Aftershock Sequences of Two Great Sumatran Earthquakes of 2004 and 2005 and Simulation of the Minor Tsunami Generated on September 12, 2007 in the Indian Ocean and Its Effect (1)

7/31/2019 Aftershock Sequences of Two Great Sumatran Earthquakes of 2004 and 2005 and Simulation of the Minor Tsunam

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O R I G I N A L P A P E R

Aftershock sequences of two great Sumatran earthquakes

of 2004 and 2005 and simulation of the minor tsunamigenerated on September 12, 2007 in the Indian Ocean

and its effect

R. K. Jaiswal A. P. Singh B. K. Rastogi T. S. Murty

Received: 5 December 2009 / Accepted: 28 September 2010 / Published online: 2 November 2010 Springer Science+Business Media B.V. 2010

Abstract We present the seismic energy, strain energy, frequencymagnitude relation

(b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the

AndamanSumatra earthquakes of December 26, 2004 (Mw 9.3) and March 28, 2005

(Mw 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and

0.365% of the energy of the respective mainshocks, while the strain release in aftershocks

was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate

normal value of about 1. All these parameters are in normal range and indicate normalstress patterns and mechanical properties of the medium. Only the strain release in af-

tershocks was considerable. The fourth largest earthquake in this region since 2004

occurred in September 2007 off the southern coast of Island of Sumatra, generating a

relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude

as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm.

TUNAMI-N2 model was used to investigate ability of the model to capture the minor

tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and

simulated tsunami generation, propagation and wave height at tide gauge locations showed

that the model was able to capture the minor tsunami phases. The directivity map shows

that the maximum tsunami energy was in the southwest direction from the strike of the

fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts

along the Indian coastlines except near the coast of epicentral region.

Keywords Aftershock Seismic characteristics Energy release Tsunami simulation Tsunami propagation Wave height Tide gauge BPR

R. K. Jaiswal

National Geophysical Research Institute (NGRI), Hyderabad, Andhra Pradesh 500007, India

A. P. Singh (&) B. K. RastogiInstitute of Seismological Research, Raisan, Gandhinagar, Gujarat 382009, India

e-mail: [email protected]

T. S. Murty

Department of Civil Engineering, University of Ottawa,

140 Louis Pasteur Ottawa, Ottawa, ON K1N 6N5, Canada

123

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DOI 10.1007/s11069-010-9637-z


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1 Introduction

The two largest earthquakes of the past 40 years ruptured a 1,600-km-long portion of the

fault boundary between the Indo-Australian and southeastern Eurasian plates on December

26, 2004 (Mw 9.3) and March 28, 2005 (Mw 8.6). The first event generated a tsunami thatcaused more than 2,83,000 deaths, fault slip of up to 15 m occurred near Banda Aceh,

Sumatra but to the north, along the Nicobar and Andaman islands, rapid slip was much

smaller. The March 28, 2005 event ruptured an adjacent 300-km-long portion of the plate

boundary (Lay et al. 2005; Subarya et al. 2006). Though this great earthquake did not

generate significant tsunami, it caused damage to property and loss of lives in Sumatra

islands of Indonesia. These two events are the largest to occur after the global deployment

of digital broadband, high-dynamic range seismometers (Romanowicz and Giardini 2001;

Butler et al. 2004; Natawidjaja and McCaffrey 2006), which recorded both the huge

ground motions from ensuing free oscillations of the planet (Park et al. 2005; Stein and

Okal 2004). A large number of aftershocks, including some strong ones, that occurred after

the earthquakes enticed lot of interest. Study of these aftershocks is expected to throw light

on tectonics of the region. The seismic energy, strain energy, frequencymagnitude rela-

tion (b-value) and decay rate of aftershock (p-value) for the complete AndamanSumatra

sequences provide a much better understanding of the physical processes responsible for

the continued aftershock activity.

The 2004 tsunami event was followed by annual occurrence of ocean wide tsunami (i.e.

those that influence areas far from the generation region) in 2005, 2006, 2007 and 2009

[McCloskey et al. 2005; Ammon et al. 2006; Fujii and Satake 2006, 2008 United States

Geological Survey (USGS)], and these tsunami have been recorded on sea level gauges inSri Lanka except the 2006 tsunami of Java (Table 1). The fourth largest earthquake in this

region occurred on September 12, 2007, with a magnitude Mw 8.4 and struck the western

coast of Bengkulu, Sumatra, Indonesia. The epicenter of the earthquake was located

150 km SW of Bengkulu (4.520S, 101.374E), off west coast of southern Sumatra, and it

generated a relatively moderate tsunami. The focal depth of this earthquake was 34 km,

and origin time was 11:10:26 UTC (16:40:26 IST), (USGS). The seismic sequence con-

tinued for the next 2 days, with the biggest event of magnitude Mw 7.1 happening on

September 13. This sequence took place in the same rupture zone where the historical

earthquakes of 1797 and 1833 occurred which generated significant tsunami (Nalbant et al.

