The Holocene tsunami history of the Arabian Sea

The Makran Subdution Zone (MSZ) is the dominating structure in the Arabian Sea. Here the Arabian Plate is subducted beneath the Eurasian Plate. The seismicity of the MSZ is comparatively low and the historic record is fragmentary and incomplete as the coastlines of the Northern Indian Ocean (NIO) are only sparsely populated. Therefore, the recurrence interval of large tsunamigenic earthquakes (MW>8) is unknown. An additional tsunami threat results from submarine slides within the basin (Hoffmann et al. 2014). We aim at reconstructing the Holocene tsunami history of the NIO and use a holistic approach by utilizing a) historic records, b) archaeological evidence and c) sedimentological archives. The only instrumentally recorded earthquake on the MSZ that triggered a larger tsunami occurred on 27th November 1945. We interviewed old fishermen along the coastline of Oman and collected additional historic information from archives to reconstruct the 1945 event in the NIO (Hoffmann et al. 2013a). The damages reported were severe along the coastline of modern Pakistan and minor along the coastline of Oman, where wave heights of 2-3m are reconstructed. Block and boulder deposits along Oman’s coastline indicate higher energetic wave events (Hoffmann et al. 2013b). The maximum mass of these deposits is estimated as 120 ton. The blocks are colonised by marine sessile organism, indicating the former position in the intertidal area. We did radiocarbon dating (n=55) of these organisms. Additional work concentrates on the analyses of fine grained deposits which are interpreted as overwash. We performed ground penetrating radar surveys (Koster et al. 2014) and conducted sedimentological investigations. These deposits are dated by radiocarbon (n=25) and OSL (n=16). Additional information is obtained from 2 archaeological sites. The detailed analyses of marine overwash deposits along the coastline of Oman are interpreted as evidence for extreme wave events within the NIO. Storm surges as well as tsunami waves may be responsible for the dislocation of the boulders and the fine grained overwash deposits. However, neither overwash deposits nor dislocation of blocks or boulders were observed during recent cyclone events, and therefore tsunamis appear to be a more likely process. A recurrence interval of 400-450 years for the Mid to Late Holocene is calculated for tsunamis generated at the MSZ. The data are preliminary and need further investigation. The data suggest a worst-case scenario for Muscat with wave run-up in the range of 15m. The resulting inundation would affect critical infrastructure such as oil refineries and desalination plants. References Hoffmann, G., Rupprechter, M., Al Balushi, N., Grützner, C. & Reicherter, K. (2013a). The impact of the 1945 Makran tsunami along the coastlines of the Arabian Sea (Northern Indian Ocean) – a review. Annals of Geomorphology, Supplementary DOI: 10.1127/0372-8854/2013/S-00134. Hoffmann, G., Reicherter, K., Wiatr, T., Grützner, C. & Rausch, T. (2013b). Block and boulder accumulations along the coastline between Fins and Sur (Sultanate of Oman): tsunamigenic remains? Natural Hazards 65, 851-873. Hoffmann, G., Al-Yahyai, S., Kociok, M. & Grützner, C. (2014): An Indian Ocean tsunami triggered remotely by an onshore earthquake in Balochistan. – Geology. doi: 10.1130/G35756.1 Koster, B., Hoffmann, G., Grützner, C. & Reicherter, K. (2014): Ground penetrating radar facies of inferred tsunami deposits on the shores of the Arabian Sea (Northern Indian Ocean). – Marine Geology 351: 13-34. /10.1016/j.margeo.2014.03.002.