Ireland is shaken by two to three earthquakes a year but no one other than a seismologist ever notices. The quakes in this region are so weak that they usually can't be felt.
A rare exception to this general rule shook the east coast in July 1984. While most quakes around our coast measure between one and two on the Richter scale, this event reached 5.4 on the Richter scale, a system devised by Charles Richter and colleagues at the California Institute of Technology in the mid-1930s.
The Sumatran earthquake was one of the biggest generally recorded, a magnitude nine event. The quake that decimated Kobe, Japan in 1995 measured above seven, while the 1906 San Francisco earthquake measured above eight. However, there is a huge difference between the energy released in these various events.
A magnitude eight event is 30 times stronger than a magnitude seven event but a magnitude nine is 900 times stronger than a magnitude seven. Estimates suggest the quake off Sumatra that caused such devastation released energy equivalent to 23,000 times that emitted by the bomb that destroyed Hiroshima.