Two scales for one earthquake
This reference pairs the two systems used to describe earthquakes. Magnitude (Richter or the modern moment magnitude, Mw) measures the energy released at the source on a logarithmic scale. The Modified Mercalli Intensity (MMI) scale rates the shaking and damage felt at a location on twelve descriptive levels. A built-in estimator converts any magnitude into an approximate energy release.
How it works
Magnitude is logarithmic. Each whole-number increase represents about ten times
larger ground-motion amplitude and roughly 31.6 times more energy released, since
energy scales with 10^(1.5 M). The energy estimate uses the Gutenberg-Richter
relation:
energy_joules = 10 ^ (1.5 * M + 4.8)
tons_TNT = energy_joules / 4.184e9Mercalli works the opposite way: it is observational. Trained assessors assign a roman-numeral level from I to XII based on felt reports and structural damage, so a single quake produces a patchwork of intensities that fade with distance from the epicentre.
Magnitude versus intensity: a practical illustration
Imagine a magnitude 6.5 earthquake in a rural area of Nevada. At the epicentre, the MMI might be IX (violent shaking, significant structural damage). Fifty kilometres away, the same event might register as MMI V (felt by most people, some dishes broken). Two hundred kilometres away, some people might feel it at MMI II–III, and many would not notice it at all. The magnitude never changes — it describes the energy source — but intensity is a local experience that varies with distance, depth, and the ground you are standing on.
This dual-scale system is why news reports sometimes say “a magnitude 7.1 earthquake was felt as far away as 500 km” — they are describing the magnitude at the source and the extent of perceptible intensity at distance.
The Modified Mercalli scale at a glance
| MMI Level | Description | What you might observe |
|---|---|---|
| I | Instrumental | Not felt; detected only by instruments |
| II | Faint | Felt by few people at rest, especially on upper floors |
| III | Slight | Felt indoors; hanging objects swing; not always recognized as a quake |
| IV | Moderate | Felt indoors by many; windows and dishes rattle |
| V | Rather strong | Felt outdoors; sleepers awakened; small objects displaced |
| VI | Strong | Felt by all; some heavy furniture moves; minor wall cracks |
| VII | Very strong | Difficult to stand; moderate damage to weak structures |
| VIII | Destructive | Steering difficult; partial collapse of poor construction |
| IX | Violent | General panic; considerable damage to well-built structures |
| X | Intense | Most masonry destroyed; rails bent |
| XI | Extreme | Few structures remain standing; bridges destroyed |
| XII | Catastrophic | Total destruction; objects thrown into the air |
Why depth matters as much as magnitude
Two quakes of the same magnitude can produce very different effects depending on focal depth. A shallow quake (less than 70 km deep) concentrates its energy near the surface and typically causes stronger shaking over a smaller area. A deep quake (over 300 km) dissipates energy through more rock and may be felt over a wide area with lower peak intensity. The 1994 Northridge earthquake (Mw 6.7, depth 18 km) was shallow and caused devastating damage in Los Angeles. A 6.7 at 200 km depth in the same location would likely have been barely felt.
Tips and notes
- A magnitude 6.0 releases roughly 1,000 times the energy of a magnitude 4.0, because the energy scale uses 10^(1.5).
- Intensity depends heavily on local geology: soft sediment and reclaimed land amplify shaking relative to bedrock, so two towns equidistant from an epicentre can report very different MMI values.
- “Great” quakes (Mw 8.0 and above) happen about once a year worldwide; Mw 9.0+ events occur only every one to several decades.
- The energy figure produced by this tool is an order-of-magnitude guide, not a precise seismological yield calculation.