends in the northwest - southeast (NW-SE) direction, only a small minority extends in the northeast - southwest (NE-SW) and sub- longitudinal directions. A common feature of the movement mechanism of faults in the late stage (Pliocene - present) is the dominance of strike-slip mechanism for all major fault zones, of which the Lai Chau - Dien Bien fault zone is sinistral strike-slip and the NW- SE fault zones are mainly dextral strike-slip. Moreover, the sub-latitudinal extensional ac- tivities of the sub-longitudinal neotectonic fracture zones under normal mechanism in the contemporary stage have occurred quite strongly (Tran Van Thang et al., 2012). Nguyen Van Hung et al. (2016) has suggested that the common features of tectonic defor- mation are the dextral strike-slip and differen- tiated normal extensional movements which are clearly demonstrated by the morphology of faults in the studied area in contemporary geodynamic conditions. Figure 1. Active faults in Pliocene - present in Northwestern Vietnam 1- Phong Tho, 2- Than Uyen, 3 - Muong La - Bac Yen - Cho Bo, 4- Song Da, 5- Son La, 6- Ma River, 7a- Fu May Tun, 7b- Quan Son, 8- Lai Chau - Dien Bien, 9- Upper Da River, 10- Muong Toong, 11- Muong Nhe, 12- Nghia Lo - Hoa Binh, 13- Bung Lao - Da Bop, 14- Tuan Giao - Nam Ty, 15- Dien Bien - Pac Nua To assess the movement potential on major faults in the studied area under the effect of contemporary tectonic stress field, we have used a dataset on slip surface attitude of the 245 245 Vietnam Journal of Earth Sciences, 39(3), 240-255 Than Uyen BC004 334 80 154 10 25 0.0 faults in the Pliocene - present at 60 survey points in the studies of Nguyen Van Hung, 2002; Nguyen Dinh Xuyen et al., 2004b; Nguyen Ngoc Thuy et al., 2006; 2008 and Bui Van Duan, 2012. The locations of survey points are shown in Figure 2. With these sur- vey points, the normals of fault planes in the studied area corresponding to each slip sur- face attitude have been calculated and pre- sented in Table 1. Figure 2. Locations and values of FMP of 60 geological survey points in Northwestern Vietnam Table 1. Results of movement potentials of major faults in Northwestern Vietnam Attitude of fault Normal line of fault plane (*) Fault Survey point Azimuth of dip direction (o) Dip angle (o) Azimuth of normal line (o) Dip angle (o) θ (o) FMP TD-59 40 75 220 15 53 0.7 TD141 50 50 230 40 72 0.6 Phong Tho BL168 40 80 220 10 51 0.7 BL534 225 68 45 22 53 0.8 TU389 70 80 250 10 80 0.3 246 246 Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) ML-09 40 80 220 10 51 0.7 ML973 40 70 220 20 55 0.8 Muong La - Bac Yen - BC124 60 75 240 15 71 0.6 Cho Bo Song Da Son La Song Ma BY438b 10 60 190 30 42 0.4 PY450 54 63 234 27 69 0.7 KB786 30 60 210 30 53 0.7 SH265 70 80 250 10 80 0.3 QN10 60 70 240 20 72 0.6 MC719 40 80 220 10 51 0.7 QN999 34 79 214 11 46 0.5 QN1007 60 60 240 30 75 0.5 SL7-8 10 70 190 20 33 0.1 SL981 30 80 210 10 42 0.4 MS692 240 80 60 10 67 0.8 MOC703 55 80 235 10 65 0.8 NQ817 36 81 216 9 47 0.6 TL774 54 60 234 30 71 0.7 NQ825 50 60 230 30 67 0.8 TC669 40 70 220 20 55 0.8 TC1016 40 80 220 10 51 0.7 MS40 200 80 20 10 28 0.0 TH907 240 79 60 11 67 0.8 TH937 236 68 56 22 63 0.9 TH941 40 75 220 15 53 0.7 TG587 50 90 230 0 58 0.9 MOC88 20 80 200 10 33 0.1 HN279 55 70 235 20 68 0.7 MA333 50 70 230 20 64 0.9 SM35 70 60 250 30 84 0.2 SM989 60 72 240 18 72 0.6 TH864 201 78 21 12 29 0.0 TH854 200 80 20 10 28 0.0 TH922 63 63 243 27 77 0.4 HN63 40 80 220 10 51 0.7 TH882 72 72 252 18 83 0.2 TH880 72 77 252 13 82 0.3 Fu May Tun DB603 40 80 220 10 51 0.7SC22 30 80 210 10 42 0.4 Pt564 270 72 90 18 85 0.2 LC596 275 80 95 10 79 0.4 Lai Chau - Dien Bien LC1023 305 80 125 10 50 0.7LC199 290 90 110 0 62 0.9 LC1029 290 90 110 0 62 0.9 LC558-316 90 85 270 5 81 0.3 Upper Da River MT114 40 80 220 10 51 0.7LC022b 220 90 40 0 49 0.6 Muong Nhe TvLC64 240 80 60 10 67 0.8 Muong Toong TvLC65 240 80 60 10 67 0.8 NT31 215 80 35 10 42 0.4 