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The hazard potential of the western segment of the Makran subduction zone, northern Arabian Sea

Article in Natural Hazards · January 2012 DOI: 10.1007/s11069-012-0355-6

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ORIGINAL PAPER

The hazard potential of the western segment of the Makran subduction zone, northern Arabian Sea

C. P. Rajendran • Kusala Rajendran • Majid Shah-hosseini • Abdolmajid Naderi Beni • C. M. Nautiyal • Ronia Andrews

Received: 3 January 2012 / Accepted: 10 August 2012 Ó Springer Science+Business Media B.V. 2012

Abstract Evaluating the hazard potential of the Makran subduction zone requires understanding the previous records of the large earthquakes and tsunamis. We address this problem by searching for earthquake and tectonic proxies along the Makran Coast and linking those observations with the available constraints on historical seismicity and the tell-tale characteristics of sea floor morphology. The earthquake of Mw 8.1 of 1945 and the consequent tsunami that originated on the eastern part of the Makran are the only his- torically known hazardous events in this region. The seismic status of the western part of the subduction zone outside the rupture area of the 1945 earthquake remains an enigma. The near-shore shallow stratigraphy of the central part of Makran near Chabahar shows evidence of seismically induced liquefaction that we attribute to the distant effects of the 1945 earthquake. The coastal sites further westward around Jask are remarkable for the absence of liquefaction features, at least at the shallow level. Although a negative evi- dence, this possibly implies that the western part of Makran Coast region may not have been impacted by near-field large earthquakes in the recent past—a fact also supported by the analysis of historical data. On the other hand, the elevated marine terraces on the western Makran and their uplift rates are indicative of comparable degree of long-term tectonic activity, at least around Chabahar. The offshore data suggest occurrences of recently active submarine slumps on the eastern part of the Makran, reflective of shaking events, owing to the great 1945 earthquake. The ocean floor morphologic features on the western segment, on the contrary, are much subdued and the prograding delta lobes on the shelf edge also remain intact. The coast on the western Makran, in general, shows indi- cations of progradation and uplift. The various lines of evidence thus suggest that although the western segment is potentially seismogenic, large earthquakes have not occurred there

C. P. Rajendran (&) K. Rajendran R. Andrews Indian Institute of Science,Á Centre forÁ Earth Sciences, Bangalore 560012, India e-mail: [email protected]

M. Shah-hosseini A. N. Beni Iranian National CenterÁ for Oceanography, Tehran 1411813389, Iran

C. M. Nautiyal Birbal Sahni Institute of Palaeobotany, Lucknow 226007, India

123 Nat Hazards in the recent past, at least during the last 600 years. The recurrence period of earthquakes may range up to 1,000 years or more, an assessment based on the age of the youngest dated coastal ridge. The long elapsed time points to the fact that the western segment may have accumulated sufficient slip to produce a major earthquake.

Keywords Makran subduction zone Hazard potential Earthquake recurrence Tsunami Tectonics Á Á Á Á

1 Introduction

The convergence between the Eurasian and Arabian plates has resulted in nearly 1,000- km-long Makran subduction zone, located in the northern Arabian Sea (Fig. 1a). Con- fined between two regions of active continent–continent collision (the Zagros and the Himalaya), the Makran subduction zone is bordered on the east by the left-lateral Ornach Nal fault and on the west by the right-lateral Minab–Zendan fault (Fig. 1a, b). Sub- duction of the oceanic crust has been occurring along the north-dipping plane since Early Cretaceous (White and Ross 1979; Platt et al. 1985). This subduction zone has a thick sediment pile, estimated as *7 km, thus forming one of the largest accretionary wedges in the world (Kopp et al. 2000; Kukowski et al. 2000). Much of the 500-km-wide sedimentary wedge is exposed in the onshore parts of Pakistan and Iran (Schlu¨ter et al. 2002; Kukowski et al. 2001). The Mw 8.1, 1945 earthquake, sourced on the eastern segment of the Makran, is the only instrumentally recorded great earthquake from this region. Compared with most other subduction zones, this region has generated fewer earthquakes since historical times, and this seems to be especially true for its western half. The apparent aseismicity on the western Makran and its potential as a source for future great tsunamigenic earthquakes have been a subject of debate. There seem to be three possibilities: one, the entire western segment is mostly deforming aseismically (Bayer et al. 2006); two, the subduction process is no longer active on this segment (Vita-Finzi 2002; McCall 2002); and three, the western segment is locked and is capable of generating a plate boundary earthquake (Byrne et al. 1992; Mokthari et al. 2008; Musson 2009). The last assumption is based on the fact that the coast on the western part appears to have been uplifted as shown by a terraced topography and that the plate convergence also shows a more or less uniform velocity (*23 mm year-1, relative to Eurasia), evidenced by GPS measurements (Masson et al. 2007). The perceived locked status of the western segment and its unknown tsunamigenic potential has important implications on the hazard and risk assessment for the North Arabian shores (e.g., Wyss and Al-Homoud 2004). Yet another possibility is that the entire Makran subduction zone (both the eastern and western segments) can rupture as a single block, as exemplified by the 2004 Andaman-Sumatra earthquake. In this paper, we will restrict our discussion only to the question whether the western Makran has the potential to generate large earthquakes.

