Pure Appl. Geophys. Ó 2020 Springer Nature Switzerland AG https://doi.org/10.1007/s00024-020-02575-0 Pure and Applied Geophysics

The Orphan Tsunami of 1524 on the Coast, Western , and Its Implications

1 2 1 3 4 C. P. RAJENDRAN, MOHAMMAD HEIDARZADEH, JAISHRI SANWAL, A. KARTHIKEYAN, and KUSALA RAJENDRAN

Abstract—In comparison to the east coast, the tsunami hazard 1. Introduction for the west coast of India remains under-recognized, despite the impact in 1945 following a Mw 8.1 earthquake in the subduction zone in the northern Arabian Sea. The previous The hosts two known tec- occurrences of tsunamis in the Arabian Sea that would have a tonically active tsunamigenic sources, one in the bearing on the west coast of India are being debated, including the North Arabian Sea and the other in the Andaman- question whether the Makran region has the potential to generate greater-magnitude earthquakes. With this in the backdrop, we Sumatra region (Fig. 1). The Sumatra–Andaman present here the historical and geological evidence of a tsunami subduction zone generated a mega-tsunami in 2004, impact zone from a site on the Konkan Coast of western India. in the wake of an earthquake of Mw 9.1 (Fujii and Located in the village of Kelshi, the impact zone is preserved within a coastal dune complex that also reveals occupation layers. Satake, 2007), with massive transoceanic impact This laterally extending 30–40-cm-thick zone, coinciding with a (Fig. 1; inset). The 1945 (Mw 8.1) earthquake sourced habitation level, displays varied sedimentary structures including in the eastern segment of the Makran subduction zone scour-fill features, and is inter-layered with shells, at a height (MSZ), located in the northern Arabian Sea, also of * 3 m from the high-tide level. We attribute these sedimentary features to a tsunami flooding event that was contemporaneous produced a massive tsunami, devastating the nearby with the transportation of shells, dated at 1508–1681 CE. The coasts (Byrne et al. 1992). geological inference matches with the description by the Por- The potential for a mega-thrust earthquake (M tuguese fleets of a sea disturbance in 1524 CE, reported from w Dabhol, not far from Kelshi, and also from the Gulf of Cambay, C 9) rupturing the complete stretch of the MSZ (both located about 500 km to the north. Precluding submarine landslide the eastern and western segments combined) is being scenarios, the modeling results suggest that the high impact in debated (e.g. Okal and Synolakis 2008). Based on Kelshi could have been generated by a Mw C 9 earthquake sourced in the Makran subduction zone. It is, however, intriguing how a modeling studies, Okal and Synolakis (2008) suggest

Mw C 9 earthquake in the Makran region finds no mention in the that the MSZ has the potential to produce tsunami- historical documentation. We underscore the need for fresh efforts genic earthquakes of Mw C 9.0. Although no records along the Makran coast to reconstruct the tsunami recurrence his- of predecessors of such earthquakes are available in tory that would generate required validating constraints on the 1524 event, if it was indeed generated by a massive earthquake among Persian archives, mega-earthquake scenarios have other mechanisms. been supported by the tectonic features from the Iranian coast (Rajendran et al. 2012). Heidarzadeh Keywords: Tsunami, west coast of India, medieval settle- ment, Makran subduction zone, earthquake, submarine landslide, et al. (2008, 2009) presented various estimates of the numerical modeling. tsunami hazard associated with the MSZ, including those originating from mega-thrust earthquakes in the

MSZ (Mw C 9). Besides the coasts of , and , the west coast of India could also face the brunt of a mega-tsunami originating from the MSZ. Aside from the source(s) of the Makran region, no alternate offshore earthquake sources have yet been 1 Jawaharlal Nehru Centre for Advanced Scientific Research, identified in the Arabian Sea as generators of tsuna- Bangalore 560064, India. E-mail: [email protected] mis that can have an impact on the west coast of 2 Brunel University London, Uxbridge UB8 3PH, UK. India. 3 National Institute of Ocean Technology, Chennai 600100, India. Among the historically reported natural phenom- 4 Indian Institute of Science, Bangalore 560012, India. ena from the western offshore region of India, an C. P. Rajendran at al. Pure Appl. Geophys.

Figure 1 Map showing the source zone of the 1945 Mw 8.1 Makran earthquake in the North Arabian Sea and the west coast of India. Red dots represent instrumentally recorded earthquakes (M 5 and above); yellow stars denote major earthquakes in the region. A fourteenth-century earthquake (moderate magnitude?) is reported from the city of Kolhapur (Graham 1854). Inset: Map showing two major sources of tsunami in the Indian Ocean region: the Makran subduction zone in the northern Arabian Sea and the Andaman-Sumatra subduction zone in the Bay of Bengal. Stars denote major earthquakes in the region. Presumptive locations of medieval earthquakes of 1483 and 1497 CE, as given in Ambraseys and Melville (1982) and Musson (2009), respectively incident of a severe sea disturbance off Dabhol (see Premasiri et al. 2015; Rajendran and Rajendran Figs. 1 and 2 for location), on the Konkan Coast 2020). Unlike the east coast of India, the geological between Goa and Mumbai in the year 1524 CE, records of ancient tsunamis on the west coast have stands out (Logan 1887, Figs. 1, 2). A somewhat been factually less attested (Prizomwala et al. 2018), similar narrative of the same event that was also even as the historically reported incidences have been reportedly occurred around the Gulf of Cambay, assigned a low confidence index (Dominey-Howes located * 500 km to the north of Dabhol (Kerr et al. 2007). Therefore, to gain authentication on the 1824; Fig. 2). As it is known from other , the historical information, focused tsunami geology scientific evidence to discriminate the specificities of studies need to be conducted along the west coast at coastal impact of such sea disturbances must come par with the east coast (Rajendran et al. 2011). from the characterization of near-shore deposits (e.g. The Orphan Tsunami of 1524 on the Konkan Coast

