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The theory of plate
tectonics is that the Earth's outer layer, the crust, is made up
of plates. These plates have moved throughout Earth's history and
continue to move as much as a few inches a year. This theory explains
the creation of mountains, the existence of earthquakes and volcanoes,
and the existence of similar animals living on continents that are
separated by the seas.
Source:
Environmental
Physical Geography
The world has nine
major plates and many smaller plates. The nine major plates are
the North American, South American, Eurasian, African, Indo-Australian,
Antarctic, Pacific, Nazca, and Cocos. The relatively smaller size
of the other plates does not diminish their significance or impact.
There are three types
of plate boundaries: divergent boundaries, convergent boundaries,
and transform boundaries.
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Plate
Boundaries
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- Divergent Boundaries:
This type of boundary is created when two plates are pulling
away from each other. Divergent boundaries can form in the middle
of a continent, but eventually the rift that is created from the
separation forms ocean basins. It is considered a constructive boundary
as the space it creates is filled with molten magma that forms below.
An example of this is the Great
Rift Valley in Africa.
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Convergent Boundaries:
This type of boundary is created when two plates collide,
and is known as a subduction zone. Mountains and/or volcanoes are
generally found along these boundaries. There are three types of
convergent boundaries: Oceanic-Continental Convergence; Oceanic-Oceanic
Convergence; and Continental-Continental Convergence. The subduction
of one plate is decided by the density of the plates. Generally
the denser plate will subduct beneath the other. An example of continental-continental
convergence would be the Himalayas.
Source: Introduction
to Plate Tectonics
- Transform
Boundaries: This type of boundary is formed by two plates
sliding horizontally by each other. These boundaries are more commonly
known as faults. They generally offset active spreading ridges on
the ocean floor with a few occurring on land. An example of this
type of plate boundary is the San
Andreas Fault in California.
Source:
Introduction
to Plate Tectonics
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Earthquakes
Plate boundaries are
important as they are the major source of earthquakes. Merriam-Webster
Online defines an earthquake as the shaking or trembling of
the earth that is volcanic or tectonic in origin. Because of friction,
the plates do not move easily past each other; instead stress builds
until it reaches the slipping point of rocks on either side of the
fault. The energy that is released results in an earthquake. Earthquakes
can happen any where in the world, but more than 90% of them happen
along plate boundaries. The map below shows the epicenter of earthquakes
in 1963-1998. The earthquake locations effectively delineate the
plate boundaries.
Source: Digital
World Tectonic Activity Map (DTAM)
Earthquakes are an everyday
occurrence all over the world, but most are minor and cause no damage.
Large earthquakes, on the other hand, can result in massive damage
and loss of life. Earthquakes not only cause the ground to shake
and sometimes rupture, but the effects of the shaking and rupturing
can cause other events to occur such as inundation (tsunami,
seiche), ground failure (liquefaction, landslide), fire, or a release
of hazardous materials.
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Social
Impacts of an Earthquake
Two earthquakes in
December 2003 demonstrate the uneven effects of earthquakes on
different economic groups. The San Simeon earthquake of California
on December 22 and the Bam, Iran earthquake on December 26, are
two earthquakes of equal strength which had disproportionate social
impacts. Both earthquakes were similar in style and intensity
but culture and geography left Iranians worse off than the Californians.
When the earthquake
hit San Simeon it had been the first of its size in the area since
1952. The earthquake was measured at a 6.5 magnitude and was caused
by reverse faulting. Located near several faults, most of the
city was prepared for an earthquake. The buildings that took the
most damage were old masonry buildings that had not been retrofitted
to withstand an earthquake, however many of them had been retrofitted
and survived the earthquake. The California Seismic Safety Commission
reported 2 deaths, 47 seriously injured, 290 homes damaged, and
191 commercial structures damaged. This earthquake caused enough
damage to be declared a disaster by the president and the rebuilding
of San Simeon is well underway with $20.1 million of federal relief
money.
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Area
of heaviest damage in San Simeon
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When the first tremors
came to Bam, Iran at 4 am Friday the 26 of December, some people
came out of their houses into the streets but shortly thereafter
went back into their houses as tremors were not that uncommon
for the area. At 5:27 am, an earthquake of 6.6 magnitude hit.
