15

CHAPTER 2

GEOLOGY AND SEISMOTECTONICS OF THE REGION

2.1 Introduction

The Kachchh peninsula, constituting the Kachchh district of the Gujarat state,

Indian through which passes the tropic of Cancer, occupies an area of 45, 612 sq km,

and has length and width extensions of 320 and 170 km, respectively. The district has

a total of ten talukas namely Lakhpat, Rapar, Bhachau, Anjar, Bhuj, Nakhtarana,

Abdasa, Mandvi, Mundra, and Gandhidham, with Bhuj town as it’s headquarter. It

has a long coastline (~1600km), along which nine active ports have developed to

carry out the commercial activates. Several creeks, the major ones being Sir, Kori,

Chukh, Kharo and Godia, dissect the coast of the Kachchh. The delta land of Sindh

(Pakistan), also known as olden times by the name Sapta Sindhhhu, or in the seven

distributaries of Indus River, borders it in the west. Its 352 km long southern margin

is demarcated by the Gulf of Kachchh, which separates the peninsula from Saurashtra

or Kathiawar. Its northern margin constitutes the international border with Pakistan

and the eastern abducts Gujarat mainland. The district has a population of 1, 526, and

321 people inhabiting 949 villages, population density of 33-persons/sq km, and a

total of 466,239 housing units (Source: census of Housing GoI, 2001). In the last on

decade, its population density has risen by 5%.

The Gujarat region is one of the most seismic prone intra-continental regions

of the world. All the four seismic zones V, IV, III and II defined in seismic zoning

map of India (Bureau of Indian Standards (BIS), 2002) falls in the Gujarat and

adjoining region. As shown in Figure 2.1, the Kachchh region falls in zone V an 80-

100 km stretch of land bordering Kachchh covering northern parts of Saurashtra and

some part of Gujarat mainland comes under zone IV, rest of Saurashtra and mainland

comes under zone III. A very small area in the eastern part comes under zone II.

The Kachchh region is seismically the second most seismically active region

in India after the Himalayas. It is situated about 500 km away from the Heart-Chaman

16

plate boundary in the west and 1000 km away from the Himalaya plate boundary in

north (Figure 2.2).

Figure 2.1: Map showing a seismic zoning map of Gujarat as per BIS 1893-2002

Figure 2.2: Map showing a broader view of regional tectonics of India. The red rectangle

represents the 2001 Bhuj earthquake region. H-C F.Z; Heart-Chaman Fault Zone (after Singh et al., 2011).

Longitude (E)

Lat

itude

(N)

Lat

itude

(N)

Longitude (E)

17

Kachchh is considered as pre-continental rifted major sedimentary basin

limited by Nagar Parker Uplift (NPU) in the North, Radhanpur-Barmer arch in the

east, North Kathiawar Fault (NKF) in the south and eastern Arabian Sea in the west

(Figure 2.3).

Occurrence of earthquakes between 885 and 1035 A.D has been inferred from

radiocarbon age data (Rajendran and Rajendran, 2001). The M7.8 SamJi, Indus delta

(MM X) of 6th May 1668 earthquake occurred in the region with an epicenter at

25.00N, 68.00E. The Mw7.8 first Kachchh earthquake of 16th June 1819, attracted

attention of international scientific community as the co-seismic uplift had evidenced

presence of the Allah Bund fault expression (Figure 2.3). The crustal block to the

south and north of the Allah Bund fault are identified as hanging-wall and footwall

respectively (Rajendran and Rajendran, 2001; Gaur, 2001).

A scarp excavated identified in a shallow trenches across this structure has

been noted as part of a fold (Rajendran and Rajendran, 2001), which resulted in

progressive migration of streams towards and formation of a lake the down dip side of

the up-thrust block. Thus, 1819 earthquake was significant as it was felt over an area

of one million square miles and large scale changes in ground took place. A bund of

80-90 km running NW-SE was formed called Allah Bund (meaning Dam-of-God) due

to this earthquake (Gaur, 2001). Furthermore, the 1819 earthquake has generated

minor tsunami that affected the Kachchh region on local scale only (Singh et al.,

2008; Jaiswal et al., 2009; Jaiswal et al., 2010). On 19th April 1845 Lakhpat

earthquake (M 6.0, MM VIII) with 60 strong aftershocks was followed by earthquake.

