Transcript Plate Tectonics - Geography at InterHigh

Plate Tectonics
Earth
qqqqqquakes!
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What are we going to look at?
How do
earthquakes
form?
Where do
earthquakes
happen?
What are
EQs like?
Differences
between
LIC & HIC
Variety of
magnitude?
Personal
management
of EQs
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To experience the drama of plate tectonics -the jostling of the giant plates that carry
continents and oceans -- try this experiment:
Sit in a comfortable chair, hold your hand out,
and watch your fingernails grow. That's about
the average speed of a tectonic plate.
But wait around long enough, and even the
tortoise crawl of plate tectonics will have
dramatic and deadly consequences.
Though plate tectonics is a global phenomenon
and virtually invisible to us in our daily lives, it
introduces enormous stresses in the crust
where we live.
From time to time, stressed-out crust releases
the stress in sudden fits: earthquakes.
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Most earthquakes happen near the
boundaries of tectonic plates, both
where the plates spread apart and where
they crunch or grind together
But large temblors also strike from time
to time in the normally stable interior of
continents.
Movement of body waves
away from the focus of
the earthquake. The
epicentre is the location
on the surface directly
above the earthquake's
focus.
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Earthquake hits North Yorkshire
The British Geological Survey (BGS) said
the 3.6-magnitude quake struck 9km
north-west of Ripon in North Yorkshire
just after 2100 GMT on Monday
(Jan 3rd 2011)
As you see the UK is not immune
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But as you can see – 3.6 is not very
big – and there are 100s of
thousands every year
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More frequently than time to time,
actually. If you imagine the Earth as a
giant bell, it's ringing with earthquakes
every second of the day -- from the
many imperceptible clinks of microquakes
to the deafening gong of very occasional
but "great" earthquakes (those of
magnitude 8.0 or greater).
It is estimated that there several million
temblors, most undetectable, happen
every day.
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These are all the earthquakes of magnitude 7.0
or greater for 2010. You can see where they are
concentrated – but even tho’ these are quite
severe, deaths only occurred 5 of them.
Red >7.5 – with ! means deaths
Blue<7.5 or wheelchair is <7.5 +
deaths
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What do earthquakes feel
like?
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There are 2 types of waves in an
earthquake
Body waves and surface waves.
Body waves travel outward in all directions,
including downward, from the quake's focus -that is, the particular spot where the fault
first began to rupture.
Surface waves, by contrast, are confined to
the upper few hundred miles of the crust.
They travel parallel to the surface, like ripples
on the surface of a pond. They are also slower
than body waves.
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Following an earthquake …
..the body waves (P-wave) strike first and are the
fastest kind. People often report a sound like a train
just before they feel a quake, which is the P-wave
moving as an acoustic wave in the air.
Then the secondary, or S-waves, arrive. A person in a
building perceives the arrival of S-waves as a sudden
powerful jolt, as if a giant has pounded his fist down
on the roof.
Finally, the surface waves strike. In very strong
earthquakes, the up-and-down and back-and-forth
motions caused by surface waves can make the ground
appear to roll like the surface of the ocean, and can
literally topple buildings over.
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In Washington State 2001
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In Washington State 2001
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In Washington State 2001
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How do we measure earthquakes?
The scale you usually hear about is the
Richter Scale.
This uses data from a seismograph (see
below) and combines it with the distance
from the epicentre (the middle of the
earthquake)
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What do the numbers mean?
Each number is a power of 10!
So a 2 is 10 x bigger than 1
And a 4 is 10x bigger than 3 and is 1000 x
bigger than a 1
So while 6 is moderate, a 7 is x10 bigger and
getting serious and an 8 is x100 bigger than a
6 and very bad indeed!
But Richter only really works within about 500
km of the epicentre, so there are now more
accurate way to measure.
But news programmes still tend to stick to
Richter as everyone knows about it!
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Ground-shaking is not the only
hazard
Some times the shaking is so violent,
that boulders can be thrown into the air
Also landslides. In 1692, in Jamaica, the
whole town of Port Royal slid into the sea
and came to rest 15 metres below the
surface. Mud quickly covered it up.
When it was investigated in 1959, they
found a buried copper kettle ( a big pot
with a lid) together with it contents,
turtle soup.
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Groundshaking is not the only
hazard
Sometimes whole chunks of land move
Near San Francisco, you can still see
where a piece of fence was moved
sideways
In Alaska, some land rose 10 metres and
the sea floor dropped nearly 15 metres!
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Ground-shaking is not the only
hazard
Also earth-slides and mudslides can happen.
If a sugar bowl has been sitting around and you
shake it, the sugar grains stay stuck together.
But if you continue shaking it side to side, the
grains split up and look a bit like a liquid moving
– and this is what can happen to the soil, and
like a liquid it can run down slopes and envelop
villages and so on
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Review
What is the first thing you might notice
about an earthquake?
Second?
Then?
Afterwards, besides collapsed building,
what are some other effects?
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Homework
Go through the slides and make sure you know
what it might be like
Then imagine there is a biggish earthquake
where you live – as you have seen it could
happen!
Then write an short article for the local paper
telling what it was like.
You might also want to include one or 2 of the
‘effects’
e.g. A crack 1 metre deep and 10 metres long
appears in the road outside Sainsbury's. The
road was closed for a week while repairs were
made
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