2005). Sea level data indicated that a minor tsunami was generated on September 12,2007. The maximum wave height registered by the Padang, tide gauge, north of the

quakes epicenter was about 60 cm. The Padang tide gauge located at 0.9S, 100.4E. A

maximum tsunami runup height of 90 cm has been reported. The travel time of the first

tsunami wave is estimated at about 20 min. The period of subsequent waves was about

Table 1 Details of Indian Ocean tsunami 20042007

Year Date Magnitude

of earthquake

Epicenter

location

Maximum tsunami

height in Sri Lanka

2004 26th December 9.2 3.32N, 95.854E 10.1 m Hambantota

2005 28th March 8.6 2.07N, 97.01E 2.7 m Kirinda

2006 17th July 7.7 9.22N, 107.32E 0.0 m

2007 12th September 8.4 4.52N, 101.37E 0.6 m Trincomallee

Source: NARA, Sri Lanka

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28 min. Pacific Tsunami Warning Center (PTWC, NOAA) and Japan Meteorological

Agency (JMA) issued ocean wide tsunami warning and watch, respectively, for the

coastlines of Indian Ocean on September 12, 2007 (PTWC and JMA). Indian National

Centre for Ocean Information Services (INCOIS), Hyderabad, issued alert for Andaman

Nicobar region and watch for Indian mainland. Finally, INCOIS downgraded alert forAndamanNicobar region and watch for Indian mainland into all clear (no tsunami threat)

after observing near-real-time minor sea level variations in Bottom Pressure Recorders

(BPRs) and tide gauges deployed in the Indian Ocean by National Institute of Ocean

Technology (NIOT), Survey of India (SOI) and Global Sea Level Observing System

(GLOSS) (Nayak and Kumar 2008). This tsunami was recorded on about 22 sea level

measurement networks (viz. tide gauges, Indian BPRs, NOAA Buoy 23401 for Thailand

etc.) deployed by various authorities and Institutions in the Indian Ocean. This tsunami

event helps tsunami modelers to quantify the size of tsunami and to validate the tsunami

propagation and inundation models for the Indian Ocean as well as to find the gaps in the

Indian Ocean tsunami Warning Systems (IOTWS) and to fill them. In view of this, an

attempt is made here to simulate the minor tsunami generated on September 12, 2007,

using TUNAMI-N2 numerical code.

2 Data

Aftershocks data for this study have been collected from USGS catalogs for the period up

to May 20, 2006, covering the region between Lat. 0S to 15N and Long. 90 to 99E.

Rupture of the 2004 earthquake propagated northward for 1,300 km up to Andaman (fromLat. 3N to 15N), while rupture of the 2005 earthquake propagated southward for 300 km

(from 3N down to 0S). Aftershocks are considered to have occurred in the respective

rupture zones. It is noticed that there were a few aftershocks of 2004 earthquake between

latitude 2.5 and 3N. Hence, shocks between latitude 2.5 to 3N until March 28, 2005, are

considered as aftershocks sequences of December 26, 2004, event, while shocks at and

below latitude 3N after March 28, 2005 are considered as aftershocks of March 28, 2005

mainshock (Fig. 1). A similar pattern is shown by Ammon (2006).

A total of 3460 aftershocks of December 26, 2004 mainshock are compiled in the

magnitude range 3.57.5. On the other hand, 1835 aftershocks of March 28, 2005 earth-

quake are compiled in the magnitude range 3.66.9 (Fig. 2). For the December 26, 2004earthquake, the two largest aftershocks were of magnitude, Ms 7.5. The first one occurred a

few hours after the mainshock near Nicobar Islands (Lat. 6.91E, Long. 92.96N). How-

ever, the second one occurred 150 km further NW on July 24, 2005 (Lat. 7.92E, Long.

92.19N). Analysis of the aftershocks activity reveals that the earthquakes are mostly

confined to the top 50 km (Fig. 2). However, events with 33 km focal depth are the model-

assumed depth, which are not properly resolved, as these are determined using stations

located at regional and teleseismic distances. Since the catalog depths are found to have

errors, the depth depends on the b-value was not attempted. Histogram of magnitudes of

the original catalog compiled by USGS shows a maximum numbers of earthquakes occurin magnitude range of 4.04.9 (Fig. 3a, b) in both cases, which is a first indication of value

of magnitude completeness (Mc) of catalog (Fig. 4).

