Earthquake hazard in East Africa

10/10/22 Asrat Worku, Associate professor, AAU

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An Overview of Earthquake Hazards

http://marrakeshnights.com/wp-content/uploads/2010/03/GB.jpg


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Introduction:Some statistics An average of 10,000 people die of EQs each year

A UNESCO study shows, from 1926 to 1950 alone, average annual damage of $400,000,000

Hundreds of villages and tens of towns and cities are damaged and destroyed annually


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Some Statistics…

Richter Magnitude

Class Description Annual Average

>8 Great Can totally destroy communities near epicenters

<1

7 - 7.9 Major Can cause serious damages

15

6 - 6.9 Strong May cause a lot of damages in densely populated areas

134

5 - 5.9 Moderate Slight damages to structures

1319

<5 Minor to Light No to minor damages

1.44 million

Worldwide Earthquake Occurrence Frequency


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Some Statistics… Destructive EQs since 1900 that caused deaths > 50,000Event Country Date M DeathsMessina Italy 12/8/1908 7.2 72,000Haiyuan China 12/16/1920 7.8 200,000Kanto Japan 9/1/1923 7.9 142,800

AshgabatTurkmeinistan/USSR 10/5/1948 7.3 110,000

Chimbote Peru 5/31/1970 7.9 70,000Tangshan China 7/27/1976 7.5 255,000W.Iran Iran 6/20/1990 7.4 50,000Sumatra Indonesia 12/26/2004 9.1 227,898Muzaffabarad Pakistan 10/5/2005 7.6 86,000E.Sichuan China 5/12/2008 7.9 87,588Haiti Haiti 11/12/2010 7 222,570

Average 138,532

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Some Statistics M: 6 to 7 in the past 30 years that caused deaths > 1000Event Country Date M DeathsS Italy Italy 11/23/1980 6.5 2,735(Dhamar) Yemen 12/13/1982 6 2,800(Erzurum) Turkey 10/30/1983 6.9 1,342(Border) Nepal-India 8/20/1988 6.8 1,000Spitak Armenia 12/7/1988 6.8 25000*Latur-Killari India 9/29/1993 6.2 9,748Kobe Japan 1/16/1995 6.9 5,502

(Border)Afghanisan-Tajikistan 5/30/1998 6.6 4,000

Colombia Colombia 1/25/1999 6.1 1,185Hindu Kush Afghanisan 3/25/2002 6.1 1,000N Algeria Algeria 5/21/2003 6.8 2,266Bam Iran 12/26/2003 6.6 31,000Indonesia Indonesia 5/26/2006 6.3 5,749

*Total damage: US$16.2 billion>annual budget of most African countries!

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Some Statistics M < 6 since 1900 that caused deaths > 1000

Event Country Date M DeathsGole Turkey 5/28/1903 5.8 1,000Zangezur Armenia 4/27/1931 5.7 2800*Ustukran Turkey 5/31/1946 5.9 1,300Cosiguina Nicaragua 8/2/1951 5.8 1,000Agadir Morocco 2/29/1960 5.7 10,000†Yanjiang China 7/25/1969 5.9 3,000El Salvador

El Salvador 10/10/1986 5.5 1000**

Hindu KushAfghanistan 2/4/1998 5.9 2,323

* 57 villages destroyed; 46 villages damaged† compare it with the M=8.1 Mongolian earthquake that killed only a few** 10,000 injured and 200,000 made homeless due to this moderate earthquake

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Some Statistics Earthquake statistics of the USA since 2000

Magnitude

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011Aver.

