Upper Mantle Shear Velocity Structure below Northwestern India Based on Group Velocity Dispersion

Summary The Discovery/Gofar transform faults system is associated with a fast-spreading center on the equatorial East Pacific Rise. Most previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multi-dimensional stacking and tracing to determine the average fundamental mode Rayleigh wave phase-velocity dispersion curve (period band 20–100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh wave phase-velocity dispersion (period band 2–25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2–100 s) and the first-higher mode (period band 3–7 s) Rayleigh wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and 200-km depth of the final model is about 4.18 km/s. There exists an ∼5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km/s. Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15–60 km depth) is 4.03–4.23 km/s (∼10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5–5 per cent) mainly occurs in the shallow upper mantle zone beneath this young (0–2 Myr) oceanic region. In the deeper portion (60–200 km depth), the Vs of a weak LVZ is 4.15–4.27 km/s (∼5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary (LAB) with ∼15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper mantle lithosphere and asthenosphere in this active ridge-transform fault region.

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Surface wave tomography of the arctic from seismic Rayleigh and Love wave group velocity dispersion data

Физика земли ◽

10.31857/s0002-33372019358-70 ◽

2019 ◽

pp. 58-70

Author(s):

L. V. Seredkina

Keyword(s):

Surface Wave ◽

Group Velocity ◽

Upper Mantle ◽

Velocity Dispersion ◽

Deep Structure ◽

Group Velocity Dispersion ◽

Dispersion Curves ◽

The Arctic ◽

Crust And Upper Mantle ◽

Earth's Crust

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

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Inversion of Rg waveforms recorded in southern Spain

Bulletin of the Seismological Society of America ◽

10.1785/bssa0870040847 ◽

1997 ◽

Vol 87 (4) ◽

pp. 847-865

Author(s):

Manuel Navarro ◽

Victor Corchete ◽

José Badal ◽

José A. Canas ◽

Luis Pujades ◽

...

Keyword(s):

Group Velocity ◽

Velocity Structure ◽

Group Velocity Dispersion ◽

Shear Velocity ◽

Dispersion Analysis ◽

Southern Spain ◽

Mountain Range ◽

Low Velocity ◽

Dispersion Data ◽

Group Velocities

Abstract Group velocity dispersion measurements of Rg waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of Almería (southern Spain). In principle, the usable frequency range of 250 to 2000 mHz allows determination of structures to depths of about 4 km. For this purpose, the main operations are a detailed dispersion analysis of high-frequency Rayleigh waves propagating along very short paths and the inversion of Rg-wave group velocities. A total of 21 seismic events were studied. These events had small magnitudes (2.0 to 2.5 approximately) and very shallow focal depths (about 100 m) and were taken from a set of 214 events that occurred in 1991 during a Spanish-Italian seismic experiment. The events were recorded at seven single-component stations belonging to the Regional Seismic Network of Andalucía at approximate distances of between 15 and 57 km from the source. These events were grouped into six seismic sources according to specific criteria. We used digital filtering techniques providing a significant improvement in signal-to-noise ratio to determine ray-path group velocities, and we inverted dispersion data via generalized inversion. In order to obtain refined dispersion data, we have carried out a further regionalization of group velocities, and thus six small subregions have been resolved in group velocities. The highest group velocity values, from 1.93 to 2.25 km sec−1, correspond to the Filabres mountain range, which is an area containing materials of the Nevado-Filábride complex of Paleozoic and Triassic age. On the other hand, low velocity values, between 1.39 and 1.56 km sec−1, correspond to the Alhamilla mountain range, which belongs to the Alpujárride complex and contains conglomerates of the Cambrian and Tortonian periods. The velocities obtained for the neogene-quaternary basin of the Andarax river, with materials of the Tortonian and Pliocene periods, are also very low, between 1.35 and 1.68 km sec−1. We inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km. According to the regional Earth models that we obtained, we find clear variations in velocity both laterally and vertically for several zones with different composition. The Filabres mountain range shows high shear velocity values: 2.14 to 2.83 km sec−1. In the opposite end, we have the Andarax basin that presents the lowest shear velocity values, consistent with its sedimentary structure: 1.56 to 2.55 km sec−1. Intermediate shear-wave velocities characterize the remaining regions: the Tabernas-Sorbas basin, the Gádor mountain range, and the volcanic region of Nijar-Cape of Gata. Although the relationship between lateral changes in Rg dispersion and geologic structure may not be straightforward, in this study, we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations.

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