Seismology is the science of the study of the seismic waves and earthquake. These seismic waves are generated either by natural processes or man-made processes. The seismic waves play an important role in the identification of the earth’s layers, mineral deposits, ground water, oil & gas, sedimentary basins, etc. An Earthquake is the result of the seismic wave in which the ground is started to shaking. The seismic waves are the elastic wave that are generated when a rock cannot bear more energy than its capacity, in such case the rock tends to break and all stored energy are released along the pre-existed fracture, fault planes or new born fracture, fault planes(active fault). These energy are started to propagate in all direction in the form of waves. Thus, these suddenly released elastic energy are called seismic waves.

1. Seismology and Earthquake
Shivam Vishwakarma

2. Basic Concepts
 Seismology is the science of the study of the seismic waves and earthquake. These seismic waves
are generated either by natural processes or manmade processes.
 The seismic waves play an important role in the identification of the earth’s layers, mineral
deposits, ground water, oil & gas, sedimentary basins, etc.
 The seismic waves are the elastic wave that are generated when a rock cannot bear more energy
than its capacity, in such case the rock tends to break and all stored energy are released along
the pre-existed fracture, fault planes or new born fracture, fault planes(active fault). These energy
are started to propagate in all direction in the form of waves. Thus, these suddenly released
elastic energy are called seismic waves.
 An Earthquake is the result of the seismic wave in which the ground is started to shaking.

3. Types of Seismic Waves
 The seismic waves have been classified into two types and their subtypes, which are
1. Body Wave
1. P-wave, Primary Wave, Longitudinal Wave, Dilation Wave, Compressional Wave
2. S-wave, Secondary Wave, Transverse Wave, Shear Wave
2. Surface Wave
1. L-wave, Love Wave
2. R-wave, Rayleigh Wave

4. Nature of Seismic Waves
 Body Wave: the body wave travels throughout the rocks or medium.
 It is non-dispersive wave which does not change its frequency and wavelength with depth.
 It is high frequency (high intensity) and short period wave.
 It is only causes for the changes in volume (dilation) or some distortion in the rocks.
 It is the fastest wave.
 Surface Wave: the surface wave travels near or over the free surface of the rocks or medium.
 It is dispersive wave which changes its frequency and wavelength with depth.
 It is low frequency (low intensity) and long period wave.
 It is the main cause for the destruction of the ground, collapsing of building, bridges, dams, etc.
 It is the slowest wave.

5. Properties of the Body Waves
1. P-wave/ Primary Wave:
 P-wave is the fastest wave among the all seismic waves.
 It behaves like a sound wave.
 When a P-wave passes through a body, the particles of the body tends to oscillated forward and
backward (push or pull) motions; in such case some parts of the body experiences the compression and
some parts experiences extension or stretching. During the oscillation of the particle the motion of the
particles is parallel to the propagation direction of the P-wave.
 It is responsible for the changes in the volume of the body.
 Velocity range in the earth: 6 – 14.0 km/s.
 It can travel though all medium like solid, liquid, and gas.

6. Properties of the Body Waves
2. S-wave/ Secondary Wave:
 It is the second fastest wave.
 It behaves like a water ripples.
 When the S-wave passes though the body, the particles of the body tends to oscillate up and down motion; in
such case the motion of the particle is perpendicular to the propagation of the S-wave.
 It only can travel though the solid medium.
 It is responsible for the changing in the shape of the body.
 The S-wave travels as two components, horizontal motion (SH) and vertical motion (SV).
 Velocity range in the earth: 3 – 8.0 km/s.
SV
SH
S-wave
S- wavefront

7. Properties of the Surface Waves
1. Love Wave (L- wave):
 The Love has the characteristics of the SH wave type of S-wave.
 When love wave passes along the free surface, the ground tends to shake horizontally (side by side
motion).
 The velocity range of the L – wave: 2.0 – 6.0 km/s.
2. Rayleigh Wave (R- wave):
 The Rayleigh wave is characterized by the P – wave and SV – wave characteristics. Therefore it the
particle of the medium show the rolling motion in the elliptical wave front.
 It is also called the rolling wave.
 It is the most destructive wave.
 The velocity range of the R – wave: 1.0 – 5.0 km/s.

