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EAST AFRICA RIFT SYSTEM, SEISMIC ACTIVITY,

GROUND DEFORMATION AND TSUNAMI HAZARD

ASSESSMENT IN KENYA COAST.

By

Odhiambo Amollo Joseph

Kenya Meteorological Department

P.O Box 98512 (80100)

Mombasa- Kenya

E-mail: [email protected]

Kenya is crossed by the East African Rift System at the central region. The East AfricanRift is seismically and volcanically active and this has resulted to ground deformation

within the Rift System. The western branch of the EARS has undergone some subsidencedue to greater seismic activities while the eastern branch has undergone some form of uplift

as a result of greater volcanic activities which has led to the formation of deeper lakes andhigh mountains on the Western and Eastern branch respectively. Tsunami hazard

assessment has shown that the Kenyan coast is vulnerable to tsunamis that are generated inIndian/pacific oceans. Near/far field tsunami simulations have shown that the region can

experience tsunami wave heights of up to 2 m high and inundation extent is greatest in theunprotected areas and can reach about 500 m inland. This study aims at understanding the

rifting and deformation processes within the EARS due to seismic and magmatic activitiesand also to map out and delineate Kenya coastal tsunami impact areas in order to put up

defensive measures to the disasters that might be caused by the possible Earthquakes andTsunamis in this region.

1.0. INTRODUCTION

Extensional regions are commonly associatedwith low level of seismic strain release and

earthquakes of relatively small magnitude (Scholz,

2002). In a synoptic assessment of global rift-related seismicity and magmatism, Parsons and

Thompson (1991) showed that magmatic riftzones in particular are characterized by low levels

of seismicity. They explained the phenomenon ofsuppressed faulting by stress equalization due to a

buildup of magma pressure counteracting tectonic

stress. In such a scenario, rift extension is thoughtto be accomplished by a combination of magmatic

dyke intrusion and active normal faulting,

accompanied by small but frequent earthquakes.In the East Africa Rift, differences in seismicity

between magmatically active versus inactive areasare well documented (Maguire et al., 1988). The

Kenya rift in the eastern branch of the rift system

is one of the most volumetrically importantmagmatic extensional areas on Earth (Latin et al.,

1993; MacDonald et al., 1994). Kenya rift ischaracterized by basaltic and rhyolitic volcanism

and pronounced hydrothermal activity along the

tectonically active rift axis attesting to thesignificant thermal deformation of physical

properties of the crust (MacDonald et al., 1994).


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Fairhead and Girdler(1971) suggested thatthermal overprinting by upwarped

asthenosphere lowers the crustal tensilestrength in this region, thus decreasing the

level of seismically released strain. Thinning

of the crust also reduces the potential down-dip rupture width, therefore reducing theseismogenic potential of any fault. In contrast

with the magmatically active Kenya rift,higher seismicity levels with historical

magnitudes as large as M 7.4 characterize thevirtually less volcanic western branch of the

East African Rift and the Tanzania rift zone(Shudofsky, 1985, Ambraseys 1991; parsons

and Thompson 1991 and Girdler andMcConnel 1994. Amongst these are the recent

M 6.8 Kalemie earthquake (2005), M 7.0Machaze earthquake (2006), and M 6.0

Cyangugu earthquake (2008). The Kenyan riftsystem can best be considered in two parts,

the division being at the Kenya-Tanzania

border approximately north and south of 2S.

In central and southern Kenya, the Rift Valleyis about 60 km wide and about 1 km deep.

The rift walls are normal faults with many

offsets. Within the Kenya rift, there is somemicroseismicity associated with the rift floor.The lack of large magnitude events and the

presence of microseismic activity along therift may suggest that stress release is at a

lower level and it is also possible that thestrain may be slowly accumulating along this

part of the rift. In contrast to the Kenya rift,there have been many earthquakes associated

with the rifting in Northern Tanzania wherethe rift fans out into a series of east facing

fault scarps. The high level of seismicactivities in the northern Tanzania is related to

the fault and is concentrated along the mainfault structure.

Figure1a, b: Seismicity of the East Africa Rift System

1.1. TECTONICS OF THE EAST

AFRICAN RIFT SYSTEM.

