Seismotectonics of 26 January 2001 Bhuj earthquake ... - Iisc.ernet.in

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Seismotectonics of 26 January 2001 Bhuj earthquake-affected region On 26 January 2001 (Republic Day), a devastating earthquake struck the Bhuj district and surrounding areas of Gujarat. It was one of the deadliest intraplate earthquakes, with conservative official estimate putting the number of human lives lost at 30,000 and the economic loss at US$ 10 billion1. The epicentre of the earthquake is located at 23.326oN, 70.317oE, 15 km northwest of Bachau and 60 km east of Bhuj; the magnitude is Mw = 7.6 and focal depth is 17–22 km (USGS). The Kachchh region has experienced large earthquakes in the historical past. The May 1668 earthquake completely destroyed the town of Samaji (25oN, 68oE), and the 16 June 1819 Allahband earthquake of magnitude > 8 in the Great Rann of Kachchh formed a mound 90 km long and up to 9 m in height. The Allahband earthquake killed 1500 people in Bhuj and 500 in Ahmedabad2. On 19 June 1848, an earthquake shook the walls of the fort at Lukput where some lives were lost, and the sea rolled up the Kores, overflowing the country westward. During the 1848 earthquake from 19 to 25 June, 66 shocks were counted. The 30 April 1864 earthquake was felt in Wagir, Ahmedabad and Surat3 . The magnitude > 7 Anjar earthquake of 21 July 1956 which killed about 150 people, was the last major earthquake in this region. The 1819 Allahband earthquake was grouped under the Stable Continent Region (SCR) earthquake, comparing it with the New Madrid earthquake4. The scarp morphology of the Allahband is interpreted as representing a growing fold related to a buried northdipping thrust fault5,6. The reactivation of rift-related normal fault into reverse fault was interpreted as the causative fault for this earthquake4. The location of Allahband Fault and probable epicentral location of Allahband earthquake, the epicentre of the 2001 Bhuj earthquake lying close to Chaman Fault system and active Makran subduction (Figure 1) and historical seismicity suggest that the Kachchh region is undergoing contraction as a result of northward convergence of India. The Allahband Fault is interpreted as dipping north5–8. The study of aftershocks of Bhuj earthquake suggests that the causative fault plane was dipping 40–50o 396

south9. But the damage pattern of Allahband earthquake reported by Wynne3 and Oldham2, is similar to that of the 2001 Bhuj earthquake, showing damage extending towards east and south from the epicentres of both the earthquakes. An alternative explanation is discussed here, suggesting that Allahband Fault may be also dipping south akin to south-dipping causative fault for Bhuj earthquake. The Kachchh region lies close (~ 250 km) to the active western margin of the Indian plate which is juxtaposed against the Eurasian plate along the Chaman Fault system on its northern part and against the oceanic Arabian plate on its southern part (Figure 1 a, b)10. Along the

Makran margin there is an active subduction zone, where the oceanic lithosphere of the Arabian plate is subducting underneath the Eurasian plate. The Indian plate as well as the Arabian plate have similar movement direction, for both the plates are converging towards north to north northeast. The Chaman Fault, and its enechlon southward continuation Ornach-Nal fault, both trend in north-south direction with left-lateral strike-slip motion (Figure 1 c)11. In Chaman Fault Zone, the north-south oriented transform fault merges westward into Makran subduction zone, changing its motion of left-lateral strike-slip into eastwest trending thrust faulting along the northward-converging Makran subduction

Figure 1. a, Plate tectonic setting of 2001 Bhuj earthquake. Epicentre of the earthquake is shown as a cross. Chaman Fault system and Makran subduction show northward active convergence of Indian and Arabian plates, respectively (adapted from Jacob and Quittmeyer10); b, Seismicity map of Kachchh and adjoining regions. Circle represents epicentre. Isoseismals of a few dated events (1819, 1909, 1945) are shown. Triangle represents the location of Karachi (adapted from Quittmeyer et al. 21); c, Major tectonic features and trends in Indian plate in Gujarat (India) and Pakistan. Left-lateral strike slip motion of Ornachnal Fault merges westward into reverse fault along Makran subduction. Arrows represent fold trends (adapted from Sarwar and DeJong12). CURRENT SCIENCE, VOL. 82, NO. 4, 25 FEBRUARY 2002

