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)

Abstract

Abstract. 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 rn 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 300-60 ø (with an average uncertainty of 12ø); the fault strikes mainly range between -60 ø and 60 ø and seem to define preferential directions oriented radially from the synm•etry 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 threedimensional Qv 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 1ow-Qp zone and low-V• (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-Qv 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 (450ø-500øC) 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 Q•, images has been interpreted by using a three-dimensional thermal conduction model assuming an extended heat source (initial temperature of 800øC) located underneath the attenuation anomalous region. The results indicate that the Qv-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.

Detailed Analysis of Wave Propagation beneath the Campi Flegrei Caldera, Italy

“…Modeling teleseismic P-S converted phases, Ferrucci et al (1992) postulated a magmatic chamber beneath the center of the caldera at about 4-5 km depth. A similar depth range was indicated by De Lorenzo et al (2001), based on extrapolation of borehole temperature data and the location of a negative Q p anomaly.…”

“…The high V p /V s anomaly revealed by Aster and Meyer (1988) and by Vanorio et al (2005) at about 1 km depth 3. The low-Q p anomaly at depth between 0 and 1 km (De Lorenzo et al, 2001) 4. The shallow high-scatter density imaged by the 3D scattering tomography of Tramelli et al (2006) 5.…”

“…The results obtained by a recent 3D Q p tomography (De Lorenzo et al, 2001) indicate in the area northeast of the town of Pozzuoli (corresponding to the Solfatara crater) a large low-Q p anomaly in the shallowest crust (0-1 km depth). In the same region, a previous tomographic study by Aster and Meyer (1988) reported a high V p /V s ratio at 1 km depth.…”

The seismic velocity and attenuation structure beneath the Tatun volcanic area, Taiwan

“…Since the rock composition, porosity as well as the fluid pressure, fracture, and temperature could have significant effect on V p /V s ratio in shallow layers (Walck, 1988). According to the experiments of rock, the fluid in crack would affect the Q p value (Tittmann et al, 1980;Winkler and Nur, 1982) and the relatively low Q p anomalies which imply that the high hydrothermal activities beneath the surface, and with residual thermal effects presently seen on the surface in the dormant volcanic area (De Lorenzo et al, 2001). Sanders et al (1995) and Zucca et al (1994) have also shown that Q p decreases with increasing porosity and fluid circulation in cracked rocks.…”

The Bay of Naples (southern Italy): Constraints on the volcanic structures inferred from a dense seismic survey

“…The analysis and interpretation of the data set recorded during the last crisis have provided most of the knowledge of the seismic structure. The main issues are the identification of a low V p , and a high V p /V s ratio volume [Aster and Meyer, 1988], associated to a low Q P anomaly [de Lorenzo et al, 2001]. The hypothesis that there is a magma body located at 4-5 km in the center of Orsi et al, 1996] and experimental layout of the study area.…”