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Wall-Palmer D.,University of Plymouth | Coussens M.,UK National Oceanography Center | Talling P.J.,UK National Oceanography Center | Jutzeler M.,UK National Oceanography Center | And 34 more authors.
Geochemistry, Geophysics, Geosystems

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass-wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high-resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density-currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption-fed and mass-wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands. © 2014. American Geophysical Union. All Rights Reserved. Source

Singh A.,Indian Institute of Technology Kharagpur | Ravi Kumar M.,National Geophysical Research Institute andhra Pradesh | Srinagesh D.,National Geophysical Research Institute andhra Pradesh | Chadha R.K.,National Geophysical Research Institute andhra Pradesh
Geochemistry, Geophysics, Geosystems

We assemble P and S waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the Indian plate from the southern tip of India to the Himalayan collision belt and generate an accurate data set of 52,050 P and 30,423 S arrival times through the multichannel cross-correlation approach. These traveltimes are then inverted to obtain 3-D P and S velocity structures of the subcontinent at a 2° × 2° lateral resolution. The heterogeneous nature of the Indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale. The key cratonic segments of the Indian shield are characterized by pockets of high velocity anomalies (∼3%) at shallow depths (<300 km), with the diamondiferous regions like Wajrakarur revealing high shear wave anomalies down to ∼300 km. In contrast to the southern Deccan volcanic province (DVP), the northwestern DVP is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of Deccan volcanism which was facilitated by the reactivation of the rift systems. Key Points The first 2° × 2° resolution tomographic image using data from 413 stations Highly heterogeneous nature of the Indian lithospheric mantle Evidence for preservation of high velocity roots beneath kimberlite provinces © 2014. American Geophysical Union. All Rights Reserved. Source

Tao Y.,ConocoPhillips | Sen M.K.,University of Texas at Austin | Sen M.K.,National Geophysical Research Institute andhra Pradesh
Journal of Geophysics and Engineering

A practical strategy to implement frequency-domain full waveform inversion (FWI) with a scattering-integral approach is presented in this paper. A typical implementation of FWI is the adjoint-state approach, where the data residual at receiver locations are back-propagated into the media to form the gradient. With the scattering-integral (SI) approach, the gradient is formed with a data-weighted Fréchet kernel, which involves calculation of Green's functions for all source and receiver locations. The computational cost and memory requirement of this approach are comparable to the adjoint-state approach using a shot-profile implementation in the frequency-domain. SI implementation can be more efficient if the number of sources is larger than the number of receivers. Sensitivity analysis can also be easily applied without additional cost because the Fréchet kernel has been calculated and stored during the gradient calculation process. We demonstrate the effectiveness of this approach with synthetic data. © 2013 Sinopec Geophysical Research Institute. Source

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