E 72° 51’ N 23° 21’ Cambay

ARABIAN SEA

N 18° 42’

Kelshi

Dabhol

N 17° 23’

Figure 2 Map of the Konkan Coast showing the locations of Dabhol (from where a Portuguese fleet reported sea disturbance in 1524 CE) and Kelshi where the study site is situated

We first discuss the historical context of the 1524 and geological context of the findings registered at event using available information from both western the site in Kelshi. Lastly, we discuss the numerically India and the Makran region. We also explore the modeled scenarios to explain the possible causes of competing triggering mechanisms associated with the the medieval sea inundation. above-water (atmospheric) as well as under-water (seafloor) disturbances that could explain the 1524 phenomenon. Then we discuss the geological and 2. Historical Data sedimentary characteristics of a near-shore dune section located in Kelshi, a village on the Konkan 2.1. Sea Disturbances from the West Coast of India Coast of western India, not far from where the 1524 During the Medieval Period sea disturbance was reported (Figs. 1 and 2), where Some descriptions of violent sea disturbance in we have identified a contemporaneous wave impact 1524 from off Dabhol (Dabul), west coast of India zone. We also present the historical, archeological C. P. Rajendran at al. Pure Appl. Geophys.

(Fig. 2), that has direct relevance to our present were cured by the freight. The viceroy, who perceived studies, can be gleaned from historical sources. that the commotion was perceived by the effects of an Relevant excerpts taken from Logan (1887, earthquake called aloud to his people, ‘‘courage my pp. 322–323) given here describe the episode in all friends, for the sea trembles from the fear of you who its terrifying details from a seafarer’s angle: ‘‘…On are on it…’’. the 11th or (perhaps) 21st of September 1524, there Multiple possibilities need to be considered by arrived at the bar of Goa D. Vasco da Gama, who way of interpreting the historical reports of the 1524 discovered India. He came in great state befitted his sea disturbance reported from some parts of the position with a fleet of fourteen ships carrying three western seaboard of India. For example, one impor- thousand men…’’. On reaching the land at Dabul: tant question is whether the 1524 event could be ‘‘…and with the wind becalmed, during the watch of explained as a ‘‘meteotsunami’’—a phenomenon daybreak, the sea trembled in such a manner, giving driven by air-pressure disturbances, often associated such great buffets to the ships, that all thought they with fast-moving weather events such as severe were on shoals, and struck the sails, and lowered the thunderstorms (Rabinovich and Monserrat 1996). boats into the sea with great shouts and cries and They are different from seismic tsunamis in that they discharge of cannon’’. On sounding they found no are meteorologically induced sub-basin propagating bottom: ‘‘…and they cried to God for mercy, because waves with periods ranging from a few minutes to 2 h the ships pitched so violently that the men could not (Linares et al. 2019). If the propagation speed of the stand upright, and the chests were sent from one end atmospheric perturbations is close to the meteot- of the ship to the other’’. According to this narrative, sunami wave speed, the height of the wave grows. the trembling came, died away, and was renewed as it Studies from Lake Michigan (USA), that saw such reads: ‘‘each time during the space of a Credo’’. After phenomena in 1954 and 2003, indicate that it can be day-break the sailors were able to see the land and enhanced through near-shore wave transformation fleet commandant Da Gama ‘‘…maintained his such as shoaling, refraction, reflection and superpo- presence of mind during the trying scene…’’ and sition (Linares et al. 2019). A recent incidence (in tried to raise the confidence of his men by telling 2017) of waves as large as 3 m and approximately them that ‘‘…even the sea trembled at the presence of 5–10 times larger than normal waves in the Persian the Portuguese…’’.. The subterranean disturbance Gulf is attributed to a similar group of phenomena reportedly lasted about an hour: ‘‘…in which the (Heidarzadeh et al. 2019). water made a great boiling up, one sea struggling In the description of the sea disturbance from with another…’’. A fast recitation of the Latin Credo Dabhol, it is mentioned that ‘‘…the wind be calmed (a Christian prayer of faith) takes about three during the watch of daybreak…’’. This implies that minutes, and therefore, it is suggested that if the sea the ship was unable to move due to lack of wind, and waves resulted from an earthquake, its moment could be the main reason why it was anchored. It is release must have been considerable (Bendick and reported that the sea started ‘‘trembling’’ afterwards, Bilham 1999). subjecting the ships to ‘‘buffets’’. It is worth com- Contemporaneous reports of the same event from paring this description of the 1524 sea-disturbance the Gulf of Cambay (with archaic spelling ‘‘Cam- with a narration of a powerful storm from the west baya’’), located * 500 km, north of Dabul, also coast, by a Jesuit historian, that struck the city of called Dabhol (Figs. 1 and 2). is reproduced here Bombay (now called Mumbai; Fig. 1) on the 15th of from Kerr (1824; p. 445): ‘‘…While in the Gulf of May 1618: ‘‘…The sky clouded, thunder burst, and a Cambaya, in the dead calm, the ships were tossed mighty wind arose. Towards nightfall a whirlwind about in so violent a manner that all on board raised the waves so high that their city would be believed themselves in imminent danger of perishing swallowed up…’’ (Ghosh 2016). and began to consider how they might escape. One The September 1524 incident was a case of ‘‘a man leapt over-board, thinking to escape by swim- dead calm sea’’ suddenly becoming rough, with no ming, but was drowned; and such as lay sick of fevers allusion to any sign of previous deterioration of local The Orphan Tsunami of 1524 on the Konkan Coast weather conditions that lasted for an hour. Thus, the 2.2. Medieval Earthquakes from the Northern reports originated form the Portuguese fleet can be Arabian Sea taken only as a description closest to a tsunami, the The only historically known source for tsunami- one of its kind reported from the west coast of India. genic earthquakes (e.g. the 1945 event) in the In a recent study, Heidarzadeh et al. (2020b) Arabian Sea is located on the eastern part of the demonstrated that the 2004 Andaman-Sumatra tsu- Makran subduction zone (Fig. 1; Heidarzadeh and nami’s run-up was 1.6–3.3 m in the Mumbai Satake 2015). The seismogenic status of the western (Bombay) area, but the tsunami damage was minor. part still remains unknown. Historical data published Therefore, in terms of impact, the 2004 event is a by Ambraseys and Melville (1982) seem equivocal poor analogue. Further, a comparable previous mega- on the size and location of the medieval earthquakes tsunami sourced from the Andaman-Sumatra region in the region. According to Ambraseys and Melville occurred in the fourteenth century according to (1982), the 1483 earthquake with an estimated geological data and circa 1343 CE as per the magnitude of 7.7 had impacted the Strait of Hormuz interpretation of historical archives (Rajendran 2019). as well as the northwest Oman, and it was considered In the absence of historical references, it would be to have originated from a source in the western beneficial to cross-check the dates of medieval Makran subduction zone. More recent interpretation shipwrecks reported from the Arabian Sea and its of the historical data suggests that two earthquakes neighboring areas as a proxy regional record of rough from this region in 1483 and 1497 CE had magni- sea conditions along the trajectory of the proposed tudes ranging from 6 to 6.5 (Musson 2009). The tsunami. According to numerical simulations pre- former was near the Island of Queshm in the Strait of sented by Maselli et al. (2020), for the full-length Hormuz, and the latter was near Qualhat in Oman rupture of the Makran subduction zone, the resultant (Fig. 1). Whether or not these events were associated tsunami would have a propagation path directed with the Makran subduction zone, and indeed asso- towards the west coast of India, Seychelles and ciated with any tsunamis, cannot be easily affirmed Maldives, apart from impacting the near-field coasts with the available data. Questions can also be raised of Iran, Pakistan and Oman (Fig. 1, for locations). on the veracity of the date of the later event that Marine archeological surveys conducted off the coast supposedly occurred in 1497 CE. Musson (2009), of Oman (where the 1524 tsunami could have had a who quotes several sources to fix the date of the high impact), for example, report a 500-year-old earthquake, highlights how mutually contradictory wreckage of a ship under the command of Vasco da the historical accounts are. For example, Miles Gama, but suggested to have been sunk in May 1503 (1919), who was the British representative in Oman (Mearns et al. 2016). Many sixteenth-century Por- during 1872–1886, states in his memoirs that, ‘‘Ac- tuguese shipwrecks from elsewhere in the Indian cording to tradition Kilhat was destroyed by an Ocean are also reported, but none could be conclu- earthquake about four centuries ago…’’, which would sively dated earlier than 1552. Of these, the mid- place the event around 1500 CE. sixteenth-century Portuguese wreck reported from As for the western coast of India, a potential Seychelles (see Fig. 1, inset for location) is worth distant target for the tsunamis from the Makran mentioning because of the time interval during which region, little contemporary historical data exists that this happened (Blake and Green 1986). From the could throw direct light on the impact of any perspective of the potential distant impacts of a medieval tsunami. Interestingly, although precise tsunami sourced from the Makran region in 1524 CE, dates are not available, a medieval port located in this wreck off Seychelles could be a candidate worth Dwarka on the Gujarat Coast, NW India, is reported exploring further. Likewise, it is also worth evaluat- to have declined between the fifteenth and sixteenth ing the medieval earthquake occurrences in the centuries (Gaur et al. 2004; Fig. 1). The ruins source region adjoining the Northern Arabian Sea, associated with a jetty and port in Dwarka are now to identify the possible trigger for a medieval located underwater, about 500 m from the shore. transoceanic tsunami. C. P. Rajendran at al. Pure Appl. Geophys.

