Far away landslide detection A mass wasting and flood event on 7 February 2021 in Uttarakhand, India, killed more than 200 people and damaged two hydropower plants. Cook et al . discovered that teleseimic signals from the beginning of this event were recorded at different stations on a regional seismic network in northern India. The signals were observed up to 100 kilometers from the disaster and demonstrate the potential for these far-away monitoring stations to be useful for early warning. This discovery suggests a different way to monitor such remote Himalayan valleys for mass wasting hazards. —BG

We present a high‐resolution 3‐D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along ~14,000 paths are made in a broad frequency range (16–250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere‐Asthenosphere boundary (LAB) derived from seismic velocity Vsv reductions at depth reveal large variations (120–250 km) beneath the different cratonic blocks. The lithospheric thickness is ~120 km in the eastern Dharwar, ~160 km in the western Dharwar, ~140–200 km in Bastar, and ~160–200 km in the Singhbhum Craton. The thickest (200–250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.

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Abstract.The issue of whether subduction is still active in the India-Burma plate boundary zone has been rather controversial. While the presence of an eastward dipping Indian lithospheric slab is undisputed, different opinions have been voiced regarding the continuance of subduction at present. Analysis of the Harvard CMT data in comparison with major subduction zones of the world demonstrates that the Burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down-dip direction. We propose a major right-lateral shearing of the Indian plate along with its subducted slab past the Burmese plate in the NNE direction.

S U M M A R YThe Koyna-Warna region in western India is the best example of reservoir triggered seismicity. The world's largest triggered earthquake of magnitude 6.3 occurred in Koyna in 1967, followed by several moderate to small earthquakes ever since. A digital seismograph network deployed for seismic monitoring during 2005 August to 2008 December has indicated a shift in concentration of seismicity towards south in the Warna region including a new zone of seismic activity to the southwest. During the observation period 13 earthquakes of magnitude 4 and greater have occurred of which 11 occurred near the Warna region while only two occurred in Koyna. In this study we modelled broad-band waveform data of six of these earthquakes near Warna using waveform inversion approach. Initially a new velocity model was determined using a joint hypocentral determination approach that simultaneously solves for the velocity structure as well as the hypocentral parameters. A trap thickness of 1.2 km with a P-wave velocity of 4.40 km s −1 and an upper crustal layer down to 10 km with a velocity of 5.96 km s −1 are obtained. The new model not only provides the minimum residual error for the traveltimes, but consistently provides the least mismatch error in the waveform inversion of all the events, performing better than any of the previously determined velocity models. In general, focal mechanisms of normal type with NS to NNW-SSE oriented fault planes are obtained for all these events that are correlated with probable faults inferred from satellite images and aeromagnetic anomalies. Focal depths in the range of 5-6 km are obtained for earthquakes in the Warna region based on the sensitivity of whole waveform inversion at local distances. It is felt that joint inversion of waveform data of several earthquakes with a wider spatial distribution, along with the velocity structure would help in precisely characterizing the faulting mechanism and seismogenic depth range for the Koyna-Warna region in future.

Stress inversion of focal mechanism data in the Burmese arc region indicates distinct stress fields above and below 90 km along the subducted Indian lithospheric slab. In the upper part, the σ1 and σ3 axes trend NNE and ESE respectively, in conjunction with the ambient stress field of the Indian plate. However, in the lower part of the slab there is no preferred orientation of the σ1 or σ2 axes, but a very well defined σ3 axis is observed, that trends steeply in the down‐dip direction. It is inferred that while the upper part is governed by the NNE oriented horizontal plate tectonic forces, the lower part is governed entirely by tensile forces due to gravitational loading on the subducted slab. A model of attempted slab detachment at the base of the lithospheric contact zone is suggested, which is supported by results of high‐resolution seismic tomographic studies in this region.

Summary The only instance of a confirmed deep lower crustal earthquake occurrence in the Indian shield region has been that of the 1938 Satpura earthquake (M 6.3) of central India, reportedly at a depth of about 40 km. Moment tensor inversion of regional broadband waveform data of the 1997 May 21 Jabalpur earthquake (Mw 5.7) confirms yet another such earthquake at about 35 km depth in the central part of the Narmada‐Son lineament (NSL) zone. The study is based on a refined velocity model obtained using a traveltime grid search method. A reverse fault mechanism is obtained which, for a palaeo‐rift valley zone, indicates the possibility of reactivation of a pre‐existing fault under the influence of the ambient stress field due to the India–Eurasia plate collision forces. The occurrence of earthquakes at lower crustal depths, quite unusual for the Indian shield region, indicates a possible causative mechanism related to crust–mantle interaction. Based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the NSL, we suggest a model of stress accumulation due to horizontally elongated or elliptical, possibly serpentinized mafic intrusives in the lower crust, to explain the occurrence of deep earthquakes in the heart of the Indian shield.

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