活断層研究 29 号 71 77 2008 71 Active Low-Angle Reverse Fault and Wide Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault, Kachchh, Gujarat, India Michio Morino a, Javed N. Malik b, Gadhavi Mahendrasinh S c, Khalid Ansari b, 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 But there is no such supportive evidence available in the historical documents; also none of recent studies except ours (Morino et al., 2007, Malik et al., 2008, and Morino et al., 2008) reported ground truth that these fault are active. 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 F1 and F2 were identified in the trench. The northern F1 shows a low-angle reverse fault with inclination of 15 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 F1 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 *a *b *c 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
72 Michio Morino Javed N. Malik Gadhavi Mahendrasinh Khalid Ansari Chandrashekhar Bhuiyan Prashant Mishra Fumio Kaneko 2008 boundary between Lower Jurassic-Upper Triassic and Middle carried out trench investigation on the left bank of Kaila River Jurassic, Middle Jurassic and Tertiary deposits, and Upper- near Jhura Village about 30 km west of Lodai site, during May Middle Jurassic and Lower Cretaceous, respectively (Biswas 2007 and December 2007 to January 2008 (Figs. 1 and 2a). and Deshpande, 1970; Biswas, 1980). Based on the occurrence A low-angle reverse fault displacing channelized overbank of uplifted Late Quaternary fluvial and alluvial fan surfaces deposits of Kaila River was identified. In this paper, we report along the northern fringe of the Northern Hill Range (NHR) and the nature of active fault, event horizons, and a net-slip during a along the Katrol Hill Range (KHR), the KMF and the KHF were single faulting event, though the dating for Optically Stimulated suggested to be active (Sohoni et al., 1999; Malik et al., 2001b). Luminescence (OSL) is still in process. The data generated from However, no active fault exposure was reported till now. Based this study will be very significant for seismic disaster assessment on the satellite photo interpretation, Malik et al. (2001a) inferred in the Kachchh region. several active fault traces along the KMF and the KHF. But the field survey and trench investigation were not carried out. 2. Geomorphology around the Jhura trench Morino et al. (2007), Malik et al. (2008) and Morino et al. (2008) were the first to undertake paleoseismic investigations and report Figure 1 shows distribution of active fault traces along evidence of active faults displacing Quaternary deposits in the the KMF and the location of trench sites at Jhura and Lodai. Kachchh region. The fault traces were identified from the interpretation of the We reexamined the satellite photo interpretation made by CORONA satellite photo (Mission No. 1025-2, photographed Malik et al. (2001a), and conducted field survey along the KMF. on 13 October 1965) and field survey. The active fault traces From trench investigation carried out during March-April 2007 striking E-W to WNW-ESE were inferred between east of near Lodai Village across the KMF, we were able to establish Lodai and west of Nirona Village. From the field survey, it is the KMF as an active fault displacing Late Quaternary alluvial suggested that the topographic boundary between the hills and fan deposits and Banni Plain sediments (Morino et al., 2007; plain (around Y of Fig. 2a) represents an erosional boundary, Malik et al., 2008). To know the further extent of the paleo- not a fault contact. The active fault traces are located 1-4 km surface rupture that was identified in the Lodai trench, we north of the NHR, viz., the Habo and Jhurio Hills (Figs. 1 Fig. 1 Active fault traces along the Kachchh Mainland Fault and trench sites of Jhura and Lodai. Active fault traces are shown by bold lines. Box in the upper right inset shows major active faults in the Kachchh region. ABF: Allah Bund Fault, IBF: Island Belt Fault, KMF: Kachchh Mainland Fault, KHF: Katrol Hill Fault, BF: Bhuj Fault. Box with dashed line in the inset represents the subsurface rupture plane area of the 2001 Mw7.6 Bhuj earthquake based on micro-earthquakes distribution (Mandal and Horton, 2007). Star shows the epicenter of the 2001 earthquake.
活断層研究 29号 Active Low-Angle Reverse Fault and Wide Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault, Kachchh, Gujarat, India 73 Fig. 2 2a: CORONA satellite photograph around Jhura Village. Arrows represent an active fault. 2b: Topographic profile made from SRTM data. The location of profile is shown in Fig. 2a. See text for details. 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 Fig. 3 Sketch showing trench sites at Jhura and generalized geology (location is shown in Fig. 2a). The KMF as an active fault was inferred along the road by the satellite photo interpretation. However, the active fault was finally identified on the northern alluvial fan by trenching. The low fault scarp is modified by cultivation. The older KMF which demarcates the geological boundary between Jurassic shale and Tertiary conglomerate was confirmed on the uplifted surface.
