Assam Earthquake Research Articles

The Great Assam Earthquake of 1950: A Historical Review

Northeast India is prone to major earthquake events due to its geographical and tectonic settings. Recurrence of the earthquakes in this part of India is frequent and associated with devastating events. Earthquake scenario in the region is very dangerous as the unplanned settlements with rise in population increased the vulnerability of human life. To understand such a risk and variability in human life and changes in natural dynamics; it is essential to know and study the past earthquake events. In northeast India, the last most devastating earthquake was the great Assam earthquake of 15th August 1950. This event brought different types of changes in the physiography which, results in havoc among the people. The changes in the river course, landslides, liquefaction and river bank changes are most influential factors in both human and natural arrangements. This brought devastation in human lives that resulted into several unsettled socio-economic issues. After the earthquake, Government and people tried together to overcome such trauma, although lack of preparedness caused some problems. This is the time to review all the scenario of major earthquakes in northeast India and realise that the necessity of preparedness is very important. The paper attempts to review the impact of the earthquake on different spheres: geographical, social and economic.

3D Crustal Velocity Model for Ground Motion Simulations in North-East India

ABSTRACT The present study focuses on developing a 3D crustal velocity model and applying it to perform ground motion simulations for North East India. The study region encompasses area between 89°E to 97°E longitude and 22°N to 30°N latitude. The calibration of the material property is based on 48 shear wave profiles available for the region along with the geotectonic features reported in the literature. The 3D material model arrived from the study is implemented in a finite element framework for ground motion simulations. The developed model is validated using the strong motion data available for four events; 1988 Mw 7.2 India–Bangladesh earthquake, 2011 Mw 6.3 India–Myanmar earthquake, 2013 Mw 5.5 Assam earthquake and Mw 5.2 Bhutan earthquake. The simulations are able to capture the prominent features of the recorded data up to 2Hz. Hence, the developed model can be implemented for estimating ground motions (< 2Hz) in the north eastern region of India. The simulated results can be used to estimate region-specific hazard and the displacement-based design of structures.

A Study on Variation in Channel Width and Braiding Intensity of the Brahmaputra River in Assam, India

The Brahmaputra River flows through Assam, India, for about 670 km along an alluvial valley as a wide braided river. The width of the river varies with time along its course. The braiding intensity of this river is estimated using the braiding index (BI) of Brice (1964), which also changes with space and time along the course of the river. Temporal changes of both width and BI have been studied using topographic maps of 1912–1928 and 1963–1975, and dry season satellite data of 1996, 2000, 2007 and 2009. The mean widths of the Brahmaputra River channel in Assam during 1912–1928, 1963–1975, 1996, 2000, 2007 and 2009 were 5949 m, 7455 m, 7505 m, 8008 m, 8308 m and 9012 m, respectively, confirming an overall increase in width with time. Both the width and variation of width are lowest in four short narrower segments of the river. Three of these segments represent hard points comprising gneissic rock, and one segment is on alluvium comprising cohesive clay. The increase in width is correlated to enormous sediment load produced by the great Assam earthquake of 1950 and large-scale deforestation in the Himalayas. The mean BIs for the Brahmaputra for 1963–1975, 1996, 2000, 2007 and 2009 were 8.59, 8.43, 6.67, 6.58 and 7.70, respectively, indicating in general a decreasing trend up to 2007. The BI showed low variation at the four narrow segments where there is also a minimum variation of the channel width. The BI has increased significantly in the upstream part of the river. Very high fluctuation of discharge (17,000 m 3 / s − 1 in 24 h) and high sediment loads of the Brahmaputra (daily mean sediment discharge of 2.0 million tonnes during monsoon), erodible alluvial banks and high width/depth ratios are the main causes of development of braiding. The interrelationship between channel width and BI of the Brahmaputra shows a positive correlation, indicating an increase in BI with increasing channel width.