Nghia Lo - Hoa Binh NT02 210 80 30 10 38 0.3 C78 240 80 60 10 67 0.8 Tuan Giao - Nam Ty DH-56 53 85 233 5 62 0.9 Dien Bien - Pac Nua TT-30 298 75 118 15 58 0.9 Quan Son HN56 35 70 215 20 51 0.7 (*) θ is the angle between the direction of maximum principal compressive stress axis (1) and the normal line of fault plane 247 247 Vietnam Journal of Earth Sciences, 39(3), 240-255 2.2.2. Maximum principal compressive stress axis in Northwestern Vietnam The compressive or tensile stress field is generated by the movement of lithospheric plates in the convergence or divergence forms, respectively. This movement induces a tectonic force field that propagates in the plates and is called the regional tectonic stress field. It does not remain in a certain form but changes according to time, space and magnitude (Andeweg et al., 1999). The contemporary tectonic stresses in geological structural units at varied locations are different; however, they still carry the typical morphology of regional tectonic stress field. The local force fields cause the local stresses which can alter the regional stress field. The local stresses possibly result from the movement of magma in active volcanic areas (Roman and Heron, 2007), the topography and density heterogeneities in the Earth’s crust (Mandal et al., 1997), the incremental stresses due to the reservoir loading (Bui Van Duan et al., 2015). The methods of conjugate joint set and superposition of compressive-tensile regions on the chart were used to determine the direction of maximum compressive stress axis (Nguyen Trong Yem, 1996). Angelier et al. (1982) calculated the stress tensor by minimizing the difference between horizontal stress and slip vectors observed on the fault surface based on the principle of Bott (1959). Bott (1959) established a mathematical model to clarify all types of faults on the assumption that the movement direction on the fault surface corresponded to the direction of maximum horizontal stress on this fault. McKenzie (1969) identified the relationship between earthquake focal mechanism and stress tensor, essentially the same as the correlation of Bott (1959); however, this is an important finding for determining the tectonic stress field based on earthquake focal mechanisms. Phan Trong Trinh (1993) used the inverse problem solution based on a set of striations on the fault planes and focal mecha- mechanisms in a specific region to determine the most appropriate stress tensor. This method can also be used to separate the different tectonic phases. The tectonic stress field in Northwestern Vietnam from the Pliocene to present has the nearly horizontal compressive stress axis (1) in the sub-longitudinal direction (Nguyen Trong Yem, 1996; Tingay et al., 2010). The results of the paleostress analysis in Ma River area in Findlay and Phan Trong Trinh (1997) also show that the axis 1 orientated from the north-northwest to the northeast is dominant. The stress perturbation often occurs around the major faults in a seismic cycle and affects the change in direction of higher-order fault zones (Maerten et al., 2002). In the modern stage, the data on earthquake focal mechanisms (the large earthquakes on major faults) is a reliable indicator for evaluating the regional tectonic stress field. The earthquake focal mechanisms are mainly regulated by the tectonic stress field; in other words, the tectonic stress field is reflected in the picture of earthquake focal mechanisms (Phan Trong Trinh, 1993). One of the simplest methods for determining the values of stress axes of contemporary tectonic stress field is based on the results of earthquake focal mechanism analysis (Sorbi et al., 2009; Moghimi et al., 2015). According to earthquake focal mechanisms in the studied area (Table 2), the direction and dip angle of average attitude of the axis 1 in Northwestern Vietnam are calculated as 170.26.7° and 7.44.0° respectively (Figure 3). This