Tuesday, November 20, 2012

Earthquakes: How Often?

Until the following question arrived, I had only thought of earthquakes as being relatively localized features, mainly occurring along continental margins and ocean-floor spreading centers or transform faults. The San Andreas fault is a mostly on-land transform fault. I hadn't really thought about how MANY earthquakes there are in the world as a whole.

Hello, I was wondering how many earthquakes happen typically in a year?
-Lauren J


The USGS estimates that several million earthquakes occur in the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The National Earthquake Information Center in Denver, CO, now locates about 50 earthquakes each day, or about 20,000 a year. There are far fewer large events than small earthquakes, and this website will show you how these are parsed out according to magnitude:

There are lots of fascinating stats here, including an interesting table (below). Like asteroid impacts and frequency, or volcanic eruption magnitude and frequency, these all seem to follow an inverse log law:
The bigger the event, the less common it is.

Frequency of Occurrence of Earthquakes

MagnitudeAverage Annually
8 and higher
7 - 7.915 
6 - 6.9134 
5 - 5.91319 
4 - 4.913,000
3 - 3.9130,000
2 - 2.91,300,000


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