Major tectonic features in the eastern andsouthern African region are mainly

controlled by the well-known geologicalstructure, the East African rift system. It

extends almost 3,000 miles (4,830kilometers) from northern Syria down the

Eastern side of the African Continent tocentral Mozambique. Further to the South a

series of rifts occur which include a Westernbranch, the Lake Albert Rift or AlbertineRift which contains the East African Great

Lakes, and an Eastern branch that roughly

bisects Kenya north-to-south on a line slightlywest of Nairobi.


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The two EAR branches are often grouped withthe Ethiopian Rift to form the East Africa RiftSystem (EARS). In addition there are several

well-defined but definitely smaller structures,called grabens that have rift-like character andare clearly associated geologically with

the major rifts. Some of these have been givennames such as the Nyanza (Kavirondo) Rift inWestern Kenya near Lake Victoria. Most of

earthquakes occur on either the western or

eastern branch of the East Africa Rift and themechanisms are all normal or strike-slip.

Fig. 2:The location and orientation of the Eastern & western branch of EARS and the focal mechanisms.

South of Ethiopia, the East Africa rift system

breaks up into two branches, the Western riftand the Eastern rift. Continental rifting startsfrom the Afar triple junction and continuestowards the south through the Ethiopian rift,

joining into the Gregory rift in Kenya.

This structure constitutes the eastern branch ofthe East Africa rift system. Further south, it

branches into the Davie Ridge (Mougenot et al.,1986). The northern sector of this rift segment

cuts across the abyssal plateau volcanic ofKenya. In eastern Tanzania, the rift structures

form a broad zone of faults defining a series of

tilted blocks with varying orientations.

Southeast of Mount Kilimanjaro, the Pare-Usambara faults define a branch of the eastern

rift which trends SE to join the fault systems of

the Davie Ridge in the Indian Ocean. Thewestern branch of the East Africa rift systemextends from northern Uganda to southern

Mozambique, encompassing the major lakes in

the region such as the lakes Albert, Edward,Tanganyika and Malawi. In the south, the main

features of the rift in the Malawi-Mozambiquesegment are border faults defining Lake

Malawi.

1.2. MECHANISM OF THE EAST

AFRICA RIFT FORMATIONOne popular model for the EARS assumes thatelevated heat flow from the mantle (strictly the

asthenosphere) is causing a pair of thermal

bulges in central Kenya and the Afar regionof north-central Ethiopia. These bulges can be

easily seen as elevated highlands on any


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topographic map of the area. As these bulgesform, they stretch and fracture the outer brittlecrust into a series of normal faults forming the

classic horst and graben structure of RiftValley.

Figure 3: Map of Africa showing in blue levels the elevations higher than 1200 m, evidence of the main

Ethiopian and KenyanTanzanian domes ( J. Chorowicz / Journal of African Earth Sciences 43 (2005) 379

410).And the formation of horst and grabens in the rift valley.

2.0. EVALUATION OF SEISMIC RISKS

The seismic activities in East Africa rift and theKenya rift are concentrated along the Eastern andWestern branch of the East Africa Rift System.

Seismicity of the Kenya rift valley of the EastAfricas areas of active tectonics has beendominated by earthquakes of low to intermediatemagnitudes originating along the two branches of

the rift system. Past studies have revealed that the

region has a relatively high activities and a stablerecurrence relation of log (N) = a b (Ms), where

N is the number of events of magnitude equal toor greater than Ms per annum. Kenya rift system isthus capable of producing a regular pattern of

earthquakes including an event of magnitude 6.9

associated with surface faulting in January 6, 1928

in Subukia. There is also evidence that smallerpotentially damaging earthquakes have occurred

in the past years.

Fig. 4: Subukia earthquake location & seismicity

distribution along the Kenya rift Valley and Nyanza

trough (1910-1990).


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The seismicity of the EARS was analyzedusing data from various catalogues which

included: USGS for the period 1973 to

2009, ISS/ISC and Seis-PC data for thehistorical period from 1900 to 2006.Seismic activity was greatest during the

period after 1960. There occurredsignificant seismic activity in the first half

of the twentieth century in both theWestern and Eastern branch of EARS(Gutenburg and Ritcher, 1954).