The cause of the earthquake was reverse faulting and strike slip
faulting within the zone of deformation. The city of Bam is an
ancient city, most of the buildings were made of adobe brick and
mud; none of the buildings were prepared to handle an earthquake
of this magnitude. After the dust had settled, according to the
International Federation of Red Cross and Red Crescent Societies,
there were 43,000 dead, 30,000 injured, and 75,000 left homeless
with over 85 percent of the buildings destroyed. Rescue workers
reported that the collapsing mud-brick structures had completely
disintegrated and buried people in piles of earth, rather than
trapping them in air pockets between building slabs, as would
happen in a concrete building collapse. Even though the relief
phase is now over it was thought unlikely that the rebuilding
of Bam would be completed within the next two years (as of August
2004). Bam is slowly being rebuilt due largely to the International
Federation of Red Cross and Red Crescent Societies which have
posted an appeal for $42 million.
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Click
on the Photo's for larger Before and After pictures of Bam, Iran
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Bam
before the earthquake
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Bam
after the earthquake
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The
Recent Earthquakes of Sumatra
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- On December 26, 2004,
exactly one year after the Bam earthquake, an earthquake of 9.0
magnitude occurred 160 km off the western coast of Sumatra along
the Burma and Indian plate boundary. Here the Indian plate is subducting
under the smaller Burma plate. It is estimated by the U.S.
Geological Survey (USGS) that the resulting rupture was 1200
km in length along the Sunda Trench and a width of over 100 km.
A large portion of the ocean sea floor was thrust upwards, resulting
in a tsunami. The earthquake itself caused initial damage in northern
Sumatra, Indonesia, and the Nicobar Islands; however the tsunami
was much more devastating and affected a larger area. The height
of the tsunami reached 30 meters in some areas.
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Another earthquake
that occured on March 28, 2005 was a 8.7 magnitude earthquake
and occured 100 miles southeast of the December 26, 2004 Sumatra
quake. It has been questioned as to whether or not this earthquake
is simply an aftershock of the December earthquake, but it occurred
on a different fault line. Initially there was fear that this
earthquake would generate a tsunami similar to the 2004 earthquake,
but fears were put to rest as the largest tsunami seen was only
approximately 3 meters in height. The map below shows the general
location of both earthquakes and the resulting aftershocks. Click
on it for a larger version.
Source:
USGS Earthquake Hazards Program - Latest Earthquakes
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USGS researchers recently
published an article titled "USGS
Research Sheds Light on Why the March Tsunami was Smaller than
the December Tsunami." The article gives four main reasons
for the smaller tsunami in March. The first was that the smaller
magnitude of the March earthquake. The magnitude is a function
of the rupture area and the amount of slip. The second was the
depth of the water where the earthquakes occured. March's earthquake
was about six-tenths of a mile underwater and December's earthquake
was 1-2.5 miles under water. The deep water of December's quake
resulted in larger amplification of the tsunami as it traveled
from the source region of the quake to shore. The third reason
given by USGS was the depth under the earth where the fault slip
occured. The majority of the falut slip was 12 to 25 miles below
surface on March 28; December's slip may have extended all the
way to the sea floor. This factor along with the greater magnitude
of the December earthquake "resulted in greater vertical
movement of the sea floor," according the USGS. The last
factor for the smaller tsunami in March was the difference in
the primary direction of the tsunami wave. December's tsunami
was focused in an east-west direction where as March's tsunami
was focused southwest, away from nearby land. The March earthquake
occured mainly under the island shelf of Sumatra, so the island
itself blocked most of the wave activity that would have been
produced.
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Quake
Shook Entire Planet
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Scientists:
Sumatra quake longest ever recorded:
Temblor
big enough to 'vibrate the whole planet '
Marsha Walton, CNN, Thursday, May 19, 2005
Dramatic new data from the December 26, 2004, Sumatran-Andaman
earthquake that generated deadly tsunamis show the event created
the longest fault rupture and the longest duration of faulting
ever observed, according to three reports by an international
group of seismologists published Thursday in the journal "Science."
"Normally, a small earthquake might last less than a second;
a moderate sized earthquake might last a few seconds. This earthquake
lasted between 500 and 600 seconds (at least 10 minutes),"
said Charles Ammon, associate professor of geosciences at Penn
State University.
The quake released an amount of energy equal to a 100 gigaton
bomb, according to Roger Bilham, professor of geological sciences
at the University of Colorado. And that power lasted longer than
any quake ever recorded.
The quake, centered in the Indian Ocean, also created the biggest
gash in the Earth's seabed ever observed, nearly 800 miles. That's
as long as a drive from northern California into southern Canada.
