Ivan Cukanić
Science 7B
2/24/2013
Earthquakes
Earthquakes are very common occurrences in nature. Averagely
7 million earthquakes occur each year, but only a few of them produce
damage. The crust, the outer thin layer of our planet is composed of
tectonic plates that are floating on a hot, melted liquid that is constantly
circulating. While beneath the Earth’s crust, this liquid is called magma, but
as soon as it is shot out of a volcano, it changes its name to lava. The
circulation of magma makes the plates move. Earthquakes are caused by the
movement of tectonic plates. The movement of tectonic plates also causes
seismic waves that make tsunamis, volcano eruptions and much more. Seismic waves
are not created just anywhere, but on the borders of these plates.
First, an earthquake is a sudden, rapid shaking of the ground,
which is caused by subterranean rock breaking and moving. This subterranean
rock is part of a tectonic plate that is constantly moving. “The underground
location where the rock first broke apart or shifted is called the focus of the
earthquake” (http://cse.ssl.berkeley.edu/lessons/indiv/davis/inprogress/QuakesEng3.html,
1995). Location on earth surface directly above
the focus is called epicenter. Energy is accumulated because of the moving plates, so when there
is enough energy, it is released onto the surface in the shape of earthquakes. If
strong enough this energy release can make tectonic plates separate. Whole
cities can be devastated by earthquakes. All of Europe, the west coast of South
and North America, parts of Asia near India and the east coast of Asia are the
most common places where earthquakes occur. This is because the boundaries of
tectonic plates are on those locations. “Most of the largest earthquakes occur
within the Pacific ‘Ring of Fire’, a horseshoe-shaped band of volcanoes and
fault lines circling the edges of the Pacific Ocean” (http://www.trust.org/alertnet/crisis-centre/subtopic/earthquakes,
2009). Briefly, earthquakes are a destructive shaking of the ground caused by
tectonic plates.
Also, waves caused by a movement of tectonic plates are seismic wave,
which include longitudinal (P waves), transverse (S waves) and surface waves. P
waves are the fastest ones, S waves are the most destructive ones and surface
waves are a combination of longitudinal and transverse waves that travel
through the surface of their medium. Seismographs are machines that
record these waves and warn us about upcoming earthquakes. When a seismograph
records P waves that is a just a warning and there is usually no destruction,
but when it records S waves, that means that the earthquake is already in
session. However, when a seismograph warns us about P waves, it is only a
matter of minutes before the destructive S waves come. If the location of the
seismograph is on the border of the tectonic plates, there would be absolutely
no time to prepare for an earthquake, for the distance the waves have to travel
is short, so the P waves would barely outrun the S waves. Summarizing, there
are three types of seismic waves: S, P and surface waves.
Third, the best way to protect ourselves and our city / town from
earthquakes is through earthquake engineering. Earthquake engineering is the
scientific field that concerns the damage done by earthquakes and how to stop
it. One issue that they are solving is designing a building that can withstand
seismic waves and not partially or completely collapse during a rare and very
destructive earthquake or a common and not so destructive earthquake. They test
out buildings through experimental or analytical/numerical assessment.
Experimental assessment is the assessment of 1:1 scale models of buildings on a
mechanical surface that shakes and simulates an earthquake. During the
experiment, they record their observations and conclude what kinds of buildings
are safer and more serviceable during and after the earthquake.
Analytical/numerical assessment is when they try to design new buildings using
beams, columns, beam-column joints, shear walls, etc. The only problem with making buildings with these accesories is that it would be very expensive and the economy of that country would probably weaken because of this, but later they would get the money back, for the buildings would endure even the most destructive earthquakes. Hence, there are two
types of earthquake engeneering: experimental and analytical/numerical.
In conclusion, humans still have not found a way to warn ourselves
or protect ourselves from very common and very destructive occurrences called
earthquakes. Our warning system goes off just a few minutes before the
earthquake, our buildings collapse and injure or kill people and we cannot save
the injured people on time, for they are usually buried under pieces of
concrete. But scientists are working on it and each day they come to an
important conclusion, so step by step they will find a way to warn us on time
and make endurable buildings.
Bibliography:
http://nisee.berkeley.edu/bertero/
http://www.trust.org/alertnet/crisis-centre/subtopic/earthquakes
http://www.ready.gov/earthquakes
