JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B8, PAGES 16,265-16,286, AUGUST 10, 2001

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B8, PAGES 16,265-16,286, AUGUST 10, 2001 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) Salvatore de Lorenzo Dipartimento di Geologia e Geofisica, Universith di Bari, Bari, Italia Aldo Zollo Dipartimento di Scienze Fisiche, Universith di Napoli "Federico II", Naples, Italia Francesco Mongelli Dipartimento di Geologia e Geofisica, Universith di Bari, Bari, Italia 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 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 ( 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 durationshow 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 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 km, during one or more recent (time of < 10 kyr) eruptiv events. 1. Introduction This paper mainly concerns the study of the anelastic properties of the very shallow crust materials at the Campi Flegrei (Italy) active volcanic area (Figure 1). The Campi Copyright 2001 by the American Geophysical Union. Paper number 2000JB /01/2000JB Flegrei caldera is inside the Campanian Plain, a graben-like structure located at the eastern margin of the Tyrrhenian Sea, where a marked lithospheric thinning is observed [see Wang et al., 1989, and references therein]. The formation of the Campi Flegrei caldera is related to the collapse that followed the Campanian Ignimbritic eruption which occurred about 35 kyr B.P. [Rosi and Sbrana, 1987]. The postformation caldera activity was characterized by several eruptive episodes occurring inside and at the border of the primary caldera. The 16,265

2 16,266 DE LORENZO ET AL.: Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE A 1 V/////I... Astroni Crater 4 Bay of Naples I / Mt. Nuovo / C ter I / D VoloaBo ß,?.,=' J,,.,,.' ß Agnano A 1 Km Figure 1. (a) Earthquak epicenters (February-June 1984) with other geophysical results superimposed: area 1, Vv/V > 1.9 at 1-km depth; area 2, Vv/V s ( 1.6 at 1-km depth; line 3, 40-cm elevation contours for the uplift (data from Osservatorio Vesuviano, Rapporto Sorveglianza 1985); line 4, elliptical trace of the hypothesized ring fault of Aster and Meyer [1988]; line 5, 1-mgal Bouguer gravity contours [after Aster et al., 1992]. (b) Hypocenter locations of 360 earthquakes obtained by using the three-dimensional velocity model of Aster and Meyer [1988] [after Aster et al., 1992]. (c) Recording of a seismic events that occurred in March 1984 at the Campi Flegrei caldera and the fit of the first P wave at each station with the synthetic P wave field through the Qp structure and the source parameters retrieved in this study. volcanological history of the caldera is documented in various the identification of a P-SV converted phase in a seismic papers [see Dvorak and Berrino, 1991, and references profile [Ferrucci et al., 1992], by the maximum depth of therein]. earthquakes [Aster et al., 1992; De Natale and Pingue, 1993], Geophysical evidence for a crustal magmatic body which is and by petrological and geochemical data (see e.g., Civetta et responsible for eruption activity after the caldera formation is al. [ 1991 ] and Annienti et al. [ sparse and rather poorly constrained. The possible occurrence Recently, two important phenomena of ground uplift of a magma reservoir at about 4 km of depth is indicated by accompanied by seismic activity occurred in the area during

3 DE LORENZO ET AL.: Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,267 c event date: ,1 5 zoom of first P waves and their fitting S21 S20 ls... 2S S12... Sll $ ---,; ls so time [s] Figure 1. (continued) and , producing a cumulative maximum uplift of about 3.5 m at the densely inhabited town of Pozzuoli, 8 km west of Naples. The first genetic models of the ground deformation pattern during unrest episodes ascribed it to a pressure increase at the top of a shallow magma chamber embedded in an elastic medium [Berrino et al., 1984; Bonasia et al., 1984; Bianchi et al., 1984, 1987; Dvorak and Berrino, 1991; De Natale and Ping te, 1993]. DInSAR images collected in the period 1993 to 1996 have a very dense space coverage and allow a reliable estimate of the Mogi-type pressure source location. It has been located about 800 m offshore SW of the town of Pozzuoli at a depth of about 2700 m [Availone et al., 1999]. Bonafede et al. [1986] considered the pressure source embedded in a viscoelastic medium, whereas Como and Lembo [1992] developed a numerical model to simulate the effects of fracturation and conductive thermal propagation on the ground deformation. All of these models require overpressures of several hundred megapascals to justify about 3 m of maximum ground uplift in an elastic medium. If a viscoelastic medium was assumed, an unrealistic insensitive to the depth of the overpressure source. The uplift episode was accompanied by intense microearthquake activity, producing more than 10,000 events in the magnitude range [Del Pezzo et al., 1987b]. During 1984, for a period of about 6 months, a local seismic network consisting of 12 three-component digital (12 bit) viscosity as low as 1016 Pa s was necessary in order to keep seismographs was operated by the University of Wisconsin in the overpressure within reasonable values [Bonafede et al., the framework of a cooperative project with Osservatorio 1986]. Finally, the sudden deflation of the area without the Vesuviano [Aster et al., 1992]. The network recorded highoccurrence of any eruption cannot be easily explained with quality signals from several hundred microearthquakes. mechanical source models. One-dimensional (l-d) and 2-D numerical models based on the application of nonequilibrium thermodynamics to the interaction between a thermal source and the circulation of groundwater in a permeable system [Gaeta et al., 1998] have shown that hydrothermal systems have the effect of amplifying the ground deformation. In fact, they transfer at shallower depth and on larger surfaces the increase of pressure occurring at the base of the layer. Therefore the motion of the front of overpressure should cause a progressive transfer to shallower depths of the inflation center. However, this phenomenon has not been detected by any of the geophysical measurements in the area. De Natale et al. [1997] argue that this is due to the caldera walls acting as rigid boundaries which make the shape of the surface deformation area rather A number of studies concerning the estimate of the

