The Richter scale was a seismic scale used to measure the intensity of an earthquake.
After pockets of subterranean carbon dioxide were released on Penthara IV by drilling holes with a phaser drill, the planet experienced two massive earthquakes. Geordi La Forge estimated that if the earthquakes had been on Earth, they would have measured about 8 or 8.5 on the Richter scale. (TNG: "A Matter Of Time")
The biosphere on Moab IV was constructed to withstand quakes of 8.7 on the Richter scale. During initial contact, Data informed Aaron Conor that the approaching stellar core fragment had a density of one hundred billion kilograms per cubic centimeter, with its passing causing tectonic shifts "well beyond 8.7 on the Richter scale". (TNG: "The Masterpiece Society")