Download Earthquakes

Earthquakes
Introduction
Earthquakes are sudden ground movements which result from the sudden release of built
up energy. This energy is released in the form of seismic waves. Earthquakes are caused
due to tectonic motions in the earth's crust.
Objectives
 To explain how Earthquakes occur
 To highlight wear Earthquakes are distributed
 To explain the effects of Earthquakes
Content
Where Do Earthquakes Occur
Earthquakes are found at all three of the major plate boundaries (convergent, divergent,
and transforms) due to the forces of collision between plates as well as the irregular
movement and build up of friction as plates move past each other.
Earthquakes also occur away from plate boundaries at weaknesses in the earth's crust
known as faults.
So where do Earthquakes occur?
Along convergent, divergent, and transform margins as well as faults.
Facts about Earthquakes
An earthquake is what happens when two blocks of the
earth suddenly slip past one another. The surface
where they slip is called the fault or fault plane. The
location below the earth’s surface where the
earthquake starts is called the hypocenter, and the
location directly above it on the surface of the earth
is called the epicenter.
Sometimes an earthquake has foreshocks. These are
smaller earthquakes that happen in the same place as
the larger earthquake that follows. Scientists can’t
tell that an earthquake is a foreshock until the larger earthquake happens. The largest,
main earthquake is called the mainshock. Mainshocks always haveaftershocks that follow.
These are smaller earthquakes that occur afterwards in the same place as the mainshock.
Depending on the size of the mainshock, aftershocks can continue for weeks, months, and
even years after the mainshock!
Why Earthquakes Occur
The earth has four major layers: the inner
core, outer core, mantle and crust. The
crust and the top of the mantle make up a
thin skin on the surface of our planet. But
this skin is not all in one piece – it is made
up of many pieces like a puzzle covering the
surface of the earth. Not only that, but
these puzzle pieces keep slowly moving
around, sliding past one another and bumping
tectonic plates, and the edges of the plates
are called the plate boundaries. The plate boundaries are made up of many faults, and most
of the earthquakes around the world occur on these faults. Since the edges of the plates
are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate
has moved far enough, the edges unstick on one of the faults and there is an earthquake.
Why Does the Earth Shake When There is An Earthquake
While the edges of faults are stuck together, and the rest of the block is moving, the
energy that would normally cause the blocks to slide past one another is being stored up.
When the force of the moving blocks finally overcomes the friction of the jagged edges of
the fault and it unsticks, all that stored up energy is released. The energy radiates
outward from tWihe fault in all directions in the form of seismic waves like ripples on a
pond. The seismic waves shake the earth as they move through it, and when the waves
reach the earth’s surface, they shake the ground and anything on it, like our houses and us!
Where Do Earthquake Occur in the Caribbean
In the Caribbean earthquakes tend to originate in the sea. The earthquake zone extends
from Grenada to St. Kitts and up to Jamaica, and west of Trinidad. (See map under 1.3:
Caribbean Plate and its Neighbors for details.)
Distribution of Earthquakes
Earthquakes occur in well-defined belts, mainly along plate margins.
 Along the Pacific Coast or Ring of Fire
 Within the West Indies (Greater and Lesser Antilles)
 From the Mediterranean to East Africa
Seismographs and Richter Scales
Seismographs are used to record the intensity of the vibrations and are measures by the
Mercalli Scale.
The Ritcher Scale measures the magnitude of an earthquake shock.
Effects of Earthquakes
We can divide the effects of an earthquake into those known as the primary effects and
those known as the secondary effects. Primary effects of an earthquake are those
resulting directly from the earthquake itself. These include; buildings collapsing; roads
cracking; bridges giving way; shattering of glass and injuries / deaths resulting from
these. Secondary effects are those that result from the primary effects. For example
ground shaking may result in the cracking of gas and water pipes (primary effects) this can
result in severe fires due to explosion from escaping gas and difficulties in putting out
fires due to lack of water from burst mains (secondary effects). Other secondary effects
include, homelessness, business going bankrupt and closing etc.
Predictions
Computer programs have been designed to predict earthquakes. Predications are based on:
seismic activity, water level changes, bulges of the Earth’s crust and changes in land
survey lines.
Precautions
 Buildings are built Earthquake resistant
 High-occupancy buildings are not built fault lines
Questions/Feedback/Multiple Choice
None: This only covers a small portion of the exam.
Summary
Earthquake - a sudden ground movement
Epicentre - this is the point on the surface directly above the focus of the earthquake
Fault - a weakness in the earth's crust where an earthquake may occur
Focus - this is the point underground where the earthquake starts
Richter Scale - a logarithmic scale used to measure the magnitude of an earthquake
Seismic Waves - waves of energy released in the event of an earthquake
Seismograph - used to measure seismic waves released during an earthquake