What is Peak Acceleration
What is "Peak Acceleration" or "Peak Ground Acceleration" (PGA)?
1. What is "acceleration"?
When you push on the gas pedal in your car, you experience the increase in velocity as a force pushing you back into your seat. Technically, then, acceleration is the rate of increase in velocity, that is, how much the velocity changes in a unit time. Personally, we are most aware of acceleration by the experience of an applied force.
So, consider a car increasing in speed from a stop to 60 miles an hour. 60 miles per hour is 88 feet per second. If the acceleration is uniform (constant) while the car increases speed, we could say that if the car reaches a velocity of 88 feet per second in 8 seconds, the velocity changes by 11 feet per second every second, and the acceleration is 11 feet per second per second. If the acceleration were not uniform, but started off small, achieved a maximum, and then decreased as we approached 60 miles an hour, the largest value of the acceleration would be the "peak" acceleration.
2. What do we mean by "peak" acceleration as a measure of earthquake ground motion?
A small particle attached to the earth during an earthquake will be moved back and forth rather irregularly. This movement can be described by its changing position as a function of time, or by its changing velocity as a function of time, or by its changing acceleration as a function of time.
Since any one of these descriptions can be obtained from any other, we may choose whichever is most convenient. Acceleration is chosen, because the building codes prescribe how much horizontal force building should be able to withstand during an earthquake. This force is related to the ground acceleration. The peak acceleration is the maximum acceleration experienced by the particle during the course of the earthquake motion.