1.1 Background

1.1.1 Case for two segments

The entire Makran subduction zone was initially considered to be structurally homoge- neous and consequently deforming uniformly (Byrne et al. 1992). It has been suggested

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Fig. 1 a Tectonic features around the Makran subduction zone (MAF Makran accretionary front, SF Sonne fault, MZF Minab–Zendan fault, ONF Ornach Nal fault, OFZ Owen fault zone, OMP Ormara microplate, DT Dalrymple trough, MR Murray ridge, LMR little Murray ridge, EM Eastern Makran, WM: Western Makran). White arrow denotes direction of plate motion NW–SE trending; arrows indicate the trend of a possible active transverse fault coincident with the suture zone of the Lut Block and Helmand Block (two Pre-Eocene accreted terrains). Inset A map of the region showing the study area. b. Structural elements of the region encompassing the North Arabian Sea. Abbreviations: AP accretionary prism, MAF Makran accretionary front, LMR little Murray ridge, OP Ormara microplate, WM western segment, EM eastern segment. Solid arrows show GPS horizontal velocities for JASK, CHAB, MUSC sites (Vernant et al. 2004) that the large amount of unconsolidated and water-saturated overpressured sediments may result in a low apparent friction on the detachment—a reason for relative aseismicity of the western Makran (Byrne et al. 1992; Bayer et al. 2006; Pacheco et al. 1993; Bilek and Lay

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2002). There are some compelling structural and geometrical considerations that suggest the segmented nature of the Makran tectonic province. Segmentation is implied by the existence of the Sonne fault, the NW-trending, left- lateral strike-slip fault, whose offshore extension is traced from swath bathymetry surveys (Kukowski et al. 2000). The apparent geometrical considerations, therefore, suggest that the Sonne fault possibly divides the Makran subduction front into western and eastern segments (Fig. 1b). The patterns of seismicity and the structural setting of the Murray Ridge suggest a separate microplate on the eastern part of the subduction zone (called Ormara microplate). This triangular-shaped microplate occurs between the NNW-trending left-lateral Sonne fault on the western side and NNE-trending Ornach Nal fault on the eastern side (Kukowski et al. 2000). The Sonne fault accommodates the differential movement between the Ormara microplate (driven northward by the Murray Ridge) and the Gulf of Oman (driven by the Sheba Ridge). The Sonne fault makes such a distinctive boundary that a single event, breaking through both the segments of Makran, is considered improbable (Musson 2009). Aside from the presence of a transverse bounding structure, strength contrast of mid-level de´collement between the eastern and western Makran has also been inferred (Byrne et al. 1992) that might inhibit a segment breaking full rupture. Seismicity, GPS and offshore morphology data suggest that the eastern and the western Makran segments have distinctive characteristics. Recent efforts have generated an extensive GPS database for the Zagros–Makran zone (Vernant et al. 2004; Bayer et al. 2006; Masson et al. 2007). GPS data from Iran suggest that Arabia converges toward southeastern Iran at velocity of 23 ± 2 mm year-1 near the western termination of the Makran sub- duction (Vernant et al. 2004), less than the time-averaged velocity of 36.5 mm/year based by the NUVEL 1 model (DeMets et al. 1990). GPS data also shows significant differential movement between Oman and Iran, with shortening of the order of 1.95 cm/year, thus confirming the active subduction process that is underway along the entire stretch of Makran tectonic belt (Masson et al. 2007). Variations in convergence rate between the eastern and western segments of the Makran subduction zone have also been observed. GPS studies also infer an eastward increase in convergence rate, from 11 ± 2 mm/year, at Jask to 19 ± 2 mm/year at Chabahar (Vernant et al. 2004). Later studies affirmed this greater convergence rate at easterly located Chabahar compared with Jask on the western side (Bayer et al. 2006). Apel et al. (2006), however, estimate a much greater convergence of 30 mm year-1 on the eastern segment, a convergence rate between Arabian and Asian plates. The presence of a nearby spreading center (Ormara microplate driven northward by the Murray Ridge) on the eastern segment probably generates a much larger convergence on this part (Musson 2009). An intriguing velocity of 21 ± 3 mm/yr observed at a semi-permanent point in Ormara (relative to Indian plate) was interpreted as due to steady-state post-1945 aseismic slip (Bilham et al. 2009). The general easterly increase in plate convergence rates observed from recent GPS data, the structural features such as the sinistral Sonne fault and the higher level of current seismicity make the eastern Makran distinct from the western segment.

1.1.2 The 1945 earthquake and ground deformation: a template for western Makran

The 1945 earthquake (Mw 8.1) is the only major earthquake known to have occurred along the Makran Coast in the historical times. The rupture due to this earthquake is believed to have occurred on the segment between Pasni and Ormara in Pakistan (Byrne et al. 1992; Fig. 1a). Thrust mechanism with focal depth of 27 ± 3 km, and a rupture length of 100–150 km and a down dip width of 100 km, on a gently north-dipping (5°) fault has

123 Nat Hazards been suggested by Byrne et al. (1992). The dislocation model of Byrne et al. (1992) suggests that the up-dip limit of rupture along the trench axis emerges offshore about 30 km from the coast and that the uplift is confined to a narrow region. Large-scale coseismic ground deformation was observed in the onshore parts of the Pakistan coast. The coasts at Pasni and Ormara were reported to have risen by 4.8 and 2 m, respectively (Snead 1967; Page et al. 1979). The possibility for greater coseismic uplift, which must have subdued later, has been implied (Page et al. 1979; Vita-Finzi 2002), but there is no direct evidence to suggest a greater coseismic uplift and a post-seismic relaxation. The 2-m coseismic uplift of the Ormara tombolo, located east of Pasni, and the reactivation of mud volcanoes on the eastern part of the rupture (Fig. 1a) might suggest a unilateral eastward directivity of the rupture (Byrne et al. 1992). Intense coseismic liquefaction and sand blow formations were also reported from Pasni and Ormara (Page et al. 1979). Coseismically raised water table causing shallow flooding of the epicentral area (Ambraseys and Melville 1982) must have resulted from the massive squeezing out of ground water. Further, the earthquake must have also generated large- scale submarine slides, which caused multiple breakages of transoceanic cables in eight places (Byrne et al. 1992). The coastal part of the town of Pasni is reported to have moved with one of the submarine slide, shifting the coast 100 m landward (Byrne et al. 1992). Independent support for submarine slides has come from the seismic reflection data, dis- cussed later in this paper. Other coseismic changes include reactivation of some previously existing mud volcanoes and creation of four new islands (Sondhi 1947; Fig. 1a), possibly a manifestation of the release of high pressured fluids from sediments.

