US Geological Survey Earth Quake Hazards Programme website:
What is an "airquake?" Earthquakes along the Hayward Fault occur at varying depths. Since we cannot crack open Google Earth to show you earthquakes at the depth which they occur, we have color coded them according to their depth. This Google Earth file contains "airquakes"(earthquakes that we have pulled up out of the ground) along the Hayward Fault to help illustrate the orientation of the fault plane below the surface. The yellow coloured earthquakes are 0-3 km deep, the orange colored earthquakes are 3-6 km deep, and the red colored earthquakes are 6-10 km deep.