Earthquake source characteristics along the arcuate Himalayan belt: Geodynamic implications. (original) (raw)
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Earth-Science Reviews, 2017
Spread over countries including Pakistan, India, Nepal, Bhutan, and China, the Himalayan mountain chain, the most spectacular result of the Indo-Eurasian plate collision, is a locus of destructive earthquakes. Past earthquakes from this region have impacted vast swathes of frontal Himalaya and stretches of alluvial plains south of the range front. Risk from future earthquakes has increased, considering the burgeoning population and an ever-expanding built-environment in the region. While considerable ambiguities exist on the locations, ruptures, and sizes of the earthquakes during the first half of the last millennium (1255, 1344, and 1505 AD), those during the latter half (1803 and 1833 AD) are quite well-documented, all reported from the central Himalayan segment comprising of eastern Nepal, Kumaun, and Garhwal. While dormancy prevailed in the central segment in the intervening period, the Himalayan arc elsewhere witnessed three large/great earthquakes in the last century, namely, 1905 Kangra (Mw 7.8), 1934 Bihar-Nepal (Mw 8.2), and 1950 Upper Assam (Mw 8.6), the last one being the largest intra-continental earthquake in the recorded history. The April 25, Gorkha (Nepal) earthquake (Mw 7.8) located in the central seismic gap terminated the period of low-level seismic productivity that followed the
Acta Geophysica, 2014
Detailed analysis of intensity for ten damaging historical earthquakes in the central arcuate belt between the Himachal and Darjeeling Himalayas was carried out in the backdrop of isoseismal eccentricity, source depth and Indian plate obliquity. Results indicate that the elongated axes of the isoseismals and strike of ruptures for shallow earthquakes are almost parallel with strike of the Himalayan arc, and clearly conformable with the obliquity. An empirical power law relationship between eccentricity and focal depth established under the present study illustrates that the deeper events are more influenced by the bending of the penetrating Indian lithosphere, whereas the shallower events are principally controlled by the obliquity. A positive correlation between eccentricities and obliquity obviously supports this inference. The present study further reveals that the constant decrease in Indian plate obliquity from Himachal to Nepal-Bihar Himalaya is well compatible with the graben structures and horizontal shearing along this arcuate segment.
The Himalayas and the Tibetan Plateau are the result of the continental collision between India and Eurasia. The Indian Plate underthrusts the Himalayan mountains and the southern Tibetan Plateau. Recorded seismicity at the Himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust-mantle boundary. The question of whether the near-Moho earthquakes are in the crust or in the upper mantle has been controversial, and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes. Earthquake locations from several experiments place seismic events in the upper mantle. Using a finite element model, we establish a link between the recorded upper mantle seismicity beneath the Himalayan collision zone and flexural bending of the Indian lithosphere. Earthquake locations, focal mechanisms, and seismic imaging results from the HIMNT experiment, combined with previous constraints on the geometry and deformation of the Himalayan collision, are used to set up the finite element models of lithospheric loading. Our purpose is to infer the mechanical state of the lithosphere beneath the Himalayas and to evaluate the role of the lithospheric mantle in the support of the loads. The pattern of mantle seismicity can be explained by modeling the response of the Indian Plate to loads corresponding to the weight of the sediments of the Ganga basin, the Himalayan mountains and the southernmost Tibetan Plateau, combined with the effects of a horizontal force per unit length acting upon the lithospheric plate. We calculated the steady-state stress field in the Indian lithosphere, where the lithospheric mantle is assumed to be viscoelastic and non-Newtonian, and the asthenosphere is modeled as viscoelastic and Newtonian. Two model suites were tested, one with an elastic crust (Model Suite 1), and one with a viscoelastic crust (Model Suite 2). Both model suites provide a good fit to the observed patterns of seismicity, but Model Suite 2 is the one that best reproduces the observations. High differential stresses concentrate in the upper mantle, and predicted principal stress orientations match those inferred from focal mechanisms in the area. Our models show that beneath the Ganga basin and the southernmost Himalaya, earthquakes at near-Moho depths do not need to show extension, nor is the lower crust required to be weak, in order to infer that the uppermost mantle yields by brittle failure. Even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes, Mohr-Coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure.
