Assessment of Earthquake Size from Rock Fracture Dimensions of Seismic Source in Geotechnical Earthquake Engineering Investigations

The paper presents two case histories of rock fractures resulting in strong ground motion. Case I describes the November 27, 1971 rock burst involving 270 m long rupture with displacement of 15 mm in Nundydroog mine in Kolar Gold Field in Karnataka State, India, which resulted in damage to buildings on the surface and underground works in the mine. The event was recorded as an earthquake by seismic stations and the computed magnitude ml = 3.9 of the rockburst compares well with magnitude mb= 4.6 determined from the recorded seismogram. Case II describes April 5, 1976 rock burst in an open cast granite quarry at Shravanbelagola, in Hassan district in Karnataka State, resulting in 60 m long fracture with estimated average displacement of 0.5 mm. The rockburst accompanied with loud noise produced strong ground motion in the area. The magnitude of the rockburst is estimated as ml= 2. The estimated intensity of ground motion in the area match well with the ground motion at the epicentre of an earthquake of similar magnitude. The two studies provide guidelines for assessment of earthquake size and indicates the importance of detection and delineation of tectonic lineaments and determination of ambient seismic activity along lineaments which are capable or have potential for earthquake occurrence