Geomorphic signatures of the neotectonic activity in Haridwar-Kotdwar region, Northwestern Himalaya, India

<p>In this study, we investigate the ongoing crustal deformation in the Haridwar-Kotdwar piedmont zone of the Northwestern Himalaya, India. The Himalayan mountain front has been actively deforming along the Himalayan Frontal Thrust (HFT) which marks the conjunction between the Siwalik hills and the Indo-Gangetic Plains. We report NNE-SSW trending left lateral strike-slip fault towards the west of the study area namely Haridwar Fault (HF) and it offsets the HFT sinistrally by ~ 9 Km. Using the satellite imagery (Cartosat-1 stereo pairs) flat-lying uplifted river terrace have been identified, which is at an elevation of ~80 m from the flood plain of Mitthawali River. Along with uplifted terraces, the HF offsets various structural features, the rivers flowing across it and manifests itself as a series of scarps and slope breaks visible in the satellite imagery. The Khoh River Fault (KRF) trends N-S and offsets HFT dextrally by ~12 Km, this controls the course of the Khoh River and forms a lateral ramp perpendicular to HFT. The KRF manifests itself geomorphically as uplifted terraces at an elevation of ~50 m from the flood plain of the Khoh River which is conspicuous in the DEM and the Cartosat-1 imagery of the area. The Haridwar-Kotdwar piedmont zone has been surrounded in the north by HFT, in the south by Najibabad Fault (NF), towards east by KRF and the western margin has been dissected by HF. The KRF and HF show signatures of neotectonic activity and offsets HFT at two locations forming two ramps in the region. The piedmont zone has been showing signatures of upwarping which causes sudden migration of the rivers flowing into the piedmont zone on a decadal scale, mainly caused by an E-W trending NF. NF is a blind fault and manifests itself geomorphically by series of knee turn bending of the rivers in the study area. The deformation caused by NF has been comprehended using the satellite imageries and Gradient Length Anomalies (GLA). The GLA results show signatures of upliftment in the piedmont zone along the NF. The Haridwar-Kotdwar piedmont zone is surrounded by neotectonically active faults from four sides, making this block a potential seismic hazard in near future.</p>

Delineation of S-wave velocity profiles across the Himalayan Frontal Thrust (HFT) using metaheuristic approaches.

<p>The continent-continent collision between the Indian and Asian Plate formed a series of major faults from north to south along the Himalayan belt. Among these Himalayan Frontal Thrust (HFT) is the southernmost and youngest one and is tectonically very active. Any information on the shear wave velocity distribution across the fault is therefore very important. In this study, we have used the Wide Angle Multichannel Analysis of Surface Wave (WAMASW) to estimate the subsurface shear wave velocity profiles across HFT at Pawalgarh in Uttarakhand, India, using widely used stochastic global search Particle Swarm Optimization (PSO) and Grey wolf Optimization (GWO) algorithms. To gain confidence on the accuracy of the inversion results, we first generated an elastic synthetic seismic shot gather with ground rolls by using the forward modelling scheme of SOFI2D for a two-layer velocity depth model overlying a half-space. The generated gather was then processed in MATLAB to generate the experimental dispersion curve using the Phase shift method. We then extracted the fundamental mode for the gather and inverted it using the standard PSO and GWO algorithms and estimated 1D shear wave velocity profile. After getting acceptable results for the synthetic dataset, we then applied the PSO algorithm to generate the 1D S-wave velocity (Vs) profile across the Himalayan Frontal Thrust (HFT). In the study area, the Rayleigh wave phase velocity for the first shot varies from 444 to 743 m/s. We then obtained the 1D shear wave velocity profiles and a jump in Vs is observed across the HFT indicating variation in the sediment stiffness across the fault.</p><p><strong>Keywords: </strong>WAMASW, dispersion, Meta- Heuristic, PSO, GWO, 1D Shear wave velocity</p><p> </p>

Crustal structure across Himalayan Frontal Thrust (HFT) using high-resolution seismic datasets

<p>The Himalayan fold-thrust belt has been developing due to the northward convergence of the Indian plate against the Eurasian plate since ~55 Ma. Three major thrust systems: Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Himalayan Frontal Thrust (HFT) are distinctly observed in the Himalayan orogeny from north to south indicating southward propagation of active deformation. These active thrust systems produced several devastating earthquakes in the past such as 1905 Kangra (Mw 7.8), 1934 Nepal-Bihar (Mw 8), and 1950 Assam (Mw 8.6) earthquakes. Presently HFT is found to be the tectonically very active zone that accommodates a strain rate of ~10-15 mm/year and is a zone for great threats in near future to the societies residing over the Himalayan foothills. The present study carried out in the lower Siwalik Himalaya near Pawalgarh in Nainital District of Uttarakhand, India with an objective to estimate the velocity model across HFT in the locality. To accomplish the objective, seismic data were acquired along three profiles of a cumulative length of ~13 km using a seismic thumper as a source and 96 vertical component geophones with the natural frequency of 5 Hz and Remote Acquisition Unites (RAUs) as sensors and data loggers, respectively, and with a group and shot interval of 20 m and near offset of 100 m. Highly uneven Himalayan terrain causes large static errors. In order to overcome this challenge, we used Real Time Kinematics (RTK) to estimate more precise source and receiver surface elevation. In the pre-processing phase of acquired seismic data, three different shots taken at the same location are vertically stacked to eliminate random non-coherent noises and improve the SNR of the data. We then applied a low-frequency array filter (LFAF) to suppress the ground roll using velocity estimates from the ambient noise tomography (ANT). We process the data by implementing conventional seismic processing techniques including normal move-out (NMO) correction, velocity analysis followed by stacking. In the stack section, we observe a northward dipping reflector extending from the surface to ~ 1- 1.25 s TWT indicating evidence of HFT. Another reflector observed at ~3-4 s TWT demarcating the extent of overlying sedimentary deposits on the top of the under-thrusting lithosphere. Rocks of the Siwalik Himalaya mainly composed of sedimentary deposits of sandstone mudstone, and alluvial deposits. Average velocity obtained from the refraction tomography ~ 2900 m/s matches well with rock type in the region. Thus, the high-resolution crustal structure across the highly active HFT can be crucial to understand the earthquake mechanism in the locality and for a better hazard assessment.</p>

