Recurrence relations of Koyna and Bhadrachalam Aftershock activity

Estimates of seismic risk require a recurrence relation between the expected number of earthquakes per year and a measure of earthquake size. The recurrence relations of Koyna and Bhadrachalam after schocks are discussed in the light of low seismicity and poor geographic distribution of seismographic stations in south India. As the definition of an ‘active fault’ may vary according to the type of land use contemplated, it is emphasized that multi instrument micro-earthquake investigations should be carried out in order to take into account even the smallest earthquakes and know not only which faults may move but how they may move.