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Understanding the behaviour of soil and site in highly seismically active Assam valley, Shillong-Mikir Hills Plateau and Eastern Himalayan Syntaxis of Northeastern India is important for seismic hazard assessment. Constrained by this tectonic interaction, we estimate the ground motion parameters of these tectonic domains through the inversion of 129 three component strong motion records of earthquakes with magnitude Mw>4.0 & Mw

Himalayan region is seismically one of the most active continental regions. It experienced at least 4 M ∼ 8 earthquakes during an active phase from 1897 to 1952. However, no such earthquake has occurred since 1952. Detailed investigations have revealed that the region is currently in a seismic quiescence phase, and enough strains have been accumulated to source several M ∼ 8 earthquakes. However, when and where such an earthquake would occur cannot be forecasted. Even if such an earthquake is forecasted to occur, can everyone leave to a safe place? That is not practical. It is therefore important to learn to live with earthquakes and develop an earthquake resilient society. Recent earthquakes have demonstrated the effectiveness of such an approach. One very effective way to develop earthquake resilient society is to create an earthquake scenario for the repeat of an earlier earthquake, estimate the losses, and then go through an exercise of imparting and sharing with public, state and central governance the ways and means of reducing the anticipated losses. The National Disaster Management Authority (NDMA), Government of India, in collaboration with other agencies, built earthquake scenarios for the repeat of the 1905 Kangra earthquake of M ∼ 8 and 1897 Shillong earthquake of M 8.7. Appropriate ground motion prediction equation was used to generate earthquake intensities. The intensities of the hypothetical Mw 8 earthquake located at Mandi in Himachal Pradesh (close to the epicenter of 1905 Kangra earthquake) were compared with the 1905 Kangra earthquake’s isoseismals and found to be satisfactory. For the 1897 Shillong earthquake, the isoseismals drawn by experts for the 1897 earthquake were used. Using the 2011 Census data for demography and building typology, it was estimated that if the hypothetical Mandi earthquake occurs in the middle of the night, the human lives lost would be ∼ 959100 combining the states of Himachal Pradesh, Punjab, Haryana and the Union Territory of Chandigarh. It may be noted that the 1905 earthquake had claimed ∼ 20,000 human lives. For the repeat of Shillong M 8.7 earthquake in the mid of the night, the number of lives lost estimated would be ∼ 423000 in the 8 north-east states (Assam, Arunachal Pradesh, Meghalaya, Sikkim, Nagaland, Manipur, Tripura and Mizoram). NDMA took up detailed preparatory exercises involving the center and state bodies for Rapid Visual Screening (RVS) of lifeline buildings; school sensitization events; Incident Response System (IRS); and Awareness Generation programs involving local celebrities and a variety of news media. National Disaster Response Force (NDRF) and State Disaster Response Force(s) (SDRF) played a crucial role. To test the preparedness, mega-mock drills were held on 13 February 2013 for the 3 north-western states and UT Chandigarh; and on 10 and 13 March 2014 for the 8 north-eastern states. Performances of all the sectors were evaluated by independent observers. There was an excellent media coverage and the very purpose of generating awareness of the losses due to foreseeable earthquakes and how to reduce them was amply demonstrated. It is important to conduct similar exercises in other regions falling under Seismic Zones V and IV of the country. Of utmost importance is educating school students on an yearly basis, ways and means of developing earthquake resilience.

