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Vol. 2 No. 2 December 2009 u 1

The Andaman - Sumatra Arc in the NE Indian Ocean is well known for its high seismic

hazard and tsunami potentiality. Seismicity is caused by eastward subduction of the

Indo-Australian plate along Andaman - Sumatra trench to about a depth of 500 km

below the NE Asian plate. The seismicity map with earthquake magnitude data (Ms/mb

≥ 4) for the time period 1906 – 2010 and its correlation to the crustal and mantle faults

for this plate margin is presented. The seismicity map shows visible earthquake clusters.

From the spatial perspective, the extents of these clusters are constrained by ‘near’ and

‘point density’ analyses. Seismic potentialities of the 11 numbers of clusters (source

zones) in terms of generation of maximum capable earthquakes are calculated throughempirical relationship between RL and magnitude (M) of individual clusters, and also by

Gutenberg-Richter (G-R) relationship. Again, these earthquake sizes are used to calculate

the maximum run-up height (ranging from 1.98 to 6.42 m) of tsunamis along the nearby

coastal tracts. To our understanding, the source zones A, B in offshore Sumatra and F,

J in south of Little Andaman and North Andaman are the most vulnerable areas with

future occurrence of earthquake magnitude greater that Mw 7.3 with a run-up height of



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Journal o South Asia Disaster Studies

tsunamis more than 6 meter. Further, the maximum magnitude earthquake of the entire

study area is calculated by Gumbels extreme value statistics for return period 400, 450

and 500 years. The return period (400-500 years) of giant palaeo-tsunami is comparable to the return period for tsunamogenic earthquake of magnitude Mw 9.2 and above. Key words : Earthquake source zone, Andaman-Sumatra subduction, Spatial statistics,

Maximum Capable Earthquake, Tsunami Run-up, Gumbels extreme value statistics.

The interelationship between occurrence of a megathrust earthquake event under the

Sea/Ocean and subsequent generation of tsunami as its after effect is known from past.

Examples of such combinations are 1960 Great Chilean earthquake (Mw 9.5), 1964 Alaskaearthquake (M

w9.2), and most recently 2004 Sumatra earthquake (M

w9.3). These giant

megathrust earthquakes with accompanied large scale tsunamis were felt all along the

coastal tracts of the Oceans across the globe, enundate and damage properties worth of

millions of dollars and most importantly incured a huge loss of life both human and

livestocks. It is known that principal cause of generation of a tsunami is by the displacement

of a huge volume of water under-sea (Haugen et al. 2005) by either earthquake, landslide,

and / or volcanic eruption (see Voit 1987; Margaritondo 2005 for details). Owing to the

tectonic loading over millions of years, the lithospheric plate in the destructive plate

boundaries placed below sea floor abruptly buckles and subsequently rebounds back asper the elastic rebound principle to generate primarily a thrust earthquake. This tectonic

process naturally displaces a large volume of water above the ruptured area on the Ocean

floor. The small amplitude and long wavelength waves generated by this ocean floor

rebound process stabilise by the earth’s gravity approaches towards the coastal land as

‘tsunami’ waves. The height of the waves along the coasts depend on many factors like

nature and material along the coast-line, configuration, slope towards sea etc., including

unusual bathymetry or irregular configurations of the coastlines. Geologically, a tsunami

is formed when a thrust fault, present under Ocean, within the subduction zone, slips

suddenly resulting into displacement of large volume of water by the vertical componentof the movement vector. Movement on normal or strike slip faults undersea may cause

displacement of the seabed, but fail to give rise to a significant tsunami due to its small

vertical movement component.

The record of tsunami in the Bay of Bengal especially in and around the Andaman –

Sumatra islands is dotted within the record book. In recent times, the earliest recorded

tsunami in the Bay of Bengal dates back to 31 December 1881 caused by an earthquake



Vol. 2 No. 2 December 2009u

3

of size ML

7.5, with its epicenter off the coast of the Car-Nicobar Island. The next record

is from the Andaman Island. It happened on 26 June 1941, caused by an earthquake

(magnitude exceeding 8.5) that created pervasive damage to the Andaman Islands. The2004 Sumatra earthquake (Mw 9.3) [Krüger and Ohrnberger, 2005] with epicentre off

the coast of Sumatra had generated the last, most prominent and probably well-studied

tsunami and its after effects along Andaman Sea. The damage and loss of life due to this

tsunami was felt in and around the coastal tracts of Indian Ocean rim countries. Before the

rupture, the edge of the NE Asian plate was being dragged downward by the descending

