Earthquakes.pptx

Geology ppt very good

1. Earthquakes

2. Earthquakes are the shaking of ground surfaces generally due to natural causes.

3. Causes of Earthquakes: Earthquakes are caused by disturbances in the interior of the earth and other causes Tectonic Movements : The disturbances inside the earth are called tectonic movements . These forces bring about changes on the earth surface and physical features like mountains , plateaus and rift valleys are formed . Most disastrous earthquakes are caused by tectonic forces .Tectonic forces create tension and pressure and the stress begins to build up inside the earth . When the stress tends to be more than what the rocks of the earth can bear ,the rocks are broken and displaced from their state of equilibrium . It is known as

4. faulting . The energy accumulated during faulting is released . This release of energy gives rise to mighty waves . These waves originate from a point called Focus in the interior of the earth and then spread out in all directions . On the surface whatever comes into their contact begins to vibrate . The chief cause of earthquakes felt often in California in USA is often the San Andreas Fault found there.

5. Volcanic Eruptions : The volcanic eruptions are often very violent and cause vibrations in the earth crust . Sometimes the vent of a volcano is blocked temporarily and explosive eruption takes place suddenly causing tremors in the earth crust . The Krakatoa that erupted in 1883 became the cause of a violent earthquake there . Other Reasons : The roofs of underground caves sometimes give way and release great force to cause minor tremors in the earth crust . Nuclear explosions also release massive energy to cause tremors in the earth crust.

6. Earthquake Waves (Seismic Waves) Two major classes: Body waves which travel inside the body of the Earth. Two sub-classes, (a) primary or P-wave and (b) secondary or S-wave also known as shear wave. Surface waves which travel through the surface of the Earth. Two sub-classes, (a) Rayleigh wave and (b) Love wave.

7. Measurement of Earthquakes

8. Intensity Scale Intensity Scale - Modified Mercalli Scale Uses Roman Numerals example: VI or X The effects of an earthquake are an indication of an earthquake intensity. . Examples: people awaking, damage to brick and stone structures, and movement of furniture The scale ranges from I, which corresponds to imperceptible events, up to XII which corresponds to total destruction.

9. No mathematical basis for the scale Ranking based on observed effects Measure of the actual effects at a certain location Subjective measure, perceived, based on qualitative descriptions.

10. Magnitude Scale Richter scale - Arabic Numerals Decimals to the tenths place Ex: Earthquake Magnitude 9.2 on the Richter Scale Earthquake magnitude is a measure of the amplitude of the seismic waves recorded on a seismogram. Magnitude scales are logarithmic based on powers of 10. Seismic wave amplitudes increase by 10 times for each unit of the scale.

11. The Mercalli Scale is a measure of the effects of an earthquake at a particular place and depends not only on strength (magnitude) of a quake, but also the distance from the place of origin and the local geology at the observation point. A given event will have only one magnitude , but many intensity values , which tend to decrease with distance from the origin , although local conditions can produce anomalies . Intensities are considerably greater over soft soils than solid rock . Ground shaking - amplitude, duration, and damage increases in poorly consolidated rocks, all of which increase the intensity.

12. Worldwide Distribution of Earthquakes Earthquakes have a definite distribution pattern. There are three major belts in the world which are frequented by earthquakes of varying intensities. These belts are as under : 1 . The Circum -Pacific Belt 2 . The Mid-Atlantic Belt 3 . The Mid-Continental Belt

13. This belt is located around the coast of the Pacific Ocean. In this belt the earthquakes originate mostly beneath the ocean floor near the coast. The Circum - Pacific Belt represents the convergent plate boundaries where the most widespread and intense earthquakes occur. This belt runs from Alaska to Kurile, Japan, Mariana and the Philippine trenches. Beyond this, it bifurcates into two branches. One branch going towards the Indonesian trench and the other towards the Kermac -Tonga trench to the northwest of New Zealand. 1. The Circum -Pacific Belt:

14.  On the eastern side of the Pacific Ocean, the earthquake belt runs parallel to the west coast of North America and moves on towards the South along the Peru and Chile trench lying on the west coast of South America. This belt has about 66 percent of the total earthquake that are recorded in the world. Most of the earthquakes occurring in this belt are shallow ones with their focus about 25 km deep.

