Active Low-Angle Reverse Fault and Wide

Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault,

Kachchh, Gujarat, India

Michio Morinoa, Javed N. Malikb, Gadhavi Mahendrasinh S c,

Khalid Ansarib, Chandrashekhar Bhuiyan b,

Prashant Mishra b, and Fumio Kaneko a

Abstract

The Kachchh region has suffered from at least four damaging moderate to large earthquakes since the 17th

century. However, none of these earthquakes except the 1819 Allah Bund earthquake accompanied surface

rupture. Even the recent 2001 Bhuj earthquake with Mw7.6 occurred on a blind fault. Several faults in the

Kachchh viz. the Island Belt Fault, the Kachchh Mainland Fault (KMF), and the Katrol Hill Fault were suggested

to be active during Late Quaternary time by previous studies. But there is no such supportive evidence available

in the historical documents; also none of recent studies except ours (Morino et al., 2007, Malik et al., 2008, and

Morino et al., 2008) reported ground truth that these fault are active.

<BR>We in our earlier paper reported faulting in Late Pleistocene to Holocene age sediment near the Lodai

Village along the KMF. To confirm further active faulting along the KMF, paleoseismic investigation near

Jhura Village about 30 km west of Lodai revealed an active fault displacing overbank deposits of Kaila River.

Two fault strands Fl and F2 were identified in the trench. The northern Fl shows a low-angle reverse fault with

inclination of 15&deg; towards the south. At least two faulting events were inferred on the basis of upward fault

termination with clear angular unconformity. The net-slip during a single faulting event considering deformation

on the hanging wall of Fl fault is over 5 m.

1. Introduction

The Kachchh region, which lies in the western part of the

Indian shield, has suffered severe damages from moderate

to large intra-plate earthquakes since the 17th century, viz.,

the 1668 Indus Delta earthquake (M7), the 1819 Allah Bund

earthquake (M7.8), the 1956 Anjar earthquake (Mw6.0), and the

recent 2001 Bhuj earthquake (Mw7.6) (Johnston and Kanter,

1990; Chung and Gao, 1995). The 1819 event along the Allah

Bund Fault (ABF) is a well documented earthquake in literature

(right top inset of Fig. 1), resulting in formation of 4-6 m

high fault scarp with rupture extending along the E-W strike

up to 80-90 km (Quittmeyer and Jacob, 1979; Johnston and

Kanter, 1990; Bilham, 1998). However, other earthquakes were

generated along blind faults. Though the magnitude of the 2001

event was Mw7.6, the rupture remained concealed below the

ground at a depth of 7-10 km (Mandal and Horton, 2007). So,

if blind faults in the Kachchh region are capable of generating

earthquakes with magnitude as large as Mw7.6, it is possible that

active faults with larger rupture area also have the potential to

generate a similar or larger magnitude earthquake.

The Island Belt Fault (IBF), the Kachchh Mainland Fault

(KMF), and the Katrol Hill Fault (KHF) except the ABF are

well-known as major E-W trending faults in the Kachchh region

(Fig.1). The IBF, the KMF, and the KHF mark geological

OYO International Corporation, Rokubancho Kyodo Bldg. 2F, 6 Rokubancho, Chiyoda-ku, Tokyo, Japan

Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208-016, India

Institute of Seismological Research, Gandhinagar 382018, Gujarat, India

boundary between Lower Jurassic-Upper Triassic and Middle

Jurassic, Middle Jurassic and Tertiary deposits, and Upper-

Middle Jurassic and Lower Cretaceous, respectively (Biswas

and Deshpande, 1970; Biswas, 1980). Based on the occurrence

of uplifted Late Quaternary fluvial and alluvial fan surfaces

along the northern fringe of the Northern Hill Range (NHR) and

along the Katrol Hill Range (KHR), the KMF and the KHF were

suggested to be active (Sohoni et al., 1999; Malik et al., 2001b).

However, no active fault exposure was reported till now. Based

on the satellite photo interpretation, Malik et al. (2001a) inferred

several active fault traces along the KMF and the KHF. But

the field survey and trench investigation were not carried out.

