1. Introduction

Focal mechanism solution determines the nature and orientation of the stress field that prevailed in the preliminary generation of an earthquake. It is the analytical result of waveforms generated by an earthquake which eventually provide information about the mode in which the seismic energy is released in the hypocentral zone and facilitates to understand the physical and the tectonic conditions. It commonly refers to fault orientation, the seismic slip in relation to plate movement, stress release patterns and the geodynamic process of seismic wave generation. To understand the stress pattern and the geodynamic processes in the eastern segment of Himalayan collision zone and its adjoining regions, a total of 94 focal mechanism solutions have been considered in the present research work. Thirty four focal mechanism solutions have been determined in present work. The remaining 60 solutions are compiled from already published literatures (e.g.[3],[4],[5],[15],[17],[18],[20],[22],[26], Centroid Moment Tensor (CMT) solution). Rastogi et al.[22] determined eleven focal mechanism solutions of earthquakes occurred in Assam-Burma region using P-wave first-motion direction data and reported that the dominant modes of deformation are thrust faulting in the region. Chandra[3] determined eighteen focal mechanism solutions for the earthquakes occurred in Himalayan region using P-wave first motion data and reported thrust faulting pattern in northeast India region. Singh and Shanker[25] have stated that the effect of Tibetan plateau on Burmese Arc tectonics and seismicity distribution cannot be ignored.

Several thrusts, lineaments, folded belts are found to be responsible for the earthquake generation in this region. It is believed that high seismic activity in the region is due to northward as well as eastward movement of the Indian plate toward the Eurasia and Burma plates, respectively.

2. Physiographic Location of Study Region

The Himalayan mountain belt extends from Nanga Parbat (8138 m) in the west to Namche Barwa (7775 m) in the east[14]. This belt trends mostly in NW-SE with a length 2400 km and width vary from 200 to 250 km covering Nepal and some parts of India, China, Bangladesh, Bhutan and Burma. The Himalaya is bounded by Tibetan plateau to the north and Indus- Ganga -Brahmaputra plain to the south. Northeast India Himalaya and its adjoining regions, the eastern part of Himalaya, are focused here to identify the present state of geodynamic processes by using focal mechanism solutions (Fig 1). This region has been considered as complex from geologic and tectonic points of view[1, 7, 9, 19]. The region is seismically potential zone as several large/great damaging earthquakes have occurred in the past. The earthquakes in this region are reported to be associated with MCT, MBT, thrusts, faults and lineaments. The main Himalayan seismic belt is mostly confined with the MCT and MBT[20]. The Indo-Burma region forms complex geology and tectonics. On the basis of physiographic structure, Burma territory has been divided as Eastern Highlands, Central lowlands and Indo-Burma mountain belt (called Indo-Burma Range). Bengal basin lies to the west of Indo-Burma range. The Arakan-Yoma, Manipur, Naga Hills, Lushai and Patkai are the most prominent structure in the Indo-Burma range. The northernmost part of Indo-Burma arc joins with the Himalayan belt and the joint region is outlined a complex structure that forms Syntaxis called Eastern Syntaxis.

Figure 1. Map of the Northeast India Himalaya and its adjoining regions (modified after[20]). The map includes the Himalayan arc, Tibetan plateau and surrounding regions showing major tectonic– structural features. MCT (Main Central Thrust); MBT (Main Boundary Thrust); MFT (Main Frontal Thrust)

3. Seismicity and Stress Pattern

The Northeast India and the Eastern segment of the Himalaya is seismically one of the most active regions in the world where several large and two great earthquakes have occurred during the past hundred years. Several thrusts, lineaments, folded belts are found to be responsible for the earthquake generation in this region. A total of 560 earthquakes have been listed in the database during the period 1808 to 1962 in the considered region. Gupta et al.[10] have considered the great Shillong earthquake (1897) as the demarcation between the historical and recent seismicity of northeast India region. The seismicity pattern of this region has been studied by a number of researchers[10, 12, 13, 23, 25]. Considering the spatial distribution of earthquakes for the period 1808-2008 and using the information from[8], five seismically active zones are delineated in the northeast India Himalaya and its adjoining regions (Fig. 2.) as: Eastern Syntaxis (I), Arakan Yoma Fold Belt (II), Shillong Plateau (III), Himalayan Frontal Arc (IV) and Southeastern Tibet (V).

The seismicity and stresses acting in Himalayan region are very complex. Eastern and western Syantaxis of the Himalaya show comparatively high seismic activity to that of the central section. Four great earthquakes have occurred in this Himalayan belt during the last one hundred years which are 1897 (M 8.7, Shillong), 1905 (M 8.6, Kangra), 1934 (M 8.4, Bihar –Nepal border) and 1950 (M 8.7, Assam). The high seismic activity in the region is due to northward as well as eastward movement of the Indian plate toward the Eurasia and Burma plates respectively.