4 16,268 DE LORENZO ET AL.: Q, IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE properties of seismic source and propagation have been 1998], global Q studies [Sato and Sacks, 1989; Mitchell, performed by using this data set. Aster and Meyer [1988] 1995; Romanowicz, 1995] reveal a high degree of correlation (hereinafter referred to as AM) used the first P and S arrival among anomalous high-heat-flow and 1ow-Q zones. times to retrieve a refined three-dimensional Vp and Vs velocity In order to discern between the different thermal and model for the upper 3 km of the crust, which roughly mechanical numerical models proposed to explain the Campi corresponds to the maximum depth spanned by Flegrei unrest phenomena, it is necessary to have a complete microearthquakes. Their results show a 1ow-Vp, low-vs and overall picture of the thermal regime actually existing in the high-vp/v anomaly located slightly northeast of the town of upper 4 km of the Campi Flegrei caldera and the Pozzuoli. This was interpreted in terms of a highly porous elastic/anelastic rock properties. The measured temperatures water-filled and cracked material filling the inner caldera region. indicate that the inner part of the caldera is actually the site of significant deep thermal anomaly (a temperature of 420øC Del Pezzo et al. [1987b] estimated the microearthquake was measured at 3 km of depth at the San Vito 1 well). source parameters from the inversion of the displacement Consistently, surface geothermal gradients of about 150 ø- spectra. They observed source radii in the range m and a rather constant stress drop Ao versus moment pattern of 200øC/km are measured in the AGIP (Azienda Italiana Generale Petroli) deep wells and in shallow boreholes inside microearthquakes (Ac of around 4 bars). Fault plane the caldera [Corrado et al., 1998]. solutions for Campi Flegrei microearthquakes have been obtained by using both the standard polarity method [Gaudiosi and Iannaccone, 1984; Aster et al., 1992] and a method [Zollo and Bernard, 1991] of joint inversion of S polarization and first P pulse polarities [De Natale et al., 1995]. These studies show a variability of fault plane In this study we imaged the three-dimensional Qp of the Campi Flegrei caldera. The comparison of Qp images with both Vp and Vs images and geochemical, geothermal, and hydrogeological data allows us to distinguish among surface fluid-filled rocks and deeper hot rocks in which the Qp variations can be primarily attributed to the variation of the orientations, which seems to be controlled by the local stress high-temperature field. rather than by the regional or induced stress by the ground On the basis of actual knowledge of the surface geothermal deformation. On the other hand, the earthquake mechanisms field [Corrado et al., 1998] and some them al measurements for the swarm sequence that occurred on April 1 define a rather coherent pattern of inferred P axes pointing out radially from the maximum uplift center thus suggesting a strong link between swarm activity and stress regime induced by the ground inflation [De Natale et al., 1995]. Seismic attenuation measurements have been performed by from deep boreholes, we calibrated the relationship between the temperature field and the Qp quality factor of the area and convened the three-dimensional Qp image in a temperature map, from which an image of the temperature field at sufficiently high depth (between 1 and 3 kin) was inferred and the thermal properties of possible hot bodies were Del Pezzo et al. [1987a] and Aster et al. [1992] by spectral investigated. analyses of coda waves and direct S waves. The spatial average Qs estimates from these studies show values in the 2. Inversion of Campi Flegrei Microearthquake range , with lower values associated with direct S Data' Estimate of P Wave Attenuation Images measurements. On the other hand, lower values for the P quality factor (in the range 10-40) have been obtained by Ortiz et al. [1992] and Vila et al. [1997] by analyzing P coda waves and body wave dispersion. The present study concerns the analysis of the 1984 The data set used for the seismic attenuation analysis in this study is composed of three-component seismograms from 87 microearthquakes recorded by a minimum of four stations in the period March-April 1984 (Figure 2). The 3-D velocity microearthquake data set, aimed at reconstructing a refined model has been obtained by the local tomography study of (lxlxl kin') seismic P attenuation image for the shallow AM. Preliminarily the events have been located in the AM 3- volume (depth range 0-3 km) beneath Campi Flegrei caldera D structure using both first P and S arrival times. The based on measurements of P pulse width. This study is motivated by the availability for this region of a quite refined microearthquakes appear located in the central part of the caldera at depths between 1 and 5 km, with the highest 3-D velocity model (AM) and a large number of high-quality concentration between 1.5 and 3 km (Figure 1). seismic records. The reconstruction of Q imaging is actually considered as a powerful tool to study thermal properties of subsurface rocks For each recorded event we measured the arrival time, rise time, and total pulse width of direct P waves by considering only those stations for which the signal-to-noise ratio is in volcanic areas and geothermal field, in defining the location greater than 4. Depending on the number of recordings, the of magma bodies and in establishing the extension of the fractured systems characterized by fluid circulation [Sanders and Nixon, 1995; Sanders et al., 1995; Zucca et al., 1994; Wu and Lees, 1996]. Usually, a high degree of cracking or high effective porosity [Zucca et al., 1994; Sanders et al., 1995] and the presence of water or gas in the fractures lower both number of data varied from 8 (four rise time and four total pulse width measurements) to 18. The average measurement error is 0.03 s for the rise times and 0.06 s for pulse widths. The total number of pulse data for our analysis is around 1000 measurements (Figure 3). Although the P wave travel distances are rather short, both Qp and Q. Laboratory measurements in the seismic frequency rise time and pulse width data show a dependence with the range [Kampfmann and Berckemer, 1985] indicate an exponential increase of intrinsic attenuation with the temperature of the rocks, according to an exponential type Arrhenius law. Despite actual uncertainties in uni fing travel time in the area, but a large scatter is also present in the data (Figure 3). Similar observations can be done when considering the P pulse data for each individual event. The numerical simulations in our companion paper [Zollo and de estimates of Q from different methodologies [Romanowicz, Lorenzo, this issue] (hereinaftereferred to as ZD) shows that

5 DE LORENZO ET AL.: Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,269 A '- I=LI _.'_ 40o48 '- B 14ob3, 14o1, LONGITUDE 40ø50 ' LONGITUDE I ' Figure 2. (a) Epicenter location of the 87 Campi Flegrei earthquakes considered in this study. (b) Horizontal ray coverage for the 87 earthquakes data set considered in this study. for microearthquakes having source radii in the range of the Campi Flegrei events ( m), the effect of source finiteness and directivity can originate the large scattering of data in pulse width versus travel time plots. Since the hypothesis of concurrent source and path effects affecting the P pulse width is not a priori verified, we decided to proceed on data inversion by comparing the average variances provided by models presenting an increasing level of complexity, i.e., from a point sourcearthquake model and homogeneous Q, to a finite dimension source model and 3-D attenuation medium. In calculating the misfit function defined by ZD, we

6 16,270 DE LORENZO ET AL.' Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE l O. lo / LLI 0.3 / 0.1 / 0.0 / m I I I I I I TRAVEL TIME {s] Figure 3. Pulse width and rise time versus travel time of the analyzedata set. Also shown is the least squares straight line corresponding to the delta-like sources and homogeneous Q structure assumption. Note the large scattering of both data sets around the best fitting straight line. This may be a priori due both to the source effects and/or to the heterogeneity of the attenuation parameters of the crust at the Campi Flegrei caldera. assigned weighting factors inversely related to uncertainties measurement due to the larger uncertainty (about twice the on measurements of P rise times and pulse widths. Depending uncertainty of rise time) related to its measurement. on the signal-to-noise ratio, we subdivided the data in five In this section we only discuss the results concerning the categories; data for which the pulse is contaminated by estimate of the attenuation parameter and its space variation. secondary arrival were discarded. The first three categories The next section will describe the analysis used to estimate the consist of high-quality recordings (14/ = 1, 0.9, and 0.8 source parameters. corresponding to a level of uncertainty of less than or equal to 5%, 10%, and 15%, respectively). The last two consist of lower-quality recording (W = 0.4, 0.3 corresponding to an 2.1. Earthquake Point Source Model and Homogeneous uncertainty of less than or equal to 40% and >50%, respectively). However, the actual data set is mainly In order to estimate the homogeneous Q, structure under constituted by high-quality data (80% with uncertainties of the assumption of point-like sources, we applied the rise time less than 15%). method [Wu and Lees, 1996]. In this case, a linear relation We also assigned to total pulse width data a weighting among rise time values q:u,o,s (or pulse width AT,;/,obs) and factor equal to the half of the corresponding rise time travel time T is assumed, according to the equations:

7 DE LORENZO ET AL.' Qp IMAGING/CAMPI FLEGREI CALDERA 'ITIERMAL STATE 16,271 POINT-SOURCE UNIFORM Q ",,, ,,,I!,,- ß o.o _ o.oo I o.ooß I"ilJ.,"" _ ' i#';-. ", O. lo == ' ' I' =1 ' I ' I ' I FINITE SOURCE 3-D Q ß ß... ß 0.4 ß ß "" I'-I ß. Hli,,, t' o. _.., I.0.2 III.0.6 'l J-I ß.0.7."..o.8 I I I I" I , 4,, ß I I I I I TRAVEL TIME [sec] Figure 4. Residuals between rise time (and pulse width) data and the corresponding data predicted by the model as a function of travel time. (left) Residuals corresponding to the assumption of a point-like source and homogeneous Q model, (fight) those corresponding to the assumption of a finite dimension seismic source and the 3-D attenuation structure.,./,,b., = o., + CiT those previously retrieved and determined the best fitting ( ) value of Q for all the events by assuming starting value /tt,.,,,,,., =,STo, ' + c r. the average one obtained at the first step. The obtained By applying equations (1), first we determined the best attenuation parameters was Q = 280 _ fitting slope of both the rise time (C/) and of the pulse width The Q parameter uncertainty was estimated by mapping (C2) versus travel time for the whole data set (Figure 3); random deviates on data in the model parameter space, which therefore for each event, we computed the best fitting value of is a classic approach to the study of the errors in nonlinear both the rise time at source %./ and of the pulse width at problems [i.e., Vasco and Johnson, 1998]. With this aim, we source AT0./. Finally, the pulse width theoretical data were carded out 50 inversions applied to an equivalent number of inferred throughout equations (1), which were used to data sets generated by adding to the measured quantities a compute the residuals with the observed data. The average uniform random en'or in the range of the estimated data residual for rise time and total pulse width are <fix'> = s uncertainty. A variance of 2.1 x 10 '3 s 2 (corresponding to an and <is(at)> = s, respectively. The residuals as a RMS of s) was obtained by assuming the homogeneous function of travel time are shown in Figure 4. Qp structure, with a reduction of about 100% in rise time and pulse width residuals with respect to the previous case of a 2.2. $ato and Hirasawa [1973] Circular Crack Source point-like, impulsive source. Model and Homogeneous Q, Model 2.3. Sato and Hirasawa [1973] Circular Crack Source The pulse width and rise time data are inverted for Model and 3-D Qt, Model obtaining the source parameters of the 87 considered events (source radius, dip, and strike of the fault) along with an In order to obtain an image of the vertical variations of Qp estimate of Qp (the apparent Qp, as defined by ZD) which can in the caldera, we preliminarily subdivided the ray-crossed be different from one event to another. area in three horizontal 1-km-thick sheets (0-1 km, 1-2 km, We retrieved highly variable apparent Qp values ranging and 2-3 km). Each sheet was further subdivided in lxlxl km blocks. from Qp = 18 to Qp = 862. This Qp variability per event reflects the heterogeneity of the attenuation structure in the The convergence of the iterative inversion procedure (ZD) area. At the second step we fixed the source parameters to was obtained after three steps. The final value of the variance