The Mw 6.0 Anjar earthquake of 21st July 1956 which caused 115 deaths was last

damaging earthquake in this region prior to 2001 Bhuj earthquake (Figure 2.3). The

focal mechanism of Kachchh earthquakes, such as 1956 Anjar, 1819 Rann of

Kachchh and few others indicated reverse faulting (Chung and Gao, 1995).

The 2001 Bhuj earthquake has been considered as the largest intraplate

earthquake occurred in modern seismology era (Figures 2.2 and 2.3). The intense

aftershock activity of 2001 Bhuj earthquake is still continuing.

18

Figure 2.3: A Map showing major tectonic features in Kachchh, Gujarat, namely the Kachchh mainland fault, KMF; Katrol Hill fault, KHF; Allah Bund fault, ABF; Island Belt fault, IBF; Gedi fault, GF; Bhuj fault, BF; Nagar Parkar fault, NPF; North Kathiawar fault, NKF; North Wagad fault, NWF. The stars mark the epicenters of the major earthquakes in the region, whose focal mechanism solutions are also shown. Elliptical areas show distribution of the intensity values (X and X+) observed in and around Anjar city during the 2001 Bhuj earthquake. The two solid gray lines, AB and CD, indicate locations of the cross-sections, which are used in next cahpters (modified after Singh et al., 2011).

After more than ten years of the aftershock activity of 2001 Bhuj earthquake, a

M5.6 event occurred along the neighboring GEDI fault (GF, 30 km north of NWF).

Its mainshock occurred of 2006 March 7 (Figure 2.3) was followed by another M5.6

event on 2006 April 6 located on the NWF. It was the second largest aftershock of the

2001 Bhuj sequence (Mandal, 2007). Until December 2010, 14 aftershocks having M

5.0-5.8, about 200 aftershocks of magnitudes M 4.0-4.9, about 2000 aftershocks with

magnitude M 3.0-3.9 and several thousand aftershocks having magnitudes M≤3 have

been recorded (Mandal and Johnston, 2006; Mandal, 2007; Rajendran and Rajendran,

2001; ISR report, 2009, 2010; 2011). Regional seismicity has also increased with

M≤5 earthquakes and associated foreshock-aftershock sequences (Table 2.1). The

Lat

itude

(N)

Longitude (E)

19

most recent of these are well located along with others known only by accounts of

local damage. It is clear from the above descriptions that Kachchh is still one of the

most seismically active areas of the northwest India. The Bhuj earthquake aftershocks

sequence provided us a unique opportunity to probe the rehology of the deep

lithosphere of the intraplate region using large and precise data recorded at ISR

network. Table 2.1 Details of the past earthquakes in Kachchh, Gujarat of M≥5.0

Year Month Date Lat(N) Long(E) Magnitude Location Ref.

1668 05 06 25.00 68.00 7.8 SamJi, Indus IMD

1819 06 16 24.00 69.00 Mw 7.8 Kachchh IMD

1845 04 19 23.80 68.90 6.3 Lakhpat OLD

1845 06 19 23.80 68.90 5.7 Lakhpat OLD

1882 06 28 23.35 70.58 5.0 Lakadia MALIK

1882 06 29 23.35 70.58 5.0 Bhachau MALIK

1903 01 14 24.00 70.00 5.6 Kachchh IMD

1921 10 26 25.00 68.00 5.5 Indus-Kachchh IMD

1950 06 14 24.00 71.20 5.3 Kachchh CHAN

1956 07 21 23.30 70.00 Mw 6.0 Kachchh IMD

1963 07 13 24.90 70.30 5.3 Thar, Pakistan IMD

1965 03 26 24.40 70.00 5.1 Kachchh IMD

1966 05 27 24.46 68.69 5.0 Thar, Pakistan ISC

1970 02 13 24.60 68.61 5.2 Kachchh USGS

1976 06 04 24.51 68.45 5.1 Allah Bund, Pakistan

ISC

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1985 04 07 24.36 69.74 5.0 Kachchh ISC

2001 01 26 23.44 70.31 Mw 7.6 Kachchh USGS

2006 02 03 23.92 70.44 Mw 5.0 Gedi, Rapar NGRI

2006 03 07 23.79 70.73 Mw 5.7 Gedi, Rapar NGRI

2006 04 06 23.34 70.39 Mw 5.6 Lakadia NGRI

OLD- Oldham (1883), CHAN: Chandra (1977), MALIK- Malik et al. (1999).

