Delhi earthquake of 25 November 2007 (Mw 4.1): Implications for seismic hazard

Abstract

The Delhi earthquake of 25 November 2007 (Mw 4.1) is the best recorded local event ever. Peak ground acceleration (PGA) at seven sites in the epicentral zone ranged between 7 and 118 Gal. An analysis of the earthquake leads to the following conclusions. (1) It occurred at a depth of 30 km and involved strike-slip faulting with some normal component, as against thrust faulting with minor strike-slip component previously reported for the region. (2) Observed PGA of this earthquake is poorly explained by the attenuation relation that has been used in the seismic hazard estimation of the Delhi region. This relation is also inconsistent with data from well-recorded Himalayan arc earthquakes. (3) Recordings from the 2007 earthquake reconfirm that there is a large and variable site effect in Delhi which differs significantly from that estimated from bore-hole penetration test data. These results, together with those from a previous study of two small events in Delhi, and an examination of the seismicity of the region, lead us to conclude that our present knowledge of seismotectonics and seismic hazard of Delhi is fraught with uncertainty, emphasizing an urgent need for improved and enlarged seismic instrumentation.