Indo-Australian plate. Upon released by the rupture, the dragged edge of the plate jumped

back up (Lay et al. 2005). This process uplifted the ocean floor and water column on the

top of it and initiated the tsunami that inundated the coastal areas. The fault that caused

the rupture had slipped as much as 15 meters at places, with an average of 7-10 metersof displacement (Lay et al. 2005; Stein and Okal 2005; Subarya et al. 2006; Banerjee et al.

2007; Rhie et al. 2007; Mishra et al. 2007). There are no other well-documented records

of Tsunami in this part of India Ocean in recent time. Though, palaeo-tsunami has been

identified in this part from the geological record of extensive deposits of sand sheets

marked within the sedimentary cores drilled from the coastal marshes in the northern

Sumatra. These sand deposits are similar in character and extent; and comparable to that

was deposited by the tsunami associated with the 2004 Sumatra earthquake at that place.

From the deposits within the drilling core, two palaeo-tsunamis of size equivalent to the

tsunami occurred after the 2004 Sumatra earthquake are delineated and that dates backto 780–990 AD and 1290–1400 AD (Monecke et al. 2008).

In this particular discourse, we have examined the earthquake distribution in the

Andaman-Sumatra subduction zone from the spatial perspective where extents of the

clusters are constrained by ‘near’ and ‘point density’ analyses. Principal outcome of our

finding is the disposition of the clusters and determination of the maximum capable

earthquakes estimated through maximum rupture lengths of individual clusters and

separately by the G-R relationship. The earthquake sizes in these source zones are then

used to calculate the run-up heights and estimation of tsunamogenic potentiality in the

coastal tracts. The return period of large earthquakes with associated tsunami comparableto the 2004 Andaman – Sumatra tsunami is also predicted through the Gumbels extreme

value statistics.

Identification of the earthquake source zones is one of the key research areas for the

earth-scientists and can be approached through different earth science disciplines, and



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also by statistics. The last one requires sufficient amount of accurate data. The source

zone for an earthquake may be outlined where earthquakes physically cluster in recent

past, identified from the last 100 years of earthquake records. This is because the areas of the clusters are zones where the strain has been accumulated through millions of years

by imposition of far-field stress associated with the plate movement. The accumulated

strain is getting released periodically in the form of earthquake shocks of variable sizes.

Our discourse is on the statistical viewpoint wherein a simple spatial statistical procedure

has been attempted. These techniques constrain the spatial extents of visually identified

clusters on ‘epicentral plot’ over a tectonic base map. The procedure is a combination

of two spatial statistical techniques, near- and point density analysis. Point density

is a statistical tool used to identify areas where data points are concentrated more or

vice versa. To calculate point density, the distance between the adjacent earthquakesis measured by near analysis. Statistically, a mean distance is then calculated from the

measured distances. This mean distance is used further as search radius to calculate area

of the circular neighborhood required for the point density calculation. Point density is a

2D statistical technique and measured as the total count of earthquake points that locate

within the proposed circular neighborhood, and normalized by dividing the count with

the area of the neighborhood. A factor resulting from the size of earthquake is also taken

into consideration for calculating point density values, e.g., 6 numbers are counted

instead of only one for an earthquake of magnitude 6 in the selected neighborhood. This

is done to offer relatively more weight to the size of the earthquakes. The measurementis then carried out in an overlapping grid pattern both along latitude and longitude

of the map area by a sliding distance equal to the search radius. The calculated point

density value is stored at the grid point placed at the center of the circle. This process

generates a 2D grid of point density values. The grid values have a mean (m) and a

standard deviation (sd). Areas with higher point density [value > (m + 3sd)] are marked

as zones of spatial clusters.