1.1.3 The 1945 tsunami: complex processes

The 1945 Makran earthquake also generated a tsunami (with a maximum wave height of 10 m) that impacted the coasts of Iran, Oman, Pakistan and the northwest India, which was recorded by tide gages maintained by the Survey of India in Aden, Karachi and Bombay (Pendse 1948; Murty and Rafiq 1991; Heidarzadeh et al. 2008). The farthest location where the tsunami was reported is Seychelles, 3,400 km to the southeast of Makran, where a wave height of 0.5 m was reported. The Bombay edition of the Indian newspaper Times of India dated November 29, 1945, reported that the tidal waves had inundated the Malad Creek of Bombay and drowned three people (Rajendran et al. 2008). The tsunami is reported to have arrived as multiple surges, the second and fourth waves being the largest at Pasni and Karachi, respectively (Pendse 1948). Reports indicate that there was a substantially long pause between the time of the earthquake and arrival of the tsunami sequence. Other workers who have examined this event have also commented on this disparity in time (Bilham et al. 2007; Rajendran et al. 2008). The tsunami is reported to have arrived as multiple surges, the second and fourth waves being the largest at Pasni and Karachi, respectively (Pendse 1948). Reports indicate that there was a substantially long pause between the time of the earthquake and arrival of the tsunami sequence. Ambraseys and Melville (1982) indicate that two damaging waves arrived 90 and 120 min after the main shock at Pasni. Although the accounts conflict to some extent, it appears that the largest pulse of tsunami at Pasni was delayed by 2–3 h. Rajendran et al. (2008) suggested that submarine landslides could have caused the delay in tsunami arrivals. In an alternate model, the generation of delayed, high waves has been attributed to excitation of trapped modes on the continental shelf (Neetu et al. 2011). The long delay in arrival of the second wave at the epicentral region (second wave arrived Karachi coast, located to the east of Pasni, at 7 a.m.) and the localization of its impact were

123 Nat Hazards considered as other factors that favor a tsunami generation mechanism that is not directly related to coseismic faulting.

1.1.4 The western Makran Coast: the unresolved question of hazard potential

A major question is whether the western segment is indeed aseismic or it has ever expe- rienced any major earthquakes in the past. The historical data represent a very limited interval, and the data published by Ambraseys and Melville (1982) seem equivocal on size and location of earthquakes. Exploring the coastal geological archives offers an alternate way of assessing the potential hazard. The coastal stratigraphy may preserve evidence of past earthquakes mainly in the form of layers liquefied sediments formed due to ground shaking. But not much paleoseismological work from this region has been published, as yet. Aim of our study is to evaluate the seismic potential of the western segment through the study of such proxies. We explored the near-shore coastal parts of western Makran in two field seasons in November 2008 and October 2010 to identify sedimentary proxies that would point to past earthquakes or tsunamis.

2 Methods

Both modern and historical earthquake data are analyzed to study the seismicity of the region. Epicentral data of events occurred prior to 1918 are adopted from Quittmeyer (1979) and Musson (2009). Other data sources are the International Seismological Sum- mary (1918–1963), except for the 1945 event for which we used relocated epicentral data from Quittmeyer (1979); Bulletin of the International Seismological Centre (1964–1972); and National Earthquake Information Center of USGS (1973–2011). Events from all depths are considered. Focal mechanisms of earthquakes are constructed from the source parameters determined by Byrne et al. 1992, Chandra 1984, Jackson and McKenzie 1984 and Quittmeyer and Kafka 1984, and the centroid moment tensor solutions provided by the Global CMT catalog. The data from NEIC have been used to develop the seismicity distribution of the region. The shallow stratigraphy of the region consisting of alternating layers of silty clay and sand with shallow ground water table of the Makran Coast provided ideal condition for sediment liquefaction in the event of a major earthquake. Assuming uniform lithologic and water table conditions, the absence of liquefaction features in an area (negative evidence) can also be used to assess whether the region has been affected by previous earthquakes (Obermeier 1996). Wherever these features are present, the shallow sedimentary sections can be used to estimate the recurrence interval and magnitude of strong earthquakes (Obermeier 1996). The negative evidence, however, may not be used universally as it is possible that deeper levels of the sections may preserve evidence of earthquake-triggered sedimentary structures of much older events. We made shallow trenches (2 m long, 1 m wide and 2 m deep) in near-shore areas, and the walls were scrapped and cleaned up to make the sedimentary structures visible. These sections were logged after the trench walls were square gridded at 1-m interval using strings. Our previous experience in regions like Rann of Kachchh in India located beyond the eastern tectonic boundary of Makran sug- gests that the shallow sedimentary sections (1.5 m deep) may provide information on earthquakes occurred during the last 1,000 years (Rajendran and Rajendran 2001). We have also estimated the average long-term uplift rate of a marine terrace at one location where the stratigraphic evidence of uplift has been unequivocal by applying the

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relation elevation U = Y1 - Y2/t (U is the rate of uplift in meters per 1,000 years, Y1 is the presently measured elevation of the coast, and Y2 is the elevation of the paleo-high level stand during which time (t) the terrace was formed. The effect of eustatic sea-level change has to be subtracted using the general sea-level curves available for this region (e.g., Lambeck 1996). Shells were collected for radiocarbon dating from terrace deposits. The shells were chosen only after it was made sure that there is no effect of recrystallization, generally shown by chalky or powdery appearance. The problems of interpretations shell dates are further discussed elsewhere. The details of age data processing and calibration are given under footnote in Table 1. Here we first discuss the historical and current seismicity patterns on the eastern and western Makran. We also reassess the existing database on the offshore morphology to bring out the differences in earthquake impacts on the western and eastern Makran. Then, we present the coastal sedimentary sections and their features (Beris to Bander Abbas;

