SEISMOTECTONICS OF GARHWAL HIMALAYA BETWEEN ALAKNANDA AND YAMUNA VALLEYS

Abstract

The Garhwal Himalaya is located in the central segment of the Himalayas, which formed by the collision between the India and Eurasia continental plates during the late cretaceous period (<57 Ma). It displays all the major structural and litho-tectonic features that have formed since the collision viz., the Himalayan Frontal Thrust (HFT), the Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the Southern Tibetan Detachment System (STDS). All of these north-dipping thrusts and detachments are sole to a common décollment known as the Main Himalayan Thrust (MHT). These structures from south to north divide the region into different lithotectonic zones viz., the Sub Himalayan Sequence (SHS), the Lesser Himalayan Sequence (LHS), the Higher Himalayan Crystallines (HHC) and the Tethyan Himalayan Sequence (THS). The region is located in the western part of the Central Seismic Gap, which has 52% probability for a great earthquake (Khattri, 1999). In this study, the earthquakes of Garhwal Himalaya that occurred between July 2007 and October 2015 have been analyzed for the source, stress and seismotectonics of the region. For which, the data from the Garhwal Himalaya Seismic Network (GHSN), consists of 10 broadband stations established in the Garhwal Himalaya have been used. Besides this network, Kumaun Himalaya Seismic Network (KHSN) consists of 4 stations and Kinnaur Seismic Network (KSN) consists of 9 stations have also used for preliminary dataset preparation and Moment Tensor Inversions, respectively. During the reporting period, ~1000 local earthquakes have recorded by the GHSN. From which, a well-located catalogue of 585 earthquakes have been prepared for analysis based on their azimuthal coverage (>180°), minimal location errors (< 5.0 km) and the smaller time residuals i.e. Root Mean Square (< 1.0). Besides this, a historical seismicity catalogue, consisting of 147 earthquakes of magnitudes M>4.0 has been prepared from the historical records, catalogues and research articles. The MC calculated for these catalogues are 1.8 and 4.1 for the present and historical seismicity, respectively. The seismicity of the region follows the regional NWN-SES trend of the Himalayan Seismic Belt located around the MCT Zone (between MCT1 and MCT2). The majority of the seismicity (83.7%) is confined to the upper crust and the micro seismicity are located in the entire crust, in which, few earthquakes of small-to-moderate size (3.0≤M≤5.0), have also occurred close to the Moho. It shows that the entire crust (both upper and lower crusts) represent a single seismogenic layer. The Frequency Magnitude Distribution of seismicity show low b-values; indicating a high stress accumulation in the region owing to the continuous underthrusting, hence, iv suggesting a high probability for a future major or great earthquake(s) occurrence. Furthermore, the region have divided into three zones, viz., NE Garhwal, Central Garhwal and SE Garhwal (Chamoli), for analysis based on the clustering properties. In which the earthquakes in the Central Garhwal is diffused and coinciding with the relatively high b-value (b=0.98) in comparison to the low b-values of 0.71 and 0.72 for the NE Garhwal and SE Garhwal, respectively. This zone is having low seismicity and coinciding with the trends of Delhi-Haridwar Ridge (DHR), which is a subsurface structure that extend deep into the Himalayan litho-tectonic zones. Further, the 2011 Chamoli earthquake of M 5.0 preceded by a quiescence period, also succeeded by a decrease in b-value, hence suggesting a increased stress levels. This is also well constrained with the increased activity of moderate size earthquakes around the Chamoli region, indicating, high seismic risk in this region. The Fractal dimension (Dc) value estimated for the present seismicity is high (Dc = 1.47), and suggesting that the region is approaching a two-dimensional space and seismically active. Moreover, it shows that the faults in this region are sparsely distributed, hence, structurally heterogeneous, may be due to the presence of local faults (e.g. Alaknanda Fault) and transverse tectonics (e.g. DHR). The present seismicity locations have relocated using a double-difference algorithm for precise locations. The seismic cross sections prepared for each three zones show a distinct trend of seismicity along the Mid Crustal Ramp (MCR). In addition, the seismic cross sections reveal that the sensu-stricto Main Central Thrust (i.e. Munsiari Thrust) in two of the three profiles and suggesting the MCT is active in