Seismology

Introduction to seismology
Seminar on
Submitted To:-
Prof.Y.K.Gupta
Submitted By:-
Awinash Tiwari
( 2017ST08)

Topic to be discussed
 Introduction to engineering seismology
 Internal structure of earth
 Theory of plate tectonics
 Seismic waves
 Size and classification of an earthquake
 Strong ground motion characteristics
 Estimation of ground motion parameter
 Seismic hazard map
 Response spectra

Introduction to engineering seismology
•Seismology is a scientific study of earthquakes and the propagation
of elastic waves through the Earth or through other planet-like bodies and
with artificially produced vibrations of the earth and of the internal
structure of earth. It includes the study of origin, geographic distribution,
effects and possible prediction of earthquakes. The field also includes
studies of earthquake environmental effects such as tsunamis as well as
diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric,
and artificial processes such as explosions.
•Seismologists not only study earthquakes as they happen, but also try to
create models to predict when and where earthquakes might occur. They
also study the effects of earthquakes, like seismic waves and tsunamis.
Tsunamis are massive ocean waves caused by an under sea earthquake,
and they can be quite destructive to coastal communities.
• Paleoseismology:- A related field that uses geology to infer information
regarding past earthquakes .
•Seismograph:- A recording of earth motion as a function of time.

Engineering seismology:-
Engineering seismology is the study and application of seismology for
engineering purposes. It generally applied to the branch of seismology
that deals with the assessment of the seismic hazard of a site or region
for the purposes of earthquake engineering. It is, therefore, a link
between earth science and civil engineering.
There are two principal components of engineering seismology. Firstly,
studying earthquake history (e.g. historical and instrumental catalogs of
seismicity) and tectonics to assess the earthquakes that could occur in a
region and their characteristics and frequency of occurrence.
Secondly, studying strong ground motions generated by earthquakes to
assess the expected shaking from future earthquakes with similar
characteristics. These strong ground motions could either be
observations from accelerometers or seismometers or those simulated
by computers using various techniques[, which are then often used to
develop ground motion prediction equations(or ground-motion models).

Internal structure of earth
• Introduction
Earth is divided into numerous shells, each with different physical and
chemical properties. Because we can’t collect samples from the center of
the Earth, we base our models on indirect evidence, such as data from
seismic events.
When differentiating the layers, geologists lump subdivisions into two
categories, either rheologically or chemically. Rheological differentiation
speaks to the liquid state of rocks under tremendous pressure and
temperature. For instance, rock will respond very differently to strain under
normal atmospheric temperatures and pressures as compared to fewer
than thousands of kilometers of rock. If we subdivide the Earth based on
rheology, we see the lithosphere, asthenosphere, mesosphere, outer core,
and inner core. However, if we differentiate the layers based on chemical
variations, we lump the layers into crust, mantle, outer core, and inner
core.
To construct a model of the Earth’s interior, we use a variety of geophysical
observations. Shadow zones of seismic waves show that the Earth has three
distinct layers ,a crust, a mantle and a core

The Crust:-
•This is where we live
It is very thin as compare to other layers
A layer of rocks that forms the earth’s outer skin including the rock under
ocean
 The thickness varies depending topography of earth, with oceanic crust
being 5-10 km and continental mountain ranges being up to 30-45 km
thick.
The temperatures within Earth's crust will vary from air temperatures at
the surface to approximately 870 degrees Celsius in deeper sections
Earth's crust "floats" on top of the soft plastic-like mantle below
About a dozen chemical elements, minerals, or rock types is all that it
takes to describe approximately 99% of the crust.
The most common chemical elements in the crust are oxygen (46.6%),
silicon (27.7%), aluminum(8.1%),iron (5.0%), calcium (3.6%), potassium
(2.8%), sodium (2.6%), and magnesium (2.1%).

Continental crust and oceanic crust:-
1. Continental crust:-
•Continental crust is the layer of igneous, sedimentary, and metamorphic
rocks that forms the continents .
•The average density of continental crust is about 2.7 g/cm3
•continental crust is considerably thicker than oceanic crust, which has an
average thickness of around 7–10 km.
•About 40% of Earth's surface is currently occupied by continental crust. It
makes up about 70% of the volume of Earth's crust.
2. Oceanic crust:-
•Oceanic crust is the uppermost layer of the oceanic portion of a tectonic
plate. It is composed of the upper oceanic crust
•Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in
iron and magnesium.
•It is thinner than continental crust, or sial, generally less than 10 kilometers
thick
• it is denser, having a mean density of about 3.0 grams per cubic centimeter

