The 2018 Sunda Strait tsunami (Indonesian: Tsunami Selat Sunda 2018) occurred on 22 December 2018 at around 21:38 local time after large parts of the southwestern side of Anak Krakatoa collapsed onto its caldera. The landslide spawned a tsunami wave that struck multiple coastal regions in Banten and Lampung, including the tourist destination of Anyer.
Lack of warnings on the arrival of the tsunami, coinciding with the Christmas and New Year holiday season in the nation and the tsunami occurring at nighttime, contributed to the large loss of life. With 426 deaths, the tsunami was the deadliest volcanic tsunami in Indonesia since the 1883 Krakatoa tsunami, which originated from the same island.
Following the tsunami, Indonesian authorities installed multiple emergency early warning systems in tsunami-prone areas. The newer system would detect any significant disturbances on water levels, including changes caused by volcanic activity or undersea landslide.
Lying on the Ring of Fire, Indonesia experiences frequent earthquakes and is home to 127 active volcanoes. One such volcano is Krakatoa in the Sunda Strait. The volcano was famous for its historic eruption in 1883, one of the most violent eruptions in recorded history. That eruption, through tsunamis and ash fall, killed more than 36,000 people, many of whom lived in the same regions where the 2018 tsunami struck. The 1883 eruption was violent enough to destroy the majority of the island, causing it to collapse into its caldera.
Approximately 45 years after the historic eruption of Krakatoa in 1883, Anak Krakatau ("Child of Krakatau") rose out of the sea where the old Krakatau volcano had collapsed. Since then, Anak Krakatau had grown to a height of around 338 m (1,109 ft) above sea level. Its frequent volcanic activity poses risks to the nearby coastal regions and seafarers. As such, in 1980, the Indonesian government set up a permanent volcano observatory in the island to observe its activity and to prevent another disastrous event in the region.
An investigation in 1928 during the volcano's re-emergence found that the western slope had grown considerably steeper than its eastern slope, which was caused by the strong current in the region and the steep wall of its basin. Researchers reported in 1995 that the subsequent eruptions of Anak Krakatau did not result in an infilling of the caldera and thus concluded that the possibility of a sector collapse could not be excluded. In 2012, a study conducted by international researchers suggested the possibility of a major sector collapse on Anak Krakatau at some point in the future, which could produce dangerous tsunami waves. The hypothetical tsunami from Anak Krakatau was calculated to reach an initial height of 43 meters before finally striking the western coast of Java and southeastern coast of Sumatra at a height of approximately 2–3 meters, considerably lower than that in 1883. However, owing to the population density in the region, insufficient mitigation measures could cause significant numbers of deaths and major damage to the area.
In the months leading up to the 2018 tsunami, Anak Krakatau had seen increased activity. Starting in June 2018, Anak Krakatau saw an increase of volcanic activity with a Strombolian eruption, producing lava flows and ash plumes as high as 5 km. One day prior to the collapse, on 21 December, an eruption was observed which lasted more than two minutes and produced an ash cloud 400 metres (1,300 ft) high.
The Volcanological Survey of Indonesia (PVMBG) recorded a dramatic increase in activity of Anak Krakatau. On 22 December, from 12:00 to 18:00 local time, a total of 423 eruptions were recorded by the agency. The danger level of the volcano was increased to Level 2. Authorities warned people not to conduct any activities within two kilometres (1.2 mi) of the volcano.
On 22 December 2018, at 21:03 local time (14:03 UTC), Anak Krakatau erupted and damaged local seismographic equipment, though a nearby seismographic station detected continuous tremors. A large part of the volcano's southwestern side broke off and fell into the sea, causing a tsunami. Officials stated that approximately 64 hectares (160 acres) of the volcano had collapsed into the ocean. The collapse caused the height of the volcano to be reduced from 338 to 110 metres (1,109 to 361 ft).
The Meteorology, Climatology, and Geophysical Agency (BMKG) detected a tsunami around 21:27 local time (14:27 UTC) at the western coast of Banten, even though the agency had not detected any preceding tectonic events. The Indonesian National Agency for Disaster Countermeasure (BNPB) spokesman Sutopo Purwo Nugroho said, "Dozens of buildings were destroyed by the wave, which hit beaches in Lampung and Banten about 21:30 local time (14:30 UTC) on Saturday." According to The Jakarta Post, authorities said that the tsunami "may have been triggered by an abnormal tidal surge due to a full Moon and an underwater landslide following the eruption of Anak Krakatau." The BMKG initially refused to label the event as a tsunami, claiming that it was a "normal" high tide and that no tsunami had occurred on the coast of Banten.
Previously, BMKG had issued a high wave warning for the waters of the strait. Tide gauges measured the tsunami at around 90 centimetres (35 in) in Serang and 30 centimetres (12 in) in Lampung, on top of the two-metre (6 ft 7 in) high tide. While Indonesia possesses a tsunami warning system for tsunamis caused by earthquakes, there is none for volcanic tsunamis. Hence there were no early warnings. On 23 December, satellite data and helicopter footage confirmed that the southwest sector of the volcano had collapsed, which triggered the tsunami, and that the main volcanic conduit was erupting from underwater, producing Surtseyan-style activity.
The waves struck about 313 kilometres (194 mi) of coastline with various heights. In Carita, the waves reportedly struck with a height of at least two metres (6 ft 7 in), while in Tanjung Lesung a height of more than five metres (16 ft) was reported. Further surveys conducted on 31 December by the Ministry of Maritime Affairs and Fisheries, with assistance from Tohoku University and Chuo University, revealed that the tsunami struck the coastline of Banten and Lampung at a height of 13.4 metres (44 ft).
Øystein Lund Andersen, a Norwegian volcano photographer who photographed the eruption of Anak-Krakatau prior to the tsunami, described the eruption as "quite heavy". Andersen was alone at the shore when the tsunami waves arrived, but managed to escape: "The wave came out of nowhere," he said, "and within seconds I had to run," he stated in an interview with Forbes. Following the event he published an eyewitness account.
The Indonesian National Board for Disaster Management (BNPB) initially reported 20 deaths and 165 injuries. By the following day, the figure had been revised to 43 deaths — 33 in Pandeglang, seven in South Lampung, and three in Serang Regencies, with 584 injured, and two missing; most of the injuries recorded (491) also occurred in Pandeglang. The death toll was further updated to 62 with 20 missing persons later that day. Missing persons were also reported from small islands that are part of Pandeglang Regency. By 13:00 local time on 23 December, BNPB had confirmed 168 fatalities and 745 injuries with 30 having been reported as missing, and the figure was further increased to 281 dead and 1,016 injured. As of 31 December, the death toll was 437, with 14,059 reported injured and 24 missing. Indonesian officials later revised the death toll to 426, with 24 reported missing and at least 7,202 injured in total.
The areas of Pandeglang Regency struck by the wave included beaches that are popular tourist destinations, such as Tanjung Lesung. The tsunami struck during peak season, where hundreds had arrived to celebrate the Christmas and New Year holiday. In Pandeglang, the popular pop group Seventeen was performing a concert when the tsunami struck without warning. A pasar malam (funfair) was also being held in Sumur District, Pandeglang. Eyewitnesses from the funfair recalled the tsunami struck with a height nearly as high as the light pole, with dozens, including children, reportedly dragged by the rushing wave.