GeologyRocks

This time from their Science and Nature - Hot Topics - Natural Disasters section on Earthquakes

Centre of the Earth
The Earth is made up of three main layers:
# The core is at the centre of the Earth
OK so far

# The mantle is a mobile semi-molten layer around the core
The mantle is a plastic solid. It is not semi-molten (except under limited circumstances)

# The outer-shell of the Earth is called the crust. Scientists call this the lithosphere - it's the part we're on now
The crust and lithosphere are distinctly different things. The lithosphere comprises the crust and the upper part of the mantle. Mantle and crust are compositional entities, lithosphere, aesthenosphere and mesosphere are mechanical ones.

The crust is made up of 12 individual tectonic plates.
The lithosphere is make up of numerous tectonic plates - how many major plates there are is a matter for debate, but overall there are far more than 12 plates

Below the sea, they can measure three to six miles (4km-9.6km) thick and under land this increases to 20-44 miles (32km-70.8km).

But the plates are lithosphere - about 220km thick.

... The point where the seismic activity occurs is the epicentre, where the earthquake is strongest. But it doesn't always end there, seismic waves travel out from the epicentre, sometimes creating widespread destruction as they pass.

The focus of the earthquake is the hypocentre. The epicentre is the point on the Earth's surface above the hypocentre.