Alarming Faultlines

In view of the disaster that struck Japan and alerted another 90 nations as far apart as 11,000 kilometres indicates the power of the tsunami, which, to the world, has come as a new phenomenon only after 2004. It was never really spoken of in historical terms but seems since seismic activity has been on the increase, especially over oceans, the resultant devastation from the silent roil of water speeding at 800 kilometres an hour is a very intimidating prospect in terms of natural disaster.

In the past seven years it is clear that our ability to predict, limit the damage, save lives and ensure that sensitive installations are not constructed on unsafe coastal areas has not achieved technical breakthrough and the warnings are either too late or after the event.

One would have imagined that technology would have been able to get us some control on the planet of what is going on under the sea but that has not happened and never is Man so helpless as when he is faced by the power 
of the surge.

It is a sobering thought that in these circumstances most of the world is on a faultline. Whether it is the San Andreas or the New Madrid line or the South American and Northern American plates or the unstable Eurasian and Indian plates — not to mention the Australian plate — the majority of the world is in some kind of danger of the post earthquake terror from the sea. What makes it that much more hazardous is that these faults are growing and moving and forming ridges and positioning themselves in confrontational sequences. Coastal areas are not only threatened in the six continents but worse, the few hundred island nations are in greater danger and not only are they facing erosion but are now can be engulfed if they happen to be in the way of a giant wall of water.

It seems necessary for the world to come together and work out a solution to the problem because it is pretty obvious it is not going away from the looks of it unless this is a major initiative and all the technology put into the meteorological research there will be a major price to pay.

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Scientific Details Of Japan’s Quake

The fifth largest earthquake ever recorded hit Japan today (March 11), sending huge tsunami waves crashing onshore and reportedly killing at least 18,000 people and many still missing.

The 8.9-magnitude earthquake struck at 2:46 p.m. local time (12:46 a.m. EST), near Honshu, Japan, an island that is home to about 100 million people. The temblor was the fifth in the past two days to hit the region, and major aftershocks can be expected for months, possibly even a year. Despite the massive foreshocks, there was no way to predict that Japan’s biggest-recorded earthquake was looming, said Paul Caruso, a geophysicist with the U.S. Geological Survey (USGS) in Golden, Colo.

“We have big quakes there all the time,” Caruso told OurAmazingPlanet. For all scientists knew at the time, the 6.3-magnitude quake that struck yesterday was the main shock, Caruso said. “Not every big earthquake has a foreshock but they all have aftershocks.”

The rule of thumb for seismologists is that an earthquake’s largest aftershock will be one magnitude smaller than the main shock, Caruso said. That means a 7.9-magnitude earthquake could hit the region even a year from now. Yet aftershocks are already hitting northern Japan now — 35 larger than magnitude 5.0 and 14 larger than magnitude 6 — according to the UGSS.

Big aftershocks are not unusual. In February, a 6.6-magnitude aftershock ruptured near Maule, Chile — almost a year after what is now the sixth largest earthquake in recorded history, a magnitude 8.8, hit in the same region.

The Japanese earthquake ruptured near the boundary between the Pacific and North American tectonic plates — huge, moving slabs of the Earth’s crust. The quake was a

megathrust earthquake, where the Pacific plate dove underneath Japan at the Japan Trench. The seafloor was pushed away from Japan sending waves roaring toward Hawaii and the West Coast of the United States.

“The tsunami wave speed in deep water, open ocean, is about the same as a commercial jet’s ground speed,” said Ken Hudnut, a USGS geologist in Pasadena, Calif.

The epicentre of today’s quake was about 15.2 miles (24.4 kilometers) deep, according to the USGS, which is near enough to the surface to set off a tsunami.

 

Violent Pacific, Due To “Ring Of Fire”

Despite its name, the Pacific has the most violent geography in the world. It is surrounded by the ‘ring of fire’ that generates 9 out of 10 earthquakes in the world.

The Pacific is the largest ocean. Its area (which various measures calculate between 150 and 200 million km.) has an area larger than the sum of all continents. There are 20 to 30 thousand islands, the majority of the ones that exist, and slightly less than the total of the liquid water on Earth.

