EARTHQUAKES : CAUSES AND PREDICTIONS

 

by

 

Prof. R. N. Tiwari

Department of Geology

Banaras Hindu University

Natural Hazard

• Worst natural hazard

• Impact over a large area

• Causing destruction on massive scale

• Loss of human life and property

• Devastation is quick and sudden

• No time to escape

Major Earthquakes

• China- 1177• Peru- 1471• Lisbon – 1755• California- 1906• Tokyo- 1923• Chili- 1960• Last two decades – (1982-2000)

26 major earthquakes- casualities-150000

• Occurrence of earthquakes- frequent

• No body can remain without being affected

• Every time people get panic

• Series of questions arise

Definition• An earthquake is vibration of earth

produced by rapid release of energy

• Energy released radiates in all directions from its source

• Energy is in the form of waves

• Sensitive instruments around the world record the event

Seismic Waves• Longitudinal (P) Waves• Highest velocity, 5-7 km./ sec.• Can travel solid and liquid both• Transverse (S) Waves• Lower velocity-3-5Km./Sec.• Do not pass in liquid and gases• Surface (L) Waves • Travel along outer part of earth with great

amplitude and slowest velocity• Cause greatest destruction

Focus and Epicentre

• Focus : The place in the earth crust where the earth shocks occur

• Epicenter : The point at the surface immediately above the focus.

Location of Epicenter

• The time intervals between the first arrivals of P and S waves are used to determine the distance between a seismograph and the earthquake epicenter

• Three or more stations are needed to determine the location of an earthquake.

Seismograph: Instrument to design to register earthquake waves

Seismogram: Recording of earthquake waves on a paper

Seismograph

Magnitude And Intensity Of Earthquakes

• The severity of an earthquake can be expressed in several ways.

• Magnitude and Intensity measure different characteristics of earthquakes.

• Intensity measures the strength of shaking produced by the earthquake at a certain location. It is determined from effects on people, human structures, and the natural environment.

• Magnitude measures the energy released at the source of the earthquake. It is determined from measurements on seismographs and usually expressed by the Richter Scale

Richter Scale• < 2 Generally not felt but recorded• 2.0 – 2.9 Potentially perceptible • 3.0 – 3.9 Felt by some• 4.0 – 4.9 Felt by most• 5.0 – 5.9 Damaging shocks• 6.0 – 6.9 Destructive in populated

regions• 7.0 – 7.9 Major Earthquakes; Inflict

serious damages• >8 Great Earthquakes; cause

extensive • destruction near epicenter• ( Maximum known 8.9 )

Magnitude / Intensity Comparison

Magnitude Typical MaximumModified Mercalli Intensity

1.0 - 3.0 I

3.0 - 3.9 II - III

4.0 - 4.9 IV - V

5.0 - 5.9 VI - VII

6.0 - 6.9 VII - IX

7.0 and higher

VIII orhigher

Effects of Earthquakes • 1.    Loss of human life and property• 2.    Displacement of roads, rails, canals,• bridges, dams, etc. • 3.    Landslides, formation of lake, change of• stream channel• 4.    Liquefaction where ground is very wet with• higher water table, common in coastal• areas 5.Tsunamis occur following violent movement of the sea floor generated by seismic sea waves.

Liquefaction

Liquefaction in Japan

Tsunami

• Underwater earthquakes

• Seismic waves moving large bodies of water

• Result from vertical displacement along a fault located on ocean floor

Tsunami Waves

• Deep water

• • In the deep ocean the waves may go by unnoticed (low wave height)

• • Wave speed 800 km/hr

– Shallow water– Height exceeds 30m.

