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EVIDENCE FOR HOLOCENE TSUNAMI-IMPACT ALONG THE SHORELINE OF OMAN

Gösta Hoffmann (1), Klaus Reicherter (2), Thomas Wiatr (2), Christoph Grützner (2)

(1) Department of Applied Geosciences, German University of Technology in Oman, PO Box 1816, Athaibah, Muscat, PC

130, Sultanate of Oman, email: goesta.hoffmann@gutech.edu.om (2) RWTH Aachen University, Inst. für Neotektonik und Georisiken, Aachen, 52056, Germany

Abstract (Evidence for Holocene tsunami-impact along the shoreline of Oman): Three independent sets of evidence of past tsunami along the coastline of Oman are reported. The rocky coastline of the Sultanate of Oman between Fins and Sur is decorated by a number of large boulders and boulder accumulations forming ramparts. The boulders occur as individual blocks of almost 50 tons weight, as imbricated sets and “boulder trains”. The coast is made up of folded Tertiary limestones and beach rock of Quaternary age. The transport distance from the fractured cliff front of 6-10 m height above mean sea level varies between several meters up to 70 m inland. We found individual blocks of recent corals and overturned blocks with oysters and pools. T-LIDAR was used to analyse geomorphologic features and for volumetric estimates of boulder weight. Tropical cyclones such as Gonu in 2007 or Phet in 2010 as well as historical tsunamis are known to have affected Oman’s coastline in the past. Coastal changes by cyclones are known to have been negligible; therefore, we interpret the boulder ridges as tsunamigenic deposits. Additionally, fine grained lagoonal sediments were analyzed. A distinct shell layer with allochthonous species is documented. A tsunamigenic origin is most likely. Although no dating evidence of the observed boulder and lagoon deposits is available at the moment we conclude that the 1945 Makran tsunami affected Oman’s coastline. This conclusion is based on interviews with local people. Key words: Oman, tsunami, boulder deposits, T-LiDAR INTRODUCTION Recent tsunami events like the Indian Ocean tsunami on 26th December 2004 and the Tôhoku earthquake and tsunami on 11th March 2011 resulted in large numbers of casualties and immense damage to infrastructure. These events underline the need for tsunami hazard research and assessment for any potentially vulnerable region. In most cases this can only be done by studying past tsunami records. The coastlines of the Sultanate of Oman are prone to various natural hazards such as tropical cyclones, landslides and tsunamis. The devastating effects of the cyclone Gonu, caused by flash floods and landslides in June 2007 illustrated the need to investigate the recurrence intervals of such events in order to assess the vulnerability and to mitigate damages. So far no scientific research concerning recurrence intervals of natural hazards has been carried out. However, studies published by Heidarzadeh et al. (2008a, 2008b, 2009) and Jordan (2008) reveal past tsunami events in the Indian Ocean with possible effects on the coastline of Oman (Fig. 1). As the population of Oman and the neighboring countries is concentrated along the coastline and large infrastructure projects are planned or already completed a holistic scientific approach to decipher the geological record of past extreme events is overdue. On 27th November 1945 an earthquake occurred in the Makran Subduction Zone offshore Pakistan and triggered a tsunami (Jaiswal et al., 2009). Up to 4000 casualties were reported along the coastlines of NW India and Pakistan, including 5 m run-up along the coastlines of the Sultanate of Oman. Donato et al. (2008, 2009)

analysed shallow sediment cores from the lagoon in Sur and recorded a 5-25 cm thick shell bed close to the surface. Based on the taphonomy and fragmentation a tsunamigenic origin is discussed as the most likely form of deposition related to the 1945 tsunami. However, there are almost no historical documents available for Oman for this period as the country was isolated with no international contacts until the 1970s, living conditions were poor and no modern technology was in use. We report geological and historical evidence for the tsunami along Oman's coastline. These evidence are: (a) fine grained lagoon sediments, which show distinct layers with allochthonous, offshore species (mollusks and foraminifera); (b) boulder deposits encountered along cliff-coastlines and (c) eyewitness-reports of old people we interviewed. OBSERVATIONS The coastal area under investigation is situated in the eastern part of Oman between the cities Quariat and Sur. The area is sparsely populated as most of the country; small fishing villages are scattered along the coast. Only since 2008 there is a paved road connecting the cities Quariat in the north and Sur in the south. The geology of the area is dominated by Paleogene to Neogene limestone formations, which rise from the coast up to 1500 m to form the Selma Plateau. Geomorphologic evidence of Quaternary land-uplift is obvious along the entire coastline: coast-parallel, wave-cut terraces are encountered up to elevations of 300 m. Within the study area these terraces are

2nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011)

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cut into Paleocene-Early Eocene limestone formations. Quaternary deposits are either of fluvial origin or ancient to subrecent littoral deposits, usually preserved as beachrock. In most cases only erosional remnants of the beachrock are found and the underlying older strata dominate along the cliff coast. Several intertidal lagoons exist in the vicinity of Sur and Ras al Hadd. These lagoons serve as geological archives with a preservation potential for palaeo-tsunami and were investigated during several field campaigns in 2010.

We collected seven sediment cores at various locations within Sur lagoon. The deepest core reaches 10 m below the present surface. The sequence is characterized by silty fine sand in the lower part (10-6 m) and fine-sand in the upper part (6 – 0 m). Within the uppermost meter several distinct shell beds were identified. The shell and foraminifera assemblage contains allochthonous species living in the subtidal zone and offshore. Additionally, we collected 4 sediment cores in the lagoon of Ras al Hadd. The longest core is 3 m long. The base of the sequence is made up of sandy gravel partly

Fig. 1: Historical tsunamis in the Indian Ocean and working area in the Sultanate of Oman.

Fig. 2: Study area along the east coast of the Sultanate of Oman. Inset shows bathymetric sections.

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Fig. 3: Study area along the east coast of the Sultanate of Oman with the observed ramparts.

cemented as beachrock, overlain by fine to medium sand. The mollusk and foraminifera assemblages in the upper 100 cm show a variety of species. Especially some bivalve species within this layer are allochthonous as their habitat is characterized as subtidal and offshore. In our 2011 survey we found boulder deposits south of the village of Fins (Fig. 2) and more distal finer-grained deposits yielding shells and coral blocks on the cliff top and to approx. 60-70 m inland (Fig. 3). The blocks are partly imbricated and reach volumes of more than 20 m3 (determined with terrestrial LiDAR scanning), corresponding to a weight of almost 50 tons. Also, we found so called tsunami boulder trains, where blocks are aligned in a row (Fig. 3). Some blocks are toppled or upright with hit marks on the surface, erosional pot holes, Lithophaga borings and attached oysters (which provide dating material, dating is in progress). The boulders form ramparts and have a wavy, lobe-like pattern. Most blocks have a platy shape, which origins in layer thickness of reworked material, mainly beach rock and Tertiary limestones. We measured the long axis (a-axis) of 60 boulders, a vast majority is oriented N30, possibly pointing towards the wave direction. The cliff tops are “cleaned”, however, drift wood of the tropical cyclone Phet in 2010 is found in height of approximately 6 m above mean sea level. Inland, boulders are found in a gravel/sand matrix with various fossil remains like shell and corals (Fig. 4). The finer-grained layers show fining-up cycles. We also interviewed old people living in the towns of Sur and Tiwi. An old man in Sur recalled an event that happened most

probably during the 1940s: first the sea retreated, then, two waves washed onshore. The event took place at 02:00 am. Another old man of the village of Tiwi reported from hearsay, as he was born in 1946. He heard about an event that destroyed the local graveyard in the 1940s. He described that the graveyard was located much further inland. Furthermore, he gave an account of fish (sardines) and mollusks (oysters) being washed into the Wadi Shab. The women who used to get freshwater from the wadi could not walk there anymore, but boats had to be used. The marine fishes lived in the wadi after the event. DISCUSSION The uppermost ~1m in the lagoon of Sur as well as in the lagoon of Ras al Hadd clearly indicates an event-layer which can be either storm- or tsunami-generated (see Kortekaas and Dawson 2007). As the lagoons are intertidal, reworking and bioturbation of the sediments is a common phenomenon that hampers a clear stratification. Boulders deposits along the east coast of the Sultanate of Oman form ramparts between Fins and Sur. Inland, boulder deposits are incorporated in finer-grained sediments. Observations of the coastline changes and modifications of the last two very strong tropical cyclones Gonu and Phet rather exclude tropical cyclones a “moving agent” for the large 50 tons boulders. This was also proven by comparing time series of Google Earth, where blocks are detectable, but remained in the position (before and after).