Several other contemporary ports along this coast Made of laterite blocks, the well was originally dug in also show signs of decline during the same period the early medieval period, evidenced by the presence (Gaur et al. 2004). Some form of a natural calamity of contemporaneous pottery found inside the well including a tsunami inundation cannot be ruled out (Marathe and Chandrashekhar, 2011). Currently, the for the decline of these ports. well remains submerged during the high tides, In the next section, we will discuss the observa- implying that the sea level was lower at the time tions from a site on the Konkan Coast of western when it was dug. India, not far from Dabhol (Fig. 2), from where the Archeological explorations revealed three types of Portuguese fleet had experienced the sea disturbance, pottery, including red, grey and glazed ware within as discussed earlier. This site located in Kelshi the sedimentary formation at Kelshi. Faunal remains Village offered a coastal dune section with tell-tale of both vertebrate and invertebrate animals along features of a sea inundation and consequent destruc- with molluscan shells and fish bones were also tion of a coastal community (Fig. 2 for locations). retrieved from this site (Joglekar et al. 1997, 2002). Some copper coins bearing the inscription ‘‘Al- Ahmad Shah Bin Ahamad Bin Al-Hasan Al-Bahmani, 3. Geological Evidence for Sea Inundation 837’’ have been reported from here. From the reference to Hijra year 837 (1433 CE), it is inferred 3.1. The Dune Formation in Kelshi that Ahmad Shah Wali Bahamani (1422–1436 CE) must have issued these coins (Marathe and Chan- Facing the coast, located on the river mouth at drashekhar 2011). Kelshi, a spatially extensive dune is being formed, The medieval settlers occupying the dune site and it spreads over an area of about 570 m2, with an were mostly maritime traders, and they were active average height of 9 m from the high-tide level only during non-monsoon periods (Deo et al. 2011). (Fig. 3a–d). Situated on the southern side of a tidal The reason for an abrupt end of this medieval coastal channel, a seaward extension of a small estuary (with outpost was debated by previous researchers. For an area of * 6km2) called Bharja, this 8- to 10-m- instance, Marathe and Chandrashekhar (2011) thick formation bears indication of human occupation hypothesized that the 1524 sea inundation, which starting from 1100 to 500 BP (Marathe and Chan- they considered as a tsunami, had impacted the coast drashekhar 2011; Deo et al. 2011). First reported by of Kelshi, but no specific evidence of the impact was Ghatpande (1993), the human artifacts identified reported. In the following section, we present the were part of a medieval settlement (Joglekar et al. observations from our field studies that led us to 1997, 2002; Deo et al. 2001, 2006). record the evidence for an ‘‘impact zone’’ within the The dune complex, separated from the beach by a dune stratigraphy at Kelshi that also must have tidal channel, is optimally situated between two rocky resulted in the destruction of the human settlement headlands to have been under a ‘‘wind tunnel here. effect’’’, creating a low-pressure region causing wind to move faster, thus facilitating faster movement of sand to form the dune. Deo et al. (2011) suggest that 3.2. Methods Adopted for the Study the rate of sand accumulation was moderate in the of the Sedimentary Section early periods and was accelerated during the latter The section at Kelshi (17.9217°N, 73.0577°E; intervals (after * 500 years BP). Along with these Fig. 3b) was initially cleaned with trowels, followed geographic elements, the stabilization of the beach as by gridding (1 m 9 1 m) using cotton strings and an active source of sand could be the triggering factor nails. Individual sedimentary units were discrimi- for the accelerated dune formation. nated based on their color, texture, lithology and A defunct and silted dug water well, 1.2 m in thickness. Stratigraphic contacts between sedimen- diameter, located at the base of the eolian deposit tary units, their continuities and breaks were traced bears testimony to changes in the sea level (Fig. 3d). along the exposed walls. The detrital charcoals The Orphan Tsunami of 1524 on the Konkan Coast

W E E W

Study site HTL Bharja Inlet Medieval well Kelshi ARABIAN SEA a c

(m)

Study Area

(m)

b d

Figure 3 a Map showing the location of the study site at Kelshi. b View of the dune formation (location as shown in a) from the tidal channel (photograph taken facing east during low tide). c View of the study site (location as shown in a); note the location of the medieval well. d Topographic profile of the region showing high-tide level (HTL) available from various sedimentary units within the 3.3. The Sedimentary Section and its Characteristics trench sections that provided accelerometer mass The part of the dune formation at Kelshi shows spectrometer (AMS) ages are reported in the 2r three successive foundations (compacted lateritic calendar ranges in CE, calibrated using the CALIB soil) of human settlements (I, II and III from bottom 6.1.1 program (see Table 1 for details). The AMS to top), each vertically separated by 40- to 50-cm- dating of charcoal samples was performed at Poznan thick eolian sand (Figs. 5, 6). The top foundation Radiocarbon Laboratory (‘‘Poz’’ as shown as sample level is about 3.5 m from the high-tide level (HTL) tag in Table 1), Poland. Radiocarbon ages were (Fig. 5). The most remarkable aspect of the stratig- calibrated using CALIB (version 7.0.4) (Stuiver and raphy is the presence of several distinctive pockets of Reimer 1993). The 2r ranges have maximum area bivalve (molluscan) shells with varying thicknesses, under the probability distribution curve. We have also extending discontinuously above the occupation level applied the correction factor DR = 163 ± 30 years, III (Figs. 5, 6). We could also record the remains of a empirically estimated for the Arabian Sea, which is bovine skeleton from level III (Fig. 5c), along with specifically used for calibrating the raw shell dates specimens of red-ware pottery and glazed pottery (Dutta et al. 2001). The region-specific DR values (twelfth–sixteenth century; Figs. 5 and 6). account for deviations due to variations in oceanic circulation and mixing (Stuiver et al. 1998). C. P. Rajendran at al. Pure Appl. Geophys.

Table 1 Radiocarbon dates from the section at Kelshi

S. no. Sample ID Lab ID Nature of the sample Depth Lab. no. 14C Age Calibrated 2r radiocarbon age in calendric years (CE)

1 M1 K-1 Charcoal 2.60 m Poz - 91036 365 ± 30 1449–1634 2 M2 a K-2 Charcoal 1.40 m Poz - 91037 470 ± 30 1410–1456 3 M2 b K-3 Charcoal 1.80 m Poz - 91038 395 ± 30 1439–1628 4 M3 a K-4 Charcoal 3.40 cm Poz - 91040 335 ± 30 1474–1641 5 M3 b K-5 Charcoal 2.20 cm Poz - 91041 375 ± 30 1446–1632 6 M4 K-6 Charcoal 2.95 cm Poz - 91042 350 ± 30 1458–1681 7 M4 shell KL Shell 3.60 cm Poz - 96253 865 ± 30 DR = 163 ± 30 1508–1681 8 KA—Shell KA Shell 3.25 m Poz - 100903 980 ± 30 DR = 163 ± 30 1432–1615 All dates have been calibrated using the CALIB Radiocarbon Calibration (version 6.0.1) and the marine09 data set (Stuiver & Reimer 1993). Two-sigma (r) ranges with the largest area under probability distribution curve. The estimated marine correction factor for the Arabian Sea: DR = 163 ± 30 is after Dutta et al. (2001). Samples with ‘‘Poz’’ tag were analyzed at the Poznan Radiocarbon Laboratory, Poland

The shell assemblage in the sedimentary section agency to a tsunami or a storm. Reinhardt et al. includes both bivalves and gastropods. The bivalves (2006) and Donato et al. (2008) have shown that include Meretrix meretrix, Paphia spp., Acanthocar- tsunami-transported shell beds contain a high con- dia lata, Perna viridis, Placuna placenta and centration of shells and articulated bivalves, as well Crassotrea cucullata. Gastropods include Umbonium as angular fragments with many allochthonous taxa. vestiarium, Natica didyma and Natica picta. The The above considerations allow us to assess distribution of these taxa is consistent with a marine- whether the shell layer represents an abrupt one-time to-saline pond environment (Joglekhar et al. 2002). deposition during an energetic sea incursion derived All molluscan species are identified as marine, except from the estuarine substratum. The shell layer occurs one that has estuarine affinity. The gastropod assem- at a height of 3.4 m from the base level (i.e. HTL; blages are dominated by Umbonium vestarium, Figs. 5a, b). The presence of articulated bivalves is usually found near the sandy beaches below the particularly noteworthy, as it denotes erosion of HTL. The presence of bivalves and gastropods estuarine or shallow sea bottom sediment and land- (Fig. 4b) that overlies the occupation level III ward transportation. The bed geometry defining a suggests winnowing from the nearby shelly mud linear distribution superjacent to the top occupation present in the estuary and subsequent transportation level, and the presence of articulated bivalves within before they were deposited at a height of * 3m the matrix of clay or mud, therefore suggests natural from the base level (i.e. HTL). The presence of shells depositional agencies rather than mobilized domestic on the top of the occupation level is uncharacteristic waste or midden generally associated with human of the ongoing eolian sedimentation processes, and settlements (Fig. 5b). therefore, its allochthonous nature needs to be Subjacent to the shell layer, though much explored. obscured by the ongoing wind-borne sand deposi- tion, outlines of a long-wavelength undulating scoured surface can be observed at occupation level 3.4. The Depositional Agency III (Fig. 6a–c). This zone is associated with a Earlier studies on the mechanism of deposition of hardened layer of reddish mud and reworked such shell sheets in near-coastal environments sug- material of potsherds and bricks from the habitation gest two transport mechanisms driven by either zone (Figs. 4 and 5b). Such ‘‘scour and fill’’ storms or tsunamis (e.g. Morales et al. 2008; Atwater sedimentary structures observed in geological sec- et al. 2011; Reinhardt et al. 2011). Previous research tions have been attributed to high-velocity flow of has shown that the shell taphonomy and the bed water generated during ebb currents produced during geometry may be useful for ascribing the depositional tsunami waves (Rossetti et al. 2000). The 2011 The Orphan Tsunami of 1524 on the Konkan Coast

Figure 4 a The face of the uncleaned part of the section at Kelshi, that shows the distribution of molluscan shell deposits of variable thickness (max. thickness: 15 cm) over the occupation level III at a height 3.5 m from HTL. The lower portions of the uncleaned section exhibit randomly occurring shells fallen or washed down from the top shell bed. b A closer view of shell deposit marked in a; Inset: a close-up of the shell deposit that contains articulated ones. c A part of a bovine skeleton from the level III. d Specimen of red-ware pottery obtained from the same level. e Specimen of glazed pottery (twelfth to sixteenth century) from the same level. The skeletal remains and pottery shown in panels c, d and e, respectively, are located * 30 m west of the section, as shown in Fig. 4a.

Tohuku-oki tsunamigenic earthquake off the north- The coastal geomorphology of the Konkan Coast, eastern coast of Japan (Mw 9.1; Satake et al. 2013) including that of Kelshi, is dominated by rocky provides some comparative live examples of mor- lateritic cliffs with spatially restricted sandy embay- phological features formed due to the tsunami ments. It is likely that, along high-relief coastlines, currents. For instance, the surveys of the Sendai tsunami backwash leading to greater erosion than coastal plain impacted by the 2011 Tohoku-oki deposition could be more dominant. One such tsunami show that the tsunami ‘‘created an extensive example is the tsunami associated with the 2006 shore-parallel scour depression in the back-beach Kurile Island earthquake (Mw 8.3) that was found to area’’ (Richmond et al. 2012). Video evidence from be dominantly erosive (MacInnes et al. 2009). This the Tohuku-oki tsunami, reported by Tappin et al. was attributed to the high-relief topography that (2012), reveals ‘‘significant turbulence at the land- accelerated tsunami outflow; therefore, the amount of ward edge of the coastal dunes’’. In such sediment eroded by the tsunami was far greater than comparable morphological settings, an initial what was deposited on land, as reported by Gelfen- shore-normal scour depression formed by the high- baum and Jaffe (2003), from their post-event field est tsunami wave could have been subsequently in- investigations of the 1998 Papua New Guinea filled with sand sourced from ebb flows. tsunami. C. P. Rajendran at al. Pure Appl. Geophys.