Further, to constrain the maximum capable size of earthquake within the clusters

(source zone), the author has employed the empirical relationship between rupture length

(RL) and size of earthquake (M) as a function of type of faulting (Thrust / Strike-slip).The strike lengths of clusters are considered as the maximum rupture length (RL) that

can be generated by an earthquake occurring there. On measuring RL, the expected size

of earthquake is empirically calculated via equations 1 and 2 (Wells and Coppersmith,

1994)

Log RL = -2.86 + 0.63M [for reverse faults] (1)

and




5

Log RL = -3.55 + 0.74M [for strike-slip faults] (2)

As stated earlier, these clusters can be considered as the probable source zones of

future tsunamogenic earthquakes in Andaman area. Further from published andunpublished data on tsunami run-up, coseismic vertical displacement, and seismologic

data, an empirical relationship between tsunami run-up and its causative earthquake

magnitude has been worked out (Nishenko et al. 2004). They opined that for most

tsunamogenic earthquakes, the associated tsunamis are generated by the large-scale

vertical coseismic displacement of the seafloor as stated earlier. They also argued that

for tectonic tsunamis, both average fault slip and maximum regional tsunami run-up

correlate with the earthquake size (seismic moment or earthquake magnitude). The

maximum run-up heights for tectonic tsunamis, which are not complicated by unusual

bathymetry or irregular configurations of the coastlines, are governed by the equation 3(Nishenko et al. 2004):

Run-up (m) = 4.99 Mw - 34.6 (3)

Seismic clusters are places of repeated rupturing by medium to large sized earthquakes.

Keeping this in mind, the possible source areas for larger tsunamogenic earthquakes in

and around Andaman – Sumatra Islands are delineated by ‘cluster analysis’ applied

on 7520 earthquake magnitude data (Ms/mb ≥ 4) for the time period 1906 – 2010

(Fig. 1). The spatial extents of the clusters are constrained by near and point densityanalysis as stated earlier. The distances between the nearby earthquakes are calculated

by near analysis and thereby an average value of these distances is delineated as 3.4

Km. This distance is taken as the search radius. The point density is carried out in an

overlapping grid pattern through a sliding distance of 3.4 Km on the map area . The

resulting point density grid has a mean (3.54) and a standard deviation (8.11). The areas

above a critical density value 27.87 (mean + 3 * standard deviation) are marked as zones

of spatial clusters and demarcated in this study as closed polygon with red outline on the

map. This process identifies 11 numbers of spatial clusters of variable sizes with numbers

A to K across the study area (Fig. 2). These clusters are the seismic source zones that hadspawned larger earthquakes in the past and have the potentiality to generate comparable

magnitude earthquakes in future (see Mukhopadhyay et al. 2010c for more implication

on Himalaya).



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Fig. 1: Epicentral map for the Andaman - sumatra Arc, period: 1906–2010. AR: Alcock

Rise; ASR: Andaman Spreading Ridge; AT: Andaman Trench; RF: Renong Fault; ST: Sunda

Trench; SF: Sumatra Fault; SR: Sewell Rise; VA: Volcanic Arc; WAF: WEST Andaman Fault.

Tectonic features are adopted after Curray (2005) and Dasgupta et al. (2003).

To constrain the size of earthquake within the clusters, the strike lengths of clusters

are measured in kilometer on the map (Fig. 2). This length is then taken as the maximumrupture length (RL) and the expected size of earthquake is empirically calculated via

equations 1 and 2 (Wells and Coppersmith, 1994). The estimates show that the capable

earthquake magnitude will range between 7.33 and 8.22 M (Table 1) in the eleven

clusters. The estimated magnitude is obviously an indication of the maximum possible

earthquake size for respective clusters when the entire strike length will rupture. Hence,

smaller rupture length will naturally yield smaller magnitude earthquake. Using M, the




7

expected Average coseismic displacement (AD) has also been calculated following the

relation, equation 4 (Wells and Coppersmith 1994):

Fig. 2: Seismic clusters (A to K) delineated by point density analysis are future source

zones for large earthquakes and tsunamis in the Andaman – Sumatra area. AR: Alcock

Rise; ASR: Andaman Spreading Ridge; RF: Renong Fault; SF: Sumatra Fault; SR: Sewell

Rise; VA: Volcanic Arc; WAF: WEST Andaman Fault. Tectonic features are adopted from

Curray (2005) and Dasgupta et al. (2003). Green star is the epicenter of 2004 Sumatra

earthquake.

Log AD = - 4.8 + 0.69 M (4)

The coseismic slip within the clusters would vary from 1.81m to 7.44m (Table 1).

