Long-term seafloor experiment with the CUMAS module: performance, noise analysis of geophysical signals, and hints towards the design of a permanent network

Iannaccone G., Vassallo M., Elia L., Guardato S., Stabile T. A., Satriano C., Beranzoli L.

Seismological Research Letters, 81(6) pp. 916-927, 2010 | doi: 10.1785/gssrl.81.6.916

INTRODUCTION:

The Campi Flegrei caldera (southern Italy) is one of the most hazardous areas in the world because it is home to several hundred thousand people live there and an important center of socio-economic activities. The caldera includes the western-most part of the city of Naples and extends into the Gulf of Pozzuoli (eastern Tyrrhenian basin; Figure 1). The main feature of the present volcanic activity of the caldera is the episodic slow and high-amplitude soil movement (bradyseism) accompanied by intense and shallow seismic activity that only occurs during the uplift phase.

The most recent strong bradyseismic episode occurred between 1982 and 1984, when there was a maximum ground uplift of more than 170 cm, which was followed by slow and continuous subsidence, accompanied by rare low-amplitude uplifts (of a few centimeters). The maximum uplift was observed in the center of the caldera, in the town of Pozzuoli, while toward the caldera margins the uplift decreased gradually. The amplitude and the areal distribution of the vertical soil displacement that occurred over time in the marine sector, which covers more than one third of the caldera area, is completely unknown. During this 1982–84 bradyseismic episode, there were more than 10,000 shallow earthquakes (<4 km) with a maximum recorded magnitude of 4.2 (Aster et al. 1992). The space distribution of the earthquake hypocenters was denser on land, where the monitoring network had been deployed (Figure 1). In contrast, only a small number of the earthquakes were located in the marine sector. This can be explained primarily by a probable lesser level of seismic activity in the marine sector than on land, and secondly by the inadequate coverage of the sea area by the monitoring network configuration in place at the time.