NOAA
Noaa: Environmental Technology Lab: Infrasonics Program
This resource focuses on research of infrasonic sounds found in the environment. It discusses such infrasonic sources as volcanoes, avalanches, severe thunderstorms, and earthquakes.
Other
Sweetwater.com: What Is Infrasonic?
This resource features a basic definition of infrasonic. It also explains the difference between infrasonic and subsonic.