8 16,272 DE LORENZO ET AL.: Q, IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE was 1.30 s 2 (corresponding to an RMS of s). The present 3-D model (Plate 1) provides a global variance reduction of 38% with respecto the previous homogeneous Q, model. As for the homogeneous Q, structure, also for the 3-D structure the Q, parameter uncertainties were estimated by mapping random deviates on data in the Q, parameter space, with the source parameters fixed to the best fit values obtained by the inversion (Plate 2). We carried out 100 inversions of an equivalent number of data sets generated by adding to the measured rise time and pulse width data a uniform random error in the range of the estimated data error. Several fundamental features can be observed from the tomographic 3-D Qp images (Figure 5): 1. A large low-qp anomaly, in the shallow layer (between 0 and 1 km in depth, in the following referred to as layer 1) located northeast of the town of Pozzuoli is observed. The ad value (=2.2) at shallow depths (0-1 km) and sharply decreases with depth (about 1.9 at 3 km in depth). AM attributed a high Vv/Vs value at shallow depths in Campi Flegrei to the effect of fluid-filled cracked rock matehals. The decrease of Vp/V, with depth and the correlation between low Qp and low Vs suggest that thermal effects are dominant in this region Comparison of Our Results With Previous Attenuation Measurements in the Area Previous studies of Q in the Campi Flegrei caldera were mainly devoted to modeling spectral properties of coda waves and direct S waves. Del Pezzo et al. [ 1987a] and Aster et al. [ 1992] interpreted the almost constant coda Q (Qc) versus frequency as the dominant effect of intrinsic against scattering attenuation, which seems to be a characteristic of volcanic areas where the intrinsic attenuation should be strongly affected by the high hoc resolution study for an anomaly having approximately the temperature fields produced by hot bodies. De Natale et al. same geometry and position indicates that this anomalous Qp [1987] determined an average value of direct S wave quality zone can be detected and sufficiently well resolved by the factor in the area (Qs = ) by using a spectral inversion of pulse width and rise time data (ZD). Moreover, technique. Comparing this result with the spatially averaged by the error map we can infer that the level of uncertainty Q, obtained the present study (Q = ), we obtain affecting this anomalous Qv body is very low (around 5%). that Qp/Qs = 2.6. This value indicates that rocks filling the 2. A more smooth variation of Q, in the intermediate layer inner caldera primarily dissipate the elastic energy by shear (in the following referred to as layer 2) between 1 and 2 km in deformation, according to various laboratory [Kampfmann depth with secondary, extremely localized low-qv values is and Berckemer, 1985] and regional Q [Sato and Sacks, 1989] observed. Synthetic tests (ZD) for small anomalies having studies which indicate Qv/Q in these cases. Also, approximately the same geometry and position indicate that, De Natale et al. [1987] showed that the kappa parameter of in this anomalous Qp zone, a high level of recovery of these the Anderson and Hough [1984] attenuation model is k = confined 1ow-Qv anomalies is expected, without the s for $H waves and s for P introduction of lateral and vertical significant smearing waves in Campi Flegrei caldera. These rather low values (3 effects. A maximum uncertainty of 15% in the Qp values is times smaller than k values measured in California) were estimated by the corresponding error map for this layer. attributed to the relatively short distances traveled by body 3. In the layer between 2 and 3 km in depth (layer 3) and waves (<10 km) in the caldera. Assuming a mean P wave in the central eastern part of the caldera, a 1ow-Qp area exists, travel distance R of 7 km and Vp = 3 km/s, we obtain an extending from the caldera center toward the East. An ad hoc equivalent Qp = R/(v k)_--290, which is consistent with the synthetic test (ZD) indicates that a 1ow-Qp body having average Qp obtained from this study. similar extension and geometry can be adequately detected by Different estimates of body wave Q have been proposed by our data set, although an overestimation of the Qp value is Ortiz et al. [1992] and Vila et al. [1997] analyzing a subset of expected. The error map for this layer indicates that resolution the Campi Flegrei microearthquake data. These authors progressively deteriorates toward the edges, whereas it is obtained rather low values for the P quality factor (in the sufficiently high at the center, in the zone where a higher ray range 10-40) by analyzing P coda waves and body wave sampling verifies (see ZD). However a maximum uncertainty dispersion and assuming a point-like source model. The of about 15% affects the low-q body located in the eastern discrepancies between results of Vila et al. [1997] and those part of the caldera. from the present study may arise from the different 4. In the central part of the area a high-qp body (Qp = assumptions the two methods are based on, i.e., the dimension 1000) separates the 1ow-Q, anomalies. An ad hoc synthetic of the propagation medium (two dimensions versus three test (ZD) indicates that a similar structure can be recovered by dimensions), the source description (point-like versus finite the inversion. However, owing to the location of this body in source), the selected wave window (P wave train including the an area poorly sampled by rays, the true shape of this body coda versus first P-pulse), and the data to be interpreted cannot be recovered, and smearing effects are expected from (phase spectrum versus first P pulse duration). the corresponding synthetic test (ZD). 5. The comparison of Qp tomographic images with seismic 3. Inversion of Carnpi Flegrei Microearthquake wave velocity maps inferred by first arrival time inversion Data: Estimate of the Source Parameters (AM) shows interesting correlations (Figure 5-). In particular, the 1ow-Q, region in layer 3, east of the town of Pozzuoli, The source parameters (fault radius and angles) have been matches well both in location and geomet 3, with the low-v, obtained by the iterative inversion scheme described by ZD area. The correlation between Qp and V maps appears weaker assuming the 3-D attenuation model. The source parameter moving toward shallower depths, althoughigh-vs are located uncertainties and resolution was instead evaluated by on the north edge of the high-qp anomaly in layers 1 and 2. In performing 200 inversion runs in each of them, the Q structure the same area the Vp-to-Vs ratio shows an anomalous high being fixed to the three-dimensional best fit model. For each

9 , DE LORENZO ET AL.' Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,273!ayerl'0<z< 1 km A - 13oo - 11oo 40o50, QP 40o48, - I 14oo3, 1 14ø11' L 300 IO0 B Iil 40o50, - layer 2' 1 <z<2km i ooo - 8oo P o48, --.-.J ' t 14Ol 1, c layer3' 2 < z < 3 km oo 40%0'-, - looo ' I 14ø11 ' Qp 600 j LONGITUDE Plate 1. Three-dimensional Q, attenuation structure of the Campi Flegrei caldera

10 _, 16,274 DE LORENZO ET AL.: QP IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE layer 1'0 < z < 1 km 0,14 A ø50, -- 40o48, -- 0,10 -o.o 6Q/Q ,04 Iil I 14ø03 ' layer 2'1 <z<2km t ' B I I 40o50, Q/Q 40o48, _ ø03 ' I 14ø11 ' layer3' 2 < z < 3 km 0.16 c o ' -- -" 'x' Q/Q I 14ø03 ' LONGITUDE 14ø11' Plate :Z. Error maps estimated by mapping random deviates of data in the Q, parameter space (see text).

