Pamir-Hindu Kush Region Research Articles

Variations of b-value and the ω-upper bound magnitude of GIII distribution of the Pamir–Hindu Kush region

Variations of b-value and the ω-upper bound magnitude of GIII distribution of the Pamir–Hindu Kush region

The Khakas-Altaian Mythonym Ker and the Proto-Yenissean Word for ‘Mammoth’. 2

The article continues the research on the names of mammoth or other mammoth-like monsters as characteristic elements of the mythological onomasticon of some Siberian traditions (Turkic, Yenissean, Tungusic). In the first part of the article, the author identified a reconstructed Proto-Yenissean form *čer / *ťèkə́r ‘mammoth (giant horned fish)’ > Ket 1tēľ ~ Yug 1čel ~ Pumpokol *kher (> Altaic, Khakass, Teleut ker, kär ‘a monster, especially a giant fish’) as a possible origin of these names. In this part, the author proposes a pre-Yenissean (Sino-Caucasian) etymology for it: *c̣HírV[ʁ]V (> North-Caucasian *c̣irVχV ‘caterpillar, snail; line’ ~ Sino-Tibetan *čaŋ ‘lizard’ ~ (?) Burushaski hargin ‘dragon, a monster that a snake turns into.’ It is assumed that the image of the mammoth-fish (as opposed to a more widely known and probably more ancient image of mammoth-bull) was formed in the Proto-Yenissean tradition after the speakers of the Yenissean proto-language migrated from the southern ancestral homeland to Siberia. It is based on ideas about the transformation of a snake, fish or similar chthonic animal into a giant dragon-like creature that existed among the Sino-Caucasian ancestors of the Yenisseans spread in the Pamir-Hindukush region, and was known to the ancient Chinese. The formation of the image of the mammoth-fish among the Yenisseans was also influenced by the Iranian motif of the “a belt dissecting a tree.” The Yenissean image of the mammoth-fish reached Siberia, particularly, it was borrowed into the shamanic tradition of the Evenks, which is evidenced by the possible etymology of the Evenki selī ‘mammoth’ (including mammoth-horned fish) from the reconstructed form of the unknown Yenissean language *seĺ (< *čer / *ťèkə́r).

Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion

The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.

Numerical modelling of the stress in the Pamir-Hindu Kush region

The current stress state of the Earth's crust in Central Asia depends entirely on the interaction of the Eurasian plate with the Indian and Arabian plates. Moreover, an essential role in this action is played by the subduction process in the Pamir-Hind Kush zone. In this region, deep earthquakes and anomalies in seismic velocities indicate subduction of the Indian Plate. To determine the effect of earthquakes on the stress state of the Earth's crust in Central Asia, we analyzed the hypocenters according to the available data over a hundred years. Taking the envelope of the hypocenters as the surface of the subducted indenter, we analyzed its penetration into the mantle. Instead of the pushing force of the indenter, the velocity of the Indian plate is taken into consideration. A model of the stresses is constructed on the equations of creeping motion of a viscous incompressible fluid. The stress variation in the Earth's crust and mantle was calculated as the stress difference before and after the earthquake. The mechanism of earthquakes is modelled by an introduction of equivalent volume forces in the equilibrium equations corresponding to a couple dipoles without a moment. The equations of creeping motion are numerically solved using boundary element methods. Numerical experiments with different physical model parameters for the Earth's crust and mantle were conducted. To introduce the negative buoyancy of the subducting plate, partial eclogitization of the rocks in the collision zone was assumed. As a result, a certain combination of stiffness and density was determined for the lithosphere and mantle, which explained the peculiarities of this region. The influence of crustal and mantle earthquakes in the Pamir-Hindu Kush region on the change in background stresses in Central Asia was analyzed which are not higher than 2–3%.

Re-appraisal of earthquake catalog in the Pamir―Hindu Kush region, emphasizing the early and modern instrumental earthquake events

Re-appraisal of earthquake catalog in the Pamir―Hindu Kush region, emphasizing the early and modern instrumental earthquake events

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Forecasting seismicity rate in the north-west Himalaya using rate and state dependent friction law