Geodetic imaging mega-thrust coupling beneath the Himalaya

The 2015 Gorkha earthquake highlights the urgent need for understanding seismicity in the Himalaya, which is envisaged to be governed by stick-slip behavior on the Main Himalayan Thrust (MHT) beneath the southern edge of the Tibetan Plateau. We integrate GPS data in southern Tibet with those public available for geodetic imaging of the frictional properties of the seismogenic fault, aiming at a better understanding of the strain buildup on the largest continental mega-thrust and its relation to great earthquakes. The postseismic effects due to viscoelastic relaxation from three large earthquakes (1934 Bihar Mw 8.4, 1950 Assam Mw 8.5 and 2005 Kashmir Mw 7.6) are corrected in the coupling modeling. The new model, constrained by all GPS velocities, spirit leveling rates and InSAR interferograms spread between the eastern and western Himalayan syntaxes confirms previous findings that the MHT is fully locked in the uppermost 15–20 km of crust, below which an abrupt decrease in coupling occurs with coefficients reduced from 0.8 to 0.2 within a narrow transition zone of 30–50 km in width. Generally, the coupling pattern is modified little after removing the postseismic contributions of the 1934 Bihar and 2005 Kashmir earthquakes from GPS velocities, meanwhile, the high-coupling region around the Eastern Himalayan Syntaxis is widened from 100 km to >140 km after removing the perturbation induced by the 1950 Assam earthquake. The spatial variation of fault coupling coincides well with the epicentral areas of historical earthquakes. In the Sikkim and Bhutan Himalaya, the locking zone is wider than elsewhere along the Himalaya. The strain buildup for the past 300 years under the Bhutan Himalaya would be sufficient to power a maximum earthquake of Mw ~8.6.

Stress transfer and connectivity between the Bhutan Himalaya and the Shillong Plateau

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.

Active tectonics in the Assam seismic gap between the meizoseismal zone of AD 1934 and 1950 earthquakes along eastern Himalayan front, India

The Assam Seismic Gap has witnessed a long seismic quiescence since the \({ Mw}{\sim }8.4\) great Assam earthquake of AD 1950. Owing to its improper connectivity over the last decades, this segment of the Himalaya has long remained inadequately explored by geoscientists. Recent geodetic measurements in the eastern Himalaya using GPS document a discrepancy between the geologic and geodetic convergence rates. West to east increase in convergence rate added with shorter time span earthquakes like the 1697 Sadiya, 1714 (\({ Mw}{\sim }8\)) Bhutan and 1950 (\({ Mw}{\sim } 8.4\)) Tibet–Assam, makes this discrepancy more composite and crucial in terms of seismic hazard assessment. To understand the scenario of palaeoearthquake surface rupturing and deformation of youngest landforms between the meizoseismal areas of \({ Mw}{\sim }8.1\) 1934 and 1950 earthquakes, the area between the Manas and Dhanshiri Rivers along the Himalayan Frontal Thrust (HFT) was traversed. The general deformation pattern reflects north-dipping thrust faults. However, back facing scarps were also observed in conjugation to the discontinuous scarps along the frontal thrust. Preliminary mapping along with the published literature suggests that, in the eastern Himalayan front the deformation is taking place largely by the thrust sheet translation without producing a prominent fault-related folds, unlike that of the central and western Himalayas.

Estimation of current seismic hazard using Nakamura technique for the Northeast India

Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, among them north-east region of India is most geologically complex one, which are found to be highly earthquake hazard prone region. This region has experienced several disastrous earthquakes like Assam earthquake of 1950 for 8.1 and Shillong earthquake of 1897 for 8.7 magnitude in the past. Several methods are used for seismic hazard measurement. Among these methods the site response analysis is found to be most valuable part of the seismic hazard assessment. In this work we have used Nakamura technique (H/V site response spectral ratio) for estimating the site amplification of seismic ground motion. Fundamental frequencies estimated are in the range 0.6–10 Hz for the 15 broadband stations of India Meteorological Department distributed over the study region. ZIRO station and its surrounding area are found to have high liquefaction indices (kg) value of 250. The horizontal-to-vertical ratio curve variation of higher amplitude with lower frequency helped to find thick soil zones in Aizawl and Agartala. Similar analysis also gave flat response at some sites near Jorhat and Dibrugarh having thin soil. The H/V rotate analysis helped us to see the variation of fundamental frequency with azimuth. Based on the observations, frequency and amplitude counter map of the north-east region represented. Further considering the site amplification due to very high fundamental frequency for Shillong, Guwahati, Imphal and Itanagar city can be considered under high seismic hazard scanner in the north-east India region.

Liquefaction potential assessment of Guwahati city using first-order second-moment method

Soil liquefaction is the failure of the soil due to the sudden increase in a pore water pressure causing the effective stress to reduce significantly thus losing shear strength, with the resulting effect causing the fluid type behavior of the soil. Guwahati city, the capital of Assam, lies in the northeastern region of India and this whole region is considered seismically very active (seismically sixth position globally). The whole region is categorized in Zone-V according to the seismic zoning map of India as per IS 1893:2002 (Part 1). This region witnessed/experienced several major and great earthquakes in the past such as 1897 Shillong and 1950 Assam earthquakes. The soil deposits of the city mainly consist of an alluvial type of Holocene age and the presence of shallow groundwater table makes it vulnerable to soil liquefaction. In this study, an assessment of soil liquefaction potential of Guwahati city is performed based on the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014) using standard penetration test data. The evaluation of liquefaction potential is carried out for 82 borehole sites considering the Great 1897 Shillong earthquake of Mw 8.1 with a peak ground acceleration of 0.36 g. In the deterministic approach, the various parameters are involved in the evaluation of liquefaction potential, and the uncertainties of the input parameters as well as model cause the dissimilarity of the result. For instance, the same input parameters for both the models show different factor of safety. Hence, a comprehensive probability approach considering the uncertainty of parameters is essential for the evaluation of liquefaction susceptibility. In this study, reliability analysis for both the models based on first- order second-moment method has been used. The Reliability Index based on input parameters such as cyclic resistance ratio and cyclic stress ratio computed for both the models and subsequent liquefaction probability are established. The result shows the city is in most vulnerable condition even up to 15 m depth considering both the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014). Therefore, extra measure should be taken while constructing structure in and around the study area.

Implications for elastic energy storage in the Himalaya from the Gorkha 2015 earthquake and other incomplete ruptures of the Main Himalayan Thrust

Rupture in the 2015 M7.8 Gorkha earthquake nucleated at the downdip edge of the Main Himalayan Thrust (MHT) near the transition from interseismic locking to aseismic creep beneath the Tibetan plateau, and propagated incompletely towards the Main Frontal Thrusts (MFT). Despite the imposition of a substantial static strain in the mid-décollement, afterslip on the MHT within a year of the earthquake had decayed to negligible levels. Earthquakes that incompletely rupture the MHT (7 < Mw < 7.9) have been relatively common in the past two centuries, and as a consequence heterogeneous patches of stored elastic strain must exist throughout the Himalaya similar to that emplaced by the Gorkha earthquake. We show that these patches of stored strain are not dissipated by creep or by subsequent updip earthquakes, with the possible exceptions of a sequence of moderate earthquakes to the west of the great 1950 Assam earthquake, and to the east of the Kangra 1905 earthquake. It is thus considered likely that mid-décollement strain newly imposed by the Gorkha earthquake, and other recent incomplete ruptures will be incorporated in the rupture of a future much larger earthquake. Incomplete ruptures (i.e. those that nucleate downdip but fail to rupture the frontal thrusts) appear to occur preferentially in parts of the central Himalaya characterized by relatively narrow transition regions of interseismic decoupling (<30 km downdip). Assuming uniform strain at failure these narrow zones are unable to store large amounts of strain energy compared to wide zones of interseismic decoupling. Since the transition from fully locked to a fully creeping rheology depends partly on temperature, to first order the width of the interseismic decoupling transition zone depends on the local dip of the MHT. Where the decoupling zone is narrow (25 km) moderate earthquakes (6 < Mw < 7) are observed to occur at intervals of a few hundred years. Where the transition zone is wide (e.g. Kashmir and Assam, 150 km) great earthquakes nucleate at long time intervals (millennia). Because the cumulative moment release of moderate earthquakes in regions of narrow seismic decoupling is insufficient to keep up with plate convergence, we conclude that megaquakes that eventually sweep through these regions are augmented by the heterogenous fossil strain of former incomplete ruptures. Because great earthquakes in the central Himalaya are inferred to nucleate from moderate earthquakes near the base of the MHT, the preparation zones of these moderate earthquakes may provide opportunities for forecasting the approach of future great earthquakes.

