- The seismicity of Gran Sasso massif
A seismic array is a set of seismographs distributed over an area of the earth's surface at spacing narrow enough so that signal waveform may be correlated between adjacent seismometer. This geophysical instrument installed in the underground physics laboratories of Gran Sasso (L.N.G.S.-I.N.F.N.), located in the seismic zone of central Apennines, Italy (fig. 8). The seismicity of the area is associated with the mainly distensive tectonics affecting the Apennines since the late Pliocene. Historical events include the 1915, MS=6.8 Fucino earthquake, which caused about 32000 casualties; recent studies have modelled this event in terms of a normal fault striking along the Apennine direction (NNW-SSE). In addition, swarms of low-to moderate-size earthquakes occur quite frequently, such as those of 1992, 1994, 1996 and 2006. The most recent activity is dominated by the MW=6.3 earthquake which, on April 6th, 2009, stroke the city of L'Aquila and surroundings, causing more than 300 casualties. This catastrophic event was part of an energetic seismic sequence which began as of early January, 2009. This relevant rate of seismicity, joint to the low-noise conditions and site response associated to the underground setting, make the Gran Sasso underground laboratories an ideal site for high-resolution seismic observations.
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Figure 8 show the epicenters of the most energetic earthquakes occurred in historical times in this zone of Apennines; the insert show the geometry of UnderSeis array inside the laboratories.
- General description
The seismic array UnderSeis is a set of seismographs distributed over an area of the earth's surface at spacing narrow enough so that signal waveform may be correlated between adjacent seismometer. Therefore, the correlation of the entire waveform of the signal is studied. The system is realized mainly for the seismic monitoring on local and regional scale. Considering geometry and the dimensions of the underground laboratories, UnderSeis has a small aperture (400 m x 600 m) and the average spacing between the short period seismographs is about 90 m. The array become fully operative since may, 2002. In its present configuration, it consists of 19 three-components short period seismometers (Lennartz 3D-LITE, 1 Hz) and 1 broad-band Guralp GMC-40 T seismometer. Seismic signals are digitized locally at each individual seismometer with a dynamic range of 24 bits and sampling frequency of 100 Hz. Data synchronization is achieved via a Master Oscillator which transmit the UTC synchronized pulses from an atomic clock to the different ADC boards. The synchronized data packets are then sent via serial cable connection to a set of five nodal PCs, which are in turn connected via an Ethernet network to a central data server and an on-line processor. The design of the UnderSeis hardware and software components began on the late 90’s; over the following years, major upgrades were developed under a technological effort jointly carried out by engineers from the University of Granada (Spain), University of L’Aquila (now the team moved to the University of Salerno), and I.N.G.V. – Osservatorio Vesuviano.
- Conclusions