Mapping the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

In this study we have tried to detect and collect later phases associated with Moho discontinuity and used them to study the lateral variations of the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuzawa, T., Hasegawa, A. 2002. Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Physics of the Earth and Planetary Interiors, 130, 31–47), having epicentral distance ranging from 60 km to 150 km. Taking the advantage of high quality broadband data now available in northeast India, we have detected 1607 Moho reflected phases (PmP and SmS) from 300 numbers of shallow earthquake events (depth ⩽ 25 km) in Shillong–Mikir Hills Plateau and its adjoining region. Notably for PmP phase, this could be identified within 0.5–2.3 s after the first P-arrival. In case of SmS phase, the arrival times are observed within 1.0–4.2 s after the first S-arrival. We estimated the crustal thickness in the study area using travel time difference between the later phases (PmP and SmS) and the first P and S arrivals. The results shows that the Moho is thinner beneath the Shillong Plateau about 35–38 km and is the deepest beneath the Brahmaputra valley to the north about 39–41 km, deeper by 4–5 km compared to the Shillong Plateau with simultaneous observation of thinnest crust (∼33 km) in the western part of the Shillong Plateau in the Garo Hills region.

► Variation of Moho depth using direct (P and S) and reflected phases (PmP and SmS).
► Total numbers of 1607 PmP and SmS phases from 300 earthquakes events are utilized.
► Travel time difference between PmP, SmS phases and first P, S arrivals are utilized.
► Moho depth is estimated thinner beneath the Shillong Plateau about 35–38 km.
► Moho depth is deepest beneath the Brahmaputra valley to the north about 39–41 km.