Table 1 Calibrated radiocarbon ages of samples from the western Makran coast

Lab code Sample code/location 14C BP (years) Marine Calibrated ages carbon (years BP) (percentage)b

NZA30852 F-1A; Free Trade Zone 856 ± 20 100 140–340 (0.98) N25°25.410/E60°35.880 240 ± 100 S-3935 P-3/M; Free Trade Zone 3170 ± 100 81 2,600–3,130 (0.99) BS-3089 N25°23.350/E60°37.520 2,865 ± 265 S-3938 BA-2/M; Baluchi Industrial Center 3,980 ± 130 62 3,640–4,270 (0.99) S-3092 N25°24.730/E60°38.030 3,955 ± 315 S-3937 BA-3/M; Baluchi Industrial Center 7,260 ± 120 52 7,570–7,997 (1) BS-3091 N25°24.730/E60°38.030 7,785 ± 215 NZA30858 BL-1A: Beris 7,361 ± 25 100 7,530–7,675 (1) N25°12.950/E61°00.070 7,605 ± 75 S-3936 INCO-2/M; sea shore side of Iranian 18,460 ± 130 89 20,790–21,990 (0.94) BS-3090 Oceanographic Institute 21,390 ± 600 N25°14.360/E60°51.520 S-3939 Calcareous sediment; Pasabander 23,910 ± 260 98 27,550–28,680 (0.98) BS-3083 N25°04.80/E61°21.120 28,115 ± 565 S-3928 INCO-1/M; sea shore side 25,700 ± 320 94 29,350–30,580 (1) BS-3082 of Iranian Oceanographic Institute 29,965 ± 615 N25°16.630/E60°41.280 S-3974 STO-3/A/M; Pasabander 26000 ± 320 82 29,570–30,900 (1) BS-3030 N25°054770/E60°18.4440 30,235 ± 665 S-3976 PB-1/M; Pasabander 46,050 ± 4250 60 42,260–50,000a (1) BS-3032 N25°04.120/E61°24.900 46,130 ± 3870 All dates have been calibrated using the CALIB Radiocarbon Calibration (version 6.0.1) and the marine09 data set (Stuiver and Reimer 1993). Two sigma (r) ranges with the largest area under probability distribution curve (values in parenthesis) are listed. At 24.83°N and 65.92°E off the coast of Makran in the Arabian Sea, two values of D R have been empirically estimated: 249 ± 23 and 212 ± 25. The value used here is the average of the two (232 ± 26), taken from http://www.calib.qub.ac.uk/marine Samples with ‘‘NZA’’ tag were analyzed at Rafter Radiocarbon Laboratory, New Zealand. Other samples were dated at Birbal Sahni Institute of Palaeobotany, Lucknow, India a 2 sigma (r) ranges are suspect due to impingement on the end of the calibration data set b Percentage of CO2 obtained from the sample. Dead CO2 has been added to the sample to make it up to 100 %, leading to higher uncertainty in the calibrated ages

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Fig. 1a) and their implications on past seismicity in the region. We also present the radiocarbon dates from the coastal terraces to constrain the uplift rates.

3 Results

3.1 Makran seismicity: historical and instrumental data

Historical data on past seismicity are probably incomplete, owing primarily to the sparse population in the region. The newly interpreted historical data indicate that two moderate events occurred in the region during AD 1483 and 1497 (Musson 2009; Fig. 2a). The source of the AD 1483 earthquake that had affected the Strait of Hormuz and northwest Oman has been attributed to the western Makran. Byrne et al. (1992) describe this as the only known earthquake that may have potentially struck the Western Makran. Musson (2009) suggested that the 1483 earthquake possibly occurred in the Island of Queshm near the Strait of Hormuz (27°40N; 56°270E; Fig. 2a) and the later event occurred near Qualhat in Oman. The 1483 earthquake reportedly affected the area around the Strait of Hormuz and thus probably sourced at a location closer to Strait of Hormuz rather than associated with the western Makran, as previously thought. If these events are not associated with the Makran subduction zone, it is reasonable to assume that no major earthquakes have occurred on the western part of Makran subduction zone at least during the last 600 years. The level of current seismicity, although sparse for subduction zone, also shows a marked difference between the two segments (Rani et al. 2011), and the western part is notably low on interplate seismicity (Fig. 2a). The *250-km-long segment between the Sonne fault and Ormara is the only active part of the subduction zone where earthquakes are noticed, both on the onshore and offshore regions. Focal mechanism solutions of earthquakes presented by Byrne et al. (1992) and those updated from CMT catalog suggest thrust faulting along the gently dipping East–West-oriented fault planes along the sub- duction front. Dip-slip motion with right-lateral component is observed along the Minab Fault zone that forms a transition between the Makran and the Zagros collision zone. Sinistral faulting occurs along the Ornach Nal fault, as observed from the fault plane solutions presented by Byrne et al. (1992) and events updated from the CMT catalog (Fig. 2b). The Ormara plate is noted for its higher level of activity toward the Sonne fault, which is attributed to a higher rate of convergence between Ormara and Eurasian plate, compared with the Asian and Eurasian plate (Kukowski et al. 2000). Although the Sonne fault does not exhibit much ongoing seismicity, we could decipher an extended NNW-trending pattern of seismicity in the onshore part. This transverse trend of seismicity is coincident with the line separating the western and eastern Makran. Further north, a continuity of this structure could be traced along the suture zone of the pre-Eocene terrains of Lut Block and Helmand (Afghan) Block. The seismicity distribution of both the western and eastern segments indicates a shallow Benioff zone in the offshore part, which gathers sudden steepness on the onshore region. Depth distribution of earthquakes between 62° and 65°E, which constitutes the currently active, eastern segment, shows a similar trend with a northward steepening of the trench (Fig. 2c). Epicentral locations of normal faulting earthquakes at intermediate depth sourced within the subducting slab projects at *300 km northward of the trench, and these are restricted to the eastern segment (see the location of the recent 2011 Mw 7.2 earthquake in Fig. 2b). The large variation in the distance between the onshore volcanic arc and the

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Fig. 2 a Earthquake (M [ 5.0) distribution of the region (sources: Quittmeyer 1979; ISS (1918–1963), ISC (1964–1972) and NEIC (1973–2011)). Open circles: M [ 6.0; closed circles: M 5.0–6.0; two moderate earthquakes occurred in AD 1483 (Island of Qeshm, off Bander Abbas, Iran) and 1497 (Qalhat in Oman) are also shown (source: Musson 2009). AA0 represents the line along which all earthquakes between 62 and 65°E are projected (c). b Focal mechanism solutions compiled from CMT catalog (1977–2011); Byrne et al. 1992; Chandra 1984; Jackson and McKenzie 1984; Quittmeyer and Kafka 1984. The NW–SE-trending suture zone between the Lut block and the Helmand block is identified by arrows. c Hypocentral locations of the earthquakes between 62 and 65°E projected along A–A0 as shown in (a)

123 Nat Hazards subduction front in the western and eastern segments (*450 and *600 km, respectively; Fig. 1a) is another manifestation of the slower subduction along the western segment.

3.2 Seismic imprints: features of offshore morphology

As the sediment input into the Makran subduction zone is very high, a thick sediment pile (*7 km thick) occurs at the deformation front. The large sedimentary cover and feeble contrasts between the accreted and overlying sediment mask distinct morphological expression of a trench as observed in most other subduction zones. However, the long period of shortening and the abundant sediment supply have led to the formation of a 350-km-wide accretionary prism (Platt et al. 1985, Kukowski et al. 2001). Wide-angle and seismic reflection data indicate that the sedimentary wedge is comprised of stack of five uniformly spaced packages, thrust along a northward-steepening de´collement (Kopp et al. 2000). The swath bathymetry data and echo sounding profiles reveal topographic details of the Makran offshore (Kukowski et al. 2000, 2001; Uchupi et al. 2002). The continental shelf off coastal Makran is 10–40 km wide, and its edge reaches up to a depth of about 100 m. The shelf is succeeded by 70–110-km-wide continental slope that reaches to a depth of 700 m. The continental slope is divided into upper and lower zones (Uchupi et al. 2002): the upper slope is smooth with a gentle terrace, but several east–west- and north–south- trending ridges, gullies and canyons characterize its seaward edge (Fig. 3a and b). These east–west ridges and the depressions are the resultant topography created during the wedge formation and, therefore, considered to have a tectonic origin (White and Ross 1979). The north–south features are attributed to erosion and mass wasting (Kukowski et al. 2000, 2001; Kopp et al. 2000; Uchupi et al. 2002; Bourget et al. 2010). Three major north–south-trending turbidity systems that cut through the ridges including the 700–1,000-m-high Makran escarpment (the frontal fold) are identified on the Makran sea floor (Fig. 3a). The largest of these, namely the ‘‘Save Canyon’’ and the ‘‘Shadi Canyon,’’ are more than 100 km long and are located close to the source zone of the 1945 earthquake. It is remarkable that the greater depth and steepness on the upper part of the Shadi Canyon, which have been identified by Kukowski et al. (2001), are also in the vicinity of the 1945 source. Highly dissected by clear topographic expressions, the shelf edges here are also characterized by major slumping (Fig. 3a). Distinct mid-slope terraces on the canyon walls, indicating possible episodic vertical movement, have also been reported by Kukowski et al. (2001). The seafloor segment west of Sonne Fault, however, shows relatively smooth and less dissected morphology. One major through-going, north–south-flowing turbidity channel has been identified west of the Sonne fault; this forms a large deep-sea fan around 59°300E (Uchupi et al. 2002). It appears to be topographically much smoother than the turbidity channels to the east of the fault. The shelf break on the western segment shows aerially well-defined relict delta formations. Many of the submarine channels originate in these deltas (Uchupi et al. 2002; Fig. 3b). The smoother walls of the turbidity channels and the better preservation of deltas at the shelf edge may indicate that no shaking events may have occurred for sometime on this part, unlike the eastern Makran.

3.3 Coastal sedimentary sections and constraints on past earthquakes

The Chabahar bay is located along the coast of Sistan province in southeastern Iran (Fig. 4). This region has undergone sea-level changes in the late Quaternary, which is reflected in the coastal sedimentary sequences and also by the presence of paleo-shorelines

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Fig. 3 a 3-D visualization of the bathymetry of the east central part of the Makran accretionary wedge showing the submarine canyons and channels (after Kukowski et al. 2001; reprinted with permission from Elsevier Publishing Company). The location of the 1945 earthquake is shown. Note the major slumping off Pasni. b Morphological features of the Makran offshore (after Uchupi et al. 2002; reprinted with permission from Springer Publishers). Note the curved shelf edge with intact delta lobes on the western Makran

(Gharibreza and Motamed 2006). It is suggested that a major marine transgression may have occurred around 4,000–5,000 years BP, and the coastal stratigraphy suggests that Chabahar bay area has been above the MSL ever since and is continuing to be uplifted (Gharibreza and Motamed 2006). The bay area may have been turned into intertidal zone ever since, as shown by the stratigraphy. Here we discuss the observations from four sections at the Chabahar bay.

3.3.1 Site-1 at Chabahar bay free trade zone

Chabahar bay area is an accretionary coast and twenty beach ridges have been identified in this area by Gharibreza and Motamed (2006). The farthest strandline is *3 km away from the present shoreline and 15 m above the MSL (Fig. 4 for location). The oldest ridge was dated at 4690 ± 40 yr BP (4655 ± 145 cal year BP), and the youngest beach ridge, about 200 m away from the present shoreline, was dated at 1250 ± 145 year BP

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Fig. 4 Satellite image showing the Chabahar bay and the study sites. Sites numbered 1–10 are listed in Table 1. Abbreviations: FZ free zone, BL Baluchi Industrial Center, INCO Unit of the Iranian National Centre for Oceanography at Chabahar. Unnumbered site is from Gharibreza and Motamed (2006). Source of the image: MDA Federal (2004), Landsat GeoCover ETM? 2000 Edition Mosaics Tile N-41-25.ETM- EarthSat-MrSID, 1.0 USGS

(622 ± 266 cal year BP). These authors suggest that the Chabahar bay region was under a regressive phase for the last 5,000 years. Lambeck (1996) suggested that the sea level for Persian Gulf region at 6,000 years BP was about 2–4 m high with respect to the present sea level. The general trend of elevation of beach can be explained only if we factor in the localized uplift of the coast by tectonic forces. The sections that we independently examined are located on the eastern side of the Chabahar bay, 500–600 m inland from the swash zone (Site 1 in Table 1, and Fig. 4). The 1-m-thick section, *3 m above the present shoreline, is constituted mostly by intertidal mud and marine sediments with gastropod and brachiopod shells. We obtained a calibrated age of 2,865 ± 265 years BP for the shell sample, which may represent the youngest age of the raised intertidal deposit preserved on this part of the bay (Table 1).

3.3.2 Site 2 at Chabahar bay free trade zone

The sedimentary sections at site 2 in the Free Zone area (Site 2 in Fig. 4) showed 50–cm- to 1-m-thick, reddish sand, which unconformably overlies the sticky grayish intertidal silty clay (Fig. 5a–d). We dug four shallow pits, and all of them showed similar stratigraphy—a top layer of dune sand over the silty clay. The cross-laminations within the reddish sand suggest that they may have been deposited by wind. This sequence is clearly indicative of the switching over of the environmental conditions from a tidal flat to continental influ- ence, which possibly signifies the ongoing uplift of the coast. If any past earthquakes have caused liquefaction in this area, the emplaced material would make abrupt cross-cutting contact with the dune sand. Thus, we considered this area as a potential site to search for liquefaction events.

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Fig. 5 a Sedimentary section near Free Trade Zone, Chabahar, showing the liquefied layer and the location the shell sample. b Sketch of the section shown in (a). c Section from an adjacent trench showing emplaced layers of liquefied silty clay. d Sketch of the section shown in (c)

A typical section shown consists of a 10-cm-thick layer of black to grayish clay overlain by reddish to yellowish sand (Fig. 5a). The top 10 cm of this exposure shows emplaced grayish mud overlying the dune sand, sourced from blackish silty clay at the bottom. A thin layer (*10 cm) of silty clay forms a sill within the dune sand just above the layer of sticky intertidal mud. This layer is discontinuous and is not observed in all the trenches. Sills and funnel-shaped dikes through which grayish silty clay intruded at different levels, into the reddish sand, were observed as in other trenches (Fig. 5a–d). These features represent evidence for widespread and intense liquefaction of the underlying silty clay due to intense ground shaking. An obvious question is how we could explain the liquefaction of silty clay as fluidization of silty clay is considered to be rare (Obermeier 1996). The observations, however, show that clay and silt-rich deposits are capable of breaking down and ‘‘flow’’ under strong ground shaking (Updike et al. 1988). The liquefaction of silty clay observed in the Makran Coast is comparable to the liquefaction of cohesive soil occurred during the Haicheng (1975) and the Tangchen (1976) earthquakes in China and several other events in Japan and Turkey (e.g., Prakash and Puri 2010). The earlier studies, mostly focused on liquefaction phenomenon of sands and fine-grained soils, never considered that silts, clayey silts and even sands with fines are liquefiable during earthquake loading. It is now established that not only sands but silts and clays can also liquefy, and plasticity index (PI \ 7) was found to be a key criterion for liquefaction susceptibility of silts and clays (Prakash and Puri 2010). We obtained a single radiocarbon date from this section. A shell obtained from the bottommost grayish mud yielded calibrated age of 240 ± 100 year BP (Fig. 4a; Table 1).

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Based on the stratigraphic relation, this may correspond to the maximum age of the liquefaction event. It is important to note that the age is based on a single shell and needs to be corroborated with additional data. Another point to be noted is that in the absence of well-constrained marine correction values, the age data may not very reliable. Thus, the large deviation between the uncalibrated age of 856 ± 20 year BP and the calibrated age (Table 1) may indicate the uncertainties in the marine corrections available for this region. The calibrated age suggests that the Chabahar liquefaction features may represent the distant effect of the 1945 Pasni earthquake on the eastern part of the Makran subduction zone.