Its ‘Ring of Fire’ in inverted horseshoe shape encompasses whole west coast of the Americas, Alaska, East Asia, Indonesia and New Zealand. This bow is about 40,000 km long and has 452 volcanoes (3 out of 4 that exist on our planet).

Shocks occurring there are caused by the movement of tectonic plates that make and shape the earth’s surface.

The plates can collide between them (as happens when the Pacific is subsumed under the slabs of Northern Australia, creating elevations, volcanoes and earthquakes of New Zealand and New Guinea and Japan, Alaska and Hawaii, respectively) separated from them by opening cracks (like those that occur between the Pacific Plate and the Nazca and Antarctica) or move in opposite directions (such as creating the fault and the earthquakes in California).

While earthquakes can generate death and destruction, they are also responsible for having developed many of today’s living conditions. The 3 most populated islands in the world (Japan, Philippines and Indonesia in Pacific Asia) are fertile (like New Zealand and the Andes), in part because the product of the lava and movements that trigger these shocks.

The two biggest mountain ranges that exist (the Himalayas and the Andes) were created as a result of the sinking of a plate under which it arose. The Pacific plate has produced the largest mass grave (the Marianas that are 11 kilometers deep, is the lowest point of the earth’s crust) and also the highest mountain (Mauna Kea in Hawaii), which measures 10 kilometers from its base to 6 kilometers under the sea. Compared to both, Everest is less than 9 kilometers above sea level.

The Pacific Ocean also produces the climatic oscillation that regularly causes devastation worldwide: El Niño. This is produced when the temperature of tropical seas is altered which creates droughts or heavy rains at its opposite ends, but can also link together several changes in climates as far away as Europe or Africa.

‘Tsunami’ is the most common Japanese word in several languages. This originated in the Asian Pacific, but people around the ocean have designated it by other names. One of them is ‘tidal wave’ as well being called Callao in 1746, which destroyed the largest port of the American Pacific, killing more than 95% of its inhabitants.

 

 

Impacts Of Nuclear Disaster Over The World

1.How long will the radiation be in the reactor area after it is finally contained? Will the radiation spread across the globe via air or sea?

Any releases of radioactivity would tend to be carried to the east across the Pacific, although day-to-day weather could spread it over considerable parts of Japan. Scientists are already forecasting the path of long-range transport through the atmosphere, as they have long practiced in order to track releases from nuclear testing and major accidents. The Chernobyl nuclear power plant accident in the Soviet Union sent radioactive dust particles hundreds and even thousands of kilometers away, with health effects occurring close in. The breadth of the Pacific would seem to be sufficient protection from detectable health effects.

2. Would it be possible to build underground emergency tsunami shelters that would be insulated from the earthquakes that would surely precede the wave?

This is technically possible but probably not a good idea. It would be psychologically difficult to persuade people to go underground to avoid a tsunami. Debris could pile up around or on top of any entrance. And, as in this case, coastal areas hit by a tsunami can remain flooded for several days, a situation aggravated in some areas by ground subsidence.

3.  What is the probability of a magnitude-7 or higher aftershock in Japan? Could the Japan quake lead to other quakes across the globe? Are we having more earthquakes than before? Why are we seeing such a dramatic increase in seismic activity globally?

From past quakes, an estimate of the likely largest aftershock can be made. A magnitude 7—far, far smaller than a 9—sounds reasonably likely. In recent years, seismologists have realized that the biggest quakes can trigger more quakes thousands of kilometers away, but these are typically small and often located at hot springs or volcanic areas already prone to small quakes. Global seismicity has not been going up in the long term; high-profile events like Haiti and Japan make it look as if it is. Quakes can trigger eruptions from nearby volcanoes; nothing from this one so far.

Globally, seismic energy has been released over the years at a fairly constant rate, with the inevitable random fluctuations. Part of the reason we have a sense of greater seismic activity is the notoriety of recent quakes. Haiti was not a big deal seismically speaking, but because of its location, it killed hundreds of thousands and captured world attention. There is a record of quakes in historical and geologic times preserved in sediments on faults or deposited by tsunamis, but it is not complete enough to address global seismicity trends.

