In January, a large-scale eruption occurred at a submarine volcano in Tonga, South Pacific, and tide level changes were observed along the coasts of various parts of Japan.
In response to this, the Japan Meteorological Agency compiled a report on the mechanism at a meeting of experts and concluded that the change in tide level was due to the pressure wave caused by the impact of the eruption.
On the other hand, as for the reason why it became so high that it caused damage, he said, "There are multiple factors that can be considered, and further investigation and research are needed."
At the submarine volcano "Hunga Tonga Hunga Ha'apai" in Tonga, a large-scale eruption occurred in January, and tide level changes of several centimeters to a meter or so were observed along the coasts of various parts of Japan. There was damage.
Initially, the Japan Meteorological Agency announced that "there may be some changes in the tide level, but there is no concern about damage", but at midnight, tsunami warnings and warnings were announced over a wide area from Hokkaido to Okinawa, causing confusion. Because of that, we discussed the mechanism at a meeting of experts.
The change in tide level is "affected by atmospheric pressure waves"
In this report, we conclude that the reason why the tide level change was observed about 4 hours earlier than the tsunami caused by the earthquake was that the atmospheric pressure wave generated by the impact of the eruption was transmitted quickly and the atmospheric pressure changed suddenly. rice field.
It is a kind of atmospheric pressure wave called "Lamb wave", which is close to the speed of sound at about 300 meters per second, and the tide level changed in the process of this Lamb wave.
When we analyzed the changes in water vapor in the atmosphere with the meteorological satellite "Himawari", we were able to confirm how the atmospheric pressure waves were transmitted.
Reason for the increase “Complex but the main factor could not be identified”
The change in tide level caused by such a phenomenon is also called "meteotsunami", but this time, the height of the tide level exceeded 1 meter at the maximum, which led to the damage.
According to the report on the reason, it is a complex phenomenon such as a phenomenon called "Proudman resonance" in which the influence of atmospheric pressure waves becomes large in deep water, and it is locally amplified in the terrain such as bays. Although it is possible, at this stage it is not possible to identify what affected it and how much it affected, and further investigation and research is needed.
Professor Kenji Satake of the Earthquake Research Institute, the University of Tokyo, who compiled the report, said, "A volcanic tsunami that occurs overseas like this time may also have an impact. Even though I do not feel the shaking of the earthquake, a tsunami warning Please keep in mind that there is a possibility that such things will come out. "
Japan Meteorological Agency to announce the time and area where the tide level change is expected to start
In response to the report, the Japan Meteorological Agency has decided to inform the time and region where changes are expected to begin, although it is not possible to specifically predict changes in tide levels when a large-scale eruption occurs overseas.
When tide level changes are observed and reach the standard, tsunami warnings and tsunami warnings will be announced.