Not all earthquakes are felt by people; however, such earthquakes are often detected on seismographs. why?

The seismic activities are usually in areas that are not populated, plus, the activity is deep in the ocean or in the earth’s core.  The shifting of the earth's plates doesn't only occur on the surface,  some occur deep down in the surface, causing us not to feel them.  Moreover, seismographs are able to detect low frequencies, causing them to be able to detect even the slightest movement of the plates down under. 

What are the following resources?