Download recordings of the amatrice seismic sequence to assess the response

GNGTS 2016
Sessione Amatrice
Recordings of the Amatrice seismic sequence to assess the response
of Cultural Heritage: the case of the San Giovanni Battista Church,
Acquasanta Terme, Italy
A. Caserta1, F. Doumaz1, A. Costanzo2, A. Gervasi2, W. Thorossian1,
S. Falcone2, C. La Piana2, M. Minasi2, M.F. Buongiorno1
1
2
Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Roma, Italy
Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rende (CS), Italy
During the moderate magnitude aftershocks following the Amatrice earthquake (Mw 6.0)
occurred on 24 August 2016, we installed eight portable seismic stations in the Acquasanta
Terme town, located at about 20 km NE of the mainshock zone epicenter.
The aim is to study the interaction between the incoming seismic wave-field and the San
Giovanni Battista Church (AD 1039), the main relevant cultural heritage in the town. Such
seismic sequence, indeed, offers an unprecedented singular opportunity to calibrate the
monument response to central Apennines earthquakes, which have been responsible for the
largest damage to the cultural heritage disseminated in the whole central Italy.
In a first step, we installed an array of five seismic stations in the town (red triangles on the
geological map in Fig. 1). Then, to study the dynamic behavior of the monumental structure
under the action of the incoming wave-field, we installed three seismic stations inside the
church (green triangles on the geological map in Fig 1). The location of these stations has been
chosen in order to study mainly the interaction between the bell tower and the rest of the church
structure.
In the same time, in order to estimate the state of the health of the monument, we coupled a
terrestrial laser scanner (TLS) campaign with some in-door and out-door drone photogrammetric
flights. Thermographic survey of the walls has been done to reveal earthquake induced structural
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GNGTS 2016
Sessione Amatrice
Fig. 1 – Geological map of the near-surface geology of Acquasanta Terme town; the red and green triangles represent
the seismic stations used in this study. Preliminary results of the spectral ratios on ambient noise (HVNSR) is shown
for each seismic station.
inconsistencies on the monument. In particular, the TLS high resolution scans allow to perform
a detailed geometrical analysis of the structure (e.g. verticality of walls, shifts), whereas the
shootings with drone allow to cover the non-visible parts from the ground and unreachable nor
inaccessible ones. The geometrical analysis has been combined with thermographic inspection
of the external walls (cf. Costanzo et al., 2015).
The Acquasanta Terme town was built on terraces constituted by variable thickness of
alluvial deposits resting on a travertine layer. This later is overlapped to the bedrock represented
by calcareous-marly formation, named marl with Cerrogna and Bisciaro (see geological map
in Fig. 1).
Temporary seismic stations were installed along a WNW-ESE profile to investigate the role
played by near-surface geology in amplifying the soil shaking. All seismic stations are equipped
with three-component 20s velocity transducer and three-component accelerometric transducer.
The horizontal components were oriented along the north-south and east-west directions. Data
were sampled at 250 Hz using 24-bit analog-to-digital converters, and time synchronism was
provided by a GPS system at each station. Furthermore, all stations were equipped with solar
panels for power supply and an antenna for wireless data transmission. Such data transmission
was based on HyperLAN 5GHz band. All data collected by seismic stations were wireless
transmitted and in real time to the server installed in the municipal offices.
Using continuous recording, we computed spectral ratios on ambient noise (HVNSR)
(Nakamura, 1989). All of the signals were processed with a detrigger algorithm (SESAME,
2004), to remove noise windows containing disturbing and transient phenomena. The single
window length has been set to 50 s, to enhance the resolution in the low-frequency range. The
Fourier spectra were smoothed with a Konno and Omachi (1998) algorithm, and HVNSR was
evaluated on the geometrical mean of the Fourier spectra of the horizontal components.
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GNGTS 2016
Sessione Amatrice
Fig. 2 – Upper panel - a, b) Images taken from the 3D reconstruction of the San Giovanni Battista Church carried out by
terrestrial laser scanning (TLS); the triangles indicate the locations of the seismic stations. Lower panel) Preliminary
results of the spectral ratios on ambient noise (HVNSR) for each seismic station installed on the building.
In Fig. 1, the HVSNR calculated on the base of four hours recordings, selected between
daytime and nighttime, are given for all investigated sites. The figure shows a relevant
variability of the amplification frequencies from left to right side of the Tronto River. At bottom
valley (ACS02 station), where is the contact between travertine and Bisciaro, the spectral ratio
shows relevant amplification for frequencies greater than 10Hz. Instead, for the other stations
the HVNSRs show amplifications on variable frequency ranges depending on the stratigraphic
sequence; however, these frequencies are compatible with those of the built up structures.
With reference to the stations located in the San Giovanni Battista Church, the HVNSRs
are shown in Fig. 2. In the upper panel of the same figure are reported two images of the
preliminary 3D model, that has been reconstructed through data carried out by terrestrial laser
scanning and aerial photographic surveys. On the pictures the location of the stations is given.
Analyzing the response of the bell tower and the church windows, we find HVSR greater than 4
for frequency ranges compatible with that obtained for the reference station (ACS01), that has
been installed in the church garden.
Acknowledgements. The authors are grateful to citizens, public administrators and priests of Acquasanta Terme for
their support during the deployment of the activities. We also thank the firefighters, who provided the technical support
for the installation of the seismic stations on the church. This work has been possible thanks to the instrumentation
funded by Ministry of Education, University and Research under the research project PON01-02710 “MASSIMO”.
References
Costanzo, A.; Minasi, M.; Casula, G.; Musacchio, M.; Buongiorno, M.F. (2015) Combined Use of Terrestrial Laser
Scanning and IR Thermography Applied to a Historical Building. Sensors 2015, 15, 194-213. doi:10.3390/
s150100194.
Konno, K., and Ohmachi T. (1998). Ground-motion characteristics estimated from spectral ratio between horizontal
and vertical components of microtremor, B. Seismol. Soc. Am., 88, 228-241.
Nakamura, Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremors and the
ground surface, Q. Rept. RTRI Japan, 30, 25-33
SESAME (2004). Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrationsmeasurements, processing and interpretations. SESAME European research project EVG1-CT-2000-00026,
deliverable D23.12, http://sesame-fp5.obs.ujfgrenoble.fr
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