03 Measuring Earthquakes

8/7/2019 03 Measuring Earthquakes

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M E RI E R KE

r t th P 2

Me as

g a t q a e s H

st y: The ea r es

seis gis

s we re the Chine se who wo rked ha rd to record thei r q akes in detail. The y even develo ped a mean s to pred ict ea rth q akes by filling a cerami c ja r to the br im with wate r an d lea ving it set. If the wate r ove rflowe d the ja r then anea rth q ake wa s imminent. Of cou rse, thi s mean s of prediction wa s un reliab le an d un ce rtain.

It i s thou ght that some animal s ma y feel vibration s f rom a qua ke befo re human s,an d that e ven minute s befo re a qua ke dogs ma y howl an d birds fly e rratically. Aristotle wa s one of the fi rst Eu ropean s to create a theo ry about the o rigin of Ea rth qua kes. He thou ght that the y we re the result of hea vy winds . T e F

st S ei sm g ap The fi rst seismo graph wa s invente d by the Chine se a st ronome r an d mathemati cian,

Chan g Hen g. He called it an "ea rth qua ke weathe rcock . It ha d eight dragon s an d ea ch of the ei ght dragon s ha d a bronze ball in it s mouth below the dragon s, at the base of theweathe r cock a re ei ght toa ds with thei r mouth s open representin g eight direction s.Whene ve r the re wa s even a slight ea rth t remo r, a me chani sm in side the seismo graph woul d open the mouth of one dragon.

The bronze ball woul d fall into the o pen mouth of one of the toa ds , ma king enou ghnoise to ale rt someone that an ea rth qua ke ha d just ha pp ene d. Im perial wat chmen could tellthe direction f rom whi ch the ea rth qua ke came by seein g which dragon' s mouth wa s em pt y.

Fig . 3 : The Firs t Sei smo graph ea rth qua ke weathe rcock A Chine se in vention.

In the 1850 Ro be rt Mallet, fi gured out a mean s to mea sure the velo cit y of seismic wa ves. Meanwhile, in Ital y, Luigi Palmie ri in vente d an ele ct roma gneti c seismo graph, one of which wa s installe d nea r Mount Ve suvius an d anothe r at the Uni versit y of Na p les. The seseismo graphs we re the fi rst seismic inst rument s capable of routinel y dete cting ea rth qua kes


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MEASURING EARTHUQAKES

Dr. N.Venkatanathan Page 3

imperceptible to human beings. In 1872 a U.S. scientist named Grove Gilbert figured out that earthquakes usually center around a fault line. It was after the 1906 earthquake in SanFrancisco that Harry Reid hypothesized that, earthquakes were likely the result of a build-upof pressure along these faults. It was about 1910 that Alfred Wegener published his theoryof plate tectonics to explain volcanic and seismic activity. Since then, seismologists havecontinued to work at a furious pace, building better instruments, computer models, theories ,

and forecast to study the causes and effects of earthquakes. M odern

eis ographs

Most seismographs today are electronic, but a basic seismograph is made of a drumwith paper on it, a bar or spring with a hinge at one or both ends, a weight, and a pen. Theone end of the bar or spring is bolted to a pole or metal box that is bolted to the ground.The weight is put on the other end of the bar and the pen is stuck to the weight. Bystudying the seismogram, the seismologist can tell how far away the earthquake was andhow strong it was. This record does not tell the seismologist exactly where the epicenterwas, just that the earthquake happened so many miles or kilometers away from that seismograph.

In a seismogram, there will be wiggly lines all across it. These wiggly lines areseismic waves that the seismograph has recorded. Most of these waves were so small(microseisms) that nobody felt them. At the time of earthquake, the P-wave will be the first wiggle, which is bigger than the microseisms. P-waves are the fastest seismic waves, andare usually the first ones that a seismograph records. The next set of seismic waves on theseismogram will be the S-waves and these are normally bigger than the P-waves. Thesurface waves ( Love and Rayleigh waves) are often larger waves marked on theseismogram.

Fig. 4: Simple schematic diagram of modern seismograph


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Fig. 5: Schematic representation of Modern Seismograph

W orking

The seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary. This is accomplished by isolating the recording device (like a pen)from the rest of the Earth using the principal of inertia. For example, if the pen is attachedto a large mass suspended by a spring, the spring and the large mass move less than thepaper which is attached to the Earth, and on which the record of the vibrations is made.