IMD- India Meteorological Department, New Delhi, India

USGS- United States Geological Survey (USGS), National Earthquake Information

Center

ISC- International Seismological Centre, U.K

GERI- Gujarat Engineering Research Institute, Gujarat, India

2.2 Geology and geomorphology setting

Figure 2.4 shows the geological map of Gujarat. The geology of Gujarat

comprises a Precambrian basement over which younger rocks of Jurassic, cretaceous,

Tertiary and Quaternary is deposited. The rocks of Paleozoic era are absent in

Gujarat. The sedimentary sequences are mainly Jurassic, Tertiary and Quaternary in

age. About 60% of the area is covered by Deccan basalt covering major parts of

Saurashtra, some portions in Kachchh and major portion of South Gujarat with

intervening Cretaceous and Tertiary rocks at many places. Stratigraphically, Mainland

Gujarat comprises of Precambrian crystalline, sedimentary rocks of Cretaceous,

Tertiary and Quaternary periods and the Deccan Trap. The Saurashtra shows

sedimentary sequence as old as upper Jurassic with majority area covered by Basalts

(Merh, 1995).

The geological structures of the Kachchh and in particular of the major

earthquakes (1819 and 2001) region is the most important to know as the hypocenter

located more than 500 km away from the nearest Indian plate boundary (Figure 2.2).

21

Figure 2.4: A geological map showing setup in Gujarat prone region

The Kachchh peninsula presents a complex geomorphic structure in the state

of Gujarat region. The surface configuration, drainage patterns and relief are

immensely affected by tectonic activity, sea level changes and the process of erosion

and deposition. The study area mainly comprises Mesozoic, Tertiary and Quaternary

sediments (Biswas, 1980). Physiographic structure of Kachchh consists of several

high hills interspersed by low plains. The rugged hilly terrain is composed of

Mesozoic rocks (Figure 2.4). The coastal plains bordering it are narrow strips of

gently dipping Cenozoic rocks. These hill ranges separated by low lying plains run

almost parallel to each other revealing the predominant control of geological factors

like folding, faulting etc. The region has been tectonically active right from Mesozoic

times. This activity has resulted in creation of new faults, changes in drainage patterns

etc. Geologically, Quaternary/Tertiary sediments, Deccan volcanic rocks and Jurassic

sandstones rest on the Archean basement mainly characterize the region (Gupta et al.,

2001). The oldest rocks exposed in Kachchh belong to the middle Jurassic age

(Jhurio/Kala Dongar formation) that occurs in the northern part (Figure 2.4). Deccan

traps are seen exposed in the southern and western parts of the Kachchh. Tertiary

sediments are distributed mainly in the western, southern and eastern parts of the

Kachchh. While Tertiary rocks are seen overlying the Deccan traps in Kachchh

Longitude (E)

Lat

itude

(N)

22

mainland, they are seen overlying the Msesozoic rocks in other uplifts. The surface

geology of the 2001 earthquake epicentral region (Figure 2.3) is constituted of

Mesozoic (135-65 million years old) sediments (~2-6 km thick) overlying an uplifted

Granitic basement (Gupta et al., 2001). The region lies just north of the basalt covered

areas of southern Kachchh, which are part of old Deccan traps, one of the largest

volcanic provinces in the world.

