Anything that isn't a man-made event is a natural phenomenon. Hurricanes, earthquakes, the aurora borealis, volcanic eruptions, floods, avalanches, meteor collisions, lightning strikes and the tidal forces caused by the moon are all natural phenomena. Biological processes and astrophysical occurrences fall under nature's purview.
Earthquakes happen because the Earth is composed of tectonic plates connected by fault lines. When the stresses between these plates grows too large and one of them slips, shockwaves are sent throughout the land and any nearby bodies of water.
Hurricanes form over the ocean when a low pressure area is powered by the heat of condensation. This heat results from water vapor rising up to form clouds, which liberates energy. If there isn't an outlet for the build-up of energy via wind shear, the winds build up. A tropical storm forms when winds are 74 miles per hour or higher.
The aurora borealis results when energetic electrons from the solar wind interact with molecules and atoms in our atmosphere. Earth's magnetic field captures part of the solar wind, and the display of lights comes from the many collisions between the particles. This natural phenomenon usually occurs in regions where the magnetic field poles define aurora ovals.