We calculated the magnitude of completeness and eliminated all the events having

magnitudes less than Mc (Fig. 5). The calculated Mc was found 4.2 in both cases using the

maximum curvature method for the catalog. The estimated Mc is based on the assumption

of a power-law GutenbergRichter relationship and taken as the magnitude where the first

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derivative of the frequencymagnitude curve has its maximum. The magnitude from where

curves became exponential is known as magnitude of completeness. Mc can be also

defined as the lowest magnitude at which 100% of the events are detected in space and

time volume (Woessner and Wiemer 2005).

For the tsunami simulation of 2007 case GEBCO (General Bathymetric Chart of the

Oceans), bathymetry data of 1 min arc (1.8 km) resolution with 5-km grid spacing has

been used. The fault parameters are taken from USGS and Fujii and Satake (2007).

3 Tectonic setting

The 2004 and 2005 earthquakes ruptured the boundary between the Indo-Australian plate,

which moves generally northward at 4050 mm/year, and the southeastern portion of the

Eurasian plate, which is segmented into the Burma and Sunda subplates (Lay et al. 2005).

The eastern margin of the Burma microplate is defined by the NS trending and Sagaing

transform fault, which separates the central low lands from the eastern high lands of

Myanmar, and the fault continues as the Andaman sea rift system (known as the Andaman

Spreading Ridge (ASR)). The Andaman Sea basin is considered to be a complex back arc

spreading center, is categorized as a pull apart or rip off basin rather than a typical

back-arc-extensional basin. Seismic reflection studies across the trench slope indicate

folding and thrusting in the accretionary prism, where a major component of convergence

occurs normal to the trench axis. Among the series of thrusts/faults in the Andaman

subduction zone, the west Andaman fault is most prominent. The thrust/fault appears to be

Fig. 1 Aftershocks of December 26, 2004, earthquake are shown by white patch of epicenters and a few red

ones from Lat. 3N and northward, the earthquake of March 28, 2005, are shown by red epicenters about

3N and southward (source EMSC). Location of the earthquake of March 28, 2005, was about 190 km SE ofthe location of mainshock of December 26, 2004

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continuous from the west of Sumatra in the south to Irrawady basin in the north, where it is

buried beneath the sediment cover. The region between the AndamanNicobar Islands and

the volcanic arc is the foredeep sedimentary trough (Dickinson and Seely 1979).

Along the Sumatra subduction zone, plate convergence is partitioned into dip-slip andright lateral strike-slip components, the former being accommodated by slip on the sub-

duction interface and the latter by the Sumatran fault. The present-day tectonic processes

are controlled by three major fault systems, the most prominent being the subduction

thrust, which outcrops in the Sunda trench. Inland, the trench-parallel Sumatra fault that

runs through the entire length of the island from Banda Aceh to Sunda Strait, accom-

modates oblique convergence through strike-slip faulting. The Mentawai fault at the outer

margin of the forearc basin is another important fault system in the Sumatra region (Sieh

and Natawidjaja 2001).

Subduction along AndamanSumatra trench system has given rise to a discontinuous

belt of the submarine ridges and volcanic seamounts. The andesite volcanoes of the Barren

and Narcondam Island are prominent among them; the Narcondam is now extinct, but the

Barren is still marked by an active volcano; it erupted last in March, 1991 after lying

dormant for about two centuries. Further south, this volcanic belt is represented by the

Barisan range in Sumatra, and in the north, the trend is correlated with the central molasse

basin of Burma.

Fig. 2 Epicentral distribution of earthquakes used in this study is shown by solid circles; in cross sectional

views with depth (left side shows distribution of events along latitude with depth, while lower shows the

distribution along longitude with depth). Depth distribution of the used events (in crosses) in the latitude and

longitude directions, respectively. Mainshock for 2004 and 2005 earthquakes are shown by red stars

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The September 12, 2007 earthquake was followed by a bigger aftershock of Mw 7.9

Bengkulu (2.50S, 100.906E, USGS), some 12 h after the onset of the mainshock, andthis also generated a small tsunami that was recorded at Pointe La Rue and Rodrigues

(Lorito et al. 2008). Both earthquakes caused a total of 25 fatalities and 161 injuries

(USGS). The seismic sequence continued during the next 2 days, including a magnitude

MW 7.1 earthquake on September 13, some 15 h after the mainshock. This sequence

occurred in the same zone where the historical giant earthquakes of 1797 and 1833 gen-

erated major tsunami (Nalbant et al. 2005).

