Ground Motion

Ground motion from an earthquake can be quite frightening. The Earth we live and build on becomes unstable. When it suddenly starts moving it can feel completely disorienting.

Earthquake shaking, or ground motion, is not inherently dangerous to people. Injuries and deaths happen when ground motion interacts with the built environment and human behavior. Seismologists measure and analyze ground motion to better understand the physical properties of earthquakes.

During an earthquake, Earth’s surface shakes with the passage of earthquake waves, which radiate energy that had previously been stored in rocks. An earthquake is essentially the release of built-up stress that is released when a fault breaks and the surrounding rocks slip against each other. Ground motion can be measured in terms of the earth’s surface velocity and acceleration, the frequency content of the shaking, and the duration of shaking.

Scientists typically use the Modified Mercalli Intensity (MMI) scale to measure the felt experience of ground motion. Ground motion intensity is often listed with “MMI” followed by a Roman numeral between I and X. For example, MMI II shaking will not cause damage and may be barely perceptible, while MMI VIII can cause considerable damage to buildings and knock people to the ground. The US Geological Survey's "Did You Feel It?" (DYFI) tool collects information from people around the world whenever an earthquake happens. If you feel an earthquake, you can share your experience of shaking with DYFI as a citizen scientist.

Seismic networks like PNSN use seismometers to record ground motion in a more quantitative manner. Seismic instruments record time histories of ground velocity and acceleration. Peak ground velocity (PGV) and peak ground acceleration (PGA) are especially useful measurements for structural engineers. The frequency content and duration of earthquake shaking are valuable for engineers and seismologists alike.