Earth Quack
PLATE TECTIONICS & EARTHQUAKE
The theory of plate tectonics explains that lithosphere is broken into plates that are In motion. These plates can be thought like cracked shell on a boiled egg which moves relative to one another along plate Boundaries.
Plate boundaries are classified in to three types:
1. Divergent.
Plates moves apart which result in creation of new ocean floor with submarine volcanoes, mid-oceanic ridge a small to moderate earthquake.
2. Convergent:
Plate move towards each other resulting destruction of ocean floor, creation and growth of mountain range with volcanoes, subduction zones .earth’s greatest earthquake and tsunamis.
3. Transform:
Plates moves sideways past each other. It causes no creation or destruction of crust and results small to large earthquake.
Earth Quake:
Shaking of the earth’s surface caused by sudden release of energy stored in the rocks beneath earth’s surface is called an earthquake.
Causes of Earth Quake:
1. Tectonic forces acting deep the earth may put a stress in the rocks .it bends and change the shape of rocks. When these rocks breaks, waves of energy are release and sent out through the earth. These are seismic waves cause to shake the ground during earthquake.
2. Volcanic activity can also cause earthquake.
3. Movement of plates (plate tectonics) may also cause earth Quake.
Terminology:
Focus:
The point within the earth where seismic waves first originate is called “Focus”. This is entre of earthquake.
Epicenter:
The point on the earth surface directly above the focus is called epicenter.
Shallow focus Earth quake
Focus ranges between 0-70 km. these EQ are large and more dangerous as they are closer to the surface of earth. They are more common and occur near date boundaries.
The account for 85% of total energy.
Intermediate Focus Earthquake
Depth 70-350 km, and account 12%cause. Causes less damage as compared to shallow focus E.Q
Deep Focus Earthquake
Focus range between 350-670 km. These occur in subduction zone. Causes less damage and are 3% of the totals because most deep rocks flow and deformed when stressed.
Seismic Waves
Two types of seismic waves are generated during earthquakes
Body waves
These are seismic waves that travel through the earth’s interior, spreading outward from the focus in all direction. These are of two types
P- Waves (primary waves)
It is compressional or longitudinal wave in which rock vibrates back and forth parallel to the direction of wave propagation. These are very fast (4 to 7km/sec) and first to arrive at a recordings. After earthquake occurrence.
S-Waves (secondary weaves)
It is shear wave and the rock vibrates perpendicular to the direction of wave propagation .these are slower than p-waves (2 to 5 km/sec) in near surface rocks. Both P-waves and s was passes easily through solid rocks. P wave can pass through a liquid .but an s wave cannot pass through liquid.
Surface waves:
Surface waves are the slowest waves generate by the earth quake. These are high amplitude waves and in general causes most property damage because they move slowly and take more time to pass .there are two types of surface waves.
Love waves
Love waves are like s-waves. The ground moves side to i.e. perpendicular to the direction the wave is travelling or propagating. Liquid and cannot be felt upon a water body .these waves knock the building off their foundations and may destroy highway bridge support.
Rayleigh waves
These behave like rolling ocean waves and cause the ground to move in an elliptical direction of wave passage Rayleigh waves are the most destructive to buildings because they produce.
Seismography:
It is recording devices that produce a permanent record of earth motion detected by seismometer.
Seismogram:
The paper record of the earth vibrations is called seismogram.
Measurement of the size of an earth quake:
The size of an earth Quake is measure two ways. One method is to find how much and what kind of damage the earth Quake has caused this determines to intensity, which is actually the measure of an earth Quake is effect on people and buildings. Intensities are expressed in Roman numerals ranging from I to IX, on the number s indicate great damage.
The second method of measuring a size of a Quake is to measure and calculate amount of energy released by the earth quake this is done by measuring the height (Amplitude) of waves on a seismogram. After measuring a specific waves and correcting for the type of seismography and for the distance from Quake scientists can assign a number called magnitude. It is measure of the energy released during the earth quake. The magnitude is reporter on the richer scale.
