What causes earthquakes?
A movement of the tectonic plates when they overlap. Most earthquakes happen at the plate boundaries.
What causes underwater
earthquakes and/or volcanoes?
There is no difference between underwater earthquakes and earthquakes on land. Volcanoes are made by weak spots in between the boundaries.
How does an underwater earthquake cause a tsunami?
A tsunami can be caused when the floor of the ocean shifts and displaces the above water from where it is. Waves are made as what determines the size is the amount of the sea floor deformation. Not all earthquakes make tsunamis. They must occur under or near the ocean to cause a tsunami.