In seismology, strong ground motion is the strong earthquake shaking that occurs close to (less than about 50 km from) a causative fault. The strength of the shaking involved in strong ground motion usually overwhelms a seismometer, forcing the use of accelerographs (or strong ground motion accelerometers) for recording. The science of strong ground motion also deals with the variations of fault rupture, both in total displacement, energy released, and rupture velocity.
As seismic instruments (and accelerometers in particular) become more common, it becomes necessary to correlate expected damage with instrument-readings. The old Modified Mercalli intensity scale (MM), a relic of the pre-instrument days, remains useful in the sense that each intensity-level provides an observable difference in seismic damage.
After many years of trying every possible manipulation of accelerometer-time histories, it turns out that the extremely simple peak ground velocity (PGV) provides the best correlation with damage.[1] [2] PGV merely expresses the peak of the first integration of the acceleration record. Accepted formulae now link PGV with MM Intensity. Note that the effect of soft soils gets built into the process, since one can expect that these foundation conditions will amplify the PGV significantly.
"ShakeMaps" are produced by the United States Geological Survey, provide almost-real-time information about significant earthquake events, and can assist disaster-relief teams and other agencies.[3]
The United States Geological Survey created the Instrumental Intensity scale, which maps peak ground velocity on an intensity scale comparable to the felt Mercalli scale. Seismologists all across the world use these values to construct ShakeMaps.
| Instrumental Intensity | Velocity (cm/s) | Perceived shaking | Potential damage | |
|---|---|---|---|---|
| I | < 0.0215 | Not felt | None | |
| II–III | 0.135 – 1.41 | Weak | None | |
| IV | 1.41 – 4.65 | Light | None | |
| V | 4.65 – 9.64 | Moderate | Very light | |
| VI | 9.64 – 20 | Strong | Light | |
| VII | 20 – 41.4 | Very strong | Moderate | |
| VIII | 41.4 – 85.8 | Severe | Moderate to heavy | |
| IX | 85.8 – 178 | Violent | Heavy | |
| X+ | > 178 | Extreme | Very heavy |
| PGV (max recorded) | Mag | Depth | Fatalities | Earthquake | |
|---|---|---|---|---|---|
| 318 cm/s | 7.7 | 33 km | 2,415 | 1999 Jiji earthquake | |
| 215 cm/s[4] | 7.8 | 10 km | 62,013 | 2023 Turkey-Syria Earthquakes | |
| 183 cm/s[5] | 6.7 | 18.2 km | 57 | 1994 Northridge earthquake | |
| 170 cm/s | 6.9 | 17.6 km | 6,434 | 1995 Great Hanshin earthquake | |
| 152 cm/s | 6.6 | 10 km | 11 | 2007 Chūetsu offshore earthquake | |
| 147 cm/s | 7.3 | 1.09 km | 3 | 1992 Landers earthquake | |
| 145 cm/s | 6.6 | 13 km | 68 | 2004 Chūetsu earthquake | |
| 138 cm/s | 7.2 | 10.5 km | 356 injured | 1992 Cape Mendocino earthquakes | |
| 117.41 cm/s | 9.1 | 29 km | 19,747 | 2011 Tohoku earthquake and tsunami | |
| 108 cm/s | 7.8 | 8.2 km | 8,857 | April 2015 Nepal earthquake | |
| 38 cm/s | 5.5 | 15.5 km | 0 | 2008 Chino Hills earthquake | |
| 20 cm/s (est) | 6.4 | 10 km | 115-120 | 1933 Long Beach earthquake |
Sources