West Coast Fault Research Articles

Magnetotelluric imaging reveals fragmented crustal blocks beneath the Konkan plains adjoining the Western Ghats, Deccan Volcanic Province, India

The Deccan plateau and the Konkan plain of the western continental margin of India are significant morphotectonic features. Seismic activity along with regional-scale geophysical and geological studies suggests that the plateau is tectonically disturbed and the traps together with the sub-basaltic terrain have been experiencing structural deformation along pre-existing linear zones. The recent Palghar swarm activity has drawn significant attention on the crustal architecture of the Konkan plain and the adjoining Western Ghats region. Here, we present the results of a magnetotelluric study carried out along a 100 km long E-W profile traversing major tectonics features between the Konkan plain and the Bhatsa swarm activity region. Two-dimensional modelling of the distortion corrected impedance tensors reveals high resistivity crustal blocks down to 35–40 km dissected by faults/fractures. These crustal blocks are underlain by a moderately conductive mantle lithosphere which is also characterized as a seismic low velocity and low density layer. The persistent elevated topography of the Western Ghats could be explained by a buoyant, low-velocity/low resistivity upper mantle coupled with reactivation of deep crustal features during the Quaternary period. The model yields some westward dipping conductive and resistive zones coinciding with the exhumed topography highs. The west coast fault in the western and the Ghod fault in the eastern part of the profile are imaged as crustal-scale steeply dipping fault zones segmenting the Eastern Dharwar craton (EDC) crust into the EDC1 and the EDC2. The model in conjunction with earlier results from the Palghar swarm activity region suggests southward continuation of the major conductivity anomalies obtained at Palghar. This study proposes that NW extension of the Kurudwadi rift and its interaction with local structural features is possibly affecting the stress state of the region leading to local seismicity.

Use of Remote Sensing and GIS Techniques in Identification of Landslide Vulnerable Zones of Shastri River Basin Along the West Coast of Ratnagiri District, Maharashtra

The atmosphere, hydrosphere, and lithosphere are subjected to different processes, leading to natural hazards like weathering, erosion, floods, cyclones, landslides, earthquakes, tectonic movements, etc. Environmental degradation is a serious aspect of the recent past, mainly due to natural and manmade interactions. The pressure for infrastructure development due to rapid urbanization has led to the expansion of construction activities. It has catapulted the frequency of landslides to dramatic proportions in recent decades, especially along western ghats. The West Coast of India (WCI) has attracted the attention of Geo-scientists due to its neo-tectonic setup, continuing seismic activities, sea-level changes, and also due to environmental degradation. It is followed that very limited attempts have been made related to the land sliding along the west coast tract of Maharashtra. The present investigations are emphasized mainly to locate the landslide vulnerable zones of Shastri River Basin (SRB), Ratnagiri district of Maharashtra by using remote sensing data, GIS techniques along field studies. The area lies within a triple junction of Koyana-Kurduwadi Lineament (KKL), West Coast Fault (WCF), and Panvel Flexure (PF). Based on the integration of data from various thematic maps viz. lithology, lineaments, slope, geomorphology, land use-land cover along with inventory map, Landslide Vulnerable Map (LVM) of the SRB has been prepared. It follows that about 29% area of the SRB forms a highly vulnerable zone for land sliding. These zones are mainly confined to steep slopes, wasteland, highly weathered basalts, and deep valleys and in the vicinity of lineaments.

Magnetotelluric study of an intraplate seismic zone in the Palghar region of the Deccan Volcanic Province, India

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of flood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate seismic activity, the most recent one being a swarm-type activity in the Palghar region, about 120 km north of Mumbai, that started in November 2018 and has produced a few thousand micro-earthquakes and a 4.3 magnitude earthquake since then. We have carried out a magnetotelluric (MT) study along a 35-km long profile across the seismic zone to delineate the subsurface structure to understand the possible cause for the seismic activity. Broadband MT data were acquired at 18 sites with average station spacing of 2 km. Impedance tensors were analyzed for distortion and dimensionality, decomposed into TE- and TM-mode, and inverted by a 2D inversion algorithm. The geoelectric structure yields an assemblage of highly resistive and moderately conductive blocks in the uppermost crust resting on a major listric-type fault, that possibly reaches the surface at the West Coast Fault from a depth of about 15 km beneath the Panvel flexure zone. In conjunction with the regional Bouguer gravity anomaly and the seismicity distribution, we infer that the upper crustal heterogeneities coupled with the basement fault and low rheological strength of the fractured upper-to-middle crust might be leading to triggering of the seismicity in the region.

