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The seismic crises at Mt. Vesuvius during 1995 and 1996
Phys. Chem. Earth (A), Vol. 24, No. 1l-12, pp. 977.-983. 1999 0 1999 Elsevier Science Ltd All rights reserved 1464-l 895/99/$ - see Front matter
Pergamon
PII:
S1464-1895(99)00145-3
The Seismic Crises at Mt. Vesuvius during 1995 and 1996 F. Bianco’,
M. Castellano’,
G. Milano’,
G. Vilardo’,
F. Ferrucci2 and S. Gresta3
‘Osservatorio Vesuviano, Via Manzoni 249, 80123 Naples, Italy ‘University of Calabria, Cosenza, Italy 3University of Catania, Corso Italia 55, Catania, Italy Received 2.5April 1997; accepted 25 July I999 Abstract. The seismic@ which affects MtVesuvius is, at present, the only clear indicator of the volcano dynamics. In the last years, two periods of increased seismic activity occurred (August-October 1995 and March-May 1996). This seismic@ was detected by the 10 analog stations of the Permanent Seismic Network as well as by up to 7 three-component temporary digital stations. A total number of about 600 events have been recorded, four of which showing magnitude m3.0. The maximum magnitude earthquake (M=3.4) was the strongest in the last fifty years and occurred on 25 April 1996. The use of three-component seismometers allowed us to obtain very reliable hypocentral locations. The focal volume of the two seismic crises does not exceed 5-6 km of depth below the crater area. Fault plane solutions of the most energetic events show focal planes oriented NW-SE and NE-SW. in agreement with the regional tectonic features. indicating that at present the seismic@ of MtVesuvius develops along pre-existing discontinuities. In addition. the occurrence of a fluid-driven source mechanism suggests a role played by the underground water on the seismic energy release. Shear wave splitting analyses confirmed the presence of an anisotropic volume related to a distribution of cracks and/or fractures parallely aligned to the main faults system of the volcano. 0 1999 Elsevier Science Ltd. All rights reserved
At present, no geophysical evidence of magma chamber is reported. whereas the presence of a probable melting zone at a depth of about 10 km beneath Vesuvms was recently suggested by 20110 et al., (1996). From the dynamical point of view seismic@ was the only significant signal of the Vesuvius activity in the last of others 25 years. whereas no notable variation geophysical parameters, such ground deformations or gravimetric anomalies, were observed (Bonasia et al.. 1985; Berrino et al., 1993). Vesuvius seismicity was characterized by low to moderate activity both in number and energy release (Fig.1). Nevertheless, some periods of increased seismicity have been recorded (Vilardo et al., 1996a; Bianco et al., 1998a). Higher rates of events were observed in 1978. 1989-1990 and, more recently, during 1995 and 1996. 600 d 5 D E 5
400 200 0 72
5E+O09
76
60
64
68
92
7
96 Years -a
7E+O06
1 Introduction OE+OOO
Vesuvius is a strato-volcano located in the Campanian plain (Southern Italy) at the intersection of two regional fault systems oriented about NNW-SSE and NNE-SSW (Oldow et al., 1993; Bianco et al., 1998a). It is formed by an ancient caldera (Mt. Somma) and a younger cone (Vesuvius). Volcanic activity started about 300-500 ka ago (Santacroce, 1987) and was characterized both by effusive and explosive activity. The last eruption of Vesuvius occurred on March 1944 and marked the beginning of the present quiescence stage. Correspondence
72
Fig.1. Seismic Mt.Vesuvius
76
occurrence in the period
60
fkquency
64
66
92
(a) and
cumulative
96 Years
strain
r&ax
(h) at
1972-1996.
In this paper we analyzed the seismic activity occurred during the last two crises: August-October 1995 and March-May 1996. These sequences were well recorded by a high number of stations and were associated to the highest energy release in the last 25 years. Data collected allow us to characterize the Vesuvian seismicity in terms of focal volumes, energy release, fault plane solutions and
to: Mario Castellano. 977
F. Bianco el al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
978
shear wave splitting. Particular attention, moreover, was made to analyze the event recorded on 25 April 1996 (M=3.4), that was the strongest earthquakes occurred in the Vesuvian area in the last f@ years.
earthquakes of the two sequences) result very low, being RMS
2 Acquisition system and Data
2.1 Seismic activity
The seismic&y of Vesuvius was recorded by instruments since the end-1700s. In the late 1971 a frequencymodulation station recording on magnetic tape was introduced (OVO station; Fig. 2). A detailed history of the seismic instruments that operated at Vesuvius is given in Vilardo et al. (1996b). At present the permanent seismic network is composed by 10 analog seismic stations (9 vertical and 1 threecomponent [OVO] stations; Fig. 2) equipped with shortperiod (1 second) seismometers, Signals are radio-telemetered to the Surveillance Center of the Osservatorio Vesuviano in Naples where they are continuously sampled by a 12 bit A/D converter with 100 Hz sampling rate and recorded on personal computer (Civetta et al., 1995).
The seismic activity of Vesuvius during the last 25 years was generally very low, with occasionally sharp increases. The time distribution of earthquake occurrence rate and strain release in the period 1972-1996 are plotted in figure 1 (a,b). Periods of higher activity occurred during 1978, 1989-90 and 1995-96. During the 1978 and 1989-90 periods, the seismic@ was characterized by the occurrence of swarms of events superimposed to a moderate background seismic@ of about 30-40 events per month. On the contrary in the last years two distinct periods of high, but time confined activity (August-October 1995 and MarchMay 1996) occurred. In particular, after several months of very low activity, on 2 August 1995 at 02:07 (U.T.) the occurrence of a M=3.1 earthquake marked a sharp increase of seismicity. This period of higher level seismicity lasted three months, during which 217 events were detected; 18 earthquakes displayed magnitude greater than 2.0, with a maximum magnitude of 3.2 (16.09.1995 09:03 U.T.). After this crisis the seismic activity became very low again. A renewal of seismicity started on 4 March 1996 at 00:12 (U.T.) with a M=2.5 earthquake. This new sequence lasted about three months, during which 375 events were recorded; ‘30 earthquakes having magnitude between 2.0 and 2.9 occurred and a single event reached magnitude 3.4 (25.04.1996 20:55 U.T.). This event was the most energetic one of the last fifty years and was strongly felt by the inhabitants of the Vesuvian area. A closer examination of the last two periods of high seismic activity put in evidence both different features and similarity. A difference concerns the magnitude-frequency distribution obtained for the two crises. In fact the b-value of the Gutenberg-Richter relationship for the 1996 crisis (l.OO~tO.02) is higher than the b-value of the 1995 crisis (0.78tO.O5), reflecting a significantly different ratios of the number of earthquakes m the low- to high-magnitude groups. However both these b-values are lower than the characteristic b-values of the Vesuvius seismicity, which usually displays values greater than 1.5 (Osservatorio Vesuviano, unpublished data). The analysis of the inter-event times distribution gives a further information about the different seismic features displayed in the two periods. We measured the clustering of the events evaluating the coefficient of variation Cv (Godano et al., 1997), defined as the ratio between the standard deviation and the average of the inter-event times. This parameter assumes values of 0, 1 and greater than 1 for a periodic, random and clustered process respectively,
14’20’
14’25’
14’30’ 0
$
i Fig. 2. Seismic network operating at Mt.Vesuvius. Both permanent seismic stations (dots) and digital temporary array (triangles) are shown.