11 DE LORENZO ET AL.: Op IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,275 B v N V ' ;. "*',, ' tl / / x ' ' 14 ø07' ' Longitude ' ' ' Figure 5. (a) Three-dimensional Vs map at the Campi Flegrei caldera [after Aster and Meyer, 1988]. (b) Three-dimensional Ve/V. map at the Campi Flegrei caldera [after Aster and Meyer, 1988]. inversion run, a new data series has been generated by adding to original data a uniform distributed random quantity in the faults are for most of the events confined in the range ø. Overall, the less resolved source parameter is the fault strike, range of observed errors in data. while few solutions show a rather well-constrained value The histograms for each parameter have been computed (error of <15ø). This can be due to the poor azimuthal and the chi square test was used to assess the reliability of the obtained parameter distribution, as detailed by ZD. The inferred source parameters are reported in table 1 and plotted coverage for the current source-receiver configuration. As derived by noise-free synthetic tests (ZD), the actual source-receiver geometry is suitable to detect source in Figure 6. Average error estimates for well-resolved source parameters at least when seismic source radii are greater than parameters (7 m for the radii of the faults, 12 ø for the dip, and 43 ø for the strikes) are consistent with the uncertainty in 30 m. Clearly, the presence of noise in data reflects itself into a certain degree of indetermination of source parameters; ZD model parameters as inferred from the resolution tests reported show that the level of noise in data does not significantly by ZD. Fault radii vary from 70 m to 230 m and do not show a well-defined pattern with the hypocentral depth. Dips of the affect the estimate of fault radius and that robust estimates of this parameter and of its uncertainty can be retrieved by the

12 16,276 DE LORENZO ET AL.: Q, IMAG1NG/CAMPI FLEGREI CALDERA THERMAL STATE Table 1. Source Parameters of the Earthquakes Date Time, p, m Error 5, deg Error q, deg Error LT p, m 5, deg q, deg 1 March 18, :35, March 15, :05, March 16, :57, March 17, :30, March 18, :27, March 19, :30, March 19, :22, March 19, :28, March 19, :35, March 19, :57, March 18, :49, April 1, :60, April 1, :21, March 19, :23, April 1, :22, April 1, :41, April 1, :60, April 1, :04, March 19, :42, March 18, :22, March 19, :09, March 19, :59, March 19, :15, March 10, :45, March 20, :43, March 22, :32, April 1, :31, April 1, :50, April 1, :54, April 1, :13, April 1, :33, April 1, :58, March 20, :34, April 1, :39, April 1, :09, April 1, :27, April 1, :38, April 1, :15, April 1, :51, March 21, :14, March 21, :51, March 21, :42, March 23, :42, April 1, :38, April 1, :04, April 1, :44, April 1, :52, April 1, :55, April 1, :44, April 1, :04, April 1, :21, April 1, :44, March 20, :44, March 19, :57, March 19, :54, March 19, :05, April 1, :13, April 1, :60, March 15, :41, March 16, :10, March 18, :35, March 19, :48, March 20, :11, March 20, :27, March 21, :38, March 21, :06, FPFIT 5, deg q, deg

13 DE LORENZO ET AL.: Q IMAGING/CAMPI FLEGREI CALDERA THEP MAL STATE 16,277 Table 1. (continued) Date Time, p, m Error 5, deg Error, de Error LT p, m i, deg, deg March 22, :55, March 22, :46, March 22, :28, March 23, :49, March 23, :21, March 24, :12, March 24, :14, March 24, :55, March 24, :51, March 24, :15, March 24, :31, March 24, :12, March 24, :61, March 26, :60, April 1, :11, April 1, :30, April 1, :09, April 1, :65, April 1, :52, April 1, :58, April 1, , FPFIT J, deg ), deg (a)fpfit [Reasenberg and Oppenheimer, 1985] represent the fault plane solutions (dip and strike) obtained from the inversion of P polarity data. actual data set. In all synthetic tests we made (ZD), we never observed a trade-offbetween source radius and the angles (dip and strike) which define the orientation of the fault, at least for source radii greater than 50 m. This should indicate the reliability of fault dimensions reported in Table 1 independently of the reliability of fault orientation. Another investigated point is the reliability of the fault angle parameters as retrieved by the rise time and pulse width data inversion (ZD). It has been shown that the adopted approach based on error mapping in the model parameter space allows us to discriminate well-constrained solutions from weak or poorly constrained solutions. For an assigned event, it may happen that resolved dip may couple with unresolved strike and conversely. It has also been shown (ZD) o '--' 60--,' o- r'w O EVENT # Figure 6, Source parameter estimates of the 87 events considered in this study a er the statistical analysis described in the text.

14 16,278 DE LORENZO ET AL.: Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE consistency between fault plane orientations as inferred from P pulse and P polarity measurements, although the number of events is insufficiento assess it quantitatively.? 1E.H9 4. Inferences on the Geothermal Field at Campi Flegrei from P Wave Attenuation Measurements ' 1B-17 1E Source radius (km) Several authors have shown that the quality factor is the seismological parameter more strictly affected by the thermal properties of the rocks [e.g., Mitchell, 1995; Sato and Sacks, 1989]. As was argued by Romanowicz [1994], the nonlinear sensitivity of Q to temperature (Arrhenius law) implies that, in principle, attenuation tomography should be able to resolve hot regions (high attenuation) better than elastic tomography. Kampfmann and Berckemer [1985](hereinafter referred to as KB) determined an exponential dependence of Q on temperature by measuring Q in laboratory experiments on rock materials undergone to forced oscillation in the range Hz. When considering materials for which Qv/Q = (as we have inferred for the Campi Flegrei case) and neglecting dispersion of body waves, the relation that they obtained was T = log Q. (3) Moreover, KB observed that, for temperatures lower than Figure 7. Scaling law for the considered seismic events. It is 900øC, the above equation well explains the dependence of T interesting to observe the extremely low values of the stress on Qp for materials characterized by very different chemical drops and their high variability in the range bar. composition. Wu and Lees [1996] imaged the temperature field at depth in Coso (California) geothermal areas by converting Q in r through equation (3). that the actual rise time and pulse width data set is more Actually, the main problem in recovering T(Qp) is the need sensitive to the dip of the fault than its strike. Only well- to discriminate the thermal effect from effects due to constrained fault angle solutions are further considered for interpretation. Seismic moments M0 for the considered events have been derived by the moment magnitude relation [Bakun and Lindth, variations of permeability of fluid-filled rocks. This problem is relevant in volcanic areas characterized by intense geothermal activity. Both laboratory and seismological studies (see, e.g., Sanders et al. [1995, and references therein] and Zucca et al. 1977] log M0 =1.2 Ma + 17, where Ma is the magnitude [1994]) have in fact shown that Q, and Qs strongly decrease duration estimated by Osservatorio Vesuviano for the same events. Stress drop estimates Ao are therefore computed by using the scaling relationship for circular earthquake sources [Keilis-Borok, 1959]: with the increasing of porosity and fluid circulation in cracked rocks. The main difficulty in interpreting T(Qp) images consists of discriminating the thermal effect from the rock rheology changes due to the presence of, not necessarily hot, fluid- A(7 = 7 M 0 (2) filled, porous/fractured rocks. This ambiguity can be relevant 16 P'0 ' especially in volcanic areas characterized by intense with the radius of the fault Po estimated by the inversion of pulse width and rise time data. Figure 7 shows that the stress drop is rather low, in the range MPa. This result is consistent with stress drop estimates (Ao = MPa) obtained in the frequency domain [Zollo and De Natale, 1986; Del Pezzo et al., 1987b] for the same area. Both the extremely low stress drop values (<1 MPa) and the characteristic swarm-like activity [De Natale and Zollo, 1986] (with only low-magnitudevents) of the Campi Flegrei earthquakesuggesthe low strength and the highly brittle behavior of the rock materials filling the caldera. The inferred fault dips usually range between 30 ø and 60 ø. geothermal activity. In the case of Campi Flegrei caldera P and S velocity estimates by local earthquake tomography provided the evidence for an anomalously high Vp/Vs region located approximately in the same area where the 1ow-Qv anomaly has been found in this study. The Vp/Vs value is higher (2.2) close to the surface (depth of <1 km) and sharply decrease with depth (1.9-2 at 2-3 km). Large V,/Vs (>1.9) values can be generally attributed to a highly fractured fluid-filled rock volume [O'Connell and Budiansky, 1974]. The observed decrease of the Vt,/V. ratio with depth suggests that fluid contribution is weaker at larger depths, where the thermal The well-constrained solutions show fault strikes effect is expected to primarily control the attenuation and preferentially oriented NW and NE. Only for a limited number of this subset we were able to compute reliable fault plane solutions by first P motion polarities by using the FPFIT code [Reasenberg and Oppenheimer, 1985]. There is a general shear wave velocity anomalies. This hypothesis is also confirmed by the values of effective porosity and transmissivity measured in the AGIP wells [AGIP, 1987; De Vivo et al., 1989] and that are observed to decrease with depth.