Figure 5 a View of the cleaned and vertically leveled section showing three occupation platforms. b Close-up of the cleaned-up section with scaled log on the right. Itemized views of the boxed areas marked on the section as shown in b. White dotted lines are drawn to highlight boundaries of features. The filled black squares denote the collection points of shell samples for dating, and corresponding age ranges including those obtained from the charcoal samples (filled black circles) are also shown

At the base of the occupation level III, we could charcoal dates range from 1410 to 1681 CE record soft-sediment deformation features like cracks (Table 1). The age of the sea inundation is estimated filled with coarse material (Figs. 5b; 6a–c), which using the dates obtained on shells as they are have been termed ‘‘diastasis’’ cracks, as defined in considered contemporaneous with the transportation Cowan and James (1992). Along the same zone, we event. As per the shell dates, this inundation event also recorded pockets of heavy minerals (Fig. 6d). must have occurred between 1432 and 1681 CE, Considering the distance of the section from the surf which overlaps the historical reports of the sea zone (* 400 m) and the height from the base l disturbance in 1524 CE. (3.4 m) the level at which these flow structures are Although no previous geological studies from the present, we argue that the top occupation zone (i.e. region have alluded to a tsunami of late medieval level III; Fig. 5b) has been impacted by high-energy period that had impacted the western coast of India, waves, with a run-up height of 5–6 m. The numerical previous exploratory work in and around the Chaba- models discussed in a later part will throw light on har (Iran) part of the Makran Coast (Fig. 1) reported a the potential mechanisms for inundation at Kelshi. liquefaction event prior to the 1945 tsunami (Rajen- dran et al. 2012). In particular, the radiocarbon date 856 ± 20 years BP of an embedded shell within an 3.5. Chronological Constraints for the Tsunami older liquefaction feature in a trench section is quite Event revelatory (see Rajendran et al. 2012). Two different The radiocarbon dates form the charcoal samples ranges of calibrated ages for this sample can be and the shells suggest that the tsunami event must estimated: 1591–1807 CE, using a marine correction have been contemporaneous with habitation. The factor DR = 249 ± 23 years (https://www.calib.qub. The Orphan Tsunami of 1524 on the Konkan Coast

Figure 6 Photographs showing sedimentary structures at the base of the occupation level III at Kelshi: Scour and fill structures (a, b and c); d Stratified patches of heavy mineral ac.uk/marine), and 1524–1681 CE, using DR = 163 5.4 m between the dug water well and the sand layers ± 30 years, after Dutta et al. (2001). The latter range can be used as an estimate of the run-up of the 1524 of dates overlaps the age constraints obtained from tsunami. However, there are some uncertainties the site at Kelshi. This single-date data only provides regarding this measurement, which include run-up a possible lead and is far from being confirmatory measurement errors (± 0.3 m; Heidarzadeh et al. evidence for a contemporary Makran-sourced earth- 2018) and the sea level uncertainties, because it quake/tsunami. A detailed survey needs to be appears that sea level was slightly lower at the time of conducted in the near-field areas of Iran and Pakistan the 1524 event. The dug well is under water during to collect further validating evidence for the medieval high tide at the current time, and it was possible that earthquake and the tsunami (e.g. Satake and Hei- it was out of water a few hundred years ago (this darzadeh 2017). difference is assumed to be less than 0.5 m). There- fore, we assume the run-up associated with the 1524 tsunami is in the range of 5 to 6 m. 3.6. Tsunami Run-up Estimation for the 1524 Event Based on the level of sand layers on the sediments section (Fig. 5), a measured elevation difference of C. P. Rajendran at al. Pure Appl. Geophys.