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The information gained from the interview in Sur is very helpful. The timing of an event resulting in sea-water entering the house was given with 02:00 am which is 22.00 GMT (local time zone is GMT +4). This fits quite well with the time of the Makran earthquake on 27.11.1945 which is reported by Pendse (1948) as 21:57 GMT. The modeled travel time of the tsunami wave is 20-30 minutes (Heidarzadeh and Kijko. 2011). The description of a retreating sea fits general tsunami descriptions. The arrival of two separate waves fits observations in India (see Rajendran et al. 2008) where also two waves with disparity in arrival time are reported. A submarine landslide is assumed for the second wave. The descriptions given in the interview in Tiwi cannot unambiguously be related to any know event. However, the accounts are more likely to be the effect of a tsunami wave rather than wadi-flooding.

Hence, we propose a tsunamigenic event being responsible for the rampart formation and the boulder deposits along this part of the coast. Dating of oysters, which have grown on the blocks and died during the deposition, is in progress.

Acknowledgements: This study was financially supported by the German Research Foundation (DFG-project Re 1361/14-1) and GUTech in Mascat. Our students Sina Rausch, Kathrin Wagner and Tobias Rausch are thanked for field and lab support, and Tobias also for graphic design. References Donato, S. V., Reinhardt, E.G., Boyce, J.I., Rothaus, R.,

Vosmer, T. (2008). Identifying tsunami deposits using bivalve shell taphonomy, Geology, 36, 199-202.

Donato, S. V., Reinhardt, E. G., Boyce, J. I., Pilarczyk, J. E., Jupp, B. P. (2009) Particle-size distribution of inferred tsunami deposits in Sur lagoon, Sultanate of Oman, Mar. Geol., 257, 54-64.

Heidarzadeh, M., Pirooz, M., Zaker, N. H., Yalciner, A. C., Mokhtari, M., Esmaeily, A. (2008a) Historical tsunami in the Makran subduction zone off the southern coasts of Iran and Pakistan and results of numerical modeling, Ocean Eng., 35, 774-786.

Heidarzadeh, M., Pirooz, M., Zaker, N. H., Synolakis, C. (2008b). Evaluating tsunami hazard in the northwestern Indian Ocean, Pure Appl. Geophys., 165, 2045-2058, Doi:10.1007/s00024-008-0415-8.

Heidarzadeh, M., Pirooz, M., Zaker, N. H., Yalciner, A. C. (2009). Preliminary estimation of the tsunami hazards associated with the Makran subduction zone at the northwestern Indian Ocean, Nat. Hazards, 48, 229-243, Doi:10.1007/s11069-008-9259-x.

Heidarzadeh, M., Kijko, A. (2011). A probabilistic tsunami hazard assessment for the Makran subduction zone at the northwestern Indian Ocean, Nat. Hazards 56, 577-593.

Jaiswal, R., Singh, A., Rastogi, B. (2009). Simulation of the Arabian Sea tsunami propagation generated due to 1945 Makran earthquake and its effect on western parts of Gujarat (India), Nat. Hazards, 48, 245-258, Doi:10.1007/s11069-008-9261-3.

Jordan, B. R. (2008). Tsunamis of the Arabian Peninsula - a guide of historic events, Science of Tsunami Hazards, 27, 31-46.

Fig. 4: Sketch of the possible tsunamigenic deposits.

Kortekaas, S., Dawson, A. G. (2007). Distinguishing tsunami and storm deposits: An example from Martinhal, SW Portugal, Sed. Geol. 200, 208-221.

Pendse, C. G. (1948). The Makran earthquake of the 28th of November, 1945, Scientific Notes, Indian Meteorological Dept. 10, 141-145.

Rajendran, C. P., Ramanamurthy, M.V., Reddy, N.T., Rajendran, K. (2008). Hazard implications of the late arrival of the 1945 Makran tsunami, Current Science 95, 1739-1743.