Seismic risk assessment of water supply systems - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 661

volumetric strain and shear distortion. The main sources of PGD include
surface faulting, tectonic uplift and subsidence, liquefaction, landslides, and
densifi cation. Surface faulting and tectonic uplift and subsidence are associ-
ated with the crustal deformation from which the ground shaking originates.
In contrast, local ground conditions are responsible for liquefaction, land-
slides, and densifi cation. Liquefaction usually occurs in loose saturated
granular soils, which transform to a liquefi ed state or condition of substan-
tially reduced shear strength when subjected to seismic loading. Inertial
forces from ground shaking may trigger mass movement of the ground, i.e.,
landslides, in the forms of rock falls, relatively shallow slumping and sliding
of soils, or relatively deep translation and rotation of soil and rock. The
earthquake-induced shaking can decrease the volume of dry or partially
saturated granular soils, resulting in ground subsidence.
24.4.3 Seismic demand on system components
For each of the 59 scenario earthquakes, strong ground motion data at 572
points were generated, and the seismic demands on the system components
in the LADWP water supply system were determined accordingly. Ground
motion contour surfaces were developed from the data at the 572 points
using local polynomial interpolation in a GIS software, and further cor-
rected for the site conditions. Then, the seismic demands on the system
components were determined according to their locations. Consider, for
example, the water pipelines, the seismic performance of which is closely
related to the peak ground velocity (PGV) they are subjected to. The PGV
was therefore utilized as the primary seismic demand parameter. The PGV
contour surfaces were interpolated from these 572 points, and site condition
corrections were applied according to the NEHRP-HAZUS procedures.
With the aid of GIS software, the spatial distribution of the LADWP pipe-
lines were superimposed on the PGV contour surfaces to determine the
seismic demand on each pipeline in accordance with its respective location.
The GIS function discretized the pipeline spatially in accordance with the
PGV contour and assigned the corresponding PGV value to each pipeline
segment subsequently. Plate III (between pages 452 and 453) shows the
LADWP pipeline system and the PGV contour surface for the Scenario
175 Verdugo earthquake. Similar procedures were applied repeatedly to
other 58 scenario earthquakes, as well as other ground motion parameters,
if necessary.
24.5 Componentresponses
The effects of TGD and PGD are evaluated for components of above-
ground and underground facilities. The performance of underground