3.3.3 Site at Baluchi Industrial Center

The sections of this area near Baluchi Industrial Center, Chabahar bay area, exposed similar sedimentary units as described above. As in the case of earlier sites, thin layers (*10 cm) of greenish clay were found to have been intruded into the brownish sand (Fig. 6a, b). Vertical pipes and vents of greenish silty mud with broken shells occur within the reddish sand (Fig. 6a). At some locations, the greenish clay has been emplaced onto the top part of the section, which occurs as a mixed layer of clay and sand (Fig. 6b). At some locations, they occur as 10–20-cm-thick lenses. Pockets of reddish sand within this grayish clay and the assorted layer of sand, shells and clasts of clay within the contact zone are suggestive of intense ground shaking. Combinations of features such as vertical pipes containing emplaced greenish clay with broken shells suggest sudden forceful ejection from the underlying clay layer as generally expected during cyclic loading. As observed in the earlier site, the liquefaction event may be related to the distant 1945 earthquake, the only event that is known to have disturbed the area historically. The calibrated shell date of 3955 ± 315 year BP obtained from greenish mud, a marine/ intertidal unit, may represent the age of the deposit (Table 1). The published sea level of the area suggests that the last marine transgression took place around 4690 ± 40 yr BP (Gharibreza and Motamed 2006). The calibrated date reported in this study, however, suggests a younger age for the last transgressive event.

3.3.4 Sedimentary sections around Jask

Jask, located *300 km west of Chabahar, is the westernmost site that we examined (see Fig. 1a for locations). We logged about 12 trenches with a maximum depth of 1.5 m around Jask (Fig. 7a). In terms of the number of exposures available to us for our examination, it was similar to what was available at Chabahar. The shallow coastal sed- imentary sections located at distances of 20–50 m from the preset shoreline exposed alternate sequences of white silty sand and greenish mud, typical of an area that has undergone changes from tidal flat to intertidal sedimentation. A typical section near Maidani Creek is shown in Fig. 7b. The sections (maximum depth 1.25 m) mostly bot- tomed on blackish intra tidal sequences succeeded by grayish supratidal facies, overlain by beach/dune sands. None of the sections showed any characteristics of emplacement (like diking or venting) of lower level sediments onto the grayish upper level supratidal sedi- mentary units. Unlike the sections at Chabahar, the shallow stratigraphic sections around Jask are devoid of any sedimentary features that are indicative of fluidization due to ground shaking. However, it should be noted that this does not rule out the potential occurrence of fluidization features at deeper levels or in the vicinity. The section thickness of 1.5–2 m may be representative of processes that took place during the last 1,000 years, assuming a

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Fig. 6 a A section near Baluchi Industrial Centre showing lenses of fluidized silty clay within the reddish sand. b Section from an adjacent trench showing thicker zone of emplaced silty clay at the top. Dotted lines mark the boundaries of various litho-units conservative sedimentation rate of 1–2 mm/year. However, it should be noted that many of the sections (from Chabahar to Jask) show a facies change from tidal flat environment to intertidal sedimentation, which, among other possibilities, suggest a gradual ongoing uplift.

3.4 Cliff section at Beris: constraint on coastal uplift

The site we examined at Beris is the easternmost, and it is about 80 km east of Chabahar (Figs. 1a and 3). Located about 15 m from the present shoreline, the site is about 10 m above the MSL (Fig. 8a). The morphology represents a retreated shoreline consisting of calcareous marl and assorted beach rocks. The section we examined is topped by a *1-m- thick sequence of assorted mix of beach rocks. The boulders are pierced by burrows made by Lithophaga, a genus of marine bivalve mollusks, which bore into the calcareous rocks (Fig. 8b). Association with this genus suggests that the boulders were initially at the swash zone close to MSL. The calibrated AMS age of Lithophaga collected from the rocks is 7605 ± 75 cal year BP, which is taken to be the depositional age of boulder deposits.

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Fig. 7 a Google Image showing the study sites around Jask located *300 km west of Chabahar. b Sedimentary section near Maidani Creek (see a for location, at the first site from the right), showing facies variations

The global sea level during circa 7,000 years ago was generally assumed to be 3 m above the current sea level (Woodroffe and Horton 2005), although this may vary at any particular coast. For example, the sea-level curve from Malay–Thai peninsula suggests that in this part, the sea was 6 m below the current level (Horton et al. 2005). According to Lambeck (1996), sea level in the Persian Gulf (our area of interest) at 6,000 years BP was about 2–4 m above the current level, and between 8,000 and 7,000 years, it was relatively lower or probably similar to the current level (Gharibreza and Motamed 2006). The average uplift rates are calculated after applying the Lambeck’s correction (2 m above the current level) and also assuming that no change in sea level has occurred during the last 6,000 years. The estimated uplift rates at this site range between 0.17 and 0.25 cm/year. These rates are comparable to what was reported by Page et al. (1979) from Pasni (0.2–0.3 cm/year), the site closest to the epicenter of the 1945 earthquake. Some of the age data reported here are also indicative of the Pleistocene or older sea- level stands ([20,000 year BP; Table 1) and are not useful to reconstruct the Holocene tectonic history. Further, any shell date greater than 20,000 years is considered as

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Fig. 8 a Cliff section at Beris located east of Chabahar showing the marine boulder beds (see Fig. 4 for location). b A close-up of marine boulder beds showing the cavities made by Lithophaga, from which samples were collected for dating minimum age on account of contamination, which could change the apparent age (Broecker and Bender 1972). These considerations and better stratigraphic control of the dated sample compel us to use the date from the boulder beds from Beris to constrain the uplift rate at the study area. The uplift rate from Beris is remarkably similar to what is observed on the eastern Makran, which essentially reveals that both eastern and the western segments of the Makran Coast are undergoing long-term tectonism more or less at the same rate.

