An article by Fairfax on the magnitude 6.3 earthquake that devastated the city of Christchurch in New Zealand includes an interactive diagram of several earthquakes which are compared by the relative sizes of circles shown on a map.
The magnitude 7.9 earthquake that occurred in Sichuan, China on 12 May, 2008, appears nearly as large as the catastrophic magnitude 9.1 earthquake near northern Sumatra on 26 December, 2004. It is not a meaningful comparison, because earthquake magnitudes are measured on a logarithmic scale. An increase in magnitude by one unit represents a tenfold increase in the maximum amplitude of the seismic waves recorded by the seismograph and about 32 times the amount of energy released. Thus the magnitude 9.1 earthquake near northern Sumatra had an amplitude 109.1 - 7.9 = 15.8 times larger than the magnitude 7.9 earthquake in Sichuan, China, and 109.1 - 6.3 = 631 times larger than the magnitude 6.3 earthquake in Christchurch.
According to an empirical relationship developed by Beno Gutenberg and Charles Richter, log10E is proportional to 1.5M, where E is the energy released by the earthquake and M is the magnitude. This means that in terms of energy released, the magnitude 9.1 earthquake was about 101.5(9.1 - 7.9) = 63.1 times stronger than the magnitude 7.9 earthquake, and about 101.5(9.1 - 6.3) ≈ 15800 times stronger than the magnitude 6.3 earthquake.
No comments:
Post a Comment