Along‐strike variation in the shear wave crustal structure of the NE Himalayan and Indo‐Burmese arc: Evidence based on surface wave dispersion analysis

Abstract

Surface wave dispersion data is used to infer the shear wave velocity structure variation in the north‐east Himalaya and Indo‐Burmese arc regions. We have used 25 earthquakes data from four groups with a magnitude range of 5.0–6.7, epicentral distance range 368–800 km, and focal depth less than 50 km. Ray paths from the earthquake location to the seismic station are transversely passing different geotectonic units of the Himalayas, Indo‐Gangetic plains, and Indo‐Burma collision zones. The weighted average dispersion curve and the path averaged shear wave velocity models are computed for the four groups located at different azimuths around Shillong seismic station. Non‐linear least‐square inversion is performed to obtain the shear wave velocity structure of the crust and uppermost mantle from joint inversion of Rayleigh and Love waves group velocities. Subsequently, Rayleigh and Love waves group velocities are inverted separately to obtain vertical and horizontal components of S‐wave velocity. Next, using these components anisotropy coefficient values at different depths for each path are also estimated. A high variation of dispersion curves and the shear wave velocity models from one group to another indicates that the region is geotectonically very complex. An approximately 80 km‐thick zone beneath the study region has shear wave velocity as low as 1.7 km/s in the uppermost crust in the southern part and ~4.7 km in the uppermost mantle beneath the Eastern Himalayan Syntaxis (EHS). Inferred velocity is also lower than that of PREM and AK135 global models with a much thicker crust beneath the study region. Radial anisotropy varies even within the northern part from the Indo‐Eurasian collision zone to EHS and northern to southern Indo‐Burma ranges. Anisotropy is comparatively stronger in the deeper part below ~40 km for the three paths, except for the EHS, where the result is contrary.

INSA National Report on Seismological Research in India: 2019 – 2022

“…They observe a thick (>20 km) transition zone beneath the Burmese Arc and close to the Tengchong volcano. Surface wave tomography reveals thick sediments represented by low shear wave velocity down to ∼ 21 km depth in the eastern Bengal Basin beneath the southern Indo-Burma Range (IBR) Chanu et al, 2022). Tomography images also report subduction of Indian plate beneath the Burmese arc with signatures of a medium of high shear wave velocity below ∼ 50 km to ∼ 75 km depth.…”

“…The Walong fault is characterized by strike-slip motion which helps to facilitate the clock-wise rotation of crustal material around the syntaxis . Radial anisotropy interpreted within the Eastern Himalaya Syntaxis (EHS) and the Indo-Burmese Ranges based on surface wave dispersion reveals stronger anisotropy in the deeper part below ˜40 km depth (Chanu et al, 2022).…”

INSA National Report on Seismological Research in India: 2019 – 2022

“…Surface wave tomography reveals thick sediments represented by low shear wave velocity down to ∼ 21 km depth in the eastern Bengal Basin beneath the southern Indo-Burma Range (IBR) Chanu et al, 2022). Tomography images also report subduction of Indian plate beneath the Burmese arc with signatures of a medium of high shear wave velocity below ∼ 50 km to ∼ 75 km depth.…”

Introduction, Part 2—Geodynamic evolution of Eastern Himalaya and Indo‐Myanmar orogenic belts: Advances through interdisciplinary studies

“…Chanu et al (2022) studied the azimuthal variation of the shear wave velocity structure around north‐east India, using surface wave data of earthquakes recorded by a single station on the Shillong Plateau. They considered that a high variation of dispersion curves and the shear wave velocity models from one group to another indicate that the region is geotectonically very complex.…”

“…Geological map of Eastern Himalaya, Indo‐Myanmar orogenic belt, northern, Central Myanmar, and adjacent areas (modified after Mitchell et al, 2007; Searle et al, 2007; Y. R. Singh, Singh, Singh, et al, 2022). Locations of study area presented in the issue include 1/S2: Kundu and Hazarika (2022); 2/S2: Y. R. Singh, Singh, Singh, et al (2022); 3/S2: Rawat and Luirei (2022); 4/S2: Kayal et al (2022); 5/S2: Choudhury et al (2022); 6/S2: N. M. Sharma et al (2022); 7/S2: Goswami, Gogoi, et al (2022); 8/S2: Bikramaditya et al (2022); 9/S2: Goswami, Kalita, et al (2022); 10/S2: Joshi (2022); 11/S2: Jha and Sharma (2022); 12/S2: Shukla et al (2022); 13/S2: Khonglah et al (2022); 14/S2: Ali and Duarah (2022); 15/S2: Mohanty et al (2022); 16/S2: Chanu et al (2022); 17/S2: Majumdar, Gogoi, and Ghatak (2022); Majumdar, Gogoi, Ghatak, Saikia, et al (2022); 18/S2: Ding et al (2022); 19/S2: Ghose et al (2022); 20/S2: Hussain and Dey (2022); 21/S2: Ozukum et al (2022); 22/S2: Jamir et al (2022); 23/S2: Chaubey et al (2022); 24/S2: Bora, Borah, et al (2022); Bora, Mukherjee, et al (2022); 25/S2: Khuman and Ibotombi (2022); 26/S2: Y. R. Singh, Singh, Singh, et al (2022); 27/S2: Thokchom and Kshetrimayum (2021); 28/S2: Barman et al (2022); 29/S2: V. Sharma and Biswas (2022)…”