Understanding the Richter Scale and Modern Magnitude Measures

The Richter Scale: Famous But Retired

Nearly everyone has heard of the Richter scale, and nearly everyone believes it is still the standard way to measure earthquake magnitude. In fact, seismologists largely abandoned the original Richter scale in the 1970s and 1980s, replacing it with more accurate and universally applicable measures. Understanding why — and what replaced it — helps make sense of the magnitude numbers you see in earthquake reports.

Charles Richter's Original Scale

Charles Richter, a seismologist at Caltech, developed his scale in 1935 specifically for measuring earthquakes in Southern California, using a specific type of seismograph (the Wood-Anderson seismometer). The scale was logarithmic: each whole number increase represents a 10-fold increase in the amplitude of seismic waves and approximately 31.6 times more energy released.

Richter's local magnitude scale (ML) worked well for its original purpose — comparing earthquakes in a limited geographic region using standardized equipment. It became problematic for several reasons: it did not work well for very large earthquakes (it "saturated" at around magnitude 7, giving similar readings for very different events), it was specific to certain distances and equipment, and it did not translate well to earthquakes in other parts of the world.

The Moment Magnitude Scale: The Modern Standard

Today, seismologists primarily use the Moment Magnitude Scale (Mw), developed by Hiroo Kanamori and Thomas Hanks in 1979. Mw is calculated from the seismic moment — a measure derived from the area of the fault rupture, the amount of slip along the fault, and the rigidity of the rock. Unlike the Richter scale, Mw does not saturate for large earthquakes and can be calculated from any type of seismograph recording.

Crucially, for earthquakes below about magnitude 7, Mw values are nearly identical to Richter values — so the numbers are comparable for the earthquakes people most commonly experience. The difference becomes significant for major and great earthquakes above magnitude 7.

What the Numbers Actually Mean

• Below 2.0: Microearthquake, not felt by humans. Detected only by seismographs. Thousands occur globally every day.
• 2.0-2.9: Very minor. Generally not felt but recorded. About 1,000+ per day globally.
• 3.0-3.9: Minor. Often felt, rarely causes damage. About 100+ per day globally.
• 4.0-4.9: Light. Widely felt, objects may shake indoors, minor damage possible in poorly constructed buildings.
• 5.0-5.9: Moderate. Can cause significant damage to poorly designed structures; slight to moderate damage to well-designed buildings.
• 6.0-6.9: Strong. Destructive in populated areas up to 100 km from epicenter. About 100-150 per year globally.
• 7.0-7.9: Major. Causes serious damage over large areas. About 15-20 per year globally.
• 8.0-8.9: Great. Causes severe damage over several hundred kilometers. About 1-5 per year globally.
• 9.0+: Rare great earthquake. Total destruction near epicenter, severe damage over thousands of kilometers. The 2011 Tohoku earthquake was 9.1; the 1960 Valdivia earthquake (largest ever recorded) was 9.5.

The Logarithmic Scale: Why the Numbers Are Deceptive

The logarithmic nature of magnitude scales means each whole number step represents a 31.6-fold increase in energy released — not a 10-fold increase as commonly misunderstood (10-fold is the wave amplitude increase, not the energy). This means a magnitude 7.0 earthquake releases about 1,000 times more energy than a 5.0 earthquake, and a magnitude 9.0 releases about 1,000 times more energy than a 7.0.

In practical terms: the 2011 Tohoku 9.1 earthquake released more energy than all other global earthquakes in the preceding decade combined.

Intensity vs. Magnitude

Magnitude measures the energy at the source. Intensity measures the shaking experienced at a specific location. The Modified Mercalli Intensity (MMI) scale — rated I through XII — captures this: a single earthquake has one magnitude but many different intensity values at different locations, decreasing with distance from the epicenter and varying with local geology. Soft sediment amplifies shaking; bedrock dampens it. This is why some neighborhoods in a city can experience severe damage from an earthquake while other neighborhoods nearby have minimal damage.