Morino et al. (2007), Malik et al. (2008) and Morino et al. (2008)

were the first to undertake paleoseismic investigations and report

evidence of active faults displacing Quaternary deposits in the

Kachchh region.

We reexamined the satellite photo interpretation made by

Malik et al. (2001a), and conducted field survey along the KMF.

From trench investigation carried out during March-April 2007

near Lodai Village across the KMF, we were able to establish

the KMF as an active fault displacing Late Quaternary alluvial

fan deposits and Banni Plain sediments (Morino et al., 2007;

Malik et al., 2008). To know the further extent of the paleo-

surface rupture that was identified in the Lodai trench, we

carried out trench investigation on the left bank of Kaila River

near Jhura Village about 30 km west of Lodai site, during May

2007 and December 2007 to January 2008 (Figs. 1 and 2a).

A low-angle reverse fault displacing channelized overbank

deposits of Kaila River was identified. In this paper, we report

the nature of active fault, event horizons, and a net-slip during a

single faulting event, though the dating for Optically Stimulated

Luminescence (OSL) is still in process. The data generated from

this study will be very significant for seismic disaster assessment

in the Kachchh region.

2. Geomorphology around the Jhura trench

Figure 1 shows distribution of active fault traces along

the KMF and the location of trench sites at Jhura and Lodai.

The fault traces were identified from the interpretation of the

CORONA satellite photo (Mission No. 1025-2, photographed

on 13 October 1965) and field survey. The active fault traces

striking E-W to WNW-ESE were inferred between east of

Lodai and west of Nirona Village. From the field survey, it is

suggested that the topographic boundary between the hills and

plain (around "Y" of Fig. 2a) represents an erosional boundary,

not a fault contact. The active fault traces are located 1-4 km

north of the NHR, viz., the Habo and Jhurio Hills (Figs. 1

and 2a). CORONA satellite photo around Jhura Village and

topographic profile extracted from SRTM data (Figs. 2a and 2b)

revealed uplifted flat surface at the base of Jhurio Hills. Vertical

displacement along the active fault has resulted in formation

of about 10 m high NNE facing low scarp demarcating the

topographic boundary between the alluvial fan of Kaila River

on the north and uplifted flat surface on the south (Figs. 2a and

2b). The southwestern uplifted surface has been incised by many

small channels. However, the top avoiding erosion maintains

almost same elevation of 35 m (Fig. 2b). A 35 m high upper

surface shows comparatively less erosion, and is underlain by

weathered Jurassic rocks with no terrace deposits. This upper

surface is inferred as strath terrace. It was assumed that the low

scarp represents fault topography, since it is linear and restrains

the head of the alluvial fan. The KMF as an active fault was

inferred along the road by the satellite photo interpretation.

However, the active fault was finally confirmed by trenching

on the northern alluvial fan. The low fault scarp may have been

modified by cultivation and shifted to the south from its original

position (Fig. 3).

The fault which demarcates the boundary between Jurassic

shale and Tertiary conglomerate was found out on the bank of an

artificially excavated pond (Fig. 3). The Jurassic shale is highly

sheared; the fault strikes N 600 W, and dips 70-80° SW. This

fault is located on the uplifted surface (Fig. 3). The sheared shale

is vulnerable. However, the fault topography such as a fault

scalp was not recognized along this fault. This fault is the older

KMF which forms the geological boundary between Jurassic and

Tertiary deposits.

3. Trench Investigation across the KMF at Jhura

site

Two trenches were excavated as shown in Fig. 3. Trench

1 was excavated on May 2007. The sedimentary succession

exposed in trench 1 revealed several packages composed of

horizontally stratified unconsolidated sand and gravel layers.

These layers showed prominent inclination of 20°-30° towards

north, and are displaced by three high-angle reverse faults

(Morino and Malik, 2008). The pattern of deformation and

around 20 m wide zone of deformation implied existence of the

major fault further north of trench 1. Therefore, trench 2 was

excavated during December 2007 to January 2008. In this paper,

we only discuss trench 2. In following text the trench 2 will be

referred to as the trench.

