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 cal dera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measureme nts by using the method described by Zollo and de Lorenzo (this issue). Sou rce radii represent the better resolved parameters with values ranging from 70 m to 230 in; 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 degrees -60 de grees (with an average uncertainty of 12 degrees); the fault strikes mainly range between -60 degrees and 60 degrees and seem to define preferential d irections oriented radially from the symmetry axis of the ground deformatio n. Stress drop estimates indicate rather low values (0.01 - 1MPa) which sug gest low strength properties of the incoherent and brittle materials fillin g the caldera (primarily yellow tuffs). The three-dimensional Q(p) images o btained from the inversion of P pulse durations show two significant low-Q( p) anomalies between 0 and I kin of depth, in the north-eastern sector and at 2-3 km of depth in the central eastern sector of the caldera. The high d egree of spatial correlation of the low-Q(p) zone and low-V-s (as inferred by Aster and Meyer (1988)) at 0- 1 kin in depth and other geophysical and g eochemical observations suggest that this anomaly can be related to the pre sence of densely fractured, porous, and fluid-filled rocks in the NE sector of the caldera. The deeper low-Q(p) anomaly is interpreted as being relate d to a dominant thermal effect. We used the surface and deep borehole tempe rature measurements available in the area to obtain a local calibration cur ve to convert Q(p) in temperature at Campi Flegrei. The retrieved T(Q(p)) m ap shows a high thermal deep disturbance (450 degrees -500 degreesC) at dep ths between 2 and 3 km in the eastern sector of the caldera, where the most recent eruptive activity is concentrated. The present-day temperature fiel d retrieved by Q(p) images has been interpreted by using a three-dimensiona l thermal conduction model assuming an extended heat source (initial temper ature of 800 degreesQ located underneath the attenuation anomalous region. The results indicate that the Q(p)-inferred temperature field can be relate d to the heat conduction effect of one or more molten bodies whose top shou ld 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 sugge sts that the present thermal state and rock rheology of the inner caldera c ould 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) erup tive events.