15 DE LORENZO ET AL.: Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,279 As we detail in the following section, we can retain that, at these depths, rocks are sufficiently compacted that only weak fluid circulation is permitted, so that the inferred attenuation/thermal anomaly could represent the effect of the thermal conductive contribution due to the presence at greater depths of melt or partially molten materials. In order to account for both the different contribution to the attenuation parameter and the eventual dependence of Q on frequency, Sato and Sacks [1989] pointed out the need to calibrate the T versus Q relation by comparing thermal data with temperatures deduced by Q. As is detailed in the following, this calibration study was possible at Campi Flegrei owing to the availability of surface and deep borehole temperature measurements ii MF1 MF2 MF5 SV1 SV Surface and Deep Borehole Thermal Data: Comparison With Q.v-Inferred Temperatures Our knowledge of the thermal state of the Campi Flegrei arises both from thermal measurements in 37 surface boreholes (up to 140-m depth) [Corrado et al., 1998] (Figure 8) and from temperature measurements in five deep AGIP boreholes (Figure 9) [AGIP, 1987]. The geothermal surface gradient map compiled by Corrado et al. [1998] provides clear evidence for strong lateral variation in the geothermal gradient, owing to the effect of groundwater motion through the surface aquifers. After removal of thermal disturbances due to the motion of the water, Corrado et al. [1998] estimated an average geothermal gradient in the range 150ø-200øC/km. The residual geothermal map, obtained by subtracting the low-pass-filtered map (cutoff frequency 3 = 10 km) from the unfiltered geothermal gradient field (cut-off frequency 3 = 10 kin), enhances small-wavelength thermal gradient highs located at Mofete, north of Monte Nuovo and at the Agnano crater (Figure 8). AGIP MF1 and MF2 deep wells are located at the top of the Mofete anomaly, and MF5 is located at its northern edge. On the contrary, the San Vito wells (SV1 and SV3) are located in an area where no local thermal anomaly is present , ,:-?,:///;+)/ \'(--':5':,, ,. I contour intervol= 20øC/kin, Longitude (km) Figure 8. High-pass-filtered map (cutoff wavelength of 10 km) of the surface geothermal gradient at the Campi Flegrei caldera [after Corrado et al., 1998] O TEMPERATURE [øc] Figure 9. Temperature measurements a function of depth in five AGIP geothermal wells. Temperature and hydrothermal mineral zonation versus depth furnish consistent information on the geothermal system existing at the Campi Flegrei caldera. A mineral zonation typical of hydrothermal systems is well developed in the Mofete wells and, although less clear, in the San Vito wells [Chelini and Sbrana, 1987]. Argillitic and illite-chlorite paragenesis are ubiquitous down to depths of m (MF1 and MF2), 1400 m (MF5) in the Mofete area and m (SV3) and (SV1) in the San Vito area. They are due to mineral transformations at temperatures lower than about 250øC. The produced rock has a very low permeability, and it forms the impervious cap rock of the geothermal system. At greater depths, neogenic minerals (K-feldspar, adularia, albite, and quartz) are dominant. They are formed by precipitation from hot water cooled by contact with the host rocks. This zone (calc-aluminum silicate zone) defines the top of the circulating fluid system. The resulting rock is often highly permeable. The temperatures measured in the wells at the depth corresponding to the transition from the illite-chlorite to the calc-aluminum silicate zone are 250øC at Mofete and 220ø-270øC at San Vito. The temperature gradients measured and inferred in wells MF2 and MF5 are much higher than those in boreholes SV 1 and SV3, consistent with the average values observed in shallow wells. MF wells have met important aquifers running on the argillitic zone ( m of depth) and within the calc-aluminum silicate zone ( m of depth) and at greater depth. No evidence for widespread aquifers was found at the SV wells, except for a very shallow water table in the yellow tuff and pyroclastics. This confirms that the present fluid circulation scarcely affects the geotherms of boreholes SV1 and SV3, which can be assumed to represent mostly a little disturbed conductive gradient typical of the area. These geotherm show a mean gradient of 130ø-210øC/km at depths smaller than 1.5 km and of 44ø-64øC/km at greater depths. This abrupt change may be explained by a change of the thermal conductivity, from typical surficial tuff values (0.85 Wm'IK ' ) [Corrado et al., 1998] to higher values expected for high-temperature hydrothermally altered rocks (2-3 Wm- K-1).

16 ß 16,280 DE LORENZO ET AL.' Qp IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE iii A oo-' Local calibration Kampfmann and Berckemer relation [1985) ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I 5 6OG (Q7p) 8 9 Qp-T conversion O LOCAL CALIBRATION ß KAMPFMANN AND BERCKEMER (1985) RELATION ], S.Vitol geotherm (eight in layer 2 and eight in layer 3) near wells SV1 and SV3, and we correlated them to the temperatures observed at the same depths (Figure 10). In the analysis we discarded the first layer, since both the Vp/Vs values and hydrogeological data [Corrado et al., 1998] coherently indicate the existence of a superficial aquifer whose recharge zone is spatially related with the 1ow-Q, body located in the northeastern sector of layer 1. This analysis provided the following empirical equation: T= log (Q,). (4) The standardeviation of Q, and the spanned temperature ranges are also shown in Figure 10, where the KB relation is shown for comparison. The shaded area represents the accuracy of Q/,. The values of In (Q/,) obtained by relation (4) are higher than those obtained by the KB equation in the range of values inferred at Campi Flegrei. This may be due to the effect of the lithostatic pressure which contributes to the increase of Q,. If this is the case, it is possible to evaluate the rate of change of Q, with pressure (o0. Since several laboratory studies indicate a type Arrhenius law for the dependence of Q on temperature and pressure [Sato and Sacks, 1989; Wu and Lees, 1996], this last effect may be roughly expressed by: A log Q, = o pgfd. (5) If we assume an average density value p = 2300 kg m -3 we obtain o = 9 x 10-9 Pa'. Plate 3 shows the temperature maps at depths between 1 and 2 km and 2 and 3 km as inferred by the Q, estimates by using the Q-T conversion formula (4). A small positive thermal anomaly located on the western caldera border (Mofete area) is present in the entire depth range, in a zone poorly resolved by the inversion, where high temperatures were measured at depth in geothermal boreholes. A weaker thermal anomaly is also present NE of the studied area, where the recharge area of the Campi Flegrei aquifer is located [Corrado et al., 1998], so that it can be related to the overestimation of the temperature due to water infiltration. In fact, there is no evidence for such a thermal anomaly at very shallow depths as inferred from the measurements of TEMPERATURE (øc) geothermal gradient by Corrado et al. [1998]. The weak Figure 10. (a) Comparison between the Kampfmann and thermal anomaly offshore may be due to seawater infiltration Berckerner [1985] relationship and the T = T(Q/,) curve [Casertanoetal., 1976]. inferred by the local calibration. (b) Comparison between At greater depths (z > 2 km) the Q,-inferred temperature T(Q,) estimated by the Kampfrnann and Berckerner [1985] image (Plate 3) shows an extended low-q, high-t anomaly, relationship and T(Qp) values inferred by the calibration oriented preferentially EW, in an area including the craters of procedure described in the text. Also shown is the geotherm of Astroni, Agnano, and Solfatara where the most recent eruptive the SV1 geothermal well. activity was concentrated [Barberi et al., 1991 ]. Interestingly, the location and geometry of this anomaly correlate well with the EW elongated region of low V observed by local earthquake tomography (AM) (Figure 5) This indicates that at the transition zone, only a slight and the position of the magma body inferred at about 4-km differencexists between the upper and the lower average heat depth by Ferrucci et al. [ 1992]. flow density (from about 144 mw/m 2 above 1.5 km to 135 The evidence for present magma emplacement at relatively mw/m 2 below this depth). On these grounds, no significant shallow depths underneath Campi Flegrei has been previously deviations from the steady state thermal regime verifies, and noticed by other authors. Bianchi et al. [ 1987] and Ferrucci et this should indicate that there is no allowance for significant al. [1992] suggested a top of a hypothetic magma chamber at thermal convection phenomena. about 4-km depth. Orsi et al. [1996] estimated an initial In order to establish a T versus O, calibration curve for volume of the melt of about 20 km 3, which represents about Campi Flegrei, we considered the values of Op obtained by double the erupted magma. Barberi et al. [ 1991 ] showed that tomographic inversion between 1 and 3 km at the 16 blocks the volcanic activity concentrated in this area occurred about