Owing to the increased seismic@ occurred since August 1995, a temporary digital array was added to the permanent network. Up to 7 digital three-component shortperiod seismic stations were operating (Fig. 2). Locations of the events in a three-dimensional velocity model (Bianco et al., 1998a) was performed using the HYPOHETERO routine (Virieux et al., 1988). Standard hypocentral errors of the best located events (about 320
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996 The tigure 3 shows the coefficient of variation as function of the magnitude threshold. As expected the values of Cv decrease as the cut off magnitude threshold increases, For the 1995 seismicity, the Cv parameter indicates a significant level of randomness for events having magnitude greater than 1.6. On the contrary, for the 1996 seismic@, the values of Cv remain always greater than 1.0, indicating the presence of a clustering degree also for larger magnitudes. 1.8
,
I
?? 0
0
00 1.0
0.8
??
-I , -0.4
??
979
In both the seismic sequences the epicenters display strong spatial clustering around the crater area (Fig. 4 a,b), even if the epicentral distribution of 1996 seismicity appears more spreaded. Focal depths do not exceed 6km b.s.1. The hypocentral location of the 1995 seismicity shows very few events in the first 3km below the crater (Fig. 4~). On the contrary the hypocenters of the 1996 seismicity also affect shallower depths (Fig. 4d), as commonly observed for the Vesuvian seismic@ (Vilardo et al.. 1996a.b: Bianco et al.. 1998a). However in both the crises the focal depths of most energetic events (M>2.0) rarely are shallower than about 2km below sea level (Fig 5). This feature suggests the presence, at this depth, of a rigidity boundary separating the shallower low-density volcanic products from the deeper and more consolidate ones. This hypothesys is in agreement with the results of a modeling of the arrival times of reflected waves recorded during a twodimensional active seismic experiment performed on Mt.Vesuvius (Zollo et a1.,1996) which put in evidence the presence of a 2km deep strong reflector.
??
,
1
I 0.0
0.4
,,, 0.8
1, 1.2
1.6
I, 2.0
Magnitude Threshold Fig. 3. Coefic~ent of variation Cv plotted as function of the magnitude threshold. Cv values during 1996 (squares) indicate a clustering degree for all the magnitude classes. During the 1995 crisis (dots) a randomness behaviour for event with M ,1.6 is evident.
6.0
'
0
I
I
0.0
1.0
I 2.0
3.0
Magnitude Fig. 5. Magnitude-depth relatmnship. Event with MX.C are mmtlv located at depth greater than 2 km b.s.i..
The seismic activity was not accompained by other geophysical or geochemical phenomena: this fact would let to exclude the influence of volcanic processes. such magma uplift. as cause of the seismicity. Moreover low-frequency events and harmonic tremor. generally related to the presence of magmatic fluids transfer. have never been observed. 2.2 Fault plane solutions
Fig.4. Epwnters of the August-October 1995 (a) and March-May 1996 (b) crises. Hypocentral distribution of the two crises: 1995 (c). 1996 (d); the istqqams represent the density distribution for depth classes of 0.5km.
Fault plane solutions for the more energetic events of the two crises were computed by means of PPFIT algorithm (Reasemberg and Oppenheiner, 1985). 48 earthquakes with at least eight P-polarity and very reliable hypocentral locations were analyzed. The diagram of figure 6, obtained by plotting the P- and T-axes orientation. shows for both
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
980
the sequences strike-slip and oblique-slip with both normal and reverse dip components.
ol
I
0
I
” I
0
I
I
0, I
I
I
I
102030406060708090
Dip angle of P axis (“) Fig. 6. Triangular diagram obtained by plotting the P- and T- axes orientations. The fields are defined by the reciprocal orientation of the axes. Pure strike-slip and strike-slip with both normal and reverse dip components are observedfor both the periods. Black squares and solid circle represent coincident solutions for di&rent events.
The distribution of focal planes (Fig.7a,b) shows a prevalent NW-SE orientation both for the 1995 and 1996 crises, even if during 1996 several different trends are also present (Fig. 7b). The results of the prevalent focal planes orientation and the observed P- and T-axes orientation agree with the feature of the whole Vesuvian seismicity at the present stage of quiescence (Vilardo et al., 1996a,b; Bianco et al., 1998a). However, the azimuthal distributions of the fault planes for the two periods suggest that, during the 1995 (Fig. 7a), seismic@ mainly affected the main NW-SE oriented fault system recognized in the area (Bianco et al., 1998a), whereas during the 1996 (Fig 7b) second order structures seems to be reactivated
a)
b)
Fig. 7. Rose diagrams showing the focal planes orientation for 1995 (a) and 1996 @) data In both period focal planes are mainly NW-SE oriented. In 1996 (b) also NE-SW and E-W oriented trends are observed. The maximum number offocal planes trending NW-SE is displayed.
