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Abstract:
The geoid is an equipotential surface that broadly mimics the mean sea level. The difference between the geoid and the reference spheroid at any location is referred to as a geoid anomaly. The geoid ‘highs’ (positive) or ‘lows’ (negative) are primarily associated with mass anomalies, thereby could offer important information about compositional and thermal properties in the Earth's interior. The maximum geoidal surplus (+85 m) is observed to the east of New Guinea whereas the largest deficit (−106 m) is observed in the Indian Ocean south of Sri Lanka – commonly known as the Indian Ocean geoid low (IOGL).
On a global geoid map, the IOGL anomaly covers an extensive circular area spanning >2000 km in diameter (Fig. 1). Several different hypotheses have been put forth to explain this enigmatic anomaly. These include effects of isostatically uncompensated crust (Ihnen and Whitcomb, 1983), depression in the core-mantle boundary (Negi et al., 1987), slab graveyards in the mantle (Spasojevic et al., 2010), anomalous variations in the mantle transition zone (Reiss et al., 2017; Rao et al., 2020) and presence of a very low-velocity material arising from the African large low shear velocity province (LLSVP) or simply known as the African superplume (Ghosh et al., 2017).
Most of these hypotheses rely upon either very sparse seismological observations, numerical modelling or remote sensing data. Global seismic tomographic models provide first-order information about the Earth's interior (Simmons et al., 2010, Simmons et al., 2012, Simmons et al., 2015). However, the uneven distribution of seismological networks has stymied production of high-resolution sub-surface images. In search of concrete causative mechanisms behind the IOGL anomaly, deep seismological observations from the Indian Ocean have been awaited for a long time. Between 2015 and 2020, the Ministry of Earth Sciences (MoES) India deployed a focused linear broadband passive seismological array comprising 17 ocean bottom seismometers (OBS) for two successive seasons comprising 14 months each (Fig. 1). These OBS stations thus continuously recorded local and teleseismic events for >28 months (Pandey, 2017). Besides, some recent studies also carried out active OBS deployments in this region to evaluate crustal and upper mantle structures (Pandey et al., 2022; Ningthoujam et al., 2022; Altenbernd-Lang et al., 2022).
This special issue was conceived to present a compilation of new field observations as well as numerical modelling studies to infer potential mass anomalies within the crust and mantle beneath the IOGL region. A collection of nine papers presented in this volume explore the role of causative sources at varying depths to explain the IOGL anomaly. In summary, scientific contributions in this special issue suggest minimal crustal contributions towards the spectacular IOGL anomaly. On the other hand, new seismological studies suggest that the IOGL anomaly can be reasonably explained by a combination of positive mass anomalies in the lower mantle and/or negative mass anomalies in the upper mantle. Varied outcomes further stress upon the need to carry out more long-term seismological observations in order to image precise mantle structure beneath the IOGL region.
