Magnetotelluric survey in the bradyseismic area of Phlegraean Fields
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A magnetotelluric survey was carried out in order to contribute to the definition of the deep structure of the Phlegraean Fields area affected by the bradyseismic crisis begun in the summer 1982. In a preliminary campaign carried out in autumn 1983, the MT measurements were made in eight sites, located between 0.3 and 5 km from the Harbour of Pozzuoli and affected by different degrees of uplift.
In a second campaign (autumn, 1984) seven of the first eight MT soundings were repeated and four new sites were investigated, one of which located in a stable zone 10 km away from Pozzuoli.
MT measurements show the presence of a thick conductive layer (ρ<1 Ωm) thinned underneath the town of Pozzuoli, where its top reaches the minimum depth (1.2–2 km). The upper horizon of this layer has a greater concavity than that produced by surface deformation. Below the conductive layer, the substratum became progressively more resistive with depth. This resistive basement is well defined in the soundings located far away from Pozzuoli. Here, on the contrary, a zone or body much more conductive than its surroundings is detected. This conductive body could correspond to a high temperature magmatic intrusion, probably already solidified, with a rough cylindrical shape with a base diameter of 1.5±0.5 km.
Introducing into the obtained model the depth of the conductive horizon desumed from the deep geothermal wells, the top of the intrusive body would lye at 3–4 km depth.
A small uplift of this body related to tectonic readjustments eventually induced by strong earthquakes recently occurred in near zones could explain the surface deformation observed during the present bradyseismic crisis.
KeywordsSedimentology Strong Earthquake Surface Deformation Conductive Layer Base Diameter
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