4 The b- and p-values of aftershock sequences

The frequencymagnitude distribution mainly defines the relative amount of smaller to

larger earthquakes through its slope-parameter b. This has been extensively studied in

laboratory experiments, as well as for synthetic and real seismicity. There are three main

natural factors, as inferred from laboratory studies that can cause significant changes of the

frequencymagnitude distribution from an average value of 1.0 (1) increased material

84

1265

1451

459

133

27 1 1 1 2 1

0

200

400

600

800

1000

1200

1400

1600

3.5

-3.9

4.0

-4.4

4.5

-4.9

5.0

-5.4

5.5

-5.9

6.0

-6.4

6.6

6.7

6.8

7.5

9.3

Magnitude Range

No.ofAftershock

45

767 742

171

32 10 1 2 2 1

0

100

200

300

400

500

600

700

800

900

3.6

-3.9

4.0

-4.4

4.5

-4.9

5.0

-5.4

5.5

-5.9

6.0

-6.4

6.5

6.8

6.9

8.7

Magnitude Range

No.ofAftershock

(a)

(b)

Fig. 3 Magnitude distribution of the aftershocks of a December 26, 2004, and b March 28, 2005

earthquake

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heterogeneity increases the b-value; (2) increase in shear stress or effective stress decreases

the b-value; and (3) increase in the thermal gradient may cause an increase in the b-value.

High b-values are generally observed for highly cracked volumes like magma chamber,

reservoir-induced or intraplate sites. However, small b-value is observed for asperities or

probable zones for future large earthquakes like plate boundaries. Another important

parameter to characterize an aftershock sequence is the p-value, which is a measure of the

decay rate of aftershocks, as described by the Omoris (1894) law. Worldwide review of

the p-value estimates suggests a range from 0.6 to 2.5, with a median around 1.1. Regional

variation of the p-value has been related to the variation in heat flow (Mogi 1962). Var-

iability of estimated p-values may be related to the structural heterogeneities, stress andtemperature in the crust.

Slope of b-value and p-value of aftershock sequences reveals the information about the

heterogeneity of the rocks and stress conditions in the earthquake zones.

The magnitudefrequency relation (b-value) is given by (Gutenberg and Richter 1944)

LogN a b M; 1

where N is cumulative number of shocks, constant a depends upon level of seismicity

and slope b reflects upon the heterogeneity of the medium and level of stress. The b-values

have been determined with least-square and maximum-likelihood methods. Using least-

square method, for the sequence pertaining to the December 26, 2004 and March 28, 2005

earthquake obtained Log N= 8.871.21M (Fig. 5a) and Log N= 7.741.08M (Fig. 5b),

respectively. The b-values are obtained 1.21 0.04 and 1.08 0.03 for the two

mainshcoks, respectively, using least-square approach.

Further, the b-value has been determined by maximum-likelihood method (Utsu 1961;

Aki 1965) using the relation, which is given below

Fig. 4 Computational domain with bathymetry and topography data, locations of sea level measurement

networks as well as focal mechanisms of recent great Sumatran earthquakes of 2004, 2005 and 2007. Color

bar shows the data are in meters. Triangle shows locations of tide gauges and circle denotes the BPRs

locations. Red stars shows recent (2004, 2005 and 2007) Sumatran earthquakes locations

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4 5 6 7

1

2

3

Log N = 8.87 - 1.22 M

LogN

M

4 5 6 7

1

2

3

Log N = 7.74 - 1.08 M

L

ogN

M

(a)

(b)

Fig. 5 The b-value of a December 26, 2004, and b March 28, 2005, earthquakes

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b 0:4343=Ma Ms; 2

where Ma is the average magnitude and Ms is the threshold value (lower limit) of the

magnitude for which the dataset is complete. The lowest magnitude is 4.2 for both the

sequences. From the above relation (2), the b-values show 0.92 and 1.07 for both the

mainshocks, respectively. The values nearly 1 indicate normal values.

The rate of decay of the aftershock sequence is empirically described with time and is

given by the Omoris (1894) law.

N t k= t c p; 3

where N(t) is the frequency of aftershocks per unit time and k, c, p are constants. N(t) was

calculated following the Ogata et al. (1993) approach for fitting the cumulative number of

aftershocks as a function of time after the mainshock. The kvalue is dependent on the total

number of aftershocks during the first time interval and c on the rate of activity in the

earliest part of sequence. The decay in aftershock activity with time reflects a decrease instress in the region. The characteristics parameter p is determined from this relation and

gives the rate of fall of aftershock activity.

The frequency of occurrence of aftershocks n (t) at time t (weekly) after the mainshock

is given as;

N t 725t1:1 R2 0:75 December 26; 2004 sequence 4

N t 281t0:9 R2 0:68 March 28; 2005 sequence 5

We estimated p-value 1.1 and 0.9 for Sumatran aftershocks (Fig. 6a, b). It shows a normal

decay and decrease in stress with time in the region.