In the present study, ground motion parameters are estimated by processing 26 number of Earthquake events which contain 128 accelerogram records with 384 components of earthquakes that originated in NE, India and its vicinity with special emphasis on 3 Jan 2016 M6.7, Tamenglong Earthquake. It is observed that the peak ground acceleration (PGA) is dependent on distance, site conditions, and site characteristics. The linear regression relations developed for GMPs with distance can be applied to characterize sites having three kinds of geology- Quaternary, Tertiary, and Precambrian. Attenuation relation curves of ground motion parameters (PGA, PGV, Ia, Ic, CAV, HI, EDA, ASI) of three different magnitudes (Mw=5.4, 5.5, and 5.8) are estimated for NS and EW components. The average response acceleration of horizontal components for all these sites is equivalent to 0.0485g for 5% damping with reference to the average period of 0.234s and for the vertical component is around 0.026g to the average period of 0.18s. January 3, 2016, Mw=6.7, Tamenglong earthquake, Manipur is one of the highest magnitudes in NER used in this study. Estimated Peak Ground Acceleration (PGA) at Itanagar, Jorhat, Kohima, Lekhapani, Mokokchung and Tadong sites are 0.0289g, 0.0145g, 0.053g, 0.00510g, 0.022g, 0.00159g respectively, while the Peak Ground Velocity (PGV) values are 0.0166m/s, 0.0097m/s, 0.050m/s, 0.0022m/s, 0.0195868m/s, 0.0009498m/s respectively. The PGA estimated for the event is found to be maximum at Kohima. Response spectra indicate that response acceleration mostly varies with periods 0.2s and 0.5s, and beyond 2s there is no effective response acceleration. The maximum effective response accelerations for all the sites are observed within the frequency range of 0.5 Hz to 5 Hz. Pseudo response acceleration (0.2802g at 1.72 Hz) along with estimated engineering parameters was observed to be higher in Kohima site situated on hard rock with a low epicentral distance of about 120 km. These relations help to forecast an average value for the GMPs estimated at a particular site at a certain distance from a source which is a prime input for seismic hazard analyses and the development of design ground motions.

An earthquake of moment magnitude Mw 5.7 shook the northeastern region of India on 3 January 2017 at 14 h:39 min:0.5 s local time. The duration of the tremor lasted for about 5–6 s and had its epicenter in Dhalai District, Tripura, India. Even though the earthquake was of moderate magnitude, it caused damage to several masonry dwellings in Tripura and triggered soil liquefaction, lateral spreading, and landslides near the epicentral area. The sand boils containing appreciable amount of silts were ejected to the ground surface at the Kanchanbari and Kumarghat area due to the liquefaction-induced upward ground water flow. This is possibly the first liquefaction evidence in India induced due to a moderate earthquake magnitude of Mw 5.7. This paper reports the field reconnaissance observations of geotechnical effects and damage to buildings following a shallow, strike-slip earthquake in northeast India on 3 January 2017. In addition, the distribution of surface peak ground acceleration of the earthquake estimated from the empirical equations based on the available data is evaluated and discussed.

The concept of seismic vulnerability is a yard-stick of damage estimation from a probable earthquake considering physical cum social dimension and enables a basis for decision-makers to develop pre...

International audience; Within the northern Indian Plate, the Shillong Plateau is a peculiar geodynamic terrane, hosting significant seismic activity outboard the Himalayan belt. This activity is often used as an argument to explain apparent reduced seismicity in the Bhutan Himalayas. Although current geophysical and geodetic data indicate that the Bhutan Himalayas accommodate more deformation than the Shillong Plateau, we aim to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfers. We compiled a map of major faults and earthquakes in the two regions and computed co-seismic stress transfer amplitudes. Our results indicate that the Bhutan Himalayas and the Shillong Plateau are less connected than previously suggested. Major earthquakes in either of the two regions mainly affect transverse faults connecting them, causing up to ~40 bar Coulomb stress change; however, this effect is clearly less on thrust faults of the either region (up to 1 bar only). The MW 8.25 1897 Assam earthquake that affected the Shillong Plateau did not cause a stress shadow on the Main Himalayan Thrust in Bhutan as previously suggested. Similarly, the Mw 8 ± 0.5 1714 Bhutan earthquake had negligible impact on stress accumulation on thrust faults bounding the Shillong Plateau. Furthermore, the main process shaping the regional stress patterns continues to be interseismic loading with complex boundary conditions in a diffuse deformation field involving the Bengal Basin and Indo-Burman Ranges. While both the Bhutan Himalayas and the Shillong Plateau exhibit a compressional regime, their stress evolutions are more weakly connected than hypothesized. Although our modelling suggests lateral increase in stress interactions, from west (less) to east (more), in the Bhutan Himalayas, a clearer picture will only emerge with better constrained fault geometries, slip rates, crustal structure, and seismicity catalogues in the entire region of distributed deformation.

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.