3 Shear wave splitting We perform shear wave splitting analyses for the seismic@ recorded at Vesuvius during the March-May 1996 crisis, and a comparison with the results of a previous study (Bianco et al., 1998a) was made to evidence whether the splitting changed or not during the 1996 seismic sequence. We considered two splitting parameters: the polarization direction of the leading split shear wave (hereafter LSPD = Leading Split Polarization Direction) and the time delay between the qS1 and qS2 phases (hereafter TD = Time Delay). We use the three-component data recorded with digital stations at sites BKE and BKN which satisfied the following requests: 1) incidence angles less than Shear Wave Window (SWW=35”) to remove the effect of the free surface interaction; 2) S wave arrival onset clear and impulsive on the horizontal components; 3) direct S wave arrival showing amplitude greater on the horizontal component than on the vertical one, to avoid scattered or converted phases. This selection furnished 436 seismograms available for our analyses. For an objective analysis of shear wave splitting dam, we used the 3x3 covariance matrix decomposition (Born and Wolf 1965) to calculate both LSPD and TD values. Following Aster et al. (1990) and Bianco et al. (1998a), we calculated TD by estimating the duration of the polarization linearity interval. starting from the qS1 onset. The TD measures performed at the sites BKE and BKN showed values hardly ever equal to zero. Values of TD at station BKE vary from 0.024 to 0.112 s, with a mean value of 0.052?0.037 s. At station BKN, the delays range from 0.032 to 0.110 s, with a mean value of 0.0581tro.034 s. The measured TD values at the two sites appear very comparable, and agree with previous results on shear wave splitting in the area (Bianco et al., 1998a). No clear trend between TD values and the depth of earthquakes is observed. The qS1 polarization eigendirections are plotted in figure 8; a clear NNW-SSE trend appear at both sites. In addition, for each site, the measured directions are very coherent, If the anisotropic body, whose presence is fully confirmed by the results presented above, was related to the presence of Extensive Dilatancy Anisotropy (EDA cracks; Crampin et al., 1984) or to some other form of anisotropy sensible to stress changes, one could find a strong correlation between the greater TD values and the stronger magnitude of the events (Booth et al., ‘1990). Our data do not show this kind of correlation being the distribution of TD values vs. magnitude (Fig. 9) trendless and uncorrelated with the occurrence of the greater magnitudes. In addition no variations both of LSPD and TD in time have been observed during the analyzed period. These observations may be easily explained with the presence of an anisotropic volume due to a distribution of cracks and/or fractures alignments parallel to the main
F. Bianco et al.:
Seismic Crises at Mt. Vesuvius during 1995 and 1996
strike slip fault recognized in the area (Bianco et al., 1998a), and completely unrelated with some form of stress aligned cracks.
BKE:
BKN
981
of the different phases results not simple to realize accurately using only the visual inspection (Pig. 10a). In order to characterize the phases content of the recorded waveforms, we performed a polarization analysis on the seismic signal recorded at three digital stations: BKN. FTC and SGV.
al
L
LT Fig. 8. Rose diagrams of the qS1 polarization eigendirections at the stations BKE and BKN. The general alignment toward NNW-SSE direction is evident. The maximum number of qS1 polarization eigendiidions trending NNWSSE is displayed.
3)
TZ
0.12 T_-
I
+
Fig.10. The M=3.4 earthquake recorded at BKN (a); the polarigrams in the several planes (b); the time behaviour of the rectilinearity coeffknt (c): the time behaviour of the incidence angle (d).
We used the technique based on the covanance matrix decomposition proposed by Born and Wolf (1965) and successively modified by Kanasewich (1981). The incidence angles at the three sites range between 18” (BKN) and 45’ (FTC; Table 1) and this leads us to well resolve the P phases in the polarization domain. We diagonalized the covariance matrix, calculated the eigenvalues h,> J.2 >h3 and studied the time behaviour of the rectilinear&y coefficient r and complanarity coefficient c defined as: 4 M=3.4
Earthquake - Polarization Analyses
The M=3.4 earthquake which occurred on 25 April 1996 at MtVesuvius, was the more energetic event in the last fifty years. It was located at about 2 km of depth beneath the crater area. Its waveform appears complex and the picking
r=
[(h&)/(hl+;L2)] 2
c = [@2-h3)/((
&+x2)/2)1
and 2
982
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
The solution of the three eigenvalue problems, for the signal recorded at the three digital stations, showed invariably Xl>> h2 >b3. In this case the linear polarization of a seismic phase results well constrained by the behaviour of the r coefficient. Station
BKN SGV FTC
incidence angle (degrees)
PIP* transition time (seconds)
180 24” 45”
0.256 0.264 0.205
Ar=Pz high rectilinearity duration (seconds) 0.088 0.084 0.080
Table I. Comparison of the incidence angles, phase delays and rectilimarity duration at three 3-D stations.
In figure 1Oc the time behaviour of the rectilinearity coefficient is showed for the BKN site. The rectilinearity pattern appears very complex. In particular two successive phases with high r values (Fig. IOc, Pi and PZ) are evident. The point P, marks the beginning of a phase with a very complex behaviour that ends with the occurrence of P2 phase. The rectilinearity values are high longer after P2 phase starts, than after PI phase starts (Fig.lOc). The transition time between PI and P2 phases does not increase with the increasing of the source-receiver distance, being shorter at the furthest station (FIG) and longer for the intermediate one (SGV; Table 1). The developmg of the P2 phase appear less complex and shows a high rectilinearity time duration (Ar) greatest for the nearest station (BKN, At=O.O88 s) and shortest for the farthest one (FTC, Ar=O.O80 s; Table 1). The PI and P2 phases are clearly polarized in the vertical plane showing the same incidence angles (Fig. IOb,d). The spectral analysis performed on the two phases evidenced a different frequency content: the Pr wave fundamental mode is about 4 Hz. whereas the P2 one is about 16 Hz. The previous observations suggest that P, and P2 phases may be P waves originated by different source mechanisms, as explained in the following section. The complex pattern of r coefficient seems not be due to a near field effects, being the wavelength of the P, and P2 phases shorter than the source-receiver distances. Moreover this pattern exists at all the stations, suggesting a source effect. A fluid-driven source mechanism could be invoked to explain this pattern. The occurrence of a fluid-driven mechanism seems to be supported by other seismological observations. In fact, Johnson and McEvilly (1995) proposed that anomalous VpiVs values, seismic attenuation and anisotropy can be evidences of the presence of fluids. Vesuvius seismic@ displays all these features: VpNs shows a peculiar depth dependent trend with the maximum value at about 2km of depth (Castellano et al., 1998); the presence of seismic anisotropy is clearly evidenced; in addition, preliminary studies suggest the presence of high body waves
attenuation (Bianco et al., 1998b). The complexity in the observed first-motion pattern (PI phase) of 25 April 1996 event might be interpreted as the development of a tiny hydrofracture immediately growing into a predominant shear event (P2 phase).