Table 2.2: A generalized stratigraphy sequence of the Kachchh region (after Biswas, 1980, 1987,)

23

The sediments are found juvenile southward as well as westward. But the

Kachchh peninsula has recorded a nearly unbroken sequence of sediments from late

Triassic onwards. The sediments fill thickness from less than 500 m in the north to

over 4000 m in the south and from 200 m in the east to over 2500 m in west (Biswas,

1987, 2005). Obvious features of Kachchh district are the vast zones of salt marshes,

island-like uplifts and E-W trending hill ranges flanked by lowlands with a sudden

drop of elevation in the north and gradual drop of elevation in the south. Early

cretaceous sedimentary formations are also seen exposed in many parts of the rocky

land mass. By and large, the Jhurio formation is dominated by limestone and shell, the

Jumara formation by gypseous shale, the Jhuran formation by sandstone and shale

inter calculations and the Bhuj formation by sandstone (Table 2.2). The areas of

Banni grassy undulations, the Rann of Kachchh alluvial or mud, salt flats and the

region between mainland uplift and Wagad uplift, described as Samakhiali Basin,

comprise unconsolidated soft sediments. The Precambrian basement rocks over which

the Mesozoic sediments were deposited but are not exposed in Kachchh. Intrusive

bodies of igneous rocks that include coarse-to-fine-grained melanocratic alkine basic

rocks to oversaturated lecocratic granophyres are seen ubtruding into the Mesozoic

sediments at many parts of Kachchh. Presence of large bodies of mafic intrusive

below the surface is confined by the gravity high recorded around the Nakhtarana,

Pachham uplift and other places.

The region reflects on the anisotropy of lithological units. The crust in

Kachchh comprise fairly thick layer of relatively ductile Mesozoic–Cenozoic

sediments (1-4 km) overlying the rigid basement rocks and several igneous bodies are

unevenly distributed all over. It is also of great importance to consider the anisotropy

within the sedimentary cover that comprises consolidated Mesozoic sediments with

alternating shale and sandstone-limestone beds, rather poorly consolidated tertiary

sediments and unconsolidated blanket of quaternary sediments (Krishna et al., 1983).

When subject to compressive stress, the response of these units would not be

identical.

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2.3 Seismotectonics of Kachchh region

Tectonic history of Gujarat begins with the formation of three major rift

systems namely Kachchh, Cambay, and Narmada basin. These basins were rifted

during break up of Gondwanaland when Indian continent broke off from African

continent (Biswas, 1988). These rift-basins are bounded by three important tectonic

trends, the NNW-SSE Dharawar trend, the NE-SW Delhi-Aravali trend and the ENE-

WEW Satpura trend (also known as Son Narmada Tapti lineaments or SONATA),

which dominate the structural fabric of the region (Oldham, 1926, Talwani and

Gangopadhyay, 2001). However, the Kachchh rift was first to form followed

sequentially by Cambay rift during early Cretaceous and Narmada rift during late

Cretaceous in clockwise direction (Biswas, 1982). Further, the Kachchh rift

developed within the Mid-Proterozoic-Aravali-Delhi fold belt by reactivation of pre-

existing faults along NE-SW trend of Delhi fold belt that swing of E-W in Kachchh

region. Figure 2.3 shows the Seismotectonics map of the Kachchh, Gujarat region. In

the Kachchh region the tectonic trend is EW along which rifting resulted in the

formation of the KRB. The Bouger gravity anomaly in the Kachchh basin is high and

the contours also have the E-W trend (GSI, 2000; Chandrasekhar and Mishra, 2002).

In addition to the major E-W faults, the basin is transacted by major N-S to NE-SW

and NW-SE tectonic lineaments that include a structural ‘Median High’ (Hing Zone)

to the west of Bhuj, a NE-SW lineaments from near Anjar through Rapar a NW-SE

lineament from Bhachau to the NW, NW-SE Banni faults and various short

lineaments and faults (Talwani and Gangopadhyay, 2001). On the other hand, the NE-

SW Aravali trend, continues and across the Cambay basin into Saurashtra horst to

form south westerly plunging Saurashtra arch. The Cambay Rift Basin (CRB) is

considered to have formed along the northern extension of Dharwar trend. The

Narmada Rift Basin (NRB) extends into the Cambay graben, and continues into the

southern part of Saurashtra (Mishra and Gupta, 1993; GSI, 2000).

The structure of the basin is styled by a series of uplifts, master faults and

upthrusts. Tilted footwall uplifts and a series of half-grabens generated in the region

due to the differential movements of discrete basement blocks due to compression

along these faults. The uplifts appear to be the basement blocks draped with marginal

monoclinal flexures of sediment cover over the faulted edges. In all the uplifts, second

25

order folds e.g., anticlines and domes of varying shapes and size, complicate the

flexures within a narrow deformation zone along the master faults. In the western part

of the basin the uplifts are tilted down to the south forming a series of step faults,

whereas in the eastern part the approach is differently tilted.

