Salvatore de Lorenzo

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-Vs (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.

Thermal state of the Campi Flegrei caldera inferred from seismic attenuation tomography

Athree-dimensional Qp image of the Campi Flegrei caldera between 0 and 3 km of depth has been inferred by the inversion of P rise time and pulse width data of 87 local earthquakes recorded during the last bradiseismic crisis by a local array deployed in the area by the University of Wisconsin. The availability of both thermal measurements in 5 deep boreholes and of a heat flow surface map of the area allowed us to calibrate the local temperature F vs. Qp relationship. The comparison of Qp, Vp and Vp/Vs images, combined with hydrogeological and geochemical data from deep boreholes, allowed us to distinguish some low-Qp anomalies related to the presence of fluids in the rocks from a deep low-Qp anomaly related to the conductive cooling of a magma reservoir. The deep anomaly is located in the same zone where several authors believe that the volcanic and magmatic activity migrated after the Neapolitan Yellow Tuff eruption. Moreover this anomaly includes the area where the existence of a magma chamber at depth between 4 and 5 km was inferred by an active seismic experiment. # 2001 Published by Elsevier Science Ltd. All rights reserved.

Surface heat flow density at the phlegrean fields caldera (SOUTHERN ITALY)

The Phlegrean Fields area is a Holocene caldera located west of Naples, southern Italy. The recent post-caldera activity is characterized by several eruptive centers inside the collapsed area. In order to investigate the still active volcanic processes, surface heat flow measurements were carried out in 1995 in 30 sites of the Phlegrean Fields, and a heat flow map compiled. Filtering of the map reveals some well-defined anomalies superimposed on a general southward-increasing trend. Local anomalies are related to small magma bodies, whereas the observed general trend has been attributed to the effect of groundwater flow. This effect was calculated and removed. The undisturbed mean value of the surface heat flow density in the eastern sector is 149 mW\m2, which is above the regional value of 85 mW\m2 assigned to the eastern part of the Tyrrhenian Sea, and which is probably influenced by a very large, deep magmatic body.

Source parameters and three-dimensional attenuation structure from the inversion of microearthquake pulse width data: Method and synthetic tests

We propose a new method to determine source parameters and attenuation structure of a three-dimensional medium based on first P rise time and total pulse width measurements from microearthquake data. The effects of fault finiteness on seismic radiation are taken into account by assuming the rupture model for a circular crack of Sato and Hirasawa (1973). Ray theory synthetic seismograms in a constant Q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters (fault radius, orientation of the fault plane, and quality factor) to the pulse width data (half and total duration of the P waveforms).The numerically built relationships are used to compute the direct problem in the framework of a nonlinear inversion scheme, based on the modified downhill Simplex method. The validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration of the seismic passive experiment conducted in the Campi Flegrei caldera (southern Italy) during the last microearthquake crisis (1982–1984). Different heterogeneous Q models have been considered in order to assess the uncertainty and resolution of source and attenuation parameters for the given acquisition layout. The results of this simulation study indicate that first pulse width data from a local network permit retrieval with sufficient accuracy of the heterogeneous Q structure and fault radii. A rather dense azimuthal coverage of the sources is instead needed to recover the angles (in particular, the fault strike) which define the fault orientation.

Thermal effects of long intrusions

The cooling of igneous intrusions into the lithosphere is treated by a time-dependent analytical model with a varying initial temperature distribution throughout the country rock. One important result is that, for long times after the intrusion, the effect of the intrusion on the surface gradient is approximately the same as for a host rock with a uniform temperature equal to the mean of the initial temperature distribution through that interval. The method is of practical importance because it allows us to study the thermal anomaly produced within the lithosphere by a hot batholith or by an astenolith of long vertical extension just after its emplacement.

A three‐dimensional Qp imaging of the shallowest subsurface of Campi Flegrei offshore caldera, southern Italy

To improve the knowledge of the shallowest subsurface of Campi Flegrei caldera, a 3-D P wave attenuation tomography of the area was performed. We analyzed about 18,000 active seismic traces, which provided a dataset of 11,873 Δt* measurements, e.g. the differential traveltimes to quality factor ratios. These were inverted through an adapted tomographic inversion procedure. The 3-D tomographic images reveal an average QP about 70, interpreted as water-saturated volcanic and marine sediments. An arc-like, low-QP structure at 0.5-1 km depths was interpreted as a densely fractured, fluid saturated rock volume, well matching the buried rim of Campi Flegrei caldera. The spatial distribution of high and low-QP bodies in the inner caldera is correlated with low-VP values and may reflect either the differences in the percentage of fluid saturation of sediments or the presence of vapor state fluids beneath fumaroles manifestations.

Modeling of Rayleigh waves dispersion in the Sannio region (Southern Italy) from seismic active refraction data

Short period surface waves, recorded during a seismic refractionsurvey in the Sannio region (Southern Italy), have been modeled to infera shallow velocity model for the area. Based on the decrease of resolutionwith depth, due to the bias on group velocity estimates arising frominterference of the Rayleigh waves with higher modes, we carried out aprocedure of fitting, with synthetic seismograms, of selected filtered traceswith a gaussian filter, having a width at half height equal to 1 Hz and acentral frequency lying in the range [1,4] Hz. We estimated the likelihoodbetween synthetic and observed seismograms by measuring their semblance.In this way we were able to infer a more refined local velocity modelcharacterized by a high Vp and Vs vertical gradient in the sedimentarycover. Two ad hoc resolution studies, based on group velocity andamplitude data respectively, indicate that the local velocity model is a goodvelocity model also for the entire studied area. The increase in the numberof available data when using amplitude information allows us to make amore selective choice in the model parameter space (Vp and Vs of eachlayer) and to solve for the Vp/Vs ratio. The inferred Vp velocity in thehalf-space is equal to 2.8 km/s. This value is in excellent agreement withthat inferred by other authors (3 km/s) by modeling P-wave travel timevs. distance. The best-fit model furnish low Vp/Vs for the sedimentarycover so indicating a high degree of the sediments compaction in thestudied area. The inferred shallow high-velocity gradient indicates thatthe shallow sedimentary layer in the area could trap and focus the energytraveling into it.