4 Discussion

The geological, seismological and geodetic considerations suggest that the Makran sub- duction zone is segmented. The transverse Sonne fault marks the boundary of these two

123 Nat Hazards segments. The eastern and western segments not only show variations in seismic pro- ductivity but may also differ in recurrence period of large earthquakes. Unlike the eastern Makran, the western segment is noted for a long-term quiescence in terms of large earthquakes. The recently uplifted coastal terraces around Chabahar, however, imply that the western segment, like its eastern equivalent, is also subjected to episodic movements associated with large earthquakes and the youngest beach ridge at Chabahar is *1,000 years old. Considering the coastal uplift in the epicentral area during the 1945 earthquake that occurred on the eastern segment as typical of a great subduction zone earthquake on this plate boundary, and using the long-term uplift rate of 1–2 mm/year at Ormara (near the epicenter of the 1945 earthquake), Page et al. (1979) suggested that the recurrence of such earthquakes in the eastern segment is 1,000–2,000 years. From the data from Konark and Jask, the uplift rate of the Makran was estimated to be much slower (1–6 mm/year), and they suggested a longer recurrence interval of 3,000–20,000 years for the western Makran. Our estimates obtained for the western segment at Beris near Chabahar (1.7–2.5 mm/year), however, are remarkably closer to the uplift rates obtained by Page et al. (1979) for the eastern segment, the source zone of the 1945 earthquake. This suggests that the recurrence period of earthquakes for both the segments may be comparable. The shallow geology and depth to water table seem to be prone to liquefaction throughout the Makran Coast. But the shallow pits farther west, such as Konarak and Jask, did not reveal any evidence for ground liquefaction, whereas such evidence was abundant at sites closer to Chabahar. The evidence for liquefaction and fluidization observed around Chabahar (*200 km) from the source suggests that the west central coastal Makran was greatly affected by ground shaking during the 1945 earthquake. Based on the absence of such features further west of Chabahar, we place this region as the farthest western limit of the meizoseismal zone of the 1945 rupture. It is also likely that the 1945 rupture may have propagated much further to west up to Sonne fault. The bathymetric features of the Makran sea floor and the seismic reflection data suggest that the eastern Makran was subjected to massive submarine slumping, most probably driven by the Mw 8.1, 1945 earthquake. The major difference is that the relief of these bathymetric features is sharper on the eastern side, indicating their recent activation during the 1945 earthquake. Like the eastern segment, the western part also shows large north– south-trending turbidity channels, but their relief is subdued. Further, the offshore delta formations also remain intact on the western segment and had not been subjected to recent breakages. Such fan build-up on the eastern segment shows only remnants of such deposits. The curved shelf edge on the western Makran is shaped due to the undisturbed prograding delta formations, in comparison with the straight-faced shelf edge on the eastern Makran. One obvious reason for sharp edge of the morphological features on the eastern part is the large-scale caving in or faulting during the 1945 earthquake. Trenches with abundant supply of sediments are much more prone to slumping and triggering submarine slides along the continental slope, which occur as a consequence of the ground shaking. The submarine landslide triggering appears to be one plausible mechanism to explain the late arrival of the 1945 tsunami (Rajendran et al. 2008). Such earthquake and tsunami mechanisms are highly probable for the western part of the Makran trench as well. Previous large tsunami events on the western Makran have been proposed on the basis of the occurrence of transported coastal boulders in the region (Shah- hosseini et al. 2011), indicating that the western Makran also holds equal potential for tsunami hazard.

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5 Conclusions

1. The boundary between the eastern and western Makran is defined by the transverse Sonne fault. Although the eastern segment is seismically more active currently, the coastal sites near Chabahar on the west show comparable long-term uplift rates indicating that western segment or at least part of it is also tectonically equally active, capable of generating large earthquakes. 2. The 1945 earthquake resulted in large-scale onshore liquefaction, and westernmost limits of the 1945 liquefaction field lie around Chababhar. The shallow coastal sedimentary sections on the western segment beyond Chabahar showed no evidence of liquefaction generated by near-field large earthquakes, precluding historically recent occurrences of earthquakes on the western segment. Non-occurrence of large earthquakes in the recent past is also evidenced by the subdued offshore morphological features on the western Makran. The recurrence period of earthquakes in the western Makran can be [1,000 years. 3. The long elapsed time shown by the western Makran may indicate that this segment is currently locked and it must have accumulated enough strain to generate a massive earthquake and a consequent tsunami. The example of 1945 tsunami suggests that the presence of large amount of sediments along the continental slope itself provides sufficient condition that could promote submarine slumping and tsunami generation. 4. Considering the earthquake and the tsunamigenic potential of the western Makran and the complicated mechanisms involved in the tsunami generation, the hazard scenario of the region needs further studies. A focused attention on the ramifications of a possible locked status of the seismogenic zone on the western Makran is required, considering the higher hazard risk owing to its location in the vicinity of coastal stretches of Persian Gulf countries that are most extensively modified.

Acknowledgments We thank the Vahid Chengini, Director, Iranian National Oceanographic Institute, Tehran, for the help and logistics to conduct fieldwork along the Makran Coast. CPR thanks the Department of Science and Technology, New Delhi, for the Ramanujan Fellowship scheme for the financial support to visit Chabahar (Iran) during November 2008. The Indian Ocean Commission, UNESCO, funded the fieldwork during October 2010. We are thankful to Jane Cunneen and Brian Atwater for all their help. We thank Eduard Reinhardt, McMaster University, for his help during our first phase of fieldwork in Chabahar, and Razyeh Lak and Ali Mohamadi of the Geological Survey of Iran and Eko Yulianto of Indonesian Institute of Sciences (Lembaga Ilmu Pengetahuan Indonesia) for their help during the second phase of fieldwork around Jask and Bandar Abbas. CPR and KR thank the Ministry of Earth Sciences, Government of India, and Indian National Centre for Ocean Information Services, Hyderabad, India, for supporting earthquake and tsunami research at the Centre for Earth Sciences at the Indian Institute of Science. The manuscript improved much from the comments of three anonymous reviewers and the editor Thomas Glade.

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