Rates of similar micro-earthquakes: Comparison between Campi Flegrei (Southern Italy) and other volcanic areas

Abstract

The precise relocation of micro-earthquakes using waveform cross-correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas. To be applied and to provide meaningful results, the technique requires, however, the presence of sufficient amounts of similar events. We examined about 870 earthquakes recorded at the Campi Flegrei (CF) caldera by a temporary digital network during the end of the 1982–1984 bradiseismic crisis. The classification of these earthquakes indicates a surprising low amount of similar events, especially according to the clustered aspect of the seismicity. The relocation technique, allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera. To determine up to which point this low similarity rate is unusual, we examined the rates for several data sets coming from various volcanic areas: Hawaii, Mount Pinatubo (Philippines) and Piton de la Fournaise (La Réunion Island). The results show for all areas, percentages of similar events significantly higher than for CF and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics. At CF, most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures. The low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress.