With the scientific purpose to monitor the micro-seismicity in the Central Apennines, we recently
deployed in Eastern Tuscany (Casentino) a temporary 12 element seismic array, composed
exclusively of 3-component seismometers. During the one month of registration we recorded an
unexpected high rate of local micro-seismicity with up to 180 events per day

Braun, T.

Dahm, T.

Krüger, F.

Ohrnberger, M.

Scherbaum, F.

Earth-prints Repository

Seismic array analysis of Tornillo-like signals recorded in Tuscany  
 
T. Braun (1), T. Dahm (2), F. Krüger  (3), M. Ohrnberger  (3) and F. Scherbaum (3)  
 
(1) Istituto Nazionale di Geofisica e Vulcanologia, Arezzo 
(2) Istitut für Geophysik, Hamburg  
(3) Istitut für Geowissenschaften, Potsdam  
 
With the scientific purpose to monitor the micro-seismicity in the Central Apennines, we recently 
deployed in Eastern Tuscany (Casentino) a temporary 12 element seismic array, composed 
exclusively  of 3-component seismometers. During the one month of registration we recorded an 
unexpected high rate of local micro-seismicity with up to 180 events per day.  
 
 
 
Fig. 1 –E,N,Z-traces of a tornillo-like seismic event recorded by a seismic array in eastern Tuscany 
 
Waveforms surprisingly resembled volcano-seismic events rather than tectonic earthquakes. Distinct P 
and S-phases with a sharp onset are missing. Seismograms show an emergent onset followed after a 
few seconds by a weakly attenuated monochromatic coda, similar to “Tornillos” known from active 
volcanoes like e.g. Galeras. More than 5000 Grapillos (Gas-Related APenninic Tornillos) have been 
recorded during the one month recording period, showing all similar amplitudes but irregular temporal 
occurrence. Locations of the Grapillos were determined by automated f-k analysis. Wavefronts reach 
the array from NW (306°) with a slowness 20 s/deg (vapp~5.32 km/s), corresponding to an epicentre 
location 13 km NW respect to the array centre. Source locations seem to remain rather stable for the 
entire duration of the array installation. 
At the end of the experiment some of the array stations were used to set up a small network around the 
epicentral area. Classical hypocentral determination - using exclusively first onsets - confirmed the 
epicentres found by f-k analysis and determine for the Grapillos a distinct focal depth of ca. 4 km. 
Concerning the origin of the unusual seismic events geothermal and volcanic activity can be excluded 
“a priori”. However, in the nearly situated Upper Tiber Valley many cold CO2 springs (Mofettes) can 
be observed on the surface since decades. Pulsation and transients in gas flux have been observed at 
known Mofettes and are possibly caused by tectonic transients. However, in the epicentral area 
currently no gas Mofettes are known. We speculate, that the origin of the “Tornillo”-like seismic 
signals could be related to transients in an unknown CO2 reservoir inside the upper crust. 
 
Acknowledgments. The field experiment was financed by rhe University of Potsdam and Roma1. 


Seismic array analysis of Tornillo-like signals recorded in Tuscany

http://www.earth-prints.org/bitstream/2122/2935/1/GNGTS-2006-CAESAR.pdf

Seismic array analysis of Tornillo-like signals recorded in Tuscany

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