8. Source: https://gpg.geosci.xyz/content/seismic/wave_basics.html
a): P – wave b): S – wave
c): L – wave d): R – wave

9. Velocity of the Seismic Wave
 The seismic wave velocity depends upon the two factors – stiffness and the density of the
medium/body.
 Velocity of the Seismic wave =
Stifness
Density
 These physical variables can be influence by the temperature, pressure, and chemical fluids.
 Stiffness of the material can be expressed in the form of the Bulk modulus (K), Compressibility,
Rigidity/Modulus of Rigidity or Shear Modulus (µ), Fluidity.

10. Properties of the Material
1. Bulk Modulus (K):
 Bulk Modulus K =
Isotropic stress or Pressure on the body (σm,P)
Volume strain or Dilation in the body (eV)
Dimensionless
 The bulk modulus is the inversely proportional to the compressibility.
 The bulk modulus means how much amounts of pressure is applied to reduce the size of the matter.
2. Compressibility:
 Compressibility means the capacity of something to reduce in size by applying pressure/stress.
 The compressibility of the material depends upon the intermolecular spaces among the molecules.
More spaced matter is easily compressed.
 Solid is incompressible matter.
 Compressibility of the matter: Gas (easily compressible) > Liquid > Solid (Incompressible).
 Bulk Modulus of the matter: Brittle > Semi – rigid > Ductile > Plastic > Liquid > Gas.

11. Properties of the Material
3. Rigidity/Shear Modulus/Modulus of Rigidity:
 Rigidity is defined as the property exhibited by the solid to change its shape , i.e. when a force is
applied on the solid matter, there won’t be any change in the shape. This means that the
molecules of the matter are tightly packed together and the attraction force among the
molecules are very strong.
 Only solid state possesses the rigidity and the liquid and gas show the fluidity.
 Modulus of Rigidity =
Tangential Stress (τ)
Shear Strain (γ)
Dimensionless
 Shear Modulus would be: Solid >> Semi – rigid > Ductile
4. 𝐏𝐨𝐢𝐬𝐬𝐨𝐧′
𝐬 𝐑𝐚𝐭𝐢𝐨 σ =
Shear Stress
Longitudinal Stress

12. P – wave and S - wave Velocities
 The P – wave velocity depends upon the three factors, which are
1. Bulk Modulus (K)
2. Shear Modulus or Modulus of Rigidity (µ)
3. Density of the material (𝜌)
 The velocity of P − wave (α) =
K+
4
3
μ
ρ
km/s
 The S – wave velocity depends upon the two factors only,
1. Shear Modulus or Modulus of Rigidity (µ)
2. Density of the material (𝜌)
 The velocity of S − wave (β) =
μ
ρ
km/s
 The relationship between P – wave and S – wave 𝛼 = 1.73𝛽

13. Relationship
 Relationship between the Young Modulus (Y), Bulk Modulus (K), Shear Modulus (µ), and Poisson’s
Ratio (σ)
 𝑌 = 3𝐾 1 − 2𝜎
 𝑌 = 2𝜇 1 + 𝜎
 𝜎 =
3𝐾−2𝜇
6𝐾+2𝜇

9
𝑌
=
1
𝐾
+
3
𝜇

14. Factors affecting the seismic velocity
1. Composition of the rocks.
2. Temperature: With increasing temperature, the intermolecular spaces tend to increase. Thus the a
matter converts into other matter (e.g. solid state changes into liquid or semi solid). With increasing
temperature, the compressibility of the matter increases and the bulk modulus decreases. The result is
that the stiffness of the body is reduced and the velocity is also reduced. We can observe within the
Low Velocity Zone (Asthenosphere).
3. Pressure: With increasing pressure the stiffness of the body increases, so the velocity will increase.
Density of the body also increases with increasing pressure.
4. The velocity generally increases the depth, if we ignore the other factors like temperature, pressure,
and chemical fluids.
5. Addition of water reduces the velocity of the seismic wave. We can observe within the Low Velocity
Zone (Asthenosphere). The addition water and temperature also change the phase of the matter, the
solid may be converted into ductile or plastic material.

