Saurabh Baruah

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.

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.

Mapping Sediment Thickness in Shillong City of Northeast India through Empirical Relationship

Modified form of Nakamura method, ratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated from ratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness and . With the help of this, the probable values for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city.

Influence of attenuation and site on microearthquakes’ spectra in Shillong region, of Northeast, India: A case study

We examine the influence of attenuation and site on the spectra of microearthquakes having origin within the Shillong region. The ratios of spectral amplitudes at lower and higher frequencies are measured for three different stations at varying epicentral distances to estimate Q value for both P- and S-wave in near and sub-surface layer. The average estimates of QP and QS are found to be 178 and 195. The ratio of QS to QP emerges to be greater than unity in major parts of the Shillong area, suggesting dominance of dry crust prevailing in Shillong region. The variation in corner frequencies for these spectra is inferred to be characteristics of the site. Besides, the disparity in spectral content with reference to hard rock site yields the inference that the incoming seismic signals get amplified considerably while traversing from southern part to northeastern part of Shillong, best outlined at 2 to 5 Hz, which is well corroborated by the existing lithology.

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).

Modelling of the Kopili Fault based on slip rate, moment rate and seismic activity in Mikir Hills Plateau of Northeastern India

ABSTRACT The recurrence period of maximum magnitude earthquake in a seismogenically active formation along the Kopili Fault has been estimated having adequate dependence on the slip rate, moment rate and its seismicity pattern. Here, the frequency–magnitude cum fault area–maximum magnitude relations play a key role with input parameters pertinent to the Kopili Fault zone. Subsequently, where the fault slip rate estimates are not available, the seismic activity is studied from the seismic moment release. The results of this study show that the return period has a strong relation with the fault length, slip rate, strain drop and rigidity. This study ascertains the activity rate in terms of the return period as ∼50 ± 5 years with the moment release of 2.12E+23 dyne-cm from the most active 80-km fault length considering Mw 5.5 as reference magnitude under the Kopili Fault zone that may produce a maximum magnitude of Mw ∼ 7.6. Finally, we conclude that these models can be used to study the rate of seismicity of the active faults in Northeast India which will provide us prime inputs for seismic hazard analysis of the region and is especially significant for estimating expected return period for poorly known faults or blind faults that lack surface expression.

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.