Figure 2. Shear velocity structure of
the upper
mantle along two profiles, obtained from joint inversion of the
surface wave
dispersion data and receiver functions. (From Jana et al., 2022) ( Reprinted from Gondwana Research, 111, Niptika Jana, Chandrani Singh, Arun Singh, Tuna Eken, Arun Kumar Dubey, Abhisek Dutta, Arun Kumar Gupta, Lithospheric architecture below the Eastern Ghats Mobile Belt and adjoining Archean cratons: Imprints of India-Antarctica collision tectonics, 209-222, Copyright (2022), with permission from Elsevier.)
Das and Rai (2019) generated a 3-D shear velocity image of the crust
through joint inversion of the P- RFs and Rayleigh wave group velocity
dispersions, derived from cross correlation of ambient noise, to study
the linkage and the boundary between the Dharwar craton and the Southern
Granulite Terrain (SGT). The study delineated a 10–15 km thick high
shear velocity layer in the lowermost crust of the southern part of the
WDC continuing into the SGT up to 40 km beyond the E-W trending southern
limb of the Palghat-Cauvery shear zone, suggesting that the Dharwar
craton continues further south of the mapped orthopyroxene boundary to
the Palghat shear zone.
Vashishtha et al. (2022) used surface wave data from the April 25, 2015,
Nepal earthquake (Mw 7.8) and its aftershocks, recorded at eleven
stations in India to estimate group velocities of both Love and Rayleigh
waves. All stations are at regional distances from the earthquake
sources. It is observed that the group velocities for both Love and
Rayleigh waves obtained from the mainshock data are lower than those
obtained from aftershock data for stations located towards W and SW of
the earthquake source region. Such a variation of group velocity
obtained from mainshock and aftershock data for different stations may
be due to source directivity for the mainshock affecting the source
group time, which in turn affects the travel time of surface waves at
different periods.
2.2 Himalaya
Kanna and Gupta (2020) studied the crustal structure of the Garhwal
Himalaya along a linear profile, using regional travel times and
receiver function analysis. Their receiver function modeling showed a
prominent intra-crustal low velocity layer with a flat–ramp–flat
geometry beneath the Main Central Thrust zone and a variation in the
Moho depth from \(\sim\) 45 km beneath the Sub Himalaya to \(\sim\) 58 km to the south
of the Tethys Himalaya. A similar study in the NW Himalaya and
Ladakh-Karakoram indicates that the Moho depth increases from
\(\sim\) 46 km below the Gangetic Plain to \(\sim\) 78
km at the southern flank of the Karakoram Fault. Several intra-crustal
low-velocity layers were seen and the MHT is mapped as an LVL (Kanna and
Gupta, 2021).
Gupta S., et al. (2022) examined the P-wave velocity (Vp) and Vp/Vs
variations using local earthquake arrival time measurements recorded
over 41 seismic stations operated during November 2006 to June 2008 in
the Kumaun–Garhwal Himalaya. In the 0–25 km depth range, the Vp and
Vp/Vs varies between 4.8–6.8 km/s and 1.55–1.85, respectively and show
a heterogeneous structure in the upper-mid crust. The seismic images
exhibit signatures of unconsolidated sediments, close to the Main
Frontal Thrust, and Klippes (Lansdowne and Almora) in the uppermost
crust. In the upper-mid crust, the observed low Vp and high Vp/Vs
(1.82–1.85) along with available conductivity values indicate
saline-rich aqueous fluid and partial melt. Using the inferred crustal
composition and constraints from earlier information, they proposed the
bottom of these fluid zones as the top of the underthrusting India
Plate, which shows a flat-ramp-flat geometry at 16–21 km depth. The
earthquakes of moderate and smaller magnitude mostly occur in the
fluid-rich zone above the mid-crustal ramp in the underthrusting Indian
Plate. Madhusudhan et al. (2022) calculated the uppermost mantle seismic
(Pn and Sn) velocities using 12 regional earthquakes recorded by 33
digital broadband seismological stations from the Gangetic plain to the
Tethys Himalaya along a \(\sim\) 160 km profile and showed that
the Moho dips in the north direction with an overall dip angle of 3.43\(^{\circ}\)-
4.2\(^{\circ}\) in the Eastern Kumaun Himalaya.
Mandal et al. (2021c) imaged the lateral variations in the Moho depths
and average crustal composition across the Kumaun–Garhwal Himalaya,
through H-K stacking of 1400 radial P-RFs from 42 broadband stations.
The modeled Moho depth and average crustal Vp/Vs values vary from 28.3
to 52.9 km and 1.59 to 2.13, respectively. They also mapped three
NNE-SSW trending transverse crustal blocks in Uttarakhand Himalaya that
extend down to the lithosphere-asthenosphere boundary. Local earthquake
tomography of the Kumaun-Garhwal Himalaya (Mandal et al., 2022b; Gupta
S., et al., 2022) shows a low velocity (Vp, Vs), high Vp/Vs mid-crustal
layer, which was identified as the Main Himalayan Thrust associated with
metamorphic fluid/partial melt.
The Himalaya-Karakoram-Tibet region characterizes a unique setup of
crustal and upper mantle structure related to present-day geodynamics.
Kumar N., et al. (2019) performed surface wave tomography studies that
reveal highly variable shear wave velocity structure indicating
signatures of underthrusting of the Indian plate beneath Eurasia. The
study reveals a NE-dipping Moho with its depth increasing from
\(\sim\) 40 km beneath the frontal Himalaya to 70-80 km below
the collision zone. Low near-surface seismic velocities indicate thick
sediments (5-6 km) in the Indo-Gangetic plains. Broader low-velocity
zone at mid-crustal depth beneath the southern parts of Tibet and
Karakoram fault is due to the presence of partial melting and/or aqueous
fluids.
