A Seismic Array in the Town of Pozzuoli in Campi Flegrei (Italy)

Abstract

Many active volcanic areas in the world are densely inhabited. Large towns are built at the foot of volcanoes, or inside volcanic fields, resulting in a very high volcanic hazard due to the combination of high volcanic risk and a large population. Vesuvius, Campi Flegrei, and Etna (Italy); Marapi (Indonesia); Teide (Tenerife, Canary Islands); Guagua Pichincha (Ecuador); Popocatepetl (Mexico); Aso, Sakurajima, and Unzen (Japan); and Mayon and Pinatubo (Philippines) are only a few of the active volcanoes sited in densely populated areas. Monitoring these volcanoes with modern technology is critical to mitigating volcanic hazard through the detection of any possible precursor phenomena. Volcanic eruptions are usually preceded by a wide variety of observable phenomena, such as an increase of seismic activity, ground deformation, and variations of the gas emissions. Therefore a continuous monitoring of seismic activity to detect the most common signals possibly related to imminent volcanic activity, such as volcano-tectonic earthquakes, low-frequency (or long-period) events, and volcanic tremor (McNutt 2005; Chouet 2003), is an important task. The use of borehole instruments in highly urbanized environments may increase the signal to noise ratio (SNR), improving the capability to detect very small signals. The Hi-net in Japan (Okada et al. 2004) and the seismic network of the Auckland volcanic field in New Zealand (Ashenden et al. 2011) are two notable examples. Volcanic tremor is observed at many volcanoes before and during eruptions. The analysis of signals characterized by emergent onset, lack of phases recognizable as direct P and S waves, and long duration (referred to as “tremor-like signals” throughout this paper) requires continuous data recorded by dense arrays. In this paper we describe a short-period seismic array, named ARF, installed since August 2010 under the town of Pozzuoli, in the center of the Campi Flegrei caldera (Italy). The aim of the ARF array is the detection of volcanic tremor, whenever should it show up. One year of continuous data have been analyzed with array techniques tuned to detect coherent low-frequency signals, possibly related to volcanic activity. We describe the main features of seismic noise, its relationship with human activity, and compare the signals recorded at 16 m depth with those recorded at surface. Detailed analysis of some local events, including volcano-tectonic earthquakes, artificial explosions, and artificial tremor-like signals, are used to check the array performances. These events provide important insight for establishing the amplitude threshold and frequency range of detectable tremor-like signals. The results of array analysis suggest that the ARF array is suitable to detect coherent signals with root mean square (rms) > 1 micron/s in the 1–5 Hz frequency band.