MadSci Network: Earth Sciences


Date: Thu Mar 7 11:38:49 2002
Posted By: Jennifer Anderson, Grad student, Geological Sciences, Brown University
Area of science: Earth Sciences
ID: 1014801280.Es

The broad question you are asking is an excellent one!  Are there more 
earthquakes where lots of people live?  In other words, are earthquakes 
caused by highly populated places?  It often seems like this might be the 
case, but it actually turns out to be a big coincidence.  Be sure to check 
out some of the webpage links that Iíve attached below that show global maps 
of earthquake locations, magnitude and depths.  

One of the things you can immediately notice from these maps (especially the 
first link) is that earthquakes donít appear randomly around the world.  
Instead, there seem to be definite patterns of where earthquakes happen.  
For example, look at all the earthquakes that happen around the edge of the 
Pacific Ocean.  Also, there are long strings of earthquakes that happen in 
lines through the middle of the oceans.  In general, earthquakes outline the 
edges of tectonic plates.  Tectonic plates are large pieces of the Earthís 
crust that are moving very slowly across the globe.  You can think of these 
plates as floating on top of a more fluid, hotter and deeper layer of the 
Earth, called the mantle.  At the center of the oceans, the tectonic plates 
are spreading apart and hot mantle material is flowing up, cooling and 
creating more oceanic crust.  At these regions, called mid-ocean ridges, 
there are small, shallow earthquakes (the lines of earthquakes in the 
centers of the oceans happen along the mid-ocean ridges).  At other places 
on the Earth, like around the edge of the Pacific Ocean, these tectonic 
plates are crashing into each other (very slowly) and the oceanic crust 
plunges beneath the continent.  In this case, you can get very large 
earthquakes that occur deep inside the Earth as the plate moving downward 
pushes against the surrounding mantle material.  These regions are called 
subduction zones and the edge of the Pacific Ocean is surrounded by 
subduction zones, so there are lots of earthquakes that happen at the edges 
of continents next to the Pacific Ocean.  The third type of boundary between 
tectonic plates is called a strike-slip zone, where the plates are sliding 
past each other.  The most famous strike-slip zone in the world happens to 
be all along the coast of California, through Los Angeles and San Francisco.  
At strike-slip zones, you can get very dangerous earthquakes happening at 
shallow depths.

OK, that was a lot of explanation just to describe the earthquake patterns 
that you see on the world maps below.  Back to your question about people 
and earthquakes.  Again, although it may initially appear like places with 
more people have more earthquakes, that isnít actually the case at all.  
Rather, it is a complete coincidence.  First, there are many earthquakes 
that happen where people donít live at all, like in the middle of the 

Second, if you just look at the United States, while there are lots of 
people who live on the west coast, there are even more people who live on 
the east coast.  But, there are many more earthquakes that happen along the 
west coast of the United States.  This is because the west coast of the 
United States is actually a subduction zone (in Washington and Oregon) and a 
strike-slip zone (in California).  Thus, you can get very intense 
earthquakes all along the west coast.  Plus, most of the worldís population 
lives at the edges of continents, where there is a beautiful ocean, warm 
temperatures and wonderful sights.  This is dangerous, especially if the 
plate boundary near by can have large earthquakes along it (which is often 
the case).  In fact, many geologists worry about the huge cities that are 
built in dangerous earthquake regions, especially if the people who build 
the cities are not careful and build buildings that can be destroyed easily.  
One particular geologist who is very worried about population growth at 
tectonic plate boundaries is named Roger Bilham and he is a geologist at 
Oxford University in England.  Iíve referenced two of his papers that might 
be interesting to you at the bottom of the list of web pages.  Just to quote 
from his letter in the journal Nature (1999), "About one-third of the 
worldís supercities Ė those with populations of more than two million Ė are 
located near tectonic plate boundaries, where damaging earthquakes have 
occurred and will recur".  

Third, I think one of the main reasons that there seems to be a connection 
between large numbers of people and earthquakes is because of the news that 
we get from around the world.  Earthquakes (especially small, not dangerous 
ones) are extremely common around the Earth.  Iíve listed a link below which 
will show you the total number of earthquakes in the past 30 days, and itís 
amazing how many earthquakes there are every day!  Luckily, often these 
earthquakes are so small that you canít even feel them.  However, when a big 
earthquake strikes, weíre more likely to hear about it in the news if it 
struck a place with lots of people, like the earthquakes that happen in 
California and recently in Turkey.  The truth is, we tend to hear more news 
if the earthquake was dangerous and damaged a lot of property and possibly 
hurt a number of people.  So, unless we dig deeper than the evening news 
shows, we often only hear about large earthquakes in populated cities.  For 
all of these reasons, geologists definitely agree that it is not high 
numbers of people that are causing earthquakes, but rather that people tend 
to like living at the edges of continents and thatís where many earthquakes 
happen.  So it is a complete coincidence.

But you also had another more subtle question and that is about the affect 
of water on earthquakes.  First, there is no evidence at all that 
earthquakes follow any kind of yearly pattern.  That is, there arenít any 
more earthquakes during the summer than the winter, or during dry seasons 
more than wet seasons.  Earthquakes happen at all times of the year, under 
all sorts of different conditions.  Second, if water somehow affected 
earthquakes, it would only affect shallow, relatively low intensity 
earthquakes in the upper few miles of the Earthís crust.  Again, there is no 
evidence that this is common.  Third, and this is a bit strange, there isnít 
any evidence to suggest that pumping water out of the ground causes 
earthquakes, but there is a little bit of evidence that suggests if you 
pump, or inject, water into the ground, small shallow earthquakes might 
happen.  Geologists studied this effect for a while as an idea for 
triggering small earthquakes (so small that you wouldnít be able to feel 
them) so that larger earthquakes could be avoided.  In the end, they decided 
that even when pumping water into the ground, they could not easily predict 
the size and location of the small earthquakes they were creating.  Also, it 
is just a bad idea in general to try and make earthquakes.  Instead, 
geologists are trying very hard to learn how to predict when a large 
earthquake might occur.  Also, engineers and geologists are working together 
to design and build buildings that will be able to withstand the forces of 
earthquakes, thereby reducing the damage to property and human life that 
occurs with large earthquakes.

Itís a fascinating topic, and this has been a very long answer.  I hope this 
helps you out.

Map of the world with earthquake locations on it:

Last thirty days of earthquake activity in the world:

All sorts of earthquake data can be found at the United States Geological 
Surveyís earthquake homepage and their National Earthquake Information 

Two articles by Roger Bilham about population and earthquakes:

Bilham, R. (1999) "Millions at risk as big cities grow apace in earthquake 
zones", Nature, vol 401, pg. 738.

Bilham, R. (1988) "Earthquakes and urban growth", Nature, vol 336, pp. 625-

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