Component

The component of shaking for an Intensity Measure (IM).

Current options:

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. A good, brief description of this component is also given on page 7 of Campbell and Bozorgnia's (2006) NGA Report.

Random Horizontal - for a randomly chosen horizontal component.

Vertical

Greater of Two Horizontal - the largest value obtained from two perpendicular, horizontal components.