4. Numerical Modeling of Tsunami the Mw C 9 earthquake (see Heidarzadeh et al. 2009). For landslide scenarios, we produced the 4.1. Data and Methodology initial 3D shape of the landslide wave (Fig. 8b–d) using the semiempirical equations of Watts et al. We applied the numerical package COMCOT (2005), based on previous work by Synolakis et al. (Cornell Multi-Grid Coupled Tsunami Model) (Liu (2002) and Okal and Synolakis (2004), and later et al. 1998; Wang and Liu 2006) for numerical successfully applied in numerous studies (e.g. Hei- modeling of the tsunami. The simulations were darzadeh et al. 2019; Heidarzadeh and Satake 2017; conducted on a three-level nested grid system having Salmanidou et al. 2019). Three large submarine grids with spatial resolutions of 30, 5 and 1 arc-sec landslides were considered in this study with volumes for Grids 1, 2 and 3, respectively. (Fig. 7). The of 33.6, 7.5 and 179 km3 (Table 4). For comparison, bathymetric data for Grid-1 and Grid-2 are based on the size of the submarine landslide responsible for the GEBCO-2014 (the General Bathymetric Chart of the 1998 Papua New Guinea tsunami was * 5km3 Oceans) data (Weatherall et al. 2015), while Grid-3 is (Sweet and Silver 2003; Synolakis et al. 2002; based on a combination of the marine hydrographic Heidarzadeh and Satake 2015). Landslides were survey conducted by the National Institute of placed at the continental slope offshore Kelshi Oceanography of India (Sindhu et al. 2007) and our (Fig. 8b). We note that the earthquake and landslide digitization of the Google Earth images. Table 2 scenarios considered in this study are worst-case gives a summary of the nested grid system. scenarios because our objective is to examine which source, either a local submarine landslide or an 4.2. Consideration of Two Potential Generators earthquake in the Makran zone, could reproduce the of the Tsunami at Kelshi estimated run-up of 5–6 m in Kelshi. If local generators like near-source offshore earthquakes and weather-related meteotsunamis, as 4.3. Results of Numerical Simulations discussed earlier, can be precluded, the causative The maximum crustal displacement as a result of factor for the 1524 tsunami could be attributed to the M 9.3 earthquake is * 10 m (Fig. 8a). Snap- either a submarine landslide or a much-traveled w shots of tsunami simulations at different times are tsunami from a distant earthquake source. Although shown in Fig. 9a. It takes about 6 h for the waves to tsunamis are most often generated by seafloor travel from the MSZ and arrive in Kelshi, covering displacements produced during submarine earth- about 1300 km travel distance. The continental shelf quakes, submarine landslides can also trigger along the west coast of India is very shallow devastating tsunamis, sometimes characterized by (\ 100 m; Fig. 8), extending up to 100–250 km. extreme localized run-ups (Synolakis et al. 2002; This is the reason that the tsunami travels slowly Okal and Synolakis 2004; Fritz et al. 2009; Gonza´lez- (velocity of * 80 km/h) as it arrives on this shallow Vida et al. 2019; Yalc¸iner et al. 2014; Heidarzadeh shelf. The maximum tsunami amplitude generated in et al. 2020a). Thus, for the 1524 event, an important Kelshi by a M C 9 earthquake in the MSZ, at the question is the nature of its source. w location of our tsunami sediment fieldwork, is 4–5 m We have considered different sources for tsunami (Fig. 10a), which is close to the estimated run-up of generation: a large tectonic tsunami from an M C 9 w the 1524 event (i.e. 5–6 m; see previous section). earthquake in the MSZ (Fig. 8a; Table 3) and land- Wave time histories at two numerical wave gauges slide tsunamis generated off Kelshi (Fig. 8b–d; (WG-1 and WG-2) are given in Fig. 10. It is noted Table 4). The analytical formula of Okada (1985) that although tide gauge records of the earthquake was applied to calculate the co-seismic crustal scenario give maximum amplitude of * 3m displacement generated by the earthquake (Fig. 8a). (Fig. 10a), the run-up calculations result in maximum The scaling equations by Wells and Coppersmith waves of 4–5 m (Fig. 10a, second panel). In the Gulf (1994) were used to define the source parameters of of Cambay, the tsunami height due to the Mw C 9 The Orphan Tsunami of 1524 on the Konkan Coast

Figure 7 Three-level nested grids used in this study for tsunami simulations. The resolution of Grids 1, 2 and 3 are 30, 5 and 1 arc-sec, respectively

from the MSZ is * 4 m (Fig. 10a). The three of * 1 m in the Gulf of Cambay (Fig. 10b, d). landslide scenarios result in maximum tsunami Although the landslide scenario LS-3 is a gigantic amplitudes of only 1-1.5 m (Fig. 10b–d), which is landslide with the volume of 179.0 km3, it is unable far less than the actual run-up of 5–6 m in Kelshi. to generate any large waves in Kelshi. This can be Such landslide scenarios generate tsunami heights attributed to the short wavelength of landslide- C. P. Rajendran at al. Pure Appl. Geophys.

Table 2 5. Discussion and Conclusions Information of the three-level nested grid system used in this study for tsunami simulations The strata-bound shell deposits over the topmost

Name Longitude range Latitude range Spatial resolution settlement horizon at Kelshi on the Konkan Coast, at (°E) (°N) (m) a height of 3.4 m from HTL and associated sedi- mentary structures, offer critical proof for an Grid- 58.0000–873.5000 17.5000–26.0000 925.0 1 energetic sea inundation over a medieval coastal Grid- 72.4000–73.3500 17.6000–18.5000 150.0 settlement. That the bovine and occasional human 2 skeletal remains are restricted to the top settlement Grid- 73.0403–73.0847 17.9208–17.9403 31.0 level suggests that the sea inundation at Kelshi was 3 highly destructive. The sedimentary characteristics found at the impact zone within the dune section suggest strong sea/estuarine bed erosion and scouring generated waves in comparison to tectonically gen- that took place at maximum flow strength during the erated waves (e.g. Okal and Synolakis 2004)in incoming waves, while the reworking and sedimen- addition to the existence of extended shallow water tation occurred in the waning phase. The presence of region offshore Kelshi. shells transported from the near-beach and estuarine floor, which forms an over-wash at the top occupation

Figure 8 a The co-seismic crustal deformation from the source of the Mw C 9 earthquake used in this study. b The initial sea level deformation for the three landslide sources (LS-1, LS-2 and LS-3) used in this study. c, d, e 3D projection of the initial landslide sources of LS-1, LS-2 and LS-3, respectively The Orphan Tsunami of 1524 on the Konkan Coast

Table 3 Source parameters for the earthquake source considered in this study

Segment Length Width Top depth Strike Dip Rake Slip Long.a Lat.b Moment number (km) (km) (km) (°) (°) (°) (m) (°E) (°N) magnitudec

1 500 150 25 265 7 90 25 66.205 24.65 9.3 2 400 150 25 280 7 90 25 61.200 24.30 aLongitude of the upper-left corner of the fault bLatitude of the upper-left corner of the fault cThe rigidity of earth is assumed as 3 9 1010 Nm-2 (3 9 1011dyn cm-2) for the Makran region according to Heidarzadeh et al. (2009)

Table 4 Source parameters for the landslide tsunami source considered in this study

Landslide Location Length Width Thickness Volume Water Travel Max. initial sea level Max. initial sea level scenario (LS) (°E) (°N) (km) (km) (m) (km3)* depth (m) distance depression (m) elevation (m) (m)

