The Sunda Strait tsunami on Dec. 22, 2018 shocked many. The tsunami, which was initially thought to be just a high wave, struck without warning. No earthquake preceded it, nor were there any significant changes in Mt. Anak Krakatau’s volcanic activities. The seawater hit shore suddenly. Hundreds of people were killed or injured as the tsunami hit tourist beaches in the provinces of Banten and Lampung.
The Meteorology, Climatology and Geophysics Agency (BMKG) alleged that the tsunami was linked to Mt. Anak Krakatau’s activities and the full moon. Even though the mountain has shown increased volcanic activity, most of the material has fallen down its slopes, with only a little falling into the surrounding seas.
Based on how quickly the huge wave formed in the Sunda Strait, it appears that the tsunami was not triggered by Anak Krakatau’s volcanic activities. The tsunami was probably not triggered by an undersea landslide, either. It could be a new tsunami phenomenon that occurred for the first time in the country’s history: a meteotsunami.
Based on information from the BMKG, the Serang tide gauge at Jambu Beach in Bulakan village, Cinangka township, located around 46 kilometers from Anak Krakatau, recorded a 0.9-meter increase in wave height at 9:27 p.m. Western Indonesian Time (WIB). The Banten tide gauge in Ciwandan Port, around 60 km from the volcano, recorded a 0.35-meter increase in sea level at 9:33 p.m. WIB. The Kota Agung tide gauge in Kota Agung, Lampung, around 113 km from Anak Krakatau, recorded a 0.36-meter increase in sea level at 9:35 p.m. WIB. The Panjang Harbor tide gauge in Bandar Lampung, around 70 km from Anak Krakatau, recorded a 0.28-meter increase in sea level at 9:53 p.m. WIB.
Tsunamis form when disruptions occur in the water column. These disruptions are commonly caused by changes in the structure of the ocean floor. Earthquakes, undersea landslides and undersea volcanic eruptions are some examples of phenomena that cause changes in the structure of the ocean floor. Such disruptions affect the water columns above it and create seeds of tsunami on the sea’s surface. These surface waves then spread in all directions, also towards land, thereby forming tsunamis that move inland.
Tsunamis are also caused by disruptions to water columns due to occurrences on or above the sea surface, like objects from space (meteorites or asteroids) that fall into the sea – as occurred when massive volcanic materials fell into the sea during the 1815 Mt. Tambora eruption – and by a phenomenon called meteotsunami.
Disruptions in air pressure
Meteotsunamis are tsunamis triggered by disruptions in the barometric or air pressure over the ocean. Changes in atmospheric pressure trigger anomalies in the air current. When an air current meets an ocean wave of equal speed, they resonate and combine to move in unison, and it can appear as though the sea wave is being pushed toward land.
Several meteotsunami are known to have occurred in the Adriatic Sea, the Mediterranean Sea and along the eastern coast of the United States. A similar phenomenon also occurred in Nagasaki Bay in Japan in 1979, when a 5-meter wave formed, and in New Jersey in 2013. A meteotsunami is generally lower in energy and height than tsunamis triggered by earthquakes. Nevertheless, in many cases, meteotsunamis have caused severe damage to harbor facilities. The highest meteotsunami on record occurred in 1978 in Vela Luka, Croatia, where a 6-meter wave formed.
Meteotsunamis are closely linked to weather disruptions. Unfortunately, the weather in Indonesia has become highly unpredictable due to climate change. Tornadoes, which used to be a rare occurrence in Indonesia, now happen frequently in the country, in both the dry and rainy seasons.
Looking at the possible threat of meteotsunamis in Indonesia, full alert and response is needed with respect to the increased weather disruptions. The phenomenon of meteotsunami would complement the kinds of tsunamis that have hit the country, including tsunamis formed by undersea quakes, volcanic eruptions, undersea landslides, shoreline liquefaction (as happened in Palu) and now, the meteotsunami on Sunda Strait.
The large variety of tsunamis that have occurred in Indonesia should serve as a warning for the (central and local) government as well as the general public on the importance and urgency of coastal spatial planning that incorporate the threats from these multitude of tsunami. Indonesia’s tsunami early warning system should be evaluated immediately with regard to the various risks of these tsunamis. (Eko Yulianto, Head of Indonesian Institute of Sciences Geotechnology Research Center; Paleotsunami and Disaster Researcher)