Why ridge has high shaking?

1. PRESENTATION ON
Prepared By:
Milan Kumar Rai
Engineering Geology, TU
Nepal
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 WHY RIDGE HAS HIGH SHAKING
DURING EARTHQUAKE?

2. WHAT ARE EARTHQUAKES?
 Earthquakes are the shaking, rolling or sudden
shock of the earth’s surface.
 They are the Earth's natural means of releasing
stress.
 The shaking or trembling caused by the sudden
release of energy.
 Usually associated with faulting or breaking of
rocks.
 Continuing adjustment of position results in
aftershocks.
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3. EARTHQUAKE TERMINOLOGY
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4. WHAT ARE SEISMIC WAVES?
 Response of material to the arrival of energy fronts
released by rupture.
 Two types:
 Body waves
P and S
 Surface waves
R and L
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5. P or primary waves
 fastest waves
 travel through solids, liquids, or gases
 compressional wave, material movement is in
the same direction as wave movement
S or secondary waves
 slower than P waves
 travel through solids only
 shear waves - move material perpendicular to
wave movement
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6. TYPES OF SEISMIC WAVES
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7. HOW IS AN EARTHQUAKE’S
EPICENTER LOCATED?
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Seismic wave behavior
 P waves arrive first, then S waves, then L and R
 Average speeds for all these waves is known
 After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance from
the seismograph to the epicenter.

8.  Time-distance graph showing the
average travel times for
P- and S-waves.
 The farther away a seismograph
is from the focus of an
earthquake, the longer the interval
between the arrivals of the P- and
S- waves.
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9. EARTHQUAKE SIZE – MAGNITUDE
9M = log A – log A0
Charles Richter
1900-1985
log of amplitude
Distance correction

10. HOW ARE THE SIZE AND STRENGTH OF AN
EARTHQUAKE MEASURED?
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Magnitude
 Richter scale measures total
amount of energy released
by an earthquake;
independent of intensity
 Amplitude of the largest wave
produced by an event is
corrected for distance and
assigned a value on an open-
ended logarithmic scale

11. DESTRUCTION
 The seismic waves reflect internally within the
wedge like geometric structure of the ridge,
combining to produce higher peak forces, a process
called destructive interference.
 The intensification effect of soft subsoil is partially
due to a shift in ground motion to longer oscillations
which are potentially more destructive in relation to
buildings.
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12. DESTRUCTION MORE IF EPICENTER IS
FURTHER THAN CLOSE ENOUGH
 This intensification effect is greater further away
from the epicenter than it is close to it.
 Depending on the thickness of the sediment layer,
there may be resonance effects which amplify
ground movements several times within a narrow
frequency spectrum.
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 The point within Earth where
faulting begins is the focus, or
hypocenter
 The point directly above the focus
on the surface is the epicenter

13. WHAT ARE THE DESTRUCTIVE
EFFECTS OF EARTHQUAKES?
 Ground Shaking
 amplitude, duration, and damage increases in
poorly consolidated rocks
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14. RESPONSE OF TALL STRUCTURES
 The building's vibrations tend to center around one
particular frequency, which is known as its natural or
fundamental frequency.
 So the shorter a building is, the higher its natural
frequency.
 The taller the building is, the lower its natural
frequency.
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15. HIGH AMPLITUDE AND LOW FREQUENCY
OF WAVE MOTION ON THE RIDGES
 The same phenomena implies in case of ridges too. 15

16. RIDGE TOPOGRAPHY
 The ridge topography can substantially influence
seismic ground motion and, in general, causes the
amplification of seismic ground motion amplitude at
ridges.
 On the base of these acceleration records, peak
ground acceleration and response spectrum ratios are
calculated, and examined and compared in order to
grasp the effect of ridge topography on ground motion.
 The findings shows that ridge topography has the
remarkable effect on seismic ground motion, in which
the amplification characteristics varies depending on
the shape of ridge topography
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17. RIDGE TOPOGRAPHY CONT…
 Significant wave amplifications occur in steep
topography, especially on ridge crests.
 Therefore Ground motion are enhanced both in
amplitude and duration which causes more
destruction.
 Reyleigh and love waves of low frequency are more
effective in causing greater damage to tall structure
since they travel on the surface .
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18. VARIATION OF TRAVEL OF LOW AND
HIGH FREQUENCY
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 Low frequency high descruction in smaller area
 Low frequency can damage to greater extent

19. TRAVELS OF FREQUENCY
 Low frequency travels far than high frequency.
 The amount of damage to a building or a tall
structure does not depend solely on how hard it is
shaken. In general, smaller structures are damaged
more by higher frequencies, so usually houses
must be relatively close to the hypocenter to be
severely damaged.
 Larger structures such as high-rises and bridges
are damaged more by lower frequencies and will be
more noticeably affected by the largest
earthquakes, even at considerable distances.
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20. CONCLUSION
 In conclusion, Due to inertial force and amplification of
magnitude of low frequency , ridge will suffer from
greater destruction.
 The wave amplifications occur in steep topography,
especially on ridge crests.
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21. THANKS FOR ATTENTION !!!
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