5 Energy and strain release in mainshocks and aftershocks

Energy released during an earthquake is proportional to its magnitude (M). The energy

release, E in ergs is estimated from the relation:

Log E 9 1:8M 6

The values of E have been calculated for the mainshocks and their aftershock sequences.Cumulative energy release for the two sequences is shown in Fig. 7a, b.

The energy released in the earthquake of December 26, 2004, was 5.49E? 25 ergs,

while the total aftershock energy was only 7.42E? 22 ergs, which reveals only 0.135%

energy of the mainshock was released in the aftershocks of earthquake of December 26,

2004. On the other hand, energy released in the earthquake of March 28, 2005, was

4.57E? 24 ergs, while the total aftershock energy was only 1.67E? 22 ergs. This

indicates only 0.365% energy of the mainshock was released in the aftershocks of earth-

quake of March 28, 2005.

The square root of energy for an individual earthquake is proportional to the strain

release (Gutenberg and Richter 1956) and is given by

Log E1=2 4:5 0:9M 7

Since energy unit is in ergs, strain release will be in the units of (ergs) 1/2. Cumulative strain

release for the two sequences is shown in Fig. 8a, b. The major strain release in the

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aftershock sequence of December 26, 2004, earthquake is by two major earthquakes of Ms7.5 on December 26, 2004, and July 24, 2005.

The December 26, 2004, earthquake released strain of 7.41 9 1012 ergs1/2, while its

aftershocks released 2.98 9 1012 ergs1/2, making 39% in the aftershocks. This figure is

0 20 40 6010 30 50 70

0

200

400

600

800

723

136

177177

740

96

637676

493938

15

473838

25292230

2120172320

2717211716

64

1816711

131212106

2022109117

145 6

1312141011713

7114 5 310

137

18166119 6 4 3 3 1

n(t) = 725 t-1.1

R2 = 0.75

Time in week

Time in week

No.ofAftershock

0

200

400

600

514

185

85

747163

47484039

26

36

2529

2227

16

30

1513171417

191012 9

149 11 9 1011

171413136 6

101114

9 10151314

714

9 61011

6 812 9

1 0

104

9

137

1816

611 8 6 4 2 3

No.

ofAftershocks

n(t) 281 t -0.9

R2 = 0.68

(a)

(b)

Fig. 6 Decay rate of aftershocks a December 26, 2004 and b March 28, 2005 earthquakes

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matching with the 30% slip in aftershocks for 1.5 months derived by Global Positioning

System (GPS) and seismic methods by Subarya et al. (2006). The March 28, 2005,

earthquake released strain of 1.74 9 1012 ergs1/2, while its aftershocks released

1.24 9 1012 ergs1/2, making it 71% strain release in aftershocks.

1 10 100

5.494

5.496

5.498

5.5

5.502

5.504

E(

10)25(ergs)

E(

10)25

(ergs)

Day

Ms 7.5(24July 2005)

Mw 9.3 & Ms 7.5(26 Dec. 2004)

1 10 100

0.301

0.3015

0.302

0.3025

0.303

0.3035

Day

(a)

(b)

Fig. 7 Cumulative energy release of a December 26, 2004, and b March 28, 2005, earthquakes

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1 10 100

7

8

9

10

11

E

1/2(10)12(ergs)1/2

E

1/2(10)12(ergs)1/2

Day

1 10 100

1.6

2

2.4

2.8

3.2

Day

(a)

(b)

Fig. 8 Cumulative strain release of a December 26, 2004, and b March 28, 2005, earthquakes

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6 Tsunami generation and propagation

An attempt has been made here to simulate the September 12, 2007, tsunami, its generation

and propagation in the Indian Ocean using TUNAMI-N2 numerical model. The TUNAMI-

N2 model basically takes the seismic deformation as input to predict the run-up heights andinundation levels at coastal regions for a given tsunamigenic earthquake (Imamura 2006;

Imamura et al. 2006). The seismic deformation for the earthquake has been computed

followed by Mansinha and Smylie (1971) formulation using the earthquake parameters

namely location, focal depth, strike, dip and rake angles, length, width and slip of the fault

plane.

