Seismic analysis

1. UNIT 1 – Part II
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2. Seismogram
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3. Seismograph
 Instrument that measures earthquake shaking
 Three components:
1. Sensor – (Pendulum mass, string, magnet & Support
together constitute)
2. Recorder – (drum, pen & chart paper constitute)
3. Timer – (motor that rotates the drum at constant speed
that forms the timer)

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Two horizontal seismographs - one of which swings from north
to south, while the other one swings from east to west and the
third dimension is caught by a vertical seismograph, also called
up-down or z-seismograph.
Seismographs

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North-South, East-West and Up-Down-Seismogram

6.  Shaking is more severe (about twice as much) at the
Earth's surface than at substantial depths. This is often the
basis for designing structures buried underground for
smaller levels of acceleration than those above the ground
 The motion at any site on ground is random in nature with
its amplitude and direction varying randomly with time.
 The motion of the ground can be described in terms of
displacement, velocity or acceleration
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Velocity is the integral of the acceleration
Integrate the velocity and compute the displacement

8. Accelerogram
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 The variation of ground acceleration with time recorded at a
point on ground during an earthquake is called an
accelerogram.
 The nature of accelerograms may vary depending on
energy released at source, type of slip at fault rupture,
geology along the travel path from fault rupture to the
Earth’s surface, and local soil.

9. Accelerogram
 They carry distinct information regarding ground shaking -
peak amplitude, duration of strong shaking, frequency
content (e.g., amplitude of shaking associated with each
frequency) and energy content (i.e., energy carried by
ground shaking at each frequency).
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10. Accelerogram
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11.  For an example, a horizontal PGA (peak ground acceleration)
value of 0.6g (0.6 times the acceleration due to gravity)
suggests that the movement of the ground can cause a
maximum horizontal force on a rigid structure equal to 60% of
its weight.
(In a rigid structure, all points in it move with the ground by the
same amount, and hence experience the same maximum
acceleration of PGA).
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12. Size of Earthquake
The size of the earthquake is normally represented by
 Magnitude (amount of energy released)
The magnitude of the earthquake is a single value for a given
earthquake
 Intensity (a qualitative measure based on degree of
destruction caused)
It is an indicator of the severity of shaking generated at a
given location which is much higher near the epicenter than
farther away.
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Focal Depth, is an important parameter in determining
the damaging potential of an earthquake.

14. Magnitude Vs Intensity
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During the same earthquake of a certain magnitude, different
locations experience different levels of intensity.

15. Magnitude scale
 At the same distance, seismograms of larger earthquakes have
bigger wave amplitude than those of smaller earthquakes
 For a given earthquake, seismograms at farther distances have
smaller wave amplitude than those at close distances.
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The Richter Scale (a magnitude scale) is obtained from
the seismograms and accounts for the dependence of
waveform amplitude on epicentral distance.
Richter scale is also called as Local Magnitude scale.

16. Magnitude scale
C.F. Richter defined the earthquake magnitude as the
logarithm to the base 10 of the largest displacement
(amplitude) of a standard seismograph** situated at an
epicentral distance of 100 km from the focus
(**Wood-Anderson Seismograph with properties T=0.8 sec;
m=2800; and damping nearly critical ≈ 0.8).
where A denotes the amplitude in micron (10-6m); and M is
the magnitude of the earthquake.
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17. Magnitude scale
When the distance from the epicenter at which an
observation is obtained other than 100 km, a correction is
introduced to the equation as given below
where M is the magnitude of the earthquake; Δ=distance
from epicenter (km), MΔ= magnitude of the earthquake
calculated for earthquake using the values measured at a
distance Δ from the epicenter.
The graphical form of this procedure is given below.
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18. Epicenter distance and the Earthquake's Magnitude
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Measure the distance between the
first P wave and the first S wave.
Find the point for 24 seconds on the
left side of the chart below and mark
that point.
According to the chart, this
earthquake's epicenter was 215 km.
Measure the amplitude of the
strongest wave. On this seismogram,
the amplitude is 23 millimeters.
Find 23 millimeters on the right side
of the chart and mark that point.
Place a ruler on the chart between
the points (distance to the epicenter
and the amplitude).
The point where the ruler crosses the
middle line on the chart marks the
magnitude (strength) of the
earthquake.
This earthquake had a magnitude of
5.0.

19. Magnitude scale
 An increase in magnitude (M) by 1.0 implies 10
times higher waveform amplitude and about 31 times
higher energy released.
 For an example, energy released in a M 7.7 earthquake is
about 31 times that released in
a M 6.7 earthquake, and is about 1000 (≈31×31) times
that released in a M 5.7 earthquake
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20. Magnitude scale
 Other magnitude scales are
 Body Wave Magnitude
 Surface Wave Magnitude and
 Wave Energy Magnitude.
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21. Intensity
The intensity of earthquake depends on
 Distance from the epicenter
 Compactness of the underlying ground
 Type of construction
 Density of population
 Duration of the earthquake and
 Depth of the focus.
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22. Intensity
The most popular intensity scale is the Modified Mercalli
Intensity (MMI) Scale.
This scale, composed of 12 increasing levels of intensity
that range from imperceptible shaking (i.e. vibration
below the limits of sensibility) to catastrophic destruction.
The higher numbers of the scale are based on observed
structural damage.
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23. Intensity
The other intensity scales are
 Mendvedev-Spoonheuer-Karnik scale (MSK 64). (This
scale is more comprehensive and describes the intensity
of earthquake more precisely. Indian seismic zones were
categorized on the basis of MSK 64 scale)
(Refer Annex-D; IS 1893 (part-1) – 2016)
 Rossi-Forel (RF) scale
 Japanese Meteorological Agency (JMA) intensity scale
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24. Zone Factors Vs Intensities
 Zone II MSK Scale VI or less
 Zone III MSK Scale VII
 Zone IV MSK Scale VIII
 Zone V MSK Scale IX and above

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A comparison of the various seismic intensity scales used worldwide

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27. Magnitude and Intensity in Seismic Design
 Can my building withstand a magnitude 7.0 earthquake???
M 7.0 earthquake causes different shaking intensities at
different locations, and the damage induced in buildings at
these locations is different.
Thus, the buildings and structures are to be designed to
resist the particular levels of intensity of shaking and not so
much the magnitude.
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28. How is an Earthquake’s Epicenter Located?
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.

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30. 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

31. How is an Earthquake’s Epicenter Located?
 Three seismograph
stations are needed to
locate the epicenter of
an earthquake
 A circle where the
radius equals the
distance to the
epicenter is drawn
 The intersection of the
circles locates the
epicenter

33. Thank you
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