Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Sujit Dasgupta ¹

Affiliations
1 Geological Survey of India, Banamalipur, Agartala 799001, IN

Abstract

Roundness of clastics is often correlated with other physical parameters. In such correlation the effects of various other factors are to be taken into account. Point bar samples from River Tista were studied to show such effects in roundness-size, roundness-distance and roundness-sphericity correlation. It is found that roundness (i) decreases with decrease of size, (ii) increases with more transportation, (iii) increases with increase of sphericity for granitic clasts and (iv) decreases with increase of sphericity for schists.

Full Text

   

Sujit Dasgupta ¹, Auditeya Bhattacharya ¹, Tapan K. Jana ¹

Affiliations
1 Geological Survey of India, 27 Jawaharlal Nehru Road, Calcutta-700016, IN

Abstract

Temporal occurrence of earthquakes of magnitude ≥ 5.5 and 6.0 was statistically analysed for the entire eastern-northeastern India which is seismically active. High seismicity in the region is related to plate margin activity in the Himalaya and Burmese arc. Non-stationary Poisson distribution provides a good fit to analyse earthquake sequence for the entire area of eastern-northeastern India and two subareas within it, viz., the Himalaya and the Burmese arc. For the entire area there is a 90% probability for the occurrence of at least one earthquake of magnitude (Mb₀)≥ 6.0 in any time window of 5 years, while the probability is 98% for an event of Mb₀ 5.5 in any two years time slot. Similarly for the Himalayan tectonic domain, the probability is 70% and for the Burmese arc it is 90% for at least one earthquake of Mb≥ 6.0 within a recurrence period of ten years. As earthquakes in the region above the threshold magnitudes of 5.5 and 6.0 follow a poisson distribution, probability density function maps for both the threshold magnitude were prepared for 1°x1° blocks with rate of occurrence(λ) for each 0.5° overlapping grid. These maps are used to calculate p~rcent probabilities with different return periods. Seismic hazard maps with percent probabilities for the occurrence of at least one earthquake with return periods of 20 and 50 years are presented for Mb₀≥6.0 and with return periods of 5 and 10 years for Mb₀≥5.5.

Keywords

Poisson Distribution, Probability Density Function, Seismic Hazard, Eastern-Northeastern India.

Full Text

     

Sujit Dasgupta ¹

Affiliations
1 Geological Survey of India, 27, Jawaharlal Nehru Road, Kolkata - 700 016, IN

Abstract

No Abstract.

Full Text

   

Basab Mukhopadhyay ¹, Anshuman Acharyya ¹, Manoj Mukhopadhyay ², Sujit Dasgupta ¹

Affiliations
1 Geological Survey of India, Central Headquarters, 27 J. L. Nehru Road, Kolkata - 700016, IN
2 Department of Geology & Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA

Abstract

An extraordinarily strong and persistent earthquake swarm (Andaman swarm 2005) originated in the Andaman back-arc following the aftershock sequences of the 26 December 2004 Sumatra earthquake. The swarm (n = 651, mb_max= 5.9) came mainly in two phases: January 26-31 and Feb. - Aug. 2005, in an area of size 90 x 40 km², at the centre of which lies a broad bathymetric depression and high gravity zone. The swarm demonstrates a complex faulting series, initially the strike-slip motion followed by normal faulting in repetitive sequences, whose representative fault planes orient at high angle to the regional faults. The swarm character as well as the distribution of stress-axes and their correlation to tectonic features lends speculation for formation of a nascent rift segment in NW-SE direction at the doorstep of the Sewell Seamount. The swarm has given rise to 21 episodes of rifting activities of variable time extent within 26 -31 January 2005. The r-t plots corresponding to the swarm data, modelled with variable hydraulic diffusivity (D) values 4, 6, 8 and 10 m²/s, suggest for excess pressure front from ascending magmatic fluid. This eventually heralded the rifting; causing pore pressure perturbations that propagated in accordance with known diffusion parabolic equations.

Keywords

Andaman Swarm 2005, Focal Mechanisms, Pore Pressure, R-T Plot, Nascent Rift.

Full Text

     

Sujit Dasgupta ¹, Manoj Mukhopadhyay ²

Affiliations
1 Geological Survey of India, 27, J.L. Nehru Road, Calcutta - 700 016, IN
2 Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004, IN

Abstract

The Barren Island volcano in central Andaman Sea erupted afresh on March 29, 1991, since its last eruptions in 1789 and 1832. Tectonic framework of the Barren Island volcano is delineated by a set of seismically mapped faults, including the West Andaman Fault, which. connects to the Semangko Fault in Sumatra. The morphology of Andaman Benioff zone as seen on a vertical section reveals a seismic gap at 90-110 km depth. Spatially it relates to the zone of current volcanism of the Barren and its neighbouring Alcock Seamount in a structurally depressed area. The seismic gap in the Benioff zone is interpreted as a zone of partial melting in the descending Indian Ocean lithosphere which prevents stress accumulation owing to the decreased viscosity. Best double couple solutions for the Barren Island earthquakes of October 10 and December 25, 1990, which preceded the volcanism suggest that rupturing through normal dip-slip faulting prevailed in the overriding Burma plate prior to the recent volcanism. Pronounced back-arc seismicity at shallower depths is seen further to the east and southeast of the Alcock Seamount; this relates to back-arc rifting under the Andaman Sea.