15. 2. The Mid-Atlantic Belt This belt is characterized by the sea floor spreading which is the main cause of the occurrence of earthquakes in it. This earthquake belt runs along the mid- oceanic ridges and the other ridges in the Atlantic Ocean. In this belt most of the earthquakes are of moderate to mild intensity. Their foci are generally less than 70 km deep. Since the divergent plates in this belt move in opposite directions and there is splitting as well, transform faults and fractures are created . All this becomes the causative factor for the occurrence of shallow focus earthquakes of moderate intensity. The sea floor spreading is the main cause for the occurrence of earthquakes in this belt.

16. 3. The Mid-Continental Belt: This belt extends along the young folded Alpine mountain system of Europe, North Africa, through Asia Minor, Caucasia, Iran, Afghanistan and Pakistan to the Himalayan mountain system. This belt continues further to include Tibet, the Pamirs and the mountains of Tien Shan etc. The young folded mountain systems of Myanmar, China and eastern Siberia fall in this belt. This belt happens to be the subduction zone of continental plates. It is in this belt that the African as well as Indian plates sub-duct below the Eurasian plate

17. It may be pointed out that more than 50 percent of all earthquakes are associated with the young folded mountains which are said to be still growing. The Andes, Himalayas and Coast Ranges of the United States are the specific examples. It is worthwhile to remember that this girdle of young fold mountains has no correspondence with the line of active volcanoes like the Circum -Pacific earthquake zone . There are some regions on the earth's surface which are relatively immune from violent and vigorous earthquakes. This is so because diastrophism and volcanism are either absent or only moderately active. But the infrequent occurrence of minor shocks in such regions is not ruled out.

18. Effects of Earthquake Earthquakes are less advantageous and more harmful to man . Damage done is chiefly in following respects : 1. Loss of Property : Severe earthquakes reduce to rubble human structures ranging from huts to palaces and single storied to multi storied buildings . Even pipelines laid under the ground and railway lines are damaged or displaced . The best example of this type of damage is Koyna earthquake in 1970.

19. 2. Liquefaction - a geologic process that affects earthquake intensity. Liquefaction is the temporary change of water saturated soil and sand from solid to liquid state.

20. 3. Changes in the course of rivers : On account of the impact of earthquakes , sometimes rivers also change their course . Consequently ,when floods come they play havoc with people's lives. 4. Land Shift: Earthquake cause cracks in earth's crust anywhere in fields , roads, parks and even hills and sometimes shifting of lands takes place along the cracks.

21. Earthquake Hazards Direct Hazards (due to ground shaking) Collapse of buildings & structures Broken/fallen power lines (electricity) Broken pipelines (water & gas) Damage to roads and bridges

22. Earthquake Hazards Indirect Hazards Fire : ground motion breaks fuel lines, fuel tanks and power lines. -Water lines are often broken, reducing the amount of water available to fight fires. Landslides : earthquakes trigger the failure of unstable slopes. Tsunami : the ocean floor rises or falls suddenly due to an earthquake, generating giant waves as much as 30 meters tall.

23. Seismic Hazard Assessment Seismic hazard and risk are fundamentally different concepts. Seismic hazard describes phenomena generated by earthquakes that have potential to cause harm, but seismic risk is the likelihood (chance) of experiencing a specified level of seismic hazard in a given time exposure. Seismic hazard occurs naturally and can be evaluated from instrumental, historical , and geological observations. Seismic risk depends not only on the hazard and exposure, but also on models used to describe the occurrence of earthquakes . High seismic hazard does not necessarily mean high seismic risk , and vice versa.

24. Thus a   seismic hazard  is the probability that an  earthquake  will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold . In the New Madrid Seismic Zone of the central United States, the mean recurrence interval of large earthquakes (~ M7.5) is about 500 years, and the risk posed by such events or their ground motions (consequences) is about 10 percent probability of exceedance (PE) in 50 years.

25. Seismic Hazard Zones Seismic Zones of India The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different . Thus, a seismic zone map is required to identify these regions. Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV and V.

26. The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismotectonics and the seismic activity in the country . The Indian Standards provided the first seismic zone map in 1962, which was later revised in 1967 and again in 1970. The map has been revised again in 2002 and it now has only four seismic zones – II, III, IV and V.

27. The areas falling in seismic zone I in the 1970 version of the map are merged with those of seismic zone II. Also, the seismic zone map in the peninsular region has been modified. Madras now comes in seismic zone III as against in zone II in the 1970 version of the map. This 2002 seismic zone map is not the final word on the seismic hazard of the country.