Transcript tectonic plates

Earthquakes & Tsunamis
Earthquakes are a shaking of the ground.
Some are slight tremors that barely rock a
cradle. Others are so violent they can tear
down mountains and cities.
Small earthquakes can be set off by
landslides, volcanoes or even just heavy
traffic. Big earthquakes are set off by the
grinding together of the vase tectonic
plates.
Forces cause the plates to move, at a rate
of 4-5 cm past each other in a year. In a
slip that triggers a major quake plates can
slip more than 1 m in a few seconds.
In most quakes a few minor tremors
(foreshocks) are followed by an intense
burst lasting just 1 or 2 minutes. A second
series of minor tremors (aftershocks)
occur over the next few hours.
Faults are lines of weakness where the
rock has been severed.
Portions of the crust are under constant
stress, like a bent bow.
When the strain becomes intolerable the
rock often collapse at a weak point, far
beneath the surface. The rock may have
been bending for hundreds of years before
the earthquake happens.
As the crust makes sudden shifts along
the fault lines, it releases pent-up energy
in powerful waves, which causes the
whole world to vibrate like a giant bell.
These waves are detected on sensitive
measuring instruments called
seismographs.
The Richter Scale (is a seismograph)
which measures the magnitude of an
earthquake on a scale of 1 to 10.
The most powerful earthquake was
measured in China in 1960 at 9.5 on the
RS. The longest was in Alaska in 1964,
lasting 4 mins.
The starting point of an earthquake below
ground is called the hypocenter or the
focus. The epicentre of an earthquake is
the point on the surface directly above the
hypocenter.
Earthquakes are strongest at the epicentre
& gradually weaken farther away.
Shock waves begin at the epicentre &
radiate outwards & upwards in circles.
The greatest damage is at the epicentre,
but the shock waves can radiate up to 400
km away.
Certain regions are called earthquake
zones, because they are prone to
earthquakes and lie along the edges of
tectonic plates.
Tsunamis
Means “harbour wave” in Japanese.
They are huge waves that begin when the
sea floor is violently shaken by an
earthquake, a landslide or volcanic
eruption.
In deep water, tsunamis travel almost
unnoticed below the surface. Once they
reach shallow coastal water the waves can
reach 30+ meters.
 Often mistakenly called tidal waves, but have
nothing to do with tides.
 Before a Tsunami arrives, the sea may recede
dramatically, like water draining from a bath.
 They can travel along the seabed as fast as a jet
plane, at 700 km/h or more.
 A Tsunami that begins as an earthquake in
Japan can arrive San Francisco 10 hours later.
 December 2004: On December 26, a 9.0
magnitude earthquake struck off the
Indonesian island of Sumatra, triggering a
tsunami that killed tens of thousands of
people in Indian Ocean coastal
communities. Much human and
environmental devastation was left in its
wake in Indonesia.