17 DE LORENZO ET AL.: Qt, IMAG1NG/CAMPI FLEGREI CALDERA THERMAL STATE 16,281 A layer 2' 1 < z < 2 km 40o50,-!11 40ø48 ' - 4ø03 ' 14ø1 1'.2<z<3km ' -- 40ø48 '-! 4o03 ' LONGITUDE j _ 14o11 ' Plate 3. (a) Temperature map at depth between 1 and 2 km as inferred by Qp -T conversion (equation (4)).(b) Temperature map at depth between 2 and 3 km as inferred by Qp-T conversion (equation (4)).

18 , 16,282 DE LORENZO ET AL.: Q/, IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE Table 2. Best Fit Parameters of the Thermal Model Time, kyr X, km Y!, km a, km bl, km Z01, km H, km T0, øc Ti, øc 5 4,25 I 1,3 0,3 1, I 1,3 0,3 1, ! 1,3 0,3 1, ,3 0,3 0, Time, kyr X2, km Y2, km a2, km b2, km Z02, km H2, km To, øc Ti, øc 5 1,25 0,25 1,2 0,5 2, ,25 0 1,2 0,4 2, I 0, I 0 1 0,4 1, (a) Xt, Yt represents the longitude and latitude, respectively, of the first body in the AM coordinate system; X2, Y2 represents the longitude and latitude, respectively, of the second body in the AM coordinate system. The other symbols are described in the text. 11 kyr B.P., giving arise to the Neapolitan Yellow Tuff (NYT) the modeled thermal anomaly, we hypothesized presence eruption. Bianchi et al. [1987] also suggesthat the magma of two parallelepiped heat source bodies whose parameters chamber could have been emplaced about 15 kyr B.P. Dvorak and location are reported in Table 2 and Figure 11. In this and Berrino [ 1991 ] noted that the horizontal extension of the example of thermal modeling the simple shape considered for magma body usually has one dimension much greater than the the intrusion is chosen arbitrarily. Given the observed shape of others. the thermal anomaly, a more complex morphology of the intrusion body cannot be excluded, although its lateral D Heat Conduction Modeling of the v-inferred extension is rather well constrained by the thermal anomaly Temperature Field shape. The most prominent feature of the Q,-inferred temperature The thermal field generated by a parallelepiped with field is the high thermal anomaly (450ø-500øC) located NE of dimensions -a < x < a and -b < y < b and Zo < z < H in a the town of Pozzuoli (Plate 3) in a well-resolved zone (see medium characterized by an initial temperature Ti is [Jaeger, ZD), between 2 and 3 km of depth. 1964; Mongelli et al., 2000] (Figure 11): We hypothesize that this thermal anomaly could represent the thermal effect at shallow depths of a deeper magma body T = l T 0 (x, a)o (y, b)i, (6) 2 (top at depths of >4-5 km), as suggested by the hypocenter where: locations of the microearthquakes that occurred during the crisis and by the evidence for a large-amplitude teleseismic P-to-S converted phase [Aster et al., 1992; Ferrucci et al., 1992]. O(x,a)= erf[2 7 -erf 2 / 7 The model of a magmatic intrusion is that of a rectangular parallelepiped at uniform temperature T that intrudes a country rock with an initial temperature To. Given the shape of and: at:r, -r0. In the previous expression, k is the thermal diffusivity and t is the time after the intrusion. The composition of the erupted products at Campi Flegrei ranges from trachybasalts to hyperalkaline phonolitic trachytes [Armienti et al., 1983] which are characterized by melting temperatures of about 800 ø- 1000øC. We assume, for the sake of simplicity, that the two sources activated simultaneously with an initial temperature T,. of about 800øC. This value is increased by L/c = 300øC [Jaeger, 1964] (L is the latent heat of solidification Figure 11. Diagrams of the two intruded bodies, and c is the specific heat) to account for the heat released parallelepiped shaped, considered in the forward modeling of during solidification. For the sake of simplicity, we also the thermal anomaly east of the town of Pozzuoli. assumed that at the time to of the heat sources emplacement in