Ground Electrical and Electromagnetic Studies in Koyna-Warna Region, India

ABSTRACT In this paper, we present results of the Ground Electrical and Electromagnetic (GEE) studies (MT, CSAMT, DRS) carried out in the Koyna-Warna zone since 1976 and discuss their relevance in understanding the seismicity of the Koyna-Warna seismic zone. Though there were not many GEE studies carried out in this region, the existing ones provided a vital information, in particular, on the subsurface crustal structure, the Deccan Trap thickness and its variation and also the nature of the basement in this region. The GEE studies rule out the presence of any subtrappean sediments in this region. An interesting feature that has been identified from MT studies is the presence of a well-defined crustal block structure, characterized by high resistive blocks interspersed with moderately conductive features. All these structural features clearly point out to the intensity of tectonic disturbance that the region was subjected to in the geological past. The conductive features are correlatable with some of the known seismogenic structural features, for e.g., the Konya Fault Zone, the west coast fault, the Donachiwada fault. The moderately resistive zone sitting over the high resistive blocks as seen both in MT& DRS models is inferred to be related to the generally fractured nature of the shallow crustal column. The conductive linear features bordering the resistive blocks represent fluid filled fracture/fault zones. It is inferred that because of the NE to NS oriented compressive stress regime in the Indian shield, due to the Himalayan collision tectonics, some of these structural features may become the locales of stress accumulation which may get released due to fluid filling of these zones under the influence of nearby reservoirs, resulting in triggering of seismicity. Results from different GEE studies conducted in the study area are found to be highly consistent with each other as well as with the Rasati bore hole data, thus bringing significant validity to the subsurface model derived. Further, the 3D modelling of the MT data acquired in the Koyna-Warna region together with airborne gravity gradient and magnetic studies carried out under the Deep drilling program would open up new gate ways to accomplish multi-parametric three dimensional modeling, that will provide still more detailed and relevant subsurface image of this important RTS zone.

Spatial variation of probabilistic seismic hazard for Mumbai and surrounding region

Mumbai city, the economical capital of India, is located on the west coast of stable intra-plate continental region of Peninsular India which has an experience of significant historical earthquakes in the past. The city stood as the fourth most populous city in the world. Recent seismo-tectonic studies of this city highlighted the presence of active West coast fault and Chiplun fault beneath the Deccan basalt. In the present study, spatial variability of probabilistic seismic hazard for Mumbai region (latitudes of 18.85–19.35°N and longitudes of 72.80–73.15°E at a grid spacing of 0.05°) which includes Mumbai city, Suburban, part of Thane district and Navi Mumbai, in terms of ground motion parameters; peak horizontal acceleration and spectral acceleration at 1.0-s period for 2 and 10 % probability of exceedance in 50 years are generated. The epistemic uncertainty in hazard estimation is accounted by employing seven different ground motion prediction equations developed for worldwide shallow crustal intra-plate environments. Further, the seismic hazard results are deaggregated for Mumbai (latitude 18.94°N, longitude 72.84°E) to understand the relative contributions of earthquake sources in terms of magnitude and distance. The generated hazard maps are compared with the zoning specified by Indian seismic code (IS1893: Part 1 in Indian standard criteria for earthquake-resistant design of structures, Part 1—General provisions and buildings. Bureau of Indian Standards, New Delhi, India, 2002) for rocky site. Present results show an underestimation of potential seismic hazard in the entire study region by non-probabilistic zoning prescribed by IS1893: Part 1 with significantly higher seismic hazard values in the southern part of Navi Mumbai.

A receiver function perspective of the Dharwar craton (India) crustal structure

SUMMARY Teleseismic data from six broad-band stations on the south Indian shield have been analysed, primarily to examine the differences in the crustal structure between the eastern and western units of the Archaean Dharwar craton. SV receiver functions for these stations have been computed and modelled down to the Moho level in terms of Poisson’s ratio and shear wave velocities. Results show that the crust for the entire Dharwar craton is mainly simple, and has a low (about 0.25) Poisson ratio. It is usually thinner and less complex than the adjacent Proterozoic crust. However, the western Dharwar craton crust (thickness ∼41 km) with a gradational Moho boundary, is substantially (>7 km) thicker than its eastern counterpart (thickness ∼34 km). No substantial differences in average crustal S velocities (3.6‐3.8 km s −1 ) were found. The eastern Dharwar crust below the Proterozoic Cuddapah basin was also found to have remained relatively simple and undisturbed. The continental margin to the west of the Dharwar craton appears to have shifted further west off the coast, where a possible west coast fault has down-thrown a continental crustal block under the seas. The crust constituting the Deccan volcanic province in the western Indian shield is found to be similar to that of the eastern Dharwar craton. The crust underneath the neighbouring Godavari graben is significantly different from the Dharwar crust and resembles that of a typical rift-valley.