Much of the damage in earthquakes can beattributed to the behavior of the soils

during earthquakes. Large settlements and

differential settlements caused bycompaction of loose soil, settlement andtilting of structures due to liquefaction of

saturated granular soil, lateral movementsof natural slopes have been observed

during earthquakes. All these types ofbehavior are influenced by the intensity of

earthquake shaking. Thus a determinationof seismic risk for a particular facility must

include an evaluation of the earthquakeground motions that are likely to be

induced by future earthquakes at the site.The Subukia 1928 earthquake didnt cause

extensive damage due to the sparsesettlement, the mud structures were also

anti-earthquake and also during this event,

the water table was too low to causeliquefaction. In order to carry out seismicassessment in Kenya, the University ofNairobi has had a seismological networksince 1963 as part of the World Wide

Standardized Seismic Network (WWSSN).Later in the 1995 five digital mobilestations were installed in Kenya. Recently

the International Monitoring System (IMS)has installed a primary seismic station

(PS24) in Kilimambogo, Kenya and the

construction of an infrasound station (IS32)is underway to monitor Earthquakes in EastAfrica as well as major events world-wide.

To monitor the local seismicity, UoNinstalled Seismic stations in various parts

of the country in collaboration withUniversity of Karlsruhe (Germany, GTZ).

Kenya Meteorological Department (KMD)had never operated any Real-time Seismic

Station to monitor earthquakes, Tsunamis,Volcano eruptions prior to the 26

December 2004 Tsunami.

An important first step in a seismic hazard

evaluation is the compilation and documentation

of the historical seismicity record of the region.

As more instruments are placed in more areas of

the East African Rift System, the usefulness of

such record will continue to increase in future

seismic hazard evaluations.

Majorhistorical seismic activity in EARS(1906-1960)


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Fig. 5 b &c: Showing the major seismic activities in the western and eastern branches of the East

African Rift System since 1900 respectively. Western branch is more seismically active.

2.1. B-VALUE ESTIMATION

The b-value of the recurrence relation for

Kenya Rift has been derived by severalScientists and their values have been in the

range of: (Ndontoni 1976; b =0.81), Shah(1986; b =1.29), and other values given by

Fairhead & Stuart (1982; 0.80 b 1.35).The seismicity of a seismogenic zone isquantified in terms of the frequency-

magnitude relationship. This relationship isa key element in estimating the probability

that a magnitude M earthquake will occurin a certain seismogenic zone within a

predefined time interval. Fortunately inKenya there are many small earthquakes

than large ones. The figures below showthe frequency of earthquakes as a function

of their magnitudes for Kenya and Easternand South Africa region during the period

1973-2009. The distribution may be fittedwith log N = a b (Ms) where N is the

number of earthquakes whose magnitude isgreater than Ms.

Figure 6a & b: b- Value estimate for the seismic activities in the East Africa Rift Systems.

3.5 4 4.5 5 5.5 6 6.5

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

f(x) =-0.79x +5.12

b - Value KENYA 1973 - 2009

MAGNITUDE

LOGN

4 4.5 5 5.5 6 6.5 7 7.5

0

0.5

1

1.5

2

2.5

3

3.5

f(x) =-1.05x +7.68

EAST & SOUTHERN AFRICA B-VALUE 1973-2009 (MAG 4.5-7.2

EARTHQUAKE MAGNITUDE

LOG

N


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While the a-value is a measure of

earthquake productivity, the b- value is

indicative of the ratio between large and

small earthquakes. Both a, andb-value aretherefore important parameters in

earthquake hazard analysis. Usually b is

close to unity. In calculating the b value

of the recurrence period for the Kenya and

East Africa rift systems earthquakes, a

value ofb = 0.79 and 1.05 were obtained.

The result for Kenya signifies that there are

many smaller earthquakes than larger ones

and this make the ratio to be less than unity.

In the entire Eastern and Southern region,

the distribution of the small and large

magnitude earthquakes is almost equal andthe ratio is slightly above unity.

Smaller and intermediate earthquakes will

still dominate the Kenyan rift region and

their frequencies and damage potential will

contribute to the deformation which has

been majorly as a result of the infrequent

and irregular pattern of large earthquakes.

30. THE MOVEMENT OF THE EAST

AFRICA RIFT SYSTEM

Although the East African Rift (EAR) is

often cited as a modern archetype forrifting and continental breakup, it remains

the least understood of all major plateboundaries. In particular, the rate of plate

divergence across it, how this divergence isaccommodated within the rift, and how the

rift connects farther south with the

Southwest Indian Ridge (SWIR) remain tobe determined.

The study involved the use of the direction ofearthquake slip vectors from the focal

mechanisms determined from body- wave form

and based on the structural frame work of theepicentral region. The earthquake slip vectors

along the Ethiopian rift were assigned to theNubian-Somalia plate boundary while the slipvectors along Eastern and Western branches

were assigned to Nubian-Victoria and Victoria-

Somalia respectively.

Figures 7a, b: The Earthquake slip vectors and relative motions along plate or block boundaries are shown in red arrowsand numbers are model velocities in mm.y-1.Relative rotation poles are shown with black stars. The first plate rotatescounter-clockwise with respect to the second except for VI-NU where Victoria rotates clockwise with respect to Nubia (D.Stamps et al., 2008.)


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Information needed to solve this problem issparse, but Stamps et al. found that

geological data covering the past 3.2

million years along the SWIR (South WestIndian Ridge) are consistent with currentgeodetic data in East Africa.

The first complete kinematic model for the

EAR show that the data are consistent withthe existence of three sub plates embedded

within the rift, Victoria, Rovuma andLwandle. The plate angular velocity found

in the model, predicts opening at the rate of1 to 4 mm. y

-1across the Western and

Eastern rifts, increasing from North to

South for the Western rift and from Southto North for the Eastern rift. This correlateswith the age of rifting initiation (from 12-

15 Ma to 8 Ma from North to south alongthe Western rift and 5 Ma to present for the

Eastern rift (Ebinger, 1989), Consistentwith a propagation process. The southward

decrease of the extension rate along theEastern branch is consistent with the

progressive disappearance of prominent

active faults as the Eastern branchpropagates into cold cratonic domains(stable/rigid and immobile part of the

continent having survived cycles ofmerging and rifting). The above model

predicts a very small motion rate (less than0.1 mm. y

-1) at the Victoria and Rovuma

boundary (a result of their opposite sense

of rotation with respect to Nubia), (Le Gallet al., 2004). The model predicts extension

across Malawi rift rates as decreasing from

4.5 mm. y

-1

in the north to 1.5 mm. y

-1

atthe Latitude of southern Mozambiquecoastal plain. Predicted motions along EAR

structures are quantitatively consistent withseismicity and active faulting with

extension directions approximately EW butvarying spatially as a function of the plates

involved.

4.0. VOLCANIC ACTIVITIES

Volcanism occurs in continental areas that are

undergoing episodes of extensionaldeformation. A classic example is the East

African Rift Valley, where the African plate isbeing split. The extensional deformation

occurs because the underlying mantle is risingfrom below and stretching the overlying

continental crust. Upwelling mantle may meltto produce magmas, which then rise to the

surface, often along normal faults produced by

the extensional deformation. Basaltic andrhyolitic volcanism is common in the Kenya

rift areas. Rift volcanoes attest to the presence

of magma reservoirs within the crust and themagma chemistry provides insights into

magma sources and storage depths.Petrologic studies in East African Rift

System documents the importance ofmagma chamber recharge, magma mixing

and volatile transport that accompany

dyking and eruption events (Espejel-

Garcia et el., 2008; Macdonald et el.,2008). Basaltic dyke intrusions reheats or

recharge existing magma chambers lyingnear or within the paths of dykes provoking

earthquakes, eruptions and or degassing(Wright et al., 2006; Baer et al., 2008;

Keir., 2009). The passage of magmathrough the crust is often marked by low

magnitude earthquakes just like the case ofEARS and is rarely detected on global

arrays (Rubin and Gillard, 1998, Keir et al.,

2009). Radar interferometric measurementsof surface displacement provide a versatilemeans to evaluate spatial and temporal

magma recharge. Interferometric syntheticaperture radar (InSAR) is a satellite based

method used to compare the phase of radarimages taken at different times to detect


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small (


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Fig 8a: Eastern branch is more magmatically

active than western branch.

Figure 8b: Distributions of Volcanoes in

Kenya Rift System

Inflation of deeper magmatic sources cause dilatation of overlying rocks, opening new and selfsealed fractures, increasing permeability, and decreasing pore pressure (Wicks et al., 1998; Chang

et al., 2007). Thus short duration episodes of deformation such as those reported above are likely to

be driven by volume changes in the magmatic system and amplified by the response of shallower

hydrothermal system.

Figure 9: The uplift and subsidence of some volcanoes in East Africa Rift System due to the active MagmaSystems in the East African Rift(Juliet Biggs 2008).


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Although seismicity levels in the Easternrift are comparatively low, multiple ground

rupture earthquakes have occurred in the

region (Zielk and Strecker, 2009). Volcanohazard analyses require deformation,seismicity and gas monitoring both on and

off volcanic edifices and determination ofhow pulses of activities are distributed intime. Seismicity and gas surveys can

distinguish between magmatic andhydrothermal induced deformation andcontinuous monitoring is required to constrain

the duration of individual episodes. The othervolcanoes of Kenya rift that show no

geodetic activity may simply be betweenepisodes: along period of observation is

required to judge their level of activity(Baer et al., 2008; N. d Oreye, 2009).

4.2. OBSERVATIONS REPORTED FROM THE VOLCANO MODELS.

The depth of deformation sources at Suswa,

Paka and Longonot (2- 4.5 km) is close tothe boundary between magmatic and

hydrothermal systems. Seismicity occurs in

swarms, and tremor and low frequencyevents indicate the attenuating effects ofone or more fluid phases. Longonot and

Suswa volcanoes show narrow negativeanomalies superposed on broader positive

anomalies corresponding to the edificesconsistent with shallow low density

chamber. The observations reported in thisstudy demonstrate the presence of active

magmatic systems beneath Suswa,Longonot, Menengai and Paka volcanoes.

In many settings, volcanic uplift isunambiguously associated with magma

intrusion in the shallow crust. However

recent observations at calderas like LongValley, Yellowstone and CampiFlegrei,

document subsidence not associated witheruption and CO2 emission levels too high

to be explained by magmatic pulses.Surface displacements may be the result of

the migration of hydrothermal rather thanmagmatic fluids (Wicks et al., 1998; Chang

et al., 2007; Hutnak et al., 2009).Differentiation between magmatic

intrusion and thermal expansion orpressurization of a caldera is clearly

fundamental to the development ofpredictive models for volcanic eruptions

and in evaluation of seismic and volcanic

risks (Battaglia et al., 2002). This may nothave been the case of the 1914 eruption atSakurajima in Japan where the prediction

might not have been based on models.

5.0. TSUNAMI HAZARD ASSESSMENT IN KENYA COAST.

5.1 NUMERICAL SIMULATION

The extent of Kenya coastline is about 600 kmand allows the country to play a major role intransport and communication along the East &

southeast African continent due to the natural

harbors along the coast zone. The location ofthe Kenya coastline in relation to Indian Ocean

makes the region vulnerable to tsunamis thatare generated within the Indian Ocean and

Pacific Ocean. This vulnerability was realizedduring the December 2004 tsunami which

caused destructions to the coastal infrastructure

and loss to human life. The record at Lamuport which was the first location at the Kenyancoast to be hit by tsunami wave showed a run

up of about 0.4 meters. This run up height was

as a result of the reduced wave energy as itpassed Seychelles Island. Archipelagos such asSeychelles or Mauritius significantly interact

with tsunami by reducing its energy and

consequently the wave height. Tsunamishaving passed such natural obstacles as Islandsare characterized by weak force and therefore


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its impact on the coast is slight or in somecases the wave completely disappears. Thiswas evident in the eastern coast of Africa (at

Lamu and Zanzibar) where the 2004 tsunami

wave completely stopped. The tsunami waveswere higher in more distant ports where the

waves had easier access from the open ocean.For example Port Elizabeth noted +2.73m rise,

East London +1.35m, Cape Town +0.96m ,

even Halifax in the North Atlantic recorded a

noticeable change of 0.43m despite the fact thatit is situated further than Lamu port ( 21317kmcompared to Lamu which is 6128 km) from the

2004 tsunami source region (B.Wisniewski,P.Wolski 2008). It is necessary to carry out

tsunami hazard assessment in the coastal area

of Kenya to understand the risk that the coastal

region, population and infrastructure are likelyto face. The understanding is of great help indeveloping appropriate hazard mitigationstrategies. For this study, 3 tide gauges stations

(Wasini Island, Kilindini-Mombasa, and

Lamu) were used as output points of tsunamisimulation, along the coast of Kenya for the2004 Sumatra source region.

