Re: How many earthquakes have ther been in the history of the world?

Your question actually breaks into two questions: How many earthquakes have 
scientists recorded in recent human history? And How many earthquakes do 
scientists estimate have occurred in the history of the world (which is a 
long time, about 4.6 billion years!)?

Basically, humans have been experiencing earthquakes for as long as humans 
have been around.  But it is only in the last hundred years that scientists 
have been able to measure and record earthquakes on a world-wide 
scale.  This is for a few reasons.  First of all, scientists needed to 
figure out how to measure earthquakes and determine how strong they are and 
where they are located.  Then, 
scientists needed to establish a number of observational stations around the 
world to record the motion of the Earth during an earthquake.  Only if three 
(or more) different observing stations record an earthquake using 
seismometers (machines used to measure the motions of the Earth during 
earthquakes) can the location of the earthquake and its magnitude be 
determined precisely enough to map it.  If you only have one station, you 
might be able to say that the earthquake happened "in that direction over 
there", but nothing very accurate.  In 1931, there were approximately 350 
seismic stations around the world.  Today there are over 8000 stations and 
so we are able to detect many more and smaller earthquakes than ever before.  
It’s easier to detect earthquakes today as well because of the internet and 
satellites, allowing scientists to measure an earthquake and talk to each 
other rapidly from the far reaches of the globe to compare their data and 
determine where the earthquake happened.  

The United States Geological Service runs an excellent site to monitor 
earthquake activity around the world and also contains a lot of statistical 
data about earthquakes at their National Earthquake Information Center 
(NEIC) (see link below).  Basically, they give information about all the 
earthquakes that have been recorded since 1977 as well as limited 
information on the largest earthquakes that have happened since 1900.  They 
also have excellent maps of earthquake locations throughout the world.  I 
highly recommend that you check out their website.  

Here are their estimates of the number of earthquakes occurring around the 
world in one year based on the size of the earthquake (I took this directly 
from their webpage):

Great (Magnitude > 8.0):	1 per year
Major (Mag = 7.0 – 7.9):	18
Strong (Mag = 6.0 - 6.9):	120
Moderate (Mag = 5.0 – 5.9):	800
Light (Mag = 4.0 – 4.9):	6,200 (estimated)
Minor (Mag = 3.0 – 3.9):	49,000 (estimated)
Very Minor (Mag < 3.0):		9,000 per day (estimated)

(For more information about what the magnitude of an earthquake means and 
its relationship to the energy and destructive power of an earthquake, the 
NEIC has a fantastic Frequently Asked Questions page.)  

These numbers help to address your question because the rate of earthquake 
activity on Earth has most likely been fairly constant in recent geologic 
history (in the last few millions of years).  Therefore, these numbers 
should give a good estimate of how many earthquakes have happened in 
recorded history throughout the world (although, remember that the vast 
majority of the small earthquakes are never recorded and earthquakes below 
magnitude 4.0 are generally not even felt by humans).

Another excellent site for looking up statistics about earthquakes that have 
occurred since 1977 is the Harvard Seismology CMT Catalog (link below), 
which has a searchable database of earthquakes.  Finally, because you are 
from New England, you may be interested in the New England Seismic Network 
that is run by Weston Observatory at Boston College (link below), which 
monitors earthquake activity in New England.  You can find out if there have 
been recent earthquakes near where you live!  For example, there was a 5.1 
magnitude earthquake in Plattsburgh, NY, on April 20, 2002.  Did you feel 
it?  I was in Rhode Island and I didn’t feel it, but some of my friends did.  
(Some people are just more sensitive to earthquakes than others.)

Now, the harder part of your question is estimating how many earthquakes 
have happened in the history of the Earth (4.6 billion years).  This is 
indeed an extremely difficult question!  In fact, I couldn’t find any 
general information about how the expected numbers of earthquakes per year 
might have changed as the Earth’s surface has evolved over its entire 
history.  In talking with some seismologists, it seems that scientists have 
not yet addressed this question.  So, even though this is a difficult 
question, the best way to estimate an answer is fairly simple.  Just choose 
what magnitude earthquakes you are interested in and multiply NEIC’s 
estimate of the number of those earthquakes per year by 4 billion (assuming 
that there were no earthquakes when the Earth was very very young)!  To 
keep the number fairly reasonable, you might want to use a magnitude cut-off 
of 4.0 and above as representative of earthquakes that can be felt by a 
human standing on the surface.  A magnitude cut-off of 6.0 and above might 
be a good estimate of earthquakes that cause damage.

I had a lot of fun answering this question and I wanted to thank you for 
being patient in my response time; I have been travelling for the last few 
weeks or else I would have responded sooner.  Hope these websites help!

USGS National Earthquake Information Center: http://neic.usgs.gov/

Harvard Seismology CMT Catalog: http://www.seismology.harvard.edu/

Weston Observatory at Boston College: http://www.bc.edu/bc_org/avp/cas/wesobs/