4 .Although the safety of the people of Japan is our main priority, what ramifications has the catastrophe in Japan had on wildlife? Especially due to the numerous cars and debris polluting the waters, are there certain Japan-exclusive or endangered species of plants or animals that could see a collapse in population, possibly extinction?

Scientist M. Sanjayan of The Nature Conservancy in Arlington, Virginia, says that the tsunami’s biggest impact on wildlife will be on coastal birds nesting on small islands rather than on the Japanese mainland where they could easily fly away. Indeed, the U.S. Fish and Wildlife Service is reporting that tens of thousands of birds were buried alive by the waves. (The famed Midway Atoll albatrosses will survive as a species, although their numbers took a beating.) The fate of some other endangered species, such as monk seals, is currently unknown. But although many large mammal casualties will certainly be seen, Sanjayan guesses that most of them were able to ride the waves out with minimal fatalities.

Impacts on wildlife from pollution and radiation leaks are a separate issue, however, and one that is continuing to unfold. Sanjayan says that while species will certainly be harmed by this pollution in the short term, history from Chernobyl and the Bikini Atoll shows that once an area is placed off-limits to human activity, wildlife has a way of rebounding within a few decades.

5. What is the highest intensity of radiation measured so far? What affect will the radiation have on the people of Japan? How many people could it kill?
Keep in mind that news outlets have been reporting the levels in two units: millisieverts (mSv), which is one thousandth of a sievert, and microsieverts (μSv), which is one millionth of a sievert. (All numbers that have been reported are per hour.) So if you’ve been seeing numbers in the thousands, check the units. 3000 μSv/h is equal to 3 mSv/h—equivalent to about 300 chest x-rays, or one head CT scan—which levels at the plant gates have reached at times during this disaster. This New York Times graphic shows how radiation levels around the plant have changed over the past few days with the various explosions that have occurred. The Japanese Atomic Industrial Forum, a nongovernmental organization for nuclear energy, is publishing several updates a day on the plant’s status. The most recent update reiterates that levels peaked at 400 mSv next to unit 3 on Tuesday.

The people most at risk from radiation at the Fukushima Daiichi plant are the workers trying to keep things under control. That’s because the effect on one’s health depends on how near one is to the radioactive sources at the power plant and how long one stays there. Peak radiation levels at the plant were around 400 mSv per hour right in the thick of things, between reactors 2 and 3. According to Peter Burns, former chief executive officer of the Australian Radiation Protection and Nuclear Safety Agency, the limit recommended by the international community for workers dealing with accidents is 100 mSv—so a worker would reach the limit in just 15 minutes. While health effects may not be immediate, that person would also have an increased risk of developing cancer later in life.

Limiting workers’ exposure is extremely important; if exposed to that peak radiation for 2½ hours (1000 mSv total dose) a person would start to feel sick. If the person stayed there for 15 hours, they would likely die within a month. But as one goes farther away from a radiation source, the dose one receives per unit time falls off exponentially, which is why the Japanese government has evacuated the area around the plant. Within the 20-kilometer perimeter that has been evacuated, levels peaked at 0.33 mSv per hour, equivalent to three chest x-rays per hour—or about 30 days worth of exposure to background radiation. If one stayed there a day, one’s total dose would be about the same as a full body CT scan.

6.Since it seems the radiation will mainly head out to sea, what will its effects be on ocean life?

Effects on marine life should be minimal if the plume is blown over the ocean. Radioactive isotopes are most dangerous when animals’ bodies absorb them, thinking they’re something else. For instance, cesium-137 mimics potassium and is absorbed by muscles, while strontium-90 mimics calcium and is taken up by bones. Since ocean water is full of potassium and calcium in the form of salts, this lowers the chance of an animal’s body taking up radioactive particles by mistake.

Furthermore, since the Pacific is so massive, radioactivity will be diluted to levels far too low to be toxic to aquatic life. A much bigger concern is the plume blowing over land and contaminating plant life or the freshwater supply, which would affect animals (including humans) further up the food chain.