Again, for each cluster, author has already calculated empirically the maximum

earthquake size (Mw). The maximum run-up height of tsunamis in meter for these

proposed earthquake magnitudes are calculated by equation 3 and tabulated (Table 1).

The run-up height of the tsunamis varies from 1.98 to 6.42 meters.



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T a b l e 1 : P r e d i c t e d e a r t h q u a k e a n d t s u n

a m o g e n i c p a r a m e t e r s i n A n d a m a n –

S u m a t r a R e g i o n

C l u s t e r

N o o f

e a r t h q u a k e s

( M a g n i t u d e

> = 4 )

‘ a ’ v a l u e

‘ b ’ v a l u e

M a g n i t u d e

o f M a x i m u m

c a p a b l e

e a r t h q u a k e

b y G - R

R e l a t i o n s h i p

( a / b )

P r e d o m i n a n t

m o v e m e n t ( f r o m

H R V D C M T d a t a ) i n

t h e c l u s t e r

M a x i m u m

m a g n i t u d e

o b s e r v e d b y

c l u s t e r ( M s /

m b ) i n t h e

t i m e f r a m e

1 9 0 6 - 2 0 0 9

S t r i k e

L e n g t h

o f c l u s t e r

( K m )

P r e d i c t e d

m a g n i t u d e *

( M ) t a k i n g

s t r i k e l e n g t h

o f t h e c l u s t e r

a s r u p t u r e

l e n g t h

A v e r a g e

c o s e i s m i c

d i s p l a c e m e n t *

( m e t e r ) a t

s o u r c e

M a x i m u m

r u n - u p

h e i g h t s ( m

e t e r ) o f

T e c t o n i c t s u n a m i s * *

a l o n g c o a s t a l t r a c t

A

9 4 7

5 . 9 4

0 . 7 7

7 . 7

P r e d o m i n a t e T h r u s t

a n d s u b o

r d i n a t e

S t r i k e - s l i p

8 . 6

2 0 9

8 . 2 2

7 . 4 4

6 . 4 2

B

6 7 6

7 . 1 7

1 . 0 4

6 . 9


a n d s u b o

r d i n a t e


7 . 3

1 8 1

8 . 1 2

6 . 3 5

5 . 9 2

C

7 8

5 . 9 5

0 . 9 9

6 . 0

P r e d o m i n a t e N o r m a l

a n d s u b o

r d i n a t e


6 . 1

7 7

7 . 5 3

2 . 4 8

2 . 9 7

D

1 1 9 8

9 . 6 1

1 . 5 3

6 . 3

P r e d o m i n a t e S t r i k e -

s l i p a n d s u b o r d i n a t e

N o r m a l

6 . 1

2 4 6

8 . 0 2

5 . 4 1

5 . 4 2

E

5 0

6 . 1 1

1 . 0 7

5 . 7

N o r m a l a

n d S t r i k e - s l i p

e q u a l p r o

p o r t i o n

5 . 6

7 4

7 . 5 0

2 . 3 7

2 . 8 3

F

6 1

6 . 0 2

1 . 0 7

5 . 6

T h r u s t

5 . 9

7 6

7 . 3 3

1 . 8 1

1 . 9 8

G

2 6 3

9 . 1 0

1 . 5 8

5 . 8


a n d s u b o

r d i n a t e


5 . 7

1 6 9

7 . 8 0

3 . 8 1

4 . 3 2

H

1 5

4 . 7 1

0 . 8 4

5 . 6


5 . 2

7 5

7 . 3 4

1 . 8 3

2 . 0 3

I

1 5 8

5 . 7 3

0 . 8 9

6 . 5


a n d s u b o

r d i n a t e


6 . 6

9 3

7 . 6 6

3 . 0 5

3 . 6 2

J

1 9

3 . 5 8

0 . 6 6

5 . 5


a n d s u b o

r d i n a t e


5 . 8

8 4

7 . 5 9

2 . 7 3

3 . 2 7

K

9

4 . 2 3

0 . 8 2

5 . 3


5 . 1

1 2 7

7 . 8 8

4 . 3 3

4 . 7 2

C a l c u l a t e d f o l l o w i n g

* W e l l s a n d C o p p e r s m

i t h ( 1 9 9 4 )

* * N i s h e n k o e t a l . 2 0 0

4 .




