I would suggest two possible paths for prediction area.

1. Circle

Note: Input SOUTH Latitudes and WEST Longitudes as NEGATIVE numbers.

Box1 Center Latitude
Box 2 Center Longitude
Box 3 Radius (km)

2. Rectangle

Note: Input SOUTH Latitudes and WEST Longitudes as NEGATIVE numbers.

Box 1 Top Latitude of Rectangle
Box 2 Bottom Latitude of Rectangle
Box 3 Right Longitude of Rectangle
Box 4 Left Longitude of Rectangle

This is how NEIC does it, and it seems to make sense to me ....
http://gldss7.cr.usgs.gov/neis/epic/epic_area.html

Michael