icenter. One every 5 to 10 years
Earthquake Magnitude Classes
Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude.
Class Magnitude Great 8 or more Major 7 - 7.9 Strong 6 - 6.9 Moderate 5 - 5.9 Light 4 - 4.9 Minor 3 -3.9
http://www.geo.mtu.edu/UPSeis/magnitude.html viewed 08/27/14
There seemed to have been an increase in earthquakes around 1900 as well as 1940, and lately. This means that though there are averages, some periods of time end up with more as the USGS indicates that earthquakes often come in "clusters" (Cherundolo G. Are Earthquakes Really Increasing? March 9, 2010. http://www.accuweather.com/blogs/news/story/25904/has-the-number-of-earthquakes.asp).
Various sources track earthquakes.
Here is a link to a list of earthquakes for every year this century:
From April 2010 to March 2011, I counted that there were 16 major and 1 great earthquakes.
From April 2011 to March 2012, I counted that there were 14 major and 0 great earthquakes.
From April 2012 to March 2013, I counted that there were 13 major and 3 great earthquakes.
From April 2013 to March 2014, I counted that there were 13 major and 1 great earthquakes.
The first three days of April 2014 had 1 major and 1 great earthquake.
In the past eight years or so, there were about 14 USGS defined great earthquakes. This exceeds the 'norm.'
Roger Meyers reported the following about the power and frequency of earthquakes on March 29, 2011:
The old Richter scale has been replaced with the Moment Magnitude scale, though results are usually similar to the Richter scale. Either earthquake measuring scale is somewhat misleading if you aren’t familiar with it. For instance, a magnitude 7.2 earthquake produces 10 times more ground motion than a magnitude 6.2 earthquake, but the energy released is 32 times greater. But the energy released in an 8.2 magnitude quake is 1000 times more than the 6.2 magnitude quake...
The United States Geological Service records hundreds of thousands of earthquakes that occur every year, though most of these aren’t noticed because they are so small in magnitude. In the last 30 days, there were roughly 400 earthquakes of magnitude 4.0 or greater. The earth is doing a lot of shaking and quaking! (Meyers R. Earthquakes in diverse places. Commentary, March 29, 2011)
In 2011, there were more earthquakes in the of 5.0-6.9 range per the USGS (http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php).
The rate of earthquakes has accelerated greatly in certain areas since 2009:
A new United States Geological Survey study has found that middle America between Alabama and Montana is experiencing an "unprecedented" and "almost certainly manmade" increase in earthquakes of 3.0 magnitude or greater. In 2011, there were 134 events of that size. That's six times more than were normally seen during the 20th century. While the changes in the area's seismicity began in 2001, the trend has really accelerated since 2009, the geologists note. (Madrigal A. Middle America Is Experiencing a Massive Increase in 3.0+ Earthquakes. The Atlantic, April 6, 2012. http://www.theatlantic.com/technology/archive/2012/04/middle-america-is-experiencing-a-massive-increase-in-30-earthquakes/255568/
A record earthquake occured on April 11, 2012 and seems to be part of an accerating trend:
LOS ANGELES (CBS) — Four large earthquakes struck around the Pacific Rim Wednesday with one setting a record, as the most powerful strike-slip quake ever recorded. (Cluster Of Large Quakes Serve As Reminder For Preparedness. April 12, 2012. http://losangeles.cbslocal.com/2012/04/12/cluster-of-large-quakes-serve-as-reminder-for-preparedness/)
In 2001, there were 137 earthquakes of magnitude 6.0 or greater and in 2011 there were 205...a magnitude 8.6 earthquake struck off the coast of Indonesia and that was rapidly followed by a magnitude 8.2 earthquake off the coast of Indonesia...Normally we only see about one earthquake of magnitude 8.0 or greater per year. The magnitude 8.6 earthquake was the most powerful strike-slip earthquake in recor