Surface rupture signatures of historical earthquakes (16th and 19th centuries) from Kumaon-Garhwal, central Himalaya: Implications for seismic hazard assessment

Earthquakes and their accompanying natural hazards (e.g., ground shaking, ground failure, surface faulting, tectonic deformation, inundation) pose a widespread threat to human activities and to man-made structures and facilities. Seismic hazard analysis, the quantitative estimation of the hazard of earthquake ground shaking at a site, provides valuable guidance for informed decision-making on mitigating the earthquake threat. Recent methodological advances in seismic hazard analysis allow marked improvements in incorporating fundamental input from geology, seismology, and earthquake engineering into the analysis in an orderly way. Rigorous estimations of seismic hazard (and of corresponding "risk" of social or economic consequences) require carefully prescribed models of the space, time, and size distribution of earthquakes, together with models of ground-motion attenuation with distance. Given these inputs, up-to-date probabilistic methods may be applied to compute the level of groundshaking hazard, expressed as the probability of not exceeding some particular level of ground motion at one or more sites during a time period of interest. Importantly, the probabilistic approaches provide a well-founded basis for representing natural variability, and they allow the treatment of uncertainties arising from incomplete knowledge. To interpret correctly the results of a probabilistic seismic hazard analysis, it is necessary to understand underlying assumptions and the basic concepts of probability.

Seismicity and seismic characteristics like b value, fractal dimension, energy release and recurrence period are assessed for the region (80°E–89°E and 26°N–31°N), which includes the 2015 large Nepal earthquake Mw 7.8 in central Himalaya. The International Seismological Centre (ISC) catalogue of earthquakes with mb ≥ 3.8 for the period August 1964 – January 2013 are used. The b values are estimated using the maximum likelihood and least square methods. The fractal dimension is estimated using the correlation integral method. The events are also used for estimating radiated energy in the region. The probability of occurrence of moderate earthquakes (mb ≥ 5.0 using ISC data and Mw ≥ 6.0 using USGS data for the period 1964–2015) during a specified interval of time is estimated on the basis of three probabilistic models, namely, Weibull, gamma and lognormal. The model parameters are estimated by the maximum likelihood estimates. It is found that there is a 99% probability for occurrence of at least one earthquake mb ≥ 5.0 in this region in a time window of 2–3 years from the year 2013, and in 20–30 years for an earthquake Mw ≥ 6.0 from the year 2015. The b value maps identified variable stressed zones, and the fractal dimension maps fractal characteristics of the active fault zones. The energy release map identified the zones of higher and lower energy release. The percent probability maps, prepared using the Poisson distribution, identify the zones of higher- and lower-probability earthquakes and thus indicate the areas of future probable earthquakes.

Mapping the extents and terminations of surface ruptures is essential for a better seismic hazard assessment, as these factors indicate the magnitude and location of future earthquakes. The Himalayas and its active Himalayan Frontal Thrust (HFT) plate boundary, have experienced numerous damaging earthquakes in both recent and medieval times. The variability of the associated surface ruptures and the extent of the medieval earthquakes remain poorly understood. This study aims to improve the understanding of rupture extents and behavior along the ~ 650 km Central Seismic Gap (CSG) segment by analyzing paleoseismic trench data from sixteen previously published trenches, including new excavations during this study at Teliwara and Goujani. Evidence from Goujani suggests a penultimate event between calAD1437 and calAD1576 and Most Recent Earthquake (MRE) between calAD1662 and AD1894 with a minimum cumulative slip of at least 7 m. The Teliwara site reveals evidence of MRE occurring between calAD1665 and calAD1956 with a cumulative slip of 12.3 m. Integrating these findings with data from other paleoseismic sites allows for a detailed reconstruction of rupture patterns and timing along the CSG segment. Analysis of age probability density functions (PDFs) indicate three significant events: Earthquake 1 dated between calAD1320 and calAD1412, likely ruptured 259–360 km of the HFT. Earthquake 2, dated between calAD1437 and calAD1576, appears to have ruptured 490 km with rupture termination near the Faizabad Ridge, suggesting it was an asperity confined rupture. Earthquake 3 occurred between calAD1748 and calAD1894. These events correspond temporally to the 1344, 1505, and 1803 historical earthquakes, or to unrecorded events of similar age. Estimated moment magnitudes (Mw) for Earthquakes 1 and 2 are approximately 8.3–8.5 and 8.7, respectively. These findings suggest that the CSG segment has experienced large ruptures influenced by structural asperities.

Expansion and hazard risk assessment of glacial lake Jialong Co in the central Himalayas by using an unmanned surface vessel and remote sensing

Earthquake recurrence in NW and central Himalaya

The Himalayan region has undergone significant development and to ensure safe and secure progress in such a seismically vulnerable region there is a need for hazard assessment. For seismic hazard assessment, it is important to assess the quality, consistency, and homogeneity of the seismicity data collected from different sources. In the present study, an improved magnitude conversion technique has been used to convert different magnitude scales to moment magnitude scale. The study area and its adjoining region have been divided into 22 seismogenic zones based upon the geology, tectonics, and seismicity including source mechanism relevant to the region. Region specific attenuation equations have been used for seismic hazard assessment. Standard procedure for PSHA has been adopted for this study and peak ground motion is estimated for 10% and 2% probability of exceedance in 50 years at the bed rock level. For the 10% and 2% probability of exceedance in 50 years, the PGA values vary from 0.06 to 0.36 g and 0.11 to 0.65 g, respectively considering varying b-value. Higher PGA values are observed in the southeast part region situated around Kaurik Fault System (KFS) and western parts of Nepal.

