Some insights on the occurrence of recent volcanic eruptions of Mount Etna volcano (Sicily, Italy)

Geophys. J. Int. (25) 163, 123 1218 doi: 1.1111/j.1365-246X.25.2757.x Some insights on the occurrence of recent volcanic eruptions of Mount Etna volcano (Sicily, Italy) Laura Sandri, 1 Warner Marzocchi 1 and Paolo Gasperini 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Via Donato Creti, 12 4128 Bologna, Italy. E-mail: sandri@bo.ingv.it 2 Sett. Geofisica, Dip. Fisica, Alma Mater Studiorum, University of Bologna, Viale Berti Pichat, 8 4127 Bologna, Italy Accepted 25 July 21. Received 25 June 22; in original form 24 October 29 1 INTRODUCTION Mount Etna volcano is one of the most extensively monitored volcanic systems of the world. Besides an almost continuous activity in the summit craters, every few years Mount Etna experiences episodes of flank activity. Due to its densely populated slopes, the Etna flank eruptions represent one of the most prominent source of volcanic damage in that area. The first step in estimating this kind of volcanic hazard is to provide a reliable model for the temporal occurrence of flank eruptions. Note that, in this work, we do not take into account the spatial density of flank eruptions, which has been previously studied, for example, by Wadge et al. (1994). Regarding the temporal aspect, previous studies provide some empirical constraints by studying the relationship between flank eruption occurrence and regional and local seismicity at a timescale of few weeks (Mulargia et al. 1991, 1992; Vinciguerra et al. 21). In these studies, seismicity is assumed to be a proxy for the local and regional tectonics, and therefore is not related to magma motion beneath the volcano. Despite some differences in the results, all of these papers suggest a tectonic control on the occurrence SUMMARY In the first part of this work, we make use of two non-parametric statistical pattern recognition algorithms and a multiple regression analysis to analyse seismic clusters occurring around Mount Etna, Italy. The aim is to determine if the onset of flank eruptions at Mount Etna is linked to variations in the regional seismicity at a timescale of few weeks. From the analysis, we find that the discrimination between clusters preceding flank eruptions and clusters not related in time to flank activity is mainly linked to the volume output of the previous flank eruption, in some cases together with the time elapsed from its end. Instead, we do not find any difference in the seismicity features characterizing different types of clusters, except for avery small contribution of the number of seismic events in the clusters. This result does not confirm the existence, suggested in the past, of a direct link between the regional state of stress at a timescale of few weeks and the occurrence of flank eruptions on Mount Etna volcano. On the contrary, the result suggests that a prominent role in the flank eruption occurrence is played by the re-charging of the feeding system. In the second part of this study we analyse the relationship between the magma volume erupted in an eruption and the interevent time following it, finding that a time-predictable model satisfactorily describes the occurrence of eruptions at Mount Etna in the last decades. The latter analysis is carried out both on the flank eruption catalogue only, and on the complete catalogue of flank and summit eruptions, with comparable results. Key words: flank eruptions, Mount Etna volcano, regional tectonic stress, statistical pattern recognition, time predictability. of flank eruptions at Mount Etna, at least over a timescale of few weeks. Alternatively, flank eruptions have been characterized studying the statistical distribution of the interevent times and sizes (Wadge & Guest 1981; Mulargia et al. 1987a; Gasperini et al. 199), but no univocal conclusions were achieved. While these studies generally agree on a steady magma output in recent years (the periods studied are 1971 1981 by Wadge & Guest 1981; Mulargia et al. 1987a and 1978 1987 by Gasperini et al. 199), they identify several, and different, change points in the interevent time distributions. Remarkably, they do not find any relationship between the size of a flank eruption and the subsequent interevent time (cf. Mulargia et al. 1987a). The organization of this work is mainly determined by the results obtained; in particular we started to investigate on a specific hypothesis, and the results obtained led us to look also into other directions. At first, we re-address the link between flank eruptions and regional/local seismicity by means of two non-parametric statistical pattern recognition (NSPR) algorithms based on very different GJI Volcanology, geothermics, fluids and rocks C 25 The Authors 123