SCIENTIFIC CORRESPONDENCE zone (Figure 1 c)11,12. In the Karachi region, the Karachi arc is made of gently folded Tertiary rocks with their axes plunging northwest and southwest, forming eastward-convex axial traces of folds12. The southern edge of the arc, south of Karachi, is demarcated by east-west trending faults13, named as the ‘Cutch fault zone’ after the marshland of Kachchh (Figure 1 c). The Kachchh region occupies a peculiar plate tectonic setting for it lies close to a triple junction of Indian, Eurasian and Arabian plates, with a complex matrix of plate motions (Figure 1 a). The northwest-southeast to east-west trending folds and thrusts system of the region indicates north-east to north-south compression, which corresponds to north to northeastward convergence of Indian as well Arabian plates. The Kachchh region of Gujarat is located between the Gulf of Kachchh in India and southern border of Pakistan in western India, extending from the Arabian Sea in the west to the Gujarat plain. Biswas14,15 has carried out extensive map-

Figure 2.

ping and has worked out the geology of the region. The earthquake geological interpretation and seismotectonics of the Bhuj earthquake are based on Biswas14–16 and Merh17. The Kachchh region is characterized by uplifted highlands and islands surrounded by plains of the Great Rann, Banni and Little Rann. The northern margins of the uplifts are demarcated by major faults, namely the Kachchh Mainland Fault (KMF), Katrol Hill Fault (KHF) and Island Belt Fault (IBF) (Figure 2)14,15. The KMF demarcates the northern margin of the mainland uplift along its entire 180 km length. It has a prominent geomorphic expression exhibiting an abrupt rise of the hill ranges from the flat Banni plain. Along the northern border of the mainland uplift, the Tertiary rocks steeply dipping, are faulted against the folded Mesozoic rocks. The KMF strikes NW–SE near Lakhpat and changes its orientation to E– W from Dudhai and further east. It is a reverse fault with regional dip towards south at steep angle near the surface, but may be flattening at depth. The KHF runs

parallel to the mainland fault in E–W to ESE–WNW direction and extends about 60 km from Nana Hill to Samprada. The fault brings up the Charwar range with profound uplift feature. The IBF, an east-west to northwest-southeast trending, demarcates the northern margin of the Pachham, Khadir and Bela uplifts (islands). A sedimentary sequence, approximately 4000 m thick, ranging from Lower Jurassic to Holocene with intervening Upper Cretaceous–Palaeocene Deccan Trap is exposed in the Kachchh region17. The fossiliferous Lower Jurassic to Middle Cretaceous rocks of marine to fluviatile nature of the sedimentary sequence are inferred to overlie the Precambrian basement, with unconformable contact. The basalts and associated intrusive mafic rocks of Upper Cretaceous to Palaeocene Deccan Trap overlie the Lower Jurassic to Middle Cretaceous strata with unconformable contact. The Deccan Trap is capped invariably by laterite horizon of Palaeocene. The laterite also marks an

Tectonic map of Kachchh region (modified after Biswas15).

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SCIENTIFIC CORRESPONDENCE unconformity with the overlying Tertiaries. The Tertiaries, about 1000 m thick, is made of marine to fluviomarine beds of Eocene to Pliocene sequence. The Pleistocene deposits are characterized by meliolite of aeolian origin. The Holocene sediments are comprised of coastal sands of mud flats, Rann sediments, alluvium and residual deposits. The Mesozoic strata have been subjected to northeast-southwest to northsouth compression in post-Mesozoic time, producing folds of several orders with regional axial traces trending northwestsoutheast to east-west. The major northwest to east-west trending faults, namely KMF, KHF and IBF and fault-propagated folds were formed in post-Mesozoic time. The KMF, KHF and IBF are interpreted as south-dipping thrust faults, with steep dips near the surface becoming gentle dipping and probably meeting a detachment fault at a depth of about 25 km (Figure 3). Although the evolution of Mesozoic basin of Kachchh is attributed to rift tectonics4,6,16, the development of fold and thrust belt system in post-Mesozoic time may be related to northward convergence and collision of northwestern part of India with Eurasia around 55–60 Ma. In Quaternary time, active tectonism is observed in reactivation of KMF and KHF, and wrapping of Quaternary rocks in Upper Pleistocene18,19. The fault plane solutions of Bhuj earthquake indicate thrust faulting on two planes, striking 292° and 60° (USGS) or 276° and 78° (ERI, Tokyo). The aftershocks data (http: //www.ceri.memphis.edu/~withers Gujarat) suggest that fault plane dips 40–50o south, their focal depths ranging from 8 to 40 km (ref. 9). The epicentre of the earth-

