Yesterday's South Iceland earthquake much greater than previously thought

The earthquake that occurred last night 6 km North East of the town of Selfoss is much greater than previously reported. The Iceland Met Office said that it was of a magnitude of 3.4 but now it reports that the tremor was actually of a magnitude of 4.1.

This is a considerable difference. Over 400 quakes have been detected with four of them measuring over 3 on the Richter scale. 
The area is known for powerful earthquakes and earthquakes up to 6.5 have been measured there in the past.

http://icelandmonitor.mbl.is/news/nature_and_travel/2017/10/21/yesterday_s_south_iceland_earthquake_much_greater_t/