What does (in which) mean?
What does (in which) that is mentioned In the following report mean?
"Nepal has declared three days of mourning for the victims of Saturday's earthquake in which more than 5,000 people are now known to have died."

If there are any mistakes in my question, please correct them.
Thank you.

Question

What does (in which) mean?
What does (in which) that is mentioned In the following report mean? 
"Nepal has declared three days of mourning for the victims of Saturday's earthquake in which more than 5,000 people are now known to have died."

If there are any mistakes in my question, please correct them.
Thank you.

Answer

In this case, "in which" basically means "in the thing already mentioned". It already has said the subject is the earthquake, so it would be redundant to mention it again in the same sentence, so "in which" quickly references back to it.

Answer

It kinda means 'during this earthquake'