Lineaments and seismicity of Kerala — a remote sensing based analysis

Seismically active lineaments of Kerala State were identified by correlating the lineaments mapped using IRS LISS-I data with the earthquake occurrences. There are 31 earthquake incidences in Kerala since 1821, out of which 22 are falling on/close to 9 major lineaments/faults (length more than 20 km) indicating the possible correlation between lineaments/faults and earthquakes. It was observed that the earthquake occurrences are mostly associated with the NNW-SSE to NW-SE trending lineaments (6 out of 9 lineaments), which are considered to be formed sympathetic to the West Coast Fault. Hence, there is a need to monitor these seismically active lineaments using advanced techniques such as GPS, SAR Interferometry etc. for better understanding of the influence of these lineaments on the seismic activities of Kerala.

Origin and evolution of ‘intracratonic’ thermal fluids from central-western peninsular India

The chemical and isotopic composition of several thermal springs and associated gas phases in a large sector of central-western peninsular India has been investigated. Such springs have meteoric isotopic signature and emerge, after very well developed convective circulation at depth, along important tectonic structures such as the Son–Narmada–Tapti rift zone and the West Coast Fault. Chemical components in both gas and liquid phases and geothermometric estimations suggest that such springs are not related to the presence of any active hydrothermal systems at shallow depth in any of the studied areas. The hottest convective water emerges at Tattapani at near boiling point for water at atmospheric pressure (>90°C) in association with an N 2-rich gas phase of clear meteoric signature. Since such fluids do not carry any corrosive components, they could be conveniently exploited for industrial purposes, such as drying processes. From a tectonic point of view, the presence of thermal emergences scattered in a wide area along geologically well defined structures, which also generate frequent moderate earthquakes, suggests that such structures are active. Although the isotopic composition of thermal springs points to a meteoric origin, their feeding aquifers are not topographically driven as in most active Alpine orogenic belts. The relative high quantity of total helium in the associated gas phase suggests also that they are really deep, old, long circulating waters. We propose for such waters the term ‘intracratonic thermal waters’ since the isotopic signature of He in the gas phase does not show any release of primordial 3He in any of the areas of spring emergence. Based on the quite low 3He/ 4He ratio in the gas phase we suggest also that, in spite of its morphological shape, the Narmada–Son–Tapti rift zone cutting the Indian subcontinent in two is more related to paleo-suture rather than to a mid-continental rift system.

Fault Segmentation and Earthquake Hazard Zones in Deccan Trap Region: Gravity Evidence

Abstract Detailed structural interpretation of the regional gravity map over the Deccan traps has brought out a number of major structural discontinuities (fault/fractures), particularly the E-W features whlch were not delineated by earlier investigators. The most significant among them is the major E-W fault from west coast to east coast along Bombay-Latur-Hyderabad-Bhadrachalam-Visakhapatnam (BLHV fault). Several features of the BLHV fault suggest it to be a major reactivated strike slip fault. The structural scenario presented in this paper will have great implications in earthquake studies in Deccan trap area. Based on some aspects of fault segmentation technique for earthquake hazard and from the structural interpretation of the regional gravity map over Deccan traps, several zones are identified which could be termed as earthquake hazard zones. These hazard zones are typically located at: (1) the segment boundaries or fault steps of BLHV and other major E-W faults, (2) in many cases are associated with gravity low nosings which possibly reflect buried granite intrusions (regarded as asperities or barriers in the context of seismicity), and (3) intersection of major E-W faults with other transverse structural trends (fault/fractures). The areas where the major E-W faults terminated against the west coast fault system can also be important in terms of future seismicity. The recent seismicity in the Deccan trap, area is inferred to be due to the reactivation of the major E-W faults due to the still continuing convergence of India with Asia. The present work, for the first time, demonstrates the importance and application of fault segmentation technique in earthquake hazard studies in Deccan trap area, and also the full potential of gravity interpretation in such studies.

An evolutionary model based on geomorphologic and tectonic characteristics of the Maharashtra Coast, India

Abstract The Maharashtra Coast is part of the central west coast of the tectonically ‘passive’ and ‘divergent’ Western Continental Margin of India. Major tectonic elements that characterize the region are the West Coast Fault and the Panvel Flexure. These features have played a major role in the development of coastal geomorphic features, which have been further modified by marine and fluvial processes. Neotectonism is expressed in associated raised marine terraces. The coastal tract of Maharashtra has been divided in three geomorphic blocks on the basis of the role of tectonic elements and spatial distribution of various geomorphic features.