9

Additionally for each cluster, we have also calculated the seismic parameters of ‘a’

and ‘b’ values (Table 1) by regression method where logarithm of cumulative frequency

numbers of earthquakes is plotted against magnitude following the Gutenberg-Richter(G-R) equation. The ‘b’ value in the cluster varies between 0.77 and 1.53. The ‘b’

value greater than 1.50 is recorded in clusters D and G that indicates their probable

volcanic connotation. The maximum capable earthquake on those clusters as per the

G-R relationship is also calculated. The value is from 5.3 (Cluster K) to 7.7 (Cluster A).

These calculated magnitude values are comparable to the largest magnitude observed

in those clusters within the time span 1906-2010 but underestimates the earthquake

magnitudes that have been derived through estimates based on the rupture lengths

(Table 1; Fig. 3).

Fig. 3: Graph showing Mmax calculated by different process against Cluster name.

Mmax (G-R) is as per Gutenberg-Richter equation, Mmax(observed) is the maximummagnitude observed within the catalogue spanning 1906-2010,

Mmax (W-C) is the magnitude calculated as per rupture length

following Wells and Coppersmith, 1994.

Further, Gumbels extreme value probability with a data bin of 10 years has been

applied on the earthquake dataset spanning for the period 1900-2010 for the entire study



10 u


area. The maximum magnitude is arranged for each data bin. From this arrangement the

ranks are assigned. The non-exceedence probability of each magnitude bin is calculated

from the rank index of the data following the principle stated in Gumbel, 1958. Thenon-exceedence probability is then plotted against the maximum magnitude (Fig. 4).

A best-fit line according to Gumbels asymptotic equation is also established. For return

periods for 400 to 500 years, the non-exceedence probabilities are separately calculated

for the data bin of 10 years. The maximum magnitude for return periods is interpolated

from the best-fit curve. From the above plot, it is observed that the maximum magnitude

(M) earthquake that can occur for 400, 450 and 500 years return period are 9.23, 9.35

and 9.45 M respectively.

Fig. 4: Estimated Maximfium Magnitude and its probability through Gumbels

extreme value statistics over return period 400, 450 and 500 years for

the entire study area.

Eleven numbers of spatial clusters (A to K, Figure 2) that are also considered as earthquake

source zones are studied.

The clusters A, B and C are seen in offshore Sumatra (Fig. 2) following the outline

of Nias and other islands. These are shallow to moderate depth clusters with very high

seismicity. Configuration for A and B clusters follows the structural outline of the outer




11

sedimentary arc of the Nias and other islands and cluster C is parallel to the trench

axis. Seismically, cluster A presents one of the most destructive regions in the world,

where December 26, 2004 Sumatra earthquake (Mw 9.3) occurs. Clusters A and B haveearthquakes belonging to both the plates (the under-thrusted Indo-Australian and

overriding NE Asian plates) with predominant thrust movement and subordinate strike-

slip movements, whereas, clusters C holds earthquakes only to the subducting Indian

plate with predominant normal movement and subordinate strike-slip movement. The

earthquakes of thrust mechanisms in clusters A and B are related to the underthrusting

of the Indo-Australian plate below the Sunda Arc and also from overriding SE Asian

Plate. The overall orientation of the thrust planes derived from composite CMT plot

is NW-SE dipping 25ο northeasterly. P-T axes orient NE-SW in close correspondence

to the arc geometry. The depth sections across cluster A and B show that subductingIndian plate and overriding SE Asian plate in this sector behave uniquely with a thrust

movement along NW-SE striking plane dipping less than 22ο towards northeast parallel

to the Sunda arc (Mukhopadhyay et al., 2010a). Similar geometry and stress partitioning

in both the plates indicate high seismic coupling, extreme compressive nature of this

zone resulting from slow subduction of the old Indian plate (Stein and Okal, 2007).

The fault unclamping along shallow dipping thrust planes (< 30ο) has resulted into the

Sumatra – Andaman earthquake of 2004 (Mw 9.3). The normal faulting in cluster C

indicates bending events with later adjustment by strike slip motion. It is also to be

mentioned that the hinge faults delineated by earlier studies (Dasgupta et al. 2003 andMukhopadhyay et al. 2009) act as barriers to the formation of the clusters. The boundary

zone between the cluster and adjacent hinge fault are area of high strain. This is well

documented in Cluster A where the December 26, 2004 Sumatra earthquake (Mw 9.3)

spawned at the interface between the cluster A and the hinge fault.