Surface waves travel a little slower than S-waves, so they tend to arrive at the

seismograph just after the S-waves. For shallow earthquakes, the surface waves may be thelargest waves recorded by the seismograph. Often they are the only waves recorded at longdistance, from medium-sized earthquakes.

Fig. 6: Wiggly lines of seismograph during an earthquake


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M E RI E R KE

r t th P 5

D ifferen t typ e s f S a e s:

Fig. 7 : Flow cha rt showin g different t ype of scale s

T e Rich t er s c a e

The ma gnitu de of mo st ea rth qua kes is mea su red on the Ri chte r scale, in vente d by Cha rles F. Richte r in 1934. The Rich t er mag ni t ! de is c a cul at e d fro m t he amp li t u de

of t he la r ge st s ei sm ic wav e recor de d for t he e a r t h qu a"

e, no matt er w h at typ e of wav e was t he st ron ge st. T he Rich t er mag ni t u de s a re # as e d on a lo ga ri t h m ic s c a le ( # as e 10).

So it mean s that fo r ea ch whole num be r, u p on the Ri chte r scale, the am plitude of

the groun d motion recorded by a seismo graph goe s up ten time s. On thi s scale, anea rth qua ke of ma gnitu de 5 woul d result in ten time s the le vel of groun d sha king as anea rth qua ke of ma gnitu de 4 an d it is 32 time s a s mu ch a s ene rgy would be relea sed.

MAGNI TUDE CHANG E

GR$

UN D MO TION CHANG E ( D I SPL ACE ME N T)

E N E RGY CHANG E

1.0 10.0 T IM E S 3.2 T IM E S

0.5 3.2 T IM E S 5.5 T IM E S 0.3 2.0 T IM E S 3 T IM E S 0.1 1.3 T IM E S 1.4 T IM E S

Ta ble 1 : Showin g com pa rison between Ma gnitu de Vs Groun d Motion & Ene rgy

For exam ple, a ma gnitu de of 6.0 ea rth qua ke produces 10 time s mo re groun d motion

than a ma gnitu de of 5.0 ea rth qua ke. The ene rgy differen ce i s about 32 time s. The ene rgy relea se i s the best in d icato r of dest ructive powe r of ea rth qua ke.

Scales

Unscientific M ethods

M ercalli ScaleM odified M ercalli

Intensity Scale

Scientific M ethod

Richter Scale


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Richter Scale(Magnitudes)

Earthquake Effects

Less than 3.5 Generally not felt, but recorded.

3.5-5.4 Often felt, but rarely causes damage.

5.5 - 6.0 At most slight damage to well-designed buildings. Cancause major damage to poorly constructed buildingsover small regions.

6.1-6.9 Can be destructive in areas up to about 100 km acrosswhere people live.

7.0-7.9 Major earthquake. Can cause serious damage overlarger areas.

8 or greater Great earthquake. Can cause serious damage in areasseveral hundred kilometers across.

Table 2: Showing possible effect for different magnitudes of Richter scale.

Co % parison bet & een Bhuj 2001 and

'

u % atra 2004 Bhuj 2001 Earthquake: Magnitude 7.7 on Richter scaleSumatra 2004 Earthquake: Magnitude 9.1 on Richter scale Since, the magnitude scale is logarithmic scale.

(10 9.1 /10 7.7 ) = 10 1.4 = 25.1189 i.e. Sumatra earthquake is 25.1189 times greater than Bhuj earthquake. In other words,Sumatra earthquake is equal to 25.1189 Bhuj earthquakes. Energy difference calculation

Based on empirical formula log (E) is proportional to 1.5M. Where, E(

is energy andM

(

is magnitude. 10 1.5 is approximately 32 times.

((10 1.5 )9.1 )/ ((10 1.5 )7.7 ) = 10 (1.5 x 1.4) = 125.8925 The energy released is equal to 125.8925 times. The M ercalli

'

cale

Another way to measure the strength of an earthquake is to use the Mercalli scale.Invented by Giuseppe Mercalli in 1902, this scale uses the observations of people whoexperienced the earthquake to estimate its intensity. The Mercalli scale is not considered as


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scientific as the Richter scale. Some witnesses of the earthquake might exaggerate just howbad things were during the earthquake. Therefore, the amount of damage caused by theearthquake may not accurately record how strong it was either. The M odified M ercalli I ntensity

)

cale

The effect of an earthquake on the earth's surface is called the intensity. Althoughnumerous intensity scales have been developed over the last several hundred years toevaluate the effects of earthquakes, the one currently used in the United States is theModified Mercalli (MM) Intensity Scale. It was developed in 1931 by the Americanseismologists, Harry Wood and Frank Neumann. This scale, composed of 12 increasinglevels of intensity that range from imperceptible shaking to catastrophic destruction, isdesignated by Roman numerals. It does not have a mathematical basis; instead, it is anarbitrary ranking based on observed effects.

