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A change in sea color Tuesday suggests submarine volcanic activity has begun about 1 km off the west coast of Miyake Island, and 2,000 residents have already evacuated to shelters on its north side.

“Volcanic activity on Mount Oyama appears to have subsided, as we believe magma activity has moved underground from the island’s volcano to the ocean floor west of the island,” said an official of the Coordinating Committee for the Prediction of Volcanic Eruptions, which advises the Meteorological Agency.

However, a magma eruption in shallow waters may lead to a large-scale explosion involving bursts of steam, the official warned, adding that a 40-meter-wide area of ocean west of the island has turned yellowish brown.

Meteorological Agency officials said the discoloration could be caused by phreatic outbursts that occur when lava comes in contact with seawater or groundwater.

The official said a possibility of an eruption at the west foot of the volcano, which sits at the center of the 55.5-sq.-km island in the Izu Island chain about 180 km south of Tokyo, has lessened.

A Maritime Self-Defense Agency plane on Tuesday afternoon spotted a landslide on one side of the 813-meter volcano facing the area of the sea that changed color. The landslide was about 20 meters wide and 50 meters long.

About 2,000 residents from three districts on the island, which has a population of 3,850, have been evacuated to local shelters, and ships are standing by for a mass evacuation if necessary.

Yoshiaki Ida, a University of Tokyo professor and chairman of the Coordinating Committee for the Prediction of Volcanic Eruptions, warned that such blasts close to shore could pose serious danger.

“If an eruption should occur in shallow water, there is the possibility of a strong magma-vapor explosion,” Ida said. “That is the greatest threat at this point.”

The chances of such an explosion on the island, which is under the jurisdiction of the Tokyo Metropolitan Government, are higher now that magma levels seem to be rising to the ocean’s floor.

Prime Minister Yoshiro Mori said Tuesday that the government is taking every precaution.

“So far, nothing serious has happened. The Self-Defense Forces and the Japan Coast Guard are taking the best steps possible by dispatching their ships,” Mori informed reporters at his official residence.

“The island’s villages, the metropolitan government and the central government are in close contact with each other. We have to keep a close eye on the volcanic activity today,” he added.

The Defense Agency, which has set up a disaster headquarters to monitor the SDF dispatches, received a call early Tuesday from Tokyo Gov. Shintaro Ishihara, requesting the dispatch of SDF disaster relief units, Defense Agency chief Tsutomu Kawara said.

The MSDF already has 11 ships on standby off the island, capable of transporting 10,000 people, according to agency officials. It also sent a ship carrying 88 people and 18 vehicles of the Tokyo Metropolitan Fire Agency from Yokosuka port in Kanagawa Prefecture that was to arrive at Miyake at around 5 p.m., they said.

Meanwhile, strong and frequent tremors up to an intensity of 4 continue to shake Miyake, which last erupted in 1983, ejecting volcanic rocks and ash into the sky. No injuries or fatalities occurred at the time.

The evacuation was triggered at about 7:30 p.m. Monday, when the Meteorological Agency warned that an eruption was imminent, perhaps within two hours.

Roughly 2,619 of the island’s 3,850 residents have been affected by the evacuation orders, forcing many of them to spend a fitful night at shelters set up at schools and welfare

centers at points along the northern coast of the island, where computer simulations predict magma flow to be least likely.

The 250 residents of the Igaya district, in the western corner of the island, were evacuated via bus Tuesday morning. About 1,250 residents of the Ako district, in the southwest of the island, and about 1,100 residents from the Tsubota district, in the southeast, had been evacuated by 11 p.m. Monday.

About 1,760 residents and 36 visitors to Miyake reported to the evacuation centers.

Central and municipal government bodies leaped into action soon after the first warnings were issued Monday night.

The Health and Welfare Ministry began to approve applications for national disaster relief so state funds could be used to secure food, water and medical supplies for the evacuees.

The government also set up a coordination office at the crisis management center in the annex of the Prime Minister’s Official Residence.

The Tokyo Metropolitan Government set up its own disaster prevention center and began dispatching medical staff and relief supplies, the earliest of which arrived early Tuesday morning via SDF helicopters.

SDF helicopters also transported 1,000 1-liter bottles of drinking water from Tachikawa on Tuesday afternoon.

Two wagons that can carry 152 tanks of drinking water were also to arrive today, along with ambulances and firetrucks. The supplies will be distributed at five evacuation centers.

The Metropolitan Police Department set up a hazard patrol office at 8:40 p.m., while the Japan Coast Guard and MSDF sent patrol boats and vessels, including destroyers, to the area.

Metropolitan officials said that in the event the entire island needs to be evacuated, sports and school facilities in six Tokyo wards may be needed as evacuation centers.

The facilities the wards have offered are more than large enough to house the 3,850 residents, according to Kenji Suzuki, head of Tokyo’s Disaster Prevention Planning Section.

“In the worst case scenario, we shouldn’t have any problems finding facilities for evacuees,” Suzuki said.

A group of residents fleeing Miyake Island of their own accord boarded a ship scheduled to arrive in Tokyo on Tuesday night, officials said.

A total of 382 residents, including 48 children, left Miyake Island on the Sutoretia-maru bound for Takeshiba port. No shelters were prepared for the residents, officials said.

Mount Oyama, the highest point on the island, last erupted on Oct. 3, 1983, destroying 413 of the approximately 500 structures in the Ako district of Miyake as lava gushed from numerous craters.

Hidefumi Watanabe, a professor at the University of Tokyo, said Mount Oyama typically erupts soon after showing signs of seismic activity. In 1983, the eruptions commenced two hours after the first earthquake.

In the past, lava has flowed from vents in the middle of the mountain rather than from the top, which increases the danger that lava may flow into residential areas, Watanbe said.

Mount Oyama erupts every 20 to 30 years. Prior to the eruption in 1983, the northeastern part of Mount Oyama had erupted in 1982, burning five houses.

In 1940, an eruption triggered massive emissions of volcanic ash and rocks that killed 11 people, injured 20 and burned 24 houses. In 1874, 45 houses were swept away by lava.