Earth's Mantle:-
•The mantle is the layer of the earth that lies below the crust.
• It is largest layer making up 84% of Earth's volume.
• The mantle starts at the Mohorovicic Discontinuity, also known as the
Moho.
•The Moho is defined as the density contrast from less dense crust to denser
mantle and where seismic wave velocities increase.
•The mantle acts similar to plastic and at very high temperatures and
pressures the rock is deformable at geologic timescales. This deformation
causes a convection like process in the mantle
•The mantle extends down to 2,890 km into the Earth's surface
• Temperatures that range from 500 to 900 degrees Celsius in the upper
portion to over 4,000 degrees Celsius near the core boundary with an
average temperature of 3200 degree celsius.
• It is predominantly solid but in geological time it behaves as a viscous fluid.
The mantle encloses the hot core rich in iron and nickel, which makes up
about 15% of Earth's volume.
•The mantle is divided into sections which are based upon results
from seismology.

•. These layers are the following :-
1.The Upper Mantle:-
• Starting at the Moho, or base of the crust around 7 to 35 km downward to
410 km, the transition zone (410–660 km)
2.The Lower Mantle:-
Depth of this layer varying from 660–2,891 km
anomalous core–mantle boundary with a variable thickness (average 200)
Lithosphere:-
•The uppermost mantle plus overlying crust are relatively rigid and form
the lithosphere, an irregular layer with a maximum thickness of perhaps
200 km. Below the lithosphere the upper mantle becomes notably more
plastic. In some regions below the lithosphere, the seismic shear velocity is
reduced; this so-called low-velocity zone (LVZ) extends down to a depth of
several hundred km
•A principal source of the heat that drives plate tectonics is the radioactive
decay of uranium, thorium, and potassium in Earth’s crust and mantle.

Earth’s Core:-
•Inner part of earth’s interior.
•The core of earth is like a ball of very hot metals.
•It starts from the bottom most fiber of lower mantle to the earth’s centre.
•Depth of core varying from 2900Km to 6400Km.
•Earth’s core are divide into two parts:-
1.Outer core:-
•Layer of the earth that lies below the mantle .The outer core is 2,300 km
thick and goes down to approximately 5,200 km into the earth.
•Seismic surveys of Earth's interior shows that the outer core is the liquid
having 80% iron, some nickel and a number of different lighter elements.
• The temperature of the outer core ranges from about 4,030 to 5,730
degrees Celsius.
•The outer core is fluid enough and low enough in viscosity that it may spins
faster than the entire Earth.
•This differential velocity of spinning, along with convection and turbulent
flow of the iron outer core, creates Earth's magnetic field

2.Earth's Inner Core:-
•The inner core is the centermost layer of Earth and is in many ways similar
to the outer core.
• It is also primarily iron and nickel and has a radius of about 1,220 km.
•The differentiation between the outer core and inner core is density
driven. The pressures become high enough that despite very high
temperatures, the inner core is solid.
•It is also enriched in unusual heavy elements including gold, silver,
platinum, palladium, and tungsten.
•Temperatures reach up to 5,400 degrees Celsius and pressures up to 360
gigapascal. This is 3,000,000 times the air pressure at sea level.
•The inner core is about 70% of the Moon's radius and is approximately
the same temperature as the surface of the Sun
•When Earth was just beginning to cool billions of years ago, heavier
elements sunk down into the center of the Earth, while less dense
elements rose to the surface. Therefore, we see a general increase in
density, as we get closer to the center of the Earth.

Theory of plate tectonics:-
•Plate tectonics is the theory that earth’s outer shell is divided into several
plates that glide over the mantle.
•Plate acts like a hard and rigid shell compare to earth’s mantle. This strong
outer layer is called as lithosphere which includes the crust and outer part
of the mantle.
•Below the lithosphere is the asthenosphere ,which is malleable or
partially malleable ,allowing the lithosphere to move around.
•There are nine major plates ,according to world atlas. These plates are
named after the landforms found over them.
•The nine major plates are North American ,Pacific ,Eurasian ,African ,Indo-
Austrelian ,Australian ,Indian ,South American and Antarctic.
•Most of it located under the ocean and it is moving northwest at a speed
of around 7cm/year.
•The largest plate is the pacific plate having area about 103,000,000 square
km.
•There are also many smaller plates throughout the world.

Working of tectonics plates:-
•It is generally accepted that tectonic plates are able to move because of
the relative density of oceanic lithosphere and relative weakness of the
asthenosphere
•Dissipation of heat from mantle is acknowledged to be the original
source of the energy requires to drive plate tectonics
•The driving force behind the plate tectonic is convection in the mantle.
•Hot material near the earth’s core rises ,and colder mantle rock sinks.
•When these plates meet ,there relative motion determines the type of
boundaries like convergent ,divergent or transforms.