15. P – wave and S –wave velocities within the Earth’s
internal layers
Layers/Discontinuity/Depths 𝐕𝐏 𝐨𝐫 𝛂 (km/s) 𝐕𝐒 𝐨𝐫 𝛃 (km/s)
Lithosphere (up to 150 km) 6.0  8.0 3.0 5.0
Asthenosphere (up to 250 km) LVZ 8.0 7.5 (decreasing) 5.0 4.5 (decreasing)
Upper Mantle part below the
Asthenosphere (250 to 410 km)
7.5  10.0 4.5  6.0
Transition Zone (410 to 660 km) 10.0 9.0 (decreasing) 6.0 5.5 (decreasing)
Lower Mantle (660 to 2700 km) 9.0  14.0 5.5  8.0
ULVZ (D” layer 2700 to 2900 km) 14.0  7.8 (decreasing) 8.0 0.0 (decreasing)
Outer Core (2900 to 5150 km) 7.8 11.0 0.0
Inner Core ~11.0 ±4.0

16. P
PKiKP
PcP
Focus
ScS
S
SKS
SS
SKIKS
PKIKP
PKP
PP
Reflection from the
inner core
P wave reflected
once by outer core P wave reflected once (underside
of crust)
P wave through outer core 1st up through
mantle
K = outer core, P = mantle
P wave through inner core and outer core
S wave though mantle, converted to P
wave through outer core, P wave through
inner core, P wave through outer core,
converted to S wave through mantle and
crust
S wave reflected once (underside of crust)
S wave converted to P in
outer core
S wave just grazes outer
core
P wave
S wave
Seismic Wave
propagation within the
earth
Inner
Core
Outer
Core
Mantle

18. Earthquake
Earthquake is the result of the suddenly released elastic strained energy from the ground and the
result can be seen in the form of shaking of the ground surface.

19. Facts about the Earthquake
 Most of the earthquake are triggered along the pre-existed faults, fracture and also along the
new born fractures and faults (active faults).
 Earthquake can be occurred either by the natural phenomenon like volcanic explosion,
landslides, tectonic processes or by manmade processes like atomic bomb explosion.
 Elastic Rebound Theory
 The elastic rebound theory suggests that if slippage along the fault is hindered such that the elastic strain
energy build up in the deforming rocks on the either side of the fault, when the slippage does occur the
energy released causes the earthquake.
 Friction between the blocks keeps the fault from moving again until enough strain has accumulated
along the fault zone to overcome the friction and generate another earthquake.
 Once the fault forms, it becomes the zone of weakness in the crust, so long as tectonic stress continue to
be present more earthquake are likely to occur on the fault. Thus the faults move in spurts and this
behavior is referred to as a strike slip.

20. Facts about the Earthquake
 Most of the deep focal earthquakes are occurred along the Circum pacific (Ring of fire) and
Mediterranean seismic zone.
 Earthquake occurs along the faults.
 Blind Fault: A blind fault is one that does not break the surface of the earth. Instead, the rocks
above the fault plane behaves in ductile fashion and folded (fault propagation fold) over the tip
of the fault.
 Active Fault: An active fault is that fault which is still active from the hundred thousands years or
past hundred years and will be activated in the future. For the earthquake point of view it is very
significant.

21. Elements of Earthquake
• Focus: the point within the earth where the fault
rupture starts or an earthquake was triggered.
• Epicenter: the epicenter is the point over the
surface of the earth which is directly above the
focus. At this point, the earthquake magnitude
is maximum.
• Anti epicenter: it is just directly below the focus.
Fault Scarp
Anti epicenter

22. Strain Rate
 The strain rate is the rate at which the deformation occurs.
 At high strain rate the material tends to fracture whereas more time is available for individual
atoms to move at low strain rate therefore the ductile behavior is favored.
 e =
e
T
where e is the strain, T is time taken to deform.

23. Measuring of Earthquakes and Locating of Epicenter
 A seismometer/seismograph is an instrument used to record the seismic vibrations (seismic waves) and the
resulting graph that shows the vibrations is called a seismogram.
 Locating of Epicenter: for the locating of the epicenter we need some information like –
 We need to have recorded seismogram of the earthquake from at least three seismographic stations at
different distances from the epicenter.
 We need an information about the time taken by the P – wave and S – wave to travel through the earth
and arrive at the seismographic station (fig. 1).
Time interval TSP = (arrival time of S − wave − arrival time of P − wave)
 Find the place on the time interval graph where the vertical separation between the P – wave and S –
wave is equal to the time interval TSP (fig. 2).
 From this positon, draw a vertical line that extends to the bottom of the graph and read the distance to
the epicenter (fig. 2).

24. Measuring of Earthquakes and Locating of Epicenter
 To find an epicenter, the seismograms from the three different stations are needed in order to
“triangulate the location”. Therefore we need to determine that two other seismic stations are
from the epicenter, using the procedure.
 Using the compass, draw a circle around each seismic stations with radius equals to its distance
from the epicenter. The point where all three circles intersects is the approximate location of the
epicenter (fig. 3).

