Graph of magnitude 5 or greater earthquakes from 1900- April 2011

The United States Geological Survey (USGS) keeps track of earthquake locations and magnitudes, accessible on the USGS website.  Modern recording equipment has been in use since about 1900. While this may seem like long ago, this only provides a VERY short reliable earthquake record. This graph shows the magnitude of earthquakes (looking only at those greater than M = 5) versus time since 1900.

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Submitted by The Ocean Portal Team on

This is a great question. Dr. Elizabeth Cottrell, a geologist from the Smithsonian Institution, answered a related question in 2011 on the Ocean Portal in response to her video about the 2011 Japanese earthquake and tsunami: http://ocean.si.edu/ocean-videos/understanding-japan-earthquake-and-tsunami. Here is an excerpt of Dr. Cottrell's response, which may answer your question:

The United States Geological Survey (USGS) keeps track of earthquake locations and magnitudes. You can get them from the USGS website. Modern recording equipment has been in use since about 1900. While this may seem like long ago, this only provides a VERY short reliable earthquake record. I’ve plotted the magnitude of earthquakes (looking only at those greater than M = 5) versus time since 1900.

The first thing to note is that, many more total earthquakes have been recorded in the last decade – that is only due to the increasing number and sensitivity of recording equipment (seismometers). We can presume that just as many earthquakes were happening in the past, but were not recorded. The second thing to note is that, over the period for which we have records, there has been no statistical increase in the number of very large (> M = 8) earthquakes in the last decade. The mean earthquake magnitude (again, looking only at earthquakes with magnitude > 5, so a small subset of all total earthquakes) on this plot is 7 and that standard deviation is 0.7 (meaning that 84% of earthquakes on this plot have magnitudes less than 7.7, and 97.6% of earthquakes on this plot have magnitudes less than 8.4). We saw “higher than average” numbers of very large quakes in the 1950s and 1960s as well and earlier in the 1900s. In fact, looking at this plot makes might make one wonder why we had so few large quakes in the 70s and 80s! So the long term record indicates that the last decade is just random statistical fluctuation in the number of earthquakes. For more on this stance, see the USGS website.

This isn’t the end of the story. Prominent seismologists have argued otherwise. On this even shorter time scale, it looks like we have an increase in the last 6 years especially.

The article I link to above proposes some mechanisms that might cause earthquakes in one location to trigger others in distant locations (to answer the “why” part of your question). However, when scientists have looked at the problem statistically, as that New Scientist article describes, there is no evidence to back this up.

-Submitted by Dr. Elizabeth Cottrell, Geologist, Smithsonian Institution

Submitted by Anonymous (not verified) on

Is it just because detection sites have increased since 1900 that seems to indicate an increase of earthquake activity nearer the 21st century???