NGUYÔN KIM L¹P, §ç CHIÕN TH¾NG, BïI VIÖT DòNG

Scientists have admitted that the occurrence of earthquakes is related to faults, so the determination of focal mechanism of earthquakes has great scientific and practical significance; it helps to understand the activity of faults in the present time. In ViÖt Nam, during the XX century 2 earthquakes were observed with intensity I_o = 8-9 (MSK-64) and magnitude M = 6.5 - 7 Richter degrees, 15 earthquakes with I_o = 7 and M = 5 - 5.9, and more than 100 earthquakes with I_o = 6-7; M = 4.5-4.9. Earthquakes on the territory of ViÖt Nam are not scattered, but distributed along different fault zones, such as: §iÖn Biªn - Lai Ch©u, M· River, §µ River, Red River, Ch¶y River, Cao B»ng - Tiªn Yªn, §«ng TriÒu, C¶ River - Rµo NËy, HuÕ - Xªp«n, along sublongitudinal fault system situated on the west of the East ViÖt Nam Sea, on the shelf of Quy Nh¬n coast, and along the fault system situated on the ThuËn H¶i - Minh H¶i coast (Fig.1). In the Tr­êng Sa (Spratly) Archipelago earthquakes occur along the nearly longitudinal fault system. This archipelago is situated offshore, far from earthquake observing stations, so the earthquakes with magnitude less than 4.5 are not recorded. In recent time 8 earthquakes with magnitude of 4.7 to 6.1 has been recorded and the focal depth of the earthquake of 27 February 1969 reached up to 68 km.

On the territory of ViÖt Nam many earthquakes have been occurring, but the focal mechanism of only some of them has been determined because of shortage in seismic observing stations. The methodology used for determining the focal mechanism of earthquake by means of the first arrival of P wave is based on the theory of fault plane by double couple forces.

Figure 1. Distribution of earthquake epicenters of ViÖt Nam in the XX century

Table 1. Focal mechanism of earthquakes determined by means of the first arrival of long P wave

The T¹ Khoa earthquake occurred on 6 October 1991 in the §µ River fault zone [4]. The major discontinuity in focus of the T¹ Khoa earthquake has the azimuth of 135^o making a dip of 72^o. The fault plane in focus of the T¹ Khoa earthquake has the mechanism of reverse strike slip. The T¹ Khoa earthquake occurred by north-south stress field and east-west strain field. This is the major stress field along the §µ River fault zone in recent time [9].

The Hoµ B×nh earthquake occurred on 23 May 1989 [6]. The discontinuity plane in focus of the Hoµ B×nh earthquake has the east-west azimuth and makes a dip of 45^o, the compressive stress is oriented north-south with the dip of 0^o, the tensional stress is oriented vertically with the dip of 90^o. According to the stress field, the discontinuity plane of focus of Hoµ B×nh earthquake occurred in the reverse fault mechanism [9].

The B¾c Giang earthquake occurred on 12 June 1961 in the §«ng TriÒu fault zone. Applying the source mechanism of Vvedenskaya, in 1973 NguyÔn Kh¾c M·o [5] studied on the focal mechanism of this earthquake. Its stress in focus is oriented nearly horizontally and latitudinally in azimuth. The discontinuity plane in focus of earthquake dips northeastward and makes a dip of 60^o. The horizontal direction component is larger than vertical one, that the fault plane is strike slip. According to the localization of stress field in the focal zone, the northeastern wing slips northwestward. The slip of discontinuity plane in focus is sinistral slip.

Earthquake occurred on 29 March 1993 in the §iÖn Biªn - Lai Ch©u fault zone [4]. The major discontinuity plane in focus has the azimuth of 155^o, dip of 75^o. The compressive stress is oriented to nearly north-south and creates a dip of 6^o; the tensional axis is oriented nearly latitudinally and makes a dip of 28^o. According to the stress field distribution in focus of earthquake, the discontinuity plane is in mechanism of dextral strike slip and overthrust, and azimuth of discontinuity in focus of earthquake nearly coincides with that of the §iÖn Biªn - Lai Ch©u Fault.

The M­êng Lu©n earthquake on 22 June 1996 occurred along the ChiÒng Kh­¬ng Fault [2, 4]. The major discontinuity of the focus of earthquake has an azimuth of 304^o, and creates a dip of 46^o. The azimuth of the compressive stress is 230^o and nearly horizontal. The tensional stresses have the azimuth of 137^o and nearly vertical. According to the distribution of the stress-strain axis, the focus of earthquake has a main effect of stress field, so the slip of the discontinuity plane is sinistral strike and overthrust.

There were 4 earthquakes in the Tr­êng Sa Archipelago whose focal mechanism has been determined. Three earthquakes happened on 4 Feb. 1966, 27 Feb. 1969 and 7 Dec. 1982 which have the discontinuity plane oriented sublongitudinally with the azimuth of 30, 35 and 165^o, and nearly coinciding with the eastern part of the 114^o longitude fault. This is a longitudinal fault which divides the Tr­êng Sa Earth crust into two separated structures and geodynamic areas. According to the distribution of stress-strain axis, the slip of fault planes in the focus of earthquake is sinistral strike-slip. Earthquake on 7 Oct. 1966 could be happened in the NW-SE fault area. The discontinuity plane is oriented to NW-SE with the azimuth of 120^o. The two stress-strain axes were oriented to horizontal and their slip is sinistral strike-slip.

On the territory of ViÖt Nam including the Tr­êng Sa Archipelago in the XX century occurred more than 100 earthquakes with the magnitude over 4.5, but the focal mechanism of only 12 earthquakes has been determined relying on the first arrivals of P wave. During recent years Vietnamese scientists have shown different methods for determining the focal mechanism of earthquake as seismic field method [10]. But the limitation of this method is the fault system where earthquake happened have to be known clearly. According to some authors, the §iÖn Biªn earthquake is considered to happen in the M· River Fault (NguyÔn §×nh Xuyªn, 1996. In Archives), but it has been put into the §iÖn Biªn - Lai Ch©u Fault by the field shaking method [10]. That is why in this paper, we study only the focal mechanism from first motion sign of P wave.

By studying the focal mechanism from first motion sign, we are able to determine the focal mechanism of the S¬n La, §iÖn Biªn - Lai Ch©u, ChiÒng Kh­¬ng, §µ River, §«ng TriÒu and Tr­êng Sa Archipelago faults. In the future, by determining exactly the fault where earthquake happens, we will give out method to identify the characteristics of fault systems in ViÖt Nam.

The authors would like to sincerely thank Prof. Dr. NguyÔn Ngäc Thñy and MS. T.B. Hoa who have been helping us in this work.

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8. NguyÔn Kim L¹p, 1989. Seismicity on the territory of ViÖt Nam. Acta Geophysica Polonica, XXXVII/3-4: 247-261. Warszawa

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