Kumar V., et al. (2022) used ambient noise cross-correlations from 530
seismological stations along with surface wave observations from 1,261
earthquakes to image the crust beneath the western Himalaya-Asia
convergence zone encompassing western Himalaya-western
Tibet-Ladakh-Karakoram-Pamir-Hindu Kush. The seismological data from the
PASSCAL experiments, the Global Seismograph Network, experiments in
Kyrgyzstan, Kazakhstan, Tajikistan, China, Nepal and western Tibet,
French deployments in western Kunlun and Kazakhstan and Indian
deployments in the western Himalaya were used in this study that
resulted in 22,726 inter-station Rayleigh wave dispersion measurements
in the period band of 5 to 60 s at a horizontal resolution of less than
0.5\(^{\circ}\)×0.5\(^{\circ}\). The 3-D shear wave velocity image revealed that the northern
limit of the Indian crust extends beyond the Qiangtang block in western
Tibet (77\(^{\circ}\)- 82\(^{\circ}\)E) and till the central Pamir farther west. The study
suggests a continuation of LVZs across the Karakoram Fault at a depth
below 20 km, indicating the fault’s upper crustal depth extent.
The seismicity in the Kumaun Himalaya is concentrated in the Chiplakot
Crystalline Belt (CCB). WIHG conducted a passive seismological study
using receiver functions (RFs) along a profile (see Fig.1 of Hazarika et al., 2021). The H-K
stacking analysis of RFs of teleseismic earthquakes reveals a
significantly high value of the Poisson’s ratio (\(\sim\) 0.28) in the Dharchula
region of the CCB which is coincident with the earthquake cluster (Hajra
et al., 2019). The RF inversion and Common Conversion Point (CCP)
stacking imaging reveals a variation of crustal thickness from \(\sim\)38 km in
the Indo-Gangetic Plain to \(\sim\) 42 km near the Vaikrita Thrust (see Fig.6 of Hazarika et al., 2021). A
ramp (\(\sim\) 20\(^{\circ}\)) structure on the MHT is revealed beneath the CCB. The
spatial and depth distribution of seismicity pattern beneath the CCB and
presence of steep dipping imbricate faults inferred from focal mechanism
solutions suggest a Lesser Himalayan Duplex structure in the CCB above
the MHT ramp (Hazarika et al., 2021; Hajra et al., 2021).
Medved et al. (2022) obtained 3D models of the crust and uppermost
mantle beneath the NW Himalaya down to a depth of 120 km by local
earthquake seismic tomography using data of India Meteorological
Department (IMD) complemented by the Global International Seismological
Centre (ISC) Catalogue. Their results suggest that the Indian Plate not
only underthrusts northwards below the Himalaya but also bends westwards
as it gets closer to the Hindukush Region. A peculiar feature of the
model is a high-velocity anomaly in the Kaurik Chango Rift, interpreted
as a remnant of the oceanic crust, left after the closure of the
Indo-Tethys Ocean. In the seismically active Delhi-Haridwar Ridge, a
low-velocity upper crustal layer is possibly associated with sediments
of the Indo-Gangetic Plain and with a large number of fault structures.
Mir et al. (2021) estimated the shear wave velocity structure, together
with Moho depths for the NW Himalaya, Hindu Kush and the Pamirs at a
potential resolution of 0.5\(^{\circ}\)×0.5\(^{\circ}\) and at 1\(^{\circ }\)×1\(\)\(^{\circ }\) in the
surrounding area (Fig.3), by inverting fundamental mode Rayleigh wave
group velocities calculated from regional earthquake (Δ \(\le\) 2500 km) data,
and also from their joint inversion with teleseismic receiver functions
at 38 out of the 59 broadband stations in the region. The results
illuminate a) the deeper root zone structures of the main geomorphic
features, b) a pervasive low velocity layer (Vs \(\sim\) 3.1
km/s) at \(\sim\) 30 km depth beneath the NW Himalaya. Another
notable result is the distinctly shallower Moho beneath the Kashmir
Himalaya apparently segmented by arc-normal shear zones that cross the
rupture zones of the 1905 Kangra and the 2005 Kashmir earthquakes, in
turn, marked by the current epoch seismicity.
A high-resolution seismic image of the crust beneath the Arunachal
Himalaya is documented by Singh A., et al. (2021), using RF analysis of
data from 32 broadband seismic stations deployed in the Arunachal
Himalaya during 2010-2016, along with data from the HIMNT, SIKKIM,
Hi-CLIMB, and GANSSER networks. Their results reveal lateral variations
in the crustal structure with the Moho depth varying from 40-60 km. They
also observe a comparatively less complex crust, absence of a prominent
mid-crustal ramp, a highly deformed layer running parallel to the Main
Himalayan Thrust, and an intermittent anisotropic low velocity layer in
the middle crust.
In a recent study (Ravi Kumar et al., 2022), receiver function images of
the detachment, mid-crustal ramp and the Moho of the underthrusting
Indian plate along four profiles in the Arunachal Himalaya are
documented (Fig.4). The results reveal a clear Moho signature in the
depth range of 40 to 65 km, with the detachment mapped in the depth
range of \(\sim\) 10 to 20 km. A mid-crustal ramp can be traced
in the higher Himalaya especially along one profile. Singh A., et al.
(2021) imaged the crust beneath the Arunachal Himalaya using teleseismic
receiver functions. A mechanically weak middle crust beneath Arunachal
Himalaya, highly deformed layer parallel to MHT, and comparatively less
complex crust beneath Arunachal than Nepal and Sikkim are some important
observations that have been reported in this study.