quake lies very close to the eastern end of the KMF. The projection of the epicentre to a focal depth of 20 km, and absence of any surface deformation features associated with the KMF suggest that the KMF is not the causative fault for this earthquake. It appears that the south-dipping fault, indicated by aftershocks data9, remained a blind reverse fault as no surface rupture was observed. However a surface rupture was observed near the small town of Manfra, and was referred as the Manfra Fault20. The fault strikes northwest for a distance of about 8 km. It shows right lateral motion with up to 30 cm of slip (Figure 4) and locally is characterized by a zone of distributed faulting having a width of 10 to 20 m. The Manfra Fault appears to be reflecting coseismic deformation of the hanging wall above the main causative fault plane20. The strike-slip motion reflected on the surface may have been produced due to transfer of motion from reverse faulting; for this happens when a reverse fault becomes steeply dipping and locking takes places, resulting in a change in relative motion from reverse to strikeslip. In Thomas Oldham’s memoir, Wynne’s account3 of damage pattern of 1819 Allahband earthquake is significant in understanding the vergence of the causative fault and its relationship to the 2001 Bhuj earthquake. In this account3: ‘the Bhooj, chief town was reduced to ruins and 2000 people perishing. At Ahmedabad, a city famous for its noble architectural remains, spires of great mosque of Sultan Ahmad (1411–1443) were overthrown; other mosques also destroyed; 500 people assembled for a wedding feast, all perished in ruins’. ‘Heavy motion of earthquake was reported from Surat and Broach’. The

Figure 3. Interpretative cross-section XY drawn from tectonic map (Figure 1). Tectonic interpretation by the author (V.C.T.). CS, Cover sedimentaries (Lower Jurassic to Holocene); Bm, Crystalline basement; * 2001, Epicentre of Bhuj earthquake; Allahband-coseismic uplift produced by a reverse fault. Causative fault for 2001 Bhuj earthquake is a blind reverse fault. 398

destruction of the great mosque by the Allahband earthquake may imply that an earthquake of this magnitude (> 8) had not occurred in the past at least 375–400 years. The uplifted feature of Allahband is described as a mound and not a scarp3. The flooding of Sindri fort area immediately after the earthquake suggests a subsidence. The Allahband, the uplifted ridge (band), and subsidence to its south were interpreted as resulting due to coseismic uplift and subsidence in a flexure due to faultpropagated folding. This style of tectonic morphology has been ascribed due to south-dipping fault6,7. However it can be also argued that Allahband uplift and subsidence to its south may have been caused by piggy-back folding along the southdipping thrust fault with synformal depression, complementary to the antiformal ridge to its north. During the 2001 Bhuj earthquake maximum intensity and extent of damage was around Bhuj, Bachau and Anjar; but Ahmedabad, 250 km east-south east of the epicentre also suffered collapse of multistoried buildings with a loss of about 1500 lives. Strong motion and minor damage were reported as far south as Surat. Ground cracking due to liquefaction and liquefaction features like sand blows were observed in the epicentral area (Figure 5). The distribution of damage pattern of Bhuj earthquake is similar to that of the 1819 Allahband earthquake. Oldham (op. cit.) did not report much damage in Karachi area and Sind to the north of epicentral area during the 1819 earthquake. The 2001 Bhuj earthquake also tells the same story of damage pattern as that of Allahband earthquake. In a thrust fault, the hanging wall suffers stronger deformation than the foot wall. This may imply that in thrustfault regime, the maximum intensity and extent of damage is on the hanging wall side of the fault. This criterion may be in conformity with damage pattern of the Bhuj earthquake, and the same may be true for the Allahband earthquake, if the Allahband Fault was dipping south. In compressional regime of conjugate fault system, the two thrust faults dip towards each other in the upper part, whereas the same thrust faults dip away from each other in the lower part. In the former case, a pop-up structure is developed in the upper part. The ground geology of Kachchh region shows development of fold and thrust system. Therefore formation of a conjugate fault system with Allahband Fault dipping north


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Figure 4. Small segment of Manfra Fault showing right lateral motion with up to 10–30 cm slip, near village Manfra.

