Constraining the active tectonic deformation of Andaman-Nicobar Arc in the background of December 26,2004 Great Sumatra-Andaman Earthquake

The Andaman-Nicobar Islands in the Bay of Bengal lies in a zone where the Indian plate subducts beneath the Burmese microplate, and therefore forms a belt of frequent earthquakes. Few efforts, not withstanding the available historical and instrumental data were not effectively used before the Mw 9.3 Sumatra-Andaman earthquake to draw any inference on the spatial and temporal distribution of large subduction zone earthquakes in this region. An attempt to constrain the active crustal deformation of the Andaman-Nicobar arc in the background of the December 26, 2004 Great Sumatra-Andaman megathrust earthquake is made here, thereby presenting a unique data set representing the pre-seismic convergence and co-seismic displacement.Understanding the mechanisms of the subduction zone earthquakes is both challenging sCientifically and important for assessing the related earthquake hazards. In many subduction zones, thrust earthquakes may have characteristic patterns in space and time. However, the mechanism of mega events still remains largely unresolved.Large subduction zone earthquakes are usually associated with high amplitude co-seismic deformation above the plate boundary megathrust and the elastic relaxation of the fore-arc. These are expressed as vertical changes in land level with the up-dip part of the rupture surface uplifted and the areas above the down-dip edge subsided. One of the most characteristic pattern associated with the inter-seismic era is that the deformation is in an opposite sense that of co-seismic period.This work was started in 2002 to understand the tectonic deformation along the Andaman-Nicobar arc using seismological, geological and geodetic data. The occurrence of the 2004 megathrust earthquake gave a new dimension to this study, by providing an opportunity to examine the co-seismic deformation associated with the greatest earthquake to have occurred since the advent of Global Positioning System (GPS) and broadband seismometry. The major objectives of this study are to assess the pre-seismic stress regimes, to determine the pre-seismic convergence rate, to analyze and interpret the pattern of co-seismic displacement and slip on various segments and to look out for any possible recurrence interval for megathrust event occurrence for Andaman-Nicobar subduction zone. This thesis is arranged in six chapters with further subdivisions dealing all the above aspects.

Comprehensive Probabilistic Seismic Hazard Analysis of the Andaman-Nicobar Regions

An updated catalog of earthquakes has been prepared for the Andaman-Nicobar and adjoining regions. The catalog was homogenized to a unified magnitude scale, and declustering of the catalog was performed to remove aftershocks and foreshocks. Eleven regional source zones were identified in the study area to account for local variability in seismicity characteristics. The seismicity parameters were estimated for each of these source zones, and the seismic hazard evaluation of the Andaman-Nicobar region has been performed using different source models and attenuation relations. Probabilistic seismic hazard analysis has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models (source models, maximum magnitude, and attenuation relationships). The hazard maps for different periods have been produced for horizontal ground motion on the bedrock level.

Geodynamics of the Andaman - Sumatra - Java Trench - Arc System Based on Gravity and Seismotectonic Study

This work aims to study the variation in subduction zone geometry along and across the arc and the fault pattern within the subducting plate. Depth of penetration as well as the dip of the Benioff zone varies considerably along the arc which corresponds to the curvature of the fold- thrust belt which varies from concave to convex in different sectors of the arc. The entire arc is divided into 27 segments and depth sections thus prepared are utilized to investigate the average dip of the Benioff zone in the different parts of the entire arc, penetration depth of the subducting lithosphere, the subduction zone geometry underlying the trench, the arctrench gap, etc.The study also describes how different seismogenic sources are identified in the region, estimation of moment release rate and deformation pattern. The region is divided into broad seismogenic belts. Based on these previous studies and seismicity Pattern, we identified several broad distinct seismogenic belts/sources. These are l) the Outer arc region consisting of Andaman-Nicobar islands 2) the back-arc Andaman Sea 3)The Sumatran fault zone(SFZ)4)Java onshore region termed as Jave Fault Zone(JFZ)5)Sumatran fore arc silver plate consisting of Mentawai fault(MFZ)6) The offshore java fore arc region 7)The Sunda Strait region.As the Seismicity is variable,it is difficult to demarcate individual seismogenic sources.Hence, we employed a moving window method having a window length of 3—4° and with 50% overlapping starting from one end to the other. We succeeded in defining 4 sources each in the Andaman fore arc and Back arc region, 9 such sources (moving windows) in the Sumatran Fault zone (SFZ), 9 sources in the offshore SFZ region and 7 sources in the offshore Java region. Because of the low seismicity along JFZ, it is separated into three seismogenic sources namely West Java, Central Java and East Java. The Sunda strait is considered as a single seismogenic source.The deformation rates for each of the seismogenic zones have been computed. A detailed error analysis of velocity tensors using Monte—Carlo simulation method has been carried out in order to obtain uncertainties. The eigen values and the respective eigen vectors of the velocity tensor are computed to analyze the actual deformation pattem for different zones. The results obtained have been discussed in the light of regional tectonics, and their implications in terms of geodynamics have been enumerated.ln the light of recent major earthquakes (26th December 2004 and 28th March 2005 events) and the ongoing seismic activity, we have recalculated the variation in the crustal deformation rates prior and after these earthquakes in Andaman—Sumatra region including the data up to 2005 and the significant results has been presented.ln this chapter, the down going lithosphere along the subduction zone is modeled using the free air gravity data by taking into consideration the thickness of the crustal layer, the thickness of the subducting slab, sediment thickness, presence of volcanism, the proximity of the continental crust etc. Here a systematic and detailed gravity interpretation constrained by seismicity and seismic data in the Andaman arc and the Andaman Sea region in order to delineate the crustal structure and density heterogeneities a Io nagnd across the arc and its correlation with the seismogenic behaviour is presented.

