Earthquake Studies and the Earth

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Earthquake Studies and the Earth's The upper part of the crust is P waves slow down and become
interior considered to have the overall refracted and the S waves
composition of granite, and is completely stop. This is referred to
Most earthquakes originate from therefore referred to as granitic. as the shadow zone.
fractures relatively close to the This means that the parent rock Since S waves cannot travel
Earth's surface, often at depths of was granite but has been through liquid, it can be assumed
30km, but can be recorded at metamorphosed/eroded/weathered. that the core, or the outer part of it,
depths of 700km. Oceanic crust is mainly basalt, and has the properties of a liquid.
The majority of earthquakes occur is referred to as being basaltic in However, the core isn't liquid
within well-defined earthquake or composition. throughout, P waves travel with an
seismic zones. Although increased velocity through its
earthquakes occur along a planar The Composition of the Mantle: central part which indicates that this
source such as a fault, some The mantle consists of more dense section of the core is solid.
geologists have considered them to rock than the crust, which enables
have emanated from a point source the P and S waves to increase their The Composition of the Core:
called the focus, which are found velocity. The core consists mostly of iron or
deep within the Earth. The point The most likely contender is iron/nickel. This is supported by
vertically above the focus is known peridotite, which has a denseness meteorite analysis since they are
as the epicentre. of 3.3 and contains the minerals part of an exploded planet much like
When an earthquake occurs, three olivine and pyroxene. our own. They are very rare,
types of waves are set off. These The reasons that peridotite is the however, and other ideas have
are known as;- likely rock type is because;- been put forward.
- Primary, Push or P waves, - Some volcanoes bring to It could be made up of silicon
- Secondary, Shear or S the surface solid pieces of compounds or even compressed
waves, the upper mantle which hydrogen. It is impossible t get
have been torn from the down there, so we really do not
- Longitudinal, Surface or L know.
waves. walls of the volcanic conduit
by the magma. In oceanic
regions the fragments had The Low Velocity Layer a.k.a
The L waves cause the most Asthenosphere:
damage, but the P and S waves are mostly been peridotite.
- There are some areas on This layer represents a reduction of
more important because they travel P and S wave velocities. These
through the Earth. Therefore, they Earth where, through violent
earth movements, parts of velocities are at their minimum at
are referred to as body waves. around 100km but there is variation
the upper mantle have been
brought close to the surface between 75-150km in depth.
The Crust: This is a zone of partial melting
Mohorovicic noted that two distinct and subsequently laid bare
by erosion. Peridotite was and would account for the reduction
sets of P and S waves appeared on of P and S waves, since the S
seismographs when an earthquake the dominate rock.
waves don't stop.
within a radius off 800km and at a - Meteorites also provide The liquid produced by the partial
depth of 40km was recorded. He evidence since metallic melting of the peridotite would be of
concluded that the waves travelled meteorites are believed to basaltic composition.
in different directions. resemble the core of an It is possible that the
He suggested that that one set of exploded planet, similar to Asthenosphere allows the plate to
waves travelled directly through the Earth. Stony meteorites move, being torn apart at
Earth, while the second were which represent the mantle constructive boundaries and
refracted, taking a longer time to of an exploded planet have colliding at destructive boundaries.
arrive. peridotitic composition.
He found that this occurred within We now have three subdivisions of
100km, where there were two The mantle forms 80% of the the Earth;-
distinct zones, one upper, less Earth's volume, from the top to the
dense zone and a lower denser bottom, from just below the mantle
- Lithosphere the crust and
zone. to the core-mantle boundary at upper mantle.
This discontinuity has been called 2900km the velocity of the P waves - Asthenosphere the low
the Moho and is accepted as a increases, the density increases velocity layer.
major dividing line in the structure of and the pressure increases. - Mesosphere the remainder
the Earth. The crust is above, and of the mantle and core.
the mantle is below. The Core:
At depths that are greater than
The Composition of the Crust: 2900km, there is a sudden change
There are three main elements in in the composition and structure at
the rocks that are dominate are this depth. This boundary is referred
oxygen, silicon and aluminium. to as the core-mantle boundary at
which these changes take place.

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