Figure 5. Sand blow-fissure emitting saline water and sand, surface expression of liquefaction feature.

and Bhuj earthquake causative fault dipping south, may not be tenable. The reactivation of older faults, pre-Mesozoic, as north-dipping Allahband Fault and the south-dipping Bhuj earthquake causative fault would imply a horst structure which is in contradiction to the fault geometry developed in the Kachchh rift basin. It is argued that both the Allahband and Bhuj earthquakes appear to be closely related in their source, with both the faults propagating from the detachment, dipping south (Figure 3). The coseismic uplift produced an uplifted ridge in Allahband during the 1819 event, whereas it remained a blind fault in the 2001 event with no observed coseismic uplift on the surface. The Bhuj region lies in zone V of the seismic zonation map of India, prepared by the Bureau of Indian Standards (BIS). In

the last 181 years, three earthquakes of magnitude 7 or more have occurred, including two major and one great earthquakes. Adequate attention was not paid in the past to seismological and crustal movement studies in the Kachchh region. To make better earthquake hazard assessment, seismological data acquisition needs to be strengthened, GPS studies need to be initiated, and active tectonic and more intensive palaeoseismological studies need to be promoted. In disaster reduction measures, microzonation of major cities and implementation of building codes must be the prime objectives. 1. Gupta, H. K., Rao, P. N., Rastogi, B. K. and Sarkar, D., Science, 2001, 291, 2101– 2102.


2. Oldham, T., Mem. Geol. Surv. India, 1883, 19, 163–215. 3. Wynne, A. B., ibid, 1872, 9, 1–293. 4. Johnston, A. C. and Kanter, L. R. Sci. Am., 1990, 262, 42–56. 5. Rajendran, C. P., Rajendran, K. and John B., Curr. Sci., 1998, 75, 623–626. 6. Rajendran, C. P. and Rajendran, K., Bull. Seismol. Soc. Am., 2001, 91, 407–426. 7. Bilham, R., in Coastal Tectonics (eds Stewart, I. S. and Vita-Finzi, C.), Geol. Soc. Spec. Publ., London, 1999, vol. 146, pp. 295–318. 8. Gaur, V. K., Curr. Sci., 2001, 80, 338– 340. 9. Rajendran, K., Rajendran, C. P., Thakkar, M. and Tuttle, P., ibid, 2001, 80, 1397– 1405. 10. Jacob, K. H. and Quittmeyer, R. L., in Geodynamics of Pakistan (eds Farah, A. and DeJong, K. A.), Geol. Surv. Pakistan, Queta, 1979, pp. 305–317. 11. Lawrence, R. D. and Yeats, R. S., ibid, pp. 352–357. 12. Sarwar, G. and DeJong, K. A., ibid, pp. 341–349. 13. Wellman, W. H., Geol. Rundsch., 1966, 55, 716–755. 14. Biswas, S. K., Geology of Kutch, KDMIPE, Dehradun, 1993, pp. 1–449. 15. Biswas, S. K., Proc. 3rd Indian Geol. Congress, Poona, 1980, pp. 255–272. 16. Biswas, S. K., Tectonophysics, 1987, 135, 302–327. 17. Merh, S. S., Geology of Gujarat, Geol. Soc. India, 1996, pp. 1–222. 18. Sohoni, P. S., Malik, J. N., Merh, S. S. and Karanth, R. V., J. Geol. Soc. India, 1999, 53, 579–586. 19. Thakkar, M. G., Maurya, D. M., Raj, R. and Chamyal, L. S., ibid, 53, 601–610. 20. Wesnousky, S. G., Seeber, L., Rockwell, T. K., Thakur, V. C., Briggs, R., Kumar, S. and Ragona, D., Seismol. Lett., 2001 (in press). 21. Quittmeyer, R. C., Farah, A. and Jacob, K. H., in Geodynamics of Pakistan (eds Farah, A. and DeJong, K. A.), Geol. Surv. Pakistan, Quetta, 1979, pp. 271–284.

Received 6 June 2001; revised accepted 14 December 2001

V. C. THAKUR*,† S. G. WESNOUSKY** *Wadia Institute of Himalayan Geology, Dehradun 248 001, India **Centre for Neotectonic Studies and Department of Geological Sciences, University of Nevada, Reno, USA † For correspondence. e-mail: [email protected] 399