Empirical Attenuation relationship for Peak Ground Horizontal Acceleration for North-East Himalaya

Empirical Attenuation relationship for Peak Ground Horizontal Acceleration for North-East Himalaya

Structural controls on topography and river morphodynamics in Upper Assam Valley, India

The basement in Upper Assam foreland basin exhibits a typical flexure pattern. An elongated continuous subsurface basement high has developed between Himalaya and Naga-Patkai ranges. A few prominent basement lows have developed adjacent to the foothill regions. It is noticed that the general topographic slope near the foothill regions is not only controlled by aggradation process, but also influenced by the flexured basement of the region. The basement lows have influenced the overlying topography significantly. Rivers flowing over those regions have shown unidirectional lateral migration. North bank tributaries like Subansiri, Jiadhal and Dikrang have been affected by the Subansiri basement low. Coseismic subsidence of sediments over Subansiri basement low had resulted subsidence of the North Lakhimpur-Ranga Nadi region in 1950 Assam earthquake. Some south bank tributaries like Disang and Dikhow have been affected by Nazira basement low. Topographic elevation along the Subansiri river is lower than that part of the Brahmaputra located south of Majuli. This typical topographic setting of the region makes the mouth of the Subansiri river and Majuli region highly susceptible to erosion.

Morphometric Analysis of Kakoi River Watershed for Study of Neotectonic Activity Using Geospatial Technology

Morphometric analysis is defined as the quantitative measurement of landscape shape. Morphometric studies of a watershed helps the researcher to compare different landform and calculate the geomorphic indices that may be useful for identifying a particular characteristic such as the level of neotectonic activity in a watershed. The present study has been carried out in Kakoi River Watershed which is a part of Lower Subansiri River Basin in Assam. The study area is geo-dynamically unstable region characterized by active faults, continuing crustal movements and complicated structural region which fall under the seismic zone V of India. In recent century, the Lower Suabsiri River Basin has gone tremendous morphological changes due to active tectonics activities. After the Assam Earthquake of 1950, some of the rivers like Subansiri and its tributaries Dirgha, Kadam and Kakoi suddenly changed their channels giving birth to new channels. The present study is to examine the neotectonic domains and the changes of river course. The geomorphic indices such as Hypsometric Integral (HI), Elongation ratio (Re), Asymmetry Factor (AF), Stream-length Gradient Index (SI), Mountain Front Sinuosity (Mfs), Basin shape index (Bs), Ratio of Valley floor width to Valley Height (Vf) and Channel sinuosity (S) etc. have been studied from remote sensing data. The results shows that the study area is under active tectonic area based on Vf (0.301), Rl (0.461), Bs (3.6), Sl (165.73) and other parameters such as S (1.38) and Mfs (2.02) indicated as moderate active tectonic region. Utilization of geospatial technology and remote sensing data in the present study becomes more reliable and helpful in analyzing, monitoring and understanding the landform changes in a watershed.