8.0 to 9.9

0 0 0 0 0 0 0 0 0 0 0 0

07.0 to 7.9

0 1 1 2 0 1 0 1 0 0 1 0

0.66.0 to 6.9

6 5 4 7 2 4 7 9 9 4 8 0

55.0 to 5.9

63 41 63 54 25 47 51 72 85 58 71 17

54<5 2273 2214 3808 2885 3523 3633 2725 2711 3524 4200 8415 1622 3461 Total

2342 2261 3876 2946 3550 3685 2783 2791 3618 4262 8495 16393520

Estimated

0 0 0 2 0 0 0 0 0 0 0 0*

Deaths* The practically zero number of deaths is quite instructive (compare with previous tables)


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Introduction The intensity of EQ shaking is best measured

using seismographs

First seismographs installed in 1932 in California after the 1925 Santa Barbara, CA, EQ

The first significant ground motion record was made in 1933, Long Beach, CA

Basic scientific works on strong-motion EQs started at around the mid 20th Century

By 2000, about 1000 seismographs have been installed in California alone


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Causes of Earthquakes An Earthquake is mainly caused by a sudden movement of adjoining plates of the earth’s crust due to breakage of a strained crustal block in the interface Focus (hypocenter): The point, at which rupture began

Epicenter: The point on the surface directly above the focus


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What makes the plates move?

Strains in blocks are accumulated as the blocks tend to hinder, for a long time, the push and pull of the floating plates due to the movement of the viscous mantle


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Causes of Earthquakes The viscous mantle is in a constant motion; as a

result, the plates afloat the mantle are forced to move in various ways: some sink under another, some move apart, some collide, some move past each other


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A schematic representation of plate interactions


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Early Hypothesis: About 300 million years ago, there was a single land called Pangea (= “all the earth” in Greek)


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parts of Pangea start moving away from each other as evidenced partly by fossils along coastal lines and partly the general coastal geology (Alfred Wegner’s theory of continental drift: 1915)

Note:Wegner froze to death at the age of 50 in Greenland during one of his several expeditions To collect evidences


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More evidence: tests on rock samples after 1950 showed crustal age increases from ocean center towards the continents

More Evidence: Global seismicity established from location of EQ epicenters


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The theory of Plate Tectonics took shape: The earth’s surface is a mosaic of 20+ thin plates under constant motion

Plate motion

Subduction zones

Collision zone


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FaultsMovement of rock bodies along a fracture accompanied by structural offsets are called faults

Length: from few meters to many kilometers long

Presence of faults in an area means past movement

Not all faults show traces on the surface

Most faults on maps are now inactive; in contrast, new faults are created during EQs


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A Typical Map of Regional Faults: Eastern Africa


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Fault Types

Strike slipDip

Reverse Fault

StrikeN

Normal fault


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Fault Examples: Strike-slip FaultAbout 3.9 m fence offset due to San Francisco 1906 EQ


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Fault Examples: Normal fault

Dixie Valley–Fairview Peaks, Nevada EarthquakeDecember 16, 1964


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Fault Examples: Reverse/Thrust faults


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Total Slip in the M7.3 Landers Earthquake

Rupture on a Fault:

The source spreads in a big volume of rock


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Dept

hInto theeart

h

Surface of the earth

Distance along the fault plane 100 km (60 miles)

Slip on an earthquake fault: Landers EQ

START

Focus

Epicenter


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Slip on an earthquake faultSecond 2.0


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Basic DefinitionsThe imaginary point from which the first waves start is called focus or hypocenter.

The projection on the ground surface is called epicenter.

About 75% of the total EQ energy is released from shallow-foci EQ (<70 km); they wreak the most devastation

Most EQ are accompanied by numerous smaller aftershocks and foreshocks


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Seismic WavesDuring EQs, seismic waves radiate from a source as opposite sides of a slipping fault rebound in opposite directions

Body waves: P- and S- waves (primary and secondary waves)

Surface Waves Rayleigh waves: surface waves with movement like rolling ocean waves; move slower than body waves

Love waves: surface waves similar to S-waves, but without vertical movement; faster than Rayleigh waves, but slower than body waves


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Seismic waves have unique features:

Body Waves


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Seismic Waves …

Surface Waves

Waves are recorded on the ground surface using seismographs in form of ground acceleration

The records, known as seismograms or accelerograms, are of immediate use to locate epicenters and determine the magnitude


