STRONG EARTHQUAKES AND FEATURES

STRONG EARTHQUAKES AND FEATURES OF TECTONIC ACTIVITIES IN SOUTHEAST ASIA REGION

NG¤ THÞ L¦

Institute of Geophysics, NCNST, Hoµng Quèc ViÖt, CÇu GiÊy, Hµ Néi

Abstract: On the basis of the Catalogue of strong Earthquakes in the Southeast Asia Region compiled by the author the features of strong earthquakes (M_s³ 5.5) in SE Asia have been examined and analyzed, and 11 strongest earthquakes (M_S³ 6.9) have been studied in detail. Especially, the processes happened in the focal zones of strongest earthquakes have been clarifield by analyzing their evolution in space and time and their fore- and aftershock series. The features of recent activities of series of faults have been shown by the analysis and synthesis of seismic and tectonic data, and their relation with seismic activity in the region has been presented.

The territory of Southeast Asia is characterized by high seismic activity, however until recently seismologic and seismotectonic researches which was executed for separate parts of this region [10, 11], only rather limitedly reflected the display laws of this activity and its connection with tectonic structures. This work is a continuation of researches, carried out earlier by the author [6-9].

The consideration of display features of strong earthquakes and the complex analysis of seismicity and geokinematics for the study territory are executed on the basis of the firstly made unified catalogue of earthquakes for 14 countries of Southeast Asia for the period from 1970 to 1996 [8]. This catalogue was made on the basis of the study and unification of the initial seismological data, received from various international sources with the use of an uniform methodical approach according to the works [4, 6, 7, 16]. It contains 6315 earthquakes with magnitude by a surface wave (M_S ³ 3.5), among them 163 strong earthquakes have the magnitude of ³ 5.5.

Table 1. Basic parameters of strongest earthquakes

No	Date	h	m	s	j ^oN	l ^oE	H (km)	M_S	L (km)	W(km)
1	01.01.96	08	05	11.8	0.77	119.95	20	7.6	151.36	33.88
2	06.11.88	13	03	18.6	22.90	99.51	20	7.4	117.49	29.51
3	17.11.84	06	49	32.8	0.25	98.07	40	7.3	104.71	27.54
4	06.08.88	00	36	23.9	25.15	95.15	105	7.2	91.20	25.70
5	05.01.91	14	57	12	23.55	95.95	15	7.2	91.20	25.70
6	04.04.83	02	51	36.3	5.65	94.68	80	7.1	80.35	23.99
7	11.07.95	21	46	40.3	22.02	99.24	10	7.1	80.35	23.99
8	24.06.83	07	18	21.6	21.83	103.32	9	6.9	62.37	20.89
9	16.09.94	06	20	20.5	22.56	118.79	20	6.9	62.37	20.89
10	06.05.95	01	59	07	25.05	95.34	113	6.9	62.37	20.89
11	03.02.96	11	14	20.5	27.33	100.33	10	6.9	62.37	20.89

Note: h- hour; m- minute; s- second; H- focal depth; M_S- magnitude of earthquakes, determined by surface wave according to [8]; L- length of the focal zone; W- width of the focal zone.

Fig. 1. Map of epicenters of strong earthquakes in the Southeast Asia
during the period from 1970 to 1996.

Earthquakes: 1) 5.5 £ M_S < 6.0; 2) 6.0 £ M_S< 7.0; 3) M_S ³ 7.0; 4) h £ 50 km; 5) 50 < h £ 100 km; 6) h > 100 km; 7) Established faults; 8) Inferred faults.

In the present work the examination of features of strong earthquakes of the study region has been carried out on the basis of the catalogue of strong earthquakes and catalogue of foreshocks and aftershocks, received from [8]. The map of epicenters of strong earthquakes is shown on Fig.1, from which it is clear, that epicenters of strong earthquakes are located basically along known and inferred tectonic destruction zones, first of all, along the Benioff zones on the active margin of plates and of large intraplate transversal shifts. Among 163 strong earthquakes there are 11 strongest earthquakes with magnitude of ³ 6.9, for which the foreshocks and aftershocks have been examined in an unified catalogue of earthquakes. The data about these earthquakes are given in Table 1.

