**3.1. Data analysis**

The 2009 seismic sequence was recorded by Rete Accelerometrica Nazionale (RAN) network, managed by the Italian Civil Defense Department, some of which located at L'Aquila (AQK station) or in the NW of it (AQG, AQA, AQM, AQV), and by the station AQU operating since 1988 as part of the Mediterranean Network (MedNet), managed by the Italian Istituto Nazio‐ nale di Geofisica e Vulcanologia (INGV). They are equipped with three-component acceler‐ ometers set to 1 or 2 g full-scale, coupled with high resolution digitizers, while AQU is also equipped with a very broadband Streckeisen STS-1.

In order to obtain VS models for the L'Aquila basin shallow crust, we have analysed about four‐ ty earthquakes (ML ≥ 2.9) recorded at the RAN and AQU stations, and rotated to get the radial and transverse component of motion. Rayleigh wave group velocities of the fundamental mode have been measured from vertical and radial components of 17 events (Fig. 1, Table 1).

As regards the shallow 30 m subsoil, the same analysis has been applied to recordings of an active seismic experiment performed in the Coppito area, about 500 m far from the AQV station (Fig. 1).

The group velocity is measured as function of period by the Frequency Time Analysis (FTAN) on single waveforms (e.g. [12,13,14]). The FTAN method allows to isolate the different phases in a seismogram, in particular the fundamental mode of surface waves. A system of narrowband Gaussian filters is employed, with varying central frequency, that do not introduce phase distortion and give the necessary resolution in the time-frequency domain. The source-receiver distance is commonly assumed to be the epicenter distance when it is much greater than the event depth. When this assumption is not valid, in order to extract the correct dispersion curve of Rayleigh waves we have to add a time delay to seismograms as Δt=h/VP, h being the source depth, VP the average P-wave velocity from surface to the hypocenter, and then analyze the seismograms by FTAN, considering the hypocenter distance (e.g. [19] and references therein).

The dispersion curves obtained in such a way can be inverted to determine S-wave velocity profiles versus depth. A non-linear inversion is made with the Hedgehog method ([20,14] and references therein) that is an optimized Monte Carlo non-linear search of velocity-depth distributions. In the inversion, the unknown Earth model is replaced by a set of parameters and the definition of the structure is reduced to the determination of the numerical values of these parameters. In the elastic approximation, the structure is modeled as a stack of N homogeneous isotropic layers, each one defined by four parameters: VP (dependent parame‐ ter), density (fixed parameter), VS and thickness (independent parameters).

**Events Time origin ML Depth Lat. (°N) ; Long. (°E) Path**

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04/06/09 01:32:41 5.9 90 42.348 ; 13.380 6 04/07/09 09:26:28 4.8 9.6 42.336 ; 13.387 7 04/07/09 21:34:29 4.3 9.6 42.364 ; 13.365 8 04/09/09 13:19:33 4.1 9.7 42.341 ; 13.259 5 04/09/09 20:47:01 3.3 10.8 42.495 ; 13.321 12 04/09/09 20:40:06 3.8 11.1 42.477 ; 13.312 12 04/11/09 19:53:53 3.0 90 42.336 ; 13013 3 04/13/09 21:14:24 5.0 9.0 42.498 ; 13.377 1-11 04/14/09 07:36:44 2.9 9.6 42.495 ; 13.395 10 04/20/09 11:43:06 3 9.7 42.272 ; 13002 9 04/24/09 22:51:29 3 10.6 42.265 ; 13005 9 05/05/09 18:03:41 3.2 9.7 42.268 ; 13001 9 05/14/09 06:30:22 30 90 42.483 ; 13.397 10 06/22/09 20:58:40 4.6 13.8 42.445 ; 13.354 2 07/23/09 22:37:33 3.1 10.1 42.250 ; 13.495 4 07/31/09 11:05:40 3.8 9.6 42.248 ; 13.495 4 08/31/09 14:09:10 3.1 9.5 42.246 ; 13.515 4

