Geoscience Reference
In-Depth Information
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18
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Parkfield
San Simeon
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Fig. 18.1. Dependence on azimuth of the
PGV/PGSa
ratio, as determined fromthe
observed
PGSa
-
PGV
least-squares best-fit lines along the directions defined in
the radar plot. Adapted fromPaolucci and Smerzini (2007)
Note that, since strains are calculated at the ground surface, the shear components in
the vertical plane are supposed to be either vanishing or negligible, and attention will be
focused only onthe axial components of strain(
PGSa
).
ReferringtoPaolucciandSmerzini(2007)formoredetails,weintroducehereaselection
of some representative results useful for applications. First, in Figure 18.1 the variation
with azimuth of the
C
values of the correlation (1) between
PGSa
and
PGV
, considering
κ
=
1, isshown for twoof theearthquakes inTable 18.1.
To obtain these plots, the strain and velocity components along a prescribed direction
have been evaluated at each station of the array:
C
is then calculated as the inverse of
the slopeof theleast-squares best-fitline connecting the
PGSa
and
PGV
pairsalong that
direction.Itisinterestingtonotethatthevariationwithazimuthof
C
ismoderate,ranging
typically from about 1200 to2300m/s.
AfurtherinterestingresultisillustratedinFigure18.2,wherethepeakvalueofthelargest
principalstrainasafunctionofthelargestabsolutevalueofgroundvelocityisshownfor
each earthquake and each station of the array. This selection of
PGSa
and
PGV
has been
chosentomaketheseparametersinvariantwithrespecttothereferencesystem,and,asa
consequence, to the specific direction selected.
The notable feature of Figure 18.2 is that all data tend to be aligned, with a relatively
small dispersion and irrespective of the earthquake magnitude, distance, site conditions
or prevailing wave type, along a linewithequation
log
10
PGSa
=
α
log
10
PGV
−
β,
i.e.
PGV
α
/
10
β
PGSa
=
(18.2)