5 Conclusions The seismic@ occurred during 1995 and 1996, even if shows the higher energy release in the last 25 years, does not display unusual features if compared with the seismic pattern which Vesuvius exhibited in the last two decades. The seismogenetic volume and the fault plane solutions concerning the 1995-1996 seismicity are consistent with the seismic features observed over the last years (Vilardo et al., 1996a; Bianco et al., 1998a). The locations of the events recorded during the two crises involve the usual extremely reduced volume, epicenters confined in the caldera area and focal depths not exceed 6km below sea level. Also, focal mechanisms display, as usually observed, strike-slip solutions with nodal planes mainly striking NW-SE with second order NE-SW and E-W strikes. These strikes are in agreement with those of tectonic faults recognized at regional scale. Moreover. meso-structural and volcanological studies (Bianco et al.. 1998a) indicate that Vesuvius is affected by the NW-SE and NE-SWstriking volcanic alignments and faults. The intersection of the NW-SE and NE-SW faults provide a means of stress build up in response to regional forces. In addition, the presence of a layer interface 2km deep, marking the upper boundary of a high rigidity zone, also affects the variation of stress in space (Sassi and Faure, 1997). Coherently. the largest density distribution of hypocenters occurs at the depth of this interface, where also the M=3.4 earthquake was located. The b-value obtained for the 1995 crisis results lower than the 1996 one. This could be due to the energy release mainly affecting the high rigidity deeper zone during the August-October 1995 sequence. The presence of an anisotropic volume at Vesuvius, related to a distribution of cracks and/or fractures parallely aligned to the main faults system recognized in the area, is confirmed using a considerable amount of data. The anisotropic volume features do not change during the 1996 seismic sequence, appearing not influenced by stress changes. This result confirm the interpretation that at Mt.Vesuvius the anisotropic volume is related to fault ‘alignments, but EDA cracks. In conclusion, the seismic@ of Vesuvius, at present, mainly develops along pre-existing structures of regional significance. Neverthless, the occurrence, supported by waveform polarization analyses, of a fluid-driven source mechanism (hydrofracture) for the M=3.4 event, suggests the underground water may play a significant role on the seismic energy release in the Vesuvian area.
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996 Acknowledgement. The autors are grateful to G.Nen and the anonymous referee for the critical review of the manuscript. Special tanks lo M.Capello for the field support during the data acquisition.
References Aster, K. C.. Shearer, P. and Berger. J.. 1990. Quantitative measurements of shear-wave polarizations at the Anza seismic network, southern California: xnplications for shear-wave splitting and earthquake prediction. J. Geophys. Res., 95: 12.449-12.473. Bwrino, G., Coppa, U.. De Nat& G. and Pingue, F., 1993. Recent geophysical investigation at Somma-Vesuvio volcanic complex. J Volcanol. Geotherm. Rex.. 58: 239-262 Bianco, F., Castellano. M., Mdano. G, Ventura, G. and Vilardo G.. 1998a. ‘The Somma-Vesuvius stress field: seismological and mesostmctural data. .I l’olcanol. Geotherm. Res. X2 (1.4). 199-218. Bianco. F.. Cast&no, M.. Del Pezzo, E. and Ibanez. J., 1998b. Measurements of intrinsic and scattering attenuation at Mt.Vesuvius. ,y?(IlI EGS General Assembly; Ann. Geophys.. 16, supp. 1: Cl52. Bona.%, V., Del Pezzo. E., Pingue. F., Scandone, R. and Scarpa R.. 1985. Eruptwe history, seismic activity and ground deformations at Mt. Vrsuvius. Italy. Annoles Geophywae. 3. 3: 395-406. Booth D.C., Lovell, J.H. and Chiu, J-M., 1990. Temporal changes in shearwave splitting during an earthquake swarm in Arkansas J. Geophys. Rex 95. 11151-11166. Born M. and Wolf, E., 1965. Principles of optics III ed.: Pergamon Press, New York. Castellano, M.. Bunco. F. and Vilardo, 6.. 1998. Variations of the VpiVs ratio at Mt.Vesuvius XXIII EGS General Assembly: Ann. Geophys.. 16, supp.I. Cl52. Civetta. L.. B&no, G., Bianco F., Capuano, P., Pingue. F., Ricco, C., Ventura, G. and Vilardo, G., 1995. Mount Vesuvius. Geophysmzl Sunwllance Report. Gsservatorio Vesuviano. Gpen file rep.. Giannini & Figb: 16. Crampin, S.. Evans, R. and Atkinsons Barry K.. 1984. Earthquake prediction: a new physical basis. Geophys .I. R. Am Sm.. 76: 147-156
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Godano, C., Alonzo, M.L. and Vilardo, G., 1997. Multifractal approach to time clustering of earthquakes. Application to Mt.Vesuvius seamicity. Pageoph.. 149: 375-390. Hyppolite, J., Angelier, J. and Rowe, F., 1994. A major change revealed by Quaternary stress patterns in the Southern Apennines. Tectonophysrcs, 230: 199-210. Johnson, P. A. and McEvilly, T.V.. 1995. Parkfield seismicity: Flud driven? J. Geophys. Res., 100: 12937-12950 Kanasewich, E. R., 1981. Time sequence analysu zn Geophysm ‘I‘he Llniversity of Alberta Press, Edmonton Alberta. Gldow, J.S., D’Argenio, B., Fermnti, L., Pappone, G.. Marsella. E. and Sacchr. M.. 1993. Large-scale longitudinal extension in the southan Apennines contractional belt. Italy. Geology, 21: 1123-l 126. Reasemberg, P. and Oppenheimer D., 1985. FPFIT, FPPLOT and FPPAGE. Fortran computer programs for calculating and displaying earthquake faultplane solutions U.S. (?eol. Surv. Open Ftle Rep.. XS (739). pp. IO9 Santacroce, R. (Ed.), 1987. Somma-Vesuvius. cXR f&ad RIL. SCI !IW) 251. Sassi. W. and Faure, J.L., 1997. Role of faults and layer interfaces on the spatial variation of stress regimes in basins: inferences from numerical modelling. Tectonophysm, 266: 101-119. Vilardo. G., De Nat&, G.. Milano, G. and Coppa L’ 1996a. The srismiaty of Mt. Vesuvius. Tectonophysrcs, 261: 127-138. Vilardo, G., Bianco, F., Castellano, M. and Milan”, G.. 1996b. The seismic monitoring oftbe Mt.Vesuvius. In: Bardi Thorkelsson (Ed.), Serm~olo~ rn Europe. Univ. of Iceland: 437-442. Virieux. J.. Farra, V. and Madariaga. R., 1988. Ray-tracing for earthquake location in laterally heterogeneous media. .J. (%ophy,<. Res.. ‘13 658% 6599. Zollo, A, Gasparim, P., Vw~eux, J.. Le Mew. H.. De Nat&. Ci., Biella F.. Boschi, E., Capuano, P.. De France, R., Dell’Aversana. P., De Matteis. R.. Guerra, I., lannaccooe. G., Mirabile, L. and Vilardo, G., 1996. Seismic evidence for a low-velocity zone in the uppercrusthen&h Mount Vesuvius. Saence. 274: 592-594.
