Instrumental Intensity & ShakeMaps
How seismometers turn ground motion into the MMI values you see on a ShakeMap, and how empirical ground motion models provide a second opinion.
What Sensors Measure
While humans observe an earthquake's effects, seismometers measure physical quantities that can be recorded objectively. Two values are especially important for estimating structural impact:
PGA: Peak Ground Acceleration
How fast the speed of the ground changes, expressed as a fraction of gravitational acceleration (%g). PGA is most relevant to short, stiff structures like low-rise buildings that respond to rapid changes in motion.
PGV: Peak Ground Velocity
The maximum speed the ground reaches during shaking, in centimeters per second (cm/s). PGV correlates better with damage to taller, more flexible structures such as mid-rise buildings and pipelines.
From Sensors to ShakeMap
Seismometers do not "see" a bookshelf fall or a chimney crack; they record acceleration and velocity waveforms. The USGS uses specific conversion algorithms called Ground-Motion–to–Intensity Conversion Equations (GMICEs) to translate PGA and PGV into MMI values.
This allows for a near-instant "snapshot" of the earthquake's impact before the first human reports even arrive. The resulting ShakeMap is a spatial grid of estimated MMI covering the affected region, interpolated from station recordings and constrained by regional ground-motion models.
Intensity Lab uses the Worden et al. (2012) GMICE to convert PGA (in %g) to MMI. ShakeMap also integrates Did You Feel It? crowdsourced reports as additional constraints, improving accuracy as more data arrive in the hours and days following an event.
The Same Scale, The Same Colors
Instrumental MMI is not a separate scale; it is the MMI scale. Seismometer-derived values and human-observation values both occupy the same I–X spectrum and use identical USGS color codes. The color you see on a ShakeMap or on this site has exactly the same meaning whether it came from a seismograph or a person's account.
| Level | Description |
|---|---|
| I | Not Felt |
| II | Weak |
| III | Weak |
| IV | Light |
| V | Moderate |
| VI | Strong |
| VII | Very Strong |
| VIII | Severe |
| IX | Violent |
| X | Extreme |
PGA/PGV ranges are approximate; values overlap between adjacent levels. Click any level for the full description.
How this relates to the observational MMI scale
Instrumental intensity values are designed to be consistent with the traditional human-observation MMI scale: a ShakeMap value of V should correspond to what people at that location would have reported on the Modified Mercalli Intensity scale. The two agree best at moderate intensities. At the highest intensities they can differ more, because few stations record the strongest shaking near the source and the conversion from ground motion to intensity is least certain there.
GMPE Comparison: A Second Opinion
Beside the ShakeMap values, Intensity Lab computes the shaking expected at each place from four NGA-West2 ground motion models. These are empirical: regressions fitted to thousands of recordings of past earthquakes, giving typical shaking for a magnitude, a distance, and the site's soil. Comparing them with ShakeMap shows where an earthquake shook more or less than usual. (Our physics-based simulations are a separate product: they drive the shaking animations.)
| Key | Authors | Year |
|---|---|---|
| ask14 | Abrahamson, Silva & Kamai | 2014 |
| bssa14 | Boore, Stewart, Seyhan & Atkinson | 2014 |
| cb14 | Campbell & Bozorgnia | 2014 |
| cy14 | Chiou & Youngs | 2014 |
Each model predicts PGA as a function of earthquake magnitude, distance, and site conditions (Vs30). Intensity Lab converts these predictions to MMI and plots them alongside the ShakeMap values on each event page. See the Computing Methods page for details on the conversion formulas and regional caveats.