Seismic clusters D to K locate between central and southern parts of the Andaman

Arc. They are shallow depth clusters with moderate magnitude shocks (Mukhopadhyay

et al., 2010a). Clusters F, J and K locate over the Andaman fore-arc, whereas, E and I

belong to the Andaman trench zone. Clusters D, G and H are found at the Andaman

sea, where clusters G and H correlate to the Andaman Spreading Ridge (ASR). Cluster D,the longest of all clusters; has a strike length of 246 km and contains maximum number

of earthquakes (1198). This cluster is clearly associated with the West Andaman Fault

(WAF) and its southern continuation to the Sumatra Fault (SF). Seismic clusters F, J and

K underlie the Andaman fore arc and clusters E and I belong to Andaman trench. The

clusters F, J and K are thrust dominated clusters indicating back-thrusting events in the

fore-arc. The shallow focuses Normal and Strike-slip events of E and I along the Andaman



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trench axis were generated by bending of subducting Indian Plate and occurred at the

leading edge of the subduction zone where the Indo-Australian plate descends.

Clusters G and H found below the ASR exhibit predominantly normal and strikeslip events that illustrate the basic tectonic pattern under the spreading arc. The large

cluster D typifies almost equal proportion of strike-slip and normal events. Distribution

of periodic occurrences of strike-slip and normal fault events in this cluster zone

actually corresponds to a complex faulting episode: normal faulting for the rift zone

and strike-slip movements for its transgressive regional faults WAF and SF. This cluster

is found in the area where a swarm of events originated following the occurrence of

great Sumatra earthquake (Mw 9.3) of 26 December 2004. The swarm (n = 651, mbmax

=

5.9) came mainly in two phases: January 26 - 31 and Feb. – Aug. 2005, in an area of size

90 x 40 sq. km, at the centre of which lies a broad bathymetric depression and highgravity zone (Mukhopadhyay et al. 2010b). Based on the evidences from seismology,

bathymetry, gravity, time dependent pore pressure perturbations, rift related volcanism

and calculations on phases of rifting, we assume that a nascent rift is in the process of

formation at this location (Mukhopadhyay and Dasgupta, 2008; Mukhopadhyay et al.,

2010b). The orientation of the nascent rift is perpendicular to regional trend of strike-slip

faults of WAF and SF.

Eleven source zones numbered A to K exist in Sumatra – Andaman area have the capabilityto spawn repeated damaging large earthquakes of magnitude range Mw 7.3 to 8.2.

Comparing the results of both seismic and tsunamogenic hazards in (Table 1), we can

conclude that out of the 11 clusters, clusters A, B, F and J are the most vulnerable because

they can generate earthquakes with predominate thrust movement, a prime requirement

for tsunamogenic earthquakes. These source zones can yield earthquake with magnitude

greater than Mw 7.3 that in turn can generate tsunamis with a maximum run-up height

of more than 6 meter. Moreover, these measurements are independent of the unusual

bathymetry or irregular configurations of the coastlines.

The potentiality of the Andaman - Sumatra zone to generate tsunamogenicearthquakes is also discussed elsewhere (Fujii and Satake 2007; Hébert et al. 2007) along

with the physical records of tsunami of last 1000 years (Monecke et al. 2008). From

the palaeotsunami record from northern Sumatra, it is apparent that damage-causing

tsunamis in this part of the Indian ocean occur infrequently during AD 780–990, 1290–

1400, and 2004 (Monecke et al. 2008). The recurrency rate of the giant tsunami is nearly

400 - 500 years which is comparable to the recurrency time of Giant earthquakes of




13

size Mw 9.2 and above measured by extreme-value statistics (Fig. 4). But within this

infrequency, moderate to large size thrust earthquakes within the ocean bottom always

pose a threat to create tsunami which can devastrate nearby coastal tracts. Hence, thecoastal structures may be constructed in such a fashion that it can withstand the ground

motion created by such an earthquake and can also nullify the damages caused by those

predicted tsunamis.

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Haugen K., Løvholt F., Harbitz C. K. (2005). Fundamental mechanisms or tsunami generation by submarine

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