Peak Acceleration
There was a moderate earthquake near Los Angeles today. I learned something new as a result.
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. –USGS
The strongest earthquake I experienced was the 2001 Nisqually earthquake, which measured 6.8 on the moment magnitude scale and a peak ground acceleration on Beacon Hill in Seattle of 13.5 % g. By comparison, the 1994 Northridge earthquake measured a peak ground accelleration of >109 % g, the strongest measured of any urban quake in the U.S.