explanation of the seismology and study of the earth's interior besides the shadow zone and the Moho. the presentation include the gravity anomalies with the definition of the isostasy.

1. seismolog
y
Bassmala Mohamed

2. Concepts
• Seismology and the Study of Earth’s Interior
• shadow zone
• The Moho
• Guttenberg low velocity
• Seismic Tomography
• Gravity Anomalies
• isostasy

3. • In the last session we stopped after concluded
how we know that the earths interior are made
from different material based on (refraction).
• . The behaviors of the rays depend on their
directions and the media they encounter

4. • Notice how ray 1 bends gently within the mantle
and emerges at the crust. All rays bend gently
within the mantle. This is because the mantle
becomes more rigid with depth.
• Ray 2 also passes through the mantle. It
approaches the core-mantle boundary at very
low angle and continues its curved path. It
reaches the crust at 103° from the earthquake
source. However, if ray 2 crossed the core-
mantle boundary, it would take a very different
path.
2a. It would cross the
boundary at a very
low angle and get
refracted strongly
toward the core. This
is because of the
physical differences
between the core and
mantle.

5. • Ray 3 also crosses the core-mantle boundary,
but at a much steeper angle than ray 2a. It is
refracted less strongly into the core. Compare
what happens to rays 2a and 3 as they exit the
core.
• ray 2a meets the boundary at a lower angle
than ray 3. Ray 2a is refracted more strongly
and reaches the surface farther than 180° from
the earthquake source.
In contrast, ray 3 strikes the core-mantle boundary at a higher
angle. It is refracted less and reaches the surface at 143°.
ray 4. It strikes both
boundaries at steep angles.
It is deflected much less
than ray 2a and 3.

6. the angle of refraction depends on the angle at which the
rays strike the boundary or the core-mantle boundary. The
lower the angle that the rays strike the boundary, the
higher they are refracted.

7. the shadow zone
• Whenever an earthquake occurs, there is always a
large area on the opposite side of Earth where
seismic waves do not travel. This is called the
shadow zone.
• s located between 103° and 143° away from the
earthquake source.
The shadow zone for P waves
The shadow zone
occurs because the core
blocks the seismic
waves. The location of
the shadow zone is as a
result of the physical
properties of the mantle
and the core.

8. • Recently, scientists have observed some very
weak P waves within the 103°–143° P wave
shadow zone. Weak P waves emerge at around
120° from the earthquake source. This
suggests that Earth’s core is divided into a
liquid outer core and a solid inner core. The
boundary of the solid inner core deflects the
weak waves into the outer core. They continue
through the mantle and to the
surface, within the
shadow zone.
Exception:_

9. • the core-mantle boundary is about 2900 km
below the surface.
The shadow zone for s waves
The core has very strong effects on whether S
waves can pass through it.
S waves, only pass-through solid material. They
cannot pass through a liquid. No S waves are
received beyond 103° from the source of an
earthquake. This creates a massive shadow
zone.
This zone is almost one
hemisphere in size.
How they know that the
reaching wase are only the
p waves ??

10. • From its frequency
• The S wave shadow zone provides evidence
that Earth’s outer core is liquid because it
blocks S waves.
• The Moho is the boundary between
the crust and the mantle in the earth. This is a
depth where seismic waves change velocity
and there is also a change in chemical
composition (much denser silicate minerals). Also
termed the Mohorovicic' discontinuity after the
Croatian seismologist Andrija Mohorovicic'
(1857-1936) who discovered it. The boundary is
between 25 and 60 km deep beneath the
continents and between 5 and 8 km deep
beneath the ocean floor.
The Moho

11. • At the base of the lithosphere is another
seismic transition, the Gutenberg low velocity
zone. At this level, the velocity of S waves
decreases dramatically, and seismic waves
appear to be absorbed more strongly than
elsewhere within the earth. Scientists interpret
this to mean that the layer below the
lithosphere is a "weak" or "soft" zone of
partially melted material (1–10% molten
material). This zone is termed
the asthenosphere, from the Greek asthenes,
meaning "weak." This transition between the
lithosphere and the asthenosphere is named
after German geologist Beno Gutenberg (1889–
1960), who made several important
contributions to our understanding of Earth's
Guttenberg low velocity

12. Seismic Tomography
The study of three-dimensional imagery from
seismic data
The tomography shows the upper mantle to be
cooler
beneath the continents and hotter beneath the
ocean floor.
The hot blobs in the mantle are located along
mid-ocean ridges. Hot blobs also appear in zones
of mantle upwelling.
In both places, seismic
waves have reduced
speeds because the
rocks are hotter and
less dense than
surrounding rock.