The model domain with bathymetry and topography data as well as locations of the

tsunami recorded by sea level measurement networks are shown in Fig. 4. The great

Sumatran earthquakes of 2004 (Mw 9.3), 2005 (Mw 8.7) and 2007 (Mw 8.4 and 7.9) show

roughly similar focal mechanisms i.e. shallow dipping thrust faults (Fig. 4). The CMT

focal mechanisms of these earthquakes using from Harvard program. In Fig. 4, triangles

show locations of INCOIS, GLOSS and NIO (Goa) tide gauges and circles denote the

INCOIS and NOAA BPRs (bottom pressure recorder) locations. INCOIS gets the tide

gauge data from NIOT (National Institute of Ocean Technology), Chennai and SOI

(Survey of India), Dehradun. Tsunami wave heights recorded by these tide gauges and

bottom pressure recorders are given in Table 2.

In order to compute co-seismic dislocation model (input for tsunami generation), the

following fault parameters were used: location = (4.520S, 101.374E), focal

depth = 34 km, strike angle = 327, dip angle = 12, rake angle = 85 (USGS), length

of the fault = 160 km, width of the fault = 80 km and slip of the fault plane = 5 m (Fujiiand Satake 2007). The dislocation model shows that the initial wave height of the tsunami

was 1.5 m at the source of the earthquake and with a minimum was 0.5 m (Fig. 9).

The maximum wave heights of tsunami were 2.27 m at Padang, 0.15 m at Christmas

Island, 0.24 m at Cocos Island recorded by the Global Sea Level Observing System

(GLOSS), 0.046 m on tsunami meter 23401 (8.9N, 88.54E) recorded by National Data

Buoy Center (NOAA), 0.60 m at Trincomallee and 0.60 m at Colombo recorded by

National Aquatic Resources Research and Development Agency (NARA), Sri Lanka.

AndamanNicobar region was in the shadow zone of the earthquake rupture zone when

compared to the Indian Mainland, the observed tsunami amplitude at Chennai and Port

Blair was 20 and 10 cm, respectively. Our simulation results are in close agreement withthe available observations.

The tsunami propagation states at every one-minute interval are simulated. In this study,

the simulation is carried out for duration of 600 min and to see the impact of the tsunami

wave propagation as well as wave height at the eastern costal part of India. The maximum

tsunami wave amplitude after the 600 min of tsunami is shown in Fig. 10. From the

Fig. 10, it is found that the maximum tsunami energy was directed toward southwest

direction from the strike of the northwest- and southeast-oriented fault plane. It is also

found that the path of the tsunami for Indian coastlines is oblique. Therefore, there were no

affect along the Indian coastlines except near the coast of epicentral region. The tsunamipropagation snapshot at time (t) 0, 11, 31, 61, 93, 121, 151, 181, 211, 241, 283, 301, 361,

421, and 582 min is shown in Fig. 11ac. At different time intervals, the wave amplitude is

shown in the bar with different colors, next to the simulation figure. The tsunami wave

propagation could affect by the bathymetry data of the Indian Ocean (Jaiswal et al. 2009).

Therefore, detailed bathymetry data are required for the accurate information about the

tsunami propagation in the Ocean. Initially, tsunami propagated with amplitude of 1 m at

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t= 0 min (Fig. 11a), and at different time interval, it started propagating in open sea into

southwest direction from the source. At t= 93 min, tsunami wave is reflecting back

toward source region from the Cocos island (Fig. 11a). Tsunami was touching the Sri

Lanka at 194 min, India and Lakshadweep Islands after 245 min, and the observed tsunami

amplitudes were few centimeters in these areas. The recorded tsunami amplitude on tidegauge was 20 cm at the coast of Chennai (Table 2). The AndamanNicobar Islands were

on the shadow zone or parallel to the strike of the fault when compared to the Indian

Mainland; this could be the cause of a bit more observed tsunami amplitude at Chennai

which was 20 cm when compared to Portblair which was only 10 cm, though Andaman

Nicobar Islands were nearer to the source region and Chennai was far from the source.

Table 2 Tsunami wave height recorded at different tide gauges and BPRs due to September 12, 2007,

tsunamigenic earthquake

Location Approx. wave height (cm) Source

Padang, Sumatra (0.95

S, 100.36

E) 227 GLOSSSibolga, Sumatra (01.75N, 98.77E) 20 GLOSS

Cocos Island, Australia (12.11S, 96.89E) 50 Nayak and Kumar (2008)

Sabang, Sumatra (5.83N, 95.33E) 30 Nayak and Kumar (2008)

Tsunamimeter 23401 (8.9N, 88.54E) 4.6 NDBC (NOAA)

TB 10 (7.000N, 87.017E) 1 Nayak and Kumar (2008)

TB 10 A (7.000N, 87.068E) 2 Nayak and Kumar (2008)



Port Blair, Andaman, India (11.64N, 92.71E) 10 Nayak and Kumar (2008)

Chennai, India (13.10N, 80.30E) 20 Nayak and Kumar (2008)