Intraplate Response to the Great 2004 Sumatra-Andaman Earthquake: A Study from the Andaman Segment

The 1300-km rupture of the 2004 interplate earthquake terminated at around 15 degrees N, in the northernmost segment of the Andaman-Nicobar subduction zone. This part of the plate boundary is noted for its generally lower level seismicity, compared with the southern segments. Based on the Global Centroid Moment Tensor (CMT) and National Earthquake Information Center (NEIC) data, most of the earthquakes of M-w >= 4.5 prior to 2004 were associated with the Andaman Spreading Ridge (ASR), and a few events were located within the forearc basin. The 2004 event was followed by an upward migration of hypocenters along the subducting plate, and the Andaman segment experienced a surge of aftershock activity. The continuing extensional faulting events, including the most recent earthquake (10 August 2009; M-w 7.5) in the northern end of the 2004 rupture, suggest the reduction of compressional strain associated with the interplate event. The style of faulting of the intraplate events before and after a great plate boundary earthquake reflects the relative influences of the plate-driving forces. Here we discuss the pattern of earthquakes in the Andaman segment before and after the 2004 event to appraise the spatial and temporal relation between large interplate thrust events and intraplate deformation. This study suggests that faulting mechanisms in the outer-ridge and outer-rise regions could be indicative of the maturity of interplate seismic cycles.

Constraining large earthquakes along the Andaman trench using deepwater turbidites: prospects and challenges

The 2004 Sumatra-Andaman earthquake was unprecedented in terms of its magnitude (M-w 9.2), rupture length along the plate boundary (1300 km) and size of the resultant tsunami. Since 2004, efforts are being made to improve the understanding of the seismic hazard in the Sumatra-Andaman subduction zone in terms of recurrence patterns of major earthquakes and tsunamis. It is reasonable to assume that previous earthquake events in the Myanmar Andaman segment must be preserved in the geological record in the form of seismo-turbidite sequences. Here we present the prospects of conducting deep ocean palaeoseismicity investigations in order to refine the quantification of the recurrence pattern of large subduction-zone earthquakes along the Andaman-Myanmar arc. Our participation in the Sagar Kanya cruise SK-273 (in June 2010) was to test the efficacy of such a survey. The primary mission of the cruise, along a short length (300 km) of the Sumatra Andaman subduction front was to collect bathymetric data of the ocean floor trenchward of the Andaman Islands. The agenda of our piggyback survey was to fix potential coring sites that might preserve seismo-turbidite deposits. In this article we present the possibilities and challenges of such an exercise and our first-hand experience of such a preliminary survey. This account will help future researchers with similar scientific objectives who would want to survey the deep ocean archives of this region for evidence of extreme events like major earthquakes.

Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (T-e) of the Andaman subduction zone. The recovered T-e map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0< T-e <3 km) in that region, where the NER is close to the Andaman trench (north of 10 N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, T-e < T-s (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low T-e and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region. (C) 2013 Elsevier Ltd. All rights reserved.

