Volume 102, Issue 2
June 1993, pages 1-413
pp 1- June 1993
pp 283-305 June 1993
Earth-science is greatly concerned with history. It is argued by some that a historical discipline is not a science. This is contrary to the conclusion from the demarcation criteria of Popper, set to separate science from formal disciplines such as metaphysics, mathematics and logic. Others have spoken of the unity of all sciences. Classification of intellectual activities is based ongenus proximum anddifferentia specifica. Hence the two viewpoints can be readily reconciled. Earth-science has been criticized variously for being descriptive, inductive, explanatory, etc. Other historical and concrete sciences have also attracted similar adverse comments. These issues are discussed at length to argue that Popper’s work should be extended further to define a demarcation of science into immanent and historical. It is also argued that the rigour of cognitive and logical determinants of science is not an adequate reason to embrace sociological models of science. Further, sociological/logical models of science present a misleading dichotomy.
pp 307-312 June 1993
Ionospheric scintillation observations of VHF radio signals from FLEETSAT satellite (73°E longitude) at Bhopal from January 1990 to December 1990 have been used to study the characteristic variations of scintillation activity. It is found that scintillation occurrence is essentially a night-time phenomenon and day-time scintillations are very rare. Annual average nocturnal variation of percentage occurrence of scintillations shows maximum at around 2100–2200 hours LT. Seasonally, scintillations are most prominent during equinoxes and least during summer. Geomagnetic disturbances tend to decrease the occurrence of scintillations in the pre-midnight period.
pp 313-328 June 1993
Quantitative precipitation forecasting (QPF) has been attempted over the Narmada Catchment following a statistical approach. The catchment has been divided into five sub-regions for the development of QPF models with a maximum lead-time of 24 hours. For this purpose the data of daily rainfall from 56 raingauge stations, twice daily observations on different surface meteorological parameters from 28 meteorological observatories and upper air data from 11 aerological stations for the nine monsoon seasons of 1972–1980 have been utilized. The horizontal divergence, relative vorticity, vertical velocity and moisture divergence are computed using the kinematic method at different pressure levels and used as independent variables along with the rainfall and surface meteorological parameters. Multiple linear regression equations have been developed using the stepwise procedure separately with actual and square root and log-transformed rainfall using 8-year data (1972–1979). When these equations were verified with an independent data for the monsoon season of 1980, it was found that the transformed rainfall equations fared much better compared to the actual rainfall equations. The performance of the forecasts of QPF model compared to the climatological and persistence forecasts has been assessed by computing the verification scores using the forecasts for the monsoon season of 1980.
pp 329-350 June 1993
Transient and stationary spectra of kinetic energy (KE), available potential energy (APE) and enstrophy (EN), and their spectral fluxes as a function of the two-dimensional wavenumbern were computed for July 1979. Triangular truncation at zonal wavenumber 42 was used for computation. The slopes of various spectra in the wavenumber range 14≤n≤25 were obtained by fitting a straight line in log-log scale by the least square method. The transientKE, APE andEN spectra in the lower (upper) troposphere had slopes −2·21 (−2·30), −2·65 (−2·64) and −0·36 (−0·46), respectively. The effect of stationary and divergent motion on the slope values was investigated. The possible correlation between the slope and percentage of transient component in the combined energy and enstrophy was examined to identify the transient motion of the atmosphere with the two-dimensional homogeneous isotropic turbulence. The vertically averaged slope of kinetic energy and enstrophy in the lower (upper) troposphere was close to the value at 700 (200) hPa level.
The spectral fluxes of kinetic energy and enstrophy in the wavenumber range 14≤n≤25 satisfied, to a very rough approximation, the criteria of inertial subrange. The stationary fluxes were small. The estimated stationary-transient component of flux was larger, comparable and less than the corresponding transient flux of APE, KE and EN.
Representative levels for computation of energy and enstrophy spectra and their fluxes in the lower and upper troposphere were identified.
pp 351-366 June 1993
Directional wave data collected during an experiment at a location on the continental shelf of the south west coast of India using a WAVEC buoy, have been analysed based on the technique of Kuiket al (1988). The observed wave spectra indicate that the wave field is composed of sea waves (with peaks around 0·18 and 0·23 Hz) travelling nearly in the wind direction (WNW-N), and lower frequency (0·09 Hz) swell waves from the South. The parameterization of the wave directional spread shows that both local wind conditions and nonlinear wave-wave interactions control the shape of the directional distribution. The directional distribution is generally bimodal in the transition region between sea and swell and at higher frequencies when rapid changes in wind speed and direction occur.
pp 367-381 June 1993
It is usually accepted that a time-predictable model of earthquake occurrences is better than the so-called slip-predictable model. Here a size-interval relation (SIR)-predictable model is proposed which combines the features of the time-predictable and slip-predictable models. Unlike a constant, and hence nonpredictive, relation between the size of the next earthquake and the inter-event interval, given by the so-called slip-predictable model, the SIR-predictable model prescribes such a relation contingent upon the size of the previous earthquake. Unlike the time-predictable model, instead of predicting the time interval, it proposes a size-interval relation. Using data about a seismogenic source called Cephalonia in Greece, the superiority of the SIR-predictable model over the time-predictable model is illustrated. The SIR-predictable model can be made more efficient by employing two-stage nonlinear estimation procedures motivated by the initial work by Stein. Introducing these procedures to seismologists is an independent objective of this paper. Also, Stein estimators have a dimensionality threshold. This work discusses two techniques of threshold extension.
pp 383-398 June 1993
The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.
pp 399-413 June 1993
On the basis of field relations, petrography and chemistry, three types of granitoids are recognized at Malanjkhand in and around the copper deposit over an area of about 200 km2. These are (i) a fine grained ‘leucogranite’ of restricted occurrence in the surrounding area (Gr-I); (ii) coarse-grained, grey in most parts, gneissose granitoid of regional extension (Gr-II); and (iii) the pink-feldspar bearing massive type hosting the mineralization with occasional representatives in the surrounding country (Gr-III). Gr-I comes out as a distinct entity on the basis of cross-cutting relation and mineralogical and chemical composition, the Rb-Sr whole rock isochron also giving a younger age than the other two groups irrespective of the regression model used. Gr-II comes out as the oldest unit but its age relationship with Gr-III cannot be established unequivocally. An uncorrelated error regression model establishes the age relationship as Gr-I<Gr-III<Gr-II, whereas a two-error regression model establishes temporal closeness between Gr-II and III.