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Recording incoming seismic wavesRecording incoming seismic waves

Courtesy USGS

The S-P interval on a seismogram and the known wave velocities are used to locate the epicenter of an earthquake event

P-waves arrive first and S-waves follow


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Epicenter LocationEpicenter Location

Courtesy USGS

1.5 2 1.75P

S

v tov

Example: An earthquake in Japan; 3 records obtained


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Epicenter Location:Epicenter Location:ExampleExample

Courtesy USGS

Station

Pusan Tokyo Akita

S-P int. (s)

56 44 71

Example: An earthquake in Japan


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Epicenter Location:Epicenter Location:Example…Example…

Courtesy USGS

Station

Pusan Tokyo Akita

S-P int. (s)

56 44 71

Dist. (km)

542 428 691

Example: An earthquake in Japan

Epicenter: KOBE!


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Epicenter Location:Epicenter Location:Example…Example…

Courtesy USGS

Station

Pusan Tokyo Akita

S-P int. (s)

56 44 71

Dist. (km)

542 428 691

The magnitude on the Richter scale is determined from the maximum amplitude and the distance to epicenter


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Richter Magnitude:Richter Magnitude:DeterminationDetermination

Courtesy USGS

The Richter magnitude is defined below and can be represented in a form of a monograph as shown

Dashed line: standard earthquake of magnitude 3

Blue lines: M of 4 and 5

A difference of unity in magnitude corresponds to tenfold difference in amplitude and 32 fold of energy release10/10/22 Asrat Worku, Associate professor,

AAU48


10 max maxlog ( ); ( )LM A D A m


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Courtesy USGS

Pusan: distance= 542 km amplitude= 90 mm

Tokyo: distance= 428 km amplitude= 208 mm


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Courtesy USGS

Akita: distance= 691 km amplitude= 30 mm


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Courtesy USGS

Kobe EQM=6.8


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EQ Magnitude – Other Definitions

Surface-wave magnitude, MS: suitable for distant EQs (>2000 km).

Body-wave magnitude, mb: convenient for deep-foci EQs dominated by body waves;

Moment magnitude, MW: introduced to alleviate the drawbacks of ML, MS, and mb in distinguishing between great EQs (saturation problem) and to effect uniformity;

it is based on seismic moment directly at work on the ruptured surface instead of on amplitudes of specific waves; Thus,

Where M0 is the seismic moment in dyn-cm

This measure is specially suited to M>7.5

7.105.1log 0 MMW


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Another Measure of EQ: Intensity

Intensity is the oldest yardstick

Based on damage to structures, ground surface, and human reactions

Can be assigned to historical EQs: useful in areas of data scarcity

Highly subjective; need updating


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EQ Intensity

1st scale developed by Rossi (Italy) and Forel (Switzerland) in 1880

A refined, 12-degree scale was devised in 1902 by Mercalli

A version is later modified by Wood and Neumann to fit CA conditions

Isoseismal map can be prepared for a given EQ using the numerical rates


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EQ Intensity

EQ Intensity: scales compared

Mercalli (US) Rossi-Forel (oldest) Omori (Japan) European

I I 0 I II I–II I II III III II III IV IV–V II–III IV V V–VI III V VI VI–VII IV VI VII VIII- IV–V VII VIII VIII+ to

IX- V VIII

IX IX+ V–VI IX X X VI X XI — VII XI XII — — XII</TD<

TR> 10/10/22 Asrat Worku, Associate professor, AAU

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http://geology.about.com/library/bl/blmercalli.htm

http://geology.about.com/library/bl/blomori.htm

http://geology.about.com/library/bl/blems.htm


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Isoseismal Map: Typical

Global Seismicity

• By plotting epicenters on a map, the seismicity of any region can be studied including that of the world

• Clear belts of EQ swarms of the world are identifiable

• 95% of all EQs occur on the plate boundaries

• Active areas include: western Americas, South Europe, South and East Asia


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Global Seismicity


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Seismicity of Africa• Our region is not seismically inactive!