Probabilistic Seismic Hazard Assessment for Northeast India Region

Northeast India bounded by latitudes 20°–30°N and longitudes 87°–98°E is one of the most seismically active areas in the world. This region has experienced several moderate-to-large-sized earthquakes, including the 12 June, 1897 Shillong earthquake (M w 8.1) and the 15 August, 1950 Assam earthquake (M w 8.7) which caused loss of human lives and significant damages to buildings highlighting the importance of seismic hazard assessment for the region. Probabilistic seismic hazard assessment of the region has been carried out using a unified moment magnitude catalog prepared by an improved General Orthogonal Regression methodology (Geophys J Int, 190:1091–1096, 2012; Probabilistic seismic hazard assessment of Northeast India region, Ph.D. Thesis, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 2013) with events compiled from various databases (ISC, NEIC,GCMT, IMD) and other available catalogs. The study area has been subdivided into nine seismogenic source zones to account for local variation in tectonics and seismicity characteristics. The seismicity parameters are estimated for each of these source zones, which are input variables into seismic hazard estimation of a region. The seismic hazard analysis of the study region has been performed by dividing the area into grids of size 0.1° × 0.1°. Peak ground acceleration (PGA) and spectral acceleration (S a) values (for periods of 0.2 and 1 s) have been evaluated at bedrock level corresponding to probability of exceedance (PE) of 50, 20, 10, 2 and 0.5 % in 50 years. These exceedance values correspond to return periods of 100, 225, 475, 2475, and 10,000 years, respectively. The seismic hazard maps have been prepared at the bedrock level, and it is observed that the seismic hazard estimates show a significant local variation in contrast to the uniform hazard value suggested by the Indian standard seismic code [Indian standard, criteria for earthquake-resistant design of structures, fifth edition, Part-I. Bureau of Indian Standards, New Delhi, 2002]. Not only holistic treatment of earthquake catalog and seismogenic zones has been performed, but also higher resolution in spatial distribution could be achieved. The COV maps have been provided with the strong ground-motion maps under various conditions to show the confidence in the results obtained. Results obtained in the present study would be helpful for risk assessment and other disaster mitigation-related studies.

A new classification of earthquake-induced landslide event sizes based on seismotectonic, topographic, climatic and geologic factors

