Internal structure of India: perspectives from a review of the seismological imaging studies from 2020 to 2023

Abstract

The Indian subcontinent is a mosaic of different provinces exhibiting diversity in age, geology, elastic strength, tectonics, and seismicity. The evolution of most of the provinces from the Archean to the present age is not yet fully understood, which is further complicated by the active continent–continent collision along the northern margin and other intraplate forces. Seismological imaging helps unravel various subsurface structural features, thereby enabling a better understanding of the tectonic evolution. In the last 4 years, several seismological imaging studies have been carried out by Indian scientists, which has resulted in a remarkable improvement in depth estimation and constraining the geometry of important discontinuities, such as the Main Himalayan Thrust (MHT) and the Moho. Since the MHT plays an important role in generating megathrust earthquakes in the Himalaya, a large number of studies have been carried out in the Himalaya and their number almost equals the combined studies carried out in the rest of India. Detailed velocity structure of the crust and mantle has been estimated by tomographic inversion throughout the Indian subcontinent, even including Tibet, Pamir and Burma in certain cases. Attempts have also been made to estimate the depth and topography of deeper structures such as Hales discontinuity, Lithosphere–Asthenosphere Boundary, Mantle transition zones, and D’’ layer over the Core Mantle Boundary. Some of the studies have developed and adopted newer methodologies to map the structure. Those studies are analysed here to provide an overview of the progress toward understanding the structure.

Appendices

Appendix A

A Brief Note on the different seismic imaging techniques frequently used in the text.

Receiver functions (RF)

A Receiver function is obtained by deconvolving the radial component waveform from the vertical component of a teleseismic (epicentral distance 30°–90°) earthquake. It contains information about phases such as the P-to-s converted phase and multiply reflected phases, which are characteristics of the discontinuity beneath the receiver.

RF modelling/inversion

RF modelling usually involves a linearized inversion scheme to obtain a 1D V_S model which could fit the observed RF waveform with a minimum misfit. The depth and velocity contrast across the discontinuity are reliable in an RF inversion, while the absolute velocity is less constrained.

RF H-κ stack

The delay time of the P-to-s converted and reflected phases is a function of the depth of the discontinuity (H) and V_P/V_S ratio (κ) of the crust. By grid-searching H and κ, the values are chosen at which the sum of the amplitudes of the converted and reflected phase is maximum.

Poisson’s ratio (σ) gives the ratio of transverse strain to longitudinal strain and is related to κ as σ = (κ²−2)/(2κ²−2).

Surface wave dispersion

Surface waves (SW) of different frequencies travel with different velocities. The relation between frequency (or time period) and velocity is called dispersion and is usually seen as a listric curve.

If group velocity of the surface wave is measured, it is called group velocity dispersion. Similarly, phase velocity dispersion is named. Similarly, if Rayleigh/Love surface waves are used, they are called Rayleigh/Love wave dispersion, and it can be more specific as Rayleigh wave group velocity dispersion, etc.

Surface wave inversion

A linearized inversion scheme to obtain a 1D shear-wave velocity model by matching the observed dispersion with a minimum misfit.

Joint inversion of RF and SW dispersion

RF is sensitive to the impedance contrast, whereas SW is sensitive to average velocity variation along the propagation path. In order to make use of both sensitivities, a joint inversion by assigning different weights to both datasets is carried out, generally, by a linearized scheme to obtain a 1D V_S model, which provides a good estimation of the depth of the discontinuities as well as their velocities.

RF common conversion point (CCP) stack

RF CCP stack involves depth migration of the RF by a 1D or 3D model. In this technique, individual RFs are projected along a given cross-section through their piercing point, and the travel-times of different phases are converted to depth according to the velocity model. RF amplitudes are stacked in small resolution bins if required.

Tomographic inversion

In tomographic inversion, a given uniform velocity model is perturbed iteratively to match the observed and predicted travel times along different paths. Based on the phase or process from which the travel-time is obtained, the tomography nomenclature is done accordingly. For example, if P-phase travel-times are obtained from local earthquakes, then the tomography can be named local earthquake body wave tomography. Similarly, there can be teleseismic earthquake tomography, Rayleigh/Love wave group-velocity/phase velocity tomography, Pn/Sn-tomography, etc. The inversion scheme may vary for different tomography studies.

Ambient noise tomography (ANT)

ANT is similar to SW tomography, where group/phase velocity travel-times are inverted to obtain a tomography map with an additional preceding step. Cross-correlation of ambient noise is performed to obtain an Empirical Green’s function, which is similar to a surface wave.

1D V_S model can be obtained by inverting dispersion curves extracted from regular grids of SW or AN tomography maps. The 1D models can be joined together to form a 3D V_S model.

Seismic attenuation

Seismic attenuation is defined as the ratio of lost energy to total energy per energy cycle. The amplitude of a seismic wave attenuates exponentially and can be obtained by modelling the spectra, as the attenuation factor is proportional to the slope of the power spectra. Attenuation of different phases viz. P-, S- and coda waves differ.

Appendix B

Full forms of the frequently used abbreviations.

ANT:

Ambient noise tomography

AN:

Ambient noise

CB:

Cuddapah basin

CCP:

Common conversion point

CGGT:

Chhotanagpur granitic gneissic terrain

CMB:

Core-mantle boundary

DC:

Dharwar craton

DVP:

Deccan volcanic province

EDC:

Eastern Dharwar Craton

EGMB:

Eastern ghat mobile belt

H–D:

Hales discontinuity

IGP:

Indo-gangetic plains

IOGL:

Indian ocean geoid low

KRZ:

Kachchh rift zone

LVL:

Low-velocity layer

LVZ:

Low-velocity zone

MHT:

Main Himalayan thrust

MTZ:

Mantle transition zone

RF:

Receiver function

SC:

Singhbhum craton

SGT:

Southern granulite terrain

SW:

Surface wave

WDC:

Western Dharwar Craton

WG:

Western ghats