The E-W intra-basinal faults control the structural trend of the Kachchh rift;

these are Island Belt Fault (IBF), Gora Dongar fault (GDF), Banni fault (BF),

Kachchh Mainland Fault (KMF) and Katrol Hill Fault (KHF), South Wagad Fault

(SWF) and Gedi Fault (GF) (Figure 2.3). These intra-basinal faults are bounding

grabens, half grabens, and horsts within the rift system. These faults are near vertical

basement faults and follow the mid-Proterozoic orogenic trend of Delhi fold belt

(Biswas, 1980, 1987; Biswas and Khattri, 2002). The great Rann sub-basin is a

narrow graben bound by NPF and IBF between Nagar Parkar Uplift (NPU) and Island

Belt Uplift (IBU), which is broken into four separate uplifts namely Pachham (PU),

Khadir (KU), Bela uplift (BU), and Chorar uplifts (CU) by cross faults. The Bouger

gravity anomaly map of the area suggests that these uplifts are mainly associated with

the gravity highs and basement upwraping/volcanic plugs (Chandrasekhar and

Mishra, 2002). Southward tilting of IBU along KMF formed the Banni Half-Graben

(BHG). Kachchh Mainland Uplift (KMU) is bounded by KMF and is tilted to the

south along KHF. The KHF divides the Mainland uplift into Mainland half-graben

(Bhuj Syncline) and Katrol Hill uplift in the southern, which is again tilted to the

south to form the Gulf of Kachchh half-graben against the Saurashtra block along

NKF

In the eastern part of the rift, Wagad uplift (WU) is bound by SWF along its

southern margin. SWF consists of a system of curved, covering and diverging faults

with associated tight folds (Biswas, 1988). The part of WU is much deformed with

complicated pattern of folds. WU is tilted against the north BU (of BU chain) which is

a horst bound by GF and IBF. The north tilting of WU formed Raper Half-Graben

(RHG) against GF. The overlapping relationship of KMF and SWF indicate that they

are segments of a strike-slip fault (Aydin and Nur, 1985; Biswas, 2003, 2005)

A unique feature of the basin is an across the basin meridional high. It is

manifested as a geomorphic high across the Benni half-graben and KMU. This high is

26

a first order basement ridge and appears to be the relict of the hing zone of the Indus

shelf basin prior to Kachchh rifting (Biswas, 1982, 1987; Malik et al., 2000). A

number of small scale extensional faults occur in the crestal region of this NNE-SSW

striking high, parallel to its axis as seen in the central part of the Mainland uplift.

These faults extend for long distances with throws varying from 1 to 100 m. Basic

dykes accompany most of them indicating theirs extensional origin.

2.4 Characteristics of the major faults in the region

The E-W striking master faults are the primary faults, which controlled the

structure formation. Uplifts themselves are extensively affected by secondary faults at

different generations; both normal and strike slip as also a few reverse ones. The

characteristics of the each fault is given below as

2.4.1 Allah Bund Fault (ABF)

The first international Kachchh earthquake to attract international scientific

community was the 16th June 1819 (Figure 2.3) Allah Bund earthquake (Rajendran

and Rajendran, 2001). It is generally believed that the co-seismic uplift evidenced by

the Allah Bund is the expression of a fault; the block to the south of Allah Bund

moving down with respect to northern blocks (Gaur, 2001). Based on the observations

in the shallow trenches across this structure, suggested this scrap may represent a fold,

as evidenced by the morphological features such as progressive migration of streams

and formation of a lake at the down dip side the up-thrust block.

2.4.2 Kachchh Mainland Fault (KMF)

KMF is the biggest and longest fault in the region and became the active

principal fault (Figure 2.3). It extends for 200 km along the northern edge of KMU.

Maximum sediment thickness recorded close to the faults is 2.2 km, thinning

gradually towards PU. About 350 m thick tertiary sediments lie unconformable over

the Mesozoic rocks below the quaternary cover of Banni Half Graben (BHG). This

shows the post Tertiary reactivation of the fault.