– Very destructive

Tsunami

Warning System

Earth’s History • Earth - one of the nine planets of solar system• Originated about 4500-4600 M.Y.• Today continents are separated by ocean• About 250 my. There was one continent and one

ocean called Pangaea and Panthalasa• About 200my. Pangaea split into two major

continents- Laurasia and Gondwana land • Indian plate travelled 9 m per century• Collided with Eurasia plate- 40-50my.• Formation of Himalaya

Earth Structure

• Seismic waves revealed – earth has radially layered structure

• Three main divisions- Crust, Mantle and Core

• Each layer has distinctive physical chemical and seismic characters

Mechanical division of Earth

• Lithosphere-• 100-150 km. below Continent• 70-100 Km. below Ocean• More rigid – forming hard outer shell• Divided in to several large fragments- called

Plates• Asthenosphere- • 150 Km. thick, lies beneath lithosphere • Lower rigidity partly molten

Tectonic Plates

• Twelve major plates

• Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, North America, South America, Pacific, Nazca and Cocos

• Earthquake occurs on plate margins • Associated with interaction between

plates

Types of Plate Margin• Constructive (divergent) plate margin• Spreading centre, plate moving a part • Destructive (convergent) plate margin• Subduction zone, One plate forced under another, Plates

bent sharply• Conservative plate margin: • Formed by transform fault• Lithosphere is neither created nor destroy• Convection current generated in the mantle moves the plates

Cause of Earthquakes

• Brittle condition of the lithosphere causes fracture when strongly stressed

• Rupture produces an earthquakes

• A violent release of elastic energy due to sudden displacement on a fault plane

Cause of Earthquakes • Moving Plates

• Release of energy from within the Earth

Earthquakes and Fault

• Movements that produced earthquakes are usually associated with large fractures in earth’s crust called faults

• Motion along faults can be explained by plate tectonic theory

Fault: A fracture or system of fractures along which rocks have been displaced.

Types of Fault

• Normal

• Reverse

• Strike slip • Right lateral,

• Left lateral

Elastic Rebound • Mechanism for earth quakes was explained

by H.F. Reid

• Rocks on both side of an existing fault are deformed by tectonic forces

• Rocks bend and store elastic energy

• Frictional resistance holding the rocks together is overcome

Earthquake Mechanism• Slippage at the weakest point (focus) occur

• Vibrations (earthquakes) occur at the deformed rock “ springs back” to its original shape (elastic rebound)

Earthquakes Distribution

• Earthquakes are generally distributed in belts. Eighty percent of the world's earthquakes occur in the circum-Pacific belt. Earthquakes also occur on the Mediterranean-Himalayan belt, and along other plate boundaries.

Earthquake mitigation

• Government

• Science/Technology

• Personal responsibility

Government

• Laws

• • Developing in seismically active areas?

• • Building codes

• • Emergency response

Technology

• Earthquake prediction

• Base isolation designs

Prediction

• • Natural

• • Scientific

• • The natural world is unpredictable!

• Natural – Animal behavior – Dogs barking excessively– – Pigs not eating climbing walls– – Cats left buildings– – Rats/mice left hidings places

• Scientific

• Seismic monitoring

• Elevation changes in land surface

• Groundwater level changes

Earthquake prediction

• Earthquake prediction may be possible by

• Measuring ground tilt,

• Fault movement,

• Seismic activity,

• Changes in the earth’s magnetic field

• Presence of radon gas in wells.

• No reliable method exist for making short range earthquake prediction

• Long term predictions based on occurrence of earthquakes on a time scale-30-100 years or more.

• Using historical record on paleoseismology

Disaster Management Programme

• Cultivate a culture of disaster preparedness.• Think about disaster management as a regular, day

to day thing• Disaster planning should begin at home• School children should be taught basic safety

measures• Radio, TV., can be used to explain these things• Designs of building should be earthquake safe and

must be approved by competent authority.• Identify bad building and get them repaired.

Preventive measures against building collapse

• Houses should have a uniform square or a rectangular plan

• The L/B ratio should be from 0.15 to 0.20• Foundation / plinth should be at least 1.5 times

thicker than the wall• Doors and windows should be placed at corners of

the room, 24 inches away from the room edges, minimum gap of 60cm. Between doors and windows

• Mortar ratio – Cement: sand = 1:6• - Cement: lime: sand = 1:2:9

• R.C.C. bands should be used at chair, door and roof levels

• Window porches and roofs should protrude minimal

• Jambs should be placed at corners between walls to provide strength and elasticity

• Iron rods should be vertically placed in corners • Houses build of stone- wall should not be thicker

than 45 cm. and R.C.C bands should used at each stage.

• Above all, the material used should be of good quality----after all it is your sweet home