The sizes (length L and width W) of the focal zone (km) are calculated by the following formulas according to [14]:

lgL = 0.55M - 2.0; lgW = 0.30M - 0.75 (1)

The formulas are chosen on the basis of comparison of series of L and W values, which are determined by various methods in accordance to macroseismic data from some of specified strongest earthquakes of Southeast Asia.

Fig. 2. Depth section (a) and time evolution of fore- and aftershocks (b) of earthquake 04/04/1983 (North Sumatra, Indonesia).

1) main shock; 2) fore- and aftershocks;
3) number of seismic events after the time
evolution (4-7) - foreshocks; 8 -35) aftershocks

During the analysis of strongest earthquakes the examination of a deep focal section (Fig. 2a and 3a) and time evolution (Fig. 2b and 3b) of foreshocks and aftershocks of each of 11 chosen events was realized. The received results have revealed specific features both in spatial - time distribution of fore- and aftershocks, and in a deep section of their focuses. For example, the earthquake 1, occuring in 01/01/96, at 08h. 05m. 11.8s. in the Makasar strait in southeast of the region (M_S = 7.6) has aftershocks located in a submeridional direction (Fig. 3b), coinciding to the extending direction along a previously inferred fault. According to the time evolution (Fig. 3b) aftershocks of the earthquake 1 have also taken place in a zone extending in this direction. All this has been showing recent activity of the fault in the southeast part of region with its development in the submeridional direction.

And the earthquake 6, occuring in 04/04/1983, at 02 h. 51 m. 36.3 s. in the northwest of Sumatra Island (M_S = 7.1) is characterized by fore- and aftershock activity in a rather complex time evolution (Fig. 2b). The first foreshocks appeared in a zone of over 120 km in length and 20 km in width, extending in NW-SE direction. Further aftershocks occurred in a wider zone of 150 km in length and approximately 70 km in width in the same direction. It is possible to suppose, that the process in the focal area of the given earthquake happened in the intersection of two directions (Fig. 2b). As it is seen in the Fig. 4, the said event was concentrated in the intersection of two systems of tectonic destruction: sublatitudinal system, which is connected with the extension in a spreading zone of the deep-water hollow of the Andaman Sea, and northwest system, which is situated in the Semanko influence zone of large right transversal shift on the Sumatra Island [1].

This confirms the high recent activity of the junction part of Sunda (Indonesia) and Andaman Island arcs. The resolution of the focal mechanism of the mentioned earthquake and the large size of its focal zone correspond to the prevalence of a spreading zone of the northwest-southeast direction.

The results of study on zones of aftershock activity of other strong earthquakes of the region show also good coincidence of seismological data with geological data (Fig. 4). As it is known, that in Southeast Asia there is a collision interaction of three main lithospheric plates: Eurasian, Indian-Australian and Pacific Ocean, a part of which is the Philippines microplate. The borders between them run along Benioff zones, systems of subsidence in collision zones, large transversal displacements and planete faults.

As supposed earlier [1], the marginal part of the Eurasian plate was disintegrated to a series of blocks moving in various directions along their regional shifts. The carried out analysis of seismicity has been confirming the majority of such movements.

So, the Indochinese Block (III in Fig. 4) in Oligocene-Miocene under the influence of collision between the Indian and Eurasian plates moved southeastward, but in 5 Ma BP the moving direction changed to northwest ward, that led to compression and to right shift in the northwest of region [5, 15]. The analysis of aftershock activity for earthquakes 2 and 7 on the border of China and Myanmar affirms a shift component of the majority of faults of this area and the compression of northwest-southeast direction. The same results have been received from the study on earthquakes 4, 5 and 10 in the Indo-Myanmar mountain range.

The analysis of processes in the focal zone of earthquake 1 and its aftershock activity (Fig. 3) in the Makasar strait in the east of the region has been allowing to follow the continuation to the north of the left shift, which is established in the central part of the Sulavesi Island. It is a serious evidence recognizing the southward displacement of the South-Chinese sea block IV (Fig. 4) along the mentioned fault and the Hainan-Natuna right extensional shift, taking place from Southeast China coast to the east end of the Sumatra Island [1].

The prevalence of the right shift along the continental margin was also discovered when studying on the surface of the sublatitudinal faults in Central ViÖt Nam [12]. With the account of data from the littoral part of Southeast China (the fault 4 on Fig. 4) it is possible to confirm the development of a gigantic recent right shift zone along the margin of the Asian continent.

Fig. 4. Schema of recent geokinematics of Southeast Asia with the distribution
of epicenters of strongest earthquakes of the central part of region.

1- epicenters of strongest earthquakes and their numbers corresponding to the Table 1: a- without the resolution, b- with the resolution of the focal mechanism (the area of compression waves is crossed); 2, 3) size and form of the earthquake focal areas with the direction of distribution of the fore- and aftershock: 2) in space, 3) after the time evolution; 4, 5) major faults, active in late Cenozoic: 4) shifts (a- established and inferred, b- planete faults); 5) large subsidences (a- Benioff zone; b- other subsidences); 6) spreading structures: a- near-shore deep troughs; b- sedimentary basins within the limits of continents and continental shelf; 7) belts of near-shore deep-water hollows; 8, 9) vectors of shift: 8) of main lithospheric plates, 9) of the blocks of southeast part of the Eurasian plate. The letters designate the main plates: EU - Eurasia, IN - Indo-Australia, P - Pacific Ocean, PH - Philippines microplate. The Roman numbers designate the blocks: I- Tibet-Himalaya, II- Southeast China, III- Indochina, IV- South China Sea, V- Central Sunda, VI- Kalimantan-Java. The Arabian numbers in squares designate the faults: 1- Main boundary subsidences, 2- Shiaojang, 3- Fancheng-Linshan, 4- Dongiang-Chingiangpu, 5- Red River, 6- Sagaing, 7- Trans-Indochina, 8- Philippines, 9- Hainan-Natuna, 10- Lupar Line, 11- Sorong, 12- Semanko, 13- Sulavesi-Palawan

Further to the north outside of the considered territory this zone is continued by the Tanlu known fault, also of basically right shift at the most recent stage [17].

The local structures related to the shown transversal shifts and their faults, as well as to more western left shift in Kunming area in South China (fault 2 on Fig. 4), that is proved to be true by the study on earthquakes 9 and 11.

Apparently, the majority of such spreading structures belong to near-fault pull-apart basin, whose development is characteristic for the east part of Eurasia (eastward about 100-102^o E) [2, 17]. The analysis of the time evolution of process in the focal areas of earthquake 8 in northwest ViÖt Nam and earthquake 3 near the coast of Central Sumatra has been revealing the distribution of aftershocks in the Northwest-southeast direction, that proves the existence of the right shifts of just this band (Red River and Semanko faults) together with the resolution of focal mechanism of these earthquakes (Fig. 4). If along the fault 1 of the Indochinese Block (III) as it is seen above, the shift is northwestward then along the Semanko fault the Central Sunda Block (V) moved from the extending axis of the Andaman Sea to southeast. It is to note, that the zone of right-transversal shift covers not only Southwest Sumatra, but spreads also to the adjoining part of the Mentawai fold arc almost just to the Sunda deep-water trough.

What are the reasons of high intraplate seismicity and of active transversal shift activity of blocks in Southeast Asia? Formerly it was supposed, that the marginal part of a continental plate under pressure of the adjoining oceanic plates plunges under it through the formation of the drift block with the division into separate blocks [14].

The study on deep sections of focal zones of earthquakes has been carried out within the limits of the considered Eurasian plate part shows that the overwhelming majority of them is intra-lithospheric section, not deeper than 150-200 km. Therefore, the seismogenic structures should be distributed in rather upper parts of the tectonosphere, whose activity is caused more by the interaction of lithospheric plates, than by the ascension of deep mantle flows.

The same conclusion has been made in the work [3] on the basis of study on late Cenozoic basalts of the Indochina region. The clear connection between intraplate riftogenic shift, volcanism, displacement of blocks according to large shifts and Cenozoic subduction and collision processes has been recently established for Central and East Asia [13].

Thus, the activity characteristics of strong earthquakes and their fore- and aftershocks have been clarified on the basis of the first made unified catalogue of earthquakes of Southeast Asia; they conform in a great degree with geotectonic schema supposing a discrete structure of lithosphere of the region and the movement of its separate blocks under the influence of collision between main lithospheric plates.

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