Geoscience Reference
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Fig. 2.9. Location ofGubbio Plain (Central Italy)and longitudinal cross-section used in
numerical analysis. The position of the digital accelerometer stationGBP isalso
displayed
A first application of the DRM approach was concerned with the 2D response analy-
sis of the Gubbio sedimentary Plain (Figure 2.9) to the main shock of the damaging
Umbria-Marche earthquake sequence of 1997 (26.09.97, 9.40h,
M
5.9), well recorded
byasingledigitalaccelerographlocatedinthemiddleofthePlain(stationGBP)at40km
epicentral distance. These analyses were intended to provide only preliminary quantita-
tive insight into the problem, since the deep structure of the Gubbio Plain has not yet
been clearly defined and ispresentlyunder study.
The source parameters and the slip distribution of the fault plane were taken from
Hernandez et al. (2004). The crustal velocity model adopted combines the description
given by the same authors, with that suggested in Mirabella et al. (2004); a smooth
decrease in the rock properties with decreasing depth was also introduced to properly
account for near-surface alteration (Cotton et al., 2006). For the valley sediments
V
s
val-
uesincreasingfrom500ms
−
1
to2000ms
−
1
werederivedfromtheinversionofRayleigh
wave dispersion curves and siteresponse estimation fromweak motions.
Shown in Figure 2.10 are the results of the numerical analysis of the longitudinal (NW-
SE) cross-section of the valley.
DRS
were computed at closely spaced receivers at the
surface and then averaged according toa subdivision of the cross-section into four zones
(see Figure 2.9). The figure shows the average
DRS
for each zone, together with the
Type I spectrum of CEN (2004, Annex A), ground category C (applicable to most valley