Pergamon
PII:
S1464-1895(99)00145-3
The Seismic Crises at Mt. Vesuvius during 1995 and 1996 F. Bianco’,
M. Castellano’,
G. Milano’,
G. Vilardo’,
F. Ferrucci2 and S. Gresta3
‘Osservatorio Vesuviano, Via Manzoni 249, 80123 Naples, Italy ‘University of Calabria, Cosenza, Italy 3University of Catania, Corso Italia 55, Catania, Italy Received 2.5April 1997; accepted 25 July I999 Abstract. The seismic@ which affects MtVesuvius is, at present, the only clear indicator of the volcano dynamics. In the last years, two periods of increased seismic activity occurred (August-October 1995 and March-May 1996). This seismic@ was detected by the 10 analog stations of the Permanent Seismic Network as well as by up to 7 three-component temporary digital stations. A total number of about 600 events have been recorded, four of which showing magnitude m3.0. The maximum magnitude earthquake (M=3.4) was the strongest in the last fifty years and occurred on 25 April 1996. The use of three-component seismometers allowed us to obtain very reliable hypocentral locations. The focal volume of the two seismic crises does not exceed 5-6 km of depth below the crater area. Fault plane solutions of the most energetic events show focal planes oriented NW-SE and NE-SW. in agreement with the regional tectonic features. indicating that at present the seismic@ of MtVesuvius develops along pre-existing discontinuities. In addition. the occurrence of a fluid-driven source mechanism suggests a role played by the underground water on the seismic energy release. Shear wave splitting analyses confirmed the presence of an anisotropic volume related to a distribution of cracks and/or fractures parallely aligned to the main faults system of the volcano. 0 1999 Elsevier Science Ltd. All rights reserved
At present, no geophysical evidence of magma chamber is reported. whereas the presence of a probable melting zone at a depth of about 10 km beneath Vesuvms was recently suggested by 20110 et al., (1996). From the dynamical point of view seismic@ was the only significant signal of the Vesuvius activity in the last of others 25 years. whereas no notable variation geophysical parameters, such ground deformations or gravimetric anomalies, were observed (Bonasia et al.. 1985; Berrino et al., 1993). Vesuvius seismicity was characterized by low to moderate activity both in number and energy release (Fig.1). Nevertheless, some periods of increased seismicity have been recorded (Vilardo et al., 1996a; Bianco et al., 1998a). Higher rates of events were observed in 1978. 1989-1990 and, more recently, during 1995 and 1996. 600 d 5 D E 5
400 200 0 72
5E+O09
76
60
64
68
92
7
96 Years -a
7E+O06
1 Introduction OE+OOO
Vesuvius is a strato-volcano located in the Campanian plain (Southern Italy) at the intersection of two regional fault systems oriented about NNW-SSE and NNE-SSW (Oldow et al., 1993; Bianco et al., 1998a). It is formed by an ancient caldera (Mt. Somma) and a younger cone (Vesuvius). Volcanic activity started about 300-500 ka ago (Santacroce, 1987) and was characterized both by effusive and explosive activity. The last eruption of Vesuvius occurred on March 1944 and marked the beginning of the present quiescence stage. Correspondence
72
Fig.1. Seismic Mt.Vesuvius
76
occurrence in the period
60
fkquency
64
66
92
(a) and
cumulative
96 Years
strain
r&ax
(h) at
1972-1996.
In this paper we analyzed the seismic activity occurred during the last two crises: August-October 1995 and March-May 1996. These sequences were well recorded by a high number of stations and were associated to the highest energy release in the last 25 years. Data collected allow us to characterize the Vesuvian seismicity in terms of focal volumes, energy release, fault plane solutions and
to: Mario Castellano. 977
F. Bianco el al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
978
shear wave splitting. Particular attention, moreover, was made to analyze the event recorded on 25 April 1996 (M=3.4), that was the strongest earthquakes occurred in the Vesuvian area in the last f@ years.
earthquakes of the two sequences) result very low, being RMS
2 Acquisition system and Data
2.1 Seismic activity
The seismic&y of Vesuvius was recorded by instruments since the end-1700s. In the late 1971 a frequencymodulation station recording on magnetic tape was introduced (OVO station; Fig. 2). A detailed history of the seismic instruments that operated at Vesuvius is given in Vilardo et al. (1996b). At present the permanent seismic network is composed by 10 analog seismic stations (9 vertical and 1 threecomponent [OVO] stations; Fig. 2) equipped with shortperiod (1 second) seismometers, Signals are radio-telemetered to the Surveillance Center of the Osservatorio Vesuviano in Naples where they are continuously sampled by a 12 bit A/D converter with 100 Hz sampling rate and recorded on personal computer (Civetta et al., 1995).