The Modified Mercalli Intensity value assigned to a specific site after an earthquakehas a more meaningful measure of severity to the nonscientist than the magnitude becauseintensity refers to the effects actually experienced at that place.

Table 3: Showing comparison between Richter scale and Modified Mercalli Intensity

I.

N ot felt , except by a very few, under especially favorable conditions.

II.

F elt only by a few persons at rest, especially on upper floors of buildings.III.

F elt quite noticeably by persons indoors, especially on upper floors of buildings.Many people do not recognize it as an earthquake. Standing motor cars may rockslightly. Vibrations are similar to the passing of a truck.

IV.

F elt indoors by many, outdoors by few during the day. At night, some awakened.Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavytruck striking building. Standing motor cars rocked noticeably.

V.

F elt by nearly e 0 eryone ; many awakened. Some dishes, windows broken.Unstable objects overturned. Pendulum clocks may stop.

VI.

F elt by all , many frightened. Some heavy furniture moved; a few instances of fallen

plaster.VII.

D a 1 age negligible in buildings of good design and construction; slight tomoderate in well-built ordinary structures; considerable damage in poorly built orbadly designed structures; some chimneys broken.

VIII.

D a 2 age slight in specially designed structures; considerable damage in ordinarysubstantial buildings with partial collapse. Damage great in poorly built structures.Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furnitureoverturned.


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IX.

D a 3 age considerable in specially designed structures; well-designed framestructures thrown out of plumb. Damage great in substantial buildings, with partialcollapse. Buildings shifted off foundations.

X.

Some well-built wooden structures destroyed; most masonry and frame structuresdestroyed with foundations. Rails bent.

XI.

Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent

greatly.XII.

D a 4 age total . Lines of sight and level are distorted. Objects thrown into the air.

5 ocating and M easuring Earthquakes Earthquake Distance:

The epicenter is located using the difference in the arrival times between P and Swave recordings, which are related to distance.

P wave velocity depends on a material's "plane wave modulus" and its density.

Where, P is Lam's constant, is shear modulus, K is bulk modulus, and V isdensity. Notice that density is in the denominator, so denser rocks should be slower.However, although the density of rock in the Earth generally increases with depth, therigidity, as expressed in the various elastic constants, increases even more rapidly withdepth. Hence, P wave velocity generally increases with increasing depth.

Since solids, liquids and gasses have a finite bulk modulus; P waves can travelthrough any of these medium.

S-wave velocity depends on a material's shear modulus ( ), and density ( V),

Since fluids (liquids and gasses have zero shear modulus, S waves cannot travel

through fluids. Comparing the velocity expressions, you can see that VP > VS for anymaterial.

It has been established that velocity of "P" waves is 1.68 to 1.75 times than "S"

waves, means "P" wave travels about 1.68 to 1.75 times faster than destructive "S" wave,velocity is depending on the particular soil construction. Since there is difference in velocity

between the waves, the arrival time these waves to a particular seismic station will alsodiffer. By knowing velocity and time, distance can be calculated by plotting distance andtime travel graph.

For example, from the fig. 8, the first P wave was arrived the seismic station on 7 th

minute and the S wave was recorded on 12 th minute. Plot the graph between travel timealong Y axis and distance to the epicenter along X axis. For the 5 minute time differenceit is found from the graph that the epicenter is 3800 km away from the particular seismic


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station. It can be verified that one cannot find the same 5 minute time gap between P wave and S - wave for another epicenter distance.

Fig. 8: Travel time graph to find epicenter distance from the seismic station

Earthquake Direction:

Three or more seismic stations can be used to find exact location of an earthquakeepicenter. Using compass, draw a circle with a radius equal to the distance measuredearlier. The center of the circle will be the location of the seismograph. The epicenter of theearthquake is somewhere on the edge of that circle. Do the same thing for the distance tothe epicenter that the other seismograms recorded. All of the circles should intersect at point. The point where all of the circles intersect is the approximate epicenter of theearthquake.

