A tsunami thought to be triggered by volcanic activity affected many residents in Indonesia during the night of December 22nd. It is believed that around 24 minutes after Anak Krakatau erupted, underwater landslides triggered a three-meter tsunami that swept ashore in several areas of Java and Sumatra. Since there was not a preceding earthquake, as is usually the case prior to a tsunami, there was no warning for residents and tourists on either side of the Sunda Strait (the body of water between Java and Sumatra).


This photo was handed out by Indonesia’s National Agency for Disaster Countermeasure (BNPB) following the tsunami on December 22nd. Source: BNPB


Unfortunately, at the time of publication, there were 373 known casualties, over 1,000 people injured and dozens of missing persons, with these numbers expected to rise. As it is the holiday season, many of those killed and injured were tourists on vacation at beach resorts. Scientists believe a combination of a full moon leading to higher tides and the lack of a warning to those nearby, led to the massive destruction from this event.


PlanetWatchers used near real-time synthetic aperture radar satellite imagery from Sentinel-1 (operated by the European Space Agency) to perform an analysis of affected areas. This analysis shows how Anak Krakatau altered its shape following changes within the volcano and which locations were most affected by flooding from the subsequent tsunami.


Image One shows an optical satellite image of Anak Krakatau and the surrounding islands as they appeared prior to this event.


Image One: Optical satellite image of Anak Krakatau and surrounding islands. Source: Yahoo Maps.


Image Two shows the Sentinel-1 image captured on December 23rd. This satellite imagery was taken around 5:30 in the morning local time (eight hours after the initial reports of waves coming ashore). As such, this analysis depicts the very first satellite analysis of the Sunda Strait and areas around Tanjung Lesung Beach, an area greatly impacted by this tsunami. Locations in red are those that were assessed as being flooded following the tsunami.


Image Two: Sentinel-1 radar image acquired eight hours after the tsunami hit these locations. Areas highlighted in red were assessed as being flooded as a result of the tsunami.


Image Three shows two Sentinel-1 images, before and after the volcanic activity and the resulting tsunami. The left image shows Anak Krakatau on December 11th and the image on the right shows the same area on December 23rd, several hours after the tsunami.


Image Three: Left- Sentinel-1 image taken on December 11th. Right- Sentinel-1 image taken on December 23rd several hours after the tsunami.


Zooming in on the image on the right, Image Four clearly shows a partial collapse of the summit of Anak Krakatau and the collapse of the western side of the island. These huge land masses crashing into the ocean created the deadly tsunami waves that hit the shores of Java and Sumatra. In this rare image we can clearly see waves approximately 100 meters apart from one another encircling the island.


PlanetWatchers worked with Dr. Ran Nof, a researcher in the Seismological Division of The Geological Survey of Israel on this analysis. Based on Dr. Nof’s analysis, these waves emanating from Anak Krakatau are evidence that there continued to be intense volcanic activity hours after the initial collapse of the volcano slope and tsunami.


Image Four: A zoomed in view of Anak Krakatau on December 23rd with the collapsed summit and western edge of the island clearly collapsed into the water. Waves can be seen surrounding the island spreading in all directions.


PlanetWatchers is providing this analysis immediately following this tragic disaster in the hopes that it will assist in the rescue and recovery efforts in the hours, days and weeks ahead. Our thoughts and prayers are with those affected by this terrible event and we hope this information helps those who need it most.

For more information about PlanetWatchers, please visit planetwatchers.com.

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