Examining the suitability of the geomorphic parameters in generating the Relative Index of Active Tectonics: an example from Himalayan Frontal Thrust, India

<p>This work emphasizes the efficient use of geomorphic parameters to form a unified index ~ Relative Index of Active Tectonics (RIAT), which has seldom been tested in areas with broader variability in the rate of deformation. This study aims to verify whether the geomorphic parameters can be used efficiently for RIAT to assess the spatial variability in deformation along the fault. The Himalayan Frontal Thrust has been chosen for morphotectonic evaluation owing to its active interplate thrust fault setting. For this purpose, we select vertical uplift sensitive geomorphic parameters viz., Mountain front sinuosity (S<sub>mf</sub>), Valley floor width-height ratio (V<sub>f</sub>), and Steepness index (K<sub>sn</sub>), as a primary tool to test the RIAT.</p><p>The result of RIAT shows the along-strike variation in response to the varying degree of deformation along the HFT. This is in fine agreement with the available long-term uplift/shortening rates and geodetic rates. Overall examination reveals RIAT being an excellent tool to assess the spatial variability in uplift rates in large tectonically active regions. However, the detailed scrutiny of individual geomorphic parameters reveals that only V<sub>f, </sub>and the K<sub>sn</sub> index are more responsive and go hand-in-hand with the RIAT variation. Whereas, S<sub>mf</sub> shows no spatial variation and function as least sensitive to such an investigation. The sensitivity of these individual parameters has implications for studies with similar settings elsewhere when quantitative rates are absent.</p>

Establishing primary surface rupture evidence and magnitude of the 1697 CE Sadiya earthquake at the Eastern Himalayan Frontal thrust, India

Historical archives refer to often recurring earthquakes along the Eastern Himalaya for which geological evidence is lacking, raising the question of whether these events ruptured the surface or remained blind, and how do they contribute to the seismic budget of the region, which is home to millions of inhabitants. We report a first mega trench excavation at Himebasti village, Arunachal Pradesh, India, and analyze it with modern geological techniques. The study includes twenty-one radiocarbon dates to limit the timing of displacement after 1445 CE, suggesting that the area was devastated in the 1697 CE event, known as Sadiya Earthquake, with a dip-slip displacement of 15.3 ± 4.6 m. Intensity prediction equations and scaling laws for earthquake rupture size allow us to constraints a magnitude of Mw 7.7–8.1 and a minimum rupture length of ~ 100 km for the 1697 CE earthquake.

New estimates of minimum geological convergence for the eastern Himalaya, India

<p>We re-investigate the geological slip along the frontal Nameri Thrust, a local name for the Himalayan Frontal Thrust in the eastern Himalaya, India. Four levels of tectonically displaced and uplifted fluvial terraces preserved along the Kameng River were dated using the Optically Stimulated Luminescence (OSL) method. The OSL ages of the terraces bracket the timing of their abandonment post ~14, 11, 7.2 and 3 ka respectively. Considering the minimum timing of vertical uplift and height of the uplifted and incised bedrock strath beneath the lowermost river terrace T1, we use trigonometric method to infer a vertical uplift rate of ~0.44 mm/a on the Nameri Thrust during the Holocene Period. The mismatch in the geodetic convergence and the geological slip rates proposed for the Himalayan Frontal Thrust in the eastern Himalaya in earlier studies provoked us to re-evaluate the scenario of geological slip in the area. Our results suggest a contrasting estimate of geological slip rate as compared to the earlier studies. Though the results are indicative of a decrease in the Indo-Eurasian convergence in the eastern Himalaya in accordance with the recent GPS observations and models proposed for the region, we, however, suggest that the lower estimation in our study compared to that reported previously could be due to the use of different dating methods for the materials obtained for assigning chronology to the landforms and events. Since the <sup>14</sup>C AMS radiocarbon dating method requires a contemporary organic component in the sediments to be dated, an overestimation of the dates is also possible if the sediment has mixed with old carbon, which makes it inferior to the OSL method in which the mineral grains are assumed to have been fully bleached before their burial. This makes the OSL method more reliable to date sediments since it does not encounter the ‘old-carbon’ error problem of overestimation of the ages. Two additional samples obtained to the south of the active mountain front yield southwardly-increasing luminescence ages of ~19 and 26 ka suggesting deposition of older sediments toward downstream by the Kameng River as a result of rampant incision in the upstream triggered by episodes of tectonic uplift prior to ~26 ka.</p>