This paper reviews the classical and some particular factors contributing to earthquake-triggered landslide activity. This analysis should help predict more accurately landslide event sizes, both in terms of potential numbers and affected area. It also highlights that some occurrences, especially those very far from the hypocentre/activated fault, cannot be predicted by state-of-the-art methods. Particular attention will be paid to the effects of deep focal earthquakes in Central Asia and to other extremely distant landslide activations in other regions of the world (e.g. Saguenay earthquake 1988, Canada). The classification of seismically induced landslides and the related ‘event sizes’ is based on five main factors: ‘Intensity’, ‘Fault factor’, ‘Topographic energy’, ‘Climatic background conditions’, ‘Lithological factor’. Most of these data were extracted from papers, but topographic inputs were checked by analyzing the affected region in Google Earth. The combination and relative weight of the factors was tested through comparison with well documented events and complemented by our studies of earthquake-triggered landslides in Central Asia. The highest relative weight (6) was attributed to the ‘Fault factor’; the other factors all received a smaller relative weight (2–4). The high weight of the ‘Fault factor’ (based on the location in/outside the mountain range, the fault type and length) is strongly constrained by the importance of the Wenchuan earthquake that, for example, triggered far more landslides in 2008 than the Nepal earthquake in 2015: the main difference is that the fault activated by the Wenchuan earthquake created an extensive surface rupture within the Longmenshan Range marked by a very high topographic energy while the one activated by the Nepal earthquake ruptured the surface in the frontal part of the Himalayas where the slopes are less steep and high. Finally, the calibrated factor combination was applied to almost 100 other earthquake events for which some landslide information was available. This comparison revealed the ability of the classification to provide a reasonable estimate of the number of triggered landslides and of the size of the affected area. According to this prediction, the most severe earthquake-triggered landslide event of the last one hundred years would actually be the Wenchuan earthquake in 2008 followed by the 1950 Assam earthquake in India – considering that the dominating role of the Wenchuan earthquake data (including the availability of a complete landslide inventory) for the weighting of the factors strongly influences and may even bias this result. The strongest landslide impacts on human life in recent history were caused by the Haiyuan-Gansu earthquake in 1920 – ranked as third most severe event according to our classification: its size is due to a combination of high shaking intensity, an important ‘Fault factor’ and the extreme susceptibility of the regional loess cover to slope failure, while the surface morphology of the affected area is much smoother than the one affected by the Wenchuan 2008 or the Nepal 2015 earthquakes. The main goal of the classification of earthquake-triggered landslide events is to help improve total seismic hazard assessment over short and longer terms. Considering the general performance of the classification-prediction, it can be seen that the prediction either fits or overestimates the known/observed number of triggered landslides for a series of earthquakes, while it often underestimates the size of the affected area. For several events (especially the older ones), the overestimation of the number of landslides can be partly explained by the incompleteness of the published catalogues. The underestimation of the extension of the area, however, is real – as some particularities cannot be taken into account by such a general approach: notably, we used the same seismic intensity attenuation for all events, while attenuation laws are dependent on regional tectonic and geological conditions. In this regard, it is likely that the far-distant triggering of landslides, e.g., by the 1988 Saguenay earthquake (and the related extreme extension of affected area) is due to a very low attenuation of seismic energy within the North American plate. Far-distant triggering of landslides in Central Asia can be explained by the susceptibility of slopes covered by thick soft soils to failure under the effect of low-frequency shaking induced by distant earthquakes, especially by the deep focal earthquakes in the Pamir – Hindukush seismic region. Such deep focal and high magnitude (> > 7) earthquakes are also found in Europe, first of all in the Vrancea region (Romania). For this area as well as for the South Tien Shan we computed possible landslide event sizes related to some future earthquake scenarios.

Millenary Mw &gt; 9.0 earthquakes required by geodetic strain in the Himalaya

AbstractThe Himalayan arc produced the largest known continental earthquake, the Mw ≈ 8.7 Assam earthquake of 1950, but how frequently and where else in the Himalaya such large‐magnitude earthquakes occur is not known. Paleoseismic evidence for coseismic ruptures at the front of the Himalaya with 15 to 30 m of slip suggests even larger events in medieval times, but this inference is debated. Here we estimate the frequency and magnitude of the largest earthquake in the Himalaya needed so that the moment released by seismicity balances the deficit of moment derived from measurements of geodetic strain. Assuming one third of the moment buildup is released aseismically and the earthquakes roughly follow a Gutenberg‐Richter distribution, we find that Mw &gt; 9.0 events are needed with a confidence level of at least 60% and must return approximately once per 800 years on average.

Deterministic and probabilistic liquefaction potential evaluation of Guwahati city

