The forms and spread of gravel predict the behavior of tsunamis

Mohammad ALHADDAD

Researchers from Tokyo Metropolitan University and Ritsomikan University have linked the circular pattern of the tsunami to the depths of Japanese tsunamis and the predictability of future wave behavior.

In the study, published in mid-July in the journal Scientific Reports, the researchers gave the first estimate of the tsunami-drenched flooding distances over the past 4,000 years on the Sanrico coast.

They also collected gravel samples from various tsunamis in Koyadori, Japan, and found a sudden change in the composition of about 40% of the sediment in the immersion distance between the coastline and the end point of the tsunami, regardless of its size.

Areas of division
The tsunami is one of the most devastating natural disasters, so understanding its magnitude and intensity is of scientific and socio-economic importance. However, despite the researchers' efforts to study and understand them, their frequent occurrence may make quantitative studies difficult, especially with the frequency of seismic tremors causing the tsunami around the division areas.

Areas of geological division occur when a tectonic plate falls below another plate, an event that occurs once in 100 to 1,000 years.

As geological divisions occur, researchers have decided to use another easier study solution to facilitate understanding of tsunamis by studying geological deposits, which may help mitigate the effects of natural disasters and future tsunamis.

Gravel turn
In the current study, the researchers conducted gravel samples from cavity holes and trenches in the city of Quiadori, in the center of the Sanrico coast.

Geological sampling corresponding to three tsunamis occurred in 1611, 1896 and 2011. The researchers were able to find out the "geometrical" measurements and dimensions of these samples, as well as determine the immersion distance, or how far the tsunami waves reach the interior.

The use of computer image analysis enabled researchers to approximate each grain of sample gravel, giving data 10 to 100 times more accurate than manual methods. By comparing the distributions with modern measurements of seashore for rivers and seas, the researchers found that they could differentiate between sea gravel and river gravel, and estimate the quantity of each.

The study revealed that the proportion of gravel suddenly changes a certain distance from the coast of the sea. This point is called the "gravel tsunami juncture," which the study suggests is likely to result from gravel waves transmitted from inland areas with seawater returning from its journey inundating those areas. Although this phenomenon occurs in different locations, the researchers found that the maximum area occupied by this phenomenon did not exceed about 40% of the area of ​​immersion distance.

The researchers believe that the same analysis can be applied to other ancient tsunami events to estimate the extent to which they have reached inland areas, which in the future will facilitate the prediction of tsunami behavior and mitigation of these disasters and effective planning for the management and protection of coastal communities.