TIL that you can have an earthquake with negative magnitude
. I couldn't recall ever having seen mention of one before, but apparently they're recorded quite frequently with modern seismographic instruments.
His motivation for creating the local magnitude scale was to compare the size of different earthquakes. 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. 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  were around magnitude 3. However, the Richter scale has no lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.
Once I saw it explained, it made perfect sense, but it hadn't occurred to me before that there would be earthquakes of magnitude less-than-zero.