Site Effects of Onna during the 2009 L’Aquila (Central Italy) Seismic Sequence: Constraints on Bedrock Depth and 1D Local Velocity Structure from Aftershock Seismograms

After the 2009 L’AquilaMw 6.1 earthquake, particular attention was paid to the large difference
of Mercalli–Cancani–Sieberg (MCS)macroseismic intensity between the nearby villages
of Onna (9.5 MCS) and Monticchio (6 MCS). Several authors estimated that in Onna, settled
in the Aterno river valley, ground motions were amplified at 2–3 Hz by up to a factor of 5
with respect toMonticchio, settled onmore competent rocks. Although there was a general
agreement that the spectral peak was caused by the resonance of the uppermost 40mlayer,
a satisfactory fit of the amplitudes was not provided. Here, we apply spectral ratio techniques
to 1437 aftershock seismograms (magnitude between 1.8 and 3.9) to compare
ground motions within Onna and between Onna and Monticchio. Spectral amplitudes at
stations located outside and inside the “red zone” of Onna show that the seismic response
was uniform, confirming that vulnerability was crucial for the heavier damage of the ancient
part of the village.We have also estimated the empirical transfer function of Onna through
the spectral ratios between Onna and Monticchio. Although in a 1D simplification, a model
with a further velocity contrast of ∼2 at 200 m of depth produces a more accurate fit of
observations. Using the new velocity profile, we modeled the mainshock ground motion at
Onna in an equivalent-linear approach. Accelerations are amplified by a factor of 2 and spectral
ordinates increase from0:7g at 0.2 s to 1g at 0.5 s, a shaking level that can be destructive
for nonductile ancient buildings. This study shows that accurate estimates of empirical transfer
functions, even in a simplified 1D approach, provide useful constraints to the deeper
velocity structure where measurements are shallow or lacking.