Gandhinagar Jan 29, 2019, (Research Matters):
With the changing climate, catastrophes like cyclones, hurricanes, earthquakes, tsunamis, and floods are devastating the planet repeatedly in the last decade. In the past 15 years, we have witnessed two catastrophic tsunamis in the Pacific and Indian Oceans in which around 250,000 people are estimated to have been killed. However, such massive tsunamis have occurred in the past too; one of them in India about a thousand year ago, according to a recent study by researchers at the Institute of Seismological Research, Gandhinagar M. S. University of Baroda, MG Science Institute, Ahmedabad and ETH Zurich, Switzerland. The study has found geological evidence for a palaeo-tsunami in the northern Arabian Sea believed to have taken place in the year 1008 AD.
Tsunamis are a series of massive ocean waves caused by underwater earthquakes. They can rise to 100 feet in height and have devastating effects on the shores when they crash inland. Such undersea earthquakes are known to happen in regions called tsunamigenic sources, where two lithospheric plates overlap each other. Geologists term these zones as subduction zones, which are more prone to underwater earthquakes and generate tsunamis.
The Makran Subduction Zone (MSZ) is one such tsunamigenic source spanning along Oman, Gujarat, Pakistan and Iranian coastlines in the northern Arabian Sea. It is one of the potential sources of the tsunami in the region and historical evidence suggest that this zone has experienced three tsunamigenic earthquakes in the past. The current study, published in the journal Scientific Reports, has found evidence of a fourth tsunami event on the coast of Kachchh in western India, which was previously undocumented.
“Based on the present findings, we can surmise that the MSZ has experienced four major tsunamigenic earthquakes during the historical period viz., 4000 year old tsunami at Muscat-Oman coastline, 326 BC tsunami event near present-day Karachi, 1008 AD event experienced at the Iranian, Oman and Indian (present day) coastlines (present study) and AD 1945 event experienced at Oman, Iran, Pakistan and Indian coastlines”, say the authors.
The researchers have studied the region and have identified a 250-km-long sand sheet deposition dating back to a thousand years—a typical indicator of an underwater earthquake and an ancient tsunami. The researchers estimate that the event could have occurred in 1008 AD. “A sand sheet has been identified based on sedimentology and geochemistry for over an extent of 250 km length, which has implications for palaeo-tsunami research worldwide as well as the hazard emanating from the MSZ”, explain the authors.
Although the MSZ stretches from Oman to Karachi, the Kachchh coastline in India has unique geomorphic features of the sediments, making it a suitable site for studying and reconstructing the history of ancient tsunamis in this region. The coastline now is a beach ridge, which has an elevated ridge along the beach consisting of sand. It is also a backswamp with deposits of fine silts and clay, merging with a shallow channel of sloping sides called a swale. When the researchers studied the samples from here, they found the presence of a 250-km-long sand sheet along the coast. Radiocarbon dating of the sediments further revealed the age of these samples to be about 1000 years, indicating the occurrence of a palaeo-tsunami around 1008 AD.
The Makran Subduction Zone is still an active tsunamigenic source and could cause tsunamis in the future, devastating the coastlines of the countries along the Persian Gulf, which is one of the world’s fastest-growing regions. Studies like the current one help us better understand the geology of these zones and predict the occurrence of future catastrophes based on historical evidence. The findings could also have implications in designing alert systems and investing in preparations for the impact of such events in the future.
“The study is of paramount importance since it aides in mitigating the catastrophic hazard arising out of tsunamis along the coastal segments. The knowledge of the existence and extent of the past tsunamis is critical in developing warning scenarios. The identification of past tsunamis along with their frequency and severity is particularly important in the implementation of alert systems, which are a must in rapidly developing coastal areas” conclude the authors.