The seismic activity of Vesuvius during the last 25 years was generally very low, with occasionally sharp increases. The time distribution of earthquake occurrence rate and strain release in the period 1972-1996 are plotted in figure 1 (a,b). Periods of higher activity occurred during 1978, 1989-90 and 1995-96. During the 1978 and 1989-90 periods, the seismic@ was characterized by the occurrence of swarms of events superimposed to a moderate background seismic@ of about 30-40 events per month. On the contrary in the last years two distinct periods of high, but time confined activity (August-October 1995 and MarchMay 1996) occurred. In particular, after several months of very low activity, on 2 August 1995 at 02:07 (U.T.) the occurrence of a M=3.1 earthquake marked a sharp increase of seismicity. This period of higher level seismicity lasted three months, during which 217 events were detected; 18 earthquakes displayed magnitude greater than 2.0, with a maximum magnitude of 3.2 (16.09.1995 09:03 U.T.). After this crisis the seismic activity became very low again. A renewal of seismicity started on 4 March 1996 at 00:12 (U.T.) with a M=2.5 earthquake. This new sequence lasted about three months, during which 375 events were recorded; ‘30 earthquakes having magnitude between 2.0 and 2.9 occurred and a single event reached magnitude 3.4 (25.04.1996 20:55 U.T.). This event was the most energetic one of the last fifty years and was strongly felt by the inhabitants of the Vesuvian area. A closer examination of the last two periods of high seismic activity put in evidence both different features and similarity. A difference concerns the magnitude-frequency distribution obtained for the two crises. In fact the b-value of the Gutenberg-Richter relationship for the 1996 crisis (l.OO~tO.02) is higher than the b-value of the 1995 crisis (0.78tO.O5), reflecting a significantly different ratios of the number of earthquakes m the low- to high-magnitude groups. However both these b-values are lower than the characteristic b-values of the Vesuvius seismicity, which usually displays values greater than 1.5 (Osservatorio Vesuviano, unpublished data). The analysis of the inter-event times distribution gives a further information about the different seismic features displayed in the two periods. We measured the clustering of the events evaluating the coefficient of variation Cv (Godano et al., 1997), defined as the ratio between the standard deviation and the average of the inter-event times. This parameter assumes values of 0, 1 and greater than 1 for a periodic, random and clustered process respectively,
14’20’
14’25’
14’30’ 0
$
i Fig. 2. Seismic network operating at Mt.Vesuvius. Both permanent seismic stations (dots) and digital temporary array (triangles) are shown.
Owing to the increased seismic@ occurred since August 1995, a temporary digital array was added to the permanent network. Up to 7 digital three-component shortperiod seismic stations were operating (Fig. 2). Locations of the events in a three-dimensional velocity model (Bianco et al., 1998a) was performed using the HYPOHETERO routine (Virieux et al., 1988). Standard hypocentral errors of the best located events (about 320
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996 The tigure 3 shows the coefficient of variation as function of the magnitude threshold. As expected the values of Cv decrease as the cut off magnitude threshold increases, For the 1995 seismicity, the Cv parameter indicates a significant level of randomness for events having magnitude greater than 1.6. On the contrary, for the 1996 seismic@, the values of Cv remain always greater than 1.0, indicating the presence of a clustering degree also for larger magnitudes. 1.8
,
I
?? 0
0
00 1.0
0.8
??
-I , -0.4
??
979
In both the seismic sequences the epicenters display strong spatial clustering around the crater area (Fig. 4 a,b), even if the epicentral distribution of 1996 seismicity appears more spreaded. Focal depths do not exceed 6km b.s.1. The hypocentral location of the 1995 seismicity shows very few events in the first 3km below the crater (Fig. 4~). On the contrary the hypocenters of the 1996 seismicity also affect shallower depths (Fig. 4d), as commonly observed for the Vesuvian seismic@ (Vilardo et al.. 1996a.b: Bianco et al.. 1998a). However in both the crises the focal depths of most energetic events (M>2.0) rarely are shallower than about 2km below sea level (Fig 5). This feature suggests the presence, at this depth, of a rigidity boundary separating the shallower low-density volcanic products from the deeper and more consolidate ones. This hypothesys is in agreement with the results of a modeling of the arrival times of reflected waves recorded during a twodimensional active seismic experiment performed on Mt.Vesuvius (Zollo et a1.,1996) which put in evidence the presence of a 2km deep strong reflector.
??
,
1
I 0.0
0.4
,,, 0.8
1, 1.2
1.6
I, 2.0
Magnitude Threshold Fig. 3. Coefic~ent of variation Cv plotted as function of the magnitude threshold. Cv values during 1996 (squares) indicate a clustering degree for all the magnitude classes. During the 1995 crisis (dots) a randomness behaviour for event with M ,1.6 is evident.
6.0
'
0
I
I
0.0
1.0
I 2.0
3.0
Magnitude Fig. 5. Magnitude-depth relatmnship. Event with MX.C are mmtlv located at depth greater than 2 km b.s.i..
The seismic activity was not accompained by other geophysical or geochemical phenomena: this fact would let to exclude the influence of volcanic processes. such magma uplift. as cause of the seismicity. Moreover low-frequency events and harmonic tremor. generally related to the presence of magmatic fluids transfer. have never been observed. 2.2 Fault plane solutions
Fig.4. Epwnters of the August-October 1995 (a) and March-May 1996 (b) crises. Hypocentral distribution of the two crises: 1995 (c). 1996 (d); the istqqams represent the density distribution for depth classes of 0.5km.
Fault plane solutions for the more energetic events of the two crises were computed by means of PPFIT algorithm (Reasemberg and Oppenheiner, 1985). 48 earthquakes with at least eight P-polarity and very reliable hypocentral locations were analyzed. The diagram of figure 6, obtained by plotting the P- and T-axes orientation. shows for both
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
980
the sequences strike-slip and oblique-slip with both normal and reverse dip components.
ol
I
0
I
” I
0
I
I
0, I
I
I
I
102030406060708090
Dip angle of P axis (“) Fig. 6. Triangular diagram obtained by plotting the P- and T- axes orientations. The fields are defined by the reciprocal orientation of the axes. Pure strike-slip and strike-slip with both normal and reverse dip components are observedfor both the periods. Black squares and solid circle represent coincident solutions for di&rent events.