segments and is a site of generation of micro-seismicity due to its reactivation by thrusting along the MCR. Similarly, the Alaknanda Fault, located in the Chamoli region may also be causing the micro-seismicity. The focal mechanism solutions of 26 upper crustal earthquakes have obtained using waveform inversion technique. In addition, 11 focal mechanism solutions from the published records have also used for stress inversions for the upper crustal region. The moment tensor solutions reveal dominatingly thrust fault mechanisms for HSB and few normal faulting earthquakes in the SHS and outer LHS. The seismic cross sections illustrate that these earthquakes with low dip angle and low P axis plunge are associated with MCR. The optimally oriented fault planes from the stress inversions also suggest that the causative fault for these earthquakes is similar to a north dipping fault plane having a dip angle between 12° and 25°, which is compatible with the dip angle of the MCR (~16°). The seismic cross sections for the moment tensor solutions, relocated hypocenters are suggesting that the ramp structure is present throughout the region, despite the presence of DHR. This has further confirmed by the v continuous presence of PT2 feature at the foot of the HHC, which has been correlated with the active existence of MCR beneath the region. The seismic cross sections for the relocated earthquake hypocenters suggest that the MCT is active in segments; however, the focal mechanisms, dip angles and PT axes are not supporting the seismogenic nature of the MCT. Hence, the MCT has considered inactive at present. The principle stress axis of the upper crustal region is a NE-SW oriented horizontal compressive stress with low plunge angle, coinciding well with the relative motion direction of the Indian plate. However, the intermediate and least stress are horizontal and vertical, respectively, suggesting a compressional tectonic regime. The stress inversions performed for different zones viz., NE Garhwal, SE Garhwal, Central Garhwal, earthquakes along the MCT zone, 0-8 km and 8-18 km depth zones for Chamoli region and for the earthquakes occurred in SHS and outer LHS. The results show similar NE-SW trend principle stress for all inversions except for the SHS and the outer LHS zone and the 0-8 km depth zone in Chamoli region, which have nearly E-W principle stress orientations. During the stress inversions, a friction coefficient value has been estimated, which is 0.85 for the region, while the Chamoli region show low values (0.65), indicating low friction. Hence, the free fluids trapped beneath the detachment have believed to be penetrating into the local faults and decreasing the frictional strength of the rocks. The origin of the earthquakes that occur around the Moho with 35 to 60 km depth has been discussed and recognized as the lower crustal earthquakes based on their crustal phases, depth location, faulting mechanisms and the earlier studies on geology, thermal, mechanical, and geodynamical aspects of the region. The focal mechanism for the earthquakes that occurred in the lower crust shows predominantly strike slip faulting mechanisms. To study the lower crustal faulting nature, kinematics and stress field, few more solutions from the published records, also for the adjacent regions have been used. The results have been compared with the kinematics of the upper crustal earthquakes and show that the thrust style of earthquakes occur only in the upper crust, close to the MCR. However, the kinematics is changing, vertically and laterally, with dominatingly normal faulting in the SHS and the outer LHS and the dominatingly strike-slip style of faulting mechanism in the lower crust. The stress inversions for the lower crustal earthquakes reveal a strike slip tectonic regime with the horizontal compressive principle (ζ1) stress towards NNE-SSW, vertical intermediate stress (ζ2) and horizontal minimum stress (ζ3). The optimally oriented fault plane identified from the instability analysis through the stress inversion also suggesting a strike-slip tectonic environment. Furthermore, the lower crustal earthquakes are located around the Moho where vi the flexure of the Indian plate begins, hence suggesting the possible role of mountain load and the bending plate stresses. The present study based on the spatio-temporal analysis, moment tensor and stress tensor inversions of the local earthquakes of Garhwal Himalaya suggest that the region is accumulating stress and the whole crust beneath the region is seismogenically active and prone to major or great earthquakes in the near future.