Transcript F5 Earthquakex

Earthquakes are caused by the abrupt release of stored
energy within the earth’s crust. This release of energy is
much like that which is released when a stretched
rubber band breaks.
The Eastern Caribbean is an example of an island
arc system formed at a convergent plate boundary
(more specifically, at a subduction zone, where two
tectonic plates meet and the denser plate is forced
beneath the lighter plate).
This is the main cause of the volcanic and seismic
activity in the Eastern Caribbean.
1. Most of the earthquakes occurring in the Eastern Caribbean
are either tectonic or volcanic in origin.
1. Tectonic earthquakes are generated when plates move as
accumulated energy is released.
1. Volcanic earthquakes are generated by the movement of
magma within the lithosphere. Since magma is less dense
than the surrounding rock, it rises to the surface, breaking
the rock as it moves, thereby generating earthquakes.
1. More than 75% of the world's earthquakes occur at
convergent plate boundaries. The countries of the Eastern
Caribbean are, therefore, highly susceptible to earthquakes
The motion of the ground during earthquakes is recorded by
instruments known as seismographs. The ground motion that
people notice comes from a release of energy that radiates outward
in all directions as seismic waves, which travel through the earth.
There are two basic types of seismic waves- body
waves and surface waves. Generally, the first jolt felt
during an earthquake is the push-pull body wave, or
P wave, as it reaches the surface. A second jolt is
another type of body wave, called an S wave.
The fastest wave, and therefore the first to arrive at a given location.
Also known as compressional waves, the P wave alternately
compresses and expands material in the same direction it is traveling.
Can travel through all layers of the Earth.
Generally felt by humans as a bang or thump.
The S wave is slower than the P wave and arrives next,
shaking the ground up and down and back and forth
perpendicular to the direction it is traveling.
Also know as shear waves
Surface waves follow the P and S
waves.
Also known as Rayleigh and Love
waves.
These waves travel along the
surface of the earth.
The “size” of earthquakes is commonly expressed in two waysmagnitude and intensity.
Magnitude is a measure of the total energy released during an
earthquake. It is determined from a seismogram, which plots the
ground motion produced by seismic waves.
Magnitude is defined as the logarithm the important thing to
remember about magnitude is that the scale is logarithmic,
which means that each step in magnitude represents a tenfold
increase in amplitude of wave motion. Therefore, an earthquake
of magnitude 6.0 has ten times the wave amplitude of an
earthquake of magnitude 5.0.
Because magnitude does not describe the extent of the
damage, its usefulness is limited to an approximation of whether
the earthquake is large, small, or medium-sized.
The Modified Mercalli (MM) Intensity Scale.
Developed in 1931 by the American seismologists
Harry Wood and Frank Neumann.
This scale, composed of 12 increasing levels of
intensity that range from imperceptible shaking to
catastrophic destruction, is designated by Roman
numerals.
It does not have a mathematical basis; instead it
is an arbitrary ranking based on observed effects.
The Modified Mercalli Intensity value assigned to
a specific site after an earthquake has a more
meaningful measure of severity to the nonscientist
than the magnitude because intensity refers to the
effects actually experienced at that place.
Earthquake Hazards:
The Effect of Ground Shaking
The first main earthquake hazard (danger) is the effect of ground shaking. Buildings can
be damaged by the shaking itself or by the ground beneath them settling to a different
level than it was before the earthquake (subsidence).
Liquefaction- Liquefaction is the mixing of sand or soil and groundwater (water
underground) during the shaking of a moderate or strong earthquake. When the water
and soil are mixed, the ground becomes very soft and acts similar to quicksand. If
liquefaction occurs under a building, it may start to lean, tip over, or sink several feet.
 Surface Waves - Buildings can also be damaged by strong surface waves making the
ground heave and lurch. Any buildings in the path of these surface waves can lean or tip
over from all the movement. The ground shaking may also cause landslides, mudslides,
and avalanches on steeper hills or mountains, all of which can damage buildings and hurt
people.
Ground Displacement
The second main earthquake hazard is ground displacement (ground movement) along a
fault. If a structure (a building, road, etc.) is built across a fault, the ground displacement
during an earthquake could seriously damage or rip apart that structure.
Flooding
The third main hazard is flooding. An earthquake can rupture (break) dams
or levees along a river. The water from the river or the reservoir would then
flood the area, damaging buildings and maybe sweeping away or drowning
people.
Tsunamis and seiches can also cause a great deal of damage. A tsunami is
what most people call a tidal wave, but it has nothing to do with the tides on
the ocean. It is a huge wave caused by an earthquake under the ocean.
Tsunamis can be tens of feet high when they hit the shore and can do
enormous damage to the coastline. Seiches are like small tsunamis. They
occur on lakes that are shaken by the earthquake and are usually only a few
feet high, but they can still flood or knock down houses, and tip over trees.
 Fire
The fourth main earthquake hazard is fire. These fires can be started by
broken gas lines and power lines, or tipped over wood or coal stoves. They
can be a serious problem, especially if the water lines that feed the fire
hydrants are broken, too. For example, after the Great San Francisco
Earthquake in 1906, the city burned for three days. Most of the city was
destroyed and 250,000 people were left homeless
http://www.gns.cri.nz/Home/Learning/ScienceTopics/Earthquakes/Earthquake-Hazards