In the recent years there are several reporting’s of anomalous seismic induced temporal changes in soil radon emanation. It is however well known that radon anomalies apart from seismic activity are also governed and controlled by meteorological parameters. This is the major complication which arise for isolating the seismic induced precursory signals. Here in the investigation the soil radon emanations temporal variability at MPGO, Tezpur, is scrutinized in the lime light of singular spectrum analysis (SSA). Further prior applying SSA Digital filter (Butterworth low pass) is applied to remove the high frequency quasi periodic component in the time series of soil radon emanation. It was scrutinized that sum of just 9 eigenfunctions were sufficient enough for reproducing the prominent characteristics of the overall variation. This perhaps also evinces that more significantly produced fluctuations are mostly free from natural variations. The variations in soil temperature was observed to be dominated by daily variations similar to radon variation which account to 97.99 % whereas soil pressure accounts for 100 % of the total variance which suggests that daily variations of soil radon (Rn-222) emanation are controlled by soil pressure in MPGO, Tezpur during the investigation period followed by soil temperature. The study concludes that SSA eliminates diurnal and semidiurnal components from time series of soil radon emanation for better correlation of soil radon emanation with earthquakes.

The Tectonic stress regime of Gulf of Guinea region has been studied by stress tensor inversion analysis for the area bounded by lati− tude −10.0 0 S to 4.0oN and longitude −25 0 W to −11.0 0 E. A total of one hundred and four focal mechanism solutions, pertaining to the earthquake events which have occurred in this region, were used for this study. In order to decipher the stress pattern of this region, we have divided the region into four fracture zones namely Romanche, Chain−Romanche, Charcot and Ascension fracture−zones based on seismicity clustering, tectonics and available focal mechanism solutions. The seismicity pattern indicates that none of the nearby coun− tries on the border line between the west Africa region and the gulf of Guinea is devoid of seismicity. Simultaneously, the stress tensor inversion in the four subzones of investigation indicates different types of stress orientations. All of these zones are characterized by vary− ing principal axial directions. The orientation of the principal axial direction along Romanche, Chain−Romanche, Charcot and Ascen− sion fracture−zones are along NE−SW, NE−SW, ENE−WSW and ESE−WNW respectively. The stress tensor inversion results indicate that Chain−Romanche, Charcot and Ascension fracture zones are characterized by extensional stress regime while Romanche fracture zone is characterized by strike−slip stress regime respectively. These patterns imply sea floor spreading activities in and around the mid oceanic ridges and the general orientation of the extensional stress regime is found towards the continents, along the line of migration / progression of earthquakes.

Mechanical deformations from within the earthquake preparation zones are believed to cause seismo-electromagnetic (SEM) emission in ultra-low frequency (ULF) band, i.e. between 0.001 and 10 Hz, and these are studied as a precursor for short range earthquake forecast. Data from highly noise-resilient, 3-component ULF induction coil magnetometers, installed at the Multi-parametric Geophysical Observatory (MPGO), Tezpur, which is situated at a very close proximity with the northern end of the Kopili Fault and the Bomdila Fault, the Main Boundary Thrust of the Eastern Himalaya, the Naga and Disang Thrust, the Assam Syntaxis Zone as well as the two tectonically active Precambrian shields – Shillong and Mikir, was used for the first time to study SEM emissions employing both polarization ratio analysis and fractal analysis in the background of an intense phase of seismicity during the campaign period of April 20 – September 3, 2019. Fifty-one events were filtered via strain radius and index of seismicity calculations to nominate credible events which were studied vis-a-vis polarization ratio and fractal dimension parameters of 3-h night time data, centered around local midnight contemporaneous with the 18–21 UT window of the KP index (global geomagnetic activity index) in the 0.03–0.1 Hz frequency band. The findings show candidate SEM emissions, in the form of enhancements in SZ/SH, associated with all the seven credible events, even as nine enhancements could not be attributed to immediately adjacent credible events. The fractal dimensions, calculated with the spectral density method, ambiguously depicted a gradual increase prior to the intense phase of seismicity.

We made an attempt to assess the shear wave velocity values V S and, to a lesser extent, the V P values from ambient noise recordings in an array configuration. Five array sites were situated in the close proximity to borehole sites. Shear wave velocity profiles were modeled at these five array sites with the aid of two computational techniques, viz. spatial autocorrelation (SPAC) and H/V ellipticity. Out of these five array sites, velocity estimates could be reliably inferred at three locations. The shear wave velocities estimated by these methods are found to be quite consistent with each other. The computed V S values up to 30 m depth are in the range from 275 to 375 m/s in most of the sites, which implies prevalence of a low velocity zone at some pocket areas. The results were corroborated by evidence of site geology as well as geotechnical information.

