Heavy rains could trigger mudslides of volcanic debris on the slopes of Mount Shinmoedake. Credit: The Yomiuri Shimbun via AP Images

Scientists in Japan are stepping up monitoring efforts of Mount Shinmoedake in southwest Japan, which has erupted intermittently since 26 January. The eruptions have thrown ash and chunks of molten rock known as volcanic bombs as far as 7 kilometres from the volcano's mouth, covering nearby towns in ash, disrupting airline schedules and forcing some residents to evacuate.

With heavy rains predicted and expectations that volcanic activity will continue for two more weeks, officials are increasingly worried about mudslides.

The 1,421-metre-high Shinmoedake, featured in the James Bond film 'You only live twice', is one of more than 20 peaks that compose the Kirishima volcano range. Over the past few years, scientists have noted small 'phreatic' explosions, in which steam and mud are expelled, and because of this, new observation sites have been established.

But despite the seismometers, tiltmeters, geomagnetic instruments, thermal imaging devices and volcanic gases monitors, the recent eruptions came as a surprise. They are the first such violent explosions from Kirishima since 1959 and the first time the volcano had spewed lava in nearly 300 years. "No one expected such a dominant magmatic eruption," says Yoshiaki Ida, a volcanologist at the University of Tokyo.

Scientists were also surprised that Shinmoedake could exhibit such explosive power. Buildings more than 10 kilometres away were damaged by shock waves. "This is quite extraordinary," says Ida.

Giant reservoirs

Toshitugu Fujii, a University of Tokyo emeritus professor and chairman of the Coordinating Committee for Prediction of Volcanic Eruptions, has been less impressed by Shinmoedake. "It is one of the andesite volcanoes, which are common in subduction zone. Several hundred years of magmatic dormancy is also common," says Fujii. He adds that shock waves have been detected at much greater distances in other cases. Shock waves generated during the eruption of Mount Pinatubo in the Philippines in 1991, for example, were detected near Nagoya in Japan.

Once the volcano wakes up, it starts to tell us about itself. Motoo Ukawa , NIED

The eruption has given scientists a chance to learn about Shinmoedake. The land ministry's Geospatial Information Authority of Japan has been analysing Global Positioning System data on land deformation. It has discovered that since last May, some 6 million cubic metres of magma had accumulated underground in a 6-kilometre-deep reservoir to the north west, and another million cubic metres had accumulated in a shallower reservoir 3 kilometres directly below Shinmoetake.

The National Research Institute for Earth Science and Disaster Prevention (NIED) analysed data from the area during the 5 days after the eruption and independently found a reservoir of roughly 5.5 million cubic metres, leading it to identify this as Shinmoedake's major magmatic source. "Quiet volcanoes do not tell us anything about the magmatic system, but once the volcano wakes up, it starts to tell us about itself," says Motoo Ukawa, a volcanologist at the NIED.

Mudslide threat

Further eruptions could be very dangerous. "We want to know the timing of the next supply of magma if [an eruption] happens," says Fujii. But predicting the kind of eruption, and thus the potential danger in the form of volcanic bombs, pyroclastic flows (fast moving expulsions of gas and rock down the sides of volcanoes), and plinian eruptions (in which large amounts of pumice, gas and ash are blasted into the air), is not easy.

"By increasing the monitoring tools, we may detect the timing of eruptions, but it is quite difficult to predict the mode of eruption before the actual eruption," Fujii says. "Still we believe we can detect some change in mode of eruption through the continuous monitoring, and may be able to detect the precursors of a huge explosive eruption."

Last week, at an emergency meeting at the Japan Meteorological Agency, Fujii's committee reported that the initially rapid contraction of the reservoirs had slowed since 31 January. Based on those estimates, the committee concluded that volcanic activity would continues for another two weeks, but did not anticipate eruptions on the scale of those seen on 26 January.

That still does not obviate the risk of a big eruption. Heavy rains that started on 7 February and are expected to continue until later in the week increase the risk of lahars, muddy flows of volcanic debris that can move quickly and envelop homes and then, having lost momentum, become solid like concrete. Fujii says that wire sensors to detect lahar generation are being set up. As the land ministry races to clear rock and debris from mudslide dams, on 8 February, it issued a warning that 35 nearby towns are at risk of lahars.

Fujii says that rainfall of more than 30 millimetres an hour on slopes covered by several centimetres of fine-grained ash will produce lahar. "Compared to the prediction of volcanic eruption, prediction of lahar generation is rather easy," he says.