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expert reaction to Indonesian tsunami

Reactions to the news of a tsunami in the Sunda Strait area of Indonesia.

Prof Bill McGuire, UCL, said:

‘Volcanic tsunamis are the poor relation of the tsunami family. This said, they are not that unusual. This is the fifth such event since a tsunami triggered by the 1883 eruption of the same volcano took more than 35,000 lives. Studies of prehistoric deposits in places like the Canary Islands reveal that tsunamis generated by volcanic landslides can be many tens of metres in height and have the potential to transmit devastation across huge distances.’

Dr Richard Teeuw, Reader in Applied Geoinformatics & Disaster Risk Reduction at the University of Portsmouth, said:

“This tsunami, affecting the Sunda Straits region was probably caused by submarine landslides that were triggered by an eruption of Anak Karakatoa Volcano. From the video footage of the tsunami hitting the nearby coastal towns, this tsunami was relatively small, with waves of probably not much more than 1 or 2 metres in height. However, such waves – laden with debris – can be deadly for coastal communities, especially is there is no warning and only seconds for people to get to safe locations above the waves. The tsunami will be even deadlier if it occurs at night, as was the case here; and if it is a time of high tides, as was also the case here.  

“This Sunda Straits Tsunami looks to be very different to the tsunami in September that caused hundreds of fatalities in Sulawesi: that tsunami was triggered by an earthquake, as are most of the tsunami that impact on Indonesia, such as the catastrophic Indian Ocean Tsunami of Boxing Day 2004. Devastating tsunami caused by volcanic eruptions are rare; one of the most famous (and deadly) was caused by the eruption of Krakatoa in 1883 and the collapse of that volcano. Yesterday’s Sunda Straits Tsunami  was a third type of tsunami, caused by submarine landslides,which are  locally devastating but typically do not extending much beyond 50 kilometers. In this case, the trigger for the submarine landslides appears to be a small eruption of Anak Krakatoa (“Child of Krakatoa”) volcano.

“The lack of warning about the tsunami hazard for the communities along the Sunda Straits appears to be due to the unusual nature of the triggers for the tsunami: submarine landslides caused by a relatively small volcanic eruption. Such triggers would probably not have been detected by Indinesia’s tsunami early warning system because that is geared-up to detect earthquake-triggered tsunami. The night-time occurrence of the tsunami would have added to the chaos, with little chance of seeing the incoming tsunami wave and running to safety; it would also have hindered communication between the emergency services about the areas hit by the tsunami and limited the effectiveness of local tsunami alerts.

“The likelihood of further tsunamis in the Sunda Straits will remain high while Anak Krakatoa volcano is going through its current active phase because that might trigger further submarine landslides. To reduce the risk of fatalities from further tsunamis in the Sunda Straits area, we need research into the actual cause of the tsunami, which will involve sonar surveys to map the seafloor around Anak Krakatoa. If submarine landslides on the flanks of that volcano caused the tsunami, are there indications of potential future landslides ?  

“Unfortunately submarine surveys typically take many months to organise and carry out. So, in the short term, the communities of the Sunda Straits need to be made more resilient, with better preparedness for potential future tsunamis. That can be done relatively rapidly by:

– updating existing maps with regard to areas of tsunami run-up and inundation areas

– mapping vulnerable features such as centres of population and their exposure to tsunami inundation

– improving the local early warning system, to include alerts about Anak Krakatoa eruptions that might cause landslide-generated tsunami

–  re-assessing critical infrastructure (ports, major roads, hospitals, emergency coordination centres) to ensure that it is “tsunami-proof”

– highlighting “pinch points” such as bridges, where emergency response could be disrupted, as has occurred with the Sunda Straits Tsunami.

Dr David Tappin, British Geological Survey, said:

“This is another tsunami to strike Indonesia. The third to strike Indonesia in 6 months – all different.

“With this tsunami it is a developing event. It struck about 2130 local time, off west Java in the Sunda Strait between Java and Sumatra.

“It seems to be related to the Krakatau volcano which lies in the Strait. Within the caldera is the active volcano of Anak Krakatau which has been active during  the past few months so the suggestion is that this activity might have resulted in a collapse of the slope of the volcano which generated the tsunami. This ties in with the information we have on the tsunami striking the coasts of both west Java and south Sumatra. There is no suggestion of an associated earthquake. 

“There would have been no warning because the warning system is based on earthquakes.

“There has been talk in the media of ‘tidal waves’. This event is not a tidal wave, it is a tsunami.

“Fortunately recent scientific research on the Krakatau tsunami of 1883 which killed 36,000 has mapped the seabed area in the region. Further surveys planned for 2019 will repeat these surveys – so we will be able to determine the cause of the tsunami. Without these surveys it would be difficult to identify the tsunami source.”

