EARTHQUAKES
What is an earthquake?
An earthquake is the shaking and vibration at the surface
of the Earth caused by underground movement along a
fault plane or by volcanic activity. Earthquakes happen
because the Earth’s tectonic plates are always moving
and floating on molten rock. Most earthquakes occur on
plate edges, especially when one plate is forced under
another. An earthquake can last from a few seconds to a
few minutes, which can be followed by after-shocks.
What causes earthquakes?
Earth's outer layer or crust is broken into pieces called tectonic plates which are constantly
moving towards, away from or past each other. Because continents are part of these plates,
they also move. An earthquake occurs when the rocks break and move as a result of
stresses caused by plate movements. Most earthquakes occur on the edge of plates.
The epicentre is the point on the Earth's surface directly above the source of the
earthquake. The source, also known as the focus, can be as deep as 700 kilometres below
the Earth’s surface. Earthquake vibrations travel very fast, up to 14 kilometres per second.
The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a
distance of almost 13,000 kilometres!
Smaller earthquakes occur much more frequently than large ones and most cause little or
no damage. A very large earthquake can be followed by a series of smaller events called
aftershocks during a period of adjustment which may last for several months.
Earthquakes also can cause a tsunami, or a series of waves which can cross an ocean and
cause extensive damage to coastal regions. In areas where there are steep slopes, vibrations
resulting from earthquakes may cause landslides.
Do earthquakes occur in Australia?
Although Australia is not on the edge of a plate, the continent experiences earthquakes
because the Indo-Australian plate is being pushed north and is colliding with the Eurasian,
Philippine and Pacific plates. This causes the build-up of stress in the interior of the IndoAustralian plate which is released during earthquakes.
There are on average 200 earthquakes of magnitude 3.0 or more in Australia each year.
Earthquakes above magnitude 5.5, such as the 5.6 magnitude event in Newcastle in 1989,
occur on average every two years. About every five years there is a potentially disastrous
earthquake of magnitude 6.0 or more.
Adelaide has the highest earthquake hazard of any Australian capital. It has experienced
more medium-sized earthquakes in the past 50 years than any capital because South
Australia is being slowly squeezed sideways by about 0.1mm per year. Although
earthquakes cannot be predicted accurately, measuring these changes and combining that
information with Adelaide's earthquake history helps to develop an understanding of when
the next big earthquake might happen.
Australia's largest recorded earthquake was in 1941 at Meeberrie in Western Australia with
an estimated magnitude of 7.2 but it occurred in a remote, largely unpopulated area. A
magnitude 6.8 earthquake at Meckering in 1968 caused extensive damage to buildings and
was felt over most of southern Western Australia. Earthquakes of magnitude 4.0 or more
are relatively common in Western Australia with one occurring approximately every five
years in the Meckering region.
Where do earthquakes occur?
No part of Earth's surface is free from earthquakes, but some regions experience them more
frequently. They are most common at tectonic plate boundaries where different plates
meet. The largest events usually happen where two plates are colliding, particularly around
the edge of the Pacific Plate in New Zealand, Papua New Guinea, Japan and the Americas.
Intra-plate earthquakes occur in the relatively stable interior of continents away from plate
boundaries. They are less common and do not follow easily recognisable patterns. This type
of earthquake generally originates at shallow depths.
A map of the world's earthquakes shows that most of them lie in narrow zones, often around the edges of
the continents, or in the middle of the oceans.
Compare this to a map showing the boundaries of the tectonic plates.
Measuring Earthquakes
The magnitude of an earthquake is measured and recorded by a device called seismograph,
which uses the Richter Scale. The size of earthquakes is determined by measuring the
amplitude of the seismic waves recorded on a seismograph. A formula is applied to these
which convert them to a magnitude scale, a measure of the energy released by the
earthquake. For every unit increase in magnitude, there is roughly a thirty-fold increase in
the energy released. For instance, a magnitude 2.0 earthquake releases 30 times more
energy than a magnitude 1.0 earthquake, while a magnitude 3.0 earthquake releases 900
times (30x30) more energy than a magnitude 1.0. A magnitude 8.6 earthquake releases
energy equivalent to about 10,000 atomic bombs of the type developed in World War II.
The effects of an earthquake depend on many factors, such as the distance from the
epicentre (the point on the Earth's surface directly above where the earthquake originated
within the Earth) and the local ground conditions. Generally, for locations near the
epicentre, the following effects may be observed:
Magnitude
less than3.4
3.5 – 4.2
4.3 – 4.8
4.9 – 5.4
5.5 – 6.1
6.2 – 6.9
7.0 – 7.3
7.4 – 7.9
greater than
8.0
Description of Effect
usually felt by only a few people near the epicentre
felt by people who are indoors and some outdoors; vibrations are similar to
a passing truck
felt by many people; windows rattle, dishes are disturbed, standing cars rock
felt by everyone; dishes break, doors swing, unstable objects overturn
some damage to buildings; plaster cracks, bricks fall, chimneys damaged
much building damage; houses move on their foundations, chimneys fall,
furniture moves
serious damage to buildings; bridges twist, walls fracture, many masonry
buildings collapse
causes great damage; most buildings collapse
causes extensive damage; waves seen on the ground surface, objects
thrown into the air
GLOSSARY OF TERMS
after shock
epicentre
focus
(hypocentre)
intra-plate
Aftershocks are earthquakes that follow the largest
shock of an earthquake sequence. They can continue
over a period of weeks, months, or years. In general, the
larger the mainshock, the larger and more numerous
the aftershocks, and the longer they will continue.
The epicentre is the point on the earth's surface
vertically above the focus, point in the crust where a
seismic rupture begins.
The focus is the point within the earth where an
earthquake rupture starts.
Intraplate pertains to processes within the plates.
A landslide is a movement of surface material down a
slope.
landslide
magnitude
Richter Scale
seismic waves
seismograph
tectonic
plates
tsunami
The magnitude is a number that characterises the
relative size of an earthquake. Magnitude is based on
measurement of the maximum motion recorded by
a seismograph.
The Richter scale was developed in 1935 by Charles F.
Richter as a device to compare the size of earthquakes.
On the Richter Scale, magnitude is expressed in whole
numbers and decimal fractions. Each whole number
step in the magnitude scale corresponds to the release
of about 31 times more energy than the amount
associated with the preceding whole number value.
A seismic wave is an elastic wave generated by an
impulse such as an earthquake or an explosion.
A seismograph, or seismometer, is an instrument used
to detect and record earthquakes. The electrical voltage
is recorded on paper, magnetic tape, or another
recording medium.
The top layer of the Earth’s crust is covered by giant
pieces called tectonic plates. There are seven large
plates and several smaller ones, all moving very slowly
at different speeds and in different directions. The
plates under the ocean are called oceanic plates. The
ones under continents are called continental plates.
A tsunami is a sea wave of local or distant origin that
results from large-scale seafloor displacements
associated with large earthquakes, major submarine
slides, or exploding volcanic islands.