His motivation for creating the local magnitude scale was to compare the size of different earthquakes.
[1] Richter, who since childhood had aspirations in astronomy, drew inspiration from the
apparent magnitude scale used to account for the brightness of stars lost due to distance.
[2] Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 µm (0.00004 in) on a seismogram recorded using a Wood-Anderson torsion seismograph 100 km (62 mi) from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned.
The smallest earthquakes that could be recorded and located at the time [1935] were around magnitude 3. However, the Richter scale has no lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.