2. Types of plate boundaries:-
•The plate boundaries mainly consists of three types
1.Divergent boundary:-
These boundary happen where two lithosphere plate
move apart.
2.Convergent boundary:-
These boundary happen where both plate moving toward each
other.
It form a zone of continental collision.
3.Transform boundary:-
Occurs where two lithosphere plates slides ,or more accurately, grind past
each other along transform fault , where plates are neither created nor
destroyed

Plates tectonics causes:-
•The relative motion of plates tectonics causes
1. Earthquake
2. Volcanism
3. Mountain building
4. Mid ocean ridge
5. Oceanic trench

Seismic waves:-
•Seismic waves are the waves of energy that travel through the earth’s layer.
•Seismic waves are a results of earthquakes, volcanic eruption, magma
movement, large landslides and large man made explosions.
•Seismic wave have low frequency acoustic energy.
•Many other natural and anthropogenic sources creates low amplitude
waves commonly referred to as ambient vibrations.
•The propagation velocity of the wave depends on the elasticity and density
of the medium.
•the velocity tends to increase with increase in density, as we move deeper
in earth velocity is going to increase.
•Seismic wave fields are recorded by the seismometer, hydrophones in
water or accelerometer
•Earthquake creates distinct type of waves with different velocities, when
reaching seismic observatories , there different travel times helps to locate
the hypocenters.
•The refraction and reflection of seismic waves are used to find the
structure of earth’s interior.

Types of seismic waves:-
•Seismic waves are broadly divided into two groups
1.Body Waves:-
•Body waves travel trough the interior of the earth along the path
controlled by the material properties like density and modulus
•Body waves are further divided into two parts
A.Primary waves(P-waves):-
• Primary waves are compressional waves that are longitudinal in
nature
• P-waves are pressure waves that travel faster than other waves
through the earth.
• These waves can travel through any type of material including fluid
and can travel faster than S-waves
• In air these wave take the form of sound waves , hence they travel at
the speed of sound.

B.Secondary waves:-
•Secondary waves (S-Waves) are shear waves that are transvers in nature.
•S-waves arrived at seismograph station after faster moving P-waves and
displace the ground perpendicular to the direction of propagation.
•Since fluid do not support the shear stresses so,S-waves can not found in
fluid.
2.Surface waves:-
•Seismic surface waves are travel along the surface of earth.
•They are travel more slowly than seismic body waves.
•In large earthquakes surface waves can have an amplitude of several
centimeters.
•Some other waves are mention below:-
•Rayleigh waves
•Love waves
•Stoneley waves
•Free oscillations of earth

Size of earthquake:-
•The size of an earthquake is given by its open ended logrithimic scale of
magnitude, often referred as the Richter Scale.
•Shocks smaller than the magnitude 2.5 are usually not felt and those with
magnitude 7 causes serious damage over large areas.
•Intensity of shaking is measured on the modified Mercalli scale, ranging
from 1 far from the epicenter to a maximum near it , which can reach 12 in
the strongest earthquake.
•Richter scale:-
•The Richter magnitude of earthquake is determind from the log of the
amplitude of waves recorded by seismograph.
•The adjustments are included for the vibration in the distance between
the various seismograph and the epicenter of the earthquake.
•On Richter scale , magnitude is commonly expressed in whole number
and decimal number.
•Richter scale is not commonly used anymore except small earthquake
recorded locally.

•Mercalli intensity scale:-
•The mercalli scale quantifies the effect of an earthquake on the earth
surface , human object of nature and man made structures on an scale
form.
•1 for not felt and 7 indicates total distruction.
•The value depend upon the distance from the earthquake with the
highest intensity value for there localions.
•Modified mercalli intensity scale:-
•The lower degree of modified mercalli intensity scale generally deals with
the manner in which the earthquake is felt by the people.
•The higher number of scale are based on observed structural damage.
•The correlation between magnitude and intensity intensity is far from
total ,depending upon several factors including the depth of the
hypocenter , terrain ,distance frome the epicenter.

Classification of earthquakes:-
•Earthquake are usually classified on the following basis:
1.Cause of origin
2.Depth of focus
3.Intensity and magnitude of earthquake
1.Cause of origin:-
a.Tectonic earthquake:-
Earthquake occur when the plates moves against one another.
This movement extercan create stress that causes the earth’s exterior
shell , the lithosphere to shift or break.
b.Non-tectonic earthquake:-
Thw non-tectonic earthquake are mainly of three types due to surface
causes , volcanic causes ,and collapse of cavity roofs.

