Showing papers in "Journal of Earth System Science in 1992"

Identification of potential areas for the occurrence of strong earthquakes in Himalayan arc region

Abstract: Morphostructural zoning (MSZ) scheme of the Himalayan arc region as obtained from a joint study of topographic, geological and tectonic maps as well as satellite imagery is analysed. Three types of morphostructures have been determined: territorial units (blocks of different ranks), linear zones limiting these blocks (lineaments) and intersections of the lineaments (knots). Comparison of MSZ scheme with the know seismicity indicates epicenters of strong earthquakes (M≥6·5) clustered around some of these knots. Pattern recognition method is used to determine seismically potential areas for the occurrence of recognition method is used to determine seismically potential, for the occurrence of strong earthquakes of magnitude ≥M 0. We have carried out two such studies for the Himalayan arc region, one forM 0=6·5 and the other forM 0=7·0. Out of a total number of 97 knots, 48 knots are found to be seismically potential for the occurrence of earthquake ofM≥6·5. The results of the study forM 0=6·5 were presented in the symposium on “Earthquake Prediction” held in Strasbourg, France, March 1991 (Gorshkovet al 1991). The epicenter of Uttarkashi earthquake of magnitude,M b=6·6 that occurred in the late hours of 19th October 1991 (UTC) lies in the vicinity of one such knot. The second study carried out subsequently shows that only 36, knots are potential for the occurrence of earthquakes ofM≥7·0, which include the knot, associated with theUttarkashi earthquake.

Spatial variability of aridity oyer northern India

Abstract: Spatial variability of aridity over northern India (north of 20°N) is studied by examining variations in the arid area. Area with an objectively determined summer monsoon rainfall (June to September total) of less than 500 mm is identified as arid area. The summer monsoon rainfall of 212 rain-gauges from 212 districts of the region for the period 1871–1984 are used in the analysis. An interesting feature of the arid area series is that it shows decreasing trend from beginning of the present century. The summer monsoon rainfall fluctuations over five subjectively divided zones over northern India are examined to understand the association between rainfall and the arid area variations. The rainfall series for northwest India shows a significant increasing trend and that for northeast India a significant decreasing trend from the beginning of this century. Rainfall fluctuations over the remaining zones can be considered intermediate stages of a systematic spatial change in the rainfall pattern. This suggested that the recent decreasing trend in the arid area is due to a westward shift in the monsoon rainfall activities. From correlation analyses it is inferred that perhaps the recent decreasing trend in the arid area and increasing trend in the monsoon rainfall over northwest India are associated with a warming trend of the northern hemisphere.

Static deformation of two welded half-spaces due to dip-slip faulting

Abstract: Closed-form expressions for the displacements and stresses at any point of either of two elastic half-spaces in welded contact caused by a dip-slip line source obtained earlier are integrated analytically to derive the elastic residual field due to a long dip-slip fault of finite width. The results are valid for an arbitrary dip of the fault. The variation of the displacement field with the distance from the fault as well as with the distance from the interface is studied numerically. It is found that the displacement field is heavily dependent on the dip angle. Contour maps showing the residual elastic field in the two half-spaces caused by a vertical dip-slip fault located in one of the half-spaces are also obtained.

Application of pattern recognition algorithm in the seismic belts of Indian convergent plate margin - CN algorithm

Abstract: The earthquake catalogue from 1964 to August 1991 is used to identify the times of increased probabilities (TIPs) of the earthquake mainshocks of magnitudes greater than or equal to 6·4 and are associated with the Indian convergent plate margins, in retrospect. In Pakistan and Indo-Burma regions, the analysis was repeated for magnitude threshold 6·2 and 7·0 respectively. All the earthquakes (except one in the Hindukush region and one in Indo-Burmese region) in Pakistan, Hindukush-Pamir, Himalaya and Indo-Burmese regions were preceded by the special activation and hence were predicted.

