Published: 20 November 2019, 07:00 | By: Johannes Seiler
15-meter high waves that pushed 100 tons boulders inland: This is more or less how one can imagine the tsunami that hit the coast of today's Sultanate of Oman about 1,000 years ago, as concluded by a recent study by the universities of Bonn, Jena, Freiburg and RWTH Aachen. The findings also show how urgently the region needs a well-functioning early warning system. But even then, coastal residents would have a maximum of 30 minutes to get to safety in a similar catastrophe. The study will be published in the journal "Marine Geology", but is already available online.
Oman lies in the east of the Arabian Peninsula. The coasts of the Sultanate are repeatedly struck by tsunamis, most recently in 2013. Even with the most severe of these in recent times, the Makran event in 1945, the damage remained comparatively low. Back then, the tidal wave reached a height of three meters.
The scientists have now discovered evidence of a tsunami which is likely to have been much more powerful, with waves of up to 15 meters. For this purpose, the researchers from Bonn, Jena and Aachen concentrated their terrain investigations on a 200-kilometer coastal strip in northeastern Oman. "There we identified 41 large boulders, which were apparently carried inland by the force of the water," explains Dr Gösta Hoffmann from the University of Bonn.
Some of the boulders were probably formed when the tsunami shattered parts of the cliffs; for one of them, the largest weighing around 100 metric tons, scientists were even able to determine the exact point at which it broke off. Others show traces of marine organisms such as mussels or oysters that cannot survive on land. "Certain methods can be used to determine their time of death," says the geologist Gösta Hoffmann. "This allowed us to establish when the boulders were washed ashore."
Dr Christoph Grützner from the University of Jena has participated in many expeditions that aimed at investigating tsunami sediments in northern Oman. "What we found there is really amazing", Grützner explains. "Those massive boulders weigh more than 100 tons and haven't been moved by the past historical tsunamis that we know of, and neither did they move during the strongest tropical cyclones on record. Therefore, we conclude that a much more violent event must have happened in the past."
Grützner is an expert on past strong earthquakes that have left their traces in the geological record. "I usually work on sediment layers that have been offset by strong quakes, and then I date these layers, for example with the radiocarbon method. Here this method obviously didn't work, because the earthquake occurred hundreds of kilometres away offshore Iran. This is why we had to use secondary evidence to trace the earthquake, that is, tsunami sediments along the northern Omani coast. Thanks to the specialists from University of Freiburg we could date these sediments, and they turned out to be around 1000 years old." Grützner supported mapping the tsunami layers in the field and he helped to determine the mass of the huge tsunami boulders. "We found the suspicious sediment layers earlier and we had investigated their thickness and extent with geophysical methods such as georadar", says the geoscientist from the University of Jena. "That's how we were able to estimate the height of the waves necessary to move the sediment and the boulders." The missing piece in the jigsaw finally was the dating of the deposits. Now it is clear that the tsunami occurred around 1000 years ago. Grützner gets excited: "This means that the causative quake must have exceeded everything we know from instrumental and historical earthquake data of that region. This has enormous consequences for our understanding of the Makran Subduction Zone."
The Arabian and Eurasian tectonic plates collide in the Arabian Sea. They move towards each other at a speed of about four centimeters per year. During this process, one plate slides beneath the other. Sometimes they get stuck in this subduction zone. This can cause tensions that intensify more and more over years and decades. If they suddenly come loose with a violent jolt, the water column above the plates starts to move. This can lead to the extremely destructive waves that are characteristic of tsunamis.
"So far it has been unclear to what extent the Arabian and Eurasian plates get stuck," says Hoffmann. At the Makran event of 1945, for example, the effects were locally confined. The current findings, however, suggest that the tensions can also build up and unload on a very large scale - there is no other feasible explanation for the enormous forces at work at the time. "It is therefore extremely important that a tsunami early warning system is put in place for this region," stresses the geologist.
Nevertheless, even a smaller tsunami would have devastating consequences today: A large part of the vital infrastructure in the Sultanate of Oman has been built near the coast, such as the oil refineries and seawater desalination plants. A well-functioning warning system can, however, at least give residents some time to get to safety. Not very much though: Tsunamis move at the speed of a passenger aircraft; in the best case, the time between the alarm and the wave's impact would therefore be little more than 30 minutes.
Gösta Hoffmann, Christoph Grützner, Bastian Schneider, Frank Preusser und Klaus Reicherter: Large Holocene tsunamis in the northern Arabian Sea. Marine Geology, DOI: 10.1016/j.margeo.2019.106068