Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Qp imaging and inferences on the thermal state of the Campi Flegrei caldera (southern Italy)

The three‐dimensional P wave attenuation structure of the Campi Flegrei caldera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measurements by using the method described by Zollo and de Lorenzo (this issue). Source radii represent the better resolved parameters with values ranging from 70 m to 230 m; the dip and strike angles defining fault orientations are usually affected by larger uncertainties and are well constrained only for 11 events. The dip fault is usually confined in the range 30°–60° (with an average uncertainty of 12°); the fault strikes mainly range between −60° and 60° and seem to define preferential directions oriented radially from the symmetry axis of the ground deformation. Stress drop estimates indicate rather low values (0.01–1 MPa) which suggest low strength properties of the incoherent and brittle materials filling the caldera (primarily yellow tuffs). The three‐dimensional Qp images obtained from the inversion of P pulse durations show two significant low‐ Qp anomalies between 0 and 1 km of depth, in the north‐eastern sector and at 2–3 km of depth in the central eastern sector of the caldera. The high degree of spatial correlation of the low‐ Qp zone and low‐ V s (as inferred by Aster and Meyer (1988)) at 0–1 km in depth and other geophysical and geochemical observations suggest that this anomaly can be related to the presence of densely fractured, porous, and fluid‐filled rocks in the NE sector of the caldera. The deeper low‐ Qp anomaly is interpreted as being related to a dominant thermal effect. We used the surface and deep borehole temperature measurements available in the area to obtain a local calibration curve to convert Qp in temperature at Campi Flegrei. The retrieved T ( Qp ) map shows a high thermal deep disturbance (450°–500°C) at depths between 2 and 3 km in the eastern sector of the caldera, where the most recent eruptive activity is concentrated. The present‐day temperature field retrieved by Qp images has been interpreted by using a three‐dimensional thermal conduction model assuming an extended heat source (initial temperature of 800°C) located underneath the attenuation anomalous region. The results indicate that the Qp ‐inferred temperature field can be related to the heat conduction effect of one or more molten bodies whose top should be at about 4‐km depth, consistent with recent seismic estimates of the magma chamber top at Campi Flegrei ( Ferrucci et al. , 1992). This study suggests that the present thermal state and rock rheology of the inner caldera could be controlled by the cooling of molten bodies that originally intruded at depths of 1.4–1.6 km, during one or more recent (time of <10 kyr) eruptive events.