Chouhan and Srivastava[6] studied the focal mechanism solutions of earthquakes in northeastern India region using P-wave first motion directions data only for two years (1965-1966) and reported strike-slip faulting in northern part and thrust faulting in the southern part of Burmese mountains, and strike-slip faulting with Dauki fault and Disang thrust. Verma et al.[27] determined six focal mechanism solutions and observed normal as well as thrust faulting in Burma region and reported compressive stress axis nearly vertical than horizontal. Pure thrust faulting in Shillong area and pure thrust and strike faulting in Tura area were observed by composite focal mechanism of microearthquakes[12]. Singh and Shanker[25] determined six focal mechanism solutions in Bengal Basin using P-wave first motion data and observed thrust faulting.

Figure 2. Seismicity of Northeast India Himalaya and its adjoining regions from 1808 -2008 over the major tectonic features MCT, MBT, IBR, and Shillong plateau. Roman letters I to V indicate seismically active regions delineated in the present study

4. Data Analysis

Focal mechanism solutions are determined by using P-wave first motion directions data of short as well as long periods from the catalogue of the International Seismological Centre (ISC) bulletin for the earthquakes with magnitudes mb 4.7-6.1 occurred from 1964-2008 in northeast India Himalaya and its adjoining regions. P-wave velocity structure model of Bhattacharya et al.[2] for the region is used to estimate the angle of incidence at each seismological station. Data related to ray parameter is taken from P–wave table of Herrin et al.[11] and estimated the angle of incidence of the ray at each recording station. The first motion directions of P–wave were plotted on an equal area projection of the lower hemisphere using the estimated values of angle of incidence (i) at the azimuth (Az) of each seismological station. The orientations of the nodal planes and the direction of P (pressure), T (tension) and B (null) axes were determined by using Wulff’s stereographic projection net. The azimuth and the plunges of the P, T and B axes and the orientation of nodal planes were measured in degrees from the north and the horizontal, respectively. For all the solutions, a double couple source has been assumed for the interpretation of earthquake mechanism.

To investigate the current geodynamic status and stress pattern in northeast India Himalaya and its adjoining regions considering 94 focal mechanism solutions (Fig. 3). And source parameters and focal mechanism solution parameters for these earthquakes are furnished in Table 1. The distribution of inferred faulting patterns from focal mechanism solution of these earthquakes, in relation to major tectonic features, is shown in Fig. 4 at their epicenter locations.

Figure 3. Spatial distribution of ninety-four mainshocks occurred during 1963-2008 in Northeast India Himalaya and its adjoining Southeast Tibet region selected for focal mechanism solutions and seismotectonic studies

5. Statistic of Faulting Patterns

The tectonics of Northeast India and its adjoining Eastern Himalaya, Southeastern Tibet and Burma regions is quite complex in which mixed faulting pattern is observed. It may be seen from Table 2. that the thrust environment is very much dominant in all the regions except Southeastern Tibet which is dominated by normal faulting. The tectonics of Arakan-yoma region is even more complex in which almost equal percentage of normal as well as thrust faulting occurs from shallow to intermediate depths. The nodal planes of thrust events orient predominantly in N to NE directions, and it is NW-SE to E-W direction of nodal planes for normal faulting events similar to that of the Southeastern Tibet. Thrust environment is predominant in Himalayan Frontal arc, Shillong plateau and Eastern Syntaxis regions but a few normal and strike-slip faulting have also occurred. The different tectonic regimes as observed from north to south are (1) extensional in Tibet region, (2) compression in Eastern Himalaya; and (3) both extensional and compressional in Arakan-yoma region to the south. The faulting pattern in the adjoining Tibet is totally different than Himalayan compression belt where normal faulting is predominant with north-south trending nodal planes leading to east-west flow of materials. The data furnished in Table 2 also suggest that most predominant mode of energy release in Eastern Himalaya is due to thrust faulting, and due to both normal as well as thrust faulting in Arakan-yoma regions, whereas mechanism for energy release in the Southeastern Tibet is solely due to normal faulting.

Figure 4. Spatial distribution of faulting patterns of the ninety-four mainshocks occurred in Northeast India Himalaya and its adjoining regions during 1963-2008

6. Discussion and Conclusions

It has already been established that the great Himalayan mountain range was formed as a result of the collision of two mega tectonic plates India and Eurasia. This collision, which happened some 65 million years ago, was brought about by the northward movement of the Indian plate that resulted in closing of the vast intervening Tethyan Ocean. Many scientists have already pointed out that collision of the Indian plate with the Eurasian plate, which occurred in the Late Tertiary[16, 21], is the sole cause for the faulting pattern being observed in the Himalaya and its adjoining region.