Given the error of the experimental phase and/or group velocity data, it is possible to compute the resolution of the parameters (parameter step), computing partial derivatives of the dispersion curve with respect to the parameters to be inverted ([14] and references therein). The theoretical phase and/or group velocities computed during the inversion with normalmode summation are then compared with the corresponding experimental ones and the models are accepted as solutions if their difference, at each period, is less than the measurement errors and if the r.m.s. (root mean square) of the differences, at all considered periods, is less than a chosen quantity (usually 60–70% of the average of the measurement errors). Being the parameter step indicative of the parameter resolution, all the solutions of the Hedgehog inversion differ by no more than ±1 step from each other. A good rule of thumb is that the number of solutions is comparable with the number of the inverted parameters. From the set of solutions, we accept as representative solution the one with r.m.s error closest to the average r.m.s error of the solution set, and hence reduce, at the cost of loosing in resolution, the projection of possible systematic errors [20] into the structural model. Other selection criteria

Dispersion curves of Rayleigh wave fundamental mode have been extracted from the vertical and/or radial components of recordings of 17 events (Table 1, Fig. 1). Average dispersion curves have been computed along 12 paths and inverted with Hedgehog method [20,14] to get VS models. A VP/VS ratio equal to 1.8 turned out to be a suitable value after a set of tests made varying it between 1.8 and 2.1. In other words, keeping all other values of the parameterization unchanged, the number of solutions maximizes for VP/VS=1.8. Moreover, the analysis of group velocity derivatives with respect to elastic parameters versus depth has allowed to define the sensitivity of the investigated periods on the S-wave velocity structure. Time corrections have

**(UTC) (km)**

**Table 1.** List of the studied events and paths. The source parameters are from INGV (http://

bollettinosismico.rm.ingv.it) and for the 04/06/2009 main shock from [1].

of the representative solution are discussed in detail by [21].

**3.2. Results**

**Figure 1.** Simplified geological map of L'Aquila basin (modified after [18]) with location of the VS investigated paths connecting the analyzed events (blue circles) and the recording INGV stations (triangles), and of the deep drilling at Campotosto (red circled cross). April 2009 mainshock (MW = 6.3) epicenter is represented by star. Detail of the geologi‐ cal map at L'Aquila is shown in the bottom with the location of the VS investigated paths and the site of the active seismic experiment (red circle).

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**Table 1.** List of the studied events and paths. The source parameters are from INGV (http:// bollettinosismico.rm.ingv.it) and for the 04/06/2009 main shock from [1].

Given the error of the experimental phase and/or group velocity data, it is possible to compute the resolution of the parameters (parameter step), computing partial derivatives of the dispersion curve with respect to the parameters to be inverted ([14] and references therein). The theoretical phase and/or group velocities computed during the inversion with normalmode summation are then compared with the corresponding experimental ones and the models are accepted as solutions if their difference, at each period, is less than the measurement errors and if the r.m.s. (root mean square) of the differences, at all considered periods, is less than a chosen quantity (usually 60–70% of the average of the measurement errors). Being the parameter step indicative of the parameter resolution, all the solutions of the Hedgehog inversion differ by no more than ±1 step from each other. A good rule of thumb is that the number of solutions is comparable with the number of the inverted parameters. From the set of solutions, we accept as representative solution the one with r.m.s error closest to the average r.m.s error of the solution set, and hence reduce, at the cost of loosing in resolution, the projection of possible systematic errors [20] into the structural model. Other selection criteria of the representative solution are discussed in detail by [21].

#### **3.2. Results**

**Figure 1.** Simplified geological map of L'Aquila basin (modified after [18]) with location of the VS investigated paths connecting the analyzed events (blue circles) and the recording INGV stations (triangles), and of the deep drilling at Campotosto (red circled cross). April 2009 mainshock (MW = 6.3) epicenter is represented by star. Detail of the geologi‐ cal map at L'Aquila is shown in the bottom with the location of the VS investigated paths and the site of the active

82 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

seismic experiment (red circle).

Dispersion curves of Rayleigh wave fundamental mode have been extracted from the vertical and/or radial components of recordings of 17 events (Table 1, Fig. 1). Average dispersion curves have been computed along 12 paths and inverted with Hedgehog method [20,14] to get VS models. A VP/VS ratio equal to 1.8 turned out to be a suitable value after a set of tests made varying it between 1.8 and 2.1. In other words, keeping all other values of the parameterization unchanged, the number of solutions maximizes for VP/VS=1.8. Moreover, the analysis of group velocity derivatives with respect to elastic parameters versus depth has allowed to define the sensitivity of the investigated periods on the S-wave velocity structure. Time corrections have been applied to the recordings by assuming the regional average VP computed from the VS model relative to the cell 1°x1° containing L'Aquila [11], assuming a VP/VS ratio of 1.8. Such value is in very good agreement with those used by INGV (5 km/s) and [1] (VP=5.5 km/s) for earthquake location and attributed to the shallow 11.1 km of crust.

horizon has VS of ~2.8 km/s (VP ~5 km/s) at depths of 1-2.5 km and of ~3 km/s (VP ~5.5 km/s)

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**Figure 2.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for northern paths 1-2-10-11-12. Z is the vertical component and R is the radial component of the rotated recorded events (Table 1). For each path, the chosen solution is represented by the solid bold red line and is shown in the table with the uncertainty for the inverted parameters. The thickness marked by \* is not a truly inverted parameter, but it satisfies the condition that the total thickness from the free surface to the top of the first fixed layer is equal to a predefined quantity. Bottom: VS pattern below

the cross section A through the middle points of the paths 12-1-11-10 (located on the left).

at 4 km of depth (detected only below the path 1).

In the following, the results (dispersion data and Hedgehog VS solutions) are presented by grouping the paths as northern paths (1-2-10-11-12), middle Aterno river valley paths (6-7-8), middle paths (3-5), and southern paths (4-9). The events are listed in Table 1 and the paths are located in Fig. 1. Moreover, the results obtained with the same methods from an active seismic experiment in the Coppito area, about 500 m far from the AQV station, are presented.

#### *3.2.1. Northern paths*

All the results obtained for the northern paths (1-2-10-11-12) are shown in Fig. 2. The dispersion curves relative to path 1 have been extracted from the recordings of the 4/13/2009 event, located nearby Campotosto, at AQG, AQV and AQM stations. Rayleigh data, sampled at periods of 1-3.8 s, have been inverted in VS models of the shallow 6 km of crust. The representative solution is characterized by velocities increasing from 0.9 to 3.0 km/s at 3.9 km of depth.

The path 2 is relative to the recording of the 6/22/2009 event at AQK station. Dispersion curves have been extracted at periods of 0.5-1.6 s and VS models of the shallow 2 km have been retrieved. The representative solution is characterized by velocities increasing from 0.9 to 2.7 km/s at 1.3 km of depth.

The dispersion curves relative to path 10 have been extracted from the recordings of 2 events at AQU station. Rayleigh data have been sampled in the 0.4-1.2 s period range and the VS models are relative to the shallow 2 km. The representative solution is characterized by shear velocities increasing from 0.9 km/s to 2.2 km/s at 0.8 km of depth.

The dispersion curves along the path 11 have been extracted from the radial and vertical components of the 04/13/09 event recording at AQU station. The average dispersion curve is defined in the 1.0-2.5 s period range and VS models of the shallow 3 km have been retrieved from it. The representative solution shows velocities increasing from 0.9 km/s to 2.9 km/s at 1.8 km of depth.

The average dispersion curve along the path 12 has been computed from the dispersion curves extracted from the vertical components of 2 earthquakes on 04/09/09 and is defined at periods between 0.7 and 1.2 s. The retrieved VS models are relative to the shallow 2 km and the represen‐ tative solution is characterized by velocities increasing from 1.1 to 2.8 km/s at 1 km of depth.

The interpretation of the obtained VS models, attributed beneath the middle points of the paths (bottom of Fig. 2), is performed by taking into account available geological data [18]. Out‐ cropping rocks along the cross section A through the paths 12-1-11-10 consist of limestone and marls, which are found with a thickness of ~1 km in the Campotosto drilling (located in Fig. 1), below a 1.2 km thick layer of clays and sandstones. The Mesozoic carbonate horizon is found at the well bottom (2.45 km). VS range between 0.9 and 1.5 km/s in the shallowest 100-200 m of weathered rocks, and reach the value of 2.5 km/s at depths between 0.6 and 1.7 km. The velocity of 2.5 km/s can be reasonably attributed to fractured limestone rocks and, based on the Campotosto drilling stratigraphy, to the top of the Mesozoic limestone horizon. Such horizon has VS of ~2.8 km/s (VP ~5 km/s) at depths of 1-2.5 km and of ~3 km/s (VP ~5.5 km/s) at 4 km of depth (detected only below the path 1).

been applied to the recordings by assuming the regional average VP computed from the VS model relative to the cell 1°x1° containing L'Aquila [11], assuming a VP/VS ratio of 1.8. Such value is in very good agreement with those used by INGV (5 km/s) and [1] (VP=5.5 km/s) for

In the following, the results (dispersion data and Hedgehog VS solutions) are presented by grouping the paths as northern paths (1-2-10-11-12), middle Aterno river valley paths (6-7-8), middle paths (3-5), and southern paths (4-9). The events are listed in Table 1 and the paths are located in Fig. 1. Moreover, the results obtained with the same methods from an active seismic experiment in the Coppito area, about 500 m far from the AQV station, are presented.

All the results obtained for the northern paths (1-2-10-11-12) are shown in Fig. 2. The dispersion curves relative to path 1 have been extracted from the recordings of the 4/13/2009 event, located nearby Campotosto, at AQG, AQV and AQM stations. Rayleigh data, sampled at periods of 1-3.8 s, have been inverted in VS models of the shallow 6 km of crust. The representative solution is characterized by velocities increasing from 0.9 to 3.0 km/s at 3.9 km of depth.

The path 2 is relative to the recording of the 6/22/2009 event at AQK station. Dispersion curves have been extracted at periods of 0.5-1.6 s and VS models of the shallow 2 km have been retrieved. The representative solution is characterized by velocities increasing from 0.9 to 2.7

The dispersion curves relative to path 10 have been extracted from the recordings of 2 events at AQU station. Rayleigh data have been sampled in the 0.4-1.2 s period range and the VS models are relative to the shallow 2 km. The representative solution is characterized by shear

The dispersion curves along the path 11 have been extracted from the radial and vertical components of the 04/13/09 event recording at AQU station. The average dispersion curve is defined in the 1.0-2.5 s period range and VS models of the shallow 3 km have been retrieved from it. The representative solution shows velocities increasing from 0.9 km/s to 2.9 km/s at

The average dispersion curve along the path 12 has been computed from the dispersion curves extracted from the vertical components of 2 earthquakes on 04/09/09 and is defined at periods between 0.7 and 1.2 s. The retrieved VS models are relative to the shallow 2 km and the represen‐ tative solution is characterized by velocities increasing from 1.1 to 2.8 km/s at 1 km of depth.

The interpretation of the obtained VS models, attributed beneath the middle points of the paths (bottom of Fig. 2), is performed by taking into account available geological data [18]. Out‐ cropping rocks along the cross section A through the paths 12-1-11-10 consist of limestone and marls, which are found with a thickness of ~1 km in the Campotosto drilling (located in Fig. 1), below a 1.2 km thick layer of clays and sandstones. The Mesozoic carbonate horizon is found at the well bottom (2.45 km). VS range between 0.9 and 1.5 km/s in the shallowest 100-200 m of weathered rocks, and reach the value of 2.5 km/s at depths between 0.6 and 1.7 km. The velocity of 2.5 km/s can be reasonably attributed to fractured limestone rocks and, based on the Campotosto drilling stratigraphy, to the top of the Mesozoic limestone horizon. Such

earthquake location and attributed to the shallow 11.1 km of crust.

84 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

velocities increasing from 0.9 km/s to 2.2 km/s at 0.8 km of depth.

*3.2.1. Northern paths*

km/s at 1.3 km of depth.

1.8 km of depth.

**Figure 2.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for northern paths 1-2-10-11-12. Z is the vertical component and R is the radial component of the rotated recorded events (Table 1). For each path, the chosen solution is represented by the solid bold red line and is shown in the table with the uncertainty for the inverted parameters. The thickness marked by \* is not a truly inverted parameter, but it satisfies the condition that the total thickness from the free surface to the top of the first fixed layer is equal to a predefined quantity. Bottom: VS pattern below the cross section A through the middle points of the paths 12-1-11-10 (located on the left).

#### *3.2.2. Middle Aterno river valley paths*

All the results obtained for the middle Aterno river valley paths (6-7-8) are shown in Fig. 3. The paths are relative to the recording of the 4/6/2009 mainshock event at AQK station (path 6), and of the 4/7/2009 events at the AQG station (paths 7 and 8).

As regards the path 6, dispersion curves have been extracted at periods of 0.9-2 s and VS models of the shallow 1 km have been retrieved. The representative solution is characterized by velocities decreasing from 0.85 to 0.7 km/s in the shallow 0.1 km, and deeper increasing from 0.9 to 1.45 km/s at 0.4 km of depth. Dispersion curves relative to path 7 are defined in the period range of 0.9-1.8 s and VS models of the shallow 1.3 km have been retrieved from their average curve. The representative solution presents velocities increasing from 0.6 to 2.1 km/s at 0.8 km of depth. The average dispersion curve relative to path 8, is defined in the period range of 0.4-1.4 s and VS models of the shallow 1.3 km have been obtained. The representative solution presents velocities increasing from 0.5 to 1.7 km/s at 0.5 km of depth. Taking into account the geological data relative to the shallowest 0.2-0.3 km [10], stratigraphies may be attributed to

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Lacustrine soils have a thickness increasing from about 0.2 km in the center of the valley (path 7) to about 0.4 km towards L'Aquila (path 6) with VS increasing from 0.5 km/s to ~0.9 km/s. Maiolica and flysch layers have an average VS of ~1.2 km/s while breccia has a VS of ~0.9 km/s. The calcarenites with VS of ~1.4 km/s deepen from 0.15 km to 0.45 km

All the results obtained for the middle paths (3-5) are shown in Fig. 4. The path 3 is relative to the recording of the 4/11/2009 event at AQU station. It crosses the Aterno valley delimited on the eastern side by the Paganica fault. Dispersion curves have been extracted at periods of 0.7-1.9 s and VS models of the shallow 2 km have been retrieved. The representative solution

The path 5 is relative to the recording of the 4/9/2009 event at AQU station and crosses the lower border of the middle Aterno valley, on the west of L'Aquila. Dispersion curves have been extracted at periods of 0.8-1.4 s and the retrieved VS models are relative to the shallow 2 km. The representative solution is characterized by velocities increasing from 0.6 to 2.5 km/s

From the comparison of the representative VS profiles (chosen Hedgehog solutions) be‐ low paths 3, 5 and 6, it turns out that the western sector is characterized by high veloci‐ ties (~1.8 km/s) at very shallow depth (~0.1 km) which are detected at 0.8 km in the eastern sector and are not found in the investigated shallow 1 km in the epicentral area

The paths 4 and 9 cross the south of L'Aquila consisting of outcropping alluvial deposits and carbonate rocks (Fig. 1). All the results are shown in Fig. 5. Both the paths are averaged on 3 events recorded at AQU station. Dispersion curves have been extracted at periods of 0.5-0.9 s, for the path 4, and of 0.6-1.2 s for the path 9. VS models of the shallow 1 km have been retrieved which show homogeneous velocities increasing from 0.9 to 2 km/s at ~0.5 km of depth.

is characterized by velocities increasing from 0.7 to 2.0 km/s at 0.8 km of depth.

the VS profiles.

towards S (path 6).

*3.2.3. Middle paths*

at 0.9 km of depth.

*3.2.4. Southern paths*

of the L'Aquila main shock.

**Figure 3.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for mid‐ dle Aterno river valley paths 6-7-8. For detailed caption see Fig. 2. Bottom: VS profiles vs depth (chosen Hedgehog sol‐ utions) obtained along the paths 6, 7 and 8 (located on the left) and attributed at the respective middle points A, B and C. Stratigraphies are based on the geological studies [10].

As regards the path 6, dispersion curves have been extracted at periods of 0.9-2 s and VS models of the shallow 1 km have been retrieved. The representative solution is characterized by velocities decreasing from 0.85 to 0.7 km/s in the shallow 0.1 km, and deeper increasing from 0.9 to 1.45 km/s at 0.4 km of depth. Dispersion curves relative to path 7 are defined in the period range of 0.9-1.8 s and VS models of the shallow 1.3 km have been retrieved from their average curve. The representative solution presents velocities increasing from 0.6 to 2.1 km/s at 0.8 km of depth. The average dispersion curve relative to path 8, is defined in the period range of 0.4-1.4 s and VS models of the shallow 1.3 km have been obtained. The representative solution presents velocities increasing from 0.5 to 1.7 km/s at 0.5 km of depth. Taking into account the geological data relative to the shallowest 0.2-0.3 km [10], stratigraphies may be attributed to the VS profiles.

Lacustrine soils have a thickness increasing from about 0.2 km in the center of the valley (path 7) to about 0.4 km towards L'Aquila (path 6) with VS increasing from 0.5 km/s to ~0.9 km/s. Maiolica and flysch layers have an average VS of ~1.2 km/s while breccia has a VS of ~0.9 km/s. The calcarenites with VS of ~1.4 km/s deepen from 0.15 km to 0.45 km towards S (path 6).

### *3.2.3. Middle paths*

*3.2.2. Middle Aterno river valley paths*

All the results obtained for the middle Aterno river valley paths (6-7-8) are shown in Fig. 3. The paths are relative to the recording of the 4/6/2009 mainshock event at AQK station (path

**Figure 3.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for mid‐ dle Aterno river valley paths 6-7-8. For detailed caption see Fig. 2. Bottom: VS profiles vs depth (chosen Hedgehog sol‐ utions) obtained along the paths 6, 7 and 8 (located on the left) and attributed at the respective middle points A, B

and C. Stratigraphies are based on the geological studies [10].

6), and of the 4/7/2009 events at the AQG station (paths 7 and 8).

86 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

All the results obtained for the middle paths (3-5) are shown in Fig. 4. The path 3 is relative to the recording of the 4/11/2009 event at AQU station. It crosses the Aterno valley delimited on the eastern side by the Paganica fault. Dispersion curves have been extracted at periods of 0.7-1.9 s and VS models of the shallow 2 km have been retrieved. The representative solution is characterized by velocities increasing from 0.7 to 2.0 km/s at 0.8 km of depth.

The path 5 is relative to the recording of the 4/9/2009 event at AQU station and crosses the lower border of the middle Aterno valley, on the west of L'Aquila. Dispersion curves have been extracted at periods of 0.8-1.4 s and the retrieved VS models are relative to the shallow 2 km. The representative solution is characterized by velocities increasing from 0.6 to 2.5 km/s at 0.9 km of depth.

From the comparison of the representative VS profiles (chosen Hedgehog solutions) be‐ low paths 3, 5 and 6, it turns out that the western sector is characterized by high veloci‐ ties (~1.8 km/s) at very shallow depth (~0.1 km) which are detected at 0.8 km in the eastern sector and are not found in the investigated shallow 1 km in the epicentral area of the L'Aquila main shock.

## *3.2.4. Southern paths*

The paths 4 and 9 cross the south of L'Aquila consisting of outcropping alluvial deposits and carbonate rocks (Fig. 1). All the results are shown in Fig. 5. Both the paths are averaged on 3 events recorded at AQU station. Dispersion curves have been extracted at periods of 0.5-0.9 s, for the path 4, and of 0.6-1.2 s for the path 9. VS models of the shallow 1 km have been retrieved which show homogeneous velocities increasing from 0.9 to 2 km/s at ~0.5 km of depth.

**Figure 5.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for southern paths 4-9. For detailed caption see Fig. 2. Bottom: chosen Hedgehog solutions obtained along the paths 4

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VS models have been also retrieved from an active seismic experiment performed in the Coppito area (Fig. 1) with geophone offsets of 64 m and by using FTAN and Hedgehog methods. The models are characterized by an average velocity (VS30) of 190 m/s in the shallow 30 m of alluvial soils (Fig. 6b). Instead a VS30 of 473 m/s is obtained from a cross-hole test, 500 m distant, performed close to the AQV station, in the same alluvial soils [22]. Such discrepancy has important consequences in the respect of the national building code as the soil classification

The comparison of the frequency of the maximum peak of the H/V spectral ratio, relative to the main shock recorded at the AQV station, with the 1D spectral amplifications, computed with SHAKE program [24], by assuming the two different VS data sets has evidenced the agreement with the VS profiles relative to FTAN-Hedgehog measurements (Fig. 6c). Once more, such comparison evidences: 1) the strong lateral and vertical heterogeneities of such alluvial soils; 2) the cross-hole (and down-hole) point-like measurements, even though quite precise, may not be representative of the average seismic path (e.g. [25,26]). We remind that the surface measurements for the FTAN analysis do not need boreholes or conventional arrays,

and 9 (located on the left) and attributed at the respective middle points A and B.

*3.2.5. Active seismic experiment*

changes from C to B [23].

hence are particularly suitable in the urban areas.

**Figure 4.** Dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for the mid‐ dle paths 3-5. For detailed caption see Fig. 2. Bottom: VS profiles vs depth (chosen Hedgehog solutions) obtained along the paths 5, 6, and 3 and attributed at the respective middle points A, B and C.

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**Figure 5.** Top: dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for southern paths 4-9. For detailed caption see Fig. 2. Bottom: chosen Hedgehog solutions obtained along the paths 4 and 9 (located on the left) and attributed at the respective middle points A and B.

#### *3.2.5. Active seismic experiment*

**Figure 4.** Dispersion curves and average with error bars (on the left) and VS models (Hedgehog solutions) for the mid‐ dle paths 3-5. For detailed caption see Fig. 2. Bottom: VS profiles vs depth (chosen Hedgehog solutions) obtained

along the paths 5, 6, and 3 and attributed at the respective middle points A, B and C.

88 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

VS models have been also retrieved from an active seismic experiment performed in the Coppito area (Fig. 1) with geophone offsets of 64 m and by using FTAN and Hedgehog methods. The models are characterized by an average velocity (VS30) of 190 m/s in the shallow 30 m of alluvial soils (Fig. 6b). Instead a VS30 of 473 m/s is obtained from a cross-hole test, 500 m distant, performed close to the AQV station, in the same alluvial soils [22]. Such discrepancy has important consequences in the respect of the national building code as the soil classification changes from C to B [23].

The comparison of the frequency of the maximum peak of the H/V spectral ratio, relative to the main shock recorded at the AQV station, with the 1D spectral amplifications, computed with SHAKE program [24], by assuming the two different VS data sets has evidenced the agreement with the VS profiles relative to FTAN-Hedgehog measurements (Fig. 6c). Once more, such comparison evidences: 1) the strong lateral and vertical heterogeneities of such alluvial soils; 2) the cross-hole (and down-hole) point-like measurements, even though quite precise, may not be representative of the average seismic path (e.g. [25,26]). We remind that the surface measurements for the FTAN analysis do not need boreholes or conventional arrays, hence are particularly suitable in the urban areas.

**4. Ground motion modeling**

source, propagation and local site effects [8,9].

the epicentre through the AQK station (Fig. 7).

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

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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

Modeling of the main shock has been done along a geological cross section at L'Aquila, from

**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]).

been considered to scale the seismogram to the desired scalar seismic moment.

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