Recently, the eruption of the Tonga volcano and the tsunami it caused have attracted global attention, and tsunami waves have also been monitored off the coast of Zhejiang, China.
How did the Tonga eruption and tsunami occur? What are the implications? Is Guangdong at risk of a tsunami? Yangcheng Evening News all-media reporter interviewed Hu Zhan, associate professor of the School of Marine Sciences of Sun Yat-sen University (referred to as "Hu") and postdoctoral Fellow Chen Yujie (referred to as "Chen"), to provide popular science answers for readers.
The Tongan tsunami was mainly caused
Caused by submarine landslides caused by volcanic eruptions
Yangcheng Evening News: What are the causes and effects of the tsunami?
Hu: Tsunamis are uncommon, but they are extremely destructive natural disasters. Tsunamis are mostly caused by earthquakes, volcanic eruptions, submarine landslides and meteorites falling into the sea, and spread to land at speeds of up to 700 to 800 kilometers per hour.
In the ocean, tsunami waves are less than one meter high, and when they spread to shallow coastal areas, the energy decay is very small, and the wave height increases sharply, up to tens of meters, and repeats it in minutes or tens of minutes, which is extremely destructive, can destroy embankments, flood land, and take away life and property.
Yangcheng Evening News: What is the cause of this Tonga volcanic eruption and tsunami?
Hu: Tonga's volcanoes are in the Pacific Ring of Volcanoes, where the Pacific plate subducts at the trenches on the east side of Tonga, and the subducting ocean crust dehydrates deeply, causing the melting of the Earth's contents (mantle wedges) and the formation of magma. The subduction continues, melt accumulates, and when the pressure reaches a critical point, a volcanic eruption occurs in the upper part of the subduction zone. Because magma contains a lot of gas, it is often in a violent state of explosion, which is very powerful.
Since the vast majority of historical tsunami events are generated by earthquakes in the sea, tsunami events directly from submarine volcanic eruptions are extremely rare, and the tsunami waves caused by the volcanic eruptions in Tonga have now spread to the whole world, which is the most serious volcanic tsunami event in human history since instruments have recorded it.
The global catastrophic impact of the tsunami is unclear. But it is clear that in areas closer to Tonga's volcanoes, large-scale coastal inundation and casualties in parts of the area are inevitable.
According to preliminary judgments, the submarine landslide caused by the volcanic eruption was the main factor causing the tsunami; in addition, the air pressure disturbance caused by the volcanic eruption was an important factor in aggravating the scale of the tsunami.
The eruption of the Tonga volcano triggered a massive transoceanic tsunami that affected the entire Pacific coast. Tsunami waves even reached the coast of China, with a maximum amplitude of about 20 centimeters. The Tsunami Warning Center of China's Ministry of Natural Resources also reminded that according to the statistical analysis of historical volcanic activity events, the current eruption activity may last for weeks or months, and it is not clear whether the peak of the eruption has been reached.
Coastal Guangdong
It is one of the regions most threatened by tsunamis
Yangcheng Evening News: What is the history of the mainland's record and research on tsunamis? Has Guangdong experienced a serious tsunami?
Chen: The tsunamis recorded along China's coast in history are sparse in time and space. Since tsunamis and storm surges are both inundation processes related to sea level, it is difficult to distinguish the causes of coastal water gain.
Studies have found that early databases on the northeastern south-east of the South China Sea have errors related to translation inaccuracies between different languages, calendars, and location names, but 23 are valid and proven events, the most devastating of which was a tsunami in 1867 that affected Keelung in northern Taiwan, killing at least 100 people.
There are three relatively reliable and relatively destructive events recorded along the coast of Chinese mainland: the 1604 Quanzhou earthquake in Fujian, the 1605 Qiongzhou earthquake, and the 1918 Nan'ao earthquake in Guangdong.
Satellite footage captured by Fengyun-4 B. Photo/ Fengyun Satellite
Yangcheng Evening News: When did the mainland begin to pay attention to the study of tsunamis, and what progress has been made so far?
Hu: In 2004, an Mw9.1 magnitude earthquake occurred in the Sumatra subduction zone of the Indian Ocean, causing more than 200,000 casualties in countries bordering the Indian Ocean, causing a huge disaster. In 2011, an Mw9.0 magnitude earthquake struck northeastern Japan, triggering a huge tsunami that devastated many parts of northeastern Japan, killing and missing nearly 30,000 people and triggering a nuclear leak at the Fukushima Daiichi Nuclear Power Plant.