Table 1. List of ninety-four earthquakes occurred during 1963-2008 in northeast India Himalaya and its adjoining Tibet region are considered for seismotectonic studies using focal mechanism parameters derived through focal mechanism solutions of these earthquakes. Focal mechanism solutions for the first 34 mainshocks (Sl. No. 1-34) have been done in the present work, and the remaining 60 solutions (Sl. Nos. 35-94) and their source parameters are taken from different published literatures. The strikes, dips, dip directions (dip dir.) and slips of both the nodal planes; and azimuths (Az) and plunges (Pl) of P, T and B-axes for each of the mainshocks are estimated/ taken and these are furnished in degrees. Dip directions are considered for both of the nodal planes related to the first 34 mainshocks, whereas it is slip angle for the remaining 60 mainshocks

Table 2. Frequency distribution of different types of faulting observed in five delineated seismogenic regions in the Northeast India Himalaya and its surrounding Tibet region using ninety four focal mechanism solutions

Orientations of pressure (P) and tension (T) axes derived through focal mechanism solutions Table. 1 and Fig 4, help in understanding the stress pattern of a region. The azimuth and dip of the P- and the T- axes are determined through focal mechanism solutions of earthquakes. In general, the tension is related with the normal faulting and the compressive stress with the thrust faulting and measurement of P- and T-plunge determines the stress regime of a region. The following criterion are considered to infer about different stress regimes: (1) Extension Regime if T <45^o and P >45^o (if T- plunge is less than 45^o and P- plunge is greater than 45^o, the region is said to be an extension regime); (2) Compression Regime if P <45^o and T >45^o (if P - plunge is less than 45^o and T- plunge is greater than 45^o, the region is said to be compressive regime); and (3) if both the axes of P- and T- plunge are around 45^o, the focal mechanism solutions are said to be strike-slip in which motion occurs past each other). In order to study the general characteristic of the stress field of a region, the composite plot of P- and T -axes will be more meaningful than deriving inferences using individual event mechanisms. Such composite solutions provide direct evidence for the main compressive stress field in a region like Northeast India Himalayan region.

In the Eastern Syntaxis the events have shallow dipping nodal planes striking NW-SE to E-W. Normal faulting is observed at a depth of 29 km (events 3 and 6) with nodal planes oriented along NE-SW to E-W indicating resultant extension along NW-SE. It may be inferred that the Eastern Syntaxis region of Himalayan range is under compression from northwest and south directions. The general trend of direction of extension from NW-SE to E-W is observed in Arakan-yoma region by analyzing a total of 17 normal faulting solutions (shallow as well as intermediate). Majority of normal solutions (~65%) provided predominantly E-W direction of extension which is especially confined in the central portion of the fold belt bounded by 21^o to 25.5^o N latitude along north-south direction whereas it is predominantly NW-SE towards east and west of it. It is also observed that the deeper level of extensional regime in the central portion changes to shallow depths on either sides of it (east and west). This trend suggests that the extensional tectonic regime occur from shallow to intermediate depths (up to 140 km) in the region which may be indicative of presence of extensive fluid (magma chamber) at these depths. The analysis of focal mechanism parameters reveals NE oriented compression and E-W oriented extension pattern in Arakan-Yoma region and these patterns exist right from shallow to intermediate depths up to 140 km.

It is inferred that N to NE is the predominant direction of compressive stresses currently acting in the Arakan-yoma region whereas direction of extension is predominantly from NW-SE to E-W. Compression and extension regimes observed at different depth levels may be due to internal relative motion of different blocks along the transverse faults coupled with intermittent upward rise of magmatic materials due to partial and differential melting in the subsurface of the leading edge of the plate subducting into the asthenosphere. The Shillong Plateau region consists of several faults with three main directions of orientation i.e. NW-SE, NE-SW and N-S. Several major faults are mapped in the Shillong plateau region such as Dauki fault to the south, Chedrang fault and Oldhan fault to the north, Kopili fault to the east and Dhubri and Tista fault to the west. Seismic activity is observed to be very feeble along these faults and strong activity is found to occur slightly away from these faults. The region is also traversed by several other faults/ lineaments especially in the eastern, southern and western portions that surround the plateau. Focal mechanism solutions of eight earthquakes have been considered to study the stress pattern in Shilllong plateau of which five solutions (events No. 20-24) of the earthquakes (1982 -1992) are determined in the present work. Majority of events with thrust faulting have moderate to steep dipping nodal planes towards NW to N. Steep dip is observed towards northwest and southeast of the plateau whereas it is moderate dip within the plateau region. From the interpretation of observed faulting patterns in Shillong plateau, it can be inferred that the region is under compression due to the stresses acting in N to NE directions.

The present geotectonic activity in the Himalayan collision zone is the result of post collisional incident. The results derived through focal mechanism solutions confirm that the geological processes, that were responsible for the formation of Himalaya, are still continuing. The orientation of minimum compressive stress, reveal that the earthquake generation process in Tibet is entirely different than that of the Himalayan compression zone. And almost horizontal minimum compressive stress in the Tibet is responsible for the earthquake generation process due to the flow of materials in east-west direction.

ACKNOWLEDGEMENTS

Author (DS) is thankful to Department of Earthquake Engineering for excellent computational facility. Authors (HNS and AP) are grateful to Prof V P Singh, Prof. U S Singh and Prof. M. Banarajee of Department of Geophysics, Banaras Hindu University Varanasi, India for their continuous encouragement to organizing the scientific knowledge. Authors are also thankful to learned referees and editor for their comments and suggestions which improved the manuscript appreciably.