ABSTRACTIn this study, rate and state Coulomb stress transfer model is adopted to forecast the seismicity rate of earthquakes (MW ≥ 5) in the north-west Himalaya region within the testing period 2011–2013. Coulomb stress changes (ΔCFF), considered to be the most critical parameter in the model, exhibit stress increase in the whole study region, excluding the Chaman fault of the Kirthar range where significant stress shadow has been observed. The estimated background seismicity rate varies in the range 0.0–0.7 in the region, which is preoccupied by low aftershock duration of <50 years. Furthermore, a low b-value that varies between 0.54 and 0.83 is observed in Kirthar ranges, Karakoram fault and Pamir-Hindukush region. However, areas like Hazara syntaxis of the northern Pakistan and northern Pamir of the Eurasian plate exhibit higher b-values in the range 1.23–1.74. Considering constant constitutive properties of the faults (i.e. Aσ = 0.05 MPa), our forecast model for variable ΔCFF and heterogeneous b-value successfully captures the observed seismicity rate of earthquakes. Results have been verified using statistical S-test. However, the model fails to capture the observed seismicity rate during the period when reconstructed for average b-value to be 0.86 and no change in ΔCFF (ΔCFF = 0).

Subducting continental lower crust and crustal thickness variations in the intermediate seismic zone of Pamir–Hindu Kush inferred from Moho underside reflection pmP

The Pamir–Hindu Kush region is an orogenic belt which formed as a result of recent continental collision between the Indian and Eurasian Plates. A comprehensive understanding of the tectonic history of this region has been hampered due to limited seismological investigations. In this study, we use the Moho underside reflection pmP phase to constrain crustal thickness variations in the intermediate-depth seismic zone (36–37°N, 69–72°E). The seismic events characterized by focal depth deeper than 100km and magnitude>5.8 (Mw) were used. The crustal thickness was determined by identifying the depth phase pP and the Moho underside reflection pmP. The measured thickness varies spatially from 58.1 to 76.2km, with uncertainties most likely resulting from deviation of the average P-wave velocities (~6.21km/s with a deviation of 0.22km/s) in the crust. The strong Moho depth variations imply a large structural deformation of the crust, which reflects a complex collision-related mountain building history. We also detected two strong reflections from deep interfaces down to ~97km underneath the southernmost Pamir. Based on our direct observations and waveform modeling, we interpret that the two reflections are possibly the manifestations of the underplating subducted Eurasian lower crust in this region. Our observations complement those of other seismic results, including receiver functions from previous studies.

Imagingb‐Value Variation beneath the Pamir–Hindu Kush Region

The frequency–magnitude distribution ( b ‐value) for the seismicity of the Pamir–Hindu Kush region from August 2008 to June 2010 is analyzed. We used a very precisely located earthquake dataset containing 9532 events with a magnitude of completeness of 2.9. We attempted both 1D and 2D approaches to resolve the b ‐value variation as a function of depth. Several pockets of high b ‐value were detected during the cross‐sectional analysis that may reflect the changes in the physical properties of the subducting slab with depth. Under the Hindu Kush sector, a high b ‐value (>1.2) is identified at the depth of 150 km, but under Pamir this zone ( b >0.9) is apparent at shallower depth. An earlier detailed seismic tomography study revealed a low‐velocity zone beneath Hindu Kush where the high b ‐value is imaged in the present study. This observation is corroborated by the existence of fluid due to the dehydration and increase in pore pressure within the lower portion of the Indian lithosphere. The relatively high b ‐value at shallower depth could be associated with the higher slab temperature, as caused by the slower subduction under Pamir.

Complex deformation pattern of the Pamir–Hindu Kush region inferred from multi-scale double-difference earthquake relocations

The wide range of focal depths and the corresponding fault plane mechanisms in the Pamir–Hindu Kush region are believed to reflect the ongoing deformation along the Indian–Eurasian continental collision zone. Here we develop a multi-scale double-difference earthquake location algorithm to combine traveltime data recorded at local, regional and teleseismic distances. The 2906 relocated intermediate-depth earthquakes of MW≥4.5 we studied beneath the Hindu Kush show two groups of seismicity, with a separation of ca. 20–30km; these two groups exhibit thrust faults with different P-axes and different locations, reflecting the lateral geometry change of the seismic zone. Earthquakes beneath the Pamir present a clear south-dipping layer and unusual horizontal T-axes. There is a clear transition in direction of stress between the Hindu Kush and the Pamir regions. Here we focus on the complex deformation pattern beneath the continental collision zone arising from a combination of compression, tension, shearing and necking states.

Intermediate-depth earthquakes beneath the Pamir-Hindu Kush region: Evidence for collision between two opposite subduction zones

We employed a double-difference algorithm (hypoDD) to relocate earthquakes within the region bounded by 66°E–78°E and 32°N–42°N in the period of 1964–2003 reported by the International Seismological Center (ISC). The improved hypocentral locations delineate a double-layered Wadati-Benioff zone in the eastern Hindu Kush intermediate seismic belt. Based on this feature and other evidences, we propose that the intermediate-depth earthquakes beneath the Pamir-Hindu Kush region may occur in two collided subduction zones with opposite dip directions.