An annotated checklist of amphibian of India with some insights into the patterns of species discoveries, distribution and endemism

An annotated checklist of 284 species of amphibians of India accommodated under 50 genera and 14 families is provided. Synonyms, English names, type localities, deposition of type specimens, type specimen availability and distributional records in India and outside India are provided for all the species. Among the 284 species of amphibians from India, 132 are endemic to Western Ghats; 29 to Northeastern India; and 5 to Andaman Nicobar islands. Species discovery patterns from the various biogeographic zones in India are discussed in detail. Cumulative discovery pattern with special reference to the genera Fejervarya (17 species), Nyctibatrachus (16 species), Indirana (10 species), Micrixalus (11 species), Philautus (46 species) and Gegeneophis (10 species) are also discussed.

Studies on phytoplankton of the estuarine and marine environments

This thesis deals with the results oi’ investigations on phytoplankton productivity and related aspects conducted in various ecoaystms such as estuarine, inshore and oceanic enviroments and certain special ecosytans including the pu.-awn culture fields and associated many-eves, mud bank and the seas around the Andaman-Nicobar Islands. This study also includes the qualitative and quantitative variations of phytoplankton production, their seasonal abundance, factors controlling the same and the magnitude of the potential resources derived 1!:-om it

Ages and relative sizes of pre-2004 tsunamis in the Bay of Bengal inferred from geologic evidence in the Andaman and Nicobar Islands

Geologic evidence along the northern part of the 2004 Aceh-Andaman rupture suggests that this region generated as many as five tsunamis in the prior 2000years. We identify this evidence by drawing analogy with geologic records of land-level change and the tsunami in 2004 from the Andaman and Nicobar Islands (A&N). These analogs include subsided mangrove swamps, uplifted coral terraces, liquefaction, and organic soils coated by sand and coral rubble. The pre-2004 evidence varies in potency, and materials dated provide limiting ages on inferred tsunamis. The earliest tsunamis occurred between the second and sixth centuries A.D., evidenced by coral debris of the southern Car Nicobar Island. A subsequent tsunami, probably in the range A.D. 770-1040, is inferred from deposits both in A&N and on the Indian subcontinent. It is the strongest candidate for a 2004-caliber earthquake in the past 2000years. A&N also contain tsunami deposits from A.D. 1250 to 1450 that probably match those previously reported from Sumatra and Thailand, and which likely date to the 1390s or 1450s if correlated with well-dated coral uplift offshore Sumatra. Thus, age data from A&N suggest that within the uncertainties in estimating relative sizes of paleo-earthquakes and tsunamis, the 1000year interval can be divided in half by the earthquake or earthquakes of A.D. 1250-1450 of magnitude >8.0 and consequent tsunamis. Unlike the transoceanic tsunamis generated by full or partial rupture of the subduction interface, the A&N geology further provides evidence for the smaller-sized historical tsunamis of 1762 and 1881, which may have been damaging locally.

Factors affecting distribution of fish within a tidally drained mangrove forest in the Andaman and Nicobar Islands, India

Mangrove forests in meso-tidal areas are completely drained during low tides, forming only temporary habitats for fish. We hypothesised that in such temporary habitats, where stranding risks are high, distance from tidal creeks that provided access to inundated areas during receding tides would be the primary determinant of fish distribution. Factors such as depth, root density and shade were hypothesised to have secondary effects. We tested these hypotheses in a tidally drained mangrove patch in the Andaman Islands, India. Using stake nets, we measured fish abundance and species richness relative to distance from creeks, root density/m(2), shade, water depth and size (total length) of fish. We also predicted that larger fish (including potential predators) would be closer to creeks, as they faced a greater chance of mortality if stranded. Thus we conducted tethering trials to examine if predation would be greater close to the creeks. Generalised linear mixed effects models showed that fish abundance was negatively influenced by increasing creek distance interacting with fish size and positively influenced by depth. Quantile regression analysis showed that species richness was limited by increasing creek distance. Proportion of predation was greatest close to the creeks (0-25 m) and declined with increasing distance. Abundance was also low very close to the creeks, suggesting that close to the creeks predation pressure may be an important determinant of fish abundance. The overall pattern however indicates that access to permanently inundated areas, may be an important determinant of fish distribution in tidally drained mangrove forests.

