Ground Acceleration vs. Damage

The degree of ground shaking (or damage) caused by an earthquake is often assigned a numerical value from Roman numeral I to XII on the Modified Mercalli (MM) Scale. The value is determined from interviews with people who felt the earthquake and from damage reports. Damage at a site depends on many factors such as the size of the earthquake, distance to the epicenter, soil conditions at a site, and type of building construction. The damage reports can be used to construct a MM intensity map.

Engineers prefer a measure of ground shaking that is more objective than MM intensity such as an instrumental measurement of ground acceleration via a strong motion seismograph. Until recently, only a few measurements of ground acceleration were available in the Pacific Northwest. Based on prior studies there is a rough relationship between MM intensity and acceleration as seen in the following table.