Earthquake Parameters
Imtiyaz A. Parvez
C-MMACS, Bangalore
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Structure of the Earth
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Structure of the earth and ray path
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Earthquakes
• There are over a million (106) earthquakes each year
most of these are insignificant.
• About 3,000 of these produce noticeable effects
(e.g. tremors, ground shaking).
• About 20 each year cause major damage and
destruction.
• On average, about 10,000 people die each year because
of earthquakes
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Current worldwide earthquakes
(December 2002)
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Significant worldwide earthquakes in
2001 (M 5-8, no. 1376)
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Some examples of earthquake damage
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What causes earthquakes?
For most recorded history
they have been attributed to
mythical beasts or the wrath
of Gods!
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Development of Scientific Ideas
Most scientists in the mid 1700’s attributed earthquakes in
some way to recently discovered electricity.
John Winthrop (1755) - an astronomer at Harvard suggested
that “esrthquakes are waves within the earth” based on the
observation of moving bricks in his chimney during 1755
Boston earthquake.
John Mitchell (1760) - an astronomer at Cambridgea) Attributed earthquakes to waves caused by shifting rocks
deep within the earth.
b) He worked out a method to calculate the location of an
earthquake.
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John Milne (1891) - an English geologist working for the
Emperor of Japan is considered to be the founder of modern
earthquake studies and the science of SEISMOLOGY.
Milne noted three types of earthquake motion:1) Back and forth waves
2) Up and down waves
3) Side to side waves
(P-waves)
(S-waves)
(L-waves)
The time interval between the first back and forth wave and
later waves was related to the distance from the earthquake
epicentre. (approximately 300 miles/minute)
(example - a 10 second delay meant the earthquake was about 50 miles
away)
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The Seismograph
Milne invented the predecessor to the modern seismograph
and hence the science of seismology
(from the Greek Seio - to shake and Seismos - earthquake)
Horizontal Movement
Vertical Movement
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Earthquake Mechanism
• Reid’s Elastic Rebound theory
• Types of Earthquakes
• TECTONIC
– Inter-plate (e.g. Himalayan Fault Zone)
– Intra-pate (e.g. Peninsular India)
• VOLCANIC (E.g., Chile, Peru, Hawaii)
• Due to Cavitation (e.g., salt mine collapses)
• Reservoir Induced (e.g., Koyna)
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Elastic Rebound Model - Reid 1910
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Tectonic Earthquakes
Most earthquakes occur at plane margins due to
tension, compression or shearing forces.
Rocks at plate margins are in constant motion
and are being pulled, pushed, bent, twisted and
folded.
Inevitably at some point they must break or crack
to produce FAULTS!!
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What is a Fault ?
• A fault is a break or fracture between two blocks
of rocks in response to stress.
• Three types of stresses produce faults
1) Tension
2) Compression
3) Shear
• One block has moved relative to the other block
• The surface along which the blocks move is
called a fault plane.
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There is a chicken and egg relationship between
faults and earthquakes
1)
2)
It was initially thought that earthquakes
caused faulting (but then what caused the
earthquakes?)
It was later realized that faulting produced the
earthquakes.
Thus earthquakes may occur because:a)
b)
Rocks are initially broken to produce a fault.
Movement or re-activation of an already
existing fault.
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Faults and Fault Movement
•
•
•
•
•
•
•
Normal
Reverse
Strike-slip
Left lateral
Right Lateral
Combination-Oblique-Slip
Graben
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Strike Slip
Faulting
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Normal and
Reverse
Faulting
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Summary of Fault Movements
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Focal mechanism in Indian region
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Types of Seismic Waves
• Earthquake releases energy that is
transformed into two major types of waves
– Body Waves –travel within the Earth’s body
• P – Waves
• S – Waves
– Surface Waves – travel on the Earth’s surface
• Rayleigh Waves
• Love Waves
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P-waves
• These are compressional waves originating at the
earthquake source. They travel through the mantle, the
inner and outer core. For earthquakes of normal shallow
depth, they travel at a fairly constant velocity vp=5km/sec.
P-waves have been observed to arrive at stations of the
same distance from the epicenter at the same time with
only 2 sec deviation in arrival times. From wave theory :
– where
λ = Lame’s constant
µ = rigidity of the earth
ρ = earth’s density
λ + 2µ
vp =
ρ
• Travel in the longitudinal direction
• Reflected and refracted as other P-, S- and L-waves
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Propagation of P-Waves
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S - Waves
• Originate at the earthquake source and travel at
slower rate than the P-waves. Travel velocity is
~3 km/sec. The velocity of shear waves can be
shown to be:
µ = rigidity of the earth
ρ = earth’s density
µ
vs =
ρ
• two types of waves are generated
– SH and SV -polarized in two directions
• do not travel through the outer core
• reflected as S- and P- waves
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S-Waves
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Comments
• Note that:
– If µ = ridigity = 0, then νS = 0 and the shear (or
rotational waves) cannot be transmitted through
such media (e.g. fluids)
– νS < νP, thus P-waves travel faster than Swaves
• S-waves resolve into
– SH = S-waves polarized in the horizontal
direction
– SV = S-waves polarized in the vertical direction
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Rayleigh Waves
• These are surface waves that are a combination of
P and SV waves
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Love Waves
• These are SH waves that are trapped by multiple
reflections within a surface layer which has a lower
body wave velocity than the underlying half space
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Recording Seismic waves
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Locating an earthquake
If the distance to an earthquake can be calculated from the arrival times of
the different shock waves then it is simple matter of geometry to locate the
precise centre of the earthquake - its EPICENTER.
All one needs is information from three different locations:-
Epicenter
of earthquake
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A real (?) example
Earthquake epicenter is:-
7000 km from San Francisco
2500 km from Tokyo
6500 km from Sydney
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Earthquake Mechanism
• Structure
–
–
–
–
–
–
–
Focus
Epicenter
Rupture Width
Rupture Length
Differential Displacement or Slip
Dip Angle
Strike Angle
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N
or
th
Fault Structure
Strike Angle
Ground Surface
Epicenter
Dip Angle
Hypocenter
W
L
Surface offset
Fault Plane
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