Colombo, Sri Lanka (06.92N, 79.83E) 60 NARA, Sri Lanka

Christmas I., Australia (10.29S, 105.67E) 15 GLOSS

Cocos I., Australia (12.0S, 96.7E) 50 Nayak and Kumar (2008)

Cillcap, Java (7.75S, 109E) 20 GLOSS

Verem, Goa (15.50N, 73.81E) 29 Prabhudesai et al. (2008)

Kawaratti, Lakshadweep (10.57N, 72.63E) 5 Prabhudesai et al. (2008)

Gan, Maldives (00.7N, 73.2E) 7 UHSLC

Male, Maldives (04.2N, 73.5E) 13 Nayak and Kumar (2008)

Hanimaadhoo, Maldives (06.8N, 73.2E) 18 UHSLC

Salalah, Oman (16.94N, 54.0E) 140 UHSLC

Ponte La Rue, Seychelles (04.67S, 55.53E) 40 UHSLC

Rodrigue, Mauritius (19.67S, 63.42E) 100 UHSLC

Diego Garcia Is. (7.23S, 72.43E) 5 UHSLC

Cochin, Kerala (09.96N, 76.26E) 25 INCOS

Trincomallee 60 NARA, Sri Lanka

Djibout, Gulf of Aden UHSLC

Benoa, Java UHSLC

GLOSS Global Sea Level Observing System (http://www.gloss-sealevel.org ), NDBC National Data Buoy Center,USA (http://www.ndbc.noaa.gov ), NARA National Aquatic Resources Research and Development Agency (http://

www.nara.ac.lk), INCOIS Indian National Center for Ocean Information Services, Hyderabad (http://www.

incois.gov.in), UHSLC University of Hawaii Sea Level Center (http://www.uhslc.soest.hawaii.edu)

20 Nat Hazards (2011) 57:726

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http://www.gloss-sealevel.org/http://www.ndbc.noaa.gov/http://www.nara.ac.lk/http://www.nara.ac.lk/http://www.incois.gov.in/http://www.incois.gov.in/http://www.uhslc.soest.hawaii.edu/http://www.uhslc.soest.hawaii.edu/http://www.incois.gov.in/http://www.incois.gov.in/http://www.nara.ac.lk/http://www.nara.ac.lk/http://www.ndbc.noaa.gov/http://www.gloss-sealevel.org/


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From Fig. 11b, at t= 283 min, the wave is getting reflected from Chagos-Laccadive

ridge and due to presence of these ridges the west coast of Somalia would not get affectedif tsunami intensity would have been great. After t= 283 min, wave is getting reflected

from Lakshadweep Islands and would have struck Kerala coast and recorded maximum

trough to crest wave height at Cochin and Verem, Goa tide gauges were around 26 and

29 cm, respectively (Table 2). The maximum trough to crest wave height at Kavaratti

(Lakshadweep) was around 5 cm. After t= 421, it can be seen that Madagascar was not

Fig. 10 Directivity map of minor tsunami triggered due to September 12, 2007, earthquake. Tsunami

amplitudes are in meters

Fig. 9 Deformation of sea floor at the source region estimated by Mansinha and Smylie (1971). The scale is

in meters

Nat Hazards (2011) 57:726 21

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getting affected by tsunami due to Island of Pointe La Rue and Rodrigues (Fig. 11c). The

maximum wave height of tsunami from trough to crest was around 2.27 m at Padang,

Sumatra (0.95S, 100.36E), as Padang station was nearer to the Source (Fig. 4), and the

second highest wave of tsunami was around 11.2 m at Rodrigue, Mauritius (19.67S,

63.42E), as this station was perpendicular to source into the southwest direction.

Fig. 11 a Tsunami wave travel time at t= 0, 11, 31, 61, 93, 121, 151 and 181 min. Tsunami amplitudes

are in meters. b Tsunami wave travel time at t= 211, 241 and 283 min. Tsunami amplitudes are in meters.

c Tsunami wave travel time at t= 301, 421, 361 and 582 min. Tsunami amplitudes are in meters

22 Nat Hazards (2011) 57:726

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7 Results and discussions

Occurrence of M 8.7 earthquake just after 3 months of M 9.3 earthquake in an adjacent

area indicates that it was most likely triggered by change in static stress due to the first

earthquake (Nalbant et al. 2005; Pollitz et al. 2006). Aftershocks of the two earthquakes are

confined to the respective rupture zones. The large shocks are confined upto Nicobar (9N).

The aftershock zone at 1015N is narrow and confined to west of Andaman Islands along

the zone of convergence.

