An earthquake measuring 5.1 on the Richter scale today rocked Assam, according to the Regional Seismological Centre. There was no immediate report of any damage or casualty, officials said here. According to a report of the Regional Seismological Centre at Shillong, the epicentre of the medium intensity earthquake, was at a place 22 kilometre from Dhing in Nagaon district. The epicentre was at latitude 26. 2 degree North and Longitude 92.5 degree East.