Japan earthquake accelerated Earth’s rotation

The massive earthquake that struck northeast Japan Friday (March 11) has shortened the length Earth’s day by a fraction and shifted how the planet’s mass is distributed.

A new analysis of the 8.9-magnitude earthquake in Japan has found that the intense temblor has accelerated Earth’s spin, shortening the length of the 24-hour day by 1.8 microseconds, according to geophysicist Richard Gross at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Gross refined his estimates of the Japan quake’s impact – which previously suggested a 1.6-microsecond shortening of the day – based on new data on how much the fault that triggered the earthquake slipped to redistribute the planet’s mass. A microsecond is a millionth of a second.

“By changing the distribution of the Earth’s mass, the Japanese earthquake should have caused the Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds,” Gross told SPACE.com in an e-mail. More refinements are possible as new information on the earthquake comes to light, he added.

The scenario is similar to that of a figure skater drawing her arms inward during a spin to turn faster on the ice. The closer the mass shift during an earthquake is to the equator, the more it will speed up the spinning Earth.

One Earth day is about 24 hours, or 86,400 seconds, long. Over the course of a year, its length varies by about one millisecond, or 1,000 microseconds, due to seasonal variations in the planet’s mass distribution such as the seasonal shift of the jet stream.

The initial data suggests Friday’s earthquake moved Japan’s main island about 8 feet, according to Kenneth Hudnut of the U.S. Geological Survey. The earthquake also shifted Earth’s figure axis by about 6 1/2 inches (17 centimeters), Gross added.

The Earth’s figure axis is not the same as its north-south axis in space, which it spins around once every day at a speed of about 1,000 mph (1,604 kph). The figure axis is the axis around which the Earth’s mass is balanced and the north-south axis by about 33 feet (10 meters).

“This shift in the position of the figure axis will cause the Earth to wobble a bit differently as it rotates, but will not cause a shift of the Earth’s axis in space – only external forces like the gravitational attraction of the sun, moon, and planets can do that,” Gross said.

This isn’t the first time a massive earthquake has changed the length of Earth’s day. Major temblors have shortened day length in the past.

The 8.8-magnitude earthquake in Chile last year also sped up the planet’s rotation and shortened the day by 1.26 microseconds. The 9.1 Sumatra earthquake in 2004 shortened the day by 6.8 microseconds.

And the impact from Japan’s 8.9-magnitude temblor may not be completely over.The weaker aftershocks may contribute tiny changes to day length as well.

The March 11 quake was the largest ever recorded in Japan and is the world’s fifth largest earthquake to strike since 1900, according to the USGS. It struck offshore about 231 miles (373 kilometers) northeast of Tokyo and 80 miles (130 km) east of the city of Sendai, and created a massive tsunami that has devastated Japan’s northeastern coastal areas. At least 20 aftershocks registering a 6.0 magnitude or higher have followed the main temblor.

“In theory, anything that redistributes the Earth’s mass will change the Earth’s rotation,” Gross said. “So in principle the smaller aftershocks will also have an effect on the Earth’s rotation. But since the aftershocks are smaller their effect will also be smaller.”

 

Japan Confronts Multiple Crises as Death Toll Climbs

Japanese authorities struggled to contain new nuclear emergencies on Tuesday — including a possible rupture in a reactor containment vessel — as the death toll continued to climb with search teams reaching towns that were flattened by last week’s earthquake and tsunami.

The National Police Agency said Tuesday afternoon that 2,722 people have died, and many thousands were still missing. Bodies continued to wash ashore at various spots along the coast after having been pulled out to sea by the tsunami’s retreat.

Some 400,000 people were living in makeshift shelters or evacuation centers, officials said. Bitterly cold and windy weather that was pushing into northern Japan was compounding the misery as the region struggled with shortages of food, fuel and water.

An explosion Tuesday morning at the Fukushima Daiichi Nuclear Station — the third reactor blast in four days — damaged the vessel containing the nuclear core at reactor No. 2 , government officials said, and there was a growing fear of a catastrophic meltdown.