19 16,283 DE LORENZO ET AL.: Q, IMAG G/C PI FLEGREI CALDERA THERMAL STATE E o N N E Itu3} 3Gnlll 9

20 16,284 DE LORENZO ET AL.: Q, IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE the caldera, the unknown temperature To of the host medium was homogeneous. The thermal diffusivity at room of events is not sufficiento make a quantitative assessment of coherency. temperature can be assumed equal to 10 '6 m2/s which reduces Three main features can be observed from the tomographic to 0.77 x 10 '6 m2/s at 250øC by considering the variation of the 3-D Q, images: (1) a large low-qp anomaly, in the shallow diffusivity with temperature [Zito et al., 1993]. The horizontal layer (between 0- and 1-km depth) located northeast of the dimensions of the body and the depth of its roof are found by a town of Pozzuoli; (2) a more smooth lateral variation of Q, in trial and error solution of equation (6). the intermediate layer between 1- and 2-km depth with The vertical extensions of the two bodies are the parameters secondary, extremely localized 1ow-Qvvalues; (3) the less constrained, mainly for the lack of thermal data at depths existence, in the layer between 2- and 3-km depth and in the greater than 3 km. These can be estimated by roughly central eastern part of the caldera, of a 1ow-Qvarea, extending assuming thathe total amount of magma was 20 km 3 [Orsi et from the caldera center toward the east. al; 1996] and should be equal to 2 x (2a x 2b x H). The In order to investigate the present thermal state of the resulting parameters are given in Table 2. Plate 4 shows the caldera, we mapped the attenuation parameter into temperature maps computed at different depths and times after temperatures by using a local calibration curve inferred by the magma emplacement time. deep borehole temperature measurements in the area. The eruptive history of Campi Flegrei presents two relevant Seismic velocity estimates of AM indicate an anomalously eruptions (about 11,000 years and 8000 years B.P.) that could high V/'V ratio (2.2) at 0-1 km of depth in the area where the have occurred after the main event associated with the caldera P wave attenuation anomaly is located in the present study. At formation (about 35 kyr B.P.) [Orsi et al., 1996]. There is larger depths, both the P wave attenuation and shear wave volcanological evidence [e.g., Orsi et al., 1996, and references velocity anomalies have similar shape and extension, and the therein] for another eruption phenomenon which occurred at V,-to-V ratio sharply decreases (1.9-2). A similar pattern of about 5000 years B.P. However, these events may also be decreasing values with depth is shown by porosity and associated with a single large magma intrusion which transmissivity measured in central caldera boreholes. All of occurred at about years B.P. [Bianchi et al., 1987]. this evidence suggests a dominantemperatureffect on rock The qualitative comparison of Plates 3 and 4 suggests that rheology for depths larger that 1 km. At shallow depths the the present thermal state is fairly consistent with the attenuation parameter is likely affected by the presence of a emplacement of at least two magma pockets in the region NE densely fractured, porous medium percolated by fluids, as of Pozzuoli at times of 5-8 k.y.b.p. An RMS residual of confirmed by independent geochemical and thermal evidence. about 35øC can be estimated between the observed thermal The increase of Qt, at depths below 1 km and the anomaly and the model corresponding to 8 kyr B.P. The conductive modeling indicates an originary depth of comparison with deep thermal data seem to indicate a higher compaction degree of rocks, probably caused by the combined km and a present depth of 4-5 km for the heat source effect of an increase in lithostatic pressure and hydrothermal body roofs. The latter estimate is consistent with the magma chamber top obtained by Ferrucci et al. [1992]. alteration. The retrieved Qv imageshow that moving outward from the caldera center (the town of Pozzuoli) a general increase of the attenuation parameter is observed. This 5. Discussion and Conclusions characteristic spatial variation is also a common feature of different geophysical and geochemical parameters measured The present study concerns the estimate of microearthquake during the ground uplift episode (seismic source parameters and of the space variation of the P wave attenuation parameter in Campi Flegrei caldera, based on the inversion of first P rise time and pulse width data. The companion paper (ZD) describes the method and its application to synthetic examples aimed at studying the velocities, seismicity, temperatures, ground deformation, helium isotopic ratio, fluid circulation measurements) [Aster e! al., 1992; Allard et al., 1991; Dvorak and Berrino, 1991 ]. The results from the present study and their implications for the thermal state of the caldera lead us to suggesthat the uncertainty and parameteresolution expected for the source recent ground uprising phenomenon and its and receiver configuration of Campi Flegrei microearthquake geophysical/geochemical manifestations could be related to data. Fault radii vary from 70 m to 230 m and do not show a well-defined pattern with the hypocentral depth. The retrieved stress drop values are rather low, in the range MPa. Both the extremely low stress drop values ((1 MPa) and the characteristic swarm-like activity [De Natale and Zollo, 1986] (with only low-magnitude events) of the Campi Flegrei earthquakesuggest the low strength and the the anomalous rheological properties of volcanic deposits filling the inner caldera, mainly controlled by the hightemperature field in a medium that is densely fractured and water and/or gas saturated. The location and geometry of the low-q/high-t anomaly observed at depths between 2 and 3 km correlate well with the EW elongated region of low-v observed by local earthquake highly brittle behavior of the rock materials filling the caldera. tomography (AM). This region lies inside the inner caldera The high variability of the stress drop values (2 orders of where recent (from 11 kyr B.P. to present) magmatic activity magnitude) can represent other evidence for the heterogeneity has been concentrated [Barberi et al., 1991; Orsi et al., in the elastic properties of the material. Dips of the faults as retrieved from our study are for most of the events confined in the range 30ø-60 ø, while strikes of 1996]. On the basis of this background, we postulate that this thermal/attenuation/velocity anomaly can be related to the the faults are less resolved parameters. For a limited number effect of elastic property weakening of rocks induced by high of events we checked the consistency between the fault orientation obtained from P polarity and P pulse data. Although the results are encouraging, the considered number temperatures originated by a more deeply located heat source. An example of 3-D heat conduction modeling of the Qpinferred temperature fields at 1.5- and 2.5-km depths suggests

21 DE LORENZO ET AL.: Qe IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE 16,285 that the presenthermal state and rock rheology of the inner ka, in the light of trace elements and isotope composition, Eur. d. caldera could be controlled by the cooling of at least two Mineral., 3, , Como, M., and M. Lembo, A thermo-mechanical model of the molten bodies that originally intruded at depths of km, inflation and seismicity of volcanicalderas: an application to the during one or more recent (time of<10 kyr) eruptive events Campi Flegrei system, in Volcanic Seisrnology, 1,4 VCEI. Proc. in and whose top is presently at about km in depth Volcanol., vol. 3, edited by P. Gasparini, R. Scarpa, and K. Aki, according to other geophysical and seismic evidence. pp , Springer-Verlag, New York, Corrado, G., S. de Lorenzo, F. Mongelli, A. Tramacere, and G. Zito, Acknowledgments. We thank P. Gasparini for helpful discussions Surface heat flow density at Phlegrean Fields caldera, southern Italy, Geothermics, 27, , regarding the hydrotl ermal state of the Campi Flegrei geothermal field. This work benefits of suggestions of the two reviewers, J. Del Pezzo, E., G. De Natale, G. Scarcella, and A. Zollo, Q. of three- Bhattacharyyand Y. G. Li. component seismograms of volcanic microearthquakes at Campi F!egrei volcanic area (southern Italy), Pure,4ppl. Geophys., 123, ,! 987a. References Del Pezzo, E., G. De Natale, M. Martini, and A. Zollo, Source parameters of microearthquakes at Phlegraean Fields Flegrei Azienda Italiana Generale Petroli (AGIP), Modello geotermico del (southern Italy) volcanic areas, Phys. Earth Planet. Inter., 7, 25- sistema flegreo, 100 pp., SERG-MESG, S.Donato, Milano, Italy, 42, 1987b De Natale, G., and F. Pingue, Ground deformations in collapsed Allard, P., P. Maiorano, D. Tedesco, G. Cortecci, and B. Turi, caldera structures, J. Volcanol. Geotherm. Res. 57, 19-38, Isotopic study of the origin of the sulfur and carbon in Solfatara De Natale, G., and A. Zollo, Statistical analysis and clustering features fumaroles, Campi Flegrei caldera, J. Volcanol. Geotherm. Res., of the Phlegraean Fields earthquake sequence (May 1983-May 48, , ), Bull. Seismol. Soc.,4m., 76, , Anderson, J. G. and S. E. Hough, A model for the shape of the Fourier De Natale, G., G. Iannaccone, M. Martini and A. Zollo, Seismic amplitude spectrum at high frequencies, Bull. Seismol. Soc. Am., sources and attenuation properties at the Campi Flegrei volcanic 74, , area, Pure,4ppl. Geophys., 125, , Armienti, P., F. Barberi, H. Bizouard, R. C!occhiatti, F. Innocenti, N. De Natale, G., A. Zollo, A. Ferraro, and J. Virieux, Accurate fault Metrich, M. Rosi, and A. Sbrana, The Phlegrean Fields: magma mechanism determinations for a 1984 earthquake swarm at Campi evolution within a shallow chamber, J. Volcanol. Geother n. Res., Flegrei caldera (Italy) during an unrest episode: Implications for 48, , volcanological research, 0 r. Geophys. Res., 100, 24,167-24,185, Aster, R., and R. Meyer, Three-dimensional velocity structure and hypocenter distribution in the Campi Flegrei caldera, Italy, De Natale, G., S. M. Petrazzuoli, and F. Pingue, The effect of collapse Tectonophysics, 149, , structures on groun deformation in calderas, Geophys. Res. Lett., Aster, R., R. Meyer, G. De Natale, A. Zollo, M. Martini, E. Del Pezzo, 24, , R. Scarpa, and G. Iannaccone, Seismic investigation of the Campi De Vivo, B., H. E. Belkin, M. Barbieri, W. Chelini, P. Lattanzi, A. Flegrei caldera, in Volcanic Seistnology, IAVCEI. Proc. in Lima, and L. Tolomeo, The Campi Flegrei (Italy) geothermal Volcanol., vol.3, edited by P. Gasparini, R. Scarpa, and K. Aki, system: A fluid inclusion study of the Mofete and San Vito fields, , Springer-Verlag, New York, 1992.,I. Volcanol. Geotherrn. Res., 36, , Availone, A., A. Zollo, B. Briole, C. Delacourt, and F. Beauducel, Dvorak, J. J. and G. Berrino, Recent ground movement and seismic Subsidence of Campi Flegrei (Italy) detected by SAR activity in Campi Flegrei, southern Italy: episodic growth of a interferometry, Geophys. Res. Lett., 26, , resurgent dome, d. Geophys. Res., 96, , Bakun, W. H., and A. Lindth, Local magnitudes, seismic moments Ferrucci, F., A. Him, G. De Natale, J. Virieux, and L. Mirabile, P-SV and coda durations for earthquakes near Oroville, California, Bull. conversion at shallow boundary beneath Campi Flegrei caldera Seismol. Soc. Am., 65, , (Italy): Evidence for the magma chamber, 0 r. Geophys. Res., 97, Barberi, F., E. Cassano, P. La Torre, and A. Sbrana, Structural 15,351-15,359, evolution of Campi F!egrei caldera in light of volcanological and Gaeta, F. S., G. De Natale, F. Peluso, G. Mastrolorenzo, D. geophysical data, J. Volcanol. Geotherm. Res., 48, 33-49, Castagnolo, C. Troise, F. Pingue, G. Mita, and S. Rossano, Berrino, G., G. Corrado, G. Luongo, and B. Toro, Ground Genesis and evolution of unrest episodes at Campi Flegrei caldera: deformation and gravity changes accompanying the 1982 Pozzuoli The role of thermal fluid-dynamical processes in the geothermal uplift, Bull. Volcanol. 44, ,! 984. system,./. Geoph. Res., 103, 20,921-20,933, Bianchi, R., A. Coradini, C. Federico, G. Giberti, G. Sartoris, and R. Gaudiosi,G. and G. lannaccone, Preliminary study of stress pattern Scandone, Modeling of surface ground deformations in the Phlegrean Fields as inferred from focal mechanisms, Bull. Phlegraean Fields volcanic area, Bull. Volcano!., 44, 14,175- Volcanol., 47, , ,186, Jaeger, J. C., Thermal effects of intrusions, Rev. Geophys., 2, 443- Bianchi, R., A. Coradini, C. Federico, G. Giberti, P. Lanciano, J. 466, Pozzi, G. Sartoris, and R. Scandone, Modeling of surface Kampfmann, W. and H. Berckemer, High temperature experiments on deformations in volcanic areas: The 1970-!972 and the elastic and anelastic behaviour of magmatic rocks, Phys. Earth crises at Campi Flegrei, Italy, J. Geophys. Res., 92, 14,139- Planet. Inter., 40, , ,150, Keilis-Borok, V. I., On estimation of the displacement in an Bonafede, M., M. Dragoni, and F. Quareni, Displacements and stress earthquake source dimension,,4nn. Geofis., 12, , fields produced by a centre of dilatation and by a pressure source Mitchell, B. J., Anelastic structure and evolution of the continental in a viscoelastic half-space: application to the study of ground crust and upper mantle from seismic surface wave attenuation, deformation and seismic activity at Campi Flegrei, Italy, Geophys. Rev. Geophys., 33, , J. R. Astron. Soc., 87, , Mongelli, F., G. Zito, and S. de Lorenzo, Thermal effects of long Bonasia, V., F. Pingue, and R. Scarpa, A fluid filled fracture as a intrusions, Geotherrnics, 29, , possible mechanism of ground deformation Phlegraean Fields, O'Connell, R. J. and B. Budiansky, Seismic velocities in dry and Italy, Bull. Volcanol., 47, , saturated cracked solids, 0 r. Geophys. Res., 79, , Casertano, L., A. Oliveri del Castillo, and M.T. Quagliariello, Orsi, G., S. De Vita, and M. di Vito, The restless, resurgent Campi Hydrodynamics and geodynamics in the Phlegraean Fields area of Flegrei nested caldera (Italy): Constraints on its evolution and Italy. Nature, 264, , configuration, d. Volcanol. Geotherm. Res., 74, , Chelini, W. and A. Sbrana, Subsurface geology, in Phlegrean Fields, Ortiz, R., A.M. Correig, J. L. Diez, and M. L. Munoz, Apparent edited by M. Rosi and A. Sbrana, Qua& Ric. Sci., 114, , variation of coda Q in Phlegrean fields during the bradyseismic crisis of , in Volcanic Seisinology, IAVCEI. Proc. In Civetta, L., E. Carluccio, F. lnnocenti, A. Sbrana, and G. Taddeucci, Volcanol., vol. 3, edited by P. Gasparini, R. Scarpa, and K. Aki, Magma chamber evolution at Phlegrean Fields during the last 10 pp , Springer-Verlag, New York, 1992.

