Shaking Component

Average Horizontal

Usually defined as the geometric average of the maximum of the two horizontal components (which may not occur at the same time).

Average Horizontal (GMRotI50)

Because Average Horizontal, above, is dependent on seismometer orientation, this was defined by Boore et al. (2006, Bull. Seism. Soc. Am. 96, 1502-1511) as an orientation-independent alternative. All the NGA Relationships use this measure. See also page 7 of Campbell and Bozorgnia's (2006) NGA Report.

Random Horizontal

A randomly chosen horizontal component.


The vertical component.

Greater of Two Horizontal

The largest value obtained from two perpendicular, horizontal components.