Seismic risk assessment of water supply systems - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 656

operational aspects include the pre-defi ned setting parameters for regula-
tion valves, such as pressure setting for pressure reducing valves, and con-
trols on links (pipes, valves, or pumps) for altering the operational status of
the links.
24.4 Seismichazards
Seismic hazard at a particular site can be estimated quantitatively by either
deterministic seismic hazard analysis, DSHA (Reiter 1990), in which a par-
ticular earthquake scenario is assumed, or probabilistic seismic hazard
analysis, PSHA (Cornell 1968; Reiter 1990), where uncertainties in earth-
quake magnitude, location, and occurrence time are explicitly considered.
The seismic hazard analysis, regardless of deterministic or probabilistic,
starts with identifi cation and evaluation of earthquake sources by reviewing
geologic evidence, tectonic evidence, historical seismicity, and instrumental
seismicity (Kramer 1996). Then, a recurrence relationship, which specifi es
an average rate at which an earthquake of some magnitude will be exceeded,
is used to characterize the seismicity of each source zone. The ground
motion produced at each site is determined with the aids of suitable attenu-
ation relationships, which describe the decrease of ground motion with
increasing distance. Finally, PSHA combines the uncertainties in earth-
quake location, earthquake magnitude, and attenuation relationships to
obtain the probability that a ground motion level will be exceeded during
a particular time period.
The United States Geological Survey (USGS) has adopted the PSHA
approach to all locations in the US, and prepared probabilistic seismic
hazard maps in 1996, with revised maps in 2002 and 2008 (available at the
USGS website http://earthquake.usgs.gov/hazards/products/). The seismic
hazard information for Los Angeles is available at the same web sites, and
the maps revised in 2002 are referred to as the USGS 2002 dataset. The
USGS 2002 dataset represents a consensus within the geo-scientifi c com-
munity regarding earthquake parameters that contribute to the seismic
hazards.
The USGS 2002 dataset is commonly used for site-specifi c seismic per-
formance evaluation, in which only earthquake ground motion at a site
is required and spatial correlation among earthquake ground motions
across many sites is not of concern. In contrast, seismic risk assessment of
water supply systems, which are distributed over a broad geographic area,
requires seismic hazard information at various sites across the system.
For example, seismic hazard information over an area of approximately
1200 km 2 is required in the seismic risk assessment of the LADWP water
supply system. The earthquake ground motions with the same annual