•The Great Rift Valley (GRV) exhibits constant extensional movement

•The Nubian and Somalian plates are emerging out of the formerly known African plate

•M >7 EQs recorded recently in the region

Asrat Worku, Associate professor, AAU

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Tectonics of Eastern Africa


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• The GRV starts from the Ethiopian Afar Depression, where it forms a triple junction with the axes of spreading along the Red Sea and Gulf of Aden.

• It crosses Ethiopia in the NE-SW direction.

• It bifurcates at around Lake Turkana; the Eastern flank crosses western side of Kenya and Tanzania in a N-S direction.



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•Spreading processes have already torn Saudi Arabia away forming the Red Sea.• •A new spreading center may be developing under Africa along the East African Rift Zone to eventually form another sea.



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•Some of the recent activities in the GEARV

•Appearance of anew system of fissures in Afar in 2005 attracted attention from the world over



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• Developments around the active Afar triple junction zone

•The Somalian plate and the Danakil block are moving away from the stable Nubian plate

•Red shades show 50 km long magmatic segments;



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• The 2005 fissure is due to a dyke of 60 km length with an opening of up to 8 m width; it stopped just a short distance away from Semera – the capital of Afar Region!



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•Some of the activities in the EARV

•A volcanic vent associated with the fissure development (compare depth and width with the people on the right edge)



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•Some of the activities in the EARV

•An older fissure near Fant ‘Ale volcano is being filled by water; the lake is increasing in size at a significant rate since recently

Fantale volcanoFissure formation



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•The fissure formation is accompanied by new volcanic activities

Fresh lava flow from the fissures (smoke at location of fissure)


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•Ert ‘Ale is a constantly bubbling active ‘volcanic furnace’ in this region



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•Ert ‘Ale with a nearly perfect geometry



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•Thousands of kilometers south, another ‘weird’ active volcano is found in Tanzania – Doinyo Lengai (Mountain of God) in Massai language

Major EQs in EA from 1989-2010


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1.August 1989; Dobi (Afar), Ethiopia; M=6.32.May 19, 1990; Sudan (South) in Mongola area: 100 km NE of Juba; M=7.2, D=shallow;.

3.Dec 5, 2005; DRC-Tanzania boarder: 55 km SE of Kalemie; M=6.8, D=13 km;

4.Feb 23, 2006; Mozambique: 215 km SE of Beira; M=7.4(!), D=11 km;

5.February 3, 2008; DRC/Rwanda EQ, M=5.96.Dec. 2009 to Feb. 2010. Karonga, Malawi: Swarm of earthquakes (more than 30) between Main EQ on 19 December; M=6.2, D=11 km; 7.

Some recent EQs in East Africa

DRC-Tanzania: 55 km SE of Kalemie; Dec 5, 2005; M=6.8, D=13 km; 2 deaths

Office workers in Nairobi panicked and rushed to safety from high buildings; cracked buildings in Nairobi reported!


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Source: BBC


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Mozambique: 215 km SE of Beira; Feb 23, 2006; M=7.4(!), D=11 km; 5 deaths

Source: USGS

The GSHPA for Mozambique region does not seem to have considered such huge events!

Mozambique: 215 km SE of Beira; Feb 23, 2006; M=7.4, D=11 km


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Andrzej Kijko, the head of the seismology department at the Council for Geoscience in Pretoria, South Africa, said: “seismologists expect events in the magnitude-7 range to occur about every 50 years in this area. In fact, an earthquake similar in size and location to Thursday's event occurred in 1940.”

