Elements of seismology - Structural dynamics and Earthquake Engineering (SD&EE)

SNS COLLEGE OF ENGINEERING
DEPARTMENT OF CIVIL ENGINEERING
UNIT 3
ELEMENTS OF SEISMOLOGY
PRESENTATION BY
SHANMUGASUNDARAM .N
ASSISTANT PROFESSOR
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CE6701-SD&EE/UNIT 3 by,
Shanmugasundaram.N

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CE6701-SD&EE/UNIT 3 by, Shanmugasundaram.N

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DATE TIME LOCATION DEATH COMMENTS M
June
26,
1941
08:50
IST
Andaman
Islands
7000
Triggered a
tsunami that
affected
eastern India
and Sri Lanka
8.1
Aug
10,
2009
01:21
IST
Andaman
Islands
26
Tsunami
Warning
issued
7.1
Dec
26,
2004
09:28
IST
west coast
northern
Sumatra
India Sri
Lanka
Maldives
283,106
Third
deadliest
earthquake in
the history of
the world, the
tsunami
generated
killed 15,000
people in
9.1
4/30

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 Study of generation, propagation and
recording of elastic waves in the earth.
 Sources and cause.
 seismic = reference to EQ
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 Study of causes
 Nature of EQ
 Seismic waves
 Measurement of seismic vibration
..San.AandreasDAM.avi
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 Sudden vibration movement of earth surface
 Release of energy in the earth crust.
 Vibration of the ground produced by forces
called seismic forces
 Father of seismology was Prof.Winthrop
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 Magnitude
◦ Quantitative measure of the amount of strain energy
released during EQ.
 Intensity
◦ Quantitative measure of the actual shaking at a location
during EQ at a different site.
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◦ Does not vary place to place
◦ Developed was Charles.F.Ritcher
◦ Using Ritcher scale
◦ EQ size Vary from wide range.
◦ M = log10A – log10A0
◦ A0 – Amplitude for zero magnitude EQ at different
epicentre distance (max 100km)
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◦ Measured based on living and non living things of the
seismic region
◦ Does not required any instruments
◦ Depends on observer
Intensity scales
1. Ten point Rossi Forrel scale
2. Twelve point modified Mercalli scale
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 Large collection of material
 Large amount of heat
 Slowly as the Earth cooled down
 Heavier and denser materials sank to the
center
 The lighter ones rose to the top
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 Inner core (radius ~1290km)
 The outer core (thickness ~2200km)
 The mantle (thickness ~2900km)
 The crust (thickness ~5 to 40km)
 At the core, the temperature is estimated to be
~2500°c,
 The pressure ~ 4 million atmospheres
 Density ~13.5 gm/cc; this is in contrast to ~25°C,
 1Atmosphere and 1.5 gm/cc on the surface of the earth.
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 The inner core is solid and consists of heavy
metals (e.g., Nickel and iron)
 The outer core is liquid.
 The mantle has the ability to flow.
 The crust consists of light materials (e.g.,
Basalts and granites).
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1. Natural sources
a. Tectonic EQ
b. Volcanic EQ
c. Rock faults
2. Man made sources
a. Controlled sources (explosives)
b. Reservoir induced EQ
c. Mining induced EQ
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 6major plates
◦ African plate
◦ American plate
◦ Antarctic plate
◦ Australian- Indian plate
◦ Eurasian plate
◦ Pacific plate
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◦ Study of deformations of earth materials and the structures
that result from deformation
◦ Refer to global scale
◦ Thick plates, 100 or above thick
◦ 120km thick in continental regions
◦ 80 km thick in ocean region
◦ Its consist of outer layer, which is cold enough to behave as a
more or less rigid shell.
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◦ Presser and stress are create due to heating and cooling
◦ Moves with different velocity and different direction because
of neighbour plate.
◦ Moved to 2 to 12cm per year.
◦ Some single rock was broken or break suddenly due to high
pressure occurred in that single rock.
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 Convergent boundaries
 Divergent boundaries
 Transform boundaries
..San.AandreasBRIDGE.avi
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 ..San.AandreasSan.Andreas fault.avi
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 Concept of possible mode of origin of tectonic EQ .
 According to Pro. Of geology H.F.Reid, materials of the earth
being elastic.
 It can withstand the certain amount of stress without deforming
permanently.
 the rock will take first deformation or strain and eventually
rupture.
 The fault is a break or fracture in the materials of the earth along
which there has been displacement.

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 After break up the rock will came to original position due to
elastic rebound occurs.
 This rebound sets the seismic waves
 Thus energy accumulated or stored in the system through
decades.
 ERT explained as three phase
1. The preparing process
2. The rupture phase
3. The post failure adjustment.