Scientists estimated the average slippage (ground movement up
and down) along the entire length of the fault was at least 5
meters (16.5 feet) -- with some places being moved nearly 20 meters
(50 feet).
Scientists have also upgraded the magnitude of the quake from
9.0 to between 9.1 and 9.3, a dramatically more powerful event.
As a comparison: the ground shook 100 times harder during December's
earthquake than what was felt in the 1989 Loma Prieta quake in
California. That 6.9 magnitude quake caused extensive damage from
Santa Cruz to San Francisco.
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Monster quake
The stunning power of Asia's earthquake and tsunamis last December
has left even veteran scientists in awe.
"I think it was humbling for everyone that analyzed the earthquake,"
said Thorne Lay, professor of earth sciences and director of the
Institute of Geophysics and Planetary Physics at the University
of California, Santa Cruz.
"We're sitting in our laboratories working on the signals
from this earthquake, trying to understand what happened scientifically,
and then watching TV at night and seeing the death toll rising
for weeks," he said.
The enormous human toll from the natural disasters spurred Lay
to organize dozens of scientists from all over the world to share
their data and analysis of the quake. The long-term goal is to
try to get more, and more accurate tsunami warning systems in
place.
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Whole planet vibrated
A wide array of instruments were used for the first time to study
the earthquake, and its many aftershocks.
Global broadband seismometers recorded the ground in Sri Lanka,
a thousand miles from the epicenter, moved up and down by more
than 9 centimeters (3.6 inches), according to the report.
But no place on Earth escaped movement.
"Globally, this earthquake was large enough to basically
vibrate the whole planet as much as half an inch, or a centimeter.
Everywhere we had instruments, we could see motions," Ammon
said.
Much of that information came from digital broadband seismometers,
a new era of instruments that the National Science Foundation
and the U.S. Geological Survey began deploying around the world
several years ago.
Lay says the equipment is sensitive enough to pick up the motion
of wind blowing through trees, or cows walking in a field, or
the massive motions produced by this earthquake.
"We'd never seen signals from an earthquake of this size,
and the availability of this instrumentation was a real breakthrough
in being able to see the complete rupture process of one of these
truly monstrous events," Lay said.
Other tools added to the scientists' understanding. Underwater
cameras documented the huge crack in the ocean floor. Tsunami
buoys, and sonar from the British Navy helped with the analysis.
And a fortunate bit of timing enabled researchers to get a view
of the tsunami they have never seen before.
"Two hours after the earthquake has occurred, the wave is
spreading out from the Bay of Bengal," Lay said. "Two
satellites went over, with the capability of measuring the elevation
of the ocean surface. The satellites saw the south-going wave
and the north-going part of the wave. "It was just good luck
that the passage of the satellites caught the tsunami in motion,"
he said.
Crunching numbers, and creating maps and models is taking on a
new urgency for some of the scientists involved in this research.
"There will be more earthquakes of this type, and with more
humans exposed to the hazard there will be more devastating losses
of life. What we hope to do is develop technologies that can minimize
that loss," Lay said.
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Sources
For
more information on this topic:
Plate Tectonics
- Wikipedia : http://en.wikipedia.org/wiki/Plate_tectonics#Types_of_plate_boundaries
Plate Tectonics:
http://www.platetectonics.com/index.asp
A
Science Odyssey: http://www.pbs.org/wgbh/aso/tryit/tectonics/intro.html
USGS Earthquake
Hazards Program: http://neic.usgs.gov/neis/eq_depot/2003/eq_031226/neic_cvad_ts.html
http://earthquake.usgs.gov/eqinthenews/2004/usslav/eqsummary.html
http://earthquake.usgs.gov/eqinthenews/2005/usweax/
Red Cross
Red Crescent - Iran: http://www.ifrc.org/what/disasters/response/iran.asp
Vingenuity
Incorporated: http://www.vingenuity.net/eq/san_simeon/2003_san_simeon_eq_home.html
BAM
- Iran's Ancient City:
http://www.farsinet.com/bam/
California
Seismic Safety Commission:
http://www.abag.ca.gov/bayarea/eqmaps/inventory/3_Turner_Presentation.pdf
USGS
Release:
http://www.usgs.gov/newsroom/article.asp?ID=697
2005
Sumatran Earthquake: http://en.wikipedia.org/wiki/2005_Sumatran_Earthquake
2004
Indian Ocean Earthquake: http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake
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