J. R. Kayal

The 2009 Bhutan and Assam felt earthquakes (Mw 6.3 and 5.1) at the Kopili fault in the northeast Himalaya region

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east–west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region.

Himalayan tectonic model and the great earthquakes: an appraisal

The best known conceptual tectonic model of the Himalayan Seismic Belt (HSB) suggests that the Basement Thrust Front (BTF) lies beneath the Main Central Thrust (MCT) with a prominent ‘ramp’. The ‘ramp’ is viewed as a geometrical asperity that accumulates the stress due to the Himalayan collision tectonics, and it has been suggested that the past great earthquakes occurred on the plane of detachment. The plane of detachment is the interface between the Indian shield and the Himalayan sedimentary wedge, also known as the Main Himalayan Thrust (MHT). The recent earthquake data from the local permanent and temporary networks and a re-examination of source processes of the great earthquakes in the Himalaya, however, do not support this model for the entire HSB. The four known instrumentally recorded great (M ∼8.0–8.7) earthquakes in the foothills of the Himalaya in India, from west to east – the 1905 Kangra, 1934 Bihar, 1897 Shillong and the 1950 Assam earthquakes – occurred by different tectonic processes, and possibly none can be explained as a plane of detachment earthquake; each occurred in its own unique complex tectonic environment. The 1905 as well as the 1934 great event may have a deeper source to the south of the MBT. The 1897 great event is argued to be a shield earthquake rather than a Himalayan earthquake and it occurred by pop-up tectonics of the Shillong plateau. The 1950 great event is argued to be caused by transform tectonics in the eastern syntaxis zone rather than by thrusting on the plane of detachment.

The September 2011 Sikkim Himalaya earthquake Mw 6.9: is it a plane of detachment earthquake?

The 18 September 2011 Sikkim Himalaya earthquake of Mw 6.9 (focal depth 50 km, NEIC report) with maximum intensity of VII on MM scale (www.usgs.gov) occurred in the Himalayan seismic belt (HSB), to the north of the main central thrust. Neither this thrust nor the plane of detachment envisaged in the HSB model, however, caused this strong devastating earthquake. The Engdahl–Hilst–Buland (EHB) relocated past earthquakes recorded during 1965–2007 and the available global centroid moment tensor) solutions are critically examined to identify the source zone and stress regime of the September 2011 earthquake. The depth section plot of these earthquakes shows that a deeper (10–50 km) vertical fault zone caused the main shock in the Sikkim Himalaya. The NW (North-West) and NE (North-East) trending transverse fault zones cutting across the eastern Himalaya are the source zones of the earthquakes. Stress inversion shows that the region is dominated by horizontal NNW-SSE (North of North-West-South of South-East) compressional stress and low angle or near horizontal ENE-WSW (East of North-East-West of South-West) tensional stress; this stress regime is conducive for strike-slip faulting earthquakes in Sikkim Himalaya and its vicinity. The Coulomb stress transfer analysis indicates positive values of Coulomb stress change for failure in the intersecting deeper fault zone that produced the four immediate felt aftershocks (M ≥ 4.0).

Introduction to the special volume on Bhuj earthquake

The papers in this volume were presented in the ‘‘International Symposium on Advances in Earthquake Science,’’ January 22–24, 2011, at the Institute of Seismological Research (ISR), Gandhinagar, Gujarat, India. The symposium had special emphasis on earthquakes in Kachchh and rest of Gujarat. The region falls in the western part of the stable continental region (SCR) of India that experienced large earthquakes in the past and most recently the January 26, 2001, devastating Bhuj earthquake in the Kachchh failed rift zone. The scientific issues associated with the earthquakes that occur away from active plate boundaries are of importance to understand the related hazards in the intraplate regions around the world. The potentially devastating consequences of one such earthquake were illustrated by the severe damages and loss of lives caused by the 2001 Bhuj earthquake in Gujarat. Topics in this symposium included the Bhuj earthquake and its aftershock sequence, intraplate seismicity, seismic hazards, and tectonic models, with emphasis on recent theoretical and observational advances as well as identification of key data sets that could be collected and/or made available to address outstanding issues relevant to hazard assessment. Several papers in this volume deal with unique data collection in the Kachchh region by some 60 broadband and 50 SMA observatories since the 2001 Bhuj earthquake and the results of data analysis using the state-of-the-art techniques. In addition, data were generated by several geophysical and geological investigations. The geophysical investigations included gravity, magnetic, MT, GPS, RTF and GPR surveys, while the geological investigations included active fault mapping and geomorphological as well as geotechnical

Ground motion parameters in the Shillong–Mikir plateau, northeastern India

Ground motion parameters for the Shillong–Mikir plateau, northeastern India are examined. Empirical relations are obtained for ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. A correlation between ground motion parameters and characteristics of seismogenic zones is established. Simultaneously, new empirical relations are derived for the attenuation of ground motion amplitudes. The logarithmic width is found to be independent of earthquake magnitude and distance. The attenuation relations estimated for the logarithmic width of the Mikir plateau are found to be a little bit higher than that of the Shillong plateau both for soft and hard ground, which accounts for geometrical spreading and anelastic attenuation.