Figures 10a, b: Some of the tide stations in triangles used as tsunami output points, Wasini tide station which

measures all marine parameters and transmit data to Meteorological headquarters.

5.2. SIMULATION RESULTS OF SUMATRA EARTHQUAKE MAGNITUDE 9.1

The computation region is from 36E to 100E

and from -20S to 25N with grid points of1921 and 1351 along the longitude and latitude

respectively. The integration time step t is 5.0s while computation time is 10 hours, the

number of time steps is 7200. The resultsobtained from the modelling shows that the

Northern coastal region of Kenya could be at a

higher risk than the Southern most parts. Thesoftware used in this simulation was developed

by Nakamura of Ryuku University. For thedisplay of the modelling results, Tsunamidisplay program developed byDoung was used.


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The results from the simulation show that thearea around Wasini Island realized the lowestwave height of 0.24 m while the observations

at Mombasa and Lamu were 0.5 m and 0.92 m

respectively. The Somalia coast realized higherwave height of up to around 5m.The results

realized in this modeling of Sumatra sourceregion could be attributed to the effect of

Islands and archipelagos on the path of the

wave as they travelled to impact points and the

long distance between the source region andEast African coast. The Seychelles Island

reduced the wave energy and also refracted thewaves to the northern part of the East African

coast towards Somalia. Most of the East Africa

coastal regions are likely to be relatively safe

from future tsunamis generated from far fieldsource regions due to energy dissipation as thewaves travel the long distance. But largerearthquakes are possible in these tectonically

active regions in the Indian and Pacific Oceans.

These could lead to a magnification of run-upheights as the waves strike the East Africancoast.

Maximum tsunami wave height plotted against distance travelled across Indian Ocean for the 2004 Sumatra

source region. The heights of the waves decreased exponentially with distance (D. Obura 2006)

Sumatra 2004 Magnitude 9.1 Earthquake Fault parameters.

FAULT PARAMETERS DIMENSIONS/MEASUREMENTS

Length of Fault plane (km) 1100

Width of Fault plane (km) 175

Strike angle (degree 340

Dip angle (degree) 15

Slip 10

Maximum dislocation (m) 11Location 05N, 92.5E

Depth (km) 28.6

Moment 4.0*1022

NM

Table 1: Source USGShttp://earthquake.usgs.gov/earthquakes/eqarchives/sopar/& Nakamura fault parameters.
http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/


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The following figures (12-17) show the results of the Sumatra Earthquake source region

simulation:

Figure 12: Initial and final stages of Sumatra source region simulation.

Figure 13: Wave propagation from the Sumatra Source region Simulation.


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Figure 14: Bathymetric and topographic data of the simulation region.

Figure 15: Tsunami travel time map for Sumatra source region in time steps of 30 minutes and 1

hour.


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Figure 16: Wave heights at observation points (tide gauge stations) for the Sumatra source region.

Figure 17:Distribution of computed maximum tsunami heights along the East African coast.

5.3.CORRELATION BETWEEN INUNDATION AND RUN-UP

Inundation of the coastal region was determined through the analysis of the simulation results of the

Sumatra 2004 earthquake magnitude 9.1 as recorded in the following table 1. In order to arrive atthe results, the following formula from Dr. Nakamura was adopted;


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Xmax (inundation) = (Hmax)1.33

n-2

k where:Xmax is maximum inundation in meters

Hmax is maximum tsunami height at observation point

K is a constant (0.06 for most tsunami waves)n is a constant and different for various scenarios considered (0.015 for flat areas,

0.03 for areas dominated by buildings and 0.07 for forested areas ).

STATION LONGITUDE LATITUDE ARRIVALTIME(Min.)