The overwhelmed operator of the nuclear plants, Tokyo Electric Power Company, confirmed there had been radiation leaks and that water was being pumped into three overheated reactors in the Fukushima complex.

A fire that broke out Tuesday morning at a fourth reactor was extinguished by mid-afternoon, although the government’s chief spokesman, Yukio Edano, later said that temperatures were now rising inside a fifth and sixth reactor in the complex.

People living within about 12 miles of the reactors at Fukushima were ordered to evacuate, and those within about 20 miles were told to stay indoors and close all windows, doors and vents. If people had laundry hanging outside, the government advised, they should not bring it inside or touch it.

Tokyo-area residents began to buy and stockpile food, water, candles and batteries as shelves at grocery stores became increasingly bare. Prime Minister Naoto Kan went on national television to implore people not to panic.

But there was plenty of panic in the stock market: Fevered selling drove down the Nikkei stock index by 10.6 percent at the close of trading.

There were scattered news reports of some foreigners fleeing Japan, and one Western diplomat said Tuesday night that “anecdotes and rumors” were swirling in the international community.

Still, there appeared to be no mass exodus. The United States Embassy, for example, was not urging resident Americans to leave.

The ambassador, John V. Roos, said that about 1,300 Americans were living in the five northern prefectures most affected by the earthquake and the tsunami. American consular officers were making their way to Sendai and other northern cities on Tuesday to conduct “welfare-and-whereabouts” checks on American citizens there.

“We are encouraging U.S. citizens to heed the instructions of the Japanese civil defense authorities,” Mr. Roos said.

The commander of American forces in Japan, Lt. Gen. Burton M. Field, confirmed that some American troops aboard three helicopters had been contaminated by radiation when they apparently flew through a radioactive plume released from the crippled nuclear complex.

“We found contamination on the clothes of several crew members, and one crew member had some on his skin,” said General Field. “The exposure rate was about the same as you would get over a monthlong period outside in the sun. We assess that as very, very low.”

He added that the crew members got a good scrubbing with soap and water and were back on duty.

Chinese health and environmental officials on Tuesday gathered for emergency meetings on how to respond in case radioactive fallout hit China. The government said it was stepping up monitoring for radiation and would swiftly report results.

China’s Meteorological Administration said prevailing winds would carry radioactive material away from China until Thursday, and possibly beyond. “China will not be affected,” the agency said on its Web site.

Air China canceled all flights to and from Tokyo and Sendai until Thursday.

Nations such as South Korea and Singapore have announced they would bolster inspections of Japanese food imports.

The United States Geological Survey revised the magnitude of the earthquake to 9.0, from 8.9, but it was the subsequent tsunami that did the most damage. The initial wave scoured away entire communities, and desperate survivors searched Tuesday for signs of friends and relatives who remained missing.

There was plenty that was missing in the fishing village of Minamisanriku: the city hall, the hospital, the shipyard, police stations — and 8,000 people.

The tsunami might have crashed most heavily into this town that once was home to more than 17,000. Situated at the back of a mountainous V-shaped cove, the town was swamped by the first surge of muck and seawater that was 30 feet high as it roared between the valley walls.

As the deluge pressed in on them, Sanae Sato, 71, said 400 townspeople rushed to the community center where she worked. They thought the five-story building would be high enough to protect them. But when the water reached the fourth floor, they all sought shelter in the attic.

From the attic window, Ms. Sato said, she saw the floodwaters hurling cars along, with drivers and passengers still inside. Houses broke from their foundations and were carried along, their owners perched on the ridges of the roofs.

“I saw people trying to balance on the rooftops like surfers,” she said. “It didn’t work. It was like hell.”

The Miyagi prefectural government said Tuesday that search teams had located 2,000 people in Minamisanriku who had been missing and presumed dead. They had fled to surrounding towns as the tsunami bashed the coastal areas of the town.

