Evidence for a high‐velocity slab associated with the Hindu Kush seismic zone

Abstract

We used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the Pamir‐Hindu Kush region in Tadjikistan and Afghanistan. Data from 29 analog seismic stations in Tadjikistan and northern Afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28° to 87° and covering a broad range of azimuths. We inverted for velocity perturbations over a rectangular grid with a block size of 99 × 99 km. The model extended to a depth of 350 km with a 50‐km‐thick first layer and two 150‐km‐thick deeper layers. The results show a strong and well‐resolved zone of high velocities in the upper mantle at depths greater than 200 km, coincident with the location of the Hindu Kush seismic zone. No clear velocity perturbations are associated with the Pamir seismic zone. Above 200 km little correlation is observed with the seismic zone, but indications of thicker crust under the Pamir and thinner crust under the Tadjik Depression are seen. The high velocities are most likely caused by the presence of oceanic lithosphere at depth.

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

“…The stronger and thinner oceanic crust results in denser and thinner W-B zone below 200 km in the east end of the Hindu Kush zone. High velocity at depth greater than 200 km and low velocity above 200 km revealed by some tomographic images (Vinnik et al, 1977;Roecker, 1982;Hamburger, et al, 1992;Mellors et al, 1995) are consistent with the idea of transition from a continental slab to an oceanic slab. However, tomographic features showed by different authors are different and need further confirmation (Koulakov and Sobolev, 2006;Huang and Zhao, 2006) Secondly, the Harvard CMT solutions in the double-layered W-B zone in Hindu Kush does not exhibit systematic differences between the two layers (Lou et al, 2007), which further validates that the earthquake hypocenters of the double-layered W-B zone in Hindu Kush region delineate two subducting slabs.…”

“…However, in the deeper part the slab seems not having this kind of difference, at least not significant. Therefore, it is more likely that the shallower and the deeper parts of the W-B zone locate in decoupled continental and the oceanic plates respectively, which was revealed earlier by geophysical inversion (Roecker, 1982;Mellors et al, 1995).…”

“…Since then, a number of seismological studies for this region have been carried out (e.g. Nowroozi, 1971;Billington et al, 1977;Vinnik et al, 1977;Chatelain et al, 1980;Roecker et al, 1980;Roecker, 1982;Fan et al, 1994;Ning and Zang, 1990;Mellors et al, 1995;Zhu et al, 1997;Pegler and Das, 1998;Liu, 1999;Pavlis and Das, 2000;Lou et al, 2007). An S-shaped intermediate-depth seismic zone in this region was identified, including two segments with obviously different strikes and dip directions.…”

“…Located at the western Himalayan syntaxis, the Pamir-Hindu Kush region is one of the most active regions of intermediate-depth earthquakes which are not obviously related to well-defined subducting slabs inspite of some geophysical evidences (e.g., Vinnik et al, 1977;Roecker, 1982;Mellors et al, 1995;Van der Voo et al, 1999;Lei and Zhao, 2002;Huang and Zhao, 2006;Negredo et al, 2007). The accurate spatial distribution of those earthquakes and their focal mechanism solutions are helpful to understanding the actual geometry as well as the stress state of possibly existing subduction zones in this region, and can provide constraints on plate interaction and tectonic evolution in the Pamir-Hindu Kush region.…”

Geodynamic Implications for the 8 October 2005 North Pakistan Earthquake

“…This surface-wave CMT methodology, which yields compatible and very similar results as in standard CMT, allows study of the complex processes of stress transfer to different fault segments that marked the unusual time evolution of the sequence (Arvidsson and Ekström 1998;Ekström et al 1998;Morelli et al 2000). Several workers (Menke and Jacob 1976;Armbruster et al 1978;Chatelain et al 1980;Seeber et al 1981;Ni and Ibenbrahim 1991;Fan et al 1994;Mellors et al 1995;Lukk et al 1995;Zhu et al 1997;Searle et al 2001;Khan 2003) have studied the seismicity and the geological processes of the northwestern part of the Indian plate, and have suggested that the seismicity in this region is apparently caused by the mutual interaction between the Kohistan Arc and the Indian plate (Fig. 1).…”