Re: What is the main cause of earthquakes and why would this be the main cause?

Dear Zebulon,

The short answer to your question is that earthquakes occur when movement 
suddenly takes place along lines of weakness in the Earth’s crust called 
faults.  Because the crust is composed of rocks that are cold and brittle, 
they behave rather like a piece of brittle toffee when you try to bend 
it:  at first the toffee does not break because it is able to withstand 
some of the stress you place on it, but if you use enough force it will 
suddenly break, and the line of breakage is, in effect, a fault.  Strain 
is the response of rocks to the stress placed upon them; an earthquake 
takes place when the strain in rocks in a fault exceeds a critical limit 
and the rocks in the fault break suddenly.

The greatest source of stress placed on rocks in the crust arises from the 
process of plate tectonics.  The crust is composed of a number of large 
slabs, called tectonic plates, which slowly move in response to forces 
applied from the mantle below.  As a crustal plate moves it exerts force 
along its boundary with neighbouring plates, which are also moving.  So it 
is no surprise that most earthquakes take place along the boundaries of 
the tectonic plates because these boundaries are the regions where rocks 
are being subjected to great and constant stress.

There are three main types of plate boundary.  The type that is most 
familiar is called a transcurrent boundary, where two plates are sliding 
past each other.  Transcurrent boundaries give rise to transcurrent 
faults, also called strike-slip faults.  The prime example is the San 
Andreas Fault in California and Mexico.  Here the Pacific and North 
American plates are moving past one another, with the Pacific Plate moving 
north.  Along many parts of the San Andreas Fault there is a constant, 
small amount of movement or creep, perhaps a few centimeters per year, 
that continually relieves strain and prevents it from building up to a 
critical level.  In other areas, however, the San Andreas Fault tends to 
lock, and the strain will build up over decades or even centuries in these 
areas until the rocks suddenly break.  Then this area of the fault moves 
violently, and the force released radiates out through the crust as a 
series of waves which we experience as an earthquake.

A second type of plate boundary where many earthquakes occur is along 
constructive plate boundaries, where new ocean crust is being formed as 
two plates move apart from each other.  In this type of setting the 
crustal forces are extensional, with the crust being pulled apart, and the 
resulting faults are called normal faults.  Most of these constructive 
plate boundaries are located on the deep ocean floors along mid-ocean 
ridges, and although they actually give rise to numerous earthquakes most 
of them occur at sea, where they tend to go unnoticed except to be 
recorded by seismographic stations.  Sometimes, however, crustal extension 
happens on the continents, creating rift zones such as the East African 
Rift, in which huge crustal blocks are downfaulted; movement on such large 
normal faults will cause earthquakes.

The third type of plate boundary which gives rise to earthquakes is along 
subduction zones, where one plate dives beneath another and plunges back 
into the Earth’s mantle.  Although geologists know where these major 
subduction zones are located, the faults associated with them tend to be 
at depth in the crust and may not appear on the surface in the obvious way 
that the San Andreas Fault does.  The movement in this case is 
compressional, where one plate descends beneath another.  These faults 
along subduction zones can give rise to enormously powerful and 
destructive earthquakes.  In North America such a region occurs along the 
Pacific coast from Oregon to British Columbia, where a small crustal plate 
called the Juan De Fuca Plate is being subducted beneath North America 
along the Cascadia Subduction Zone.  Earthquakes here are much less 
frequent than along the San Andreas Fault to the south, but are larger and 
more powerful when they do occur.  Again, it is the tendency of rocks in a 
subduction-related fault zone to lock, allowing strain to build up until 
it is suddenly released, that gives rise to earthquakes.

Although most large earthquakes take place along plate boundaries as 
described in the preceding paragraphs, there are two other settings where 
earthquakes occur.  Earthquakes can occur deep within tectonic plates 
(intraplate earthquakes), far away from boundary regions.  The strain 
caused in tectonic plates as they move against each other is not only 
released along the plate boundaries, but is transmitted through the plates 
into their interiors.  These strains can build up over very long periods 
of time, centuries or thousands of years, only to be released in huge but 
very rare earthquakes in the interiors of plates where one might not 
expect them.  Such earthquakes are often found subsequently to be 
associated with extensional faulting deep in the crust, where continental 
rifting may be in its early stages.  A prime example is the New Madrid 
Fault, which gave rise in 1811 and 1812 to a series of huge earthquakes in 
the region of New Madrid, Missouri.  In 1886 Charleston, South Carolina 
was struck by a major intraplate earthquake.  Smaller earthquakes can also 
occur on old faults in plate interiors.  The British Isles, which are 
located well away from any plate boundary, experience minor earthquakes of 
this sort from time to time.  One occurred in 1990 in the county of 
Shropshire, which was felt as far away as London about 150 miles away.

Earthquakes are also associated with volcanic activity.  It is very common 
for earthquakes centred on a volcano to provide advance warning of an 
impending eruption, perhaps months or a few years in advance.  As magma 
rises through the plumbing system of a volcano it forces its way up by 
exploiting lines of weakness in the crust, opening up cracks and fissures, 
and this frequently gives rise to swarms of earthquakes.  Some of these 
can be moderately strong.  It was, for example, one such earthquake that 
triggered the explosive eruption of Mount St. Helens in 1980, because the 
earthquake caused the mountainside to give way, releasing the pressure on 
the magma chamber inside the volcano.

The cause of earthquakes is a very large subject, and I have not done more 
than scratch the surface.  But I hope my answer has sketched out the main 
scenarios for you.

Best wishes,

David Scarboro