Earthquake Intensity (MMI) Estimator
Estimate the Modified Mercalli Intensity of earthquake shaking from peak ground acceleration, using a published empirical ground-motion relation.
🌍 What is the Modified Mercalli Intensity Scale?
Modified Mercalli Intensity (MMI) is a qualitative scale describing felt shaking and observed damage from an earthquake at a specific location, expressed in Roman numerals from I (not felt) to XII (total destruction). This estimator converts an instrumentally recorded peak ground acceleration (PGA) into an estimated MMI using the published empirical relation MMI = 3.66 x log10(PGA) - 1.66 (Wald et al., 1999).
Emergency responders, engineers, and seismologists use ground-motion-to-intensity relations like this to rapidly map expected shaking severity across a region from instrumental data alone, without waiting for the slower process of collecting felt-shaking damage reports, supporting faster post-earthquake response planning.
A common point of confusion is treating intensity as interchangeable with earthquake magnitude. They aren't, magnitude is one number describing the whole earthquake's size, while intensity is a location-specific description of shaking severity that varies with distance, local soil conditions, and building quality, the same earthquake can produce very different intensities in different places.
This calculator estimates MMI from your entered peak ground acceleration, reports both the numeric estimate and the nearest whole intensity level with its plain-language description, and plots how estimated intensity grows with PGA on a logarithmic relationship.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Moderate Shaking
PGA = 100 cm/s²
Example 2 — Very Strong Shaking
Example 3 — Light to Moderate Shaking
❓ Frequently Asked Questions
🔗 Related Calculators
What is Modified Mercalli Intensity (MMI)?
Modified Mercalli Intensity is a qualitative scale (Roman numerals I through XII) describing the felt effects and damage caused by earthquake shaking at a specific location, based on observations rather than instrument readings. Unlike magnitude, which is a single number for the whole earthquake, intensity varies by location.
What is the formula used to estimate MMI from ground motion?
This calculator uses the Wald et al. (1999) empirical relation MMI = 3.66*log10(PGA) - 1.66, where PGA is peak ground acceleration in cm/s squared (gal). It was derived by statistically relating instrumentally recorded PGA to intensity values reported by observers during California earthquakes.
What is peak ground acceleration (PGA)?
PGA is the maximum acceleration experienced by the ground during an earthquake, measured directly by seismic instruments (accelerometers). It's commonly expressed in cm/s squared (gal) or as a fraction of Earth's gravity (%g, where 1g = 981 cm/s squared).
Why does intensity vary across a region from a single earthquake?
Intensity depends on ground shaking at each specific location, which is affected by distance from the rupture, local soil and geological conditions (soft sediments amplify shaking, hard rock generally doesn't), and building construction quality, all factors that vary geographically even for a single earthquake.
How reliable is this estimator outside the calibrated MMI V to IX range?
Less reliable, the Wald et al. relation was fit primarily to observations in the moderate-to-strong shaking range (roughly MMI V through IX). This calculator flags results below MMI V or above MMI IX as extrapolated, since the original calibration data was sparser at those extremes.
What does each MMI level roughly correspond to?
Approximately: I-III not felt to weak, IV-V light to moderate (felt by most, some rattling), VI-VII strong to very strong (felt by all, minor to moderate damage), VIII-IX severe to violent (considerable to great damage), X-XII extreme (widespread destruction). This calculator reports the nearest whole intensity with its plain-language description.
Does this account for soil amplification effects?
No, this calculator estimates intensity purely from a given peak ground acceleration value using a single empirical relation. If your PGA value already reflects local site conditions (as recorded by a nearby instrument), the estimate accounts for that indirectly; it does not separately model soil amplification from a generic reference PGA.
Are there other MMI estimation formulas?
Yes, this is one of several published ground-motion-to-intensity conversion equations (GMICEs). Others use peak ground velocity (PGV) instead of PGA, or are calibrated to different regions and earthquake datasets, which can produce somewhat different intensity estimates for the same ground motion.
Is this the same as earthquake magnitude?
No, magnitude (Richter, moment magnitude, etc.) is a single number describing the earthquake's overall size at its source. Intensity describes the shaking effects at a particular location, which depends on magnitude, distance, depth, and local site conditions all together.
What units does this calculator use?
Peak ground acceleration is entered in cm/s squared (also called gal), the standard seismological unit. The result, estimated Modified Mercalli Intensity, is reported both as a decimal number and rounded to the nearest whole Roman numeral with its plain-language description.