3.1 Stratigraphy identified in the trench

To confirm the existence of major fault and to understand the

wide zone of deformation revealed by the inclined sedimentary

units, a 28 m long, 2 m wide and 2.5 m deep trench was

excavated (Figs. 3 and 4). The trench revealed several packages

of medium-coarse sand and fine gravel representing channelized

overbank deposits of Kaila River. They are displaced by two

south dipping low to high angle reverse fault strands Fl and

F2 (Fig. 4). The sedimentary succession is marked by typical

upward fining sequence with gravel or coarse sand at the

base and medium to fine sand in the upper part. Based on the

repetitive sequence and angular unconformity with respect to

the faulting, the exposed sedimentary succession was divided

into 6 units (1 to 6). Units 2 to 5 were further divided into sub-

units like a, b, c, d. Each unit represents an individual cycle of

deposition. Regarding the boundary of units 2 and 3, the angular

unconformity is recognized between units 2b and 2c. However,

unit 2c shows similar facies to units 2a and 2b, and is classified

as unit 2.

Unit 1 is stratified medium to coarse sand with gravel at the

base. Since this unit shows a horizontal structure, and covers all

the units exposed in the trench, it is suggested to be the recent

small channel deposits. Unit 2 is divided into subunits 2a to 2c:

unit 2a - massive fine sand, 2b - stratified fine sand, and 2c - fine

sand with scattered fine gravels. It is suggested that unit 2 was

deposited only on the downthrown side of the fault. The present

inclined unit 2b with a dip of about 3° due north represents

the original surface. Unit 3 shows fining upward cycle and is

divided into subunits 3a to 3c: 3a and 3b - fine gravel to coarse

sand, and 3c - coarse to medium sand. This is suggestive of

deposition under overbank environment. These units are inclined

10°-15° to the north. The dragging deformation resulting from

dip-slip is clear on the hanging wall of Fl fault (Figs. 4 and

5). A gentle syncline and an anticline are recognized on the

footwall of Fl fault. Unit 4 also shows upward fining sequence

with medium to coarse sand representing overbank deposition,

and subdivided into units 4a to 4d. Units 4a to 4d exhibit typical

fault-propagated-folding and dragging movement near the fault

tip on the hanging wall of Fl strand (Figs. 4 and 6). Units 4a

to 4d are inclined about 25° towards the north between E6 and

El0 horizontal markers on the hanging wall, whereas, it shows

higher inclination of about 60° due north on the footwall of Fl

strand. This probably occurred due to intense folding during

deformation, which finally got faulted along Fl strand. Unit 5

is divided into subunits 5a to 5d: unit 5a - gravel and medium

to coarse sand, 5b - massive well-sorted medium sand with

scattered cobbles, 5c - poorly sorted fine to medium sand, and

5d - stratified medium sand. Unit 6 is weakly consolidated sandy

silt with coarser angular fragments representing older debris

deposits probably derived from the hanging wall.

3.2 Paleoseismic interpretations

Two prominent fault strands Fl and F2 dipping towards the

south were identified in the trench (Fig. 4). Fl strand is a low-

angle reverse fault with inclination of about 15°. Two seismic

events are properly understood by dividing Fl fault into two

fault strands F1-1 and F1-2. F1-1 strand extends from E8 to

20 cm north of E6 horizontal marker. F1-2 strand extends

from E6 to E4 (Fig. 5). Upper two faults observed around E6

are not subsidiaries of Fl fault. These faults are inferred to be

the extension of F1-1 strand. F1-1 strand displaces units 4a to

4d, and is covered with unit 3c. Furthermore, the F1-1 strand

reactivated after the deposition of units 2c to 3c, and the F1-2

strand propagated northward from E6. F1-2 strand displaces

units 2c to 3c, and is covered with unit 2b. The layers on the

hanging wall of Fl fault are deformed widely between E4 and

E23. The width of the deformation is about 20 m. Another set of

high-angle reverse faults (F2-1 and F2-2) with inclination of 50

° towards the south were identified around E16 to E17 . Judging

from the 20 m. wide deformation associated with Fl fault, it is

suggested that the F2 fault may be a secondary fault.