LS-1 70.230 15 15 500 33.6 1500 2000 -8.6 6.5 18.352 LS-2 70.526 10 10 250 7.5 1500 2000 -3.2 2.4 17.950 LS-3 69.400 40 30 500 179.0 1500 4000 -5.2 4.1 19.480 *Calculated using the equation by Enet and Grilli (2007) level, and their taphonomy and association with flow For numerical simulations of sea inundation at structures, can be further explained as a consequence Kelshi, we considered the following as the possible of a high-velocity inundation. sources: a large tsunamigenic earthquake (Mw C 9) The tsunami at the Kelshi coastal settlement must in the Makran subduction zone, and three large-vol- have occurred between 1432 and 1681 CE, as infer- ume non-tectonic submarine landslides off Kelshi. red from the farthest shell dates in the minimum and Although two landslide scenarios considered here maximum ranges. These age constraints correlate have extreme mass volumes (vol- with the charcoal dates, indicating that the latest time ume = 7.5–179 km3), the tsunamis generated by window of the occupation ranges from 1410 to 1681 these large landslides are negligible (approximately CE. The radiocarbon chronology overlaps with the 1–1.5 m). The maximum tsunami amplitude gener- ‘‘sea disturbance’’ on the 7th (or 24th) of September ated in Kelshi by an earthquake of magnitude

1524, as reported by the Portuguese fleet. Further, a Mw C 9 from the MSZ is 4–5 m. Obviously, a similar description of sea disturbance is reported from localized submarine slide scenario cannot be recon- the Gulf of Cambay, located * 500 km north on the ciled with the reports of ‘‘sea disturbance’’ from two west coast of India. That the reports of the 1524 sea distant locations. Another alternative mechanism is a disturbance originated from two sites separated by major submarine landslide somewhere along the about 500 km suggests that this event may not have Makran sedimentary prism owing to a local earth- been a localized meteorological phenomenon. Fur- quake, but it is very unlikely that any landslide- ther, this occurred in the month of September, a generated waves in Makran could produce noticeable period generally marked by mild sea conditions in the waves in Kelshi or Cambay, because such waves Arabian Sea, and thus it was also unlikely to be have short wavelengths and lose their heights and associated with any monsoon-related activity. energies rapidly. C. P. Rajendran at al. Pure Appl. Geophys.

Figure 9 a, b Snapshots of tsunami simulations at different times for the earthquake and landslide (LS-1) sources, respectively

Previous numerical modeling by several authors Although, based on our modeling results, we tenta- showed that the earthquakes such as the 1945 Makran tively conclude that the likely trigger for the 1524

Mw 8.1 and a hypothetical Mw 8.6 along the MSZ are tsunami was an earthquake of magnitude Mw C 9 incapable of producing large waves along the west sourced from the Makran subduction zone (a worst- coast of India (e.g. Heidarzadeh et al. 2009; Hei- case scenario), the alternative possibilities (e.g. a darzadeh and Satake 2015). But the far-field tsunami 1945-type earthquake on the eastern part with local heights could also be controlled by the directivity, as amplifying factors) need to be tested in future studies. well as edge waves and local resonance of bays Unlike the report of the Cascadia orphan tsunami (Satake et al. 2020). The models presented here do of 1700 CE (Atwater et al. 2005), which made use of not inform about the roles, if any, played by those spectacular details of an unexplained tsunami in the factors in amplifying the wave heights at Kelshi. Japanese archives and of geological evidence in The Orphan Tsunami of 1524 on the Konkan Coast

Figure 10 a From top to bottom: maximum tsunami amplitudes at each computation grid point during the tsunami simulations for the Mw [ 9 earthquake source at Grid-1 and Grid-3, and tsunami waveforms at two artificial wave gauges WG-1 and WG-2. b, c, d Same as (a) but for landslide scenarios LS-1, LS-2 and LS-3, respectively

North America, our data of an ‘‘orphan’’ Konkan Acknowledgements tsunami is limited to a single site in western India. Taking a cue from the Cascadia event, a high-mag- The work is conducted as a part of the project nitude transoceanic tsunami sourced off the Makran ‘‘Tsunami risk for the Western Indian Ocean: Steps Coast must have left its geological imprints not only toward the integration of science into policy and in the epicentral areas, but also along the far-flung practice’’, funded by Natural Environment Research littoral coasts of the Arabian Sea. Since the 1524 Council, Global Research Fund, United Kingdom earthquake/tsunami does not find a place in the his- (NE/P016367/1). Revathy Parameswaran and Thulasi toriography of the source region, it becomes Raman participated in the first phase of the fieldwork. imperative to collect spatially representative geolog- MH was also funded by the Royal Society, the United ical data of the proposed tsunami. The earlier Kingdom, under grant number: CHL\R1\180173. We mentioned caveats also need to be addressed in future thank Serge Guillas, Department of Statistical researches as such results will have bearing on the Science, University College London, for arranging validation of the worst-case scenario presented in this financial support. CPR, JS and KR acknowledge study. partial funding from the Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India. Comments by Rob Witter on an earlier version of the manuscript were helpful. We C. P. Rajendran at al. Pure Appl. Geophys. have benefitted from detailed review and critical Dutta, K., Bhushan, R., & Somayajulu, B. L. K. (2001). DR cor- comments from Emile Okal (Northwestern Univer- rection values for the northern Indian Ocean. Radiocarbon, 43, 483–488. sity, USA), Alexander Rabinovich (the Editor-in- Enet, F., & Grilli, S. T. (2007). 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(Received February 20, 2020, revised August 17, 2020, accepted August 20, 2020)