This third installment in EBI’s series on seismic risk assessments focuses on magnitude, peak ground velocity (PGV), and peak ground acceleration (PGA).

These concepts can be complex, but significant to understanding the nuances of seismic risk for particular buildings.

**Magnitude**

Beginning this post with magnitude seems natural, as it is a term most people are familiar with. Essentially, magnitude is the relative size of an earthquake, or how much energy it exerts. There are different scales available for measuring magnitude, however, the USGS recommends the Moment Magnitude Scale (MMS), which was developed to address the shortcomings of the better known Richter Scale (no longer used by seismologists). To give an idea of what the scale is, “an increase of one step corresponds to a 10^{1.5} (about 32) times increase in the amount of energy released, and an increase of two steps corresponds to a 10^{3} (1,000) times increase in energy. Thus, a magnitude 7.0 earthquake releases about 32 times as much energy as one of 6.0 and nearly 1,000 times that of 5.0.” [i]

While magnitude can be a predictor of seismic loss, scientists have found that damage to buildings and infrastructure during earthquakes relates more to *ground motion* than to magnitude itself, and there is no certain magnitude at which more damage will occur. Hence, other metrics have developed to more specifically determine building damageability – PGV and PGA.

**Peak Ground Velocity (PGV)**

To understand this concept, let’s first begin by defining velocity as the speed of an object in a given direction (e.g. 50mph to the east). In regards to an earthquake, “velocity is how fast a point on the ground is shaking as a result of an earthquake,” and the peak ground velocity is the greatest speed of shaking recorded at particular point during an earthquake. [iii]

**Peak Ground Acceleration (PGA)**

Similar to the approach above, first let’s define acceleration. Like when driving a car, the speed, or velocity, as it changes from one to another is the acceleration (faster), or deceleration (slower). The *change* in speed is the acceleration, or *how much* the velocity changes in a unit time.

When the ground is shaking during an earthquake, “it also experiences acceleration. The *peak* acceleration is the *largest* increase in velocity recorded by a particular station during an earthquake.” [iv] PGA is most typically “expressed as a fraction of gravitational acceleration, g, 32.2 ft/s2.” [v]

**Application in Seismic Risk Assessments**

One might suppose that the size of an earthquake, its magnitude, matters greatly in considering building damageability. However, building and infrastructure damage from earthquakes is more closely related to ground motion, which PGA and PGV calculate, so they are more accurate and relevant metrics to use.

PGA has been more commonly used in earthquake engineering and seismic hazard maps used for building codes because, unlike PGV, it can be related to a force, making it much simpler to use in building design. The acceleration of the seismic waves “setting the building in motion, determines the percentage of the building mass or weight that must be dealt with as a horizontal force.” [vii] PGA also is a good index in determining seismic hazard for shorter buildings (7 stories or less) as it has strong correlations when plotted against measures of demand like inter-story displacement. [viii]

That said, PGV is the better index in determining seismic hazard for taller buildings. It can help estimate macroseismic intensity and is often applied in determining liquefaction potential and in the seismic design and assessment of buried pipelines. [ix]

[i] https://en.wikipedia.org/wiki/Moment_magnitude_scale#Definition

[ii] Graphic courtesy Geocoops.com

[iii] https://earthquake.usgs.gov/learn/glossary/?term=velocity

[iv] https://earthquake.usgs.gov/learn/glossary/?term=acceleration

[v] ASTM E2026-16a

[vi] Graphic courtesy USGS.gov

[vii] https://www.fema.gov/media-library-data/20130726-1556-20490-0102/fema454_chapter4.pdf

[viii] https://earthquake.usgs.gov/hazards/learn/technical.php#accel

[ix] https://www.tandfonline.com/doi/abs/10.1080/13632460609350586