27

The KMF is exposed in sediments along the northern escarpment of KMU

whenever the associated flexure is ruptured. Where exposed, steeply upturned (ankle

fold) Tertiary strata are seen juxtaposed with steeply folded (Knee fold) Mesozoic

strata. A narrow zone at highly disturbed rocks indicates the fault. At places abrupt

truncation of sharply placated beds along a line indicate the fault. In the western part

the fault is well exposed from north of Lakhpat to Jara. The fault is again exposed

along Jhuran to Khirasra. North of Dudhai Tertiary beds resting against steeply

dipping Cretaceous beds indicate faulting. At the eastern end north of Bhachau,

steeply dipping cretaceous beds and overlying trap flows indicate gradual fading out

the fault while at the western end the faults appear to continue under the cover of

Rann sediments of indicated by sub-vertical beds of Eocene limestone’s standing

against the tidal flat of Kori creek and narrow strips of steep Miocene beds.

In the fault zone, beds are mostly vertical but frequently show erratic dips due

to tilted horses. Between Ghuveri and Sahera, the faults dip 800 to 850 towards south

with over turned forelimbs of the flexure. In the middle sector in Jhurio-Habo region,

it dips 700 towards north to 090 . In the eastern part it dips again 700-800 to the south

with overturned fold near Kas Hilll - Jawahar Nagar area but beyond Dudhai it seems

to be north-dipping normal fault again. A 4.0 km wide zone of no reflection in BHG,

close to the faulted escarpment of KMU, is seen in the seismic data (ONGC Source).

This zone of no reflection suggest near vertical attitude of the fault at depth. From this

it is concluded that KMF is a planar domino fault bounding a large basement block.

The fault zone is seen at places, e.g., north of Jhuneri, to splay out into general

synthetic faults. The fault extends discontinuously in an echelon pattern with

approaching or overlapping ends. The strike of fault changes from NW near Lakhpat

in the west to E-W near Jhurio hill in the middle and continues as E-W fault up to

Wondh at the eastern plunge of the uplift where it fades out. This shift of strike takes

place by successive southward shift of the fault along transverse wrench faults. These

factures-sub-vertical fault zone braiding or diverging in profile, changing dip and

strike, coexistence of faults with normal and reverse separation, echelon

approaching/overlapping pattern, splay out, accompanying parallel folds, folds at

28

bends (restraining bends), offset by external faults and narrow, linear or curvilinear

deformation zone along the up through side in map view, or evidence of strike-slip

movement along KMF (Wilcox et al., 1973; Christie – Blink and Biddle, 1985). This

is further confirmed by the overlapping step over of the SWF near the eastern end,

north of Dudhai –Bhachau sediment. The relative position of KMU and WU blocks

suggests right lateral strike slip striations and slicker-slides seen locally on exposed

fault plane surfaces (e.g., near Ghuneri and Jhuran) support this setup since its

movement.

2.4.3 South Waged Fault (SWF)

SWF is the eastward continuation of KMF after its left sidestepping with shift

of uplift from south to the north. SWF marks the southern uplifted margin of WU,

which is tilted down to the north forming Rapar Half Graben (RHG). SWF is a fault

zone consisting of two corned, semi-concentric marginal faults, which converge and

diverge bounding folds of varying dimensions. A transverse tear fault, Kidianagar

faults, dislocates the marginal fault in the middle. The concentric fault system

resolves into two parallel faults, east of Kidianagar fault and continues to the east

across the little Rann. Such complex deformation zone with converging/diverging

corned fault traces and folds in restraining bends suggest convergent wrench

movement (Wilcox et al., 1973; Biswas, 1980; Biswas and Khatri, 2002).

The overlapping tips of the both KMF and SWF dip at 800-850 towards each

other. The overcastted step over zone thus appears to be a convergent transfer zone

(Morley et al., 1990). The KMF fades out and SWF continues eastward as the

Permanent Danger Zone (PDZ). It is likely that at depth KMF could be either

antithetic or a down dip step over (Aydin and Nur, 1985) with relation to SWF

(Figure 2.3). With left stepping of right lateral fault the overlap zone is transgressions.

