7/30/2019 Lesson notes Earthquakes.doc
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Lesson notes Earthquakes
What is an earthquake?An earthquake is a sudden movement of the
earths crust, usually along a fault (thedividing line between two
parts of the crust). The study of earthquakes is called
seismology.Aftershocks are more minor earthquakes that represent
the settling down of the platesafter an earthquake.
What causes earthquakes?Stresses build up along lines of
structural weakness until the friction is overcome in asudden
movement. The point at which the stress is released is named the
focus (orsometimes the hypocentre), while the point directly above
that on the earths surface is theepicentre. The shockwaves released
from the earthquake are called seismic waves. Theseradiate out in
all directions, just like throwing a stone in a pond causes ripples
on thesurface, decreasing in intensity as they spread further from
the focus.
These stresses tend to built up along plate boundaries, and
earthquake patterns are very,very closely relate to them. The
different plates result in different earthquakes:
Constructive plate boundaries along ocean ridges. Mainly shallow
earthquakes.
Caused by volcanic activity as magma rises, pushing plates
apart. Earthquakes occurbelow water, so causing little threat to
people, although some do occur on land e.g.
East African Rift Valley. Destructive plate boundaries involving
at least one oceanic plate being subducted
the compressing of the subducted oceanic plate causes deep and
severeearthquakes in the Benioff Zone which is the area where the
plate is being pusheddirectly underneath the other plate. These
plates are the most common cause oftsunami.
Collision boundaries (two continental plates colliding) shallow
earthquakes over awide area.
Conservative/transform plate boundaries (plates moving past one
another, not
towards, e.g. San Andreas fault) shallow earthquakes along a
narrow zoneAbout 15% of earthquakes occur away from these plate
boundaries and are known as intra-
plate earthquakes. They may be caused by stresses within plates,
or by isostatic recoil (thesame process that has lead to sea level
change in previously glaciated areas) which revivesancient faults
that are no longer actually on a plate boundary, e.g. the fault on
which LochNess sits.
What physical factors make the damage from an earthquake
increase?
Depth of focus - The more shallow the focus deep earthquakes may
occur up to
700km below ground level, but those less than 40km deep cause
the most damageas the seismic waves as the seismic waves havent
dissipated by the time they reachareas of human habitation.
Magnitude Earthquakes are measured using the Richter Scale (see
below). The
stronger the earthquake, the more likely it is to do damage. The
Modified MercalliScale may also be used.
Duration Earthquakes may last a fraction of a second, or may go
on for several
minutes if you include aftershocks.
Distance from epicentre the further away, the less severe the
seismic waves (in the
same way as depth of focus).
Underlying geology softer rocks, muds and clays may cause more
damageincluding liquefaction (see related hazards)
Other factors time of daymay influence damage to people (night =
more deaths as
people are asleep and less likely to get out of falling
buildings etc.). Quality ofbuildings, and other human factors, are
of course equally important as any physicalfactors.
How are earthquakes measured?A seismometer records the movement
of the earth, producing a seismogram orseismograph, which is what
we commonly see on TV etc. There are two main scales:
