Volume 98, Issue 1
April 1989, pages 1-132
pp 1-5 April 1989 Editorial
pp 7-24 April 1989
We present a preliminary tectonic chart of the Indian Ocean based on a joint compilation of bathymetric data, magnetic anomaly data and Geosat altimetry data. Satellite altimeters such as Geosat map the topography of the equipotential sea surface or marine geoid. Our interpretation of the GEOSAT data is based on an analysis of the first derivative of the geoid profiles (i.e. deflection of the vertical profiles). Because of the high correlation between the vertical deflection (at wavelength <200 km) and the seafloor topography, the Geosat profiles can be used to delineate accurately numerous tectonic features of the ocean floor such as fracture zones, seamounts and spreading ridges. The lineations in the Geosat data are compared with bathymetric data and combined with magnetic anomaly identifications to produce a tectonic fabric chart of the Indian Ocean floor.
pp 25-30 April 1989
Recently Brune and Singh (1986) reported evidence for anomalous continent-like crustal structure beneath the Bay of Bengal, possibly caused by perturbation in the temperature-pressure regime and consequent phase chage, partial melting, or mass transport (e.g. convection or underplating). Recent refraction results indicate the existence of an anomalous lower crustal or subcrustal layer ofP-wave velocity about 7·3 km per second along the eastern North America passive margin, possibly a result of underplating of the oceanic crust just after initial rifting. We have searched for other evidence of anomalous crustal structure. The data suggest some mechanism may cause a general increase in the anomalous thickness of the crust with increasing thickness of the accumulated sediments, up to a thickness of about 6–7 km. On the other hand, anamolous crustal structure may in fact be transitional between oceanic and continental, or may have been modified by aseismic ridges, thus requiring no sediment related structure modification mechanism. The explanation for all the data may require more than one mechanism, all probably involving severe temperature perturbations. The general tendency is for perturbations of normal oceanic crust to make it more continent-like, suggesting that normal oceanic crust is an unstable end-member in crustal states.
pp 31-60 April 1989
Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period.
pp 61-70 April 1989
The four major earthquakes that have occurred in the Himalayan region since 1897 seem to have ruptured as little as about 15% or 20% to perhaps as much as 45% of the thrust zone separating the underthrusting Indian Shield and the overthrusting Himalayan crystalline nappes. Because of various difficulties in estimating the rupture zones for each of these earthquakes, we cannot place a tight constraint on the fraction of the Himalayan belt for which the risk of an imminent great earthquake is high. If a slip between the Indian Shield and the Himalayan crystalline nappes occurs largely by slip associated with major earthquakes, then recurrence intervals of such earthquakes are likely to be between 200 and 500 years, with a likely value of 300 years.
pp 71-89 April 1989
The Himalayan mountains are a product of the collision between India and Eurasia which began in the Eocene. In the early stage of continental collision the development of a suture zone between two colliding plates took place. The continued convergence is accommodated along the suture zone and in the back-arc region. Further convergence results in intracrustal megathrust within the leading edge of the advancing Indian plate. In the Himalaya this stage is characterized by the intense uplift of the High Himalaya, the development of the Tibetan Plateau and the breaking-up of the central and eastern Asian continent. Although numerous models for the evolution of the Himalaya have been proposed, the available geological and geophysical data are consistent with an underthrusting model in which the Indian continental lithosphere underthrusts beneath the Himalaya and southern Tibet. Reflection profiles across the entire Himalaya and Tibet are needed to prove the existence of such underthrusting. Geodetic surveys across the High Himalaya are needed to determine the present state of the MCT as well as the rate of uplift and shortening within the Himalaya. Paleoseismicity studies are necessary to resolve the temporal and spatial patterns of major earthquake faulting along the segmented Himalayan mountains.
pp 91-109 April 1989
This paper reports data pertaining to 90 local earthquakes recorded during 1984–86 using seismographs in arrays of 5–7 stations deployed near the Main Central Thrust between Bhagirathi and Alakhananda valleys. The results which are also compared with 162 earthquakes recorded in 1979–80 provide a local view that refines and complements information recorded at distant seismic stations.
pp 111-123 April 1989
Seismicity of the Himalayan arc lying within the limits shown in figure 1 and covering the period 1964 to 1987 was scanned using M8 algorithm with a view to identifying the times of increased probabilities (TIPs) of the occurrence of earthquakes of magnitude greater than or equal to 7·0, during the period 1970 to 1987. In this period, TIPs occupy 18% of the space time considered. One of these precedes the only earthquake in this magnitude range which occurred during the period. Two numerical parameters used in the algorithm, namely the magnitude thresholds, had to be altered for the present study owing to incomplete data. Further monitoring of TIPs is however warranted, both for testing the predictive capability of this algorithm in the Himalayan region and for creating a base for the search of short-term precursors.
pp 125-132 April 1989
The high seismicity of portions of the Indian peninsula, together with the high density of population and industrial growth, results in a significant seismic risk in many parts of the subcontinent. Large construction projects throughout the peninsula require an adequate basis for earthquake-resistant design. Thus, as well as strong scientific arguments, there are major practical reasons why a substantial programme to record strong seismic ground motion should be carried out in India. This paper first reviews the history of strong motion instrumental recording, beginning with the important accelerograms obtained in the Koyna earthquake of 11 December 1969 through the recent increase in strong motion instrumentation, particularly in association with construction of large dams. It is argued that there is a pressing need for further extension of strong motion accelerograph coverage of India, especially along the seismically active regional thrust faults of the Himalayan region. Such programme expansion should follow deliberate strategies of site selection, designed to optimize the scientific and practical returns, given the requirements of minimum costs, reliable maintenance and accessible data.