Modeling the evolution of continental subduction processes in the Pamir–Hindu Kush region

Several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the India–Eurasia collision zone, with steep northward subduction of Indian lithosphere beneath the Hindu Kush and southward subduction of Asian lithosphere under the Pamir. We investigate the geometry and the timing of these subduction processes. Seismic tomography images are used to constrain the geometry of the two slabs under the Pamir and the Hindu Kush. By measuring the tomographically inferred length of the Indian slab under the Hindu Kush region and by comparison with paleomagnetic reconstructions, we estimate that the process of Indian slab break-off most likely occurred at the early stage of the collision, at ca. 44–48 Ma. We infer that after slab break-off, western India continued its northward motion until its northern boundary reached the present Hindu Kush region, where it began to subduct. We estimate an age for the initiation of subduction in this region of about 8 Ma. We apply 2D thermo-kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the Hindu Kush and Pamir regions. For the Hindu Kush we introduce parameters reproducing fast and near vertical subduction, whereas in the Pamir region we use the results from published studies to reproduce slower subduction. Modeling results indicate that faster subduction under the Hindu Kush results in a deeper brittle region, which is consistent with observed deeper seismicity than in the Pamir region.

A tomographic image of Indian lithosphere break-off beneath the Pamir-Hindukush region

SUMMARY The P and S seismic anomalies in the upper mantle beneath the Pamir‐Hindukush collision zone and surrounding areas are investigated using more than 4 × 10 5 traveltimes from 5775 events located in the study region, recorded by the stations of the worldwide seismic network and reported in the ISC catalogue. All available epicentral distance ranges of seismic rays are considered. All sources are relocated using an algorithm, which includes the double-difference method. In addition, source parameters are corrected in the main inversion step, simultaneously with the velocity models. The high reliability of the resulting P-velocity images is confirmed by various tests. The S model provides much smoother patterns, which are, however, generally consistent with P anomalies. Our model shows a complex image of lithospheric plate under the study region. Under SW Hindukush, the northwards subducting Indian plate is imaged throughout the upper mantle and transition zone. Further to the NE, within a 300-km-wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at ∼250 km depth. The tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern Tien Shan down to at least 250 km depth. To the north of Tarim, we observe a northward-dipping of the Tarim plate down to ∼500 km depth. At the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults, which indicate complex crustal structure in this region.

Seismotectonics of the Himalaya and its vicinity from centroid-moment tensor (CMT) solution of earthquakes

The centroid-moment tensor solutions of more than 300 earthquakes that occurred in the Himalayas and its vicinity regions during the period of 1977–1996 are examined. The resultant seismic moment tensor components of these earthquakes are estimated. The Burmese arc region shows prominent east–west compression and north–south extension with very little vertical extension. Northeast India and Pamir–Hindu Kush regions show prominent vertical extension and east–west compression. The Indian plate is subducting eastward beneath the northeast India and Burmese arc regions. The overriding Burmese arc has overthrust horizontally with the underthrusting Indian plate at a depth of 20–80 km and below 80 km depth, it has merged with the Indian plate making “Y” shape structure and as a result the aseismic zone has been formed in the region lying between 26°N–28°N and 91.5°E–94°E at a depth of 10–50 km. Similarly, the Indian plate is underthrusting in the western side beneath the Pamir–Hindu Kush region and the overriding Eurasian plate has overthrust it to form a “Y” shape structure at a depth of 10–40 km and below 60 km depth, it has merged with the Indian plate and both the plates are subducting below 60–260 km depth. Further south, the overriding Eurasian plate has come in contact with the Indian plate at a depth of 20–60 km beneath northwest India and Pakistan regions with left lateral strike slip motion.

Large-scale three-dimensional seismic tomography of the Zagros and Pamir-Hindukush regions