Northeastern region of India is one of the six most seismically active regions of the world. Guwahati, the major city in the region is growing rapidly in every aspect. The 1897 great Assam earthquake and the 1950 Assam earthquake both of magnitude 8.7 are two of the many great earthquakes that had occurred in this region. There have been reports of extensive liquefaction occurring during these two earthquakes. These instances of liquefaction necessitate the need to evaluate liquefaction potential of the area. In the present study, an attempt has been made to predict the liquefaction susceptibility of Guwahati city based on corrected SPT values using deterministic as well as probabilistic performance based approach. The deterministic and probabilistic version of Toprak et al., Youd et.al, Idriss and Boulanger and Cetin, standard penetration test (SPT)- based liquefaction triggering correlations have been used. The results obtained in the probabilistic approach are then compared with their deterministic approach. Standard Penetration Test (SPT) N values, engineering properties of the soils and depth of water table were taken from a data base of 200 boreholes upto 30 meter depth covering an area of 262 km2 in Guwahati city. Liquefaction susceptibility from the methods is presented as maps showing zones of levels of risk of liquefaction. Comparison of the three methods in the deterministic approach have shown that there is a difference in the factors of safety in the same depth and comparison in the probabilistic approach have also shown a difference in the values of probabilities in the same depth. The SPT- based liquefaction evaluation procedures have been found to yield significantly different predictions. This study feels that the issues and uncertainties regarding the use of both the deterministic and probabilistic liquefaction triggering relationships in practice must be resolved so as to arrive at a common consensus to determine the same.

Probabilistic Models for Forecasting Earthquakes in the Northeast Region of India

Northeast India and its adjoining areas are characterized by very high seismic activity. According to the Indian seismic code, the region falls under seismic zone V, which represents the highest seismic-hazard level in the country. This region has experienced a number of great earthquakes, such as the Assam (1950) and Shillong (1897) earthquakes, that caused huge devastation in the entire northeast and adjacent areas by flooding, landslides, liquefaction, and damage to roads and buildings. In this study, an attempt has been made to find the probability of occurrence of a major earthquake (M-w > 6) in this region using an updated earthquake catalog collected from different sources. Thereafter, dividing the catalog into six different seismic regions based on different tectonic features and seismogenic factors, the probability of occurrences was estimated using three models: the lognormal, Weibull, and gamma distributions. We calculated the logarithmic probability of the likelihood function (ln L) for all six regions and the entire northeast for all three stochastic models. A higher value of ln L suggests a better model, and a lower value shows a worse model. The results show different model suits for different seismic zones, but the majority follows lognormal, which is better for forecasting magnitude size. According to the results, Weibull shows the highest conditional probabilities among the three models for small as well as large elapsed time T and time intervals t, whereas the lognormal model shows the lowest and the gamma model shows intermediate probabilities. Only for elapsed time T = 0, the lognormal model shows the highest conditional probabilities among the three models at a smaller time interval (t = 3-15 yrs). The opposite result is observed at larger time intervals (t = 15-25 yrs), which show the highest probabilities for the Weibull model. However, based on this study, the IndoBurma Range and Eastern Himalaya show a high probability of occurrence in the 5 yr period 2012-2017 with >90% probability.

The Shillong Plateau and the great 1897 Assam earthquake

AbstractPrevious analysis of triangulation data of the Survey of India concluded that the great 1897 Assam earthquake occurred on a south dipping fault near the northern edge of the Shillong Plateau, which was named the Oldham fault. This attribution has been questioned on geological and geodetic grounds. We refine the triangulation data, adding recently discovered observations, and demonstrate that they require average slip of 25 ± 5 m on a fault that dips south at ∼40° beneath the plateau. The best fitting solution to the geodetic observations gives a rupture length of 79 km. However, the Chedrang fault, immediately to the west of the Oldham fault, appears to have slipped as a subvertical tear fault during or shortly after the 1897 earthquake, with over 10 m of down‐to‐the‐west normal‐sense slip. This observation suggests that the western end of the main rupture approached within a few kilometers of the Chedrang fault, giving a length of 95 km for the rupture. This range of parameters gives a magnitude 8.15&lt;Mw&lt;8.35 for the earthquake. The triangulation data cannot be satisfied by slip on the north dipping faults that border the southern edge of the Shillong Plateau nor by slip on a south dipping fault that has been postulated in the Brahmaputra Valley. GPS velocities show that up to 5 mm/yr of shortening is taken up across the plateau and its borders; this suggests, via moment‐frequency relations, that the interval between great earthquakes in the region is several thousand years but that earthquakes of magnitude 7 or greater should occur roughly once per century.

