Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

J. R. Kayal ¹, Sagina Ram ², O. P. Singh ¹, P. K. Chakraborty ¹, G. Karunakar ³

Affiliations
1 Gelogical Survey of India, Central Geophysics Division, 27, J.N. Road, Kolkata - 70001 6, IN
2 Geological Survey of India, Northern Region, Lucknow - 226 024, IN
3 Geological Survey of India, Eastern Region, Salt Lake, Kolkata - 700 094, IN

Abstract

The aftershock sequence of the Chamoli earthquake (m_b 6.3) of March 28, 1999 provides an opportunity to study the aftershock characteristics and the seisrnotectonic structure in the Garhwal Himalaya. Detailed analysis of the spatial variation, the frequency-magnitude distribution (b-value), the activity decay-rate (p-value) and fault-plane solutions of the aftershocks throw new light on the aftershock characteristics and seismotectonics of the region. The observations suggest that the aftershock generating process and the b- and p- values changed with time, space and magnitude-ranges. The main shock occurred on the Basement Thrust by a thrust-fault mechanism. The aftershocks, on the other hand, are triggered by the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust as well as by strike-slip faulting, and all the events occurred above the plane of detachment.

Keywords

Main shock, Aftershocks, Seismicity, Fault-Plane Solution, Seismotectonics, Chamoli, Garhwal Himalaya.

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J. R. Kayal ¹, Reena De ², Sagina Ram ³, B. V. Srirama ⁴, S. G. Gaonkar ⁴

Affiliations
1 Geological Survey of India (GSI), Central Geophysics Division, 27 J.L. Nehru Road, Kolkata - 700 016, IN
2 GSI (ER), Kolkata, IN
3 GSI (NR), Lucknow, IN
4 GSI (CR), Nagpur, IN

Abstract

Fourteen seismographs were deployed by the Geological Survey of India for monitoring aftershocks of the 26 January, 2001 Bhuj earthquake (M_w 7.5) in the Kutch district of Gujarat State, western India. About 3000 aftershocks (M≥ 1.0) were recorded during the period from 29 January to 15 April, 2001. The aftershocks attenuated with time following the power law t^-P, where p = 0.91. The frequency-magnitude relation of the aftershocks also followed the power law with b-value = 1.21.

About 800 events (M≥ 2.0) are well located with an average RMS ≤ 0.2s and average horizontal and depth error ≤3 km. The epicentre map shows an aftershock-cluster area, about 60 km × 30 km, between 70.0°-70.6° E and 23.3°-23.6° N, which reflects the source area of the main shock and aftershocks at depth. The epicentres show two major trends - in a NE direction and a NW direction. Depth-sections of the aftershocks indicate that the events are mostly generated at 15 to 38 km depth. Some events occurred at depth ≤ 10 km; they follow the NW trend only. A south dipping seismogenic plane is revealed in the depth-sections

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Composite fault-plane solutions of the best-located and selected events are studied. The deeper (25 to 38 km) as well as the mid crustal (15 to <25 km) aftershocks show reverse faulting with a large left-lateral strike-slip component along the NE trending inferred fault, which are comparable with the main-shock solution. Along the NW trending inferred fault, on the other hand, the shallower aftershocks (depth <10 km) show reverse faulting with right-lateral strike-slip component, and the mid crustal and deeper aftershocks show almost pure reverse faulting. These solutions with the NW trending inferred fault are not comparable with the main shock solution. It is inferred that intersection of the two faults has been the source area for stress accumulation to generate the main shock and the aftershocks. The main shock generated rupture propagation by left-lateral strike-slip along the NE trending fault, and by pure reverse to right-lateral strike-slip along the NW trending fault.

Keywords

Aftershocks, Earthquake, Seismotectonics, Seismology, Bhuj, Gujarat.

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