dr. n. venkatanathan [email protected]
TRANSCRIPT
Earthquakes are natural ground motions caused as the Earth releases energy.
One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake.
Seismology is derived from Greek words “seismos” (earthquake) and “logos” (study).
The scientific study of earthquakes and the propagation of elastic waves through the Earth.
Seismic sources
The events that generate seismic waves are called seismic sources.
The most common seismic sources are plate tectonics and volcanoes.
The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes.
Most Destructive Known Earthquakes
S.
NO
:
Date Region Ma
g
Death
toll
1 January 23,
1556
Shaanxi, China 8.0 830,
000
2 July 27, 1976 Tangshan, China 7.5 255,
000
3 October 11,
1138
Aleppo, Syria - 230,
000
4 December 26,
2004
Sumatra, Indonesia 9.1 228,
000
5 January 12,
2010
Haiti 7.0 316,
000
6 December 22,
856
Damghan, Iran 8.0 200,
000
7 December 16,
1920
Haiyuan, Ningxia,
China
7.8 200,
000
The elastic rebound theory
• As tectonic plates move relative to each other, elastic strain energy builds up along their edges in the rocks along fault planes.
• Since fault planes are not usually very smooth, great amounts of energy can be stored as movement is restricted due to interlock along the fault.
When the shearing stresses induced in the rocks on the fault planes exceed the shear strength of the rock, rupture occurs.
This theory was discovered by making measurements at a number of points across a fault.
Prior to an earthquake it was noted that the rocks adjacent to the fault were bending.
These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.
A vibration of the Earth produced by a rapid release of energy, reason for the earthquake.
Focus and Epicenter
The source of an earthquake is called the focus or hypocenter, which is an exact location within the Earth were seismic waves are generated by sudden release of stored elastic energy.
The point on Earth's surface directly above the hypocenter is called the epicenter.
SEISMIC WAVES
Seismic waves are produced due to sudden breaking of rocks either due to natural cause or due to explosion.
These waves propagate in the form of energy that travels through the earth and is recorded on seismographs.
Seismic waves move in different ways with different speeds, based on its movement they can be categorized into different kinds.
Seismic Waves
Body Waves(Pass through the interior of
the earth)
P – Waves(Primary Waves)
Propagate in the form of compressions and dilations
S – Waves(Secondary Waves)
Propagate in the form of Crests and Troughs
Surface Waves(Pass through the surface
of the earth)
Love WavesPropagate in the form of
Crests and Troughs parallel to the ground
Rayleigh WavesPropagate in the form of ocean waves and roll the ground up and down and
also side to side
P - WAVE
P - WAVE
P wave or Pressure waves or Primary waves are longitudinal waves that travel at maximum velocity.
The first wave originates as body wave from the focus of the earthquake.
This is the fastest kind of seismic wave and recorded at a seismic station first.
The P wave can move through solid rock and the liquid layers of the earth.
It propagates as a longitudinal wave, so it pushes and pulls the rock as it moves through them.
Due to their pushing and pulling nature, P waves are also known as compressional waves.
When P waves are propagating through rocks, they move the particles of the rocks in the same direction of wave propagation.
These waves travel in water slowly with the approximate speed of 1 km/s.
At the base of mantle they are travelling at high speed, which is approximately 14 km/s.
P wave velocity depends on a material's "plane wave modulus (M)" [It is defined as the ratio of axial stress to axial strain in a uniaxial strain state. One of the elastic moduli available to describe isotropic homogeneous materials. Otherwise called as the P-wave modulus (or) longitudinal modulus] and its density.
Where, “λ” is Lamé's constant, “μ” is shear modulus, “K” is bulk modulus, and “” is density. Notice that density is in the denominator, so denser rocks should be slower.
Although the density of rock in the Earth generally increases with depth.
The rigidity - increases even more rapidly with depth.
Hence, P wave velocity generally increases with increasing depth.
Since solids, liquids and gasses have a finite bulk modulus;
It can travel through any of these medium.
S - WAVE
S - WAVE
Another type of body wave originated from an earthquake, shear waves or secondary wave or S wave.
These waves are travels slower than the P wave. S wave can only move through solid rock, not
through any liquid medium like water since, shear waves do not exist in fluids.
S waves propagate in the form of transverse wave, so it moves rock particles up and down or side-to-side.
So the particle displacement is perpendicular to the direction of wave propagation.
S-wave velocity depends on a material's shear modulus (μ), and density (),
Comparing the velocity expressions, you can see that VP > VS for any material.
Depending upon soil type "P" wave travels about 1.68 to 1.75 times faster than destructive "S" wave.
•S-wave velocity depends on a material's shear modulus (μ), and density (),
•Comparing the velocity expressions, you can see that VP > VS for any material.•Depending upon soil type "P" wave travels about 1.68 to 1.75 times faster than destructive "S" wave.
SURFACE WAVE
• These waves propagate only through the crust.
• Compared to body waves these waves are low frequency waves.
• As these waves propagate at low velocity, they are responsible for the damage and destruction.
• Since surface waves are propagating only through the surface of the earth’s crust, the strength of the surface waves are more in shallow earthquakes than the deep seated earthquakes.