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◦ Hot magma moves slowly through underground passages
under pressure.
◦ More damage and intensity waves are produced
◦ Rock take pressure when the magma came out
◦ The EQ will occurred due to this action.
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◦ In week portion the rock will suddenly moved and elastic
strain energy realised.
◦ Due to realise of strain energy the rock will rapid slippage
along fault.
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Based on the direction of movement of blocks, 3 types
1. Dip-Slip fault
◦ Normal fault
◦ Reverse fault
2. Strike-Slip fault
3. Oblique-Slip fault
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◦ Human produced EQ, like nuclear devices.
◦ Moderately create damage 50km away.
◦ Rock blasting in quarries also produced small seismic
EQ.
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◦ Weight of the water increased pressure of rock below the
valley surface.
◦ Rupture of rock.
◦ example : Koyna Dam (magnitude 6.5)
6. Mining EQ
Underground caves and mine collapse
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 San.AandreasWAVES 1.avi
 San.AandreasWAVES 2.avi
1. During EQ large strain energy is released which generate
elastic waves at the focus
2. Also called seismic waves and they travel in all directions
through the earth surface.
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 Body waves
◦ Primary waves (P waves)
◦ Secondary waves (S waves)
 Surface waves
◦ Love waves
◦ Rayleigh waves
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◦ Longitudinal waves similar to sound waves.
◦ Fastest waves.
◦ Moves through solid and fluids.
◦ Also known as compression waves because of pushing and
pulling action.
◦ Particles moves in same direction.
◦ Velocity of the waves is 4.8km/sec.
◦ Shortest wavelength and high frequencies.
◦ Sometimes animals can hear the P-Waves EQ.
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◦ Shear waves (S) also known as transverse waves.
◦ Moves in perpendicular to the direction of particle
motion.
◦ Moves only through solid rocks.
◦ Slow waves comparing to P wave.
◦ Short wavelength and high frequencies.
◦ Dangerous waves. larger then P waves
◦ Produced vertical and horizontal motion
◦ Velocity 3km/sec. Do not travel through liquid.
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◦ First kind of surface waves.
◦ Named after A.E.H. Love who explained the mechanism
of generation of love waves in 1911.
◦ Transfer vibration and confined to the outer skin of the
crust.
◦ Faster surface waves
◦ Produced horizontal motion.
◦ Move the ground from side to side
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◦ The other kind of surface waves.
◦ Named for john william strutt, in 1885
◦ Roll along the ground
◦ Moves up and down and side to side in same direction
that the wave is moving
◦ Most shaking due to rayleigh waves.
◦ Larger then other waves.
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 Seismograph
◦ Seismograph is an instrument used to recording motions
of the earth surface caused by seismic waves as a
function of time.
 Seismogram
◦ Seismogram are the records produced seismographs used
to calculate the location and magnitude of an EQ.
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 Its represents in terms of
◦ Displacement
◦ Velocity and
◦ Acceleration
 Factors influencing Ground motion
o Magnitude of EQ
o Epicentre distance
o Local soil conditions
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 According to plate boundaries
 According to its depth of focus
 According to the origin of the EQ
 Based on magnitude
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 According to plate boundaries
◦ Inter plate EQ – along the boundaries of tectonic plates.
◦ Intra plate EQ – occurred at within the plate
 According to its depth of focus
◦ Shallow focus EQ – 70 to 80 km
◦ Intermediate focus EQ – 70 to 300 km
◦ Deep focus EQ – grater then 300 km
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 According to the origin of the EQ
◦ Tectonic EQ
◦ Volcanic EQ
◦ Submarine EQ (Costal area, Tsunami)
 Based on magnitude
◦ Micro EQ ( m<3)
◦ Intermediate EQ ( 3<m<4)
◦ Moderate EQ ( 5<m<5.9)
◦ Strong EQ ( 6<m<6.9)
◦ Major EQ ( 7<m<7.9)
◦ Great EQ ( m>8)
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 ..San.AandreasTsunami.avi
 A wave train or series of waves generate in a body of
water by an impulsive disturbance that vertical
displaces the water column.
 A very high, large wave in the ocean that is usually
caused by an earthquake under the sea and that can
cause great destruction when it reaches land.
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 Tsunami is a japanes word with the English translation
harbour waves.
 Tsu-harbour
 Nami-Waves
 Tectonic earthquake is important roll of Tsunami
 when the EQ occurred beneath the sea, the water above
the deformed area is displaced from its equilibrium
position.
 The influence of gravity attempts to regain its
equilibrium.
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LEARN WELL
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