also
 

While asleep earlier today, an earthquake hit Japan. It woke me up.


BBC News - Earthquake jolts north-east Japan - no tsunami warning

More than 20,000 people died or were reported missing as a result of the disaster in March, which also crippled nuclear reactors in the Fukushima area.

Is the "also" necessary? Does the sentence sound strange without the word?

Re: also
 

Is the "also" necessary?

No, because we recognise that the adjective clause (which...) provides additional information.

Does the sentence sound strange without the word?

No, but I think it does sound a little better with the word 'also' for emphasis.

Re: also
 

I think it's also because "also" disambiguates the relative clause, pointing out that it is defining rather than non-defining. Compare :

More than 20,000 people died or were reported missing as a result of the disaster in March which crippled nuclear reactors in the Fukushima area.

Like that - no comma and no "also" - the relative clause becomes defining : it's telling you which disaster in March we're talking about.

With the comma/also it is, as emile says, just giving extra info about a disaster which you are presumed to have already identified. The "also" signposts this and thus disambiguates the clause function.