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|Title:||INVERSION OF MICROEARTHQUAKE ARRIVAL TIME DATA- A GENERALIZED INVERSE APPROACH|
|Authors:||Katiyar, G. C.|
EARTH SCIENCE ENGINEERING
|Abstract:||Man's ingenuity has enabled him to travel into outer space and watch the earth below him or to explore other pla-nets of the solar system. But our efforts of exploring the earth interior have as yet failed to unravel much except for a mere 8 km of the 6371 km journey to the centre of the earth and the earths interior still remains by and large inscrutable to direct observations. Fortunately, seismic waves that set out from an earthquake source travel through the earth's inte-rior and return to the surface with valuable details of their reconnoitering journey.. Our present knowledge of earths zoned interiors has enlarged mainly from what these waves have divu-iged. Travelling through the earth these waves bear signatures of the elastic properties of the materials traversed as well as of their disposition, which can be decoded from the records which they bring to the surface. These waves furnish us with a powerful means to unravel enigmatic structure and processes of the regions not accessible otherwise. Major earthquakes which release enormous energies stimulate large volumes of the earth and bring forth infor-mation relating to earths structure over a wide region. Waves issuing from them sample a vast space and are therefore capa-ble of yielding only 4 gross information regarding the earth structure involved. 2 However, when one is interested in delineating finer details of the earth structure, one must turn one's attention to physical processes which are 'spatially con-fined. One such possibility is furnished by microearth-quakes which are extremely small earthquakes involving a much smaller region and releasing very little energies of the order of 105 to 108 joules. These quakes like:, small sources of sound unheard beyond distances, are perceptible within a rather narrow zone around the source and become imperceptible to even the most sensitive instruments at large distances. They are largely the manifestations of a progressive failure of the rock masses of a fault undergoing slow deterioration under strain and generally prevail in the hiatus between major quakes. In the vast tracts of the Himalaya, an active young tectonic belt on the face of the earth, the structure of the crust has been rendered extremely complicated by unrelenting tectonic stresses active since the collision of the Indian subcontinent with the Tibetan Plateau over 0 million years ago. This intricate structural feature of the crust repre-senting complex interactions between the lithospheric plate and the underlying asthenosphere must in turn have produced unusual kinematic conditions leading to the development of equally unusual geochemical, magmatic and geomorphological environments especially conducive to the emplacement of rare earth resources, whose potential except for that of the water 3 power, remains largely unrealized. The kinematical con-text of the region, if understood adequately could thus provide a viable base for a ground strategy of earth resources planning and eventual development of the region. Besides, a study of the kinematics of active region has the added attraction of sheding significant light on the physics of the earthquake processes which could eventually bring us nearer to the dream of predicting earthquakes. Systematic investigations of the kinematics of the Himalayan region using an array of recorders recently under-taken by the Deptt. of Geology and Geophysics, University of Roorkee are a sequel to this realization and the present work is an attempt to provide software support to some as-pects of this project. An array of microearthquake recorders has been installed in the Garhwal Himalaya to investigate the beha-viour of its various lineaments. Microearthquakes falling on the high frequency end of the logarithmic frequency - magnitude law, occur fairly well spread in space and time, thereby illuminating the Theological response of rock masses of active lineaments. Besides they yield revealing informa-tion regarding the failure processes which might provide significant clues to earthquake prediction and exciting prospects for hypothesis confirmation, 'Fbssil'tectonic zones have been found to correlate well with earthquake activity which, however, being few and far during historical periods would be unable to furnish a definitive picture. Micro- earthquakes, on the other hand, with their exuberant number r;1 are capable of elucidating these zones in sufficient detail. Continuation of weak planes in the vertical dimension may highlight aspects of associated minerali-zation or geothermal energy derived from the upper mantle. A panorama of seismotectonic activity in. its various aspects can help evolve a kinematic - tectonic model of complex regions such as the Himalaya which whilst delineating a rationale for the planning and management of earth resources of the region could highlight certain enigmatic aspects of their origin. One of the most important task in any microearthquake investigation is to locate precisely their sources as well as their magnitudes and study their mechanism. The problem of determining the spacio - temporal coordinates of seismic events from the observed arrival time data is basically a geophysical inverse problem wherein one is concerned with deciphering the distribution of material property of the medium and the location and the nature'cf the source from the physical field created by it and recorded at number of points. With the advent of modern high speed digital com-puters, investigators in various fields of geophysics have resorted increasingly to automated techniques for solving geophysical inverse problems. One of these techniques, powerful and versatile, that came into limelight in this decade is the generalized inverse technique. This techni-que is capable of providing somewhat meaningful answers to geophysical inverse problems even when the data may be 5 insufficient or inaccurate. Mathematical simulation models of real field conditions incorporating geolo-gical and geophysical information already gleaned can be used to illuminate through numerical experimenta-tion, the structure and characteristics of the various data elements, as well as the morphology of the problem. This imparts us a prescience regarding the vital data points, their information content, their capacity for resolving unknown parameters and efficacy in error elimi-nation through properly designed 'response' of the genera-lized inverse. A pre-knowledge of information distribu-tion expedites optimal selection of data density for econo-mizing the information/cost ratios, resolution of parameters and noise filtering. A knowledge of the information distri-bution among various data points and resolution of parameters and uncertainties in data, facilitates the sifting of signi-ficant data points to be incorporated in the analysis with-out sacrificing the substance of the solution. This help us to optimize interpretation/cost ratios.|
|Appears in Collections:||MASTERS' DISSERTATIONS (Earth Sci.)|
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