Text S1.
Data and Network Geometry.
In this study, we used 44445 P- and 14801 S-wave arrivals from a high quality sub-set of about
1276 earthquakes with ML ≥ 1.9, occurred in the period April-June 2009 and recorded at a very
dense seismic network of both temporary and permanent seismic stations (Figure S1). We
obtain starting 1D earthquakes locations by using the Hypoellipse code [Lahr, 1989]. We set
parameters differently for different time intervals modulated on the network geometry
evolution. In the location procedure we changed the weighting scheme with distance and the
azimuth coverage, accordingly. We also used the elevation correction option using the first layer
velocity up to a mean topography of 500m, namely the mean altitude of the stations position.
Selected earthquakes have rms < 0.2 s, ERH-Z < 1km, gap < 200° and more than 8 P- and 4 Sreadings. About 33% of P and S readings have weight 0 (reading error ≤ +/-2 samples at 125Hz
of sampling rate) and the entire dataset has weight ≤ 2.
The temporary network was installed soon after the L’Aquila earthquake and is composed by 40
seismic stations integrating the permanent network managed by the INGV. The average spacing
between stations is about 4-6 km. After several tests, an horizontal grid spacing of 5 km and a
vertical layer spacing of 2 km proved to be the tradeoff choice between details level and model
resolution for the present dataset (see grid nodes distribution in Figure S1). A very good
sampling of the fault volume is ensured by the high number of well located earthquakes along
and around the plane and by the dense vertical layering of the model. The maximum distance of
grid nodes from the fault plane is in fact ≤ 2.5 km in the horizontal direction and ≤ 1 km in the
vertical direction.
Model Resolution.
After four iterations, unweighted and weighted data variance are reduced from 0.062 and 0.030
(earthquakes locations in the 1D starting model, iteration 0) to 0.035 (-44%) and 0.010 (-66%)
respectively. Model rms is reduced of ~44% from 0.14 after iteration 1 to the final value of 0.08.
F-test shows that the final velocity model would not benefit of further iterations.
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Model resolution has been verified thorough the analysis of the Resolution Matrix, via the
Spread Function (spf) and averaging vectors (see Toomey and Foulger [1989], Michelini and
McEvilly [1991], Reyners et al., [2007] for a complete description of the procedure). Figure S2
shows the spf and the smearing of the inverted nodes for the VP and VP/VS models, respectively,
in vertical sections across the 2009 fault. To visualize the direction of smearing in our model,
we show the volume around each node where 70% of the velocity parameter value belonging to
that node is included (the averaging vector, see Reyners et al., [2007]). For a well-resolved
node, the 70% contour line contains only that node, while the contour line comprises several
nodes for poorly resolved ones. Gray contours represent nodes with Spread Function (spf) in the
range 2.0 – 1.5, whereas black contours represent nodes with spf < 1.5. Nodes without
contouring are not inverted or poorly resolved (spf > 2.0). Figure S2 demonstrates that nodes
with spf <= 2.0 have very low to null smearing and that almost the entire VP and VP/Vs models
show compact averaging vectors and a high resolution down to 12-14 km depth. Y in Figure S2
is the distance of the vertical sections from the center of the model, along the latitude direction.
The resolution decreases at 16 km depth. In the figure, the thick black lines indicate the
intersection of the fault plane with the vertical sections. The fault is located entirely in the wellresolved model volume.
Poisson Ratio reference value
The starting VP/VS value used in the tomographic inversion is 1.88 according to the Wadati
[1933] diagram computed using P- and S-readings from all the earthquakes of the inverted
dataset (see Figure 1). Poisson ratio is calculated at each point of the grid, from the VP/VS value
according to the following formula:
 =[(VP/VS)2/2-1]/[(VP/VS)2- 1].
The relationship between  and VP/VS is approximately linear for  < 0.35 and VP/VS < 2.0
[Christensen, 1996], that is within our minimum and maximum values in the resolved region.
The value  = 0.30 that we assume as a reference for Poisson ratio in the target volume
corresponds to the value resulting from VP/VS=1.88 [Lucente et al., 2010; Chiaraluce et al., in
press]. Such value is within the range of Vp/Vs (1.8-2.2) mesured in laboratory experiments for
rocks, in wet and dry conditions, typical of this sector of the Apennines [Trippetta et al., 2010].