124 L. Sandri, W. Marzocchi and P. Gasperini approaches, and by means of a multivariate regression analysis. The statistical pattern recognition algorithms used have been previously tested on synthetic data that mimic the basic properties of our data set of seismic clusters (Sandri & Marzocchi 24). Also, the original seismic and volcanic catalogues available for this study are much more reliable and complete than those used in past works (cf. Mulargia et al. 1991, 1992). We analyse groups of seismic events that we call seismic clusters (see definition in the next section) occurring in a large area around Mount Etna. The goal of this part of the study is to check whether there is a recurrent pattern in this regional seismicity preceding a flank eruption at Mount Etna, by considering seismicity as a proxy for regional tectonics (Lay & Wallace 1995). Note that the low-average magnitude of the earthquakes considered here (see next section) do not seem indicative of a direct causal relationship between earthquake occurrence and the triggering or promoting of flank eruptions, which has been demonstrated for much larger seismic events (e.g. Marzocchi 22; Marzocchi et al. 24). Through multivariate regression analysis we study how the characteristics of the seismic clusters vary as the next flank eruption is approaching, without grouping the data into classes. We test the validity of the patterns found by applying them to an independent data set. The second part of the work is mainly motivated by the results obtained in the previous point, that show the prominent importance of the re-charge of the feeding system, apparently independent of the regional tectonic stress (at least over the timescale investigated). In particular, in the second part of the work we study in detail the relationship between interevent time (defined as the time between the onset of two successive eruptions) and size of eruptions, providing some empirical constraints for the mechanism that generates eruptions. 2 THE DATA We analyse two different catalogues: one for seismic events and one for flank eruptions of Mount Etna. Regarding the seismic data, similarly to Mulargia et al. (1991), we take into account seismic events that occurred in a circle of radius 12 km, centred over the summit of Mount Etna (Fig. 1). The catalogue from which we extract the seismic events consists of the combination of different data sets. From 1981 to 1996 we make use of the Catalogo Strumentale dei Terremoti Italiani dal 1981 al 1996 (CSTI) (CSTI Working Group, 21) where the instrumental local magnitudes have been re-valuated, according to Gasperini (22), by the comparison with a data set of true and synthetic Wood Anderson estimates. We actually use a slightly improved version of CSTI where the hypocenter depths of events having large azimuthal gaps (>18 ) are fixed to 1 km as well as the magnitude estimates associated with large P-wave residuals (>5 s.) are discarded. From 196 to 198, we essentially refer to the catalogue of the Progetto Finalizzato Geodinamica (PFG) (Postpischl 1985) but applying an empirical magnitude shift of.3 units from 196 to mid-1976 and of.5 units from mid-1976 to 198 in order to make it homogeneous with the re-calibrated section described above (see Lolli & Gasperini 23). Finally, for the earthquakes since 1997, we use the hypocentral locations reported by the INGV Seismic Bulletin (1997 22), with magnitudes recomputed from duration and amplitude data according to Gasperini (22). Before analysing any seismic catalogue, it is necessary to establish its completeness for the magnitude threshold we are inter- Figure 1. Map of the Etnean area. The circles represent the centroids (whose coordinates are the trimmed latitude and longitude, that is, LAT and LON, see text) of the seismic clusters extracted from the seismic catalogue since (a) 1974 (M c = 3.), (b) 1983 (M c = 2.5). In both panels the circle of radius of 12 km, centred over Mount Etna volcano (star), is shown. ested (see e.g. Albarello et al. 21). We evaluate the completeness of the merged catalogue obtained by visually inspecting the cumulative number of earthquakes (Mulargia