Two clear angular unconformities (shown by thick lines in

Fig. 5) were observed along F1-1 and F1-2 fault strands. One

unconformity is marked by prominent variation in inclinations

between units 4a to 4d with dip of about 25° and that of units 2c

to 3c about 10°-15°. Also the thickness of unit 3c between E6

and E9 is about 20 cm, whereas, between E5 and E6 it is about

60 cm. This suggests that unit 3c was deposited after a faulting

event occurred along F1-1 strand displacing unit 4. The unit 4

became inclined by the activity of F1-1 fault. Similarly, units

2a and 2b covering units 2c to 3c mark another major angular

unconformity.

Based on the stratigraphic cross-cutting relationship, variation

in inclination of the units and angular unconformities, at least

two seismic events are inferred. The F1-2 fault displaces unit 2c

and is covered unconformably with unit 2b. The latest seismic

event occurred after the deposition of unit 2c and before the

deposition of unit 2b. Also the F1-1 fault displaces unit 4 and is

covered unconformably with unit 3c. Therefore, it is suggested

that the penultimate seismic event occurred after the deposition

of unit 4a and before the deposition of unit 3c.

F2-1 strand displaces the lower part of unit 4d. However, the

fault dies out upward in the unit 4d, without showing upward

fault termination against an unconformity. The event occurred

after the deposition of unit 4d is only inferred. The F2-1 strand

may have activated accompanied by the latest or penultimate

event along Fl fault. However, unit 4d seems to cover unit 5a

unconformably, though the depth of the trench is not enough. If

the unconformity was formed by the activity of Fl fault, then the

third-to-the-last event may have occurred after the deposition of

unit 5a and before the deposition of unit 4d.

3.3 Net-slip on Fl fault considering fault drag

The dip separation of each unit across Fl fault is shown in

Fig. 5. The dip separation of the top of units 3a to 3c and 4a,

which represents that during a single event, is 60-70 cm. The

dip separation of the top of unit 4b is 144 cm, which is twice

the amount of dip slip observed on F1-2 fault during the latest

event. The dip separation of 144 cm on F1-1 fault means two

time displacements accompanied by the latest and penultimate

events. The dip separation shown in Fig. 5 indicates only the

displacement on Fl fault. However, the zone of deformation on

the hanging wall of Fl fault is very wide. This clearly indicates

that most of the displacement accompanied by the activity

of Fl fault is accommodated by folding at shallow depths on

the hanging wall. Therefore, we considered the wide zone of

deformation to estimate the true net-slip.

Since units 2c, 3a, and 3b are eroded, we discuss the

deformation of unit 3c for calculating net-slip during a single

event. The estimated net-slip is shown in Fig. 7. Because unit 3c

is deformed both on the hanging wall and on the footwall, the

true net-slip should be calculated by restoring the deformation

to original sedimentary inclination. On the hanging wall of Fl

fault, the net-slip is estimated as the vertex of the observed fault

and the line inclined 3° towards the north drawn from the top

of the deformation. On the footwall of Fl fault, unit 3c shows

a gentle syncline and an anticline. Therefore, we assumed that

the middle line between the top and bottom of the deformation

is regarded as original sedimentary surface of unit 3c. The top

of the deformation of unit 3c on the hanging wall is unknown,

since the unit 3c is eroded accompanied by the deposition of

unit 1. If the top of the deformation of unit 3c is the vertex of the

top of unit 3c and the bottom of unit 1, the net-slip is about 5 m.

However, it represents a minimum one, since the deformation of

unit 3c on the hanging wall is broader.

4. Conclusion

From the present study we are able to draw the following

conclusions:

1) Our study shows that the KMF has been active during

Late Quaternary time and has the potential to produce large

magnitude earthquakes in the future. There are no historical

records suggesting occurrence of earthquakes along the KMF

during historic past.

2) Trench data revealed occurrence of at least two major

faulting events on the KMF. The faults in young alluvium

terminate dip-slip against clear angular unconformities in the

Jhura trench. The time of events is expected to be decided with

narrow range.

3) The KMF is a low-angle reverse fault with inclination of 15°

towards the south.