Dried, mature leaves of Aegle Marmelos tree were converted to a powder, which was used as a biosorbent for dyes in water with methylene blue as a case study. The biosorbent had a surface area of 52.63 mg/g, and FTIR spectra showed the presence of –COOH, –NH2, –R–SC=O (thioester) and R1–S(=O, =O)-N(–R2, –R3) groups on the surface. The particles were found to be porous in nature from scanning electron micrographs, and EDX measurements showed the elements C, O, Na, Mg, K, Ca and Fe on the surface. Batch adsorption experiments showed that the adsorption of the dye was preferred at near-neutral conditions. Adsorption equilibrium was achieved in ~120 min with maximum dye uptake of 19.9 mg/g. Investigation into the kinetics of adsorption indicated that second-order kinetics gave the best fit to the experimental data, and a rate coefficient of 8.0 × 10−2 to 32.3 × 10−2 g mg−1 min−1 was obtained.

Tectonic stress regime in the Shillong plateau, northeast region of India, is examined by stress tensor inversion. Some 97 reliable fault plane solutions are used for stress inversion by the Michael and Gauss methods. Although an overall NNW-SSE compressional stress is observed in the area, the stress regime varies from western part to eastern part of the plateau. The eastern part of the plateau is dominated by NNE-SSW compression and the western part by NNW-SSE compression. The NNW-SSE compression in the western part may be due to the tectonic loading induced by the Himalayan orogeny in the north, and the NNE-SSW compression in the eastern part may be attributed to the influence of oblique convergence of the Indian plate beneath the Indo-Burma ranges. Further, Gravitational Potential Energy (GPE) derived stress also indicates a variation from west to east.

he Mikir Hills plateau is encompassed by two prominent faults – Bomdila Fault to the east and the Kopili Fault to the west characterized by strike-slip kinematics. The Kopili Fault has a dip of 75 o towards NE. Simultaneously, the Bomdila Fault dips with 50-55 o towards the NNE. An integrated approach based on seismotectonics, gravity and magnetic data is utilized to understand the tectonic activity of the Kopili and Bomdila Faults. The bottom of seismogenic zones is inferred to be 45 + 2 km and 50 + 2 km for the Kopili Fault and the Bomdila Fault region respectively. So far gravity anomaly is concerned; it varies between -110 to +60 mgals along the Kopili –Bomdila Fault regions from the Belt of Schuppen to the MCT. The low gravity values over the Bomdila Fault area indicate presence of thick alluvial deposits while along Kopili Fault lesser sediment thickness is observed. Simultaneously, basement being at shallower depth, lower magnetic values indicate presence of thick alluvial deposits in and around Bomdila Fault. The curvatures and closures of the gravity contours along the fault lines indicate structures involving basement and indicate influence of Bomdila Fault up to the basement. Simultaneously, it is observed that Kopili and Bomdila faults are neotectonically active. All these are the prime input to the seismic hazard assessment of the region.

Modified form of Nakamura method,H/Vratio, 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 fromH/Vratio 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 andVS. With the help of this, the probableVSvalues 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.