et al. 1987b) as a function of time for two different magnitude thresholds: M c = 3. (Fig. 2a) and M c = 2.5 (Fig. 2b). The completeness can be assumed since 1974 and since 1983, respectively, although for the latter threshold it is actually possible to distinguish a slight intermediate change in the slope (Fig. 2b), roughly between 1983 and 1988. We decide not to consider this minor effect, because it is much less marked than the change in slope since 1983. In the following we will consider both data sets, as the former is longer, but with fewer events, while the latter is more detailed but only on the most recent period. The seismic events extracted for the two data sets are grouped to obtain seismic clusters lasting at most 1 week. In particular, the C 25 The Authors, GJI, 163, 123 1218

On the occurrence of eruptions of Mount Etna volcano 125 (a) cumulative number 1 8 6 4 2 M c =3. Cumulative number of seismic events in the catalog (b) cumulative number 196 1965 197 1975 198 1985 199 1995 2 25 years 25 2 15 1 5 M c =2.5 Cumulative number of seismic events in the catalog 196 1965 197 1975 198 1985 199 1995 2 25 years Figure 2. Cumulative curve of the number of seismic events listed in the original seismic catalogue available, from 196 to 21. In panel (a) the magnitude threshold used is 3., while in panel (b) it is 2.5. The arrow shows the year from which the catalogue is complete (for the threshold magnitude considered). first cluster consists of the first seismic event in the considered area and of all the seismic events occurred in the same day and in the following 6 days. The other clusters are formed, in the same way, starting from the first event following each cluster. We remark that we are in a volcanic area, without clear cluster sequences in the usual seismological meaning, and that could be defined using clustering algorithms such as the one proposed by Reasenberg (1994). The definition of our clusters is instead a simple grouping of earthquakes occurring in a time window of a selected length. The spatial distribution of such clusters will be considered in the analysis and discussed later. We obtain, in total, 218 seismic clusters for the first data set (since 1974, M c = 3., see Fig. 1a) and 389 for the second one (since 1983, M c = 2.5, see Fig. 1b). Concerning the flank eruption data, we refer to Behncke et al. (25) for the complete list of eruptions at Mount Etna. However, we first have to define what flank eruptions at Mount Etna are, and then extract them from this list. We borrow the definition of flank eruptions at Mount Etna from Gasperini et al. (199), according to which flank eruptions are those linked to the opening of magmafilled fractures on the slopes of Mount Etna. In this respect, we define a maximum altitude (29 3 m asl) above which the rocks at Mount Etna are so incoherent and unconsolidated that cannot fracture. As a consequence, all the eruptions occurring below this height are considered flank eruptions, and are listed in Table 1. The time-series of the seismic clusters and flank eruptions in the Etnean area are displayed in Fig. 3 superimposed to the sequence of the clusters detected for both the above data sets. All but one the flank eruptions are preceded by at least a seismic cluster in the 1 days preceding the onset of the volcanic event. The only flank eruption occurring without precursory seismicity in a period of 1 days is the one starting on 1975 November 29. Table 1. Catalogue of flank eruptions at Mount Etna volcano used in this work. Eruption Onset End Total volume output number (YYYY MM DD) (YYYY MM DD) (Lava and Tephra) (1 6 m 3 ) 1 1971 4 5 1971 6 12 47.2 2 1974 1 3 1974 2 17 4.4 3 1974 3 11 1974 3 29 3.2 4 1975 2 24 1975 8 29 6. 5 1975 11 29 1977 1 8 35.1 6 1978 4 29 1978 6 5 27.5 7 1978 8 25 1978 8 3 4. 8 1978 11 23 1978 11 3 11. 9 1979 8 3 1979 8 9 7.5 1 1981 3 17 1981 3 23 21.3 11 1983 3 28 1983 8 6 79. 12 1985 3 1 1985 7 13 3. 13 1985 12 25 1985 12 31.9 14 1986 1 3 1987 3 1 6. 15 1989 9 27 1989 1 9 26.2 16 1991 12 14 1993 3 31 235. 17 21 7 17 21 8 8 33.1 18 22 1 27 23 1 28 85.5 3 SEARCHING FOR RECURRENT PAT T ERNS IN THE SEISMICITY PRECEDING FLANK ERUPTIONS The primary goal of this section is not to predict flank eruptions, which might be better forecast by tracking magma motion beneath the volcano by means of microseismicity. In this part of the paper we rather look for recurrent patterns in local and regional seismicity preceding flank eruptions. Considering such seismicity as a proxy C 25 The Authors, GJI, 163, 123 1218

