Natural disaster: An event caused by natural processes of the Earth, such as earthquakes, floods, droughts, or wildfires, resulting in widespread damage, loss of life, or environmental disrupt

Here's the differentiation between the two concepts:

• Natural disaster: An event caused by natural processes of the Earth, such as earthquakes, floods, droughts, or wildfires, resulting in widespread damage, loss of life, or environmental disruption.
• National Disaster: A disaster, which can be natural or human-made, that is of such severity and magnitude that it affects a significant portion of a country, overwhelming local and provincial resources and requiring a coordinated national response and declaration by the government.