Geoscience Reference
In-Depth Information
The MCE estimation, realized through four steps, uses a deterministic method
[DUB 03]:
− Identifying the seismic hazard zones: construction of a seismotectonic model
that includes the active faults or, if not available, the precise location of
homogeneous tectonic zones (a fault is active if some sliding took place during the
Quaternary).
− Characterizing the source zones by their “maximum seismic potential”, the
historically known major earthquake, applied at any location along the fault.
− By seismotectonic synthesis: evaluating the seismic hazard level in the site
from the different maximum seismic potentials. The maximum possible earthquake
is the most destructive earthquake that could potentially occur at the site. This level
is characterized by the epicentral intensity, the magnitude, the depth and the
spectrum. If an active fault is found at the projected location of the dam, it is
strongly recommended to change the building location. If an active fault crosses the
reservoir, it is essential to define the induced rejection in order to determine the
referential wave that could have an impact on the dam.
− Choosing the reference earthquake according to its maximum acceleration and
its spectrum. These two factors can be accessed from regional correlations
(earthquake type, distance, nature of the foundation soil). Correlations give two
types of pseudo-acceleration spectrums issued from one degree of freedom
oscillators for a 5% damping ratio: one corresponding to hard rock foundations with
shear wave velocity higher than 800 m/s; the other for altered or soft soil
foundations with shear wave velocity between 300 and 800 m/s. If the velocity is
lower than 300 m/s, it is necessary to conduct a specific analysis (site effect). It
should be noted that the most destructive earthquake for embankments is rarely the
closest earthquake or the one with the highest acceleration on site [JAN 76]. Studies
are underway to examine this aspect. In the next section, we suggest some
alternative approaches.
10.4.4.
Choiceofaccelerographs
Six methods are used, which will be discussed in the following sections.
10.4.4.1.
Empirical method
The empirical method is the most frequently used method. It consists of
choosing, in the databases of recorded accelerographs, those with a spectrum similar
to the one corresponding to the reference earthquake. The maximum acceleration of
a null period defined at the rocky substratum is then introduced in the selected
accelerographs. A third parameter has to be added in order to reduce the scatter: the
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