The tsunami also caused a 30-60 cm amplitude shock along China's eastern coast. Previously, the tsunami disaster in Chinese mainland was not fully paid attention to for two main reasons:
First, most coastal areas have a wide continental shelf in front of them, and the shallower continental shelf is thought to dissipate most of the tsunami wave energy; second, adjacent subduction zones, such as the Southwest Open Sea Trough, the Ryukyu Trench and the Manila Trench, have relatively low seismic activity and fewer recorded well-defined tsunami events.
The mainland joined the Pacific Tsunami Warning and Mitigation System in 1983, but the real attention to tsunami warning was after the Indian Ocean tsunami in 2004, in 2006 the State Oceanic Administration opened a 24-hour tsunami warning business, the International Ocean intergovernmental organization officially agreed to rely on the China Tsunami Warning Center to build a South China Sea Regional Tsunami Warning Center, and in 2018 the South China Sea Regional Tsunami Warning Center was officially awarded.
After the tsunami in Northeast Japan, the mainland began to urgently consider the impact of similar events in the Pacific Rim on China's coastal margins, including the National Marine Environment Forecasting Center, the Ministry of Natural Resources Key Laboratory of Marine Disaster Forecasting and other academic circles have carried out active research. At present, the mainland has begun a series of surveys and research projects aimed at mapping tsunami inundation at the community level based on worst-case scenarios. These efforts are being carried out in an orderly manner.
Yangcheng Evening News: What important findings has Sun Yat-sen University made in tsunami research in recent years?
Hu: In recent years, Professor Li Linlin of the team of Academician Zhang Peizhen of the School of Earth Sciences and Engineering of Sun Yat-sen University has conducted a series of studies on various types of potential tsunami sources in the south China Sea of the mainland, pointing out that the South China Sea is the sea with the highest risk of tsunami disasters on the mainland, and there are many types of potential tsunami sources such as the Manila subduction zone, the activity fracture in the northern part of the South China Sea, and the landslide on the seabed.
The Manila subduction zone earthquake is the most likely tsunami source in the South China Sea to trigger a regional tsunami, affected by the direction of tsunami wave propagation and the topography of the seabed, the south China coast, Taiwan and internal islands and reefs on the mainland including Guangdong, Fujian, Hainan, Guangxi and other places will be affected by different degrees of catastrophic impact, of which the Guangdong coast is one of the most serious areas threatened by tsunamis.
At the same time, the closed basin structure of the South China Sea causes that once the tsunami occurs, the tsunami wave will oscillate and remain in the continental shelf area and the ocean basin for a long time, bringing a long-term tsunami disaster impact to the south Coast of China. However, at present, the understanding of the characteristics of the pregnant earthquake in the Manila subduction zone, seismic activity, coupling degree and seismic mechanism of shallow offshore trench faults in the subduction zone is limited, and there is still a lot of uncertainty in the existing tsunami disaster assessment results, and a lot of research is still needed in the later stage.
In addition, with the continuous deepening of seabed exploration, a large number of submarine landslides widely distributed on the continental slopes of the South China Sea have been gradually identified. According to the study, it is also entirely possible that such landslides will produce a tsunami as destructive as the Manila Trench earthquake, with a serious impact on southern Chinese cities, but it is less likely to occur than the Manila Trench earthquake.
At present, CUHK has obtained geological geochemical data of geological representations in the South China Sea and its periphery, and proposed regional effects and kinetic models of major geological events. The current crustal deformation and strong seismic activity of the Global Navigation Satellite System (GNSS) in the South China Sea area were studied, revealing the speed of movement of the boundary of the South China Sea plate and the interior of the South China Sea.
The coastal fault zone and the northern land slope fault in the South China Sea were found as the main seismic structures along the south China coast and the Guangdong-Hong Kong-Macao Greater Bay Area, and the seismic mechanism and danger of future earthquakes were studied and evaluated. The potential tsunami hazards along the southeast coast and the Guangdong-Hong Kong-Macao Greater Bay Area were systematically assessed, using strong earthquakes in the Manila Trench and coastal fault zones and potential submarine landslides as potential sources.
Text | Yangcheng Evening News all-media reporter Chen Liang
Source: Yangcheng Evening News