EVALUATION OF EARTHQUAKE PARAMETERS IN KOYNA AND DAKPATHER REGIONS

Abstract

"Investigations of earthquake hazards to important engineering projects in seismically active regions of India have been gaining increased imoortance. The Koyna Hydro electric Project in the Peninsular Shield, in the Maharash tra State and the Yanuna River Valley Project in the Himala yas, in the Vlestern Uttar Pradesh are two such projects and therefore investigations were planned in these two regions with a view to studying the following:- (1) The response of existing engineering structures during various earthquakes and the average response spectra for the Koyna region on the basis of three mailable accelerograms, (ii) Possible source mechanism 3nd the cause of the recent seismic activity in the Koyna region. (iii) Order of the tectonic stress field, if any, operative in the Dakpather region. (iv) The rate and nature of creep displacements along the known planes of recent movements in the Dakpather region. As, geophysical instruments needed for such studies were not readily available, it was first necessary to design and develop some of these, which constitute the major oart of the work. The instruments designed and developed for the purpose were as follows:- (ii) (i) Multiple Structural Response Recorder and Roorkee Seismoscope: These devices produce records to give the maximum relative displacement resoonse of idealised engin eering structures which are dynamically equivalent to their pendulums. (il] Portable Water Tube Tiltmeter: This system is capable of measuring small scale seoular ground tilt associated with elastic straio energy accumulation and release as well as with rock ereep or movements across known faults or thrusts. For the studies in the Koyna region, all these instru ct were used whereas in the D,kpather region only the Por table Water Tube Tiltmeter and Multiple Structural response Recorders were employed for measurements. The data thus obtained though useful for the evaluation of earthquake hazards, was not in itself sufficient to permit a complete analysis and therefore, all other relevent data available for the two regions were also studied. During the period of investigation, the Koyna region has exhibited greater seismic activity as compared with the Oakoather region. It was, therefore, possible to make a more comolate study for the former. However, for the Dakpather region a qualitative analysis was possible leading to some broad conclusions. In the Koyna region, one event was simultaneously recorded by the MuUiple/^sp'ons^ Recorder, the Roorkee Seismo scope and aUnited ilectrodynamics XR-240 Accelerograoh. The two dimensional plots of the computed relative displacement

response in the horizontal plane were prepared and compared to the records of the seismoscopes. The comparison brings out the potential of these simplified devices for collection of Structural response data. Strong motion records were available for three of the Koyna earthquakes, on the basis of which average velocity response spectra for the Koyna region have been computed. Further more, analyses of these records have.been carried out to permit comparison of important characteristics. A model h^s been proposed for the source region which is in Conformity with the xoove common features as well as with Oth§r observations nade during the earthquakes. The seismic activity has been explained as arising from the tectonic re orientation af dykes due to shear. In the Dakpat:aer region, the Portable Water Tube Tilt-. Meter was employed for the Measurement of small displacements due to tectonic stress field, if any, across a plane of subrecent movement and the Nah^n thrust. Other indirect evidence of recent movements ?nd these observations have been discussed. The temporary seismological observatory in the area recorded some events with epicentres in the region. But the tilt measure ments do not give any conclusive evidence of tectonic movements. Some estimates of creep and seismic disolacement were made on the basis of recorded data. It is felt that continued measure ments over a number of years may provide a better understand ing of the tectonic activity."