Gondwana Research, 2012
Subduction of bathymetric features, such as ridges, seamounts, fractures etc., on the subducting plate influences the arc morphology and earthquake ruptures. We analyse their effect on the development of the arcuate shape of the Himalayan arc and on the ruptures of great and major Himalayan earthquakes. Besides the two most prominent ridges in the Indian Ocean, namely the Chagos-Laccadive-Deccan ridge and the 90°E ridge, which are assumed to extend up to the Himalayan arc, at least three major subsurface ridges have been mapped on the underthrusting Indian plate under the Indo-Gangetic plains. It appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the Himalayan arc. The interaction and subduction of the other subsurface ridges probably influenced the Himalayan arc morphology by causing a localised cusp in the frontal topography. Also, these ridges probably acted as barriers to the ruptures of the major and great Himalayan earthquakes.
Mantle fault zones beneath the Himalayan collision: Flexure of the continental lithosphere
Tectonophysics, 2009
The Himalayas and the Tibetan Plateau are the result of the continental collision between India and Eurasia. The Indian Plate underthrusts the Himalayan mountains and the southern Tibetan Plateau. Recorded seismicity at the Himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust-mantle boundary. The question of whether the near-Moho earthquakes are in the crust or in the upper mantle has been controversial, and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes. Earthquake locations from several experiments place seismic events in the upper mantle. Using a finite element model, we establish a link between the recorded upper mantle seismicity beneath the Himalayan collision zone and flexural bending of the Indian lithosphere. Earthquake locations, focal mechanisms, and seismic imaging results from the HIMNT experiment, combined with previous constraints on the geometry and deformation of the Himalayan collision, are used to set up the finite element models of lithospheric loading. Our purpose is to infer the mechanical state of the lithosphere beneath the Himalayas and to evaluate the role of the lithospheric mantle in the support of the loads. The pattern of mantle seismicity can be explained by modeling the response of the Indian Plate to loads corresponding to the weight of the sediments of the Ganga basin, the Himalayan mountains and the southernmost Tibetan Plateau, combined with the effects of a horizontal force per unit length acting upon the lithospheric plate. We calculated the steady-state stress field in the Indian lithosphere, where the lithospheric mantle is assumed to be viscoelastic and non-Newtonian, and the asthenosphere is modeled as viscoelastic and Newtonian. Two model suites were tested, one with an elastic crust (Model Suite 1), and one with a viscoelastic crust (Model Suite 2). Both model suites provide a good fit to the observed patterns of seismicity, but Model Suite 2 is the one that best reproduces the observations. High differential stresses concentrate in the upper mantle, and predicted principal stress orientations match those inferred from focal mechanisms in the area. Our models show that beneath the Ganga basin and the southernmost Himalaya, earthquakes at near-Moho depths do not need to show extension, nor is the lower crust required to be weak, in order to infer that the uppermost mantle yields by brittle failure. Even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes, Mohr-Coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure.
