Module 1&2 Flashcards
study of the
generation, propagation and measurement of seismic waves through earth and the sources that
generate them.
Seismology
The word seismology originated from Greek words,
‘seismos’ meaning
earthquake and ‘logos’ meaning science.
earth’s shape is an oblate
spheroid with a diameter along the equator of
about
12740 km
polar diameter as
12700km.
The higher diameter along equator
is caused by the
higher centrifugal forces
generated along the equator due to rotation
of earth
pecific gravity of
materials that constitute the surface of earth
is only about
2.8,
e average specific gravity
of earth is about
5.5
very heavy materials towards interior of earth.
The interior of the earth can be classified into
three major categories
Crust, Mantle and
Core
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. The oceanic crust is constituted by basaltic rocks and continental part
by granitic rocks overlying the basaltic rocks. Compared to the layers below, this layer has high
rigidity and anisotrop
Crust
e 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. The specific gravity of mantle is about 5. The mantle has an average
temperature of about 2200degree Celsius and the material is in a viscous semi molten state.
The mantle act like fluid in response to slowly acting stresses and creeps under slow loads. But
it behaves like as solid in presence of rapidly acting stresses, e.g. that caused by earthquake
waves.
Mantle
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). The core is composed of molten iron, probably mixed
with small quantities of other elements such as nickel and sulfur or silicon. The inner solid core
is very dense nickel-iron material and is subjected to very high pressures. The maximum
temperature in the core is estimated to be about 3000 degree Celsius. The specific gravity of
outer core is about 9-12 where as that of inner core is 15.
Core
proposed the hypothesis that the continents
had once formed a single landmass before breaking apart and drifting to their present locations.
German scientist Alfred Wegener, in 1915
He proposed that a large
continent termed Pangae existed in earth around
200 million years ago
The theory of plate tectonics, presented in early
y 1960s
lithosphere is
broken into seven large (and several smaller) segments called
plates
The upper rigid layer,
lithosphere
, is about 100 km
thick below the continents, and about 50 km under the oceans, and consists of Crust and rigid
upper-mantle rocks.
lithosphere
The lower layer
asthenosphere
extends down to about 700 km
depth.
asthenosphere
are areas along the edges of plates move
apart from each other
Spreading ridges or divergent boundaries
are located beneath the
oceans
spreading ridges
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.
convergent 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)
Subduction boundaries
The junction where the two
plates meet, a trench known as
oceanic trench
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
Collision Boundaries
occur along the plate margins where two plate moves past each
other without destroying or creating new crust,
Transform boundaries
is used to describe a discontinuity within rock mass, along which
movement had happened in the past
fault
are
mappable linear surface features and may reflect subsurface phenomena
Lineaments
Types of faults
Normal Fault
Reverse fault
Strike-slip fault
Oblique Fault
There are two important parameters associated with describing faults
dip and
strike
is the direction of a horizontal line on the surface of the fault
strike
measured in a vertical plane at right angles to the strike of the fault
dip
refers to the upper rock surface along which
displacement has occurred
hanging wall of a fault
The vertical
shift along a fault plane is called the
throw
horizontal displacement is termed as
heave
the slippage
occurred along the dip of the fault
dip-slip fault
the
movement has taken place along the strike
strike-slip fault
The movement occurs diagonally
across the fault plane in case of an
oblique slip fault
Based on relative
movement of the hanging and foot walls faults are classified into
normal, reverse and wrench
faults
the hanging wall has been displaced downward relative to the footwall
normal fault
the hanging wall has been displaced upward relative to the
footwall,
reverse fault
the foot or the hanging wall do not move up or
down in relation to one another
wrench fault
which are a subdivision of
reverse faults, tend to cause severe earthquakes.
Thrust faults
are nucleating surfaces for seismic activity
Faults
The region on the fault, where rupture initiates is known as the
e focus or hypocenter of
an earthquake
is the location on the earth surface vertically above the focus.
Epicenter
Distance from epicenter to any place of interest is called the
epicentral distance
The depth of
the focus from the epicenter is the
focal depth
e is the vibration of earth’s surface caused by waves coming from a source of
disturbance inside the earth
Earthquake
arriving at a site on the surface of the earth are a result of
complex superposition giving rise to irregular motion.
seismic waves
These waves propagate by longitudinal or compressive action, which mean that the
ground is alternately compressed and dilated in the direction of propagation
primary waves, push-pull waves, longitudinal waves
are the fastest among the seismic waves and travel as fast as 8 to 13 km per second.
P
waves
these are the first waves to reach any seismic station
and hence the first to be
recorded.
P
waves
These are transverse or shear waves, which mean that
the ground is displaced perpendicularly to the direction of propagation
shear waves, secondary waves, transverse waves
These waves are capable of
traveling only through solids
Secondary (S) Waves
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.
Secondary (S) Waves
When the vibratory wave energy is propagating near the surface of the earth rather
than deep in the interior
Surface Waves
wo other types of surface waves
Rayleigh and Love waves
their journey is confined to the surface
layers of the earth only
Surface Waves
s travel through the earth crust and does not propagate
into the interior of earth
Surface Waves
are the slowest among the seismic waves
Surface Waves
They travel at
the rate of 4 to 5 km per second
Surface Waves
hese waves are capable of travelling through solids and liquids
Surface Waves
are tension-compression waves similar to the P-waves
expect that their amplitude diminishes with distance below the surface of the ground
Rayleigh surface waves
are the counterpart of the “S” body waves
Love waves
Generally, the rupture causing earthquakes initiates from a point, termed
hypocenter
or focus
The point on
earth’s surface diametrically opposite to the epicenter is called the
anti-center
An imaginary
line which joins the points at which the earthquake waves have arrived at the earth’s surface at
the same time is called a
‘co-seismal’
The seismograph has three components
the sensor, the recorder and the timer
The instruments measure the ground displacements and are called
seismographs
The
record obtained from a seismograph is called a
seismogram
e devises that measure the ground accelerations are called
accelerometer
e refers to the degree of destruction caused by it
intensity
An imaginary line joining the points of same intensity of the earthquake is called an
iso-seismal
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
magnitude of an earthquake