Studies on the seaweeds of andaman and nicobar group of islands

The importance of marine algae, often referred to as seaweeds, has been felt over a long time and is appreciated more and more in modern times. The economic value of marine algae is understood both indirectly and directly. The indirect benefit is due to the role of marine phytoplankton as well as the benthic macrophyte biomass along the shore and in the continental shelf, in primary production of the sea. Direct benefit includes the use of marine algae as food, feed, fertilizer and as source of various products of commercial importance such as agar and alginic acid. Hence to understand the potential resources of seaweeds, their distribution, density, standing crop and interrelated environmental parameters, a detailed study (survey and ecological work) was carried out for a period of 20 months from August 1988 to March 1990 in South Andaman, North Andaman, Middle Andaman, Havelock, Neil, Car Nicobar, Terassa, Chowra and Bumpoka islands. However in South Andaman, data were collected from five fixed stations fortnightly during this period for the purpose of modelling and system analysis.

The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate

The similar to 1300-km-long rupture zone of the 2004 Andaman-Sumatra megathrust earthquake continues to generate a mix of thrust, normal, and strike-slip faulting events. The 12 June 2010 M(w) 7.5 event on the subducting plate is the most recent large earthquake on the Nicobar segment. The left-lateral faulting mechanism of this event is unusual for the outer-rise region, considering the stress transfer processes that follow great underthrusting earthquakes. Another earthquake (M(w) 7.2) with a similar mechanism occurred very close to this event on 24 July 2005. These earthquakes and most of their aftershocks on the subducting plate were generated by left-lateral strike-slip faulting on north-northeast-south-southwest oriented near-vertical faults, in response to north-northwest-south-southeast directed compression. Pre-2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern. Post-2004, left-lateral faulting on the subducting oceanic plate clusters between 5 degrees N and 9 degrees N, where the 90 degrees E ridge impinges the trench axis. Our study observes that the subducting plate off the Sumatra and Nicobar segments behaves similarly to a chip of the India-Australia plate, deforming in response to a generally northwest-southeast oriented compression, an aspect that must be factored into the plate deformation models.

Population Recovery of Nicobar Long-Tailed Macaque Macaca fascicularis umbrosus following a Tsunami in the Nicobar Islands, India

Natural disasters pose a threat to isolated populations of species with restricted distributions, especially those inhabiting islands. The Nicobar long tailed macaque. Macaca fascicularis umbrosus, is one such species found in the three southernmost islands (viz. Great Nicobar, Little Nicobar and Katchal) of the Andaman and Nicobar archipelago, India. These islands were hit by a massive tsunami (Indian Ocean tsunami, 26 December 2004) after a 9.2 magnitude earthquake. Earlier studies Umapathy et al. 2003; Sivakumar, 2004] reported a sharp decline in the population of M. f. umbrosus after thetsunami. We studied the distribution and population status of M. f. umbrosus on thethree Nicobar Islands and compared our results with those of the previous studies. We carried out trail surveys on existing paths and trails on three islands to get encounter rate as measure of abundance. We also checked the degree of inundation due to tsunami by using Normalized Difference Water Index (NDWI) on landsat imageries of the study area before and after tsunami. Theencounter rate of groups per kilometre of M. f. umbrosus in Great Nicobar, Little Nicobar and Katchal was 0.30, 0.35 and 0.48 respectively with the mean group size of 39 in Great Nicobar and 43 in Katchal following the tsunami. This was higher than that reported in the two earlier studies conducted before and after the tsunami. Post tsunami, there was a significant change in the proportion of adult males, adult females and immatures, but mean group size did not differ as compared to pre tsunami. The results show that population has recovered from a drastic decline caused by tsunami, but it cannot be ascertained whether it has reached stability because of the altered group structure. This study demonstrates the effect of natural disasters on island occurring species.

Present status and future scope of fisheries in the Andaman group of islands

The coastal area of approximately 2000 km and the water-bodies in between the Andaman and Nicobar islands are rich in fishery potential which range from 0.012-0.47 million tonnes. The fishery is dominated by catches of sardines, perches, carangids, mackerels, Leiognathus elasmobranchs, seerfish, mullets and tunas. About 2050 fishermen, with 1150 country craft, 113 mechanised boats and 1367 different kinds of nets and lines are engaged in active fishing in the island. Numerous bays, lagoons and creeks are available among the group of islands for mariculture activities. The mangroves of these islands provide feeding and nursery grounds for juveniles of penaeid prawns, crabs and finfishes.

A preliminary list of the plants of the Andaman Islands.

Mode of access: Internet.