HEIGHTINMETERS

INUNDATIONIN FLATAREA (M)

INUNDATIONHIGHGROUND (M)

WASINI 39.52 -4.75 560 0.24 39.96 9.99

MOMBASA 40.15 -3.59 550 0.52 111.75 27.94

LAMU 41.18 -2.15 500 0.92 238.67 59.67SOMALI A 45.41 1.98 480 1.12 310.05 77.51

SOMALI B 46.76 3.11 480 5.06 2304.03 576.01

Table 2: run-up height and inundation level.

The figures above show the direct relationshipbetween inundation extent and run-up level

along the Kenyan coast. Severe inundation and

run-up heights may affect the coastal

topography by transporting offshore sediments

on to the beach and redistributing the

sediments within the beach. Shuto (2001)

observed similar conditions along theJapanese coast where tsunamis have caused

severe erosion and have altered the coastal

topography in many locations.Narayana et

al. (2005) studied the nature of

sedimentation induced by tsunami along

0.0 500.0 1000.0 1500.0 2000.0 2500.0

0.00

1.00

2.00

3.00

4.00

5.00

6.00

INUNDATION AND RUN-UP HEIGHT

INUNDATION EXTENT (M)

RUN-UPHEIGHT(M)

0.00 1.00 2.00 3.00 4.00 5.00 6.00

0.0

500.0

1000.0

1500.0

2000.0

2500.0

RUN-UP HEIGHT AND INUNDATION

RUN-UP HEIGHT (M)

INUNDATION

LEVEL

(M)


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the Kerala coast of India and found that

tsunami surge had transported and

redistributed black sands (heavy minerals)

from the continental shelf to the coast.

It has therefore been demonstrated that

tsunamis have the potential to transport,

redistribute and deposit sediments far into

the inland areas. The low-lying coasts like

the Kenyan coast are the prime victims of

tsunami surge as they are unable to put up

any resistance against accelerating tsunami

waves. The results of a tsunami inundation

study include information about the maximum

wave height and maximum current speed as a

function of location, maximum inundation line,as well as time series of wave height at

different locations indicating wave arrival time.

This information can be used by emergency

managers and urban planners primarily to

establish evacuation routes and location of vital

infrastructure. In addition it can be used to

understand and delineate the impact of

tsunami on coastal areas.

7.0. Conclusions.The East African Rift System especially the

Eastern branch that bisect Kenya almost north-

south is dominated by smaller and intermediate

earthquakes which have contributed to the

ground deformation within the rift system. The

trend of the seismic activities in the region is

increasing and therefore requires a continuous

monitoring so as to be able to detect and assess

the impending dangers that this might cause to

the existing infrastructure and economicactivities within the region. This requires the

establishment of a dense network of seismic

stations to record the earthquake activities.

Research has also shown that the volcanoes

within the Rift System are active. Some of

these volcanoes are located near the cities

within and around the Rift System and in

the event of eruptions, the existing structures

can be destroyed by lava flow and volcanic ash.

There is therefore a need to monitor the activity

of Volcanoes such Menengai, Suswa, Paka,

and Longonot through InSAR to detect and

determine the temporal and spatial magma

migration or movement. Due to the

vulnerability of the coastal region to Tsunamis,

coastal protection through the construction of

sea walls and also vegetation conservation isnecessary in order to provide resistance to the

tsunami wave energy and also inundation

potential. Bathymetric data acquisition and

analysis through establishment of more tide

gauge stations along the Kenyan coast will

improve the ability to detect and issue timely

warnings of tsunamis.


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6.0. OUTLINE OF MY ACTION PLAN FOR THE PERIOD 2010 TO 2013

OVERALL GOAL: SEISMICI ACTIVITY MONITORING IN EAST AFRICA RIFT

SYSTEMAND TSUNAMI HAZARD ASSESSMENT IN KENYA COAST FOR

EARLY WARNING SYSTEM

OBJECTIVES ACTIONSREQUIRED

EXPECTEDRESULTS

RESPONSIBILITY

Real time seismicactivitymonitoring

Establishment ofseismic networksto monitorearthquakes

Dense seismicnetworkestablished, realtime data acquired& processed

-Government ofKenya.-Developmentpartners support.