Strike slip faults tend to detach within middle or upper crust (Christie-Blick and

Biddle, 1985). Further steep faults are expected to flat at depth toward the aseismic

semi-brittle zone below the seismogenic layer (Jackson and White 1989). In flatten in

mid crust region and detach at the top of aseismogenic layer (Biswas, 2003; 2005).

29

2.4.4 Island Belt Fault (IBF)

IBF is not well exposed along the island chain of uplift vexing concealed

under Rann sediments. The faulting is indicated by steeply dipping beds of the Ron

limbs of drape folds and the impressive escarpments facing north. At the foot of

northern escarp of Pachham (Kaladongar Hills) hard sandstone beds dipping 600-800

to the north into the Rann sediments indicates the fault. High and erratic dips along

the margin of the uplifts bordering Rann indicate faults. The faults appear to have

been dislocated by left lateral NE-SW strike slip faults, which separated IBU into four

discrete blocks (PU, KU, BU and CU). These blocks were rotated anticlockwise and

shitted progressively westward as indicated by their axial orientation (Figure 2.3).

Pande (2003) has considered the ABF as the western extension of the IBF or one of

the sympathetic off shoots.

2.4.5 Katrol Hill Fault (KHF) and Gedi fault (GF)

KHF and GDF are post-dispositional second order fault within the uplifts

KMU and PU rasp. KHF strikes parallel to KMF (Figure 2.3). To the west it splays

out into two faults, one continues to the west in the same strike and the other strikes

NW as Vigode fault and its splay outs-vigodi-Khirasta-Gugriana-Netra faults (VKGF)

(Biswas, 2003). The latter faults meet the KMF near Lakhpat (Figure 2.3). The west

striking KHF is dislocated and shifted south ward by NE-SW Jarjok faults. The

western half of KHF dips 600-800, locally 450 to the south. The western half east of

leer, it dips 700 to the north to 900, thus the fault has a reverse separation in the

western part associated over folding of beds and a normal separation with drape folds

in the eastern part. The fault plane is very sharp between homodyne beds of

cretaceous rocks to the north and sharply folded Jurassic beds to the south. The fault

plane is discontinuous with on echelon shifts, approaching and overlapping,

displaying the characteristics of strike-slip faults. Conjugate and synthetic sets of

faults are region where MH crosses it is intensely faulted. NNE-SSW faults, parallel

to the crest of MH, dominate the fault population. In general two sets of faults

predominate the fault system in all uplifts-1) NW-SE (varying to NN-SSE) and 2)

NE-SW (varying to MME-SSW). N-S striking faults are also common of these NE-

SW faults show horizontal slips displacing the other faults including the primary ones.

30

The longitudinal strike faults are relatively rare but they are conspicuously

predominant in Kaladongar anticline (northern part of PU) parallel to the IB. the

maximum intensity of faulting and most complicated pattern are seen in KMU. The

MH appears to be bound by NE-SW faults. The eastern flank fault seems to be

passing between PU and KU with a left lateral slip. This interpretation could not be

confirmed since the subsurface data are wanting. Some of these faults match with the

extensive lineament seen in the satellite imageries and seem to extend across BHG

and KMU. The conjectured faults displacing the IBU follow such lineaments. In the

KMU a few extensive NE-SW faults cut across the entire length of the uplift

dislocating both KMF and KHF, eg. Kas Hill and Bharasar fault.

The KRB is very complex in nature in the perspective of seismotectonics and

geodynamics. Presently, no one is able to give a unique tectonic model to explain the

complex structure and occurrence of shallow earthquakes for this region because of

the limited and unavailability of reliable data as well as some constrains in

methodologies. The review of the literatures indicates that the KRB is still active and

causes of the 2001 Bhuj earthquake still puzzle and questionable. Several researchers

are putted their hypothesis for different way. The nature of faulting are different in

different plates however, the overall seismic activity of the region may be assumed to

be depend on stress generated and resistance offered by drifting of plates towards

each other. It is also observed that the Kachchh region seems to be more hazardous

than the other two region of Gujarat namely Saurashtra and Mainland.

In the present thesis a sincere effort has been made to understand the extent of

seismogenic potential and its inter relationship with these stated geological faults. The

role of structural heterogeneities is also deciphered to see how these faults get

reactivated and resulted major earthquakes in the KRB.