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By Dhiman Ranjan Mondal
First published in The Daily Star on October 5, 2018 and accessible at: https://www.thedailystar.net/opinion/environment/news/what-can-we-learn-the-indonesia-earthquake-1642579
The island of Sulawesi in Indonesia was struck by a 7.5-magnitude earthquake on September 28, 2018 followed by 10-ft high tsunami waves. The death toll climbed to 1,424 as of Thursday. Indonesia is one of those countries that have adopted the tsunami warning system. This warning system contains about 150 buoys anchored to the seafloor sensors. The sensors send signals when they sense a sudden change of water level because of a tsunami.
It is shocking that despite having cutting-edge technologies offshore, Indonesia went through this devastating experience. It turns out that the authorities had issued a tsunami warning right after the earthquake as a routine procedure, but they eventually lifted it after 34 minutes. The rest of the story is known to all.
The tsunami crashed onto the shores of densely populated Palu and left the area destroyed. You might ask why the authorities had lifted the tsunami warning. The responsible body, Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG), blamed the early detection system, saying that the buoys from the Palu bay did not record any evidence of high water level. The fact is that the system of 22 buoys in the Palu bay has not been working since 2012. It was not properly maintained due to a lack of funding. What is surprising is that someone actually decided to cancel the warning despite being aware of broken buoys in the Palu bays. One could say that ignorance is to be blamed.
Let’s shift our focus to the coasts of Bay of Bengal, especially the southeast coast of Bangladesh and west coast of Myanmar. This coastline experienced one of the largest disasters on April 2, 1762 in which more than 200 deaths were reported in the aftermath of the earthquake. A recent scientific study published in Tectonophysics journal concluded that the 1762 Arakan earthquake raised the Saint Martin’s island by 2.5m and perhaps the Teknaf coast by 2.5m. The same earthquake raised the west coast of Myanmar by three to six meters.
Some researchers suggest that the coastline also experienced a tsunami after the 1762 earthquake and it is still possible for there to be an earthquake and/or tsunami in the future. But the problem is that we do not have the exact time when it might happen. Although the repeat time of this earthquake is about 500 years, we should start preparing now. It does not mean that we should start installing buoys starting tomorrow, but we should at least keep it on our radar. There are ongoing debates about whether or not the Bay of Bengal can be impacted by a tsunami. One might argue that the shelf break and shallow water level may help to dampen the effect of the tsunami. But I would say since we do not know if that is the case, we should be taking all precautions.
The tsunami in Palu is thought to have been caused partly by the submarine landslide although research is still going on and data is still being collected. However, if this is the case then it is one of those unique earthquakes where submarine landslides contributed to the tsunami. Some of the scientific studies suggested that submarine landslides may have contributed to the tsunami during the 2011 Tohoku earthquake in Japan which is well-documented. Now the question is, do we have that same possibility in the Bay of Bengal? That remains uncertain. Some evidence of small submarine landslides in the Bay of Bengal was documented by scientists. However, the continental slope angle is stiff enough to cause a landslide. Whether or not it can cause a tsunami is yet to be studied. Several studies elsewhere have addressed this issue. The research community in Bangladesh should study this and take precautionary measures accordingly.
Liquefaction was another after-effect of the Indonesia earthquake that caused building collapse. It is a geological phenomenon where water escapes from the space between individual sand particles due to agitation caused by seismic shaking. According to a recent report published in the New York Times, the tsunami waves did not do much to the buildings in the Palu area; rather the ground turned into jelly which could not hold the structures above. An area called Balaroa of Palu experienced the worst damage.
In Bangladesh, we may have to deal with a similar situation if an earthquake occurs. Some of the housing complexes and high-rise buildings were built after filling lakes and rivers with loose sand. How do we know that we will not see liquefaction in those areas? We saw the aftermath of the 5.5-magnitude earthquake in Tripura last year. Tremors felt in Shamshernagar and Kamalganj of Moulvibazar district in Sylhet caused cracks in the ground and liquid sand to come to the surface as revealed from our fieldwork. The newly built auditorium at Kamalganj partly collapsed due to the tremors caused by a small earthquake. We later came to know that the auditorium was built after filling up a pond.
After reading articles in local newspapers and watching reports on local channels regarding earthquake hazard in Bangladesh, I get the feeling that people are more concerned about the magnitude of the earthquake, where it is located or where the plate boundary is. Some researchers in Bangladesh are debating these issues. But I think from the hazard mitigation perspective, they should be concentrating more on how an earthquake can affect a certain area which seismologists refer to as “intensity”. The intensity of an earthquake can be much higher further away from the epicenter based on geologic conditions. It’s not the epicentral region that always experiences the worst damage. The epicenter of Indonesia’s earthquake was about 100km away from Palu. Likewise, the intensity can be much higher even with a low-magnitude earthquake. For example, the magnitude of the 2010 Haiti earthquake was seven—killing 220,000 to 316,000 people.
Therefore, research efforts are necessary to update the seismic hazard map. This exercise should involve geologists, engineers, policymakers, and government officials if we want to avoid an earthquake-related catastrophe in the future. Let’s not just assume that it will not happen in our country. Ignorance is not the right path. When it comes to natural disasters, we should be looking at each and every known possible scenario and make a logical decision about taking precautionary measures with sincere efforts. Let’s not just reject a hypothesis without proving it wrong.
Dr. Dhiman Ranjan Mondal is a postdoctoral associate at Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