25. P – wave S – wave
Surface – wave
Arrival time of
P – wave
Arrival time of
S – wave
End time of
P – wave End time of
S – wave
Time (minutes)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Fig. 1
If D is the distance travelled by the seismic wave
tP = D
vPor α
tS = D
vS or β
Time interval tSP = D(
1
β
−
1
α
)
𝐓𝐒𝐏 (𝐦𝐢𝐧𝐮𝐭𝐞𝐬)
𝐃𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐟𝐫𝐨𝐦 𝐭𝐡𝐞 𝐞𝐩𝐢𝐜𝐞𝐧𝐭𝐞𝐫 (𝐤𝐦)
S –wave curve
P –wave curve
e.g. TP = 4 min e.g. TS = 7 min
TSP = 7 − 4 = 3 min
Epicentral
distance ∆ 𝑒
7
4
TSP = 7 − 4 = 3 min
Fig. 2
epicenter ∆ 𝑒1
∆ 𝑒2∆ 𝑒3
S1
S2S3
Fig. 3

26. Earthquake Size (Richter Scale)
 The size of the earthquake is usually given in the terms of “scale” called Richter Scale or Richter
Magnitude.
 Richter Magnitude is the scale of the earthquake size developed by a seismologist Charles F. Richter.
 The Richter involves measuring the amplitude (height) of the largest recorded wave at the distance
from the earthquake.
 1 Richter Magnitude (M = 1), there is 10 – fold increases in the amplitude of the wave.
 A better measurement of the size of the earthquake is determined in the form of amount of energy
released by the earthquake. The amount of the energy released (E) by an earthquake is related to the
Magnitude scale (M).
log10E = 11.8 + 1.5M erg
 Each increase in 1 magnitude represent the 31 – fold increase in the amount of energy released.

27. Richter Magnitude Scale

28. Moment Magnitude Scale
 Moment Magnitude:
MW = 2
3 (log10M0 − 9.1) (S.I. unit)
MW = 2
3 (log10M0 − 16.1) (C.G.S.)
Where, MW is the moment magnitude and M0 is the seismic moment (Nm or dyne cm).
 Body wave magnitude:
Mb = log10
AP
T + Q(∆, h)
Where, 𝐴 𝑃 is the maximum amplitude of the ground associated wit the P – wave, T is the time period (<3 seconds),
𝑄(∆, ℎ) is the empirical correction for signal attenuation due to epicentral distance (∆) and focal depth (h).
 Surface wave magnitude:
Ms = log10
AS
T + 1.66log10 ∆ + 3.33
Where, 𝐴 𝑆 vertical component of the ground motion (µm) determined from the maximum R – wave amplitude, T is the
period of the wave (18 – 22 seconds), ∆ is the epicentral distance (20° ≤ ∆ ≤ 160°) where the earthquake has focal depth
<50 km.

29. Seismic Intensity Scale
 Mercalli Intensity Scale: This scale is applied after the earthquake by conducting surveys of people’s response to the
intensity of the ground shaking and destruction.
 From a scientific standpoint, the magnitude scale is based on seismic records while the Mercalli is based on
observable data which can be subjective. Thus, the magnitude scale is considered scientifically more objective and
therefore more accurate. For example a level I-V on the Mercalli scale would represent a small amount of observable
damage. At this level doors would rattle, dishes break and weak or poor plaster would crack. As the level rises
toward the larger numbers, the amount of damage increases considerably. Intensity X (10) is the highest value on the
MMI (source: https://www.usgs.gov/faqs/what-difference-between-magnitude-and-intensity-what-modified-
mercalli-intensity-scale?qt-news_science_products=0#qt-news_science_products) .