22 16,286 DE LORENZO ET AL.: Q, IMAGING/CAMPI FLEGREI CALDERA THERMAL STATE Reasenberg, P., and D. H. Oppenheimer, FPFIT, FPPLOT and FPPAGE: FORTRAN computer program for calculating and displaying earthquake fault-plane solutions, Rep , U.S. Geol. Surv., Washington, D.C Romanowicz, B., Anelastic tomography: A new perspective on upper mantle thermal structure, Earth Planet. Sci. Lett., 128, l l3-121, Romanowicz, B., A global tomographic model of shear attenuation in the upper mantle,,/. Geophys. Res., 100, 12,375-12,394, Romanowicz, B., Attenuation tomography of the Earth's mantle: A review of current status, in Q qf the Earth: Global, Regional and Laboratory Q Studies, edited by B. J. Mitchell and B. Romanowicz, Pure Appl. Geophys., 153, , Rosi, M. and A. Sbrana (Eds.), Phlegrean Fields, Quad. Ric. Sci., 114(9), 175 pp., Sanders, C. O. and L. D. Nixon, S wave attenuation structure in Long Valley caldera, California, from three-component S-to-P amplitude ratio data, d. Geophys. Res., 100, 12,395-12,404, Sanders, C. O., S.C.?onko, L. D. Nixon, and E. A. Schwartz, Seismological evidence for magmatic and hydrothermal structure in Long Valley caldera from local earthquake attenuation and velocity tomography, d. Geophys. Res., 100, , Sato, H. and I. S. Sacks, Artelasticity and thermal structure of the oceanic upper mantle: temperature calibration with heat flow data, d. Geophys. Res., 94, , Vasco, D. W., and L. R. Johnson, Whole earth structure estimated from seismic arrival times, d. Geophys. Res.,103, , Vila J., A.M. Correig and R. Carbonell, New evidence of azimuthal variations at Campi Flegrei: Attenuation and predominant frequency, Pure and Appl. Geophys., 150, , Wang, C.Y, W. T: Hwang, and Y. Shi, Thermal evolution of a riff basin: The Tyrrhenian Sea, d. Geophys. Res., 94, , Wu, H., and M. Lees, Attenuation structure of Coso geothermal area, California, from wave pulse widths, Bull. Seismol. Soc. Am., 86, , Zito, G., F. Mongelli, and M. Loddo, Temperature dependence of the thermal parameters of some rocks, Boll. Geofis. Teor. Appl., 140, , Zollo, A., and P. Bernard, Fault mechanisms from near source data: joint inversion of S polarizations and P polarities, Geophys. d. R. Astron. Soc., 104, , Zollo, A. and S. de Lorenzo, Source parameters and three-dimensional attenuation structure from the inversion of microearthquake pulse width data: Method and synthetic tests, d. Geophys. Res., this issue. Zollo, A., and G. De Natale, Scaling of source and peak ground motion parameters from microearthquakes at Campi Flegrei (southern Italy) volcanic area, in Seismic Hazard in Mediterannean Regions, edited by J. Bonnin, M. Cara, A. Cisternas, and R. Fantechi, pp , Kluwer Acad. Norwell, Mass., Zucca, J. J., L. J. Hutchings, and P. W. Kasameyer, Seismic velocity and attenuation structure of the Geysers geothermal field, California, Geothertnics, 23, , S. de Lorenzo and F. Mongelli, Dipartimento di Geologia e Geofisica, Universith di Bari, Campus Universitario, via Orabona 4, Bari, Italia.(delorenzo geo.uniba. it; f. mongelli geo.uniba. it) A. Zo!1o, Universith di Napoli "Federico ll", Dipartimento di Scienze Fisiche, Complesso Universitario di Monte S. Angelo, via Cinthia, Napoli, Italia. (aldo.zollo na. infn.it) (Received September 13, 1999; revised June 30, 2000; accepted November 1,2000.)