Photo Credit: Indonesian National Armed Forces

Tragedy in Indonesia: A Tsunami without warning

Can researchers get better at predicting when a tsunami will occur?

NGSS Standards:

  • MS-ESS2-2. Construct an explanation based on evidence for how geoscience processes have changed Earth’s surface at varying time and spatial scales.

Key Vocabulary: tsunami, volcano, tectonic plate, magnitude, seismic, earthquake, subduction

Tsunami Article Guide

On December 22, 2018, a tsunami hit several coastal regions in Indonesia killing over 400 people and injuring over 1400 (as of December 31). The nation of over 17,000 islands lies in a region where the Australian, Pacific, Philippine, and Sunda plates (in addition to smaller plates) interact in a very complex way. Sitting along the Pacific Ring of Fire, Indonesia has chains of active volcanoes and is one of the most seismically active regions in the world. In fact, this region is where one of the deadliest natural disasters occurred in 2004 when a magnitude 9.1 earthquake triggered a catastrophic tsunami killing 228,000 people.

Since then, there have been other tsunamis in Indonesia, but none as large. Due to this tragedy, research has been focused on better understanding tsunamis triggered by large earthquakes and developing better ways to provide a tsunami warning system. The December 2018 tsunami struck with no warning, nor was it triggered by an earthquake, showing scientists that there is still much to be learned about the hazards in Indonesia.  

Map of tectonic plates and plate boundaries (blue=subduction zone, red=divergent, green=transform, purple=convergent). Star shows the location of Anak Krakatau volcano that triggered the tsunami in December 2018. Photo Credit: Eric Gaba via Wikimedia

What are tsunamis?

Tsunamis are the rapid movement of large masses of water, appearing as a series of large waves. There are a variety of causes for displacing large amounts of water in the form of a tsunami. The most common is fast up and down movement of the seafloor from earthquakes in subduction zones – what caused the 2004 tsunami. Luckily, not all earthquakes in subduction zones cause tsunamis. Only those that are shallow, underneath the ocean and generate a large vertical displacement of the seafloor are capable of producing a tsunami. Other causes are underwater landslides, often from earthquakes, from the collapse of a hillside into the sea, an underwater volcanic explosion, or a meteor impact. About 80% of tsunamis occur in the Pacific ocean due to the Ring of Fire, but they are possible wherever there are large bodies of water, including lakes.

Tsunamis behave differently in deep and shallow waters, and the size and speed of them depend on how much water is displaced. For example, if you throw a bigger rock in a pond, ripples will be larger and travel faster. In the open ocean, tsunami waves are widely spaced only a few feet high and travel at speeds around 500 mph. In some cases, tsunami waves may die out before they even reach the shore. If tsunami waves reach the shore, they can cause a lot of damage! When they reach shallow water near the coast, the front of the wave slows down (~40 mph) while water behind the wave is still moving pretty fast. This causes the waves to bunch together and to grow to heights of up to 30 ft or more (in extreme cases over 100 ft high!). A tsunami does not just consist of a single wave, but rather a series of waves with the largest wave typically not even coming first. Sometimes, a tsunami causes the sea to recede at first, exposing a large part of the ocean floor. Tsunamis are so dangerous because they travel at speeds faster than people can run and can carry a large amount of debris with great force. These destructive waves can travel 0.5 miles or more inland, devasting low-lying coastal areas. As a result, it is important to move away from the beach and get to higher ground when you know a tsunami is imminent.

Diagram showing how an earthquake along a subduction zone can cause a tsunami. Photo Credit: USGS

What caused the December 2018 tsunami?

Unlike the tsunami in 2004, the December 2018 tsunami was related to the partial collapse of the Anak Krakatau volcano. Anak Krakatau is a volcanic island and one of Indonesia’s 127 active volcanoes. This volcano sits in the Sunda Strait between the islands of Java and Sumatra and has had increased level of activity over the last few months. In the 2 days prior to the tsunami, a new surge of magma led to increased explosive activity probably reaching its peak on December 22. The lava flows eventually reached the sea and the weight of the rapidly accumulated lava on the sides of the volcano caused the slope to become unstable and the eventual collapse of its side.

When 64 hectares of the volcano, an area approximately equal to 6 Ellis Islands, collapsed into the sea, displacing a lot of water and generating a tsunami. Like ripples from dropping a rock into a pod, when the side of the volcano collapsed, a tsunami wave radiated outward. Waves from the tsunami affected over 190 miles of coastline and in some places, the waves reached up to 42 ft. The collapse of the volcano greatly changed the shape of the volcano, where it decreased in height almost 750 ft. More work still needs to be done, but scientists continue to study this event in order to better understand what had happened.

Diagram showing Anak Krakatau is thought to have generated the tsunami in December 2018. Photo Credit: Science Over Everything

There is a warning system for tsunamis generated by earthquakes, where the global seismic network is used for rapid earthquake identification in order to determine the earthquake’s size, depth, and location. Knowing where and how big an earthquake is helped to scan for when tsunamis are possible. There are also buoys in the sea monitored by satellites to identify actual tsunamis. Indonesia possesses a tsunami warning system for earthquake-generated tsunamis but not for those triggered by volcanoes. As a result, there was no warning for the tsunami waves that struck that December night. There was prior seismicity reported, but not large enough to alarm people. There are buoys in place near the plate boundary to detect tsunamis generated by earthquakes, but none next to the volcanoes. Even if there were buoys next to Anak Krakatau, they would have been too close to the coastline and would have only provided a minimal warning.

Global Impact

More than 400 structures were destroyed by the tsunami with some villages completely wiped out. The tsunami in Indonesia in December 2018, in addition to the one in September 2018, ignited fear and hit the tourism and travel industries hard as a majority of reservations were canceled, leading to a large hit to the local economy.

Tsunami research has been focused on subduction zones and the big earthquakes they can produce. Research has been driven by the tsunamis caused by the 2011 magnitude 9.0 Japan, 2004 magnitude 9.1 Sumatra, and 2010 magnitude 8.9 Chile earthquakes. However, the recent tragedy in Indonesia shows that there is still more that needs to be understood about tsunami hazards and the stability of volcanoes. More focus needs to be placed on those hazards which are unexpected so to prevent casualties and minimize damage by providing ample warning and allowing for timely evacuations. Furthermore, it is important to inform people about what the different causes of tsunamis are and how dangerous they can be such that people are able to recognize the risks and know what to do to stay safe.


2 Comments Posted

  1. This is a very good article! It helped me learn more about Tsumanis and how they form and what they do when they do form. It also helped educate me on how a volcano colassps thnaks!

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