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EP and IP phases location
We show in Figure S3 as an example, the three components of the accelerogram recorded at the
AQK station (Figure 1) and the computed ground displacement. We have used 33 and 18 high
quality arrival times of EP and IP phases, respectively, in order to locate the points on the fault
where they are radiated. The inferred 3D velocity model allows constraining high quality
locations for both phases having formal errors around 0.1 km in latitude, longitude and depth.
Hypocentral coordinates and errors are listed in Table S1. For the example shown in Figure S3
the difference between arrival times of EP and IP (0.6 s) is smaller than the difference between
their origin times (0.87 s) because this station is closer to the IP location than to the EP.
Origin Time
 [s]
Lat. [°]
 [km]
Lon. [°]
 [km]
Dep. [km]
 [km]
N. Phs
rms
0,0074
42,3548
0,0730
13,3813
0,062
8,5500
0,115
33
0,09
0,0076
42,3577
0,1000
13,3822
0,128
6,6400
0,081
18
0,06
2009,04,06
EP
01:32:40.74
2009,04,06
IP
01:32:41,61
Table S1. 3D location parameters for EP and IP pulses of the main rupture. 3D location errors 
in km (latitude, longitude, and depth) and seconds (origin time) are shown.
References
Chiaraluce L., C. Chiarabba, P. De Gori, R. Di Stefano, L. Improta, D. Piccinini, A.
Schlagenhauf, P. Traversa, L. Valoroso and C. Voisin (2011), The April 2009 L’Aquila
(Central Italy) Seismic Sequence, Boll. Geofis. Teorica App, in press.
Christensen, N. I., (1996), Poisson’s ratio and crustal seismology, J. Geophys. Res. (101) 3139–
3156.
Lahr, J.C., 1989. HYPOELLIPSE/version 2.00: a computer program for determining local
earthquakes hypocentral parameters, magnitude and first motion pattern. U.S. Geol.
Surv Open-File Rep., 89 (116).
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Lucente, F.P., P. De Gori, L. Margheriti, D. Piccinini, M. Di Bona, C. Chiarabba, N. Piana
Agostinetti (2010), Temporal variation of seismic velocity and anisotropy before the 2009 M
(sub W) 6.3 L'Aquila earthquake, Italy, Geology (Boulder), 38 (11), 1015-1018.
Michelini, A., and T. V. McEvilly (1991). Seismological studies at Parkfield, part I:
Simultaneous inversion for velocity structure and hypocenters using cubic B-splines
parameterization, Bull. Seism. Soc. Am. 81, 524–552.
Reyners, M., D. Eberhart-Phillips, and G. Stuart (1999), A three-dimensional image of shallow
subduction: Crustal structure of the Raukumara Peninsula, New-Zealand, Geophys. J. Int.,
137, 873-890.
Toomey, D.R., and G.R. Foulger (1989), Tomographic inversion of local earthquake data from
the Hengill-Grensdalur central volcano complex, Iceland, J. Geophys. Res., 94, 1749717510.
Trippetta, F., C. Collettini, S. Vinciguerra, and P.G. Meredith (2010), Laboratory measurements
of the physical properties of Triassic Evaporites from Central Italy and correlation with
geophysical data, Tectonophysics, 492, 121-132, doi:10.1016/j.tecto.2010.06.001.
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101–111.
Figures Captions
Figure S1. Model parameterization of the tomographic inversions. a) the seismic stations
locations are shown by grey triangles, and aftershocks by black dots. The NW-SE oriented
rectangle displays the surface projection of the main shock fault plane (same of Figure 1).
Black crosses are the grid nodes used in the inversion. The dashed black line indicates the
position of the cross section used for Figure 3.
Figure S2. EW oriented vertical cross-sections at different distances (Y) from the center of the
tomographic model (latitude 42.38°). The fault trace in each panel is indicated by the dashed
line. Contour lines depict the spread function and smearing of the inverted nodes for VP and
VP/VS models. Crosses and nodes are inverted and not inverted model nodes respectively. Grey
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contouring indicates values of the spread function between 2.0 and 1.5, while black contouring
indicates values between 1.5 and 0.
Figure S3. Strong motion waveforms recorded at station AQK. a) three components of
acceleration time histories band-pass filtered; b) zoom of the first 2 seconds to show the picking
of the EP phase and, after 0.6s, of the IP phase; c) vertical ground displacement obtained by
double integration of acceleration waveform, showing that the EP phase has almost no
displacement, while the IP shows about 20 cm of displacement in the first 2 seconds.
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