The University of Tokyo

Curiosity and Innovation

Not Just GPS:
Basic Physics Research Opening Doors to New Worlds 3

[Earth Sciences] Muons and Volcano Imaging

Particle physics research is, as a matter of fact, also contributing to the field of earth sciences. It was the Center for High Energy Geophysics Research (CHEER) of the Earthquake Research Institute at the University of Tokyo that opened up the path to further development of earth sciences, using muon particles that are found in cosmic rays to create a new technology known as muography that is able to visualize the internal structure of volcanoes. Until this discovery seismic waves had been used as a means of surveying the internal structure of volcanoes, but this was merely like searching the human body using only a stethoscope. The development of muography has dramatically enhanced the precision of surveys of volcanic structures.

“Muons pass through our atmosphere constantly. To put it in quantitative terms, one muon passes through the palm of the hand every second, or one muon per minute through a fingertip. In the space of a single night, a million muons pass through the human body. As they have strong penetration power, they can pass through several hundred meters or several kilometers of bedrock,” says the Head of CHEER, Professor Shuhei Okubo, who adds with a laugh that due to his title, he is “UTokyo’s only male ‘CHEER’ leader.”

Muons travel in straight lines. When they pass through an object, given the same distance travelled through the object, they will be well-absorbed in the higher density portions of the object, but this absorption decreases for less dense parts of the object. If the number of muons and the direction in which they are traveling is measured, it is possible to understand the density of the place through which they pass. If the event rate in different directions is measured precisely, it is then possible to create an image “looking through” the internal structure of a volcano.

It had been hypothesized many years ago that it should be possible to see into the internal structure of a volcano using muons. A successful experiment, however, proved to be elusive. It was Professor Tanaka of CHEER who astounded the world in 2006 by becoming the first person to successfully image the inside of a volcano. Professor Tanaka positioned a detection device at the foot of Mt. Asama and took measurements over a period of two months. The result was a visualization of the internal structure of the volcano and an image of the magma pathway in the volcano.

“Before coming to the Earthquake Research Institute I was engaged in positron research,” Professor Tanaka says. “It was probably a good thing that I came to the institute as it enabled me to transform my research ideas.”

After imaging Mt. Asama, Professor Tanaka has gone on to uncover the internal secrets of many volcanoes using muography. In Japan he has surveyed the Meiji-Shinzan and Showa-Shinzan lava domes of Mt. Usu as well as the Satsuma-Iwojima Volcano. Also, in other countries, the technique has imaged Mt. Etna in Italy, Puy de Dome in France, and La Soufriere in Guadeloupe. His next target is Mt. Kirishima in Kagoshima.

“As Mt. Kirishima is in a national park it will not be possible to set up the detection device close to the mountain. The theme of my research this time will be to ascertain at what distance it is possible to see through the internal structure of the volcano. As there are normally restrictions on entering the mountain area of an active volcano, this will also be a very effective experiment from the perspective of disaster prevention.”

Professor Tanaka says that in the future he would like to go beyond the boundaries of the Earth and image the internal structure of Mars. His willpower may very well be as strong as the penetration power of muons!

Hiroyuki Tanaka

Hiroyuki Tanaka

Professor, Center for High Energy Geophysics Research (CHEER), Earthquake Research Institute