FEMA 302. The concerns of the design profession
GROUND MOTION AND
regarding the probabilistic maps included:
GEOLOGICAL HAZARDS ASSESSMENT
(1) The 10 percent probability of exceedance
in 50 years ground motion generally used as a basis
Specification of Ground Motion.
of seismic codes did not adequately capture the
hazard due to large, but infrequent, events in some
General. This document prescribes two
areas of the eastern and central U.S.
ground motions: Ground Motion A and Ground
Motion B, as defined in the following paragraphs.
Probabilistic values near major active
The ground motions are expressed in terms of
faults tended to be very high because of the high
spectral ordinates at 0.20 sec (SDS) and 1.0 sec (SD1).
rates of activity.
These spectral values are derived from various
Probabilistic values in some areas that
Geological Survey (USGS) and the Building Seismic
appeared to be unreasonably low could be attributed
Safety Council (BSSC) of the National Institute of
to lack of sufficient data regarding source zones and
Building Sciences (NIBS).
USGS Seismic Hazard Maps. At the request
To address these concerns, the Design Values Group
of the BSSC, USGS prepared probabilistic spectral
developed the MCE maps for spectral ordinates at
acceleration maps for ground motions with 10
0.2 sec (denoted as SS) and 1.0 sec (denoted as S1).
These maps are generally based on the USGS
exceedance in 50 years. For each of these ground
probabilistic maps for ground motion with 2 percent
motions, probabilistic spectral ordinate maps were
2,500-year return period), but with deterministic
values near major active faults and higher threshold
Additionally, deterministic spectral ordinate maps
values in selected areas of low seismicity. As
were developed for areas adjacent to major active
indicated below, the design spectral ordinates were
selected as two-thirds of the site-adjusted MCE
values. The traditional seismic risk level considered
Maximum Considered Earthquake (MCE)
by most model building codes is 10 percent
Maps. In response to concerns regarding the use of
the USGS maps by the building design professions,
of about 500 years). Because the value of the ground
BSSC convened a nation-wide Design Values Group
motion for other risk levels is a function of the shape
to review the maps and prepare design values for
of the site-specific hazard curve, a valid comparison
of the ground motion specified by prior codes with