What caused the tsunami?
The most powerful earthquake recorded in Japanese history, magnitude 8.9. The
tremors were the result of a violent uplift of the sea floor 80 miles off the coast of
Sendai, where the Pacific tectonic plate slides beneath the plate Japan sits on. Tens
of miles of crust ruptured along the trench where the tectonic plates meet. The
earthquake occurred at the relatively shallow depth of 15 miles, meaning much of its
energy was released at the seafloor.
How does the earthquake compare with others?
This was the sixth largest earthquake in the world since 1900, when seismological
records began. The most devastating earthquake to strike Japan was in 1923, when
a magnitude 7.9 tremor devastated Tokyo and Yokohama and killed an estimated
142,800 people. The Kobe earthquake in 1995 was a magnitude 6.9 and caused
more than 5,000 deaths and injured 36,000 others. The earthquake that wrecked
Christchurch in New Zealand last month was a magnitude 6.3 event. Around 30
times more energy is released as the magnitude of an earthquake increases one
unit, for example from magnitude 8 to 9.
Why is the area so prone to earthquakes?
The Pacific plate moves fast in tectonic terms, at a rate of 9cm (3.5 inches) a year.
This leads to the rapid buildup of huge amounts of energy. As the Pacific plate
moves down, it sticks to the overhead plate and pulls it down too. Eventually, the join
breaks, causing the seafloor to spring upwards several metres. The plate tectonics of
the region are complex, and geologists are not sure which plate Japan sits on.
Candidates include the Eurasian plate, the North American plate, the Okhotsk plate,
and the Honshu microplate.
How big were the waves?
The largest waves measured by instruments in the water were 7 metres (nearly 23ft)
high in the north-east of Japan, according to the Pacific Tsunami Warning Centre
(PTWC) in Hawaii. Other estimates put the wave height at 10 metres. Waves
reached 4 metres around the coast of Japan. As the tsunami spread across the
Pacific, the wave height dropped to around 40cm in Guam and the nearby Marianas.
The most powerful waves appeared to be moving south-west from Japan. Some
countries may experience waves up to 2 metres, according to PTWC forecasts.
How much damage has been caused?
Japan has invested heavily in coastal protection and buildings that can withstand
tremors. Nevertheless, ports were pounded by the tsunami and the airport in Sendai
was inundated. Nuclear power plants were shut down across the country and a state
of emergency declared at the Fukushima nuclear power plant, where a cooling
system failed. Modern buildings in Japan are designed to absorb the violent
sideways shaking that can devastate cities. High-rise buildings can still be damaged,
but are more likely to remain standing. There are concerns for low-lying islands in
the Pacific.
How long will the waves take to reach other countries?
The tsunami moves across the Pacific at a speed of 500mph, with waves expected
to reach the island of Fiji and Cairns in Australia at 3.28pm GMT. From then, waves
are due to reach Acapulco in Mexico at 7.59pm, Chile at 10.55pm, Ecuador at
11.31pm, Colombia at 11.47 and Peru at 12.33am.
Will there be aftershocks?
Regular aftershocks have already hit Japan as the Earth's crust continues to rupture
along the Japan trench. Those tremors are expected to be weaker and are less likely
to produce another tsunami. The release of energy along the subduction zone
between the Pacific and North Atlantic plates will transfer stress to other parts of the
faultline, which could easily generate more earthquakes in the region in coming
months
Related
27 Sep 2003
Hundreds hurt in Japanese quakes
26 Sep 2003
Powerful quake shakes northern Japan
12 Apr 2001
Geologists find fault to solve quake mystery
26 May 2003
about 10 minutes ago
Japanese authorities will release radioactive vapor to ease pressure at damaged
nuclear reactor
about 26 minutes ago
• Read more tweets from our foreign news team
• Follow all our correspondents on a Twitter list
News
Japan on new nuclear alert as quake toll rises
11 Mar 2011
An 8.9 scale earthquakes has struck Japan triggering tsunami as high as 10 metres
and threatening nuclear reactors
6 Sep 2004
14 injured as earthquakes rock western Japan
26 Dec 2004
Chronology: tsunamis
17 Jul 2007
Seven dead and 400 hurt in Japan quake
18 Jan 1995
Quake toll nears 2,000 as rescuers dig through rubble
Christchurch quake was aftershock from 2010, scientists say
22 Feb 2011
Last September's New Zealand earthquake was more energetic but caused less
damage