Rheological thickness and strength of the Indian continental lithosphere

Abstract: The estimates of rheological thickness and total lithospheric strength for the Indian continental lithosphere have been obtained based on the representative rheological properties of upper crust, lower crust and upper mantle, and some of the available heat flow and heat generation data. The rheological thickness, computed at different locations in the Indian shield, shows lateral variation ranging from 79km in the southern part to 65 km in the northern part for a strain rate of 10-14 s-1. The total strength of the continental lithosphere is of the order of 1013 Nm-1 for the same value of strain rate and decreases northward. The computations carried out for a range of strain rates show an increase in the rheological thickness and strength of the lithosphere with increasing strain rate. These results would be important in understanding the flexural response of the Indian continental lithosphere to surface and subsurface loading, and response to tectonic forces acting on it.

Estimation of the compounding distribution in the compound Poisson process model for earthquakes

Abstract: Compound Poisson process models have been studied earlier for earthquake occurrences, with some arbitrary compounding distributions. It is more meaningful to abstract information about the compounding distribution from the empirical observations on the earthquake sequences. The difinition of a compound distribution can be interpreted as an integral transform of the compounding distribution. The latter distribution can therefore be estimated by inverting the integral transform. Alternatively, from the moments of the observable random variablesviz. (a) the number of earthquakes per unit time or (b) the waiting times for subsequent earthquakes, the moments of the compounding distribution can be obtained. This information can be converted into a statement about the compounding distribution.

On the validity of time-predictable model for earthquake generation in north-east India

Abstract: North-east India is seismically very active and has experienced many widelydistributed shallow, large earthquakes. Earthquake generation model for the region was studied using seismicity data [(1906–1984) prepared by National Geophysical Data Centre (NGDC), Boulder Colorado, USA]. For establishing statistical relations surface wave magnitudes (M s≥5·5) have been considered. In the region four seismogenic sources have been identified which show the occurrences of atleast three earthquakes of magnitude 5·5≤M s≤7·5 giving two repeat times. It is observed that the time interval between the two consecutive main shock depends on the preceding main shock magnitude (M p) and not on the following main shock magnitude (M f) revealing the validity of time predictable model for the region. Linear relation between logarithm of repeat time (T) and preceding main shock magnitude (M p) is established in the form of logT=cM p+a. The values ofc anda are estimated to be 0–36 and 1–23, respectively. The relation may be used for seismic hazard evaluation in the region.

Neogene deep sea benthic foraminiferal diversity in the Indian Ocean: Paleoceanographic implications

Abstract: The species diversity indices, as defined by the number of species,S; Shannon-Wiener index,H(S) and Buzas-Gibson index,E, of DSDP sites 219, 220, 237 and 238 were measured to determine the benthic foraminiferal diversity patterns in the Indian Ocean deep sea sequences during the Neogene. The Time-Stability hypothesis could satisfactorily explain the observed diversity patterns. The general patterns of diversity suggest environmental stability during the Neogene. However, few small fluctuations in diversity during the Middle Miocene (c.14·8 Ma), Late Miocene (c.6·0 Ma) and Late Pliocene (c.2·0 Ma) may possibly be the effects of Antarctic Bottom Water (AABW) activity in this region. The benthic foraminiferal diversity in the tropical Indian Ocean is more than the high latitudinal areas with comparable depths.

Deformation of an elastic layer coupling in different ways to a base due to a very long vertical strike-slip dislocation

Abstract: The closed-form analytic expressions for the displacement and stresses at any point of an elastic layer lying over a base due to a very long vertical strike-slip dislocation are obtained. The interface between the layer and the base is assumed to be either ‘smooth-rigid’ or ‘rough-rigid’ or ‘welded’. The variations of displacement and stresses with the horizontal distance from the fault for different types of coupling of the layer with the base have been studied. It is found that the displacement for ‘welded interface’ lies between the displacements due to ‘smooth rigid’ and ‘rough-rigid’ interfaces for different positions of the observer and different values of the ratio of rigidities of the layer and half-space.

