GPS-derived Total Electron Content (TEC) is an integrated quantity; hence it is difficult to relate the detection of ionospheric perturbations in TEC to a precise altitude. As TEC is weighted by the maximum ionospheric density, the corresponding altitude (hmF2) is, generally, assumed as the perturbation detection altitude. To investigate the validity of this assumption in detail, we conduct an accurate analysis of the GPS-TEC measured early ionospheric signatures related to the vertical surface displacement of the Mw 7.4 Sanriku-Oki earthquake (Sanriku-Oki Tohoku foreshock). Using 3D acoustic ray tracing model to describe the evolution of the propagating seismo-acoustic wave in space and time, we demonstrate how to infer the detection altitude of these early signatures in TEC. We determine that the signatures can be detected at altitudes up to ~130 km below the hmF2. This peculiar behaviour is attributed to the satellite line of sight (LOS) geometry and station location with respect to the source, which allows one to sound the co-seismic ionospheric signatures directly above the rupture area. We show that the early onset times correspond to crossing of the LOS with the acoustic wavefront at lower ionospheric altitudes. To support the proposed approach, we further reconstruct the seismo-acoustic induced ionospheric signatures for a moving satellite in the presence of a geomagnetic field. Both the 3D acoustic ray tracing model and the synthetic waveforms from the 3D coupled model substantiate the observed onset time of the ionospheric signatures. Moreover, our simple 3D acoustic ray tracing approach allows one to extend this analysis to azimuths different than that of the station-source line.

Analysis of SKS/SKKS phases in the northeast India region provides the first results using digital data, from the southern side of the India‐Asia collision zone. Our analysis reveals detectable anisotropy in the study region, contrary to negligible anisotropy reported in southern Tibet. The direction of anisotropy is E‐W within the Himalaya and its foredeep, N‐S in the Indo‐Burma convergence zone and NE‐SW close to the Shillong plateau. While lithospheric deformation due to finite strain induced by collision seems to be the source of anisotropy in the Himalaya and its foredeep, an asthenospheric source is invoked as a causative for the mantle strain in the plate interior.

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The Proterozoic Eastern Ghats belt of India is often believed to be the ancient analogue of the present-day Himalayas. However, geological and geophysical signatures that can be traced and linked to the Eastern Ghats belt orogen due to a Precambrian collisional episode are sparse and evidence of such a geotectonic process in the deep lithosphere remains elusive. Utilizing the P and S receiver function imaging technique, we present depth signatures of this convergence event and its lateral extent. Approximately 2000 P and S receiver functions that predominantly sample the Eastern Dharwar craton-Eastern Ghats belt reveal the presence of two distinct westerly dipping interfaces at depths centered on 150 km and 200 km in the study region. Drawing analogy from similar tectonic settings of Proterozoic age and younger Paleozoic times around the globe, we interpret these boundaries to represent remanent structures fashioned by the collisional processes that affected this region. Recent geological, geochemical, and geochronological evidence from the region strongly favors interpretation of our delineated dipping structures as possible vestiges of a Proterozoic collision event that are preserved due to their coherent translation with the overlying lithosphere. Due to this long-lasting record of Proterozoic tectonics, our results add a complication to simple models of the Indian subcontinent in which relatively thin lithosphere underwent rapid transit during the Cretaceous.

[1] Images of the crust and mantle beneath northeast India obtained by 2D migration of $1000 broadband P-receiver functions clearly trace a northward dipping Moho from the Himalayan foredeep reaching depths up to 50 km further north beneath the Himalayan convergence zone. Also, these images reveal presence of largely coherent 410-km and 660-km discontinuities that conform to the IASP91 model. Marginal variations in the depth of the 410-km interface are observed, that appear region specific. The thickness of the mantle transition zone does not deviate significantly from a global average of $250 km. Interestingly, our results reveal consistent presence of a signal from an interface around 300 km. Origin of such a boundary, known as X-discontinuity and unrelated to the Lehmann discontinuity, is discussed. Possible presence of the X-discontinuity from the Indian region is reported here for the first time. Citation: