**4. Ground motion modeling**

0 200 400 600 800 1000 1200 Vs (m /s)

> H edgehog solutions Fixed V <sup>S</sup> C H profile

5 0

4 0

3 0

Depth (m)

Al

M a

(a) (b)

**Figure 6.** a) Rayleigh waves group velocity dispersion curves of fundamental mode extracted by FTAN method from signals of active seismic experiment at Coppito (location in Fig. 1); (b) VS velocities obtained from the non-linear inver‐ sion (Hedgehog method) of the average dispersion curve (a) as compared with Cross-Hole (CH) measurements at the AQV station site [22] (location in Fig. 1); (c) Comparison between the resonance frequency estimated from the spectral ratio H/V of the main shock recorded at the AQV station and the 1D amplifications [24] computed by assuming the VS

0 2 4 6 8 1 0 Frequency (H z)

(c)

0

0.00 0.10 0.20 Period (s)

profiles in (b). Legend: Al = Alluvial soils; Ma = Maiolica.

C oppito 64 m offset A verage

Group

velocity (m/s)

200

100

300

2

4

A

m

plific

ation

6

8

1 0

90 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

2 0

1 0

0

AQ V station Spectral ratio H /V m ain shock 1D w ith VS m easurem ents at C oppito 1D w ith VS cross-hole m easurem ents Simulations of the 2009 L'Aquila main shock have been performed with the Neo-Deterministic Seismic Hazard Analysis (NDSHA), an innovative modeling technique that takes into account source, propagation and local site effects [8,9].

This approach uses a hybrid method consisting of modal summation and finite difference methods. The path from the source up to the region containing the 2-D heterogeneities is represented by a 1-D layered anelastic structure. The resulting wavefield for both SH- and P-SV- waves is then used to define the boundary conditions to be applied to the 2-D anelastic region where the finite difference technique is used. Synthetic seismograms of the vertical, transverse and radial components of ground motion are computed at a predefined set of points at the surface. Spectral amplifications are computed as response spectra ratios, RSR, i.e. the response spectra computed from the signals synthesized along the laterally varying section (2D) normalized by the response spectra computed from the corresponding signals, synthe‐ sized for the bedrock (1D). A scaled point-source approximation ([27] as reported in [28]) has been considered to scale the seismogram to the desired scalar seismic moment.

Modeling of the main shock has been done along a geological cross section at L'Aquila, from the epicentre through the AQK station (Fig. 7).

**Figure 7.** Location of the geological cross section from the main shock (star)[1] to L'Aquila town, passing through the AQK recording station. A new interpretation of gravity data has been performed along the blu lines (modified from [10]).

station, realistically assumed on the basis of geological considerations, in order to fit the observed response spectra and the frequency of the main peak of the H/V spectral ratio obtained from the main shock recording at the AQK station (Fig. 9). A good fit results between observed and synthetic response spectra despite the fictitious greater distance (6 km) of the

0

Transverse com ponent

0 1 2 3 4 Period (s)

**Figure 9.** Comparison at the AQK station of the computed and recorded main shock: (top) H/V spectral ratio; (bot‐

Acceleration time series for P-SV and SH-waves, including surface waves, have been computed along the cross section (Fig. 10), using a grid spacing of 3 m since the lowest seismic velocity is 200 m/s and 10 points are requested for a good sampling of the minimum wavelength corresponding to 7 Hz frequency. They are shown at an array of sites along the cross section, with 50 m spacing. The alluvial and colluvial soil cover is responsible for the amplification of the peak values and duration of accelerations along the radial and transverse components.

Spectral amplifications computed for the vertical component of ground motion show maximum values of 10 at frequencies lower than 1 Hz, in correspondence of the layer of megabreccias and of about 5 at 4 Hz in correspondence of the Aterno river alluvial sedi‐ ments (Fig. 11). As regards the radial and transverse components, spectral amplifications

2

Spectral ratios H/V

4

6

0 2 4 6 Frequency (H z)

VS Crustal Models and Spectral Amplification Effects in the L'Aquila Basin (Italy)

0 1 2 3 4 Period (s)

Vertical com ponent

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cross section in the computation of the ground motion for a point source.

0 1 2 3 4 Period (s)

This amplification is higher along the vertical component.

tom) response spectra computed for 5% damping.

R adial com ponent

A Q K station R ecorded C om puted

0

200

Response

spectra (cm/s2

)

400

600

**Figure 8.** Computing cross section at L'Aquila through the AQK station (location in Fig. 7) with the physical parame‐ ters attributed to lithotypes.

Along the cross section, the outcropping units are represented by megabreccias except in the Aterno river, where recent fluvial sediments are present. At engineering scale, after the 2009 seismic sequence, geological and geophysical studies, beside several drillings with down-hole tests generally reaching depths around 25-30 m, have been performed at L'Aquila to recon‐ struct the shallow 200-300 m of subsoil [10]. The new interpretation of available gravity data indicated a meso-cenozoic carbonate unit (average density=2.6 g/cm3 ) lying at maximum depth of 200-300 m in the Aterno valley, below flysch or breccia unit (average density=2.4 g/cm3 ) and Quaternary products including alluvial and lacustrine deposits and fan alluvial material (average density=1.9 g/cm3 ). Strong lateral and vertical geological heterogeneities have been evidenced which, in the center of L'Aquila, are mainly due to a discontinuous stiff top layer of breccias, called megabreccias, overlying soft lacustrine sediments. The geometry of the vertical and lateral passage of the lacustrine to megabreccia deposits is still poorly known due to the shallow drillings. Taking into account the available geological cross sections [10], a computing cross section has been prepared (Fig. 8). We have attributed VS of 0.2 km/s to the thin silt deposits according to surface measurements at Coppito (Fig. 6). Taking into account the VS profile obtained for the path 6 (Fig. 3), we have assigned velocities of 0.9 km/s both to megabreccias and sandstones, and of 1.45 km/s both to marls and calcarenites. We are in agreement with [17] as regards velocities of megabreccias, instead a strong discrepancy results for velocities of calcarenites (our 1.45 km/s against 2.5 km/s). Literature VS [17] have been attributed to Aterno river recent deposits (colluvium and alluvial deposits) and to upper and lower lacustrine soils.

The 1-D reference model has been chosen according to the regional model [11]. A parametric study has been done for the best dip angle between 43° and 60°. A dip of 56° turned out to be able to fit the observed response spectra and is in agreement with the geometry of the seismogenic fault [29]. A small valley of colluvium had to be hypothesized beneath the AQK station, realistically assumed on the basis of geological considerations, in order to fit the observed response spectra and the frequency of the main peak of the H/V spectral ratio obtained from the main shock recording at the AQK station (Fig. 9). A good fit results between observed and synthetic response spectra despite the fictitious greater distance (6 km) of the cross section in the computation of the ground motion for a point source.

**Figure 8.** Computing cross section at L'Aquila through the AQK station (location in Fig. 7) with the physical parame‐

Along the cross section, the outcropping units are represented by megabreccias except in the Aterno river, where recent fluvial sediments are present. At engineering scale, after the 2009 seismic sequence, geological and geophysical studies, beside several drillings with down-hole tests generally reaching depths around 25-30 m, have been performed at L'Aquila to recon‐ struct the shallow 200-300 m of subsoil [10]. The new interpretation of available gravity data

of 200-300 m in the Aterno valley, below flysch or breccia unit (average density=2.4 g/cm3

Quaternary products including alluvial and lacustrine deposits and fan alluvial material

evidenced which, in the center of L'Aquila, are mainly due to a discontinuous stiff top layer of breccias, called megabreccias, overlying soft lacustrine sediments. The geometry of the vertical and lateral passage of the lacustrine to megabreccia deposits is still poorly known due to the shallow drillings. Taking into account the available geological cross sections [10], a computing cross section has been prepared (Fig. 8). We have attributed VS of 0.2 km/s to the thin silt deposits according to surface measurements at Coppito (Fig. 6). Taking into account the VS profile obtained for the path 6 (Fig. 3), we have assigned velocities of 0.9 km/s both to megabreccias and sandstones, and of 1.45 km/s both to marls and calcarenites. We are in agreement with [17] as regards velocities of megabreccias, instead a strong discrepancy results for velocities of calcarenites (our 1.45 km/s against 2.5 km/s). Literature VS [17] have been attributed to Aterno river recent deposits (colluvium and alluvial deposits) and to upper and

The 1-D reference model has been chosen according to the regional model [11]. A parametric study has been done for the best dip angle between 43° and 60°. A dip of 56° turned out to be able to fit the observed response spectra and is in agreement with the geometry of the seismogenic fault [29]. A small valley of colluvium had to be hypothesized beneath the AQK

). Strong lateral and vertical geological heterogeneities have been

) lying at maximum depth

) and

indicated a meso-cenozoic carbonate unit (average density=2.6 g/cm3

92 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

ters attributed to lithotypes.

(average density=1.9 g/cm3

lower lacustrine soils.

**Figure 9.** Comparison at the AQK station of the computed and recorded main shock: (top) H/V spectral ratio; (bot‐ tom) response spectra computed for 5% damping.

Acceleration time series for P-SV and SH-waves, including surface waves, have been computed along the cross section (Fig. 10), using a grid spacing of 3 m since the lowest seismic velocity is 200 m/s and 10 points are requested for a good sampling of the minimum wavelength corresponding to 7 Hz frequency. They are shown at an array of sites along the cross section, with 50 m spacing. The alluvial and colluvial soil cover is responsible for the amplification of the peak values and duration of accelerations along the radial and transverse components. This amplification is higher along the vertical component.

Spectral amplifications computed for the vertical component of ground motion show maximum values of 10 at frequencies lower than 1 Hz, in correspondence of the layer of megabreccias and of about 5 at 4 Hz in correspondence of the Aterno river alluvial sedi‐ ments (Fig. 11). As regards the radial and transverse components, spectral amplifications of 2-3 are computed for a wide frequency range (1-7 Hz), along the cross section. Taking into account that the majority of the buildings, generally 2-5 floor, at the historical center of L'Aquila suffered serious damage, we can argue that structures lying on soils suffered amplifications of 2-3 along the horizontal components and up to 5 along the vertical component of the ground motion.

**Figure 10.** Acceleration time series for SH and P-SV waves computed for the 2-D structural model.

**Figure 11.** Spectral amplifications (RSR 2D/1D) along the cross section at L'Aquila. Response spectra are computed for

VS Crustal Models and Spectral Amplification Effects in the L'Aquila Basin (Italy)

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95

5% damping. From the top vertical, radial and transverse components of the computed ground motion.

of 2-3 are computed for a wide frequency range (1-7 Hz), along the cross section. Taking into account that the majority of the buildings, generally 2-5 floor, at the historical center of L'Aquila suffered serious damage, we can argue that structures lying on soils suffered amplifications of 2-3 along the horizontal components and up to 5 along the vertical

**Figure 10.** Acceleration time series for SH and P-SV waves computed for the 2-D structural model.

component of the ground motion.

94 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Figure 11.** Spectral amplifications (RSR 2D/1D) along the cross section at L'Aquila. Response spectra are computed for 5% damping. From the top vertical, radial and transverse components of the computed ground motion.
