Lecture 1
Dr. A. B. Danie Roy
Faculty of Civil Engineering
Thapar University, Patiala
SEISMOLOGY
A study of earthquake engineering calls for a good
understanding of geophysical process that causes earthquakes
and various effects of earthquakes.
Seismology is the study of the generation, propagation and
measurement of seismic waves through earth and the sources
that generate them. The word seismology originated from
Greek words, seismos meaning earthquake and logos
meaning science.
Earth top surface
Earth
Interior
Internal Structure of Earth
The earths shape is an oblate spheroid or ellipsoid with a
diameter along the equator of about 12756 km with the polar
diameter as 12700km.
Crust: or the lithosphere, is the outer part of the earth is where
the life exist. The average thickness of crust beneath continents
is about 40km where as it decreases to as much as 5km
beneath oceans.
Mantle: is a 2900 km thick layer. The mantle consists of 1)
Upper Mantle reaching a depth of about 400 km made of olivine
and pyroxene and 2) Lower Mantle made of more
homogeneous mass of magnesium and iron oxide and quartz.
No earthquakes are recorded in the lower mantle.
Core: has a radius of 3470 km and consists of an inner core of
radius 1370 km and an outer core (1370 km < R < 3470 km).
Continental Drift and Plate Tectonics
German scientist Alfred Wegener, in 1915, proposed the
hypothesis that the continents had once formed a single
landmass before breaking apart and drifting to their present
locations.
Pangae happened in earth around 200 million years ago and
was surrounded by an ocean called Panthalassa.
Plate tectonics
The theory of plate tectonics, presented in early 1960s,
explains that the lithosphere is broken into seven large (and
several smaller) segments called plates as shown in Figure.
Tectonic plate map of the world
Movement of Plate Boundaries
Owing to the difference in movement between the plates that are in
motion, three types of plate boundaries are found to exist along their
edges: Spreading ridges, Convergent boundaries &Transform boundaries
1) Spreading ridges
Spreading ridges or divergent boundaries are areas along the edges of
plates move apart from each other
A cross-section of the divergent plate boundary
This is the location where the less dense molten rock from the mantle rises
upwards and becomes part of crust after cooling.
Convergent boundaries
The convergent boundaries are formed where the two plates move toward each
other. In this process, one plate could slip below the other one or both could collide
with each other.
a. Subduction boundaries
These boundaries are created when either oceanic lithosphere subducts
beneath oceanic lithosphere (ocean-ocean convergence), or when oceanic
lithosphere subducts beneath continental lithosphere (ocean-continent
convergence), Figure The junction where the two plates meet, a trench
known as oceanic trench is formed.
b. Collision Boundaries
When two plates with continental lithosphere collide, subduction ceases
and a mountain range is formed by squeezing together and uplifting the
continental crust on both plates, Figure. The Himalayan Mountains
between India and China were formed in this way.
3) Transform boundaries
Transform boundaries occur along the plate margins where two
plate moves past each other without destroying or creating new
crust, Figure
Faults
The term fault is used to describe a discontinuity within rock mass,
along which movement had happened in the past. Plate boundary is
also a type of fault.
There are two important parameters associated with describing
faults, namely, dip and strike,. The strike is the direction of a
horizontal line on the surface of the fault. The dip, measured in a
vertical plane at right angles to the strike of the fault, is the angle of
fault plane with horizontal.
Types of faults (Arrow shows direction of relative displacement) (a) Normal fault;
(b) Reverse fault; (c) Strike-slip fault; (d) Oblique fault.
Earthquake
While the edges of faults are
stuck together, and the rest of
the block is moving, the energy
that would normally cause the
blocks to slide past one another
is being stored up. When the
force of the moving blocks
finally overcomes the friction of
the jagged edges of the fault and
it unsticks, all that stored up
energy is released.
An earthquake is the result of a sudden release of
energy in the Earths Crust that creates Seismic waves
Hypocenter - the location below the earths surface where the earthquake starts
Epicenter - the location on the surface of the earth directly above the hypocenter
Figure General depiction of an earthquake rupture scenario
Earthquake is the vibration of earths surface caused by waves coming from a
source of disturbance inside the earth. Most earthquakes of engineering significance
are of tectonic origin and is caused by slip along geological faults.