The distribution of focal planes (Fig.7a,b) shows a prevalent NW-SE orientation both for the 1995 and 1996 crises, even if during 1996 several different trends are also present (Fig. 7b). The results of the prevalent focal planes orientation and the observed P- and T-axes orientation agree with the feature of the whole Vesuvian seismicity at the present stage of quiescence (Vilardo et al., 1996a,b; Bianco et al., 1998a). However, the azimuthal distributions of the fault planes for the two periods suggest that, during the 1995 (Fig. 7a), seismic@ mainly affected the main NW-SE oriented fault system recognized in the area (Bianco et al., 1998a), whereas during the 1996 (Fig 7b) second order structures seems to be reactivated
a)
b)
Fig. 7. Rose diagrams showing the focal planes orientation for 1995 (a) and 1996 @) data In both period focal planes are mainly NW-SE oriented. In 1996 (b) also NE-SW and E-W oriented trends are observed. The maximum number offocal planes trending NW-SE is displayed.
3 Shear wave splitting We perform shear wave splitting analyses for the seismic@ recorded at Vesuvius during the March-May 1996 crisis, and a comparison with the results of a previous study (Bianco et al., 1998a) was made to evidence whether the splitting changed or not during the 1996 seismic sequence. We considered two splitting parameters: the polarization direction of the leading split shear wave (hereafter LSPD = Leading Split Polarization Direction) and the time delay between the qS1 and qS2 phases (hereafter TD = Time Delay). We use the three-component data recorded with digital stations at sites BKE and BKN which satisfied the following requests: 1) incidence angles less than Shear Wave Window (SWW=35”) to remove the effect of the free surface interaction; 2) S wave arrival onset clear and impulsive on the horizontal components; 3) direct S wave arrival showing amplitude greater on the horizontal component than on the vertical one, to avoid scattered or converted phases. This selection furnished 436 seismograms available for our analyses. For an objective analysis of shear wave splitting dam, we used the 3x3 covariance matrix decomposition (Born and Wolf 1965) to calculate both LSPD and TD values. Following Aster et al. (1990) and Bianco et al. (1998a), we calculated TD by estimating the duration of the polarization linearity interval. starting from the qS1 onset. The TD measures performed at the sites BKE and BKN showed values hardly ever equal to zero. Values of TD at station BKE vary from 0.024 to 0.112 s, with a mean value of 0.052?0.037 s. At station BKN, the delays range from 0.032 to 0.110 s, with a mean value of 0.0581tro.034 s. The measured TD values at the two sites appear very comparable, and agree with previous results on shear wave splitting in the area (Bianco et al., 1998a). No clear trend between TD values and the depth of earthquakes is observed. The qS1 polarization eigendirections are plotted in figure 8; a clear NNW-SSE trend appear at both sites. In addition, for each site, the measured directions are very coherent, If the anisotropic body, whose presence is fully confirmed by the results presented above, was related to the presence of Extensive Dilatancy Anisotropy (EDA cracks; Crampin et al., 1984) or to some other form of anisotropy sensible to stress changes, one could find a strong correlation between the greater TD values and the stronger magnitude of the events (Booth et al., ‘1990). Our data do not show this kind of correlation being the distribution of TD values vs. magnitude (Fig. 9) trendless and uncorrelated with the occurrence of the greater magnitudes. In addition no variations both of LSPD and TD in time have been observed during the analyzed period. These observations may be easily explained with the presence of an anisotropic volume due to a distribution of cracks and/or fractures alignments parallel to the main
F. Bianco et al.:
Seismic Crises at Mt. Vesuvius during 1995 and 1996
strike slip fault recognized in the area (Bianco et al., 1998a), and completely unrelated with some form of stress aligned cracks.
BKE:
BKN
981
of the different phases results not simple to realize accurately using only the visual inspection (Pig. 10a). In order to characterize the phases content of the recorded waveforms, we performed a polarization analysis on the seismic signal recorded at three digital stations: BKN. FTC and SGV.
al
L
LT Fig. 8. Rose diagrams of the qS1 polarization eigendirections at the stations BKE and BKN. The general alignment toward NNW-SSE direction is evident. The maximum number of qS1 polarization eigendiidions trending NNWSSE is displayed.
3)
TZ
0.12 T_-
I
+
Fig.10. The M=3.4 earthquake recorded at BKN (a); the polarigrams in the several planes (b); the time behaviour of the rectilinearity coeffknt (c): the time behaviour of the incidence angle (d).
We used the technique based on the covanance matrix decomposition proposed by Born and Wolf (1965) and successively modified by Kanasewich (1981). The incidence angles at the three sites range between 18” (BKN) and 45’ (FTC; Table 1) and this leads us to well resolve the P phases in the polarization domain. We diagonalized the covariance matrix, calculated the eigenvalues h,> J.2 >h3 and studied the time behaviour of the rectilinear&y coefficient r and complanarity coefficient c defined as: 4 M=3.4
Earthquake - Polarization Analyses
The M=3.4 earthquake which occurred on 25 April 1996 at MtVesuvius, was the more energetic event in the last fifty years. It was located at about 2 km of depth beneath the crater area. Its waveform appears complex and the picking
r=
[(h&)/(hl+;L2)] 2
c = [@2-h3)/((
&+x2)/2)1
and 2
982
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996
The solution of the three eigenvalue problems, for the signal recorded at the three digital stations, showed invariably Xl>> h2 >b3. In this case the linear polarization of a seismic phase results well constrained by the behaviour of the r coefficient. Station
BKN SGV FTC
incidence angle (degrees)
PIP* transition time (seconds)
180 24” 45”
0.256 0.264 0.205
Ar=Pz high rectilinearity duration (seconds) 0.088 0.084 0.080
Table I. Comparison of the incidence angles, phase delays and rectilimarity duration at three 3-D stations.