For example, in Fig. 9, three seismic stations located at different distances from the

epicenter, are kept as the centre of circle and circles are drawn, after finding the distancesby using above mentioned method. The intersection three circles indicates that theearthquake was triggered along the mid Atlantic ridge, i.e. in between South America andAfrica.

The epicenter is the point on the Earth's surface that is directly above the hypocenteror focus, the point where an earthquake or underground explosion originates. In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and


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in most cases, it is the area of greatest damage. But for larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone.

Fig. 9: Finding epicenter of an earthquake

For example, in the magnitude 7.9, 2002 Denali earthquake in Alaska, the epicenterwas at the western end of the rupture. But the greatest damage occurred about 330 kmaway at the eastern end of the rupture zone. F inding the 6 agnitude of an earthquake

Fig.10: Showing reference scale to find location and magnitude an earthquake


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Measure the amplitude of the strongest wave. The amplitude is the height of thestrongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters onthe right side of the chart and mark that point. Place a ruler (or straight edge) on the chart between the points you marked for the distance to the epicenter and the amplitude. Thepoint where your ruler crosses the middle line on the chart marks the magnitude (strength)of the earthquake. This earthquake had a magnitude of 5.

F ocal D epth of an earthquake

Earthquakes can occur anywhere between the Earth's surface and about 700kilometers below the surface. For scientific purposes, this earthquake depth range of 0 - 700km is divided into three zones: Shallow, Intermediate, and Deep.

Shallow earthquakes are between 0 km and 70 km in deep. Intermediate

earthquakes, normally occur at the depth of 70 - 300 km and Deep seated earthquakesoccur at a depth of 300 - 700 km. In general, the term "deep-focus earthquakes" is appliedto earthquakes deeper than 70 km.

The focal depth can be calculated from measurements based on seismic wavephenomena. As with all wave phenomena in physics, there is uncertainty in suchmeasurements that grows with the wavelength. So the focal depth of the source of theselong-wavelength waves is difficult to determine exactly. Very strong earthquakes radiate alarge fraction of energy is released in seismic waves. This is associated with very longwavelengths. Therefore a stronger earthquake involves the release of energy from a largermass of rock. Calculating D epth of an earthquake

The most obvious indication on a seismogram that a large earthquake has a deepfocus is the small amplitude of the recorded surface waves. The surface waves do generallyindicate that an earthquake is either shallow or may have some depth. The most accuratemethod of determining the focal depth of an earthquake is to read a depth phase recordedon the seismogram.

The depth phase is the characteristic phase ( pP). pP initially goes up from theearthquake source, reflects off the Earth's surface. Then it follows closely behind the P waveto arrive at the seismograph. At distant seismograph stations, the pP follows the P wave bya time interval that changes slowly with distance but rapidly with depth. This time interval,pP - P ( pP minus P), is used to compute depth-of-focus tables. Then the additional traveltime for pP is simply twice the vertical travel time from hypocenter to the surface.

(i.e.) The extra travel time as ( pP - P) = 2d/v,Where,(pP - P) is the travel time difference,d is hypo central depth, andv is the average P wave velocity above the source.

Using the time difference of pP-P as read from the seismogram and the distance

between the epicenter and the seismograph station, the depth of the earthquake can bedetermined from published travel-time curves or depth tables.


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EFF ECT7

OF EA 8 T 9 Q @ AKE7

Earthquakes really pose little direct danger to a person. People can't be shaken todeath by an earthquake. Some movies show scenes with the ground suddenly opening upand people falling into fiery pits, but this just doesn't happen in real life.

The Effect of Ground7

haking Ground Shaking: The first main earthquake danger is the effect of ground shaking. Buildingscan be damaged by the shaking itself. Also it can be due to the ground beneath themsettling to a different level than it was before the earthquake, called as subsidence .

Shaking of the ground caused by the passage of seismic waves near the epicenter of the earthquake is responsible for the collapse of most structures. The intensity of groundshaking depends on distance from the epicenter and on the type of bedrock underlying thearea.

In general, loose unconsolidated sediment is subject to more intense shaking thansolid bedrock. Damage to structures from shaking depends on the type of construction.Concrete and masonry structures, because they are brittle are more susceptible to damage

than wood and steel structures, which are more flexible.