126 L. Sandri, W. Marzocchi and P. Gasperini (a) Number of Events in the Cluster 5 4 3 2 1 1/1/1974 1/1/1978 1/1/1982 1/1/1986 1/1/199 1/1/1994 1/1/1998 1/1/22 Time (b) Number of Events in the Cluster 15 1 5 M C =3. M C =2.5 1/1/1983 1/1/1985 1/1/1987 1/1/1989 1/1/1991 1/1/1993 1/1/1995 1/1/1997 1/1/1999 1/1/21 1/1/23 Time Figure 3. Time-series of the seismic clusters (pulses) extracted from the seismic catalogue (left axis, the number of events in each cluster is plotted). The bars with dashed borders cover the periods of occurrence of the flank eruptions at Mount Etna volcano. The height of the bars corresponds to the volume output of each eruptive episode (scale on the right axis). Panel (a) is since 1974 and for M c = 3., panel (b) since 1983 and for M c = 2.5. for regional and local tectonics (Lay & Wallace 1995), we therefore try to provide some insights on the possible tectonic control of flank eruptions over a specified time interval. For this purpose, we perform two multivariate statistical analyses by means of two different methods. 25 2 15 1 5 3 2 1 3.1 Pattern recognition analysis Pattern recognition (PR) is a powerful multivariate analysis technique allowing, in principle, the identification of possible repetitive schemes among the objects belonging to distinct classes. While usual data analysis takes into account only one variable of the process at a time, PR is able to extract information from any possible combination (linear or not) of variables that are suspected to have an influence on the process. Moreover, PR does not need the construction of a theoretical model, but is usually based on a sole basic hypothesis, that is, the assumption that the phenomenon under study is governed by a finite number of complex, but repetitive patterns of the variables. These appealing properties led PR to be applied with success in several and diverse disciplines, which share the study of complex systems (e.g. Fukunaga 199). For the same reasons, we believe that PR, and especially NSPR, might be a very promising tool also in Earth Sciences. Until now the only few remarkable efforts in this direction are M8 and CN algorithms (Keilis-Borok & Kossobokov 199; Keilis-Borok et al. 1988), and applications to volcanology (Mulargia et al. 1991, 1992; Vinciguerra et al. 21; Sandri et al. 24). With respect to these applications, our study represents an improvement because we make use of algorithms that have been previously tested to verify their performance on data sets with similar properties to the real ones (Sandri & Marzocchi 24; Sandri et al. 24). Technically, the main goal of PR is to classify objects. Every object is represented by an array of qualitative or quantitative variables. The procedure of analysis consists of three different steps: the learning phase, the voting phase, and the control experiments. In the learning phase, a set of known and classified objects is analysed in order to recognize all the possible patterns that characterize each class, that is, the combinations of variables that allow to discriminate the objects belonging to different classes. This step turns out to be very useful because it leads to the recognition of the most important variables for the process under study. In the voting phase, the patterns identified during the learning phase are used to classify new objects, whose class is unknown to the algorithm. Finally, control experiments check the stability of the results by repeating the learning and voting phases using different values for the algorithm parameters. We use two statistical NSPR algorithms named binary decision tree (BDT) and Fisher analysis (FIS). Both of these algorithms have been previously tested on synthetic data (Sandri & Marzocchi 24). Such simulations have shown that both BDT and FIS provide reliable results when applied to data sets that mimic the basic features of our seismic cluster data set, that is, few objects, each represented by many features, often not normally distributed. For a more complete definition of the algorithms, see Appendices A and B, while, for a very detailed description, see Duda & Hart (1973) for FIS, and Rounds (198), Mulargia et al. (1992) for BDT. In this paper we perform the learning phase both on the totality of available data and on a randomly chosen subset (consisting of 8 per cent of the available data). In the former case we attempt to recognize, as a first step, all the possible patterns in our data set. In the latter case, we reserve part of the available data for the voting phase. Before performing the learning phase, we first have to define the objects to be analysed and the classes involved in the problem. In our study, the objects are the seismic clusters. Any object is represented by a vector that contains all the measurements (the features) that we Volume Output (1 6 m 3 ) Volume Output (1 6 m 3 ) C 25 The Authors, GJI, 163, 123 1218

