The Earthquake Risks in Hawaii

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The Earthquake Risks in Hawaii
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Mantle plumes and hotspots Not all volcanic
activity can be related to present day active
plate margins. The Pacific Ocean shows a number
of sub-parallel chains of volcanic islands which
run diagonally across the Pacific plate (see map
below). Isotopic dating of the lavas making up
the volcanic islands shows that in all cases, the
age of the islands increases from SE to NW. The
youngest islands in the chains are all
volcanically active, but as the islands increase
in age they become extinct. The islands are also
much younger than the ocean crust that they are
built on. Dating of islands in the Pacific chains
gave an average plate movement of approximately
99mm per year over the last 90 million years,
although there is no evidence to suggest that
this rate of plate motion has been constant over
time. Changes in the alignment of some of the
volcanic chains preserves evidence of a change in
the direction of plate motion about 45 million
years ago
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Sequences in the formation of a volcanic island
above a mantle plume.
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What causes an earthquake? The crust of the Earth
is broken into 12 enormous 'tectonic' plates that
stretch over entire continents and oceans. These
plates of solid rock are in constant motion.
Where they meet is known as a fault line. As
these plates drag past or over each other, huge
amounts of stress can build up. When the stress
is too great the rocks force apart like an
expanding spring.
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Where are the most dangerous places for
earthquakes? Major fault lines cross the planet
and it is there that major earthquakes take
place. Two of the most earthquake-prone faults
are the San Andreas in California and the North
Anatolian which runs along the north of Turkey.
These are both more than 1000km long. Other areas
of high earthquake activity are Japan, the Middle
East and along the west coast of South America.
All lie on the boundaries of the Earth's
continental plates.
How much stress is needed to trigger an
earthquake? The amount of stress needed to
trigger a following quake does not have to be
great. Studies of over 20 faults have convinced
many scientists that earthquakes can be triggered
by as little as one eighth of the pressure
required to inflate a car tire.
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What is the difference between an earthquake and
earthquake storm? An earthquake is a single
isolated event in which the Earth's crust pulls
apart due to the build up of stress. The phrase
earthquake storm was coined by Stanford Professor
of Geophysics, Amos Nur, and describes a series
of earthquakes where one triggers another and
then another. The phenomenon is based on a
scientific theory known as the stress triggering
model. Scientists believe that on some faults, as
an earthquake strikes, the stress released does
not simply disappear. Instead it must be
redistributed to another region of the fault
which sets off another earthquake. This series of
earthquakes is known as an earthquake storm.
Although still in its infancy the stress
triggering model has identified possible series
of earthquakes in California, Japan and Turkey.
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1960The largest ever earthquake is measured in
Chile. At 9.5, it reaches the maximum value on
the Richter scale. More than 2,000 are killed,
3,000 injured and 2 million made homeless
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Asian Plate
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Istanbul quake more likely but unpredictable
                                            
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Earthquake Storms Earthquakes are among the most
devastating natural disasters on the planet. In
the last hundred years they have claimed the
lives of over one million people. Earthquakes are
destructive mainly because of their unpredictable
nature. It is impossible to say accurately when a
quake will strike but a new theory could help
save lives by preparing cities long in advance
for an earthquake. The surface of the Earth is
made up of large 'tectonic' plates. These plates
are in slow but constant motion. When two plates
push against each other friction generates a
great deal of energy. For this reason earthquakes
occur most frequently on tectonic fault lines,
where two plates meet. However these fault lines
run for thousands of kilometres predicting
exactly where a quake will occur is nearly
impossible
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Stress lines In 1992, Dr Ross Stein was
monitoring a large earthquake in a town in
California called Landers. Three hours later,
there was another quake 67km away at Great Bear.
Stein believed that this was not simply an
aftershock, instead he theorised the event at
Landers had set off the earthquake at Big Bear.
Stein believes that when an earthquake occurs the
stress that has built up along the fault, is in
part, transferred along the fault line. It is
this energy transfer that causes other quakes to
occur hours, days or months after the original.
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What is the difference between an aftershock and
an earthquake storm? All major earthquakes are
followed by a series of aftershocks. These are
smaller tremors that follow a main event. A
series of aftershocks following a large
earthquake is not considered to be a storm. An
earthquake storm is when large independent
earthquakes are triggered over long distances and
over long periods of time. However aftershocks
have helped scientists identify the mechanism
which triggers an earthquake storm and have given
them a clearer understanding of how the transfer
of stress along a fault takes place. How many
people die a year in earthquakes? There is no
definitive figure for this but every year
earthquakes can kill many thousands. In 1999, the
Izmit earthquake in Turkey killed over 25,000. In
January 2001 the Indian State of Gujarat was
struck by a major earthquake killing over 30,000
people. The single most catastrophic quake of
recent times was the Tungshan earthquake in China
in 1976 - an estimated 650,000 people were killed.