74 Michio Morino Javed N. Malik Gadhavi Mahendrasinh Khalid Ansari Chandrashekhar Bhuiyan Prashant Mishra Fumio Kaneko 2008 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 60 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 scarp 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 F1 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 subunits 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 Fig. 4 Log of the eastern wall of trench 2 at Jhura site. The sedimentary succession in the trench is composed of fine to coarse sand and fine gravel representing overbank deposits of Kaila River. These deposits are divided into units 1 to 6 considering mainly angular unconformities related to faulting. Units 2 to 5 are divided into sub-units like a, b, c, d (viz. 4a, 4b, 4c, 4d) by the deposition cycle of upward fining sequence. F1 and F2 fault strands are identified in the trench. The F1 shows a low angle reverse fault with inclination of about 15, and the layers on the hanging wall of the F1 fault is widely deformed. E0 to E28 represent horizontal markers with interval of 1m. See text for details.
活断層研究 29号 Active Low-Angle Reverse Fault and Wide Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault, Kachchh, Gujarat, India 75 inclined unit 2b with a dip of about 3 due north represents and subdivided into units 4a to 4d. Units 4a to 4d exhibit typical the original surface. Unit 3 shows fining upward cycle and is fault-propagated-folding and dragging movement near the fault divided into subunits 3a to 3c: 3a and 3b - fine gravel to coarse tip on the hanging wall of F1 strand (Figs. 4 and 6). Units 4a sand, and 3c - coarse to medium sand. This is suggestive of to 4d are inclined about 25 towards the north between E6 and deposition under overbank environment. These units are inclined E10 horizontal markers on the hanging wall, whereas, it shows 10-15 to the north. The dragging deformation resulting from higher inclination of about 60 due north on the footwall of F1 dip-slip is clear on the hanging wall of F1 fault (Figs. 4 and strand. This probably occurred due to intense folding during 5). A gentle syncline and an anticline are recognized on the deformation, which finally got faulted along F1 strand. Unit 5 footwall of F1 fault. Unit 4 also shows upward fining sequence is divided into subunits 5a to 5d: unit 5a - gravel and medium with medium to coarse sand representing overbank deposition, to coarse sand, 5b - massive well-sorted medium sand with Fig. 5 Mosaic photograph of the eastern wall around F1 fault. F1-1 fault displaces unit 4a to 4d and is covered with unit 3c. F1-2 fault displaces unit 2c to 3c and is covered with unit 2b. Bold lines represent angular unconformities. Observed dip separation across F1-1 and F1-2 faults is shown in the figure. The dip separation of the top of units 3a to 3c and 4a is 60-70 cm. This represents the dip slip during a single faulting event. The dip separation of the top of unit 4b which shows twice faulting is 144 cm. See text for details. Fig. 6 Photograph showing the deformation on the hanging wall of F1 fault. The layers on the hanging wall of F1 fault is deformed with arch-shape and shows the dragging structure close to the fault.
76 Michio Morino Javed N. Malik Gadhavi Mahendrasinh Khalid Ansari Chandrashekhar Bhuiyan Prashant Mishra Fumio Kaneko 2008 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 F1 and F2 dipping towards the south were identified in the trench (Fig. 4). F1 strand is a lowangle reverse fault with inclination of about 15. Two seismic events are properly understood by dividing F1 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 F1 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 F1 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 F1 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 F1 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 F1 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 F1 fault considering fault drag The dip separation of each unit across F1 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 F1 fault. However, the zone of deformation on Fig. 7 Net-slip considering fault drag of unit 3c. 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 during a single faulting event is estimated to be 5 m. However, it represents a minimum one, since the deformation of unit 3c on the hanging wall of F1 fault is broader. See text for details.
活断層研究 29 号 Active Low-Angle Reverse Fault and Wide Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault, Kachchh, Gujarat, India 77 the hanging wall of F1 fault is very wide. This clearly indicates that most of the displacement accompanied by the activity of F1 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 F1 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 F1 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. 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78 Michio Morino Javed N. Malik Gadhavi Mahendrasinh Khalid Ansari Chandrashekhar Bhuiyan Prashant Mishra Fumio Kaneko 2008 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