Although the M=w8.7, 1950 Assam earthquake endures as the largest continental earthquake ever recorded, its exact source and mechanism remain contentious. In this paper, we jointly analyze the spatial distributions of reappraised aftershocks and landslides, and provide new field evidence for its hitherto unknown surface rupture extent along the Mishmi and Abor Hills. Within both mountain fronts, relocated aftershocks and fresh landslide scars spread over an area of ≈330 km by 100 km. The former are more abundant in the Abor Hills while the later mostly affect the front of the Mishmi Hills. We found steep seismic scarps cutting across fluvial deposits and bounding recently uplifted terraces, some of which less than two thousand years or even a couple centuries old, at several sites along both mountain fronts. They likely attest to a minimum 200 km-long 1950 surface rupture on both the Mishmi and Main Himalayan Frontal Thrusts (MT and MFT, respectively), crossing the East Himalayan Syntaxis. At two key sites (Wakro and Pasighat), co-seismic surface throw appears to have been over twice as large on the MT as on the MFT (7.6 ± 0.2 m vs. >2.6 ± 0.1 m), in keeping with the relative, average mountain heights (3200 m vs. 1400 m), mapped landslide scar numbers (182 vs. 96), and average thrust dips (25–28° vs. 13–15°) consistent with relocated aftershocks depths. Corresponding average slip amounts at depth would have been ≈17 and ≈11 m on the MT and MFT, respectively, while surface slip at Wakro might have reached ≈34 m. Note that this amount of superficial slip would be out of reach using classic paleo-seismological trenching to reconstruct paleo-earthquake history. Most of the 1950 first arrivals fit with a composite focal mechanism co-involving the two shallow-dipping thrust planes. Their intersection lies roughly beneath the Dibang Valley, implying forced slip parallel to GPS vectors across the East Himalayan Syntaxis. Successive, near-identical, terrace uplifts at Wakro suggest near-characteristic slip during the last two surface rupturing earthquakes, while terrace boulder ages may be taken to imply bi-millennial return time for 1950-size events. As in Nepal, East-Himalayan mega-quakes are not blind and release most of the elastic, interseismic shortening that accumulates across the range. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38 km thick crust beneath the Shillong Plateau which increases to ∼37–38 km beneath the Brahmaputra valley and ∼46–48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8 km along with a low velocity zone within ∼8–13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18 km and low velocity zone within ∼18–36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

Attempts have been made to investigate the relationships between various earthquake magnitude scales for the region of the Gulf of Guinea, namely moment magnitude (Mw), surface wave magnitude (Ms), body wave magnitude (mb) and local magnitude (ML). The study involves a sum of 535 earthquake data from the period of 1918 to 2016. It was found that M s − M w ; M w − M L and m b − M w scales are in fairly good agreement while m b − M s magnitude scale differs by a magnitude unit of 1.45. Since the region of study is seismically active and mainly under the ocean, comprising of the mid-Atlantic ridges and the plate boundary, the seismo-tectonics of the study region is inferred from the obtained Gutenberg-Richter (GR) recurrence parameters. The inferences made are the prime input to the seismic hazard assessment of the region.

Seismic data for 100 years (1918–2018) were analysed for probabilistic analysis in the forecast of probable future earthquakes above M w ≥ 5.0 in North East India (20°–30°N and 86°–98°E) and its vicinity. The best distribution for seismic data allows probabilistic analysis to ascertain mean occurrence period E(t) for earthquakes of M w ≥ 5.0. Here, Kolmogorov–Smirnov statistics has been utilized constrained by Weibull distribution to achieve the best fit on the dataset. E(t) is found to be 74 days approximately with 50% probability. Similarly, cumulative probability function indicates a time period of 140 days with 80% probability, while 400–500 days of recurrence time period is embedded with 90–100% probability for an earthquake of M w ≥ 5.0 to recur following the occurrence of the last earthquake.

Strong ground motion parameters for the Guwahati city area, the capital city of the state of Assam in northeast India, are examined with the help of data accrued from local as well as worldwide network. Empirical relations are proposed for the ground motion parameters as a function of earthquake magnitude, distance, fault type, source depth and velocity characteristics of medium. Seismotectonics of the study region is examined, and a maximum credible earthquake M S ~ 8.0 is presumed from the Brahmaputra fault, the nearest source zone in the city area. Such great/major event may cause intensity of the of 9.3 with a probability of 0,95 in the Guwahati city during time interval of 500 years. Further, the design spectrum with 67 % confidence level and the synthetic three-component accelerograms are constructed. These results are much relevant and useful for structural engineering to mitigate seismic hazards in the region.

Some 455 events (mb ⩾ 4.5) in the Indo-Myanmar subduction zone are compiled using the ISC/EHB/NEIC catalogues (1964–2011) for a systematic study of seismic precursors, b-value and swarm activity. Temporal variation of b-value is studied using the maximum likelihood method beside CUSUM algorithm. The b-values vary from 0.95 to 1.4 for the deeper (depth ⩾60 km) earthquakes, and from 0.85 to 1.3 for the shallower (depth

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.

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.