result is consistent with the previous studies. Tingay et al. (2010) evaluated the contemporary tectonic stress field in Southeast Asia, in which the direction of maximum horizontal stress axis has a value of 173° in Khorat (Thailand) that does not change significantly in Northwestern Vietnam. Phan Trong Trinh (1994) applied a 248 248 Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) variety of methods to study the Cenozoic stress field in Northwestern Vietnam and indi- indicated that the second tectonic phase related to the tectonic stress field has a north - south compressive axis. This axis is consistent with the emission of stress originating in the eastern Himalayas, which is the result of the collision between Indian and Eurasian plates. Table 2. Focal mechanism solutions of Northwestern Vietnam Date Location Ms P T RemarkLat.(o) Lon.(o) Trend(o) Plunge(o) Trend(o) Plunge(o) 01/11/1935 21.08 103.25 6.8 336 10 67 27 13/8/1958 20.02 105.57 5.2 170 5 265 32 Nguyen Van 24/6/1983 21.29 106.31 6.7 166 10 74 11 Luong, 1996 22/5/1989 20.8 105.28 4.9 175 15 279 8 06/10/1991 21.38 104.16 4.9 178 2 91 29 Le Tu Son, 2000 19/02/2001 21.32 102.87 5.3 352 3 88 69 Le Tu Son, 2004 26/11/2009 21.32 104.15 3.9 167 6 257 8 Ha Thi Giang,19/9/2010 20.22 104.94 4.2 351 7 259 19 201230/12/2010 20.83 103.49 4.8 177 9 270 13 Figure 3. Rose diagrams of P-axis directions (a) and T-axis directions (b) from 9 focal mechanisms in Northwestern Vietnam The stress on each fault or in each small area is the result of effects of not only regional factors but also local factors (if any). The local stress can change in a short period of time when the local forces affecting the stress field change. Meanwhile, the regional stress is stable in the long period of time and has the continuous effect. Then, the fault state (with or without the effect of local stress) that is determined through geological surveys will be affected by the regional stress field. Therefore, the movement potential at geological survey points along major faults is calculated by using the axis 1 of the regional tectonic stress field. 3. Results 3.1. The movement potential on the faults The elastic rebound theory was formulated by Reid (1910, 1911) to explain the movement of ground around the San Andreas fault that was caused by the San Francisco earthquake with Mw = 7.8 in 1906 (Reid 1910, 1911). From the measurements, Reid concluded that the earthquake must have been the result of the elastic rebound of previously accumulated elastic strain energy in the rocks on either side of the fault under the effect of the regional tectonic stress field. Thus, the condition for earthquake occurrence is the 249 249 Vietnam Journal of Earth Sciences, 39(3), 240-255 simultaneous appearance of two factors which are the fault with movement potential and the stress accumulation under the effect of the regional tectonic stress field. The locations in which the fault surface is extremely rough, rugged, undulating (horizontally or vertically) and the lithological environment is stable (highly mechanical properties in the rocks, low level of tectonic fracture) must be the ideal conditions for the stress accumulation of faults. In this case, it is assumed that the geological environment is homogeneous and isotropic, which means that the potential of stress accumulation of all faults is the same. Then, only the fault movement potential should be considered. Under the effect of regional tectonic stress field, which locations on the faults have the high movement potential or the low movement potential? In order to answer this question, the formula (3) is applied to evaluate the movement potential on major faults in Northwestern Vietnam. The results of FMP are presented in Table 1 and Figure 2. The calculated results of FMP at survey points along major faults in the studied area show that the movement potential of these faults is mainly from medium