FAQ - Earthquake Effects & Experiences

Q: How do you classify/measure the shaking that you feel during an earthquake? Sharp jolts versus rolling motion?

A: The short answer is - a rolling motion means you are probably far away from the earthquake; a sharp jolting motion means you are probably close to the earthquake.

Three factors primarily determine what you feel in an earthquake:

1. Magnitude (you feel more intense shaking from a big earthquake than from a small one; big earthquakes also release their energy over a larger area and for a longer period of time. In most cases, only 10-15 seconds of shaking that originate from the part of the fault nearest you will be very strong



2. Distance from the fault (earthquake waves die off as they travel through the earth so the shaking becomes less intense farther from the fault)



3. Local soil conditions (certain soils greatly amplify the shaking in an earthquake. Seismic waves travel at different speeds in different types of rocks. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard
   rock at the same distance from the same earthquake. The looser and thicker the soil is, the greater the amplification will be, (e.g, Loma Prieta earthquake damage area of Oakland and Marina (SF) were 100 km (60 mi) and most of the Bay Area escaped serious damage).

The ground shaking produced by an earthquake is actually very complex--hard, gentle, long, short, jerky or rolling--and not describable with one number. Motions are described by the PEAK VELOCITY (how fast the ground is moving); PEAK ACCELERATION (how quickly the speed of the ground is changing); FREQUENCY (energy is released in waves and these waves vibrate at different frequencies just like sound waves); and DURATION (how long the strong shaking lasts).

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