Table: - Comparison of earth quake Magnitude , Description , Intensity and expected annual world occurrence
Richer Magnitude
Description
Max expected Marcella intensity at Epicenter
Annual Expected
2.0
Very minor
I usually detected by instrument only
600,000
2.0 – 29
Very minor
I – II Felt by some indoors , especially on upper floors
300,000
3.0 – 3.9
Minor
III Felt floors, standing motor cars may rock slightly, not felt by many peoples
49,000
4.0 – 4.9
Light
IV – V Felt by most slight damage at night some awakened, windows, dishes spoken, plastic some time cracked.
6,200
5.0 – 5.9
Moderate
VI – VII Felt by all, damage minor to moderate, heavy furniture’s moved, felt by driving persons, minor in well-designed but large in poor designed buildings.
800
6.0 – 6.9
Strong
Everyone runs out, moderate to major damage. Minor in designed and great in poorly designed building driver disturbed
266
7.0 – 7.9
Major
IX – X Major damage, partial collapse of buildings. Ground cracks, underground pipes broken landslides, rail bent, buildings shifted off foundations
18
8.0 or Heigher
Great
X – XII Major and total Damage, bridges destroyed, Broad cracks in ground, Rails
1 or 2
bent completely, waves seen on ground surface, objects thrown upward
Seismography is usually required to measure magnitude of the earth quake. This measure has many advantages over intensity because magnitude is an indicative of earth quake strength. A worldwide network of standard seismography stations (seismic observatories) has been established and the media report magnitude of all the earth quakes happening in the world. A single number can be assigned to a single earth quake (Magnitude), whereas intensity varies for a single earth quakes, depending on the amount and kind of local damage.
Earth Quake Classification
a) Intra-plate E.Q
They occur where the crust is fracturing within the plates.
b) Tectonic E.Q
These are caused by sudden release of energy stored within the rock along a fault. The released energy is produced by the strain on the rock due to movement within the earth called tectonic deformation.
c) Volcanic E.Q
These are caused by upward movement of magma under the volcano which strain the rock locally and leads to an earth Quake. This E.Q occurs in areas that are associate with volcanic eruption.
Effects of Earth Quake
(1) Destroy man-made structures
(2) Roads are subsided
(3) Fire after an earth quake due to broken gas pipe lines
(4) Shortage of drinking water due to broken pipe lines.
(5) Landslides occur and block path of streams, roads and produce artificial lakes.
(6) Tsunami affects coastal areas.
(7) E.Q can damage Dams, Bridges, Railway lines, Roads and Dis-manila irrigation system.
Earth Quake Engineering
(1) Damage and loss caused by earth quakes can be reduced if civil structures are built considering effects of earth quakes.
(2) Siting of structures on solid bad rocks or dense solids.
(3) Following strict seismic building codes may reduce the risk of loss caused by earth quakes.
(4) Regional tectonic history studies and engineering design following this history in an area may reduce losses.
(5) Before designing a civil engineering structures proper geological studies may reduce the seismic risk.
(6) Building height and mass be designed keeping in view the tectonic forces and subsurface strata of the area.
Tsunami
Tsunami is a Japanese word which means great sea wave .The sudden movement of the sea floor during an earth quake in the sea or ocean can generate very large sea waves. Tsunami is also called seismic waves.
Causes of Tsunami:
They usually are caused by earth quakes of magnitude ∞+. These earth quakes disturb the sea floor and produce large sea waves. They can also reduce from sub marine landslides and volcanic eruptions. When a large reaction of sea floor suddenly rises and falls during a quake, all water over the moving area is lifted and dropped and causes Tsunamis.
Speed and Magnitude:
A tsunami wave can gain a speed of 1000 km/h and a height of 50m – 80m. Generally height is the range of 5 to 15m but it can raise high in larger Tsunamis. The speed of an ordinary wave slows as it moves through shallow water. But a tsunami can hit the shores as a very large fast wave. Due to long wave length of tsunami the water does not withdraw quickly. The long duration and great height of a tsunami can cause widespread destruction to the entire shore zone.
Emergency Preparedness Plans:
(1) Communication strategies are adopted.
(2) Proper structural designs are observed.
(3) Education and awareness.
(4) Supports for shelves to avoid falling objects.
(5) Earthquakes survival kit.
(6) Shatter under a heavy table.
(7) Stay outside after earthquake and wait aftershocks.
(8) Evacuate coastal areas to reduce tsunami damage.