30. Modified Mercalli Scale
An Intensity Scale
Source: USGS

31. Seismic Intensity Scales
Country (use) Intensity Scales
China Liedu scale(GB/T 17742-1999)
Europe European Macroseismic Scale (EMS-98)
Hong Kong Modified Mercalli scale (MM)
India Medvedev–Sponheuer–Karnik scale (MSK or MSK-64)
Israel Medvedev–Sponheuer–Karnik scale (MSK-64)
Japan The Japan Meteorological Agency (JMA) Seismic Intensity Scale
Kazakhstan Medvedev–Sponheuer–Karnik scale (MSK-64)
Philippines PHIVOLCS Earthquake Intensity Scale (PEIS)
Russia Medvedev–Sponheuer–Karnik scale (MSK-64)
Taiwan Central Weather Bureau Seismic Intensity Scale
United States Modified Mercalli scale (MM)
Source: https://en.wikipedia.org/wiki/Seismic_intensity_scales

32. Seismic Shadow Zone
 The seismic shadows are the effect of seismic waves striking the core-mantle boundary. P and S waves radiate
spherically away from an earthquake's hypocenter (or focus) in all directions and return to the surface by many
paths. S waves, however, don't reappear beyond an angular distance of ~103° (as they are stopped by the liquid)
and P waves don't arrive between ~103° and 140° or 143° due to refraction at the mantle-core boundary.
 The seismic shadow zone is the rea of the Earth's surface where seismographs cannot detect an earthquake after the
waves have passed through the earth.
 P waves are refracted by the liquid outer core and are not detected between 104° and 140° or 143°.
 S waves cannot pass through the liquid outer core and are not detected beyond 104°.
 This information led scientists in the early 1900s to deduce a liquid outer core.
Source: Incorporated Research Institutions for Seismology (IRIS)

33. 103° 103°
143°143°
P – wave
Patterns
S – wave
Patterns
103° 103°
S – wave Shadow Zone P – wave that passed through
the core
Epicenter Epicenter
Core
Core
Mantle
Outer Core
Outer Core
Mantle

34. Reflection and Transmission Coefficients
 RC = Amplitude reflected (A1)
Amplitude incident (A0) =
𝑉1 𝜌1−𝑉2 𝜌2
𝑉1 𝜌1+𝑉2 𝜌2
𝑉1 𝜌1= 𝑍1 is acoustic impedance of layer 1.
𝑉2 𝜌2 = 𝑍2 is acoustic impedance of layer 2.
 Polarity of reflected wave depends on sign of reflection coefficient (unchanged polarity
means compression remains compression, dilatation remains dilatation),
If 𝑉2 𝜌2 > 𝑉1 𝜌1: polarity of the wave unchanged
If 𝑉1 𝜌1 > 𝑉2 𝜌2: polarity of the wave reversed
Where, V is the P – wave velocity and ρ is the density of the layer.
 𝑇𝐶 =
2𝑍1
𝑍1+𝑍2
=
2𝑉1 𝜌1
𝑉1 𝜌1+𝑉2 𝜌2
𝑉1 𝜌1
𝑉2 𝜌2
A0
A1

35. Classification of Earthquake
 Classification of Earthquake based on the focal depth,
 Shallow focal depth earthquake: < 70 km depths
 Occurred along all seismological active zones.
 Shallow earthquake can also be occurred along the mid oceanic ridges.
 The largest proportion (~85%) of the annual release of seismic energy released is liberated in the shallow focal
earthquake.
 Intermediate focal depth earthquake: 70 – 300 km depths
 12 % occurred per year.
 Deep focal depth earthquake: > 300 km depths
 Approximately 3% occurred per year.

36. Wadati-Benioff Zone
 The Benioff Zone, sometimes referred to as the seismic zone or
seismic plane, is a dipping planar concentration of earthquake
hypocenters that extends up to 700 km into the earth. It is named
after H. Benioff, who first described it in detail (Benioff, 1949). Benioff
zones occur beneath modern arc systems beginning immediately
beneath oceanic trenches.
 Dip angles range from about 30 to 90°, averaging about 45°. In
terms of plate tectonics, the Benioff Zone is the site of plate
consumption and is often referred to as a subduction zone.
Although less frequent than shallow earthquakes, the deeper
earthquakes in Benioff zones range in magnitude up to 8.
Source: TY - CHAPAU - Kukowski, NinaED - Harff, JanED - Meschede, MartinED - Petersen, SvenED - Thiede, JÖrnPY - 2016DA - 2016//TI - Wadati-Benioff-ZoneBT -
Encyclopedia of Marine GeosciencesSP - 925EP - 932PB - Springer NetherlandsCY - DordrechtSN - 978-94-007-6238-1UR - https://doi.org/10.1007/978-94-007-6238-
1_108DO - 10.1007/978-94-007-6238-1_108ID - Kukowski2016ER -

38. Global map of subduction zones, with subducted slabs contoured by depth
Source: https://www.usgs.gov/media/images/global-distribution-models-included-slab2

39. Thanking you