Seismological evidence for intra-crustal low velocity and thick mantle transition zones in the north-west Himalaya.

The Himalayan region witnesses several natural hazards like earthquakes and landslides due to the continental collisions between the Indian and Eurasian plates. This has given rise to extreme topographic variations throughout the Himalayan belt. The Kumaun–Garhwal region is a classic example of such geological consequences and is prone to several earthquakes. High-quality three-component teleseismic waveform data recorded at seven seismological stations operated by the Wadia Institute of Himalayan Geology (WIHG) are used to investigate the detailed subsurface structure of the crust, the intra-crustal low-velocity layer (LVL), and the upper mantle discontinuities beneath the Kumaun–Garhwal, north-west Himalaya. The results, derived from the inversion of individual station's stacked P-receiver functions (PRFs) using the neighbourhood algorithm approach, show that the crustal thickness varies from 44 to 54 km beneath the study region. The depth of LVL observed beneath six stations from individual and stacked PRFs, varies from 9 to 24 km. The LVL zone with a high Vp/Vs ratio may be due to fluid or partial melt, leading to shallow seismic activity within the study region. The presence of fluid or partial melts in the LVL may be due to the shear heating, cooling, and decompression. The 2D PRF migration image depicts a thick mantle transition zone due to the elevated 410 km discontinuity with respect to the global average values predicted by the IASP91 velocity model. The present research suggests that this might be due to the colder transition zone in this region, indicating the cool underthrust Indian plate with respect to the ambient mantle has reached down to the upper mantle transition zone. [ABSTRACT FROM AUTHOR]