We estimate the source parameters (seismic moment, source radius, stress drop, and source displacement) and scaling laws for local earthquakes that occurred in the Shillong–Mikir plateau, Assam Valley, and Arunachal Himalaya in northeast India during 2001–2008. The source parameters were determined using the spectral analysis of P waves from the vertical component seismograms, after correction for attenuation. Seismic moments are observed within the range from 9.51×10 12 to ; stress drop ranges from 4×10 5 to 9×10 7 Pa for the Brune model and 7×10 5 to 1×10 8 Pa for the Madariaga model. Seismic events in this study are prominent with an average stress drop of 0.1–10 MPa. The effect of site geology may be a contributing factor for such a variation in stress drop. Source dimensions are, however, found to be smaller within the major part of the plateau. It is suggested that local earthquakes in the region are associated with a brittle shear‐failure mechanism on fault segments and/or the presence of weakened zones, and earthquakes are triggered by low deviatoric stress. Empirical relations between M w – M L and M 0 – M L are developed leading to the future prediction of calibration coefficients for the local earthquakes in the Shillong–Mikir plateau and its adjoining region. Online Material: Tables of source parameters.

Microearthquake spectra from the Shillong region are analyzed to observe the effect of attenuation and site on these spectra. The spectral ratio method is utilized to estimate the Q values for both P- and S-waves in the subsurface layer, wherein the ratio of spectral amplitudes at lower and higher frequencies are taken into consideration for three stations at varying epicentral distances. Average estimates of Q P and Q S are 178 and 195. The ratio of Q S to Q P is estimated to be greater than 1 in major parts of the Shillong area, which can be related to the dry crust prevailing in the Shillong region. Typically, the variation in corner frequencies for these spectra is inferred to be characteristic of the site. Simultaneously, observations from spectral content of local earthquakes recorded at two different stations with respect to the reference site yield greater amplification of incoming seismic signals in the frequency range of 2–5 Hz, which is found to be well supported by the existing local lithology pertinent to that region.

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. Conversio...

In this study, an attempt is made to estimate the depth of mid-crustal discontinuity beneath the Shillong Plateau in northeast India region using broadband seismogram of local earthquakes. Principle of the technique is to relate the seismic travel times of the reflected phases (SxS) with the crustal thickness above the discontinuity. Though mid-crustal discontinuity (or Conrad discontinuity) is reported in some parts of the world, no such study was undertaken in the present study area due to complexity in analogue seismograms recorded before 2001. The digital waveforms of the local seismic events recorded by broadband digital network in the study area, however, make it possible precise detection of the seismic phases that are reflected at this discontinuity. The results show that the mid-crustal discontinuity exists at a depth 18±0.5 km beneath the Shillong Plateau, which provide a better understanding of the crustal velocity structure of the region for future study.

An attempt has been made to examine an empirical relationship between moment magnitude (M W) and local magnitude (M L) for the earthquakes in the northeast Indian region. Some 364 earthquakes that were recorded during 1950–2009 are used in this study. Focal mechanism solutions of these earthquakes include 189 Harvard-CMT solutions (M W ≥ 4.0) for the period 1976–2009, 61 published solutions and 114 solutions obtained for the local earthquakes (2.0 ≤ M L ≤ 5.0) recorded by a 27-station permanent broadband network during 2001–2009 in the region. The M W–M L relationships in seven selected zones of the region are determined by linear regression analysis. A significant variation in the M W–M L relationship and its zone specific dependence are reported here. It is found that M W is equivalent to M L with an average uncertainty of about 0.13 magnitude units. A single relationship is, however, not adequate to scale the entire northeast Indian region because of heterogeneous geologic and geotectonic environments where earthquakes occur due to collisions, subduction and complex intra-plate tectonics.

The tectonic model of the Shillong plateau and Assam valley in the northeast India region, the source area for the 1897 great earthquake (Ms ~ 8.7) and for the four (1869, 1923, 1930 and 1943) large earthquakes (M. ≥ 7.0), is examined using the high precision data of a 20-station broadband seismic network. About 300 selected earthquakes M ≥ 3.0 recorded during 2001–2009 are analysed to study the seismicity and fault plane solutions. The dominating thrust/reverse faulting earthquakes in the western plateau may be explained by the proposed pop-up tectonics between two active boundary faults, the Oldham–Brahmaputra fault to the north and the Dapsi–Dauki thrust to the south, though the northern boundary fault is debated. The more intense normal and strike-slip faulting earthquakes in the eastern plateau (Mikir massif) and in the Assam valley, on the other hand, are well explained by transverse tectonics at the long and deep rooted Kopili fault that cuts across the Himalaya and caused the 2009 Bhutan earthquake (Mw 6.3). It is conjectured that the complex tectonics of the Shillong plateau and transverse tectonics at the Kopili fault make the region vulnerable for impending large earthquake(s).

