Not all earthquakes are felt by people; however, such earthquakes are often detected on seismographs. Why?
seismographs is a robot - meaning it's not a human being. like many robots we know (computers, phones, etc), it is able to go beyond human capacity. that's probably why seismosgraphs is able to detect the earthquakes. :)
Seismographs register various waves that an earthquake can produce. Here is a good description of the different types of waves.http://www.geo.mtu.edu/UPSeis/waves.html