13. Anomaly is a word used to describe variations of what would
normally be expected.
Gravity Anomalies
geoid : the hypothetical shape of the earth, coinciding with
mean sea level and its imagined extension under (or over)
land areas.
Geologists refer to a deviation of the observed (real)
geoid from the reference geoid as a gravity anomaly.
Over a positive anomaly, the pull is stronger, whereas
over a negative anomaly, the pull is weaker. A positive
anomaly indicates that
there is extra mass
below the site, perhaps
due to a body of
particularly dense
rock underground,
whereas a negative
anomaly means that
there is a deficit of mass,
perhaps due to the
presence of less-dense rock

14. isostasy
Gravity anomalies also provide key information about the nature
of the Earth’s interior. To see why, we need to introduce the
concept of isostasy, whose basis stems from the work of a Greek
mathematician, Archimedes (ca. 287–212 b.c.e.). Archimedes had
been puzzling over the question of why some objects float and
others sink, until one day when, according to legend, he noticed
the water level rise in a tub as he stepped in to take a bath. He
realized that this meant that an object displaces water as it sinks
into it, and that if the density of the object exceeds that of water,
it sinks, but if it is less dense, it floats. Further, a floating object
sinks into water only until it has displaced an amount of water
whose mass equals the mass of the whole object. These
relationships are now known as Archimedes’ principle.

15. • According to Archimedes’ principle, a cargo ship
anchored in a harbor floats at just the right level so that
the mass of the water displaced by the ship equals the
mass of the whole ship. Even though the ship consists
of heavy steel, the inside of the ship is filled with air, so
the ship, in effect, is a steel-sheathed bubble—it floats
because its overall average density is less than that of
water.
• When the ship remains empty, the distance that its deck
lies above the water, its so-called “freeboard”, is large.
Addition of heavy cargo causes the ship’s keel to sink
deeper into the water, the deck to move down, and the
freeboard, therefore, to decrease. This movement can
take place because the water beneath the ship can flow
out of the way as the ship settles downward. When the
freeboard of the ship is just right for a given cargo, we
say that the ship is in “isostatic equilibrium,” or that a
condition of isostasy exists.

16. The application of this principle on our earth
(1) In the first case: when the mass above the
earth crust increase for example because of
the volcanos or ice land formation. They will
make a force on the asthenosphere. Then the
asthenosphere raise like a water in the tub to
let the crust down as its density increase
(2) In the second case: when the mass above the
earth crust decrease (like when the ice land
melt ) then It will not rise then the crust will
raise according to Archimedes principle
for ex: According to NASA, measurements
between 2003 and 2006 showed a decrease in
gravity over the Antarctic ice sheet. This
indicates a decrease in ice mass. This ice mass
has decreased by about 150 billion tons per year.
‫ده‬ ‫و‬ ‫ترتفع‬ ‫قشرة‬ ‫ال‬ ‫يدوب‬ ‫التلج‬ ‫لما‬ ‫ان‬ ‫المفروض‬ ‫مش‬ ‫بيحصل‬ ‫ده‬ ‫ازاي‬
‫؟؟‬ ‫ثابتة‬ ‫تبقا‬ ‫الجرافيتي‬ ‫و‬ ‫توازن‬ ‫يعمل‬

17. • ‫االجابة‬
:
• ‫للتوازن‬ ‫يوصل‬ ‫عشان‬ ‫جدا‬ ‫جدا‬ ‫طويل‬ ‫وقت‬ ‫بياخد‬ ‫ده‬ ‫االرتفاع‬
‫في‬ ‫انحرف‬ ‫عندنا‬ ‫بيبقا‬ ‫يتحقق‬ ‫ده‬ ‫التوازن‬ ‫ما‬ ‫لغاية‬ ‫كده‬ ‫عشان‬
‫بالنقص‬ ‫او‬ ‫بالزياده‬ ‫ده‬ ‫فالمكان‬ ‫االرضية‬ ‫الجاذبية‬

19. Bassmala Mohamed
3A