Seismotectonic studies of the coastal areas of India, Pakistan, Bangladesh, and Burma

Major geotectonic elements that are seismically active in the near-shore areas of the Indian subcontinent are the Mekran fault off the coast of Pakistan, the western part of the Narmada-Son lineament, the West Coast Fault off the west coast of India - a southward extension of the Cambay Rift, the Palghat Gap, the Godavari and Mahanadi grabens, transecting rather at an angle to the eastern coast of India and the Arakan-Yoma arcuate belt of Burma, which is a part of the global Alpine-Himalayan orogenic belt, continuing southwards into the Andaman-Nicobar island complex and the Java-Sumatra trench on the ocean floor of the advancing Indo-Australian Plate.

Integrated geophysical modelling of terranes and other structural features along the western Canadian margin

An integrated geophysical data set has been used to develop structural models across the continental margin west of Vancouver Island, Canada. A modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous Crescent and Pacific Rim terranes. These terranes in turn abut against the pre-Tertiary Wrangellia terrane that constitutes most of the island. Gravity and magnetic anomaly data, constrained by seismic reflection, seismic refraction, and other data, were interpreted to determine the offshore positions of these terranes and related features. Iterative 2.5-dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features. An average model was then developed to represent this part of the margin. The Pacific Rim terrane appears to be continuous and close to the coastline along the length of Vancouver Island, consistent with emplacement by strike-slip motion along the margin. The Westcoast fault, the boundary between the Pacific Rim and Wrangellia terranes, is interpreted to be 15 km farther seaward than in previous interpretations in the region of Barkley Sound. The Crescent terrane forms a thin landward-dipping slab along the southern half of the Vancouver Island margin, and cannot be confirmed along the northern part. Model results suggest the slab has buckled into an anticline beneath southern Vancouver Island and Juan de Fuca Strait, uplifting high-density lower crustal or upper mantle material close to the surface to produce the observed intense positive gravity anomaly. This geometry is consistent with emplacement of the Crescent terrane by oblique subduction.

Origin and Significance of Hemavathi-Tirthahalli Mega-Lineament

Abstract LANDSAT MSS False Colour Composite on 1 : 537,000 scale has been interpreted to study the lineaments in the study area (Frame 145-051 covering Hassan, Chikmagalur, Kodagu, Shimoga, Mysore, Dakshina Kannada, Tumkur, Chitradurga Districts of Karnataka and Kasaragod District of Kerala). The most prominent lineament is about 190 km long and is named Hernavathi-Tirthahalli mega-lineament. The direction of this mega-lineament is almost parallel to the west coast. This mega-lineament with considerable length and almost parallel orientation to the west coast indicates that it developed at the time of the West Coast Fault. Major lineaments like Yagachi, Tunga and a good number of other lineaments are also parallel to the west coast. As most of the earthquakes (between 1967 and 1977) in Southern Peninsula occurred in the vicinity of NNW-SSE to NW-SE trending mega lineaments, the Hemavathi-Tirthahalli mega-lineament should be studied to understand its seismic status.

Regional electromagnetic induction around the Indian peninsula and Sri Lanka; a three-dimensional numerical model study using the thin sheet approximation

Abstract Further to other recent numerical studies of geomagnetic induction in southern India, the thin sheet algorithm developed at the University of Victoria has been used to investigate the three-dimensional problem of induction by a uniform source over the Indian peninsula and Sri Lanka. With this algorithm it is not necessary to assume that the region is surrounded by a normal structure, but the limitations imposed by grid size and the given conductivities for the crust and mantle have restricted our study to an inducing field of 20-min period. Solutions have been computed for two different models: (1) with only the conductivity contrast arising from land and seawater of variable depth present; and (2) with additional sub-surface conductors included in the Palk Strait/Gulf of Manowar region, underneath or near the Comorin Ridge and along the west coast fault. Results are presented for uniform magnetic sources oriented in both the northward and westward directions to distinguish the effect of different polarizations. The correspondence between the observed and computed Parkinson arrows at the southern array stations suggests the existence of the three sub-surface conductors.

Structure and evolution of the western continental margin of India deduced from gravity, seismic, geomagnetic and geochronological studies

Available geochronological, gravity, seismic and geomagnetic data on the western continental margin of India between 8–20°N are presented. Based on the data three models describing the evolution of the crustal structure are developed. Model 1 describes the evolution of the crustal structure along 19°N. The main basic igneous activity that accompanied the tectonic events since the Late Cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure. The acid volcanic event that preceded the above activity at about 93 Ma ago is responsible for the observed crustal structure along 13°30′N shown in model 2. This event resulted in the formation of the St. Mary group of islands along this latitude. Counterclockwise rotation of the Indian plate during the Tertiary period caused rifting of the continental blocks giving rise to the Laccadive-Kerala graben bounded by the Chagos-Laccadive ridge on the west and the west coast fault on the east. Vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault. This tectonic event was accompanied by upwarping of the mantle-crust thus giving rise to the present day structure shown in model 3.