to high (40/60 points with FMP = 0.6 - 0.9). In particular, the faults in Dien Bien Phu area (Son La, Lai Chau - Dien Bien, Bung Lao - Da Bop, Tuan Giao - Nam Ty, Ma River, Muong Nhe, Muong Toong and Dien Bien - Pac Nua faults) have the highest movement potential (FMP = 0.8 - 0.9). In addition, there are some locations with FMP = 0 such as the segment of Son La fault in Moc Chau, Yen Chau, the segment of Ma River fault in Lang Chanh and Than Uyen fault in Than Uyen. By using the geostatistical method of Kriging regression, the FMP values distributed over the studied area are expressed as the color spectrum (Figure 4). 3.2. The relationship between movement potential and seismic activities of the faults When evaluating the movement potential of major faults in Hong Kong - China, Lee et al. (1997) suggested that the faults with high movement potential have been likely to generate large earthquakes and strong seismic activities. Thus, the seismic activities have a close relationship with the movement potential of faults. To examine this relationship in Northwestern Vietnam, a catalog of earthquakes in the period of 1277 - 2016 has been established, including 177 earthquakes with M = 4.0-6.8 (Appendix). The epicenter locations of these earthquakes have been represented on the color spectrum map of calculated results of FMP in the studied area (Figures 5a, b). The results in Figures 5a, b show that at the locations with high movement potential of faults, the earthquakes occur frequently and strongly (M = 5.0 - 6.8). Particularly, on the segment of Son La fault zone in Tuan Giao, Tua Chua, the movement potential has the highest value (FMP = 0.9), corresponding to the occurrence of Tuan Giao earthquake with M = 6.8 in 1983. Similarly, in Bung Lao - Da Bop and Tuan Giao - Nam Ty fault zones, intersections with Son La fault zone and Muong Ang area, the seismic activities occur frequently. On Lai Chau - Dien Bien fault zone, the earthquakes are mainly concentrated from the southwest of Dien Bien Phu city to the border with Laos and at the intersections with Son La and Upper Da River fault zones. The segment of Muong La - Bac Yen - Cho Bo fault zone in Muong La and Hoa Binh city has the relatively high movement potential (FMP = 0.7) which is completely consistent with recent seismic activities such as the Ta Khoa earthquake with M = 4.9 in 1991 and the induced earthquake related to the water accumulation-discharge of Hoa Binh hydropower reservoir with M = 4.9 in 1989. On the contrary, there are weak earthquakes or even no earthquakes at the locations with low movement potential of faults (FMP < 0.3) such as Than Uyen area on Than Uyen fault, Nghia Lo area on Nghia Lo - Hoa Binh fault, the areas near the border with Laos on Ma Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) 250 River and Son La faults and near the sea on the segment of Ma River fault (Figure 5a). Although the number of survey points in this study is not large (60 survey points) and their spatial distribution is uneven, the calculated results of FMP demonstrate that the fault movement potential is closely related to the seismic activities in the studied area. Figure 4. Map of fault movement potential in Northwestern Vietnam 4. Discussion In Figure 5b, along Lai Chau - Dien Bien fault zone and some areas, the direction of fault is almost unchanged; however, FMP is very high at some locations (the area between Muong Lay and Muong Cha) and very low at other locations (Sin Ho area or the south of Muong Cha). It is because FMP depends not only on the direction of fault but also on its slip surface attitude. Therefore, even if the fault segments have the same direction but different slip surface attitudes, the FMP values will be different. In addition, on the small-scale map (Figure 5a), it can be seen that the fault extends and its direction is almost unchanged. Nevertheless, at the survey points on various fault