India is vulnerable to differentnatural hazards due to its proximityto geodynamically active locales andunique climatic pattern.Both these factors in differentcombinations lead to the occurrenceof disasters resulting from naturalhazards like floods, earthquakes,draught, cyclones and landslides indifferent parts of the country atfrequent intervals.It is estimated that about 60% oflandmass of the country is vulnerableto earthquakes of differentmagnitudes.
Sikkim experienced a majordestructive earthquake of 6.9 (Mw)heating badly causing deaths of noof people and d.At least 20 aftershocks atmagnitudes of 5.7, 5.1, and 4.6 (Mw)within 30 minutes of the initialearthquake occurred back-to-backthroughout the night created panicin the Gangtok.
Q:What is earthquake?A:” When the edge of two pieces of the earthgrind together and cause rumbling and shakingunderneath the surface of the earth. Theshaking or trembling caused by the suddenrelease of energy is earthquake. Usually it isassociated with faulting or breaking of rocks”.
Q:What are the causes of earthquake?A:”The earth is divided into three main layers – ahard outer crust, a soft middle crust, and acenter core. The outer crust is broken intomassive irregular pieces called “plates”. Theseplates are moving very slowly, driven by energyforces deep within the earth. Earthquakesoccur when these moving plates grind andscrape against each other.”
What is the Elastic Rebound Theory?Explains how energy isstored in rocks– Rocks bend until thestrength of the rockis exceeded– Rupture occurs andthe rocks quicklyrebound to anundeformed shape– Energy is released inwaves that radiateoutward from thefault
Causes of EarthquakeTwo Types aftershock and foreshock” An aftershock is a small earthquake thatfollows the main earthquake.A foreshock is a small earthquake that oftenprecedes a major earthquake”.
Focus, Epicenter, and FaultThe point within Earth’s interior where the energywaves are produced and where faulting begins is thefocus, or hypocenterThe point on Earth’ssurface directly abovethe earthquake focusis EpicenterFault is fracturein Earth wheremovement hasoccurred
Locating an EpicenterEpicenter Location– If information is receivedfrom the earthquake at threestations, then the epicentercan be located– A circle is drawn around eachstation on a map• The radius is equal to thedistance from the station tothe epicenter– The point in which all threecircles intersect is theearthquake epicenter
Types of Faults” Strike-slip Fault– Earth’s plates move sideways to one another– Creates a shear force– Rocks on either side of the fault surface aremoving past each other without much upwardor downward movement”.
Q: Where are earthquakes most prone to occur?A: ‘’There is no technology available in any partof the world to determine the place, time, ormagnitude, of an earthquake. However, theyare most likely to occur where the plates meet.They are most prone to the fault line of theplates. Because the plates are in constantmotion.
Q:Where Do Earthquakes Occur andHow Often?A: 80% of all earthquakes occur inthe circum-Pacific belt– most of these result fromconvergent margin activity– ~15% occur in theMediterranean-Asiatic belt– remaining 5% occur in theinteriors of plates and onspreading ridge centers– more than 150,000 quakesstrong enough to be felt arerecorded each year
•Record seismic waves•A drum with a sheetof paper vibrates anda stationary penmarks the vibrationson the paper•The height of thelines are used tomeasure the energyreleased from theearthquake called themagnitude•The magnitude ofSikkim Earthquakewas 6.9 on RichterScaleSeismograph
SeismogramsSeismogramsare traces ofamplified,electronicallyrecorded groundmotion made byseismographs
Q:What are Seismic Waves?A: Response of material to the arrival ofenergy fronts released by rupture. Twotypes:– Body waves• P and S– Surface waves• R and L
Body Waves: P and S waves• Body waves– P or primary waves• fastest waves• travel through solids,liquids, or gases• compressional wave,material movement isin the same directionas wave movement– S or secondary waves• slower than P waves• travel through solidsonly• shear waves - movematerialperpendicular to wavemovement
Surface Waves: R and L waves• Surface Waves– Travel just below or along the ground’s surface– Slower than body waves; rolling and side-to-side movement– Especially damaging to buildings
•Past seismicity of the regionHistorical and instrumentally recorded data on earthquakesshow that the Sikkim and adjoining area lies in a region proneto be affected by moderate to great earthquakes in the past.Some noteworthy earthquakes that have affected the regionare:Cachar earthquake of 10.01.1869 (M: 7.5),Shillong plateau earthquake of 12.06.1897 (M: 8.7),Dhubri earthquake of 02.07.1930 (M: 7.1),Bihar-Nepal Border earthquake of 15.01.1934 (M: 8.3),Arunachal Pradesh – China Border earthquake of 15.08.1950(M: 8.5),Nepal-India Border earthquake of 21.08.1988 (M: 6.4)Sikkim earthquake of 14.02.2006 (M: 5.7)Bhutan earthquake of 21.09.2009 (M:6.2)
Landslides are one of the naturalhazards that affectat least 15% of land area of ourcountry exceeding 0.49million km2. Landslides of differenttypes occur frequentlyin geodynamically active domains inHimalaya,
TremorsThe strong tremor jolted buildings across northern andeastern India, including Delhi, Uttar Pradesh,Uttarakhand, Jharkhand, Bihar, West Bengal, Assamand Mizoram, prompting people to run out of theirhomes in panic.
Destruction from EarthquakesLandslidesWith many earthquakes, the greatest damageto structures is from landslides and groundsubsidence, or the sinking of the ground triggeredby vibrationsFireIn the San Francisco earthquake of 1906, mostof the destruction was caused by fires thatstarted when gas and electrical lines were cut.
Tsunami Movement: ~600 mph in deep water~250 mph in medium depth water~35 mph in shallow waterMovement of a Tsunami
TsunamisCaused by the movement of the ocean floor– Causes a disruption in the water– Some are so wide that a large ship cantravel over the wave without knowing– Recent earthquake in the Indian Oceancreated a tsunamis that was 100 feethigh and moving at 500 mph
NepalThe magnitude of the earthquake was5.8Mw in the region.BangladeshEarthquake was 10 km (6.2 miles) deep.
The Economics and Societal Impacts of EQsDamage in Oakland, CA,1989• Building collapse• Fire• Tsunami• Ground failure
Effects of EQ on Sikkim Govt.1. The Sikkim Government has disbursed Rs 14.21 crore(142 million Rs)as relief among quake-hit people in Sikkim.2. The India govt. has asked seismic experts from theIIT-Roorkee to conduct "earthquake impact studies" atthe dam sites of state-owned NHPC Ltd and NorthEastern Electric Power Corp. Ltd (Neepco).4.Many roads are blocked by landslides 5. Indian Prime Minister Manmohan Singh immediatelyconvened a meeting of the National Disaster ManagementAuthority Chamling to express his full support.6. The Indian Air Force flew five cargo planes with reliefmaterial and rescue personnelfor deployment in West Bengal and Sikkim.