Due to occurrence of two M 7.5, some 30 of M 66.8 and about 600 of M 55.9aftershocks of December 26, 2004, earthquake, it was perceived by many that it is an

unusually strong aftershock activity with lot of energy released by them. The magnitude

difference of the mainshocks and respective largest aftershocks is within two magnitude

units, which is as expected for large earthquakes in the Indian region (Lay et al. 2005). The

percentage of energy released by aftershocks due to two mainshocks is 0.135 and 0.365%

Fig. 11 continued

Nat Hazards (2011) 57:726 23

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of the energy of the respective mainshocks, while the strain release in aftershocks of

December 2004 and March 2005 earthquake is 39 and 71%, respectively, which are quite

large amounts.

The least-squares and the maximum-likelihood methods were used to determine the

b-value. The maximum-likelihood method gives better estimates of the b-values than the

least-square method, and the estimate is stable for the number of events exceeding 50. The

b-value for the December 2004 aftershocks sequence was (Fig. 5a, b). Thus, the b-value for

the December 2004 aftershocks zone is smaller than that for the aftershocks zone of the

March 2005 event, suggesting a relatively greater probability of large earthquakes in the

aftershock zone of the December 2004 earthquake in comparisons with zone of March 28,2005. Using the Gutenberg and Richter (1944) relation, b-value is estimated for both the

cases, as they varied between 0.9 and 1.0. This indicated that the rupture processes is

basically governed by the material properties (Ramana et al. 2009).

The p-value for the aftershock sequence of March 28, 2005, was 0.88 (Fig. 6a, b).

Hence, it can be inferred that the estimated p-value of 0.88 or 0.99 for the Andaman

Sumatra earthquakes is less than the global median of 1.1 (Utsu et al. 1995). The estimated

low p-value for the aftershocks sequence of 2004 and 2005 events support the low decay of

the aftershock activity.

The 2007 earthquake did not rupture the whole source zone of the 1833 event norreleased the likely accumulated moment since then. This poses a serious threat for a future

big earthquake that will possibly occur in the unruptured area extending between

South Pagai and the south of Siberut Island (Lorito et al. 2008). This earthquake did not

generate ocean wide tsunami. Only minor tsunami generated in local areas near to the

epicentral zone. The tsunami propagated in the southwest direction from the northwest- and

Fig. 11 continued

24 Nat Hazards (2011) 57:726

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southeast-oriented fault plane (Srivastava et al. 2007). Tsunami warning and watches were

issued for all the coastlines of Indian Ocean (NOAA, PTWC and JMA).

8 Conclusions

Seismic characteristics of the aftershock sequences of giant/great Sumatra earthquakes of

December 2004 and March 2005 have been studied. The 2004 earthquake has shown an

unusually strong aftershock activity with lot of energy released by these aftershocks. It is

found that the energy release by aftershocks is not much. The energy released due to

aftershocks sequences of 2004 and 2005 earthquakes was 0.135 and 0.365% of the energy

of the respective mainshocks, while the strain release in respective aftershock sequences

was substantial being 39 and 71%, respectively. The b-value and p-value indicate normal

values of about 1. The b-value and p-value indicate normal heterogeneity, stress distri-

bution and normal rate of decay of aftershocks. It is also observed from p-value that the

aftershocks sequence attenuation was slow as the magnitude was large.

The 2007 earthquake did not generate destructive ocean wide tsunami may be owing to

shallow water depth at the epicenter and small amount of upward slip of sea floor of about

2 m and rupture area of around 160 9 80 km. Tsunami generated by this earthquake only

affected the coastal areas near to the epicentral zone. Though moderate intensity of tsunami

generated into Indian Ocean and it was recorded throughout the Indian Ocean of order of

few centimeters, the tsunami propagated into the southwest direction from the northwest-

and southeast-oriented fault plane. The maximum tsunami energy is perpendicular to the

fault plane. The directivity map shows that the maximum tsunami energy was southwestdirection from the strike of the fault. Since the path of the tsunami for Indian coastlines is

oblique, there were no affect along the Indian coastlines except near the coast of epicentral

region. The water depth below the epicenter was shallow (*135 m or so) also responsible

for generation of moderate intensity of tsunami. To generate destructive Ocean wide

tsunami, water depth at the epicenter of the earthquake should be more and focal depth

would be shallow enough in order to displace the ocean floor.

Acknowledgments The first author is thankful to Dr. V. P. Dimri, Director, NGRI, Hyderabad for his kind

permission to publish this work. We also appreciate the comments of two anonymous reviewers, which

greatly improved the manuscript.

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Documents

Aftershock Sequences of Two Great Sumatran Earthquakes of 2004 and 2005 and Simulation of the Minor Tsunami Generated on September 12, 2007 in the Indian Ocean and Its Effect (1)