Updated: Apr 30, 2021
Jnana R. Kayal
The Indian sub-continent is currently moving northward at about 40 mm/year and colliding with the Eurasian Plate (Tibetan Plateau). The collision with Asia began in the Middle Eocene era about 50-55 Myr ago (Figs. 1 & 2).
The region of north India is seismically very active. Major earthquakes pose a continuing threat to communities of that region. In addition, there is intra-plate earthquake activity south of the Himalayan Front that poses a threat to communities in other parts of the country, in particular in west India.
Peninsular India constitutes one of the largest Precambrian shield areas of the world (Fig. 3). The Indo-Gangetic Alluvium Plain (IGAP) separates the Himalaya to the north and the Peninsular Shield to the south (Figs. 3 & 4). The Shillong Plateau in northeast India constitutes an outpost separated from the main shield by the Bengal Basin and from the Himalaya by the Brahmputra River.
The Peninsular Shield of India is made up of three main cratonic regions (Fig. 4); the Aravalli, the Dharwar and the Singhbhum which are separated by Proterozoic rifts and mobile belts. The major prominent rifts that separate the southern and northern blocks of the shield are the Narmada Son Lineament (NSL) and the Tapti Lineament (TL), together called the Son-Narmada Tapti lineament (SONATA). The other rift basins are the Kutch, Cambay, Godavari, Cuddapah etc. (Fig. 4).
The Himalayan region is very much associated with a high degree of seismicity in comparison to that of Peninsular India (Fig. 5), and making the Himalayan region seismically more vulnerable to earthquake damage (Zone V) than that of Peninsular region (Fig. 6).
To gain a better understanding of the crustal architecture of India and the threats posed by earthquakes, the Geological Survey of India (GSI) has, in recent years, determined 3-D seismic velocity structure in different parts of the Himalayan region and in Peninsular India using seismic inversion techniques with the P- and S-wave arrival times recorded by local and regional seismic networks. The aftershock sequences of recent damaging earthquakes (1993 Killari earthquake, Mw 6. 3; 1999 Chamoli earthquake, Mw 6.3; the 2001 Bhuj earthquake, Mw 7.7) have been studied in more detail to provide seismic crustal images of the earthquake source areas.
The northeast Himalayan region of India is one of the most seismically hazardous zones in the south Asia. GSI has determined the 3-D seismic velocity (Vp) structure of the crust of that region using selected arrival time data from two groups of shallow to intermediate-depth local earthquakes recorded by two different seismic networks (Fig. 8) by applying the 3-D tomography method of Zhao et al. (1992).
One group of earthquakes consisted of 1011 shallow and intermediate-depth local earthquakes recorded during 1984-2000 by 113 temporary and permanent seismic stations in NE India (Figure 8). The other group of earthquakes consisted of 169 shallow to intermediate-depth earthquakes that occurred from 1964 to 2000 and were reported in International Seismological Center (ISC) Bulletins. These earthquakes were recorded by a network of 29 seismic stations (Fig. 8).
The interpreted cross-sectional velocity images of the lithosphere in NE India (Fig. 9) demonstrate a good correspondence to the local and regional tectonic structures (Fig. 10) (Mishra et al., 2005a). The low-velocity zone down to 20 km depth in the region of the Bengal Basin corresponds to the thick sediments within the Bengal Basin, while high-velocity anomalies in the same depth range possibly indicate the presence of dense crystalline rocks under compressional stress that cause the seismicity in the region. The subducted Indian lithosphere is imaged as a high-velocity zone beneath the Burma platelet (Fig. 9).
Kayal and Zhao (1998) imaged 3-D seismic velocities Vp and Vs, and the Poisson's ratio structures using microearthquake data recorded between 1983 and 1986 by a local seismic network (Fig. 11), which support the regional tomographic results of Mishra et al. (2005a) (Fig. 9).
In the western Himalayan region, a detailed 3-D seismic image determined by Mukhopadhyay and Kayal (2003) beneath the 1999 Chamoli earthquake (M 6.3) epicenter area (Fig. 12) clearly shows that the major thrust zones; the mainshock and its aftershocks were in high-Vp zone of the "fault end" (Fig.13).
Recently, Mishra et al. (2005b) determined a high-resolution 3-D P-wave velocity structure in the western Himalaya using P- and S-wave phase arrival data from a total of 1042 earthquakes consisted of 534 aftershocks of the 1991 Uttarkashi and the 1999 Chamoli mainshocks recorded by local temporary and permanent seismic stations, and 508 earthquakes reported by ISC bulletins (Figs.12, 14). This study revealed high velocity structures at source areas of the 1905 Kangra earthquake (M 8.0), the 1991 Uttarkashi earthquake (M 6.4), and the 1999 Chamoli earthquake (M 6.3) (Fig.15).
In Peninsular India, detailed seismic imaging has been done using P- and S-wave arrival times from the aftershock sequence of the 1993 Killari earthquake (M 6. 3) that occurred in the southern Archaean shield and using the sequence of the 2001 Bhuj earthquake (Mw 7.7) that occurred in the Kutch Rift Basin (Fig. 4).
The 1993 Killari Earthquake region: 3-D imaging of the 1993 Killari earthquake source area in central Peninsular India (Fig. 3) was done using the local earthquake tomography method of Thurber (1983). The seismic images show that the mainshock occurred at the boundary between a high and a low-Vp zone (Kayal and Mukhopadhyay, 2002) and it supports their intersecting fault model shown in Figures16 and 17.
The 2001 Bhuj Earthquake region: Kayal et al. (2002), Mishra and Zhao (2003), Mishra et al. (2005b) estimated a detailed seismic structure using high precision P- and S-arrival times from a total of the 368 aftershocks (Fig. 18) using 3-D tomography method of Zhao et al. (1992).
The lateral and vertical heterogeneities in the seismic velocities Vp and Vs, and in the Poisson's ratio(s) determined at the 2001 Bhuj mainshock hypocenter suggests that the mainshock was a fluid-driven earthquake (Figs. 19, 20).
The 3-D tomographic models are well correlated with a simulated model of the intersecting fault geometry derived from the fault plane solutions of the Bhuj earthquake sequence (Kayal et al., 2002b) (Fig. 21).
Deep seismic sounding (DSS) investigations have been made by the National Geophysical Research Institute (NGRI) group in many different parts of India (Fig. 24).
The results of the DSS investigations using seismic refraction and wide angle reflection techniques reveal many deep and shallow subsurface features within the Earth's crust (Figs. 25 - 27):
Seismic receiver function (RF) studies have been conducted out using teleseismic earthquake waves recorded at 10 broadband stations spread over Peninsular India (Fig. 28) and 5 stations over the northeast India regions (Fig. 29).
A crustal thickness of 33-39 km is determined below the South India Archaean shield. The Deccan Trap basalts have not significantly affected the underlying crust. The predominant Proterozoic crust in the northern and eastern part of the shield, on the other hand, exhibits a complex character. The Moho conversions are considerably weaker compared to the Archaean terrains, and crustal thickness is 40 km and more (Kumar et al., 2001).
In the northeast India region, thinner (33 km) Archaean crust is reported below the Shillong Plateau with the thickness increasing to ~40 km below the Brahmaputra valley and to ~50 km below the northeastern Himalaya to the north (Ramesh et al., 2005).
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J. R. Kayal,
CGD, Geological Survey of India