• M Radhakrishna

      Articles written in Journal of Earth System Science

    • An analysis of Indian tide-gauge records

      P K Das M Radhakrishna

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      The paper presents an analysis of four Indian tide-gauge records. The stations were: Bombay, Madras, Cochin and Vishakhapatnam (Vizag). They were selected because of their reliability.

      There was no evidence of a monotonic rising trend at all four stations. The test by Mann and Kendall (loc. cit.) showed a rising trend at Bombay from 1940 to 1986 and at Madras from 1910 to 1933. The other records did not reveal a significant trend.

      The records reveal evidence of long-period cycles (50–60 year period), with shorter cycles (4.5 to 5.7-year period) riding on them. Spectral peaks corresponding to shorter cycles passed a false alarm probability test at 95% level of significance. The peaks were identified by computing periodograms and by maximizing the entropy of the time series.

      ARIMA models suggest a third order autoregressive model for Bombay and Madras (1953–1986). The remaining records only had a moving average component.

      Monthly tide-gauge data of Bombay reveal a 13.4-month cycle which was statistically significant. This was close to the 14.7-month Chandler wobble. But, an interaction between a 13.4-month and an annual cycle could not fully explain the observed short period cycles.

      Finally, the paper summarizes evidence to indicate that a pattern exists between fluctuations of monsoon rain and relative sea level at Bombay.

    • Trends and the pole tide in Indian tidegauge records

      P K Das M Radhakrishna

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      This paper studies tidegauge records of stations on the Indian coastline. An analysis of trends did not reveal a monotonie trend. Trends were seen for limited periods at only five of the eight stations on the Indian coast. A spectral analysis of annual records produced evidence of long period cycles with shorter cycles riding on them. The shorter cycles had a period of 5.0 years. The spectra of monthly records revealed evidence of a pole tide and an annual cycle. The amplitude of the pole tide was estimated to be around 7.5 mm. This was larger than the equilibrium tide. A spectral analysis of monthly rainfall at Bombay, a station on the Indian west coast, also showed a 13.9 month cycle and a (3,1,0) autoregressive model. But the coherence between monthly rainfall and relative sealevel fluctuations was low.

    • Development of the negative gravity anomaly of the 85°E Ridge, northeastern Indian Ocean – A process oriented modelling approach

      K M Sreejith M Radhakrishna K S Krishna T J Majumdar

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      The 85°E Ridge extends from the Mahanadi Basin, off northeastern margin of India to the Afanasy Nikitin Seamount in the Central Indian Basin. The ridge is associated with two contrasting gravity anomalies: negative anomaly over the north part (up to 5°N latitude), where the ridge structure is buried under thick Bengal Fan sediments and positive anomaly over the south part, where the structure is intermittently exposed above the seafloor. Ship-borne gravity and seismic reflection data are modelled using process oriented method and this suggest that the 85°E Ridge was emplaced on approximately 10–15 km thick elastic plate (Te) and in an off-ridge tectonic setting. We simulated gravity anomalies for different crust-sediment structural configurations of the ridge that were existing at three geological ages, such as Late Cretaceous, Early Miocene and Present. The study shows that the gravity anomaly of the ridge in the north has changed through time from its inception to present. During the Late Cretaceous the ridge was associated with a significant positive anomaly with a compensation generated by a broad flexure of the Moho boundary. By Early Miocene the ridge was approximately covered by the postcollision sediments and led to alteration of the initial gravity anomaly to a small positive anomaly. At present, the ridge is buried by approximately 3 km thick Bengal Fan sediments on its crestal region and about 8 km thick pre- and post-collision sediments on the flanks. This geological setting had changed physical properties of the sediments and led to alter the minor positive gravity anomaly of Early Miocene to the distinct negative gravity anomaly.

    • Gravity anomalies, crustal structure and rift tectonics at the Konkan and Kerala basins, western continental margin of India

      Sheena V Dev M Radhakrishna Shyam Chand C Subrahmanyam

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      Litho-stratigraphic variation of sedimentary units constructed from seismic sections and gravity anomaly in the Konkan and Kerala basins of the western continental margin of India (WCMI) have been used to model processes such as lithospheric rifting mechanism, its strength, and evolution of flank uplift topography that led to the present-day Western Ghats escarpment. Based on the process-oriented approach, two lithospheric models (necking and magmatic underplating) of evolution of the margin were tested. Both, necking and underplating models suggest an effective elastic thickness (Te) of 5 km and 10 km along Konkan and Kerala basins, respectively and a deep level of necking at 20 km at both basins. Model study suggests that the necking model better explains the observed gravity anomalies in the southern part of the WCMI. A synthesis of these results along with the previously published elastic thickness estimates along the WCMI suggests that a low-to-intermediate strength lithosphere and a deeper level of necking explains the observed flank-uplift opography of the Western Ghats. Process-oriented gravity modeling further suggests that the lateral variations in the lithospheric strength, though not very significant, exist from north to south within a distance of 600 km in the Konkan and Kerala basins along the WCMI at the time of rifting. A comparison with previous Te estimates from coherence analysis along the WCMI indicates that the lithospheric strength did not change appreciably since the time of rifting and it is low both onshore and offshore having a range of 5–15 km.

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