Seismic Investigation of the Campi Flegrei: A Summary and Synthesis of Results

Campi Flegrei, Italy is a 10-km-diameter Quaternary explosive caldera near Naples and Mount Vesuvius. Numerous eruptive vents lie within it, the last of which formed in 1538 a.d. An elevation change of ≈10m has occurred within the caldera during the past 2000 years. Recently (mostly 1982–1985), following more than 400 years of subsidence, there has been approximately 2 m of uplift in a region 2.5 km in diameter in the central portion of the caldera. Local earthquakes in the magnitude range 0.6 ⩽ MD ⩽4.2 and depth range 1.5 < z < 5 km associated with this uplift were recorded by a portable 3-component digital network and by a telemetered network of single-component stations. The results of seismological studies arising from this data set are considered in consort with geodetic measurements, geothermal drilling, and gravity data to constrain the structure and dynamics of the shallow caldera. The largely aseismic central region, where the greatest uplift occurred is spatially correlated with a 10mgal gravity low and with a zone of low seismic velocities with a high vP/vs ratio. This area is interpreted as an incompetent, highly fractured, water saturated zone of low-density material which extends to at least 3 km. The shear-wave Q of the central caldera measured from direct arrivals is 110 ± 50, somewhat lower than the range of coda-Q estimates for the region (120–250). Hypocenter locations indicate a minimum magma chamber depth of 3.5 km and suggest the presence of an inward-dipping, elliptical ring fault. Fault plane mechanisms are highly heterogeneous, even for highly restricted regions, and thus do not indicate coherent reverse faulting at the perimeter of the ring. Moment tensor inversion indicates that seismic sources are well-modeled by low stress drop brittle fracture due to uplift; no clear indications of non-double couple focal mechanisms are seen which might indicate a significant seismogenic role for intrusion.

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