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Shear wave velocity structure beneath the eastern Indian Ocean from Rayleigh wave dispersion measurements

Rehman Haseeb, Sharma Jyoti, Subrahmanyam Mangalampalli, Begum Shaik Kareemunnisa

Acta Geophysica

2023

Vol. 71, no. 3

1187--1201

The Eastern Indian Ocean is a tectonically and geodynamically active region that has experienced deformations due to rifting, uplifting, and plume activity. The earlier Rayleigh wave studies in the East Indian Ocean were mainly focused on the structure of the Bay of Bengal, Ninety East Ridge, and Broken Ridge. The structure of other region of the East Indian Ocean is not much explored. In the present study, Rayleigh wave dispersion analysis is performed to observe the signatures of upper mantle deformation in terms of shear wave velocity of the East Indian Ocean using global search method. The fundamental mode Rayleigh wave group velocities are estimated between 15 and 100 s using the multiple filter technique. The group velocities of the raypaths that traverse the same region are clustered (E1–E8) to get an average dispersion curve. Using a genetic algorithm, each cluster's group velocities are inverted for shear velocity structure. The observed dispersion curve of E6, E7, and E8 indicates the lower group velocities between 35 and 100 s relative to E1, E2, E3, E4, and E5, with an average variation of about 0.07–0.18 km/s. The crustal thickness obtained in the study region is ~ 26 km and is due to the increased thickness of the lower crust (9.1–12.4 km) having Vs 3.95–4.04 km/s. The theoretical Vs have been calculated for serpentinite rock at uppermost lithospheric conditions and found to be similar to the Vs of the lower crust in the present study. Hence, it is assumed that unusual crustal thickness is due to the progressive development of the upper lithosphere formation (Ultramafic rock) into material (serpentinite rock) with crustal-like shear velocity or moderately lower than sub-Moho shears velocity. The undeformed lithosphere is evidenced by the high-velocity (Vs 4.62–4.77 km/s) layer beneath the Moho, whose thickness ranges from 41.3 to 51.6 km. The high-velocity lithosphere is followed by a low-velocity zone that extends up to 160 km; however, the variation in Vs (4.57–4.31 km/s) indicates that the low-velocity zone is deformed.

A short note on the natural and anthropogenic variations in the water storage changes at Visakhapatnam, Andhra Pradesh, India

Mathews Gathala Vinod, Rajesh Rekapalli, Begum Shaik Kareemunnisa

Acta Geophysica

2022

Vol. 70, no 4

1847--1854

We analyze the periodic and stochastic/random dynamics in the water storage changes at Visakhapatnam, Andhra Pradesh, India. We used time-variable gravity data in terms of Equivalent Water Thickness (EWT) measured from Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellite missions for the period 2002 to 2021 along with average rainfall data to assess the natural and anthropogenic impacts on water storage changes. We employ Wavelet Spectrum and Singular Spectrum Analysis (SSA) methods to analyze the non-stationary variation of spectral power and principal components. The water storage in the study area shows a significant positive trend with a peak correlation of 0.52 with the rainfall data at a lag of 3 months. The first, second, third, fourth, and fifth principal components depicting the monotonic trend and oscillations together contribute 69.48% to the total water storage changes. The wavelet spectrum of the SSA reconstructed signal from the first four principal components revealed non-stationary annual and 1.3 to 8 years periodicities associated with natural solar and El-Nino Southern Oscillations (ENSO) respectively. The phase plot of the residual signal of ~ 30% variance suggests the random dynamics. Thus the study suggests: (i) an increasing groundwater trend in the study area, (ii) nearly 70% of the water storage changes are linked with natural solar and ENSO variations, and (iii) 30% of water storage changes are with random dynamics possibly linked with anthropogenic activities and catastrophic climatic episodes of shorter duration at Visakhapatnam, Andhra Pradesh, India.