Delineation of partial melts and crustal heterogeneities within the crust beneath Kumaon Himalaya, India from Lg wave attenuation

Abstract

The crustal seismic attenuation or the Q structure is studied by using the Fourier spectra of Lg-wave along the Tanakpur- Dharchula- Dharma transect in the Kumaon Himalaya. The 1 Hz Lg Q (Q0) values are computed between different pairs of two stations and the observed values are later utilized to calculate the lateral variation in the Q0 values by following a back projection algorithm. This computation of Q0 values utilizes five regional distance earthquakes having moment magnitude (Mw) ≥ 4.0, which lie along the great circle path of the transect. Three of the five earthquakes occurred in the Tibetan plateau and the and the others occurred to the southwest on the Indian shield and are well recorded at all the 32 broadband seismographs operated between September 2018 and March 2022. The estimate Qo values range from 63 ± 2 and 203 ± 25, with the lowest value in the Lesser Himalaya and the highest across part of the Indo Gangetic Plain and Siwalik Himalaya. The Q0 model has low values ∼200 along the profile in the Indo Gangetic Plain and the Siwalik Himalaya, and are correlated with 2–5 km thick sedimentary layers below the Himalaya and the adjoining Indo-Gangetic Plain. We observe two distinctly different Q0 values to the northeast in the Lesser Himalaya tectonic unit. The region lying between the South Almora Thrust (SAT) and the Berinag Thrust (BT) shows extremely low Q0 values (∼60) but increases further north towards the Vaikrita Thrust (VT) to ∼200. The possible explanation for observing such huge variation of the Q0 values within a single tectonic unit may be the presence of fluid rich ramp structures, which introduces crustal heterogeneities and traps the aqueous fluids or partial melts lying within the crust. The Lg Q0 values decrease to the North and become ∼166 for station pairs in the Higher Himalaya and Tethys Himalaya tectonic units. The low Q0 values observed in this region may be correlated with low viscous partial melts in the form of Miocene leucogranite plutons, which resulted out of the Indo-Asian collision. The attenuation structure along the profile in the Kumaon Himalaya can be used to estimate ground motions of future earthquakes in the area and can contribute to seismic hazard assessment in the Himalaya and neighbouring regions.