The typical characteristics of earthquake depends on
1. Stress drop during the slip
2. Total fault displacement
3. Size of slipped area
4. Roughness of the slipping process
5. Fault shape( Normal fault, Reverse fault, Strike slip fault)
6. Proximity of the slipped area to the ground surface
7. Soil condition
Earthquake Waves
Seismic Wave Types
Body Waves Surface Waves
Earthquake vibrations originate from the point of initiation of rupture and
propagates in all directions. These vibrations travel through the rocks in the
form of elastic waves. Mainly there are three types of waves associated with
propagation of an elastic stress wave generated by an earthquake. These
are primary (P) waves, secondary (S) waves and surface waves. In addition,
there are sub varieties among them. The important characteristics of these
three kinds of waves are as follows
Primary (P) Waves
These are known as primary waves, push-pull waves, longitudinal waves,
compressional waves, etc. These waves propagate by longitudinal or compressive
action, which mean that the ground is alternately compressed and dilated in the
direction of propagation. P waves are the fastest among the seismic waves and
travel as fast as 8 to 13 km per second. Therefore, when an earthquake occurs,
these are the first waves to reach any seismic station and hence the first to be
recorded.
These waves are capable of traveling through solids, liquids and gases.
The P-waves propagates radial to the source of the energy release and the velocity
is expressed
where E is the Youngs modulus; n is the Poissons ratio (0.25); and r is the density.
Secondary (S) Waves
These are also called shear waves, secondary waves, transverse waves,
etc. Compared to P waves, these are relatively slow. These are transverse
or shear waves, which mean that the ground is displaced perpendicularly to
the direction of propagation. In nature, these are like light waves, i.e., the
waves move perpendicular to the direction of propagation. Hence,
transverse particle motion is characteristic of these waves. These waves are
capable of traveling only through solids.
The shear wave velocity is given by
They travel at the rate of 5 to 7 km per second. For this reason these waves
are always recorded after P waves in a seismic station.
Surface Waves
When the vibratory wave energy is propagating near the surface of the earth rather
than deep in the interior, two other types of waves known a Rayleigh and Love waves
can be identified. These are called surface waves because their journey is confined to
the surface layers of the earth only. Surface waves travel through the earth crust and
does not propagate into the interior of earth unlike P or S waves.
Surface waves are the slowest among the seismic waves. Therefore, these are the
last to be recorded in the seismic station at the time of occurrence of the
earthquake.
They travel at the rate of 4 to 5 km per second.
These waves are capable of travelling through solids and liquids. They are complex
in nature and are said to be of two kinds, namely, Raleigh waves and Love waves.
Earthquake Terminology
Various distance measurements associated with earthquake
The place of origin of the earthquake in the interior of the earth is known as focus or
origin or center or hypocenter
The place on the earth's surface, which lies exactly above the center of the
earthquake, is known as the 'epicenter'.
Seismographs
Earthquakes are recorded by
instruments called seismographs.
The recording they make is called a
seismogram. The seismograph has
a base that sets firmly in the
ground, and a heavy weight that
hangs free. When an earthquake
causes the ground to shake, the
base of the seismograph shakes
too, but the hanging weight does
not. Instead the spring or string that
it is hanging from absorbs all the
movement. The difference in
position between the shaking part of
the seismograph and the motionless
part is what is recorded.
seismograph
Determination of Hypocenter or Earthquake Focus
Seismologists use the elapsed time between the arrival of a P-waves and S-
waves at a given site to assist them in estimating the distance from the site
to the center of energy release. The distance of focus from the observation
station is determined by the relative arrival times of the P and S waves. The
distance from hypocenter to observation point is given by
where, T=difference in time of arrival of P and S waves at an observation point; S=
distance from hypocenter to observation point; and Vp and Vs are the velocity of P
and S waves, respectively
Size of Earthquakes
1. Magnitude
The magnitude of an earthquake is related to the amount of energy released by
the geological rupture causing it, and is therefore a measure of the absolute size
of the earthquake, without reference to distance from the epicenter.
While earthquake intensity is depicted in Roman numerals and is always a whole
number, magnitude is depicted in Arabic numerals and need not be a whole
number. Similar to intensity scales, over the years, a number of approaches for
measurement of magnitude of an earthquake have come into existence.
2. Richter Magnitude, ML
A workable definition of magnitude was first proposed by C.F. Richter. He based on
the data from Californian earthquakes, defined the earthquake magnitude as the
logarithm to the base 10 of the largest displacement of a standard seismograph
situated 100 km from the focus
where A denotes the amplitude in micron (10-6m) recorded by the instrument located
at an epicentral distance of 100 km; and M is the magnitude of the earthquake.