can associate to the object. For each seismic cluster we retrieved the following measurements: (1) maximum magnitude recorded in the cluster (MXM), (2) number of events in the cluster (NEV), (3) trimmed mean 4 per cent (i.e. the mean on the data between the 2th and the 8th percentiles) of the latitudes of the events in the cluster (LAT), (4) trimmed mean 4 per cent of the longitudes of the events in the cluster (LON), (5) season of occurrence of the cluster (SEA), (6) time elapsed (in days) from the previous cluster (TPC), (7) time elapsed (in days) from the end of the previous flank eruption (also called time-since-eruption, TSE), (8) maximum magnitude recorded in the previous cluster (MXP) and (9) volume output of the previous flank eruption in millions of m 3 (VOP). The first five measurements (MXM, NEV, LAT, LON and SEA) are related to the cluster itself; in particular, LAT and LON describe the spatial distribution of the clusters. The latter four (TPC, TSE, MXP and VOP) are related to the recent volcanic and seismic history of the system. Due to the large differences between the maximum and minimum measurements in the catalogue for the TSE, for the time elapsed from the previous clusters (TPC) and for the volume output of the previous eruption (VOP), we decide to use the logarithm of these values. Thus, each object has the following components: MXM, NEV, LAT, LON, SEA, Log(TPC), Log(TSE), MXP and Log(VOP). For the sake of clarity, from now on we call time-to-eruption the time to the following flank eruption, and, as mentioned above, time-since-eruption the time between the end of last flank eruption and the beginning of the cluster (i.e. TSE). We identify three classes of clusters: Type Type Type A clusters time-to-eruption τ, (1) B clusters time-since-eruption τ, (2) C clusters otherwise, (3) where τ is the time window for defining precursory clusters (see below). Type A clusters are precursors to flank eruptions, while type B occur right after the end of a flank eruption. Type C clusters are not temporally associated with flank eruptions (Fig. 4). If both inequalities (1) and (2) are verified, then the cluster is not considered in the analysis, as well as clusters occurring during flank eruptions. A A B C C C A B B time Flank Eruption Seismic Cluster Figure 4. Attribution of seismic clusters (circles) to either type A (precursory), or B (following), or C ( away ), depending on their proximity to flank eruptions (asterisks). On the occurrence of eruptions of Mount Etna volcano 127 Clearly, the attribution of an object to one of these types depends on the parameter τ. Wearbitrarily define that a cluster has to be classified as A if it occurs at most about 3 months before the following flank eruption, that is, τ = 1 days. This choice is related to the temporal scale of the tectonic interactions we are investigating. The timescales considered in our NSPR analysis (weeks to few months) are typical of the elastic response of the lithosphere to irreversible perturbations. An example of mechanism occurring at these temporal scales is the typical aftershock occurrence following the irreversible deformation due to the mainshock (Dieterich 1994). Hereinafter, we use the term quasi-elastic to indicate the time, and spatial, scale used, being too short for the viscoelastic responses (from years to decades, e.g. Piersanti 1999; Kenner & Segall 2). With the selected value for τ, about 12 per cent of the clusters contained in our data set are of type A. In the control experiments, we will check the stability of our results to changes of the value of τ. By means of such definitions (conditions 1 3) we have, in principle, a three-class problem. Since we are mainly interested to a possible regional tectonic stress pattern favouring flank activity at Mount Etna, we take into account only the precursory clusters (type A, from now on representing class 1) and the clusters occurring away from flank eruptions (type C, from now on representing