Impact of Neotectonism in the Discussion of Geomorphological Processes as a Feedback System: North Bengal Foothills, West Bengal

The ongoing research paper is concerned about the sensitive issue like neotectonism and its impact on the landscape development. It is a detail and micro level study of North Bengal Himalayan foothills where the effect of tectonism is precisely observed. The term, neotectonism was introduced by Obruchen (1948) to summarize the active tectonic processes including the last major tectonic configuration change and establishment of modern stress. The North Bengal Himalayan foothill are very much concerned about the neotectonism activities at last to present geological periods. It is the one of the active unstable area of NE India where faults/thrusts and presence of synformal axis are predominantly controlling the geomorphological landscape development. This frontal region of an active orogenic beltlikethe NE Himalaya is characterized by complex terrace morphology with fluvial aggradation, degradationandstrath terraces. The detail study is mainly about the lineaments, drainage sequences, thrusting zone and synformal depressed zone of the area. Near the Neora-Mal river valley, close to to the mountain front a thick supported boulder bed with huge angular boulders of gneisses are exposed near Neora valley of Samsing area. Concerning the fault zone area of Matiali scarp, it is very clearly observed the extended part of Garubathan thrust (MBT) under the Matiali formation near the upper part of Kurtiriver. But, in the middle part of the river, where the synformation axis is present, river terraces are magnificiently developed over two sides of R. Kurti near Matiali hat. Here, two terraces are well virtualized but the upper most flat terrain is designated as fan surface which is made of mainly by the sediments of Matialiformation. In this region, slope of the fan surface is southerly 2.1˚. In south of the Matiali scarp on the banks of R. Neora, Kurti, Murti raised terraces are distinctly observed as T1 and T2.There kinds of features have developed due to the active tectonic activities from last Pleistocene to recent Holocene. As an evidence , we can consider the recent seismic study with in meizoseismal zone of both the 1934 Bihar-Nepal and 1950 Assam earthquakes but last surface rupture with a recorded displacement of 14m of Chalsafault related to the historical earthquake in Nepal around 1100 AD (Kumar et.al. 2011). It is a positive feedback system where neotectonism and geomorphological landscape are well related and in a classical model of fault propagation; geomorphic evolution through the structure/process is present in respect of time and space.

Kinematics and strain rates of the Eastern Himalayan Syntaxis from new GPS campaigns in Northeast India

Newly acquired GPS data along transects across Himalaya in Eastern Himalayan Syntaxis (EHS) reveal a clockwise rotation of rigid micro-plate comprising part of Brahmaputra valley, NE Himalaya and Northern Myanmar that rotates about a pole located at 14.5°N, 100.8°E at an angular rate of 1.75±0.12°/Myr. The EHS is being torn-off from the main Indian Plate as a rigid block around which the kinematic clockwise rotation of Tibetan GPS sites toward the Sichuan-Yunnan region occurs in the Eurasia fixed frame. The residual velocity field of the newly acquired data estimated after removing the rotation that minimizes the GPS rates around EHS show a clear NE motion of the EHS sites, indentation of the rigid Indian plate into a less rigid area of the Eurasian plate. The most extensive EHS zones of compression and shortening are in the direction of indenter convergence, with average values ranging between ~50–100 nanostrain/year. Along the frontal segment of EHS, from NW to SE, the shortening rate is reduced from the local maximum value of 160 to ~80 nanostrain/year, thus indicating a possibly locked fault patch of Mishmi or Lohit thrusts, the southernmost part of segment activated during the large 1950 Assam earthquake, Mw 8.6.An elastic block-model was invoked to infer the average slip rates of sections around EHS and to estimate an average locking depth of ~15km. The slip rate perpendicular to the locked sector of EHS reaches 32.4mm/year and permits to roughly infer a recurrence time of ~200year for an earthquake as energetic as the 1950 Assam event.