The nature of the deep seismic structure of the Zagros and Pamir-Hindukush regions is inferred from the three-dimensional P-wave velocity images obtained using the Aki, Christofferson and Husebye (ACH) inversion technique. We use teleseismic (25 ⩽ Δ ⩽ 90°) P-wave arrival times, recorded at 24 seismic stations during the period 1977–1986. A study of the azimuthal variation of residuals reveals that despite the crustal doubling, the Pamir-Hindukush region is characterized by faster arrivals (≈ − 1.0 s) of teleseismic P-waves. The Iranian plateau, on the other hand, exhibits delayed arrivals (0.4–1.0 s) from teleseisms. The results of the ACH inversion reveal contrasting P-wave velocity characteristics in the upper mantle of these deformed zones. In the depth interval 0–200 km, the Pamir-Hindukush region is characterized by 1–4% faster velocities relative to the Iranian plateau. Accounting for the anomalous crustal thickening in the Hindukush results in the subcrustal mantle (up to 200 km) exhibiting a significant higher velocity (≈ 2%), comparable to that of the Indian plate. However, in the depth interval 200–450 km, the Pamir-Hindukush region has 1–3% lower velocities relative to the Indian plate, while the upper mantle beneath the central Iranian plateau is 2–4% slower relative to the surrounding region. The high-velocity upper mantle till a depth of 200 km beneath the Hindukush region, reflects the possible presence of both the oceanic lithosphere and the crust and uppermost mantle of the Indian plate, which subducted beneath Eurasia. The low velocity observed beneath central Iran could be a reflection of the upper mantle thermal anomalies related to the Pliocene-Quaternary volcanism in this region.

Source retrieval from broadband regional seismograms: Hindu Kush region

A method of relocation and source characterization of small earthquakes using one modern regional station is developed and tested. First, we model teleseismic body waves of two events, which are used as masters. Short period depth phases, pP and sP, are used to establish the epicentral depth, and the events are relocated using calibrated stations and a mantle model derived for this region, TIP. The events moved upward by 12 and 31 km, respectively. The regional waveforms recorded at the IRIS station GAR from the best-determined source are forward modeled to establish a local crustal model. A four-layered model with a thickness of 65 km proves effective. Synthetics from this crustal model can then be compared with the data from other events where the depths are poorly known, i.e. PDE depths of 33 km. Forward modeling of the short period phases allows better estimates of depth and makes relocation possible. Next, we perform a long period inversion of whole and partial waveform data to obtain source mechanism and moment, and repeat the procedure if necessary. This technique is applied to events in the tectonically active Pamir-Hindu Kush region to test its usefulness. Five crustal events sampling various azimuths are presented as examples of relocation and determination of source mechanisms from small events. The smallest event has a moment of 5.1 × 10 22 dyn cm. For the seven events we studied, the average depth correction to PDE is 19 km, and the location correction is 11 km on average. The method can be used to identify earthquakes and thus it lowers the threshold below present m b: M S discrimination, since these events are too small to be seen teleseismically. Secondly, these events can be used as ‘masters’ in the calibration of other systems, case-based event characterizations, etc.

Structure and evolution of the Pamir—Hindu Kush region lithosphere

The deep structure of the Pamirs—Hindu Kush region is considered. Using the geological and tectonic data a conclusion is reached on the sliding of the 30 km uppercrustal layer and its disharmony relative to the upper crust. On the basis of the methods of the three dimensional wave velocity model of the lithosphere, obtained by the seismic tomography algorithm, and of the investigations of the recurrency law of the earthquakes in the Pamirs—Hindu Kush mantle focal zone, developed by the author's, a considerable horizontal inhomogeneity is recognized in the structure of the region up to a depth of 300 km. All these data indicate that the movement of the lithosphere plates is brought about not by simple subduction, but by their twisting, differentiated at various levels, occurring in the upper mantle notless but perhaps even more intensively than in the Earth's crust. The mixing of the crustal and mantle materials accompanying this process can account for the decrease in the upper mantle average density in certain parts of the region.

The spectral content of Pamir-Hindu Kush intermediate depth earthquakes: Evidence for a high-Q zone in the upper mantle

Very high frequencies (5–10 Hz) are recorded at Garm, Tadzhikistan (Δ∼200 km), near Toktogul, Kirgizia (Δ∼600 km) and at other close stations from intermediate depth earthquakes in the Pamir-Hindu Kush region. The seismic phase, Sn, is recorded with large amplitudes and high frequencies at stations in Pakistan and India from intermediate depth earthquakes. Such high frequencies require either extremely high average values of Q(several thousand) or very high stress drops of earthquakes (kilobars) or possibly both. Regardless of the stress drops, we infer that Q ≳ 1000 along paths through a portion of the mantle where Q is usually low. Thus these data indicate a discontinuity, or marked thinning, of the asthenosphere. Even if Q were infinite, the spectral content of the signals at Garm, in addition, could also be interpreted as evidence for relatively high stress drops for the intermediate depth earthquakes. If Q is equal to 3000, the highest calculated stress drops are for events with depths between about 50 and l80 km and range from several tens to several hundred bars. Tables 1 to 3 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document JTT-OOl; $1.00. Payment must accompany order.

Examination of relocated earthquake hypocenters in the Pamir- Hindu Kush seismic zone using 3-D plots

DOI = 10.3126/hjs.v5i7.1248 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.43