Seismic risk assessment of water supply systems - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 657

probability of exceedance differ, but are correlated at various sites over
the LADWP service area. The USGS 2002 dataset is for the aggregated
hazard at a particular site associated with potentially thousands of earth-
quakes. If it was used simultaneously at a number of different sites dis-
tributed throughout a functioning system, it would overestimate the
earthquake effects on system performance because the spatial correlation
is ignored that exists among different sites when affected by the same
earthquake.
One potential approach to account for the spatial correlation is to simu-
late water supply system performance for a comprehensive set of scenario
earthquakes, each of which is associated with an annual frequency of occur-
rence. Each scenario earthquake represents one branch of the logic trees
in PSHA, and denotes a discrete choice of input parameters and models.
However, since detailed models of a large lifeline system are often quite
complex, it may not be practical to simulate the system response for an
exhaustive set of scenario earthquakes (which may contain thousands of
scenario earthquakes for high seismic hazard areas, such as Los Angeles).
This raises a challenge on how to account for the large number of scenario
earthquakes omitted and their contribution to the probabilistic hazard and
risk analysis results.
Chang et al. (2000) proposed an approach in which a number of scenario
earthquakes are chosen based on their combined contribution to the aggre-
gate hazard, and the contribution of omitted earthquakes is accounted for
by optimizing the annual frequencies of the earthquakes. Using this
approach, a new set of event annual frequencies is sought such that the
relevant ground shaking hazards computed from the selected set at the
principal sites of interest match the target probabilistic hazards, such as
those associated with the USGS 2002 dataset.
24.4.1 LADWP seismic hazard characterizations using 59
scenario earthquakes
An approach was developed by Lee et al. (2005), similar to the one pro-
posed by Chang et al. (2000), to characterize the seismic hazards in the
LADWP water supply system by choosing a suite of 59 scenario earth-
quakes so that their optimized annual frequencies of occurrence match the
USGS 2002 probabilistic seismic hazards in the LADWP service area.
Figure 24.2 shows spatial distribution of the faults for the 59 scenario earth-
quakes, together with the LADWP water supply system. Table 24.2 sum-
marizes characteristics of the 59 scenario earthquakes.
The seismic hazard associated with the 59 scenario earthquakes was
adjusted to match that of the USGS 2002 dataset over the LADWP water
supply system by a multivariate, nonlinear optimization process. The 59