Revisiting the 1991 Uttarkashi and the 1999 Chamoli, India, earthquakes: Implications of rupture mechanisms in the central Himalaya

Evidence of blind thrusting in the frontal part of the Central Himalaya and its significance for rupture evolution and size of a medieval earthquake

Revisiting the earthquake sources in the Himalaya: Perspectives on past seismicity

Glacial lake outburst floods are among the most serious natural hazards in the Himalayas. Such floods are of high scientific and political importance because they exert trans‐boundary impacts on bordering countries. The preparation of an updated inventory of glacial lakes and the analysis of their evolution are an important first step in assessment of hazards from glacial lake outbursts. Here, we report the spatiotemporal developments of the glacial lakes in the Poiqu River basin, a trans‐boundary basin in the Central Himalayas, from 1976 to 2010 based on multi‐temporal Landsat images. Studied glacial lakes are classified as glacier‐fed lakes and non‐glacier‐fed lakes according to their hydrologic connection to glacial watersheds. A total of 119 glacial lakes larger than 0.01 km2 with an overall surface area of 20.22 km2 (±10.8%) were mapped in 2010, with glacier‐fed lakes being predominant in both number (69, 58.0%) and area (16.22 km2, 80.2%). We found that lakes connected to glacial watersheds (glacier‐fed lakes) significantly expanded (122.1%) from 1976 to 2010, whereas lakes not connected to glacial watersheds (non‐glacier‐fed lakes) remained stable (+2.8%) during the same period. This contrast can be attributed to the impact of glaciers. Retreating glaciers not only supply meltwater to lakes but also leave space for them to expand. Compared with other regions of the Hindu Kush Himalayas (HKH), the lake area per glacier area in the Poiqu River basin was the highest. This observation might be attributed to the different climate regimes and glacier status along the HKH. The results presented in this study confirm the significant role of glacier retreat on the evolution of glacial lakes. Copyright © 2014 John Wiley & Sons, Ltd.

Integrated hazard assessment of Cirenmaco glacial lake in Zhangzangbo valley, Central Himalayas

Structural interpretation of the great earthquakes of the last millennium in the central Himalaya

The objective is to understand incessant seismic activities in Northwest and Central Himalayan regions. GPS data acquired (2017–2020, Nepal; 2015–2019, Uttarakhand) from 65 GNSS stations are used to generate velocity solutions with respect to International Terrestrial Reference Frame 2014 & Indian fixed reference frame to determine the site’s precise position. These velocities are further used to calculate the strain rate and prevailing convergence rate by the respective Triangulation method and Okada’s formulation. The estimated mean maximum and minimum principal strain rate are 12.19 nano strain/yr. and − 102.94 nano strain/yr. respectively. And the respective mean shear strain and dilatation are 115.13 nano strain/yr. −90.75 nano strain, which implies that Higher Himalaya observes high compression rate compared to Outer and Lesser Himalayan region. Estimations have also elucidated presence of extensional deformation in the Northwestern part of the Himalayan arc. Accordingly, in Central Himalaya, paleoliquefaction investigations have deciphered turbidites, confirming that the seismic ruptures did not reach the surface during the 2015 Gorkha earthquake. The best-fit locking depth of 14 km and convergence rate of 21 mm/yr. (Nepal) & 18 mm/yr. (Uttarakhand) are obtained. The strain budget analysis indicates that Northwest and Central Himalaya can beckon a megathrust earthquake in the future.

Seismites in the Kathmandu basin and seismic hazard in central Himalaya

The Northwestern Himalayas have been host to many earthquakes, with the recent 1905 Kangra Mw 7.8. Studies suggest the seismic gap in the Northwestern Himalayas to be more than Mw 7.8 in general, and of about Mw 8.4 in the Nahan region of the Northwestern Himalayas. There are studies suggesting the rupture of the Himalayan Frontal Thrust (HFT) and hinterland subsidiary faults by the Earthquakes in the region. In this study, we focused the Khetpurali Taksal Fault (KTF), which is one of the corroboration. It is an out-of- sequence ~ 250 km long dextral strike-slip fault with an NNW-SSE trend. KTF, which is bounded to the west by the Nahan Salient, and in the east by the Dehradun re-entrant; marks the boundary between the Central Himalayas (convergence rate ~ 18 ± 1 mm/y and obliquity ~0°) and the Northwestern Himalayas (convergence rate ~13.6–14 ± 1 mm/yr and obliquity ~ 15°-30°), which runs through the ~ 100km locked width of the Main Himalayan Trust (MHT). The dextral strike-slip motion has caused the displacement of some quaternary deposits along the KTF. It plays a key role in the slip partitioning between the active thrust and the oblique faults with the HFT displaying the thrusting and oblique component in the Pinjore Garden fault, Jhajra fault, and Barsar fault of the same region in the Northwestern Himalayas. This study is focused on the active fault along the KTF. We prepared the geomorphic map and delineated the extent of the KTF using the high-resolution Cartosat-1 data. Our studies show the presence of displaced terraces, lateral offset of streams, sag ponds, and pressure ridges along the KTF. A displacement of 250m to 1350m has been observed. Samples from the displaced terraces are analyzed by OSL dating technique to find out the slip along the KTF. Understanding the slip along KTF will enhance the understanding of slip partitioning taking place in the Nahan Region as a whole, which will help understand the geodynamics of the region and thus in seismic hazard assessment.