As water soaks under ground, it first passes through a rock layer that has pores that are mostly filled with air. what is this zone called

Zone of Aeration- Its the layer in the crust of the Earth that is above the Hydrosphere. The layer of dirt/rock that's filled with air in the pores. 

If you need this here ya go- Hydrosphere- The layer of crust that has most of the aquifers and is filled completely with water around the pores.

Geologists use  seismic waves to  locate an earthquake's epicenter..