Santanu Baruah

State of Tectonic Stress in Northeast India and Adjoining South Asia Region: An Appraisal

An attempt is made to map the spatial variation of the tectonic stress pattern in northeast India and its adjoining south Asia region using stress tensor inversion of some 516 fault‐plane solutions. The Bhutan Himalaya and the Arunachal Himalaya are mapped with north–south to north‐northwest–south‐southeast compression. The eastern Himalaya syntaxis zone, on the other hand, shows a clockwise rotation; a north‐northeast compression is dominant. To the south, in the intraplate part of the region, the Shillong plateau, Assam valley, Bengal basin (Bangladesh), and Tripura fold belt exhibit north‐northwest to north‐northeast compression. Orthogonal horizontal extension is dominant in southern Tibet, Bhutan, and partly in the syntaxis zone, and the same is also observed in the Shillong plateau and Assam valley area of the intraplate region. The Indo–Burma ranges and the Sagaing fault in the Myanmar region show a northeast–southwest compression; an orthogonal horizontal northwest–southeast extension is also observed in the Sagaing fault zone. A depth variation of the tectonic stress is observed below the Indo–Burma ranges; it changes from north–south to northeast–southwest in the southern part, and from northeast–southwest to north‐northeast–south‐southwest in the northern part in the deeper seismogenic zone. The stress inversion results of clusters of events in individual zones, though mostly conformable with the average observations, indicate a variation in the Shillong plateau due to heterogeneity and tectonic complexity.

Moment magnitude – local magnitude relationship for the earthquakes of the Shillong-Mikir plateau, Northeastern India Region: a new perspective

An attempt has been made to examine the empirical relationship between moment magnitude (M W) and local magnitude (M L) of earthquakes recorded in the Shillong-Mikir Plateau of Northeastern India. Moment tensor solutions of 106 earthquakes recorded during the period 1976–2009 are used. The focal mechanism solutions of these earthquakes include 1 Harvard-CMT solution (M W ≥ 4.0), 54 solutions from different publications and 51 solutions obtained for the local earthquakes (2.0 ≤ M L ≤ 5.0) recorded by a 20-station permanent broadband network during 2001–2009 in the region. The moment tensor solutions of these local earthquakes are obtained by the discrete wave number method. The M W –M L relationship in the region is determined by generalized orthogonal regression analysis, which is found to be M W = M L (1.00 ± 0.02) + (0.02 ± −0.05). It is observed that, on average, M W is equivalent to M L with an uncertainty of about (0.02 ± −0.05) magnitude units for earthquakes of the Shillong-Mikir Plateau. Conversion of M L to M W is recommended for seismic hazard analysis and tectonic studies in the region.

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).

Crustal imaging of the Northwest Himalaya and its foredeep region from teleseismic events

ABSTRACT Over 450 receiver functions from 8 broadband stations located in the Indo-Gangetic plain and Northwest Himalayan region are analyzed to examine the crustal properties across the contiguous region. We identified the P-to-S phase beneath each station and estimated the crustal thickness from time delay of this phase with respect to the direct P arrival. With the help of the slant stacking technique, we determined bulk crustal chemical properties and validated our estimate of crustal thickness. The Moho was encountered in the Indo-Gangetic plain at an average depth of 33 km and thickened towards the Northwest Himalaya with the Moho depth varying from 37 to 52 km. The thickest crust matched the highest topography, which is strong evidence of the occurrence of a crustal root of the mountain range. The time domain iterative linearized inversion technique is used to invert radial receiver functions to wave velocity structures for both tectonic regimes. From the forward modelling, we found mid-crustal low-velocity layers at different patches at a depth of 10–30 km in the Northwest Himalaya region. The presence of melts may be inferred in the mid crust with high values of Poisson ratio (σ ≥ 0.260) for the stations in the Northwest Himalaya. Towards south in the Indo-Gangetic alluvium plain, we estimated a medium to higher value of Poisson ratio (0.240 ≤ σ ≤ 0.290), but velocity modelling implies absence of an intracrustal low-velocity zone around the region.

Coulomb Stress Changes in the Area of December 2013–January 2014 Sannio-Matese Seismic Sequence (Southern Italy)

The Italian Apennines are seat of extensional deformation, concentrated along the inner part of the mountain belt due to the opening of the Tyrrhenian back-arc basin during the late Miocene and the following rolling back subduction of Adriatic plate with an extension velocity of about 3 mm year−1 (D’Agostino et al. 2008; Faccenna et al. 2004). Apennines chain is a zone of high seismic hazard (http://zonesismiche.mi.ingv.it; “Mappa di pericolositasismica del territorionazionale”; D’Amico et al. 2013a) and it has been affected by a number of earthquakes in the past century suffering intensity X or higher several times in the past centuries (Boschi et al. 2000; CPTI Working Group 2004). The most recent examples are the 1980, M = 6.9, Irpinia events (Pino et al. 2008; Secomandi et al. 2013); the 1997–1998 Umbria-Marche (Caccamo et al. 2007) and the 2009 L’Aquila (D’Amico et al. 2010a, 2013b) sequences.

Waveform Modelling of 2009 Bhutan Earthquake of Magnitude 6.1 (Mw) Using Local Network Data of North East India

A strong earthquake of Mw 6.1 struck Bhutan on September 21, 2009 with casualties of several people. The epicentre of the event was given at latitude 27.34° N and longitude 91.41° E, and depth ~ 10 km (USGS report; http://earthquake.usgs.gov). Shaking from the earthquake was felt in the Bhutan, Tibet and in the adjoining North East region of India including Bangladesh.

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.