Fig. 11: Showing destruction due to ground shaking during 2010 Haiti earthquake


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Fig. 13: Buildings got toppled and sunk due to an 1964, Niigata, Japan earthquake. About 1/3 of the city subsided by as much as 2 meters as a result of sand compaction. PhotoCredit: National Geophysical Data Center

Ground D isplace A ent The second main earthquake hazard is ground displacement (ground movement)along a fault. If a structure (a building, road, etc.) is built across a fault, the grounddisplacement during an earthquake could seriously damage or rip apart that structure.

Fig. 14: Soils failed and moved down slope. The soil failure left a part of the school onunmoved ground and dropped the remainder into a wide trough during 1964 Prince WilliamSound, Alaska earthquake Photo Credit: National Geophysical Data Center.


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M E RI E R KE

r t th P 5

Floo d in g

The thi rd main haza rd is flooding. An ea rth qua ke can rup tu re (brea k) dam s or levee s alon g a rive r. The wate r f rom the rive r or the rese rvoir woul d then floo d the a rea, dama ging buildings and ma ybe swee p ing awa y or drownin g peo ple. T sunami al so can cau se floo ding,when ea rth qua ke o ccurred in o ceani c or in coa stal region.

Ph ys ic s of Ts un am i

The te rm t sunami come s f rom the Ja pane se, meanin g "ha rbor" (t su, ) an d "wa ve"(nami, ). Thi s t ype of ti dal wa ve i s a se ries of wate r wa ve s cau sed by the d isp lacement of a la rge volume of a body of wate r, u sually an o cean, sometime s in la rge la kes, is rapidly disp laced on a ma ss ive scale.

Gener at ion m ech a ni sms

The pr incipal gene ration me chani sm (or cau se) of a t sunami i s the disp lacement of asubs tantial volume of wate r or pe rtu rbation of the sea. Thi s d isp lacement of wate r is usually att ribute d to ea rth qua kes, lan ds lides, volcani c eruption s, o r mo re ra rely by meteo rite s an d nu clea r te st s. The wa ves forme d in thi s wa y a re then sustaine d by gravit y. It i s important tonote that ti des do not p lay an y pa rt in the gene ration of t sunami s; hen ce refe rring tot sunami s as 'tidal wa ve s ' is ina ccurate.

Tsunami s can be gene rate d when the sea floo r abrup tly defo rms an d ve rtically disp laces the o ve rlying wate r. Te ctoni c ea rth qua kes are a pa rticula r kind of ea rth qua ke that a re a ssociate d with the ea rth' s crustal defo rmation ; when the se ea rth qua kes occur beneaththe sea, the wate r above the defo rme d a rea i s disp laced f rom it s equilibr ium position. Mo resp ecifically, a t sunami can be gene rate d when th rust fault s associate d with con ve rgent o r dest ructive p late boun daries mo ve a bruptly, re sultin g in wate r disp lacement, due to theve rtical com ponent of mo vement in volved. Movement on no rmal fault s will also cau sedisp lacement of the sea bed, but the size of the la rgest of such e vent s is no rmally too smallto give rise to a significant t sunami.

Fig. 15 : Ve rtical downwa rd mo vement due to subd uction cau ses sudd endisp lacement of wate r


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Theory of W a F es

Waves propagates by transferring energy, they does not move particles in thedirection of their motion. The relation between energy, frequency and velocity of a wave canbe understood from the following equation.

E = h R = hv / P

Fig. 17: showing basic physical measurements for a wave

Ocean W a G es (Tidal H a G es)

Energy from the oceans is also available in the form of oceanic waves or sea-waves.Due to the blowing of wind on the surface of ocean, very fast sea-waves (or water waves)move on its surface. As wind grabs the water molecules, the friction causes ripples. Windcontinues to push against these ripples in a snowball effect that eventually creates a large

wave. The size and momentum of the waves increases, as they approach the sea shore.Simply saying, the wave energy moves on top of the water.

Fig. 18: As wind hits the water, ripples form. Then, wind gets an even better grip andcontinues to push the ripples until they grow to be large waves. Photo Curtsey: How stuff

works


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M E RI E R KE

r t th P

Ch a r a c t eri st ic s of ts un am i wav e s

Tsunami s a re cha racte rized as shallow-wate r wa ves. Shallow-wate r wa ve s a redifferent f rom win d-gene rate d wa ves, the wa ves man y of u s ha ve o bs erved on a bea ch.Wind-gene rate d wa ves usually ha ve pe riod (T) of 5 to 20 secon d an d a wa velen gth (P ) of about 100 to 200 mete rs (300 to 600 feet ). The ti dal wa ve Hei ght regula r wind gene rate d wa ves a roun d 3 met res (10 feet ) an d the wa ve sp ee d a re in the ran ge between 16 an d 32km/h r (10 to 20 mile s/h r) .

