Geoscience Reference
In-Depth Information
(BoozAllen&HamiltonInc.,1999).Repair costsforbridges wereprovided byCaltrans
(personal communications).
3.1. HAZARD ASSESSMENT
The seismicity in the San Francisco Bay Area is dominated by the San Andreas and
Haywardfaults.Magnitudes,theirfrequencyofoccurrenceandrupturelocationsarewell
documented in a recent report by USGS (2003). For the purposes of our application,
earthquakes of moment magnitude,
M
w
= 6.75 are considered to be appropriate lower
threshold. The upper threshold values are 8.0 and 7.5 for the San Andreas and Hayward
faults,respectively(USGS2003).Consideringvariousrupturelocationsalongeachfault,
a total of 56 scenario events are identified and used in the risk assessment (see Stergiou
and Kiremidjian, 2006 for further detail).
The Boore et al. (1997) ground motion attenuation model is used to predict site ground
motions.ForthatpurposethelocalsoilconditionsareassessedaccordingtotheCalifornia
Geological Survey (CGS). Ground motions are estimated at each bridge site in the
network system with corresponding annual rate of occurrence (i.e. an
IM
value with a
rate
ν
IM
)
.
Information on liquefaction susceptibility is obtained from the U.S. Geological Survey
Open File Report 00-444 (USGS, 2000) and the methodology for liquefaction and land-
slide analysis provided in HAZUS (1999) is used to estimate liquefaction and landslide
ground deformations at bridge locations.
3.2. DAMAGEASSESSMENT
There is a significant ongoing effort to develop bridge fragility functions; however, none
of these functions provide a comprehensive description of all bridge classes to enable
complete application to a region. Thus, the damage functions provided in HAZUS are
usedinthispaperforillustrativepurposes.Theconclusionwillnotchangewithdifferent
fragilityfunctions,althoughtheabsolutevaluesoflossmay.IntheHAZUSmethodology,
bridges can be in one of five damage states defined as: none, slight, moderate, extensive
and complete. The probability of being or exceeding a damage level is characterized
through a cumulative lognormal distribution conditional on the hazard level,
IM
. Such
fragility functions are available for each of the 28 bridge classes inHAZUS.
3.3. STRUCTURAL LOSS
The loss from damage to bridges from each scenario event is estimated by multiplying
the expected damage state of a bridge by its replacement value. Figure 19.2 shows the
lossfromdamagetobridgesduetogroundshaking,liquefactionandlandslidesresulting
from the potential occurrence of events on the San Andreas Fault. Similar results were
also obtained for the scenario events on the Hayward fault and can be found in Stergiou
