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. 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.
Correlation with the Mercalli scale
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.
InstrumentalIntensity 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
Notable earthquakes PGV (max recorded) Mag Depth Fatalities Earthquake 318 cm/s 7.7 33 km 2,415 1999 Jiji earthquake 215 cm/s 7.8 10 km 62,013 2023 Turkey-Syria Earthquakes 183 cm/s 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
See also Arias intensity Long period ground motion Peak ground acceleration
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