4) The amount of net-slip at depth during a single event is

estimated to be more than 5 m, which is much larger than the

slip measured in trench. This is because most of the deformation

is accommodated by drag folding in young alluvium at shallow

depths.

Acknowledgements

The authors are thankful to Rajesh Kishore, Chief Executive

Officer, GSDMA, for his permission to publish this work.

The authors are grateful to Dr. B. K. Rastogi, Institute of

Seismological Research, Prof. S. K. Jain, India Institute of

Technology Kanpur, and Dr. Alpa Sheth, VMS Consultants

Private Limited, for the discussion on this study.

References

Bilham, R., 1998, Slip parameters for the Rann of Kachchh, India,

16 June 1819 earthquake quantified from contemporary accounts.

in Coastal Tectonics Stewart, I. S. and Vita-Finzi, C. (editors),

Geological Society of London, 146, 295-318.

Biswas, S. K., 1980, Structure of Kutch-Kathiawar Region, Western

India. Proc. 3rd Ind. Geol. Congr. Pune, 255-272.

Biswas, S. K. and Deshpande, S. V., 1970, Geological and tectonic

maps of Kutch. Bulletin of Oil Nat. Gas Comm., 7, 115-123.

Chung W-Y. and Gao H., 1995, Source parameters of the Anjar

earthquake of July 21, 1956, India, and its seismotectonic

implications for the Kutch rift basin. Tectonophysics, 242,

281-292.

Johnston, A. C. and Kanter, L. R., 1990, Earthquakes in stable

continental crust. Scientific American, 262, 69-75.

Malik, J. N., Nakata, T., Sato, H., Imaizumi, T. Yoshioka, T., Philip,

G., Mahajan, A. K., and Karanth, R. V., 2001a, January 26, 2001,

the Republic Day (Bhuj) earthquake of Kachchh and active faults,

Gujarat, western India. Active Fault Research, 20, 112-126.

Malik J. N., Sohoni, P. S., Merh, S. S., and Karanth, R. V., 2001b,

Active tectonic control on alluvial fan architecture along the

Kachchh Mainland Hill Range, Western India. Zeithschrift fiir

Geomorphologie, 45, 1, 81-100.

Malik, J. N., Morino, M., Mishra, P., Bhuiyan, C., and Kaneko,

F., 2008, First active fault exposure identified along Kachchh

Mainland Fault: evidence from trench excavation near Lodai

village, Gujarat, Western India. Journal of the Geological Society

of India, 71, 201-208.

Mandal, P. and Horton, S., 2007, Relocation of aftershocks, focal

mechanisms and stress inversion: Implications toward the

seismo-tectonics of the causative fault zone of Mw7.6 2001 Bhuj

earthquake (India). Tectonophysics, 429, 61-78.

Morino, M., Malik, J. N., Mishra, P., Bhuiyan, C., and Kaneko, F.,

2007, Trench survey across Kachchh Mainland Fault at Lodai

village, Kachchh, Gujarat, India. Journal of the Geological

Society of Japan, 113, pictorial, 15-16.

Morino, M. and Malik, J. N., 2008, Field trip guide to international

workshop on active fault in Kachchh (18-19 January 2008 at Bhuj, Kachchh). Zip.

Morino, M., Malik, J. N., Mishra, P., Bhuiyan, C., and Kaneko, F.,

2008, Active fault traces along Bhuj Fault and Katrol Hill Fault,

and trenching survey at Wandhay, Kachchh, Gujarat, India.

Journal of Earth System Science, 117, 181-188.

Quittmeyer, R. C. and Jacob, K. H., 1979, Historical and modern seismicity of Pakistan, Afghanistan, North-western India and

South-eastern Iran. Bulletin of the Seismological Society of

America, 69, 773-823.

Sohoni, P. S., Malik, J. N., Merh, S. S., and Karanth, R. V., 1999,

Active tectonics astride Katrol Hill Zone, Kachchh, Western

India. Journal of the Geological Society of India, 53, 579-586.

Received : June 9, 2008

Accepted : September 18, 2008

Key words : active fault, trench investigation, Kachchh Mainland Fault, Gujarat