Ash Chap 1 Final

7/21/2019 Ash Chap 1 Final

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CHAPTER 1

INTRODUCTION

The earthquakes have been interest of the people throughout the world from early time. It

has attracted large attention due to huge loss in the recent years. Recently a great

earthquake hit the Andaman-Sumatra subduction one on !ecember "#$"%%& at #'(% a.m.

)Indian Standard Time$ IST* of magnitude +.% ),w*. This mega thrust earthquake

originated approimately #% km west of northern Sumatra. This earthquake is one of the

four largest earthquakes since +%%.The other three are the ,w/ +.#$+#% 0hilean$ the

,w / +.& +#& Alaska$ and the +12 earthquake in the Andreanof Island of Alaska

earthquakes and have occurred more than thirty years ago )Stein and 3kal$ "%%14 Tsai et

al.$ "%%1*. This earthquake generated the tsunami which killed more than "$"%%%%

persons. There was loss of property of more than several billion 5S dollar. The

earthquake occurred in a comple tectonic region$ along the boundaries of the Indo-

Australian and 6urasian plates$ the Sunda and 7urma micro plates and the Andaman sub-

plate )8igure .*. It ruptured the subduction one mega thrust plate boundary on the

Sunda Trench. This earthquake has been given many names$ and will be referred here as

the ,w +.% 9orth Sumatra earthquake of "%%&. Its epicenter )latitude (.(#: 9$ longitude

+1.;1&: 6$ 5S<S*$ at a depth of (% km$ was on a segment of the Andaman=Sunda

subduction one that is near the eastern end of the convergent boundary between the

Indian and Australian plate. In the vicinity of the epicenter$ the Indian and Australian$

both plates are moving approimately # cm. per year to the northeast relative to the

overriding 7urmese plate. The fault ruptured propagated northward from the epicenter

along approimately (%% km long$ 1% km wide section of the Andaman-Sumatra

subduction one$ which is the interface between the subducting Indian plate and

overriding 7urmese plate. The surface epression of this subduction one is known as the

Sunda Trench. The Andaman Islands$ 9icobar Islands and the northern part of Sumatra$which all lie on the 7urmese plate4 mark the boundary between the 7ay of 7engal to the

west and the Andaman sea to the east.

,agnitude of an earthquake is the most commonly used parameter of earthquake sie.

<utenberg and Richter have provided a simplest earthquake reoccurrence or magnitude=

frequency relation as$ log% N / a-bM where N is the number of earthquakes in the group



having magnitudes larger than M $ a and b are constants. The estimated coefficient b is

known as the b-value$ varies mostly from %.1 to .1$ depending on the tectonics of the

region. The variability of b-values in different regions may be related to structural

heterogeneity and stress distribution in space. )7hattacharya et al$ "%%"*. The b-value

represents a statistical measurement of the relative abundance of large and small

earthquakes in the group. A higher b-value means that a smaller fraction of the total

earthquakes occur at the higher magnitudes$ whereas a lower b-value implies a larger

fraction occur at higher magnitudes. The higher levels of motion at a site are dominated

by occurrences of the larger earthquakes. If b is large$ large earthquakes are relatively

rare. It has been observed that the b-value shows systemic variations in the period

preceding a ma>or earthquake. ),a$ +2;4Smith$ +;4,a>umdar ? 7iswas$ +++* @ence

it is the most investigated equation in seismology$ observationally as well as theoretically.

)7hattacharya et al$ "%%"*.

8ractal properties of seismicity$ a stochastic self- similar structure in time and space

distribution of earthquakes$ can be measured by fractal dimension which is introduced as

a sophisticated statistical tool to quantify dimensional distribution of seismicity$ its

randomness and clusterisation )@irata$ +;+4 agan and nopoff$ +;%$+;4 3gata$

+;;*. @irata )+;+* reported temporal variations in fractal dimension to quantify the

seismic process. Shimaaki and 9agahama )++1* studied the Bapan region and correlated

the active fault systems with the fractal dimension. Time variations of spatial fractal

dimension also suggest that there may be a positive correlation between b-value and

fractal dimension )3uchi ? 5ekawa$ +;#4 ,ain$ ++*. Therefore it is important to

study the frequency-magnitude relation and fractal dimension of seismicity of a

tectonically active region.

The Andaman-Sumatra region$ situated in the northeast Indian 3cean region is

one of the most tectonically active regions of the world. !′Amico et al )"%%1* have

calculated b-value$ p-value and the cumulative energy and the fractal dimension !% and

!" to high light the clustering of events using the data for the time period +2( to "%%&

for Sumatra area. Biang et al )"%%1* eamined the pre-shock cumulative moment release

in different segment of Sumatra region$ however 7ilham et al )"%%1* studied the rupture

process in this region. 9o systematic studies have been done on b-value and fractal

"



Fig.1.1: Tectonic map of the study region )Andaman arc-Sunda trench* along with free

air map. 0,T solutions for few events are also shown in the fig. )taken from !as et al"%%#* Smallest black dot represents earthquake before "#th !ecember "%%&. Chite star

represents earthquake after the great earthquake of "#th !ecember "%%&

CA8/ Cest Andaman 8ault

(



dimension in Andaman-Sumatra region to understand their tectonic implication and

correlation.

OBJECTIVE OF THE PRESENT WORK:

The available data for the period +2( to Ban (%$"%%# are systematically analyed to

estimate the b-value and fractal dimension in different tectonic block of the region. The

whole region is also mapped in more detail to visualie the spatial variation of the b-

value$ fractal dimension and constant DaE.

&

Ash Chap 1 Final

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