segments, the directions of these segments are actually different. It indicates that the calculated result of FMP is the useful information for fault segmentation in the stage of contemporary tectonic activity. On the color spectrum map of FMP and earthquake epicenters in Figures 5a, b, the Vietnam Journal of Earth Sciences, 39(3), 240-255 251 251 difference between the fault movement poten- potential and seismic activities can be seen at some locations. It may be due to the effects of the following factors: - There is a lack of survey points and they are unevenly distributed on the faults. This leads to errors in the data interpolation when representing the FMP distribution by color spectrum. In particular, there is no survey point on Fu May Tun fault near the Dien Bien earthquake in 1935 (M = 6.7); however, the interpolation value shows that the movement potential of this fault zone is approximately 0.6. The FMP value is quite small compared to the magnitude of this earthquake. In the east of Hoa Binh city, the FMP value is relatively high according to the result of data interpolation that is affected by the high values of FMP at survey points in Nghia Lo - Hoa Binh fault zone, Da River fault and Son La fault (Figure 2). Figure 5. (a) Relationship between the movement potential of faults and the seismic activity in Northwestern Vietnam. Small rectangle bordering the Dien Bien Phu and Tuan Giao areas is depicted in Figure 5b; (b) Detail map of the movement potential of faults and the seismic activity in Dien Bien Phu and Tuan Giao areas - The period of time in the catalog of earthquakes is not long enough; in addition, not all the earthquakes have been observed by instruments, several felt earthquakes have been recorded in the historical document and surveyed in public. Therefore, the determination of epicenter locations of these earthquakes is not highly accurate. Bui Van Duan, Nguyen Anh Duong/Vietnam Journal of Earth Sciences 39 (2017) 252 Figure 5b. Detail map of the movement potential of faults and the seismic activity in Dien Bien Phu and Tuan Giao areas Despite these limitations, the calculated results of FMP in this study provide the information on the morphological characteristics of faults, the direction of contemporary compressive stress field and the medium (homogeneous, isotropic, elastic) inside and on the faults. The movement potential on various fault segments is different. This feature is due to the morphology of faults and the regional tectonic stress field. With the complete survey dataset on slip surface attitude of faults, the Vietnam Journal of Earth Sciences, 39(3), 240-255 253 253 determination of FMP is significant for as- assessing the level of seismic activities on each part of the faults, serving the fault segmentation, establishment of seismogenic regions in the studied area, maximum earthquake prediction and seismic hazard assessment. 5. Conclusions In Northwestern Vietnam, the movement potential of major fault zones such as Son La, Ma River, Lai Chau - Dien Bien, Da River faults is assessed to be closely related to the seismic activities. The calculated results of FMP at 60 survey points along major faults in Northwestern Vietnam show that the fault movement potential is mainly from medium to high (40/60 points with FMP = 0.6 - 0.9). At the locations with high movement potential of faults (FMP = 0.8 - 0.9, corresponding to the movement potential of 80-90%), the seismic activities occur frequently, e.g. Kim Tan (Thanh Hoa), Tuan Giao (Dien Bien), Muong Ang (Dien Bien), Tua Chua (Dien Bien), Muong Nhe (Dien Bien) and the southwest of Dien Bien Phu city. At the locations with low movement potential of faults, weak earthquake or even no earthquake occur, e.g. Than Uyen (Lai Chau), Nghia Lo (Yen Bai), Mu Cang Chai (Yen Bai), Moc Chau (Son La), Yen Chau (Son La), Lang Chanh (Thanh Hoa). 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