Dr Simon Boxall, National Oceanography Centre and University of Southampton, said:

“The tsunami that hit Java and Sumatra yesterday was caused by events around the ongoing eruptions of Anak Krakatau (the volcanic island that appeared after a 1927 Krakatoa eruption).  It seems the volcano has seen significant activity since June this year and there were big eruptions a day before the tsunami.  These didn’t cause the tsunami – but it is likely the either (i) a pyroclastic flow followed – a large and very fast moving flow of molten rock into the sea or (ii) the eruption triggered a submarine land slide. In both cases large volumes of water are pushed by the moving sea floor or pyroclastic flow which results in a tsunami.  The waters around the region into the Java sea are relatively shallow (10-50 m. much like the English Channel) but to the south of Anak Krakatau they depend to c. 300m and on to the open ocean in excess of 5km.  In the shallow water the wave moves relatively slowly (about 30 mph) but in the deeper water it moves at anything between 80 and 500 mph.  The wave that hit Sumatra would have taken about 20 mins to hit land, the java wave would have taken over an hour. 

“However there was no earthquake as such and so there would have been limited warning. The region is also in spring tide at the moment and it would appear that the wave hit some of the coastal areas at the highest point of this high tide, exacerbating the damage done – the wave itself was reported to be around 3 metres (unconfirmed by scientific records at this stage).

“There will be an outcry as to why an early warning system didn’t kick in. The same criticism was levelled after the September Palu tsunami which killed 2,000 people.  These tsunamis are very localised and to cover the Indian Ocean with sufficient sensors to warn against all such eventualities would require many thousand buoys on the network. In shallow water the energy of the tsunami is quickly dispersed and so in this case the wave didn’t travel as far from source as the very destructive 2004 Boxing Day tsunami which occurred in the deep ocean.”

Dr Dave McGarvie, Lancaster Environment Centre, said:

“Growing volcanoes are unstable and collapses are common. When parts of volcanic islands collapse into water, tsunami waves can be generated (as water is displaced by the moving mass of volcanic debris). When people live on coastlines near volcanoes that are growing offshore, they are particularly vulnerable to tsunami waves generated from collapses of these volcanoes. They can be devastating for two main reasons: (1) they can occur with no warning; (2) distances can be too short to allow time for warning and evacuation.

“Although it has not been officially confirmed that this is what happened in the Sunda Strait, the growing volcanic island of Anak Krakatau is currently the chief culprit. This is because all growing volcanic islands are prone to collapse, on scales from small to massive. Only the larger collapses will displace enough water to trigger a tsunami wave capable of sweeping inland, damaging buildings, and killing people. To generate a damaging tsunami wave, the amount of material that is collapsing must be large enough to displace a substantial mass of water – and it must be quick! Slow and gentle collapses of even huge amounts of material will not trigger a tsunami wave, as the energy release is too protracted.

“Landslides also occur at mature volcanic islands, so it’s not just young and growing volcanic islands that trigger tsunami waves. But the younger volcanic islands are inherently more unstable and hence will experience more collapses.

“Regions like Indonesia and Japan, where there are numerous active and old volcanic islands, are where these hazards are most likely to be encountered.

“Recently, this hazard was highlighted at Hawai’i, when people were advised to stay well away from the new land that had formed when lava entered the sea. Simply because this new land was unstable and liable to collapse, and could trigger tsunami waves as well as explosions.”

Prof Dougal Jerram, University of Oslo, said:

“Tsunamis can be caused by volcanically induced landslides above or below water, and by volcanic eruptions themselves. Unlike tsunamis caused by earthquakes, such volcanically induced tsunamis may not trigger warning systems that are designed to alarm after large quakes, and thus may provide little warning, unless observed directly or detected by other devices such as wave buoy warning systems. Particularly when the volcano in question, as with Anak Krakatau, is already active and displaying activity such that its eruption is not a new or a surprise event.”

Prof David Rothery, Professor of Planetary Geosciences at The Open University, said:

“Anak Krakatau, the volcano on the site of Krakatau (Krakatoa) that was destroyed by the devastating eruption of 1883, has not suddenly come to life as some reports have implied. It has been erupting continually throughout much of this year, as part of the process of volcanic regrowth.

“In 1883 over 30,000 of the deaths were caused by the tsunami resulting from the explosive destruction of the former volcano. Today’s tsunami appears to have been caused by an underwater collapse of part of the new island and that has been forming as the volcano grows.

“Tsunami warning buoys are positioned to warn of tsunamis originated by earthquakes at underwater tectonic plate boundaries. Even if there had been such a buoy right next to Anak Krakatau, this is so close to the affected shorelines that warning time would have been minimal given the high speeds at which tsunami waves travel.”

Dr Ilan Kelman, Institute for Global Health and Institute for Risk and Disaster Reduction at University College London, said:

“The tragedy in Indonesia shows the ever-present challenges we face in developing warning systems for hazards like tsunamis. It must be a continual process of working with people at risk to determine possible future threats and how quickly everyone could be informed and act when one manifests. With the short timeframes we see here, we have a lot of work to do with those directly affected for improving warning systems to save lives.”

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