Quake-volcano links probed

From Daily Yomiuri Online: Quake-volcano links probed

It is not unusual for dormant volcanoes to erupt several months or years after a great earthquake. But is there a causal relationship between massive earthquakes and volcanic eruptions? Will the Great East Japan Earthquake affect volcanoes in this country? Researchers have been trying to answer these questions.

Two days after a magnitude-9.5 earthquake struck Chile in 1960, the Puyehue volcano in southern Chile erupted. The volcano erupted again in June this year, following a magnitude-8.8 temblor in February last year.

It is difficult to establish a statistical correlation between massive earthquakes and volcanic eruptions given the low frequency of such events. But research conducted by Masaaki Churei, a former chief of volcanology of the Meteorological Agency, shows a historical correlation between the two phenomena.

"Volcanic eruptions in the Tohoku region spiked before and after great earthquakes off the Sanriku coast in the region," Churei said.

According to a research paper by Churei published in 2002, 13 volcanic eruptions, including those of Mt. Chokai and Mt. Azuma, occurred in six prefectures of the Tohoku region over a period of 156 years from 1841 to 1996.

During that period, four magnitude-8-class earthquakes--including the Meiji Sanriku Earthquake of 1896 and the Ansei Hachinohe Earthquake of 1856--occurred in the Japan Trench.

Of the 13 eruptions, 12 occurred within eight years before or after the massive earthquakes, Churei's research found. It also showed that volcanoes became active three or four years after the major earthquakes.

Churei found similar phenomena in the Hyuganada sea off eastern Miyazaki Prefecture.

Toshitsugu Fujii, professor emeritus at the University of Tokyo and chairman of the Coordinating Committee for the Prediction of Volcanic Eruptions, said, "If accurate statistical data could be compiled based on exact eruption records, correlations between volcanic eruptions and earthquakes could be seen in various parts of the world."

A link between volcanoes and massive earthquakes can be found under the sea.

In the sea off the eastern coast of the Tohoku region, for example, the Pacific tectonic plate moves westward and subducts beneath the North American continental plate to create the Japan Trench, which extends from north to south. The westward-moving plate contains much moisture and transfers some of it to the continental plate during subduction.

In the presence of moisture, it is believed that rocks at a certain depth tend to turn into magma when subject to high temperature and high pressure. Indeed, just above where this magma is generated is a "volcanic front" along a north-south axis in the Tohoku region in parallel with the Japan Trench.

A major earthquake occurs when the edge of an oceanic plate suddenly slides under a continental plate. The positional and dynamic relations of the two plates change, possibly affecting magma formation. Thus, earthquakes and volcanoes essentially are closely linked.

But it is estimated to take from several thousand years to tens of thousands of years for magma created deep in a plate to rise to the Earth's surface. Therefore, a different mechanism seems to be at work when a massive earthquake affects volcanic activities shortly after its occurrence.

In the aftermath of the March 11 earthquake, underground volcanic activity spiked at 20 volcanoes throughout the country, including below Mt. Yakedake straddling Nagano and Gifu prefectures, Mt. Hakone on the border of Kanagawa and Shizuoka prefectures and Mt. Aso in Kumamoto Prefecture. Earthquakes that can be sensed by humans also occurred.

Commenting on the cause of this phenomenon, the Meteorological Agency said: "Magma chambers below the volcanoes were shaken by seismic waves, causing gases in magma to create magma bubbles. This resulted in earthquake swarms."

Many volcanologists are focusing on the hypothesis that crustal movements triggered by earthquakes squeeze out magma to cause volcanic eruptions. One such scientist is Eisuke Fujita, a senior researcher of the National Research Institute for Earth Science and Disaster Prevention. He has researched how the gravitational pressure from rocks around the magma chamber of Mt. Fuji changed in the aftermath of the March 11 earthquake compared with that before the disaster.

Mt. Fuji erupted 49 days after the Hoei Earthquake of 1707. A simulation conducted by Fujita showed that if a spherically shaped magma chamber with a radius of three kilometers existed 18 kilometers underground, movements in faults near the chamber would change its shape, squeezing out magma.

But Fujita assumes that the deformation would not be enough to create significant movement of magma.

He plans to conduct similar research of volcanoes in Tohoku, including Mt. Iwate in Iwate Prefecture.