What was the cause of the quake?
Basic geology: an enormous build-up of seismic pressure jolted the tectonic plates underneath Japan. The "megathrust" pushed the Pacific plate under the Eurasian plate to the west, forcing waves of energy up to the Earth's surface.
Where, precisely, did it occur?
The epicentre – where most tectonic energy was released – was well out to sea, 130km (81 miles) east of Sendai, the capital of Japan's north-eastern Miyagi prefecture, but at a relatively shallow depth below the seabed of 24km. Scientists believe the impact lifted the ocean floor up by about 10m, causing a fault line to rupture along as much as 500km of the Pacific plate.
How powerful was it?
At 8.9 magnitude, the quake radiated energy equivalent to exploding 1.5 billion tons of TNT (or 1,500 one-megaton nuclear bombs) beneath the seabed. Or, to put it another way, roughly enough power to meet the US's needs for one month, according to the US Geological Survey. The impact shifted the Earth's axis by nearly 25cm, and moved the Japanese coast several metres.
Were there any warning signs?
Seismologists now point to several foreshocks, including a 7.2 magnitude quake on 9 March, which was only 40km from Friday's epicentre. Amateur pundits, however, think the 50-odd melon-headed whales that washed up on the eastern Kashima shore on 4 March match the shocks as a precursor. More than 100 pilot whales were beached on New Zealand's South Island less than 48 hours before the Christchurch quake.
Is Japan the world's most quake-prone area?
The country certainly sits within it: an arc of earthquake and volcanic zones dubbed the "Ring of Fire" that encircles virtually the entire Pacific Rim, where some 90 per cent of the world's quakes occur. What's more, Japan accounts for about 20 per cent of the world's quakes of magnitude 6 or above, clocking up some 1,000 tremors every year.
How does this quake compare with others?
It was Japan's most powerful since the country started keeping records 140 years ago; more than 10 times as powerful as Kobe's 1995 quake at 7.3 magnitude, which killed 6,500; and surpassing even the Great Kanto quake of 1923, which claimed more than 140,000 lives. It ranks as one of seven quakes rated at magnitude 8 or higher since 1891 in Japan, and was among the 20 or so major quakes measuring 7 or above in the world each year. The world's most powerful recorded quake was the magnitude 9.5 one that hit Chile in 1960, killing 4,485.
How does a quake cause a tsunami?
As the quake jolts the seabed, it displaces hundreds of cubic kilometres of water above it, causing large waves that race through the ocean away from the epicentre at about 885kmph (550mph). As the waves near shallow water, they slow and compress, which makes them increase in height, forming a tsunami. The word the Japanese bequeathed to the world means "harbour wave". The water may be sucked back before huge waves break on shore.
How big was this tsunami?
Wave heights varied along the coast, hitting almost 10m in some places. Surging as much as 10km inland, the waves, which turned into a mudslide, swept away homes, crops, vehicles and many hundreds of bodies. Even out in the deep ocean, the specialist tsunami warning buoys recorded wave amplitudes of a metre, which is considerable.
What, briefly, is the damage it has done in Japan?
With tens of thousands missing, it is too early to calculate the toll on human life but domestic media reported an official estimate of at least 1,700 people dead as of yesterday. An explosion at the Fukushima nuclear power plant sparked fears of a potential nuclear disaster, while elsewhere more than 200 fires raged. All the main ports were closed, hitting exports of cars and imports of raw materials.
How about the rest of the world?
Despite initial fears that the tsunami would drown entire Pacific islands, the eventual effects on shores as far away as Hawaii were not as severe as feared, partly thanks to massive evacuations along the Chilean and Ecuadorean coastlines. One man in California died when he was swept away photographing the waves.
How does it compare with past tsunamis?
While too early to calculate the tsunami's impact in Japan itself, it paled compared with the Boxing Day Indian Ocean tsunami in 2004 that killed an estimated 230,000 people in 12 countries. The most recent worst tsunami followed the magnitude 8.8 quake that shook Chile in February 2010, killing 524.
Why did Japan survive the quake – as opposed to the tsunami – so well?
In short, thanks to hard lessons learnt and tough building regulations that mean high-rise blocks, which are safer than low buildings, are designed to sway, not shake. Foundations are loaded with rubber and steel shock absorbers, and joints are all reinforced. Earthquake drills are a fact of life from primary school onwards: children know to hide under a desk and not to venture outside, where flying debris can be fatal. In contrast, a wall of water several metres high rushing in from the sea is harder to resist.
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