Troopers from Japan’s Self-Defense Forces cleared roadways into the village on Tuesday as a long line of fire trucks waited to enter. Closer to shore, teams of searchers rummaged through the crushed houses and collapsed shops. They peered into cars that had been swallowed by the mud, hoping to find survivors. Searched cars were marked with yellow tape.

One gruesome discovery was a mud-caked woman hanging by her head from the roof of a gas station. She was brought down, covered in a blue plastic tarp, and her body was laid by the station to await collection by another disaster team.

Rescue teams from 13 countries pressed on with the searches in other towns, some assisted by dogs. In the air, helicopters shuttled back and forth, part of a mobilization of some 100,000 troops, the largest since World War II.

Because Fukushima have been lost to the national power grid, Tokyo Electric announced plans for rolling blackouts across the region to conserve electricity — the first controlled power cutbacks in Japan in 60 years.

The first set of blackouts Tuesday morning began in four prefectures outside Tokyo. The utility, which provides service to 45 million people in the region, said the cuts could continue for six weeks.

Public conservation of electricity was significant enough, the company said, that the more drastic blackout scenarios were being scaled back. Still, anticipating deep and lengthy power cuts, many people were stocking up on candles, water, instant noodles and batteries for radios.

 

Japan Faces Potential Nuclear Disaster as Radiation Levels Rise

Japan’s nuclear crisis verged toward catastrophe on Tuesday after an explosion damaged the vessel containing the nuclear core at one reactor and a fire at another spewed large amounts of radioactive material into the air, according to statements from Japanese government and industry officials.

In a brief address to the nation at 11 a.m. Tokyo time, Prime MinisterNaoto Kan pleaded for calm, but warned that radiation had already spread from the crippled reactors and there was “a very high risk” of further leakage. Fortunately, the prevailing winds were sweeping most of the plume of radioactivity out into the Pacific Ocean, rather than over populated areas.

The sudden turn of events, after an explosion Monday at one reactor and then an early-morning explosion Tuesday at yet another — the third in four days at the plant — already made the crisis at the Fukushima Daiichi Nuclear Power Station the worst nuclear accident since the Chernobyl reactor disaster a quarter century ago.

Engineers at the plant, working at tremendous personal risk, on Tuesday continued efforts to cool down the most heavily damaged unit, reactor No. 2, by pumping in seawater. According to government statements, most of the 800 workers at the plant had been withdrawn, leaving 50 or so workers in a desperate effort to keep the cores of three stricken reactors cooled with seawater pumped by firefighting equipment, while crews battled to put out the fire at the No. 4 reactor, which they claimed to have done just after noon on Tuesday.

That fourth reactor had been turned off and was under refurbishment for months before the earthquake and tsunami hit the plant on Friday. But the plant contains spent fuel rods that were removed from the reactor, and experts guessed that the pool containing those rods had run dry, allowing the rods to overheat and catch fire. That is almost as dangerous as the fuel in working reactors melting down, because the spent fuel can also spew radioactivity into the atmosphere.

After an emergency cabinet meeting, the Japanese government told people living within about 20 miles of the Daiichi plant to stay indoors, keep their windows closed and stop using air conditioning.

Mr. Kan, whose government was extraordinarily weak before the sequence of calamities struck the nation, told the Japanese people that “although this incident is of great concern, I ask you to react very calmly.” And in fact, there seemed to be little panic, but huge apprehension in a country where radioactivity brings up memories of Hiroshima and Nagasaki, the haunting images of post-war Japan.

Radiation measurements reported on Tuesday showed a spike of radioactivity around the plant that made the leakage significantly worse than it had been, with levels measured at one point as high as 400 millisieverts an hour. Even 7 minutes of exposure at that level will reach the maximum annual dose that a worker at an American nuclear plant is allowed. And exposure for 75 minutes would likely lead to acute radiation sickness.

The extent of the public health risk depends on how long such elevated levels persist, as well as how far and fast the radioactive materials spread, and whether the limited evacuation plan announced by the government proves sufficient.

In Tokyo, 170 miles south of the plant, the metropolitan government said Tuesday it had detected radiation levels 20 times above normal over the city, though it stressed that such a level posed no immediate health threat, and that levels had dropped since then.

The government said later Tuesday that radiation levels at the Fukushima plant also appeared to be falling sharply.

But worryingly, temperatures appeared to be rising in the spent fuel pools at two other reactors at the plant, No. 5 and No. 6, said Yukio Edano, the chief cabinet secretary. Meanwhile, workers continued to pump seawater into the No. 1 and No. 3 reactors, where cooling systems remained unusable.

Japan has officially requested assistance from the United States Nuclear Regulatory Commission, as well as the International Atomic Energy Agency. But on Tuesday, the United States Forces Japan said the Fukushima plant had turned away two fire trucks that had made their way to the plant to offer assistance.

“They said they didn’t need them,” said Sgt. Maj. Steve Valley of the military public affairs office. “So they came right back.”

The succession of problems at Daiichi was initially difficult to interpret, with confusion compounded by incomplete and inconsistent information provided by government officials and executives of the plant’s operator, Tokyo Electric Power Company.

But industry executives in close contact with officials in Japan expressed extreme concern that the authorities were close to losing control over the fuel melting that has been ongoing in three reactors at Daiichi, especially at the crippled No. 2 reactor where the containment vessel was damaged.

Tokyo Electric Power said Tuesday that after the explosion at the No. 2 reactor, pressure had dropped in the “suppression pool” — a section at the bottom of the reactor that converts steam to water and is part of the critical function of keeping the nuclear fuel protected. After that occurred, radiation levels outside No. 2 were reported to have risen sharply.

“We are on the brink. We are now facing the worst-case scenario,” said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. “We can assume that the containment vessel at Reactor No. 2 is already breached. If there is heavy melting inside the reactor, large amounts of radiation will most definitely be released.”

Another executive said the chain of events at Daiichi suggested that it would be difficult to maintain emergency seawater cooling operations for an extended period if the containment vessel at one reactor had been compromised because radiation levels could threaten the health of workers nearby.

If all workers do in fact leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material — by far the largest accident of its kind since Chernobyl.

Even if a full meltdown is averted, Japanese officials have been facing unpalatable options. One was to continue flooding the reactors and venting the resulting steam, while hoping that the prevailing winds did not turn south toward Tokyo or west, across northern Japan to the Korean Peninsula. The other was to hope that the worst of the overheating was over, and that with the passage of a few more days the nuclear cores would cool enough to essentially entomb the radioactivity inside the plants, which clearly will never be used again. Both approaches carried huge risks.

While Japanese officials made no comparisons to past accidents, the release of an unknown quantity of radioactive gases and particles — all signs that the reactor cores were damaged from at least partial melting of fuel — added considerable tension to the effort to cool the reactors.

“It’s way past Three Mile Island already,” said Frank von Hippel, a physicist and professor at Princeton. “The biggest risk now is that the core really melts down and you have a steam explosion.”

The sharp deterioration came after a frantic day and night of rescue efforts focused largely on the No. 2 reactor. There, a malfunctioning valve prevented workers from manually venting the containment vessel to release pressure and allow fresh seawater to be injected into it. That meant that the extraordinary remedy emergency workers had jury-rigged to keep the nuclear fuel from overheating no longer worked.

As a result, the nuclear fuel in that reactor was exposed for many hours, increasing the risk of a breach of the container vessel and more dangerous emissions of radioactive particles.

By Tuesday morning, Tokyo Electric Power said that it had fixed the valve and resumed seawater injections, but that it had detected possible leaks in the containment vessel that prevented water from fully covering the fuel rods.

Then an explosion hit that reactor. After a series of conflicting reports about what level of damage was inflicted on the reactor after that blast, Mr. Edano, the chief cabinet secretary, said, “there is a very high probability that a portion of the containment vessel was damaged.”

The steel containment vessels that protect nuclear fuel in reactors are considered crucial to maintain the integrity of the reactor and the safety of the fuel.

Mr. Edano, however, said that the level of leaking at the No. 2 reactor remained small, raising the prospect that the container was sufficiently intact to protect the nuclear fuel inside.