Parametric wave prediction model based on time delay concept. An evaluation

Abstract: In the early stages of wave growth it is seen that wave heights are underestimated by presently available models especially in a low wind regime. Parametric wind-sea relationships of significant wave height (H2) and zero-crossing period (T2) for slight to moderate sea-states were proposed earlier on an analysis of wind and wave data. This model is based on the concept of time delay between the wind speed (U) and wave evolution process. It is simple and requires less computational effort compared to the spectral method. The present paper attempts to test and evaluate the performance of the proposed model with additional field data of wind and waves measured off the Indian coast. MeasuredU,H2, andT2 ranged between 1 and 15 m/s, 0·5 and 2·7 m and 4 and 10 s respectively. By and large, the comparison between model output and field observations are encouraging. A hindcast study was carried out earlier using a spectral wave prediction model (TOHOKU) for Indian Seas using field measurements which include the data sets utilized in this study. Comparison between these two models reveals a good agreement.

Anomalous granulite crust of South India―signatures from converted teleseismic waves

Abstract: Analysis of teleseismic waves usingS-P converted phases, travel time-terms and residual travel times point to the presence of an anomalous thick (4—5 km thicker) low velocity (−3%) crust beneath Kodaikanal (KOD) on granulites characterized by an oriented inhomogeneity inferred as possibly due to Mylonites in contrast to the nature of crust beneath the adjoining precambrian granite-gneiss terrain. The observed seismic signatures in the South India granulites (represented by KOD) offer an opportunity to discriminate between the competing hypotheses of tectonic thickening and magmatic underplating to explain the origin of the granulites of South India. This analysis lends support to the hypothesis of a continent-continent collision origin for the granulites in the study region.

Critical evaluation and geological correlation of teleseismic phase data from Indian seismological stations

Abstract: We evaluated the quality of seismic phase data from Indian seismological stations through the analysis of teleseismic travel times reported during 1976–83 and infer that only WWSSN stations (NDI, SHL, POO, KOD) apart from GBA and HYB can be rated satisfactory while the majority of stations (more than 40) produce very poor quality data sets. Detailed analysis of teleseismic P-wave travel time residuals shows that while the average structure of the upper mantle beneath India has high velocity (negative residuals) there are marked lateral variations. In particular, three zones of anomalous positive residuals (low velocity) are observed: one beneath the north western part of the Deccan trap, the second covering the southernmost peninsula (granulite terrain) and a third rather localized one, to the north of Delhi coinciding with Delhi-Haridwar ridge. New Delhi exhibits strong negative residuals in the E-SE quadrant along with negative station anomaly, implying that it is underlain by an anomalous high velocity crust/upper mantle. The negative residuals observed over India, continue beneath the Himalaya till the south of Lhasa but change sign further northward, suggesting the northern limit of the Indian upper mantle structure.

Internal wave characteristics in the eastern Arabian Sea during summer monsoon

Abstract: The time series BT profiles and surface winds and atmospheric pressure, collected in the deep waters off Ratnagiri and Karwar during summer monsoon were utilized to document the characteristics of internal waves (IW). Low-frequency (≤2, cycle per day (cpd)) IW off Ratnagiri are found to propagate at 83 cm/s with wavelengths of 45 km and wave heights upto 40 m. These parameters for high-frequency (>2 cpd) IW off Karwar correspond to 99 cm/s, 3 km and 23 m. The IW off Karwar appear to leave the station at 70° (±10°) (measured from the horizontal). The data sets were further analysed to address the harmonic composition of the IW and identify the possible sources for the observed IW fields. Power spectra of the IW indicated energy peaks at inertial (0·6 cpd) and tidal (1 and 2 cpd) frequencies off Ratnagiri and in the high-frequency band of 0·5–2·0 cycles per hour off Karwar. The coherence between the IW and wind/tide is found to be good at several frequencies within the IW spectrum. This feature probably suggests tides as a source for the IW of tidal frequencies and winds and tides as a joint source for the IW at the remaining frequencies.

Application of the Linear Prediction filters in equatorial electrojet studies

Abstract: Wiener filters are derived from the horizontal field data of two adjacent equatorial electrojet stations, Adis Ababa and Trivandrum from several sequences of quiet days. The time invariant property of the filter is established and the filter is applied to conditions marked by afternoon counter electrojet events. The prediction efficiency is shown to be consistently high. Possible uses of this technique in studies related to generating mechanisms of counter electrojet events and the day-to-day variability in electrojet currents are indicated. Inter-relationships and day-to-day variability of different components of the fields at the two stations are highlighted.