• Solid lines marked P are compressional waves• dashed lines marked S are shear waves. • S waves do not travel through the core but may be converted to compressional waves (marked K) on entering the core (PKP, SKS). Waves may be reflected at the surface (PP, PPP, SS).
LOVE WAVE
LOVE WAVE
• It is named after A.E.H. Love, a British mathematician given the mathematical model for this wave in 1911.
• It fastest of all surface wave and propagates in the ground from side-to-side.
• Since these waves are confined to the surface of the crust, they produce entirely horizontal motion.
The amplitude of the waves get reduced with the depth, this gives differential particle motion at various heights. This is the one of the reason for the collapsing of buildings.
RAYLEIGH WAVE
RAYLEIGH WAVE
Rayleigh waves are mathematically predicted by John William Strutt Lord Rayleigh in 1885.
Like an ocean waves, Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
The shaking produced by these waves is much larger than the other type seismic waves.
Since it rolls, it moves the ground up and down and side-to-side in the same direction of wave propagation, most of the shaking felt from an earthquake is due to the Rayleigh wave.
The up and down rolling get decreased with the depth. These waves also confined to the surface only.
Where Do Earthquakes occur?
• Earthquakes occur along plate edges and along faults.
• The earth's crust (the outer layer of the planet) is made up of several pieces, called plates.
• The plates under the oceans are called oceanic plates and the rest are continental plates.
Plate Tectonics• The plates are moved around by the
motion of a deeper part of the earth (the mantle) that lies underneath the crust.
• These plates are always bump into each other, pull away each other, or sliding past each other.
• The plates usually move at about the same speed that your fingernails grow.
• Earthquakes usually occur where two plates are running into each other or sliding past each other.
Plate Tectonic Theory
• This theory was the outcome of the hypothesis of continental drift proposed by Alfred Wegener in 1912.
• In his hypothesis, he suggested that the present continents once formed a single land mass which had drifted apart thus formed separate the continents.
• which is much similar to the "icebergs“ floating on a sea.
Schematic view of the interior of Earth
1. continental crust 2. oceanic crust 3. upper mantle 4. lower mantle 5. outer core 6. inner core 7. A: Mohorovičić discontinuity - B:
Gutenberg Discontinuity - C: Lehmann discontinuity
Structure of Earth
• Based on chemical properties the earth can be divided into three parts, namely,
• Core, • mantle and • crust.
Since the earthquake generation mechanism mostly depends on the physical behaviour of the earth, the structure of the earth can be divided into four parts.
The inner part of the earth (i.e.) the core can be divided into two, namely, inner core and outer core.
The outer part of the earth’s interior can be divided into two parts, lithosphere and asthenosphere.
The division of the outer part of the earth into lithosphere and asthenosphere is based on mechanical differences and in the ways that heat is transferred.
The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker.
Also, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.
The lithosphere contains both crust and some mantle.
A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength.
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere.
Plate motions are at different range, it varies from typical 10 - 40 mm yr-1 to about 160 mm yr-1.
The plates are around 100 km thick and consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust and continental crust.
The continental crust is thicker than oceanic crust, 50 km and 5 km respectively.
The distinction between continental crust and oceanic crust is based on the density of constituent materials.
Oceanic crust is denser than continental crust because of presence of large amount silicon.
Oceanic crust is denser because it has less silicon and heavier elements.
As a result, oceanic crust generally lies below sea level, while the continental crust is above the sea level.
Plate Boundaries
The interface between one plate and another plate is called as plate boundary.
Most of the earthquakes are occur along the plate boundaries
And most of the topographic features like mountains, volcanoes and oceanic trenches are present near the plate boundaries.
The best example is Pacific Plate's Ring of Fire which is most active and widely known.
All the plates appear to be moving at different relative speeds and independent of each other, but in reality the plates are interconnected.
No single plate can move without affecting others, and the activity of one can influence another thousands of miles away.
For example, as the Atlantic Ocean grows wider with the spreading of the African Plate away from the South American Plate, the Pacific sea floor is being consumed in deep subduction trenches over ten thousand miles away.
Measuring rates of motion
The average rates of plate separations can range widely.
There are several methods in measuring the rate of motion of plates.
Evidence of past rates of plate movement can be obtained from geologic mapping studies.
If a rock formation of known age -- with distinctive composition, structure, or fossils - mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion.
Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements.
Using laser instruments the plate movement can be measured in very precise manner.
Using space geodesy precise, repeated measurements of carefully chosen points on the Earth's surface separated by hundreds to thousands of kilometers.
The three most commonly used space-geodetic techniques are very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS)
Major plates & Minor Plates
African Plate, Antarctic Plate, Arabian Plate, Australian Plate, Caribbean Plate, Cocos Plate, Eurasian Plate, Indian Plate, Juan de Fuca Plate, Nazca Plate, North American Plate, Pacific Plate, Philippine Plate, Scotia Plate and South American Plate
Aegean Sea Plate, Altiplano Plate, Anatolian Plate, Banda Sea Plate, Burma Plate, Caroline Plate, Conway Reef Plate, Easter Plate, Futuna Plate and Galapagos Plate,