Earthquake Magnitude Calculator

How to Use This Calculator

1. Enter the amplitude (mm) from the seismogram and distance from epicenter.
2. The calculator returns Richter local magnitude and severity category.
3. Use Moment Magnitude tab for large earthquakes (enter seismic moment M₀).
4. Use Compare Magnitudes tab to see the energy ratio between two events.
5. Professional mode adds Mercalli intensity estimate and saturation warning.

Formula

Example

Frequently Asked Questions

• What is earthquake magnitude and how is it measured? ▼
  Earthquake magnitude is a logarithmic measure of the energy released by a seismic event. The Richter local magnitude (Mₗ) is calculated from the maximum amplitude of seismic waves recorded on a seismograph, corrected for distance from the epicenter using the formula M = log₁₀(A) + 3·log₁₀(8·Δt) − 2.92, where A is amplitude in millimeters and Δt accounts for travel time. Seismologists deploy networks of sensitive instruments called seismometers that record ground motion in three axes. Modern measurement uses digital broadband seismometers capable of detecting motion as small as a nanometer. The resulting seismogram is processed to extract wave amplitudes and arrival times. Multiple stations record the same event and their readings are combined to produce a reliable magnitude estimate. The USGS and regional seismic networks continuously monitor global seismicity, cataloging tens of thousands of earthquakes per year, most below the threshold of human perception.
• Why is moment magnitude (Mw) preferred over Richter today? ▼
  The moment magnitude scale (Mw), developed by Hiroo Kanamori and Tom Hanks in 1979, is now the scientific standard because it does not saturate for large earthquakes — a critical flaw of the original Richter scale. The Richter scale was calibrated for a specific type of seismograph (Wood-Anderson torsion) at distances of 100–600 km in Southern California; outside those conditions its accuracy degrades. More importantly, the Richter scale saturates around magnitude 7–7.5: truly massive earthquakes like Chile 1960 (Mw 9.5) or Alaska 1964 (Mw 9.2) cannot be distinguished from slightly smaller ones on the Richter scale because the seismograph needle pegs. Moment magnitude is derived from the seismic moment M₀ = μAD (shear modulus × fault area × average slip), which captures the actual physical size of the rupture without saturation. It also agrees with Richter values in the moderate range (M4–7), making the transition seamless for public communication.
• What is the difference between magnitude and intensity? ▼
  Magnitude is an objective, single-number measure of the energy released at the earthquake source — it does not change regardless of where you are. Intensity, measured on the Modified Mercalli Intensity (MMI) scale (I to XII), describes the actual shaking and damage experienced at a specific location. A magnitude 7.0 earthquake can produce MMI X (extreme damage) directly above its shallow epicenter but only MMI IV (light shaking, felt indoors) 300 km away. Factors that control intensity include distance from the epicenter, focal depth (shallow quakes cause more surface damage), local geology (soft sediments amplify shaking dramatically compared to bedrock), and building quality. The USGS ShakeMap product automatically generates intensity maps within minutes of major earthquakes, combining measured ground motion with predictive models to show the spatial pattern of shaking across a region.
• How much more energy does a magnitude 7 release than a magnitude 6? ▼
  Each whole-number step up the magnitude scale corresponds to approximately 31.6 times more energy released — this comes directly from the formula E ∝ 10^(1.5×M). So a magnitude 7 releases about 31.6× more energy than a magnitude 6, and a magnitude 8 releases about 1,000× more energy than a magnitude 6 (31.6²). The ground motion amplitude — how hard the ground shakes — scales by a factor of 10 per magnitude unit. This is why the relationship between magnitude and damage is so nonlinear: a magnitude 8 does not feel twice as bad as a magnitude 7, it produces roughly 10 times stronger shaking and releases roughly 32 times more energy. The 1960 Chile earthquake (Mw 9.5) released more energy than all other earthquakes recorded in the 20th century combined, illustrating just how extreme the high end of the scale is in absolute energy terms.
• Why was the Richter scale developed and what are its limitations? ▼
  Charles Richter developed his local magnitude scale in 1935 specifically to quantify and compare Southern California earthquakes objectively, replacing purely subjective felt-intensity reports. Working with Beno Gutenberg at Caltech, he defined magnitude using the Wood-Anderson seismograph amplitude corrected for distance, choosing a logarithmic scale so the enormous range of earthquake sizes could be expressed in manageable numbers. The scale was a breakthrough for its era but carries several limitations: (1) It was designed for shallow local earthquakes at 100–600 km distance in a specific geological setting; (2) It saturates around M7–7.5 because seismographs clip on very large events; (3) It is sensitive to instrument type — different seismographs give different readings; (4) It does not directly represent physical quantities like fault area or slip. Modern seismology uses Mw as the primary scale, body-wave magnitude (mb) for deep-focus earthquakes, and surface-wave magnitude (Ms) for teleseismic events, each optimized for different event types and distance ranges.

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