2.Classification based on depth of focus:-
a. Surface earthquake:-
•Surface earthquake are those in which the depth of the focus is less than
tha 10km.
b. Shallow earthquake:-
•The earthquake with the hypocenter at a depth of 10 to 50 km
c. Deep focus earthquake:-
•The deep focus earthquake or plutonic earthquake are those with
hypocenter located at depth more than 300 km
•Majority of deep focus earthquake are originated between 500 to 700
kms.

Strong ground motion characteristics:-
•The propagation of seismic waves and resulting ground displacement during
an earthquake is picked up even at far off places. But scientists had noticed
that the damages caused by earthquakes were restricted to within few
hundreds of kilometers from the causative fault.
•Earthquakes motion can be recorded in terms of ground displacement,
velocity or acceleration. During earthquakes, the ground movement is very
complex, producing translations in any general direction combined with
rotations about arbitrary axes.
•Several earthquake parameters are reported in the literature for
quantitatively describing the various characteristics of the ground motion.
These cover characteristics such as amplitude of motion, frequency content of
motion, duration of motion, etc.
•Loading effect of earthquake ground motion at a site is generally represented
by three ground motion (GM) parameters viz. peak ground acceleration,
response spectrum and acceleration time history.
•The combined influence of the amplitude of ground accelerations, their
frequency content and the duration of the ground shaking on different
structures is represented by means of response spectrum.

Estimation of ground motion parameter:-
•Peak Ground Acceleration:-
•The earthquake time history contains several engineering characteristics of
ground motion and maximum amplitude of motion is one of the important
parameter among them. The PGA is a measure of maximum amplitude of
motion and is defined as the largest absolute value of acceleration time
history.
•The response of very stiff structures (i.e., with high frequency) is related to
PGA. Though PGA is not a very good measure of damage potential of
ground motion; due to its close relation with response spectrum and
usability in scaling of response spectrum, PGA is extensively used in
engineering applications.
•Generally, at distances several source dimensions away, vertical PGAs are
found to be less than horizontal PGA though at near source distances it
could be equal to higher than the corresponding horizontal PGA. For
engineering purposes, vertical PGA is assumed to be two thirds of the
horizontal PGA.

Peak Velocity :-
•Peak velocity is the largest absolute value of velocity time history. It is
more sensitive to the intermediate frequency components of motion and
characterizes the response to structures that are sensitive to intermediate
range of ground motions, e.g. tall buildings, bridges, etc.
Peak Displacement:-
• Peak displacements reflect the amplitude of lower frequency
components in ground motion. Accurate estimation of these parameters is
difficult as the errors in signal processing and numerical integration greatly
affect the estimation of amplitude of displacement time history.

Seismic hazard map:-
•A seismic hazard is the probability that an earthquake will occur in a given
geographic area, within a given window of time, and with ground motion
intensity exceeding a given threshold . With a hazard thus
estimated, risk can be assessed and included in such areas as building
codes for standard buildings, designing larger buildings and infrastructure
projects, land use planning and determining insurance rates.
•the latest version of seismic hazard map of India is given in earthquake
resistant design code IS 1893:2002(part-1).
•This code assigns four level of seismicity for dwellers India in terms of zone
factors. In other word the earthquake zoning map of India divides india into
four seismic zone (zone 2 , 3, 4, &5) unlike its previous version, which
consist of five zone.
•According to present zoning map, zone 5 expected the highest level of
seismicity where as zone 2 is associated with the lowest level of seismicity.
•Indian subcontinent has a history of devastating earthquakes.
•The major reason for high frequency and intensity of the earthquake is that
the Indian plate is driving in to Asia at a rate of approximately 47mm/year.
•Geographical statistics of India show that almost 54% of land is vulnerable
to earthquake

Response spectra:-
•A response spectrum is a plot of the peak or steady state
response(displacement , velocity or acceleration) of a series of oscillators
of varying natural frequency that are forced into the motion by some base
vibration or shoks.
•The resulting plot can be used to pick off the response of any linear
system, given its natural frequency of oscillation.
•One such use is in assessing the peak response of building to earthquake.
•If the input used in calculating a response spectrum is steady state
periodic , then the steady state result is obtain.
•For transient input, the peak response is reported.
•Response spectra are very useful tools of earthquake engineering for
analyzing the performance of structures and equipment in earthquakes,
since many behave principally as simple oscillators(also known as single
degree of freedom systems).
• Thus, if you can find out the natural frequency of the structure, then the
peak response of the building can be estimated by reading the value from
the ground response spectrum for the appropriate frequency.

•The ground response spectrum is the response plot done at the free
surface of the earth. Significant seismic damage may occur if the building
response is 'in tune' with components of the ground motion (resonance),
which may be identified from the response spectrum

Seismology

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