When the distance from the epicenter at which an observation is obtained other than
100 km, a correction is introduced to the equation as follows:
where M is the magnitude of the earthquake; =distance from epicenter (km), M=
magnitude of the earthquake calculated for earthquake using the values measured
at a distance from the epicenter
3. Moment magnitude
Over the years, scientists observed that different magnitude scales had saturation
points and the magnitudes estimated by different approaches did not point to a
unique value of earthquake size The Richter magnitude saturates at about 6.8, and
the surface wave magnitude at about 7.8. In addition, these magnitude estimates did
not have a linear relation with the energy released due to earthquake rupture. To
address these short falls, Hanks and Kanamori, in 1979 proposed a magnitude scale,
termed as moment magnitude, based on the seismic moment due to earthquake
rupture. The moment magnitude is given by
where Mw is the moment magnitude, M0 is the seismic moment in N-m.
4. INTENSITY
The intensity of an earthquake refers to the degree of destruction caused by it.
In other words, intensity of an earthquake is a measure of severity of the
shaking of ground and its attendant damage.
Damage that takes place to a construction at a given place depends on many
factors. Some of these factors are: (i) distance from the epicenter, (ii)
compactness of the underlying ground, (iii) type of construction (iv)
magnitude of the earthquake (v) duration of the earthquake and (vi) depth of
the focus. Intensity is the oldest measure of earthquake
MODIFIED MERCALLI INTENSITY (MMI) SCALE OF EARTHQUAKE INTENSITIES
The Modified Mercalli Intensity scale is the most widely used scale
and it indicates damage to nonstructural elements in intensities from
I to XII. Table as per IS-1893:1984.
Grade I. Not felt by people. Grade VII. Standing upright is very
Grade II. Felt only by persons at rest on difficult. Damage is negligible in buildings of
the upper floors of buildings. Suspended very good design and construction. Damage
objects may swing. to the poorly designed structures
Grade VIII. Damage is slight in specially
Grade III. Felt by some people The designed earthquake-resistant structures
vibration is similar to that caused by the Poorly built or designed buildings
passing of light trucks. Hanging objects experience partial collapses. Interiors
swing. experience heavy damage.
Grade IV. Felt by many people who are Grade IX. Well-built ordinary buildings
indoors. At night some people are suffer severe damage with partial collapses
awakened. Dishes, windows and doors are Unreinforced masonry buildings collapse.
disturbed; walls make cracking sounds; The ground cracks and some underground
stationary cars rock noticeably. pipes are broken.
Grade X. Most masonry and many frame
Grade V& VI. The direction and duration structures are destroyed. Some well-built
of the shock can be estimated by people
bridges are destroyed. Large landslides are
outdoors. At night, sleepers are awakened
triggered by the shocks.
and some run out of buildings. Small,
Grade XI. Few, if any masonry structures
unstable objects and some furnishings are
remain standing.
shifted or upset. Doors close or open.
Grade XII. Total Damage
Another intensity scale
Mendvedev-Spoonheuer-Karnik scale (MSK 64). This scale is more
comprehensive and describes the intensity of earthquake more precisely. Indian
seismic zones were categorized on the basis of MSK 64 scale.
Rossi-Forel (RF) scale,
Japanese Meteorological Agency (JMA) intensity scale
Over the years, researchers have tried to develop more quantitative ways for
estimating earthquake intensity. One of such relationships correlating earthquake
intensity to peak ground velocity is given by
where Vg is the peak ground velocity in cm/sec.
Another such relation reported by Wald et.al, (1999) based on Californian
earthquake database is
In addition to peak ground velocity, empirical relationships correlating peak ground
acceleration to MMI has also been reported. For e.g.,
MMI = 3.66 log (Peak Ground Acceleration in cm/sec/sec) 1.66
Seismic Zones
in India
Zone II
Zone III
Zone IV
Zone V
Strong Motion and Estimation of Seismic Hazard
Earthquake Ground Motion
For the design of structures to resist earthquakes, it is necessary to have
some knowledge of ground motions. 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.
Modern strong motion accelerographs are designed to record three
translational components of ground acceleration, switching on by
themselves automatically once an earthquake ground motion reaches a
certain threshold level usually about 0.005 g.
The first complete record of strong ground motion was obtained during
the 1940 El-Centro earthquake in California