Gondwana Research, 2006
The recent earthquake of 8 October 2005 in the Muzaffarabad region in western Himalaya destroyed several parts of Pakistan and the north Indian state of Jammu and Kashmir. The earthquake of magnitude 7.6 claimed more than 80,000 lives, clearly exposing the poor standards of building constructiona major challenge facing the highly populated, earthquake prone, third world nations today. In this paper, we examine variations in the stress field, seismicity patterns, seismic source character, tectonic setting, plate motion velocities, GPS results, and the geodynamic factors relating to the geometry of the underlying subsurface structure and its role in generation of very large earthquakes. Focal mechanism solutions of the Muzaffarabad earthquake and its aftershocks are found to have steep dip angles comparable to the Indian intra-plate shield earthquakes rather than the typical Himalayan earthquakes that are characterized by shallow angle northward dips. A low p-value of 0.9 is obtained for this earthquake from the decay pattern of 110 aftershocks, which is comparable to that of the 1993 Latur earthquake in the Indian shieldthe deadliest Stable Continental Region (SCR) earthquake till date. Inversion of focal mechanisms of the Harvard CMT catalogue indicates distinct stress patterns in the Muzaffarabad region, seemingly governed by an overturned Himalayan thrust belt configuration that envelops this region, adjoined by the Pamir and Hindukush regions. Recent developments in application of seismological tools like the receiver function technique have enabled accurate mapping of the dipping trends of the Moho and Lithosphere-Asthenosphere Boundary (LAB) of Indian lithosphere beneath southern Tibet. These have significantly improved our understanding of the collision process, the mechanism of Himalayan orogeny and uplift of the Tibetan plateau, besides providing vital constraints on the seismic hazard threat posed by the Himalaya. New ideas have also emerged through GPS, macroseismic investigations, paleoseismology and numerical modeling approaches. While many researchers suggest that the Himalayan front is already overdue for several 8.0 magnitude earthquakes, some opine that most of the front may not really be capable of sustaining the stress accumulation required for generation of great earthquakes. We propose that the occurrence of great earthquakes like those of 1897 in Shillong and 1950 in Assam have a strong correlation with their proximity to multiple plate junctions conducive for enormous stress build up, like the eastern Himalayan syntaxis comprising the junction of the India, Eurasia plates, and the Burma, Sunda micro-plates.
Earthquake Shocks Around Delhi-NCR and the Adjoining Himalayan Front: A Seismotectonic Perspective
Frontiers in Earth Science, 2021
An increase in the number of earthquakes and subsequent clustering in northwest India, particularly around the Delhi-National Capital Region (NCR) and adjacent NW Himalayan front, provides a good opportunity to understand the underpinning tectonic controls and the likelihood of any large earthquake in the future. The 2001 M w 7.7 Bhuj, 2011 M w 6.9 Sikkim and 2015 M w 7.8 and 7.3 Nepal earthquakes (and 2004 M w 9.2 Sumatra event) are important in this context. We analyzed the seismicity around the Delhi-NCR and the adjoining Himalayan front, including event clustering and the spatio-temporal distribution of b-values, in the context of kinematics and the regional geodynamics. The overall moderate-to-low b-values, both in time and space, since 2016, provide information regarding an increase and subsequent stabilization of the stress field in the study area. The analysis led to the identification of (1) a structurally guided stress field in the region between the Kachchh and the NW Himalaya that coincides with the direction of Indian plate convergence and (2) frequent occurrences of earthquakes particularly in the Delhi, Kangra and Uttarkashi areas. We propose that faults in western Peninsular India, which pass through the margins of the Aravalli Range, the Marwar basin, and the isostatically over-compensated Indo-Gangetic Plains beneath the under-plated Indian lithosphere, act as stress guides; concentrating and increasing stress in regions of lithospheric flexure. This enhanced stress may trigger a large earthquake.
State of Tectonic Stress in Northeast India and Adjoining South Asia Region: An Appraisal
Bulletin of the Seismological Society of America, 2013
The paper investigates to study components of seismicity and prevailing tectonic stress regimes of the considered region by analyzing the earthquake data that occurred during the last 200 years (from 1808 to 2008). For the purpose, northeast India Himalaya and its adjoining regions have been divided into five active regions namely Eastern Syntaxis, Arakan-yoma fold belt, Shillong plateau, Himalayan Frontal arc and Southeastern Tibet by taking into consideration the spatial distribution of seismicity its tectonic complexity. The minimum compressive stress is almost horizontal in the Tibet which indicates that the earthquake generation process is due to the flow of materials in east-west direction. The prevailing regional stress conditions at shallower levels in compression as well as in extension zones extend up to the deeper levels in to the upper mantle especially in southeastern Tibet and Arakan-yoma region. The present findings provide additional information on the seismicity, tectonics, the faulting pattern and the associated ongoing geodynamic processes in the region.