Basic Introduction to Earthquake engineering and seismology

What is Earthquake
An Earthquake is the result of a sudden release of energy in the Earth’s crust that creates seismic
waves.
Or
An Earthquake is the vibration of Earth produced by the rapid release of accumulated energy in
elastically strained rocks.
Fault: A fracture in the rocks that make up the Earth’s crust
Epicenter: The point at the surface of the Earth above the focus
Plates: Massive rocks that make up the outer layer of the Earth’s surface and whose movement along
faults triggers earthquakes
Seismic waves: Waves that transmit the energy released by an earthquake
Focus (Hypocenter): The point within the Earth where an earthquake rupture starts
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A number of smaller size earthquake take place before and after a big earthquake.
Foreshocks :
Those occurring before the big one are called foreshocks.
Aftershocks :
Smaller size earthquakes occurring after the main shocks are called aftershocks.
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SEISMIC WAVES
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Body Wave: p wave and S wave
•Primary waves are the fastest body waves (twice the speed of s-waves) and are the first to reach during an
earthquake. S waves can not pass through liquid.
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Surface Wave: Rayleigh waves and Love waves
All surface waves travel slower than body waves and Rayleigh waves are slower than Love waves.
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Shadow zone or no light/wave area
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SEISMOGRAPH
• The energy released during the earthquake travels as waves.
• Device used to measure an earthquake is called ‘seismograph’. The seismograph has
three main devices, the Richter Magnitude Scale, the Modified Mercalli Intensity
Scale, and the Moment Magnitude (MW) Scale.
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Principal Types of Earthquake Damages
Structural
Liquefaction
Landslide
Tsunamis

Strongest and deadliest Earthquakes in World History

List of Costliest Earthquakes of Century

World Tectonic Plates

Continental drift
Theory that continents and plates move on surface of the earth proposed by Alfred Wegener in 1915.

Seismology
Seismology is the branch of Geophysics concerned with the study and analysis of Earthquakes
and the science of energy propagation through the Earth’s crust.
The first such earthquake measuring instrument called Di-Dong-Di was invented by Cheng
Heng (132 A.D.).
Device used to measure an earthquake is called ‘seismograph’.
The seismograph has three main devices, the Richter Magnitude Scale, the Modified Mercalli
Intensity Scale, and the Moment Magnitude Scale.

Seismographic stations around the world function together to:
• Determine the location of earthquake
• Determine the strength of the earthquake
Earthquakes usually occur at some depth below the ground surface.
Earthquake are described as:
– Shallow: less than 70 km depth
– Intermediate: 70 - 300 km depth
– Deep: 300 - 700 km depth
90% of earthquake focus are less than 100 km deep
Large earthquakes are mostly at < 60 km depth
No earthquakes occur deeper than 700 km

Magnitude and Intensity of an Earthquake
Intensity
• How strong earthquake feels to observer.
• Qualitative assessment of the kinds of damage done by
an earthquake.
• Depends on distance to earthquake & strength of
earthquake.
• Determined from the intensity of shaking and damage
from the earthquake.
• Expressed using modified Mercalli scale.
Magnitude
• Related to energy release.
• Quantitative measurement of the amount of
energy released by an earthquake.
• Depends on the size of the fault that breaks.
• Determined from seismic records.
• Expressed on various magnitude scales like
Richter scale.

Modified Mercalli scale

Magnitude of Earthquake
Magnitude of an earthquake is the measure of energy released during an
earthquake.
Some of the common magnitude scales are:
– ML - Local (Richter) magnitude
– MW - Seismic Moment magnitude
– MS - Surface wave magnitude
– mb- Body wave magnitude

MW - Seismic Moment magnitude
MW= log10A + 1.66 log10 Δ +2.0
A = Maximum ground displacement in micrometers
Δ= Distance of seismograph from the epicenter, in degrees.
ML - Local (Richter) magnitude
ML= log10A(mm) +3 log10[8Δt (sec)]-2.93
Here A is the amplitude, in millimeters
The S-P time, in seconds, makes Δt.
mb- Body wave magnitude
mb = log10A – log10T +0.01Δ + 5.9
A = Amplitude of P-waves in micrometers
T = period of P-wave
Δ = Distance of seismograph from the epicenter, in degrees.
MW - Seismic Moment magnitude
Moment Magnitude, Mw = 2/3 [log10M0(dyne-cm) –16]
Moment Magnitude, Mw = - 6.0 + 0.67 log10M0(N.m)
M0= DAμ
where D is the average fault displacement in m,
A is the total area of the fault surface in m2, and
μ is the shear modulus of material along the fault plane in N/m2 (=
3x1010 N/m2
for surface crust and 7x1012 N/m2 for mantle)
M0 is measured (Note: 1 dyne-cm = 1 × 10–7 newton-meter)

40076 km = 360°

S-P (S minus P) time formula:
D=
𝑉𝑃𝑉𝑠
𝑉𝑃−𝑉𝑠
(𝑇𝑠 − 𝑇𝑝) T=
𝐷
𝑉
Ts > Tp
Vp > Vs
Ts=
𝐷
𝑉𝑠
Tp=
𝐷
𝑉𝑝
S-P Time difference
Ts-Tp=
𝐷
𝑉𝑠
−
𝐷
𝑉𝑝
(Ts-Tp)=D(
𝑉𝑝−𝑉𝑠
𝑉𝑝𝑉𝑠
)
Where
Vp= velocity of p wave
Vs= velocity of s wave
Tp= time period of p wave
Ts= time period of s wave

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Basic Introduction to Earthquake engineering and seismology

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Basic Introduction to Earthquake engineering and seismology