In this study we have tried to detect and collect later phases associated with Moho discontinuity and used them to study the lateral variations of the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuzawa, T., Hasegawa, A. 2002. Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Physics of the Earth and Planetary Interiors, 130, 31–47), having epicentral distance ranging from 60 km to 150 km. Taking the advantage of high quality broadband data now available in northeast India, we have detected 1607 Moho reflected phases (PmP and SmS) from 300 numbers of shallow earthquake events (depth ⩽ 25 km) in Shillong–Mikir Hills Plateau and its adjoining region. Notably for PmP phase, this could be identified within 0.5–2.3 s after the first P-arrival. In case of SmS phase, the arrival times are observed within 1.0–4.2 s after the first S-arrival. We estimated the crustal thickness in the study area using travel time difference between the later phases (PmP and SmS) and the first P and S arrivals. The results shows that the Moho is thinner beneath the Shillong Plateau about 35–38 km and is the deepest beneath the Brahmaputra valley to the north about 39–41 km, deeper by 4–5 km compared to the Shillong Plateau with simultaneous observation of thinnest crust (∼33 km) in the western part of the Shillong Plateau in the Garo Hills region.

Ground motion parameters for Shillong–Mikir Plateau of 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. Correlation between ground motion parameters and characteristics of seismogenic zones are established. Simultaneously, new empirical relations are derived for attenuation of ground motion amplitudes. Correlation coefficients of the attenuation relations depend on the site classifications that are identified based on average shear wave velocity and site response factors. The attenuation relation estimated for logarithmic width of Mikir Plateau found to be a little bit higher than that of Shillong Plateau both for soft and hard ground which accounts for geometrical spreading and anelastic attenuation. Simultaneously, validation are made studying the seismic microzonation process related to geomorphological, geological subsurface features for thickly populated Guwahati city of India under threat from scenario earthquake.

We compile a data set of 585 weak, broadband, and strong ground-motion recordings from 184 earthquakes covering hypocentral distances ranging from 9 to 436 km in northeast India and the northwest Himalayas. Source-normalized vertical-component Fourier amplitude S -wave spectra are used to study path-attenuation characteristics, namely, geometrical spreading and quality factor, in the two regions. A trilinear model of geometrical spreading of R -1.0 for hypocentral distance R R 0.0 for R between 125 km and 140 km, and R -1.0 for R >140 km approximates the decay of low-frequency (1.0 Hz) amplitudes in northeastern India better than the conventional theoretical bilinear geometrical spreading of R -1.0 for R R -0.5 otherwise. In the northwestern Himalayas, the geometrical spreading is found to be R -1.0 , which is the same as that of a bilinear model at hypocentral distances R f , given by in the frequency range of 0.9–20 Hz.

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.

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.

In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18–20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.

SUMMARY In Northeast India, three major plates interact along two convergent boundaries: the Himalayas and the Indo–Burma Ranges, which meet at the Assam Syntaxis. To clarify this tectonic interaction and the underlying dynamics, we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5. We then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements. North–south compression, in a direction consistent with India–Eurasia convergence, prevails in the whole area from the Eastern Himalayas to the Bengal Basin, through the Shillong–Mikir Massif and the Upper Assam Valley. E–W extension in Tibet is related to this N–S India– Eurasia convergence. Not only does the major N–S compression affect the outer segments of the Indo–Burma Ranges, it also extends into the descending slab of Indian lithosphere below these ranges, although stresses at depth are controlled by bending of the slab beneath the Burmese arc. The existence of widespread N–S compression in the Bengal Basin, far away from the Himalayan front, is compatible with the previously proposed convergence between a Shillong– Mikir–Assam Valley block and the Indian craton. E–W compression inside this block supports the hypothesis of a component of eastward extrusion. Stress inversion of focal mechanism solutions in the Indo–Burma Ranges reveals a complex stress pattern. The Burmese arc and its underlying lithosphere experience nearly arcperpendicular extension with ESE–WNW trends in the northernmost, NE-trending segment and ENE–WSW trends in the main N–S arc segment. Such extensional stress, documented from many arcs, is likely a response to pull from and bending of the subducting plate. At the same time, the Indo–Burma Ranges are under compression as a result of oblique convergence between the Sunda and Indian plates. The maximum compressive stress rotates from NE–SW across the inner and northern arc to E–W near the Bengal Basin. This rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors, from a SSW-directed Sunda–Burma motion to a WSW-directed Burma–India motion. As a consequence of this partitioning, the major belt-parallel fault zones show a variety of movements across the main N–S arc segment, from right-lateral slip in the inner ranges to oblique reverse-dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt.