In figure 1Oc the time behaviour of the rectilinearity coefficient is showed for the BKN site. The rectilinearity pattern appears very complex. In particular two successive phases with high r values (Fig. IOc, Pi and PZ) are evident. The point P, marks the beginning of a phase with a very complex behaviour that ends with the occurrence of P2 phase. The rectilinearity values are high longer after P2 phase starts, than after PI phase starts (Fig.lOc). The transition time between PI and P2 phases does not increase with the increasing of the source-receiver distance, being shorter at the furthest station (FIG) and longer for the intermediate one (SGV; Table 1). The developmg of the P2 phase appear less complex and shows a high rectilinearity time duration (Ar) greatest for the nearest station (BKN, At=O.O88 s) and shortest for the farthest one (FTC, Ar=O.O80 s; Table 1). The PI and P2 phases are clearly polarized in the vertical plane showing the same incidence angles (Fig. IOb,d). The spectral analysis performed on the two phases evidenced a different frequency content: the Pr wave fundamental mode is about 4 Hz. whereas the P2 one is about 16 Hz. The previous observations suggest that P, and P2 phases may be P waves originated by different source mechanisms, as explained in the following section. The complex pattern of r coefficient seems not be due to a near field effects, being the wavelength of the P, and P2 phases shorter than the source-receiver distances. Moreover this pattern exists at all the stations, suggesting a source effect. A fluid-driven source mechanism could be invoked to explain this pattern. The occurrence of a fluid-driven mechanism seems to be supported by other seismological observations. In fact, Johnson and McEvilly (1995) proposed that anomalous VpiVs values, seismic attenuation and anisotropy can be evidences of the presence of fluids. Vesuvius seismic@ displays all these features: VpNs shows a peculiar depth dependent trend with the maximum value at about 2km of depth (Castellano et al., 1998); the presence of seismic anisotropy is clearly evidenced; in addition, preliminary studies suggest the presence of high body waves
attenuation (Bianco et al., 1998b). The complexity in the observed first-motion pattern (PI phase) of 25 April 1996 event might be interpreted as the development of a tiny hydrofracture immediately growing into a predominant shear event (P2 phase).
5 Conclusions The seismic@ occurred during 1995 and 1996, even if shows the higher energy release in the last 25 years, does not display unusual features if compared with the seismic pattern which Vesuvius exhibited in the last two decades. The seismogenetic volume and the fault plane solutions concerning the 1995-1996 seismicity are consistent with the seismic features observed over the last years (Vilardo et al., 1996a; Bianco et al., 1998a). The locations of the events recorded during the two crises involve the usual extremely reduced volume, epicenters confined in the caldera area and focal depths not exceed 6km below sea level. Also, focal mechanisms display, as usually observed, strike-slip solutions with nodal planes mainly striking NW-SE with second order NE-SW and E-W strikes. These strikes are in agreement with those of tectonic faults recognized at regional scale. Moreover. meso-structural and volcanological studies (Bianco et al.. 1998a) indicate that Vesuvius is affected by the NW-SE and NE-SWstriking volcanic alignments and faults. The intersection of the NW-SE and NE-SW faults provide a means of stress build up in response to regional forces. In addition, the presence of a layer interface 2km deep, marking the upper boundary of a high rigidity zone, also affects the variation of stress in space (Sassi and Faure, 1997). Coherently. the largest density distribution of hypocenters occurs at the depth of this interface, where also the M=3.4 earthquake was located. The b-value obtained for the 1995 crisis results lower than the 1996 one. This could be due to the energy release mainly affecting the high rigidity deeper zone during the August-October 1995 sequence. The presence of an anisotropic volume at Vesuvius, related to a distribution of cracks and/or fractures parallely aligned to the main faults system recognized in the area, is confirmed using a considerable amount of data. The anisotropic volume features do not change during the 1996 seismic sequence, appearing not influenced by stress changes. This result confirm the interpretation that at Mt.Vesuvius the anisotropic volume is related to fault ‘alignments, but EDA cracks. In conclusion, the seismic@ of Vesuvius, at present, mainly develops along pre-existing structures of regional significance. Neverthless, the occurrence, supported by waveform polarization analyses, of a fluid-driven source mechanism (hydrofracture) for the M=3.4 event, suggests the underground water may play a significant role on the seismic energy release in the Vesuvian area.
F. Bianco et al.: Seismic Crises at Mt. Vesuvius during 1995 and 1996 Acknowledgement. The autors are grateful to G.Nen and the anonymous referee for the critical review of the manuscript. Special tanks lo M.Capello for the field support during the data acquisition.
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Godano, C., Alonzo, M.L. and Vilardo, G., 1997. Multifractal approach to time clustering of earthquakes. Application to Mt.Vesuvius seamicity. Pageoph.. 149: 375-390. Hyppolite, J., Angelier, J. and Rowe, F., 1994. A major change revealed by Quaternary stress patterns in the Southern Apennines. Tectonophysrcs, 230: 199-210. Johnson, P. A. and McEvilly, T.V.. 1995. Parkfield seismicity: Flud driven? J. Geophys. Res., 100: 12937-12950 Kanasewich, E. R., 1981. Time sequence analysu zn Geophysm ‘I‘he Llniversity of Alberta Press, Edmonton Alberta. Gldow, J.S., D’Argenio, B., Fermnti, L., Pappone, G.. Marsella. E. and Sacchr. M.. 1993. Large-scale longitudinal extension in the southan Apennines contractional belt. Italy. Geology, 21: 1123-l 126. Reasemberg, P. and Oppenheimer D., 1985. FPFIT, FPPLOT and FPPAGE. Fortran computer programs for calculating and displaying earthquake faultplane solutions U.S. (?eol. Surv. Open Ftle Rep.. XS (739). pp. IO9 Santacroce, R. (Ed.), 1987. Somma-Vesuvius. cXR f&ad RIL. SCI !IW) 251. Sassi. W. and Faure, J.L., 1997. Role of faults and layer interfaces on the spatial variation of stress regimes in basins: inferences from numerical modelling. Tectonophysm, 266: 101-119. Vilardo. G., De Nat&, G.. Milano, G. and Coppa L’ 1996a. The srismiaty of Mt. Vesuvius. Tectonophysrcs, 261: 127-138. Vilardo, G., Bianco, F., Castellano, M. and Milan”, G.. 1996b. The seismic monitoring oftbe Mt.Vesuvius. In: Bardi Thorkelsson (Ed.), Serm~olo~ rn Europe. Univ. of Iceland: 437-442. Virieux. J.. Farra, V. and Madariaga. R., 1988. Ray-tracing for earthquake location in laterally heterogeneous media. .J. (%ophy,<. Res.. ‘13 658% 6599. Zollo, A, Gasparim, P., Vw~eux, J.. Le Mew. H.. De Nat&. Ci., Biella F.. Boschi, E., Capuano, P.. De France, R., Dell’Aversana. P., De Matteis. R.. Guerra, I., lannaccooe. G., Mirabile, L. and Vilardo, G., 1996. Seismic evidence for a low-velocity zone in the uppercrusthen&h Mount Vesuvius. Saence. 274: 592-594.