Publication Date:
2017-04-04
Description:
We have studied two velocity-depth models with the aim of outlining the behavior of a velocity reversal in the top layer, which is associated with the stiff Brecce de L’Aquila unit (BrA). In this setting, the SMTH model is topped by a
layer with about 2:1 impedance contrast with the underlying layer while the NORV model
has no velocity reversal. We have simulated the propagation of SH and P-SV wavefields in
the range 0–10 Hz for incidence 0◦ –90◦ . Earthquake spectral ratios of the horizontal and
vertical components at six sites in L’Aquila downtown are compared to corresponding syn-
thetics spectral ratios. The vertical component of P-SV synthetics enables us to investigate a
remarkable amplification effect seen in the vertical component of the recorded strong motion.
Sites AQ04 and AQ05 are best matched by synthetics from the NORV model while FAQ5
and AQ06 have a better match with synthetics spectral ratios from the SMTH model. All
simulations show this behavior systematically, with horizontal and near-horizontal incident
waves predicting the overall pattern of matches more clearly than vertical and near-vertical
incidence. The model inferences are in agreement with new geological data reporting lateral
passages in the top layer from the stiff BrA to softer sediments. Matches are good in terms
of frequency of the first amplification peak and of spectral amplitude: the horizontal compo-nents have spectral ratio peaks predominantly at 0.5 Hz in the simulations and at 0.7 Hz in the
data, both with amplitudes of 4, while the vertical component spectral ratios reach values of
6 at frequencies of about 1 Hz in both data and simulations. The vertical component spectral
ratios are very well matched using Rayleigh waves with incidence at 90◦ . The NORV model
without the velocity reversal predicts spectral ratio peaks for the horizontal components at
frequencies up to 6 Hz. The reversal of velocity acts as a low-pass frequency filter on the
horizontal components reducing the amplification effect of the sediment filled valley.
Description:
Published
Description:
761-781
Description:
4.1. Metodologie sismologiche per l'ingegneria sismica
Description:
JCR Journal
Description:
reserved
Keywords:
site effect, 2D synthetic seismograms, spectral ratios, reversal of velocity, L'Aquila
;
01. Atmosphere::01.02. Ionosphere::01.02.05. Wave propagation
;
04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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