1. Earthquakes Rehana Rasool Assistant Professor Department Of Geography
Government Degree College Tral

2. An earthquake is a shaking of the ground caused by a sudden release of energy in the Earth’s crust.
More than a million of them occur each year. However, most are too small to be felt or to cause damage.

3. Some definitions associated with earthquakes
Seismic waves = Energy moving outward from the focus of an earthquake
Focus= point on the fault where rupture occurs and the location from which seismic waves are generated.
Epicenter= spot on Earth’s surface directly above the focus.

4. Where do earthquakes occur
Most earthquakes occur along the edge of the oceanic and continental plate
So what are these plates, this can be better explained by the plate tectonic theory

5. What are Plate Tectonics
Earth’s surface is made up of many plates that float on the soft rock of the mantle. As the mantle moves, the plates also move.
Plates move only a few centimeters each year.
When one plate moves, it affects the other plates.
As plates move around, they cause great changes in the Earth’s surface, such as mountains, valleys, volcanoes, and earthquakes!

7. What Happens When Plates Move?

8. Types of Boundaries

10. What Happens When an Earthquake Starts?
The sudden release of energy from an earthquake sends out several different shaking movements, or seismic waves.

11. Seismic waves: forms P-waves: S-waves:
called compressional, or push-pull waves
It travels through the interior of the Earth rather than over the surface
Propagate parralel to the direction in which the wave is moving
Move through solids, liquids
S-waves:
Called shear waves
Propagate the movement perpendicular
to the direction in which the wave is
Moving
S-waves can only travel in solid material.
Surface waves (L-waves or long waves).
Complex motion, with ripples of energy that spread outward when rocks slip past each other along a fault, just like throwing a stone into a calm pond
Up-and-down and side-to-side
Slowest
Most damage to structures, buildings
P-waves: the bell analogy: waves felt by both sound

12. How Long Does an Earthquake Last?
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake.
The largest, main earthquake is called the mainshock.
Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months after the mainshock!

13. Measuring earthquakes
Seismometers: instruments that detect seismic waves
Seismographs
Record intensity, height
and amplitude of seismic
waves
Earliest seismograph was invented in China ~ 2000 years ago (or 136 AD)
Easy to make; can make your own at home

14. Earthquake size: two ways to measure
Magnitude: Richter Scale
Measures the energy released by fault movement
related to the maximum amplitude of the S wave measured from the seismogram
Logarithmic-scale; quantitative measure
For each whole number there is a 31.5 times increase in energy
eg. an increase from 5 to 7 on the Richter scale = an increase in energy of 992 times!!
Write Table 11-1 on the board, to help relate to Richter scale

15. 2) Intensity: Mercalli Scale:
What did you feel?
Assigns an intensity or rating to measure an earthquake at a particular location (qualitative)
I (not felt) to XII (buildings nearly destroyed)
Measures the destructive effect
Intensity is a function of:
Energy released by fault
Geology of the location
Surface substrate: can magnify shock waves e.g. Mexico City (1985) and San Francisco (1989)

16. Types of earthquakes on the basis of magnitude

18. Frequency of Occurrence of Earthquakes
Descriptor
Magnitude
Average Annually
Great
8 and higher
1 ¹
Major
17 ²
Strong
134 ²
Moderate
1319 ²
Light
13,000 (estimated)
Minor
130,000 (estimated)
Very Minor
1,300,000 (estimated)
¹ Based on observations since ² Based on observations since 1990.

19. Largest earthquake in the world
More than 2,000 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile
tsunami caused 61 deaths
$75 million damage in Hawaii;
138 deaths and $50 million damage in Japan;
32 dead and missing in the Philippines;
and $500,000 damage to the west coast of
the United States.
Chile : 1960 May 22 19:11:14 UTC Magnitude 9.5

20. Most Destructive Known Earthquakes on Record in the World
Date
Location
Deaths
Magnitude
Comments
May 31, 1970
Peru
66,000
7.9
$530,000,000
damage, great rock
slide, floods.
July 27, 1976
China, 
Tangshan
255,000 (official)
7.5
Estimated death toll as high as 655,000.
Sept 19, 1985
Mexico
Michoacan
9500
(official)
8.0  
Estimated death toll as high as 30,000
Old lake bed magnified shock waves by 500%
2001 Jan 26
India
20,023
7.7
166,836 injured,
600,000 homeless .
2004 Dec 26
Sumatra
283,106
9.0
Deaths from earthquake
and tsunami

21. So how safe are we
The Indian subcontinent has a history of devastating earthquakes . The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year.
Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes.  
The latest version of seismic zoning map of India [2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version, which consisted of five or six zones for the country.
According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.

22. SEISMIC ZONES
ISR © 2007, All right reserved

23. Earthquake damage Ground Failure - constructions collapse
Fires - from broken gas and electrical lines
Landslides - occur in hilly/mountainous areas.
Liquefaction - water-saturated, unconsolidated materials flow
Tsunami (seismic sea waves; "tidal" waves) - can grow up to 65 m

24. India, Gujarat earthquake Jan 26, 2001

25. OCT 2005 ,Kashmir

26. Thanks