Keywords

Seismicity, Andaman, Barren Island, Volcanism.

Full Text

     

Basab Mukhopadhyay ¹, Manoj Mukhopadhyay ², Sujit Dasgupta ³

Affiliations
1 Geological Survey of India, Central Headquarters, 27, J.L. Nehru Road, Kolkata - 700 016, IN
2 Department of Geology and Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA
3 Geological Survey of India, Central Headquarters, Kolkata, IN

Abstract

The plate margin features defining the Arabian Sea Triple Junction (ASTJ) are: the Aden Ridge (AR), Sheba Ridge (SR) with their intervening Alula-Fartak Transform (AFT), Carlsberg Ridge (CR) and Owen Fracture Zone (OFZ). Exact nature of ASTJ is presently debated: whether it is RRF (ridge-ridge-fault) or RRR (ridge-ridge-ridge) type. A revised seismicity map for ASTJ is given here using data for a period little more than a century. "Point density spatial statistical criterion" is applied to short-listed 742 earthquakes (mb≥4.3), 10 numbers of spatio-temporal seismic clusters are identified for ASTJ and its arms. Relocated hypocentres help better constraining the cluster identification wherever such data exist. Seismic clusters actually diagnose the most intense zones of strain accumulation due to far field as well as the local stress operating at ASTJ. An earthquake swarm emanating from a prominent seismic cluster below SR provides an opportunity to investigate the pore pressure diffusion process (due to the active source) by means of "r-t plot". Stress and faulting pattern in the active zones are deduced from 43 CMT solutions. While normal or lateral faulting is characteristic for these arms, an anomalous thrust earthquake occurs in the triangular 'Wheatley Deep' deformation zone proximal to ASTJ. The latter appears to have formed due to a shift of the deformational front from OFZ towards a transform that offsets SR. Though ASTJ is still in the process of evolution, available data favour that this RRF triple junction may eventually be converted to a more stable RRR type.

Keywords

Revised Seismicity Map, Foreshock-Mainshock-Aftershock Sequence, Aspect Ratio and B-Values, Seismic Clusters, CMT Solutions, R-T Plot, RRR Plate Margin Kinematics.

Full Text

     

Sujit Dasgupta ¹, Basab Mukhopadhyay ², Manoj Mukhopadhyay ³

Affiliations
1 Ex Geological Survey of India, Kolkata, IN
2 Geological Survey of India, Central Headquarters, 27 J.L. Nehru Road, Kolkata-700016, IN
3 Department of Geology & Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451, SA

Abstract

The Burmese Arc seismic activity is not uniform for its ~ 1100 km length; only the Northern Burmese Arc (NBA) is intensely active. Six large earthquakes in the magnitude range 6.1 - 7.4 have originated from the NBA Benioff zone between 1954 -2011, within an area of 200 × 300 km² where the Indian plate subducts eastward to depths beyond 200 km below the Burma plate. An analysis on seismogenesis of this interplate region suggests that while the subducting lithosphere is characterized by profuse seismicity, seismicity in the overriding plate is rather few. Large earthquakes occurring in the overriding plate are associated with the backarc Shan-Sagaing Fault (SSF) further east. The forecasting performance of the Benioff zone earthquakes in NBA as forerunner is analysed here by: (i) spatial earthquake clustering, (ii) seismic cycles and their temporal quiescence and (iii) the characteristic temporal b-value changes. Three such clusters (C1- C3) are identified from NBA Benioff Zones I&II that are capable of generating earthquakes in the magnitude ranges of 7.38 to 7.93. Seismic cycles evidenced for the Zone I displayed distinct quiescence (Q_1, Q₂ and Q₃) prior to the 6^th August 1988 (M 6.6) earthquake. Similar cycles were used to forecast an earthquake (Dasgupta et al. 2010) to come from the Zone I (cluster C1); which, actually struck on 4 February 2011 (M 6.3). The preparatory activity for an event has already been set in the Zone II and we speculate its occurrence as a large event (M > 6.0) possibly within the year 2012, somewhere close to cluster C3. Temporal analysis of b-value indicates a rise before an ensuing large earthquake.

Keywords

Northern Burmese Arc (NBA), Precursor, Seismic Clusters, Seismic Cycles, Temporal Quiescence, Temporal B-Value.

Full Text