Monitoring of thedeformations &Displacementswithin the EARS.

Establishment ofGPS networkalong the EARS

GPS data acquiredand displacement,& EARSmovementanalyzed

-GoK-Developmentpartners-J ICA

Real timeTsunamimonitoring

Installation of moretide gauges forspatial &temporalresolution dataacquisition

Real time Tsunamiinformation &dataavailable forprocessing

GoK (MEMR/KMD)-KEMFRI-Developmentpartners-J ICA

Designation ofEvacuationcenter and routesat the Kenyacoast

Mobilize coastalcommunity todetermine andmap out mostvulnerability areas

Evacuation centersestablished androutes designated-Tsunami drillscarried outregularly

-GoK (OP/SP andProvincialadministration-Local authority-Developmentpartners

Networking withtsunami

internationalcommunity

Participate inICG/IOTWS

activities inestablishingregional tsunamiWarning system.

Regional tsunamiearly warning

established

-KenyanGovernment (GoK)

-ICG/IOTWS.


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8.0. RECOMMENDATIONS:

In view of this study, the following are recommended:

The establishment of GPS stations along the East African Rift System in order to monitorthe displacement along the rift and also the rift movement velocity in relation to the plate

and sub-plate motionsFor major projects such as power plants, dams and offshore platforms, seismic hazard

evaluation should be carried out through detailed site and regional specific studies in order

to minimize the cost of property damage by future severe earthquakes

The enforcement of local building codes which contain a seismic zone map that includes

minimum required seismic design parameters which are intended to mitigate collapse of

buildings and loss of life.

The establishment of community based Early warning System along the coast to issue alerts

and information about earthquake generating tsunami and arrival time of the tsunami at the

coast.The establishment of Evacuation routes and Center in the coastal cities of Kenya especially

in Lamu area which has shown greatest risk of tsunami waves. Public awareness and

education which should incorporate tsunami drills should also be administered to the

vulnerable communities in order to equip them with relevant skill to save their lives in the

event of disasters.

The setting up of proper channels of communications like the FM radio stations at the

village level is necessary to relay information concerning the impending disasters like

tsunami as soon as they are detected.

9.0 ACKNOWLEDGEMENT

I would like to acknowledge the contributions of Professor F. Kimata in organizing the

relevant lectures, giving directions and guidance concerning the Research and this Action

Plan. And to all other Lecturers who shared to me the relevant knowledge and ideas during

the training I acknowledge your input. I acknowledge the role played by JICA as a sponsor

of this course and Kenyan government through the Director Kenya Meteorological

Department who nominated me for this particular training. I recognize and appreciate the

contributions of the JICA Program Officer Ms Imayoshi Moeko and JICE Training team

whose efforts ensured that the training and my stay here in Japan was successful. The

support, understanding and cooperation of my co-participants was highly appreciated and

acknowledged. Special thanks go to my family for their understanding, tolerance andencouragement during the period I was away for this particular training.


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10.0 REFERENCES

J. Biggs, E. Y. Anthony and C.J. Ebinger 1997-2008: Multiple inflation and deflation

events at Kenya Volcanoes-East Africa Rift

D. Stamps et al., 2008, Eric Calais: Kinematic Model for East Africa Rift

N. N. Ambraseys 1990: Earthquake Hazards in Kenya Rift

Y. Ishikawa: SEIS-PC for Windows95, ISS/ISC data catalogue (USGS), M. Nakamura.,

2006: Source Fault Model for Numerical Tsunami Simulations

Weiss P. and W.H.F. Smith, 1998: New improved version of the Generic Mapping Tools.

Chorowicz J, (2005): The Mechanisms of the East African Rift System formation-Journal

of African Earth Science 43, 379-410.

Aniruddha Sengupta: Evaluation of Seismic Risk in Engineering Practice-Chapter 29-

SP103.

Olaf Zielk and Manfred R. Strecker: Bulletin of the Seismological Society of America Vol.

99 No. 1, pp 61-70. Recurrence of large earthquakes in Magmatic Continental Rifts(Laikipia-Marmanent Fault, Subukia Valley, Kenya Rift)

B. Wisniewski and T. Wolski 2008: Threats to the safety of Navigation resulting fromTsunami.

ea seismicity

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