The Richter Scale

The Richter scale is the most famous way of measuring how big an earthquake is. It was invented in 1935 by American seismologist Charles Richter to give a single simple number for the size of each earthquake. The scale is logarithmic: every whole number is 10 times bigger than the one below in terms of ground shaking, and roughly 32 times more energy. So a magnitude 6 earthquake is not just slightly worse than a magnitude 5: it is enormously more powerful. Modern scientists now usually use a more accurate version called the moment magnitude scale, but the basic numbers are similar and people still often call it "the Richter scale".

  • Invented1935By Charles Richter
  • Each whole step is10x ground motionAnd approx. 32x more energy
  • Mag 2Barely noticeableFelt only with sensitive instruments
  • Mag 5Significant damage possibleHeavy items moved, windows broken
  • Mag 7DevastatingBuildings collapse
  • Strongest ever measuredMag 9.5Valdivia, Chile, 1960

What the numbers mean

  • Magnitude 1-2: barely noticeable. Felt only by sensitive instruments. Happens millions of times a year worldwide.
  • Magnitude 2-3: not usually felt by humans, but recorded.
  • Magnitude 4: felt by most people, but no damage. Crockery may rattle.
  • Magnitude 5: significant damage possible in populated areas. Heavy items may fall, walls may crack.
  • Magnitude 6: damaging in built-up areas. Many buildings damaged.
  • Magnitude 7: devastating. Most buildings damaged, many destroyed.
  • Magnitude 8: catastrophic. Most modern buildings destroyed.
  • Magnitude 9+: extreme. Only the most massive earthquakes ever recorded reach this.

Why each step is so much bigger

The Richter scale is logarithmic. That means each whole step represents a tenfold increase in measured ground motion, and roughly a 32-fold increase in energy released. A magnitude 7 earthquake has approximately:

  • 10 times the ground motion of a mag 6.
  • 32 times the energy of a mag 6.
  • 100 times the ground motion of a mag 5.
  • 1,000 times the energy of a mag 5.
  • 1 million times the energy of a mag 3.

This is why a magnitude 8 earthquake is so much more dangerous than a magnitude 6: not twice as bad, but thousands of times more powerful.

How earthquakes are measured

Earthquakes are measured with seismometers: instruments that detect tiny ground vibrations. A seismometer typically has a heavy weight on a spring, plus a recording mechanism. When the ground shakes, the recording paper (or sensor) moves with the ground, while the heavy weight tries to stay still because of inertia. The relative motion creates a wavy line called a seismograph.

Networks of seismometers around the world record every earthquake, and software combines the data to calculate exactly where and how big it was. For big earthquakes, multiple agencies report magnitude estimates within minutes.

Fact The strongest earthquake ever measured was the 1960 Valdivia earthquake in Chile, at magnitude 9.5. It released roughly 178 billion tonnes of TNT worth of energy: more than 10,000 times all the bombs dropped in World War II combined. It lasted approximately 10 minutes and shifted the entire South American continent very slightly westwards.

Richter vs moment magnitude

The original Richter scale was designed for small to medium earthquakes in California, measured by one specific type of seismometer. It does not work well for very large earthquakes (it tends to "saturate" around magnitude 7 and underestimate bigger ones), or for earthquakes far from California.

Modern seismologists usually use the moment magnitude scale (Mw), which uses the same number range but is based on the actual mechanical work done by the rocks slipping. It gives more accurate values for very large earthquakes. When you hear "magnitude 7.5" in news reports today, it almost always refers to moment magnitude, not the original Richter scale. The numbers are similar in the lower ranges, so most people just call it "magnitude" or still call it "Richter scale".

Did you know? The Mercalli intensity scale is a different way of measuring earthquakes. Instead of recording how much energy was released, it records how strongly people felt the shaking in a particular place. It ranges from I (not felt) to XII (catastrophic). The same earthquake will have one magnitude but many different intensity ratings depending on how close you are and what kind of ground you are standing on.
Deeper dive: how often each magnitude happens worldwide

The pattern of earthquake sizes is amazingly consistent. Every year, around the world, there are typically:

  • Approximately 1 to 2 earthquakes of magnitude 8 or greater.
  • Approximately 15 earthquakes of magnitude 7 to 7.9.
  • Approximately 130 earthquakes of magnitude 6 to 6.9.
  • Approximately 1,300 earthquakes of magnitude 5 to 5.9.
  • Approximately 13,000 earthquakes of magnitude 4 to 4.9.
  • Approximately 130,000 earthquakes of magnitude 3 to 3.9.
  • Approximately 1.3 million earthquakes of magnitude 2 to 2.9.

The pattern is so regular that scientists give it a name: the Gutenberg-Richter law. It states that for every increase of 1 in magnitude, there are approximately 10 times fewer earthquakes. This pattern is found all around the world and across all geological time. It is one of the most reliable statistical relationships in geology.

The law also tells us that the biggest possible earthquakes are extremely rare but not impossible. The strongest earthquake ever recorded was magnitude 9.5. A magnitude 10 earthquake is not impossible in theory but would require a fault rupture larger than any known on Earth and may not be physically possible. Even so, the next "big one" in many earthquake-prone regions is overdue and will eventually arrive.

For more, see what is an earthquake and tsunamis.