Fig. 20 : D etail s of regula r wind gene rate d wa ve

Unlike a no rmal wa ve, ene rgy of a t sunami mo ve s th rou gh the wate r, not on to p of it. A t sunami can ha ve a pe riod in the ran ge of 10 minute s to 2 hou rs an d a wa velen gth inexcess of 500 km (300 mile s). It i s becau se of thei r long wa velen gth s that t sunami s beha veas shallow-wate r wa ve s. From e quation (g iven in page 18 ), one can un derstan d that a wa veis cha racte rized as a shallow-wate r wa ve when the ratio between the wate r depth an d it s

wa velen gth get s ve ry small. Hen ce in ve ry dee p wate r, a t sunami will t ravel at hi gh sp ee ds an d t ravel great t ran soceani c d istan ces with limite d ene rgy loss . For exam ple, when theocean i s 20,000 feet (6100 m ) dee p, unnoti ced t sunami t ravel a bout 550 mile s pe r hou r (890 km/h r) , the sp ee d of a jet ai rplane. An d the y can mo ve f rom one side of the Pa cific Ocean to the othe r side in le ss than one day.

The sp ee d of a shallow-wate r wa ve i s equal to the sq ua re root of the product of theaccele ration of gra vit y (9.8 m/ s2) an d the depth of the wate r.

Velocit y

As a t sunami lea ves the dee p wate r of the o pen sea an d propagate s into the mo re

shallow wate rs nea r the coa st, it un de rgoe s a t ran sformation. Sin ce the sp ee d of thet sunami i s relate d to the wate r depth, a s the depth of the wate r decrea ses, the sp ee d of thet sunami diminishe s. The rate at whi ch a wa ve lo se s it s ene rgy is inve rsely relate d to it s wa velen gth. Sin ce a t sunami ha s a ve ry large wa ve len gth, it will lo se little ene rgy as it propagate s.

Ene rgy loss rate = 1/P

d g v v!


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Therefore, the speed of the tsunami decreases as it enters shallower water, and theheight of the wave grows. Because of this "shoaling" effect, a tsunami that wasimperceptible in deep water may grow in height. Since the energy is conserved, the changeof total energy of the tsunami to be remains constant. As the kinetic energy decreases thepotential energy increases. Also from the height it attained, when come down its velocity

increases due to transformation of energy from P.E. to K.E. So with the sheer weight of water and velocity the tsunami pulverize objects in its path, often reducing building s to theirfoundations. Large objects such as ships and boulders can be carried several miles inlandbefore the tsunami subsides.

Fig. 21: Details of tsunami wave in Deep Ocean

Fig. 22: Details of the tsunami wave, when it approaches the shore

Fig. 23: Showing change in amplitude and wavelength of a tsunami wave


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Dr N Venkatanathan Page 1

Detecting tsunamis is a very difficult thing to do. Wave begins with the height of about 12 - 23 inches and look like nothing more than the gentle rise and fall of the seasurface. Tsunamis in deep water can have a wavelength greater than 300 miles (500kilometers) and a period of about an hour. As they approach the shore the height of thewave increases due to decrease in water depth and wave length. When a tsunami moving

away from the beach, it is fast and short amplitude, therefore its velocity v andwavelength P increases, so its amplitude A get decreased. When a Tsunami movingtoward the beach, it is slow and gained higher amplitude, therefore its velocity v andwavelength P decreases, with its amplitude A get decreased.

F ire

The fourth main earthquake hazard is fire. These fires can be started by broken gaslines and power lines, or tipped over wood or coal stoves. They can be a serious problem,especially if the water lines that feed the fire hydrants are broken, too. For example, afterthe Great San Francisco Earthquake in 1906, the city burned for three days. Most of the citywas destroyed and 250,000 people were left homeless.

Fig. 24: Fire at Valdez, Alaska. The tank fire was triggered by failure of oil storage tanks at the Union Oil tank farm. Photo Credit: National Geophysical Data Center.

The photograph was taken around 10:30 p.m., about 5 hours after the quake; thewhole waterfront was burning furiously. Some buildings along Front Street and StandardOil's pumping control station also caught fire. The Union Oil tank farm continued to burn fortwo weeks.

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03 Measuring Earthquakes