This is a stern warning not well reflected by the GSHPA map (previous slide)


Source: Geotimes

http://www.waveofdestruction.org/photos/phuket/37884271_VA7U0477?full=1

DRC/Rwanda EQ, February 3, 2008; M=5.9


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Source: NASA

http://eoimages.gsfc.nasa.gov/images/imagerecords/8000/8458/drc_srtm_2008034_lrg.jpg

Some Recent East-African EQs


78Source: Columbia University

Karonga, Malawi: Swarm of earthquakes (more than 30) between Dec. 2009 to Feb. 2010. Main EQ on 19 December; M=6.2, D=11 km; 3 deaths

Some Recent East-African EQs

> 4000 people lost shelter Makeshift schools were necessary


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“While the world is fixated by the horror in Haiti little attention has been paid to at least 30 earthquakes that have rocked Malawi in the past month -- the largest measuring 6.2 on the Richter scale.”


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Source: Columbia University

Source: Mail & Guardian

World Seismic Hazard Map


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• A UN sponsored study in 1991 to 1999 (GSHAP) led to a very helpful global seismic hazard map shown below – the first of its kind

Seismic Hazard Map of Africa•The GSHAP map for Africa is as shown•Nations with 0.8<PGA<1.6m/s2 include Eritrea, Djibouti, Ethiopia, Kenya, South Sudan, Uganda, Rwanda, Burundi, DRC, Tanzania, Zambia, Malawi and Mozambique•Ethiopia, Eritrea Djibouti and Tanzania exhibit PGA>1.6m/s2; with properly recorded future events, more countries can be expected to join this group.


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Seismic Hazard Map of Kenya (GSHAP, 1999)

•As per the GSHAP, Kenya can exhibit a PGA up to 1.3 m/s2


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Seismic Hazard Map of Ethiopia in EBCS-8, 1995


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Return Period=100 Yrs

Seismic Hazard Map of Ethiopia (GSHAP, 1999)•Ethiopia can exhibit PGA up to 2.4 m/s2

•The Capital Addis Ababa itself, several federal capitals and major towns can experience PGA up to 1.6 m/s2

•The capital of the Afar Regional State, Semera, is in the heart of the most seismic region with PGA reaching up to 2.4 m/s2

(Not to forget the recent dams of Tendaho and


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Earthquake damage mechanismsEQ can directly affect life in various ways such as throughGround shaking: serious and widespreadChange in physical properties of foundation soils: settlement, liquefaction

Direct fault displacement Landslides and other surface movementsEQ induced firesWater waves: tsunamis and seiches; they can be devastating (E.g.: 2004 Indonesia; 2011 Japan)


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Damages due to ground shaking

Northridge, CA, 1994


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…ground shaking

Loma Prieta, CA, 1989


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…ground shaking

Kobe, Japan, 1995


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…Ground shaking

Kobe, Japan, 1995


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Damages due to soil liquefaction

Nigata, Japan, 1964


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Damages due to direct fault displacement

Landers, CA, 1992


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Damages due to Landslide

Turnnagin Heights, Alaska, 1964


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Damages due to associated fire

Loma Prieta, CA 1989


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Earthquake effects: Tsunamis

2004 Indonesian Tsunami



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Earthquake effects: TsunamisJapan Tsunami

http://www.youtube.com/watch?v=w3AdFjklR50

Codes of Practice Seismic provisions in the rest of the world

are revised every 3 to 5 years: e.g. American, European, Japanese, Chinese, Indian, etc.

A few local codes have seismic provisions in the seismic region of E. AfricaKenyan (1973)Ethiopian (1978, 1983, 1995)Others ?

Significant growth of knowledge exists Urban settlement growing fast To work with old codes is unacceptable Revision is mandatory


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Codes of Practice


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“Vice-Chairperson of the East and Southern Africa Region Seismological Working Group Gladys Kianji, cites the 1928 Subukia quake as the ‘most significant in Kenya’. The magnitude 6.8-quake is said to have rolled cars off the roads in Nairobi, about 200km away.”

“Chair of the Architects Chapter of the Architectural Association of Kenya, Waweru Gathecha, says buildings are designed in accordance with a whole range of standards including those on structural design. "Kenya falls within an earthquake zone, for which there exists an Earthquake Design Code, which has existed since the early 1970s," he says.”

The Standard (13/03/2010)


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THE END

MANY THANKS

Earthquake hazard in East Africa

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