Strong ground motion parameters for Shillong plateau of northeastern India are examined. Empirical relations are obtained for main parameters of ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. Correlation between ground motion parameters and characteristics of seismogenic zones are established. A new attenuation relation for peak ground acceleration is developed, which predicts higher expected PGA in the region. Parameters of strong motions, particularly the predominant periods and duration of vibrations, depend on the morphology of the studied area. The study measures low estimates of logarithmic width in Shillong plateau. The attenuation relation estimated for pulse width critically indicates increased pulse width dependence on the logarithmic distance which accounts for geometrical spreading and anelastic attenuation.

Coda wave attenuation quality factor Qc is estimated in the northeastern region of India using 45 local earthquakes recorded by regional seismic network. The quality factor Qc was estimated using the single backscattering model modified by Sato (J Phys Earth 25:27–41, 1977), in the frequency range 1–18 Hz. The attenuation and frequency dependence for different paths and the correlation of the results with geotectonics of the region are described in this paper. A total of 3,890 Qc measurements covering 187 varying paths are made for different lapse time window of 20, 30, 40, 50, 60, 70, 80, and 90 s in coda wave. The magnitudes of the analyzed events range from 1.2 to 3.9 and focal depths range between 7 and 38 km. The source–receiver distances of the selected events range between 16 and 270 km. For 30-s duration, the mean values of the estimated Qc vary from 50 ± 12 (at 1 Hz) to 2,078 ± 211(at 18 Hz) for the Arunachal Himalaya, 49 ± 14 (at 1 Hz) to 2,466 ± 197 (at 18 Hz) for the Indo-Burman, and 45 ± 13 (at 1 Hz) to 2,069 ± 198 (at 18 Hz) for Shillong group of earthquakes. It is observed that Qc increases with frequency portraying an average attenuation relation \(Qc=52.315\pm 1.07f\,^{\left( {1.32\,\pm \,0.036} \right)}\) for the region. Moreover, the pattern of Qc − 1 with frequency is analogous to the estimates obtained in other tectonic areas in the world, except with the observation that the Qc − 1 is much higher at 1 Hz for the northeastern region. The Qc − 1 is about 10 − 1.8 at 1 Hz and decreases to about 10 − 3.6 at 18 Hz indicating clear frequency dependence. Pertaining to the spatial distribution of Qc values, Mikir Hills and western part of Shillong Plateau are characterized by lower attenuation.

Microearthquake spectra from Jubilee Hills, Hyderabad are analyzed to observe the effect of attenuation and site on these spectra. The ratios of spectral amplitudes at lower and higher frequency 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. Average estimates of Qp and Qs are 235 and 278 respectively. Value of Qs/Qp larger than 1.0 suggests dry crust for most of the Jubilee Hills region. The near-surface low Qp and Qs for 0 km to 0.9 km depth coincide with the soil layer, top and semi-weathered and highly fractured zone. In contrast, at a shallower depth beneath the Jubilee Hills area, Hyderabad, we obtain high Qp and Qs zone, which corresponds to the dense and high velocity rocks of the region. The varying corner frequencies for these spectra are inferred to be characteristics of site. Comparisons of disparity in spectral content with reference to hard rock site conclude that lithology of the northwest part of Jubilee Hills area amplify about twice the incoming seismic signal, as compared to the southern part best outlined at 8 to 10 Hz only.