UNDERSEIS: The Underground Seismic Array

An important research facility for subnuclear particle physics and astrophysics has been operating in central Italy since 1986. These laboratories (Laboratori Nazionali del Gran Sasso: LNGS), promoted by the Italian National Institute of Nuclear Physics, are located in an underground tunnel about 120 km east of Rome, alongside a motorway tunnel 10.5 km long. The tunnel is shielded by 1,400 m of limestone rocks (Gran Sasso massif). The LNGS are located very close to one of the major seismic faults of the central Apennines. The seismic activity of the central Apennines and in particular in the region of the Gran Sasso massif is presently relatively low compared to seismically active areas of Europe, such as central Greece. In the last decade three seismic swarms occurred: August 1992, June 1994, and October 1996, with the largest earthquake having ML 4.2. These swarms constituted the largest events in this region since 1985. The area has, however, experienced destructive earthquakes in the past, including a magnitude ∼7 earthquake in 1703. The 1915 ( Ms 6.8) Avezzano earthquake occurred close to the region, causing more than 15,000 deaths (Amoruso et al. , 1998). On average, about one microearthquake per day above ML 1 occurs within a 20-km radius of LNGS. The laboratory facilities, in addition to the seismotectonic features, make them an excellent site for studies related to the physics of earthquake sources and wave propagation in a complex medium, and to development of high-sensitivity techniques to search for earthquake precursors. In this paper we describe a dense small-aperture seismic array, or seismic antenna, designed to study wave-propagation phenomena and source processes of microearthquakes occurring in this region. A seismic array is a set of seismographs distributed over an area of the Earth's surface with sufficiently small spacing that the signal …