New England Seismic Network News

March 29, 2011

Are we in a period of increased earthquake activity ?

Seismologists normally expect, on average, one magnitude 8 earthquake to occur per year, somewhere in the world, and, again on average, just one magnitude 9 earthquake per decade. While these numbers are just expectations of averages, the recent Japan earthquake (Mw 9.0), following just 7 years after the 2004 Mw 9.1 Sumatra earthquake, and just 1 year after the 2010 Mw 8.8 Maule, Chile earthquake, raises the question: Are we in a period of increased global earthquake activity ?

To answer this question, I produced a spreadsheet using the largest (Mw >= 8.5) earthquakes in the world since 1900 (which is around when we began recording earthquakes instrumentally so that we can accurately measure earthquake magnitudes).

When you go to clean up your computer disk, you probably start by arranging the files in order of decreasing size, with the largest files at the top. You do this because it’s much more efficient to remove a single, large file, then to track down and remove many small files. In the same way, a single large earthquake (e.g., Mw >= 8.5) releases much more energy than all of the smaller earthquakes put together, and accounts for most of the seismic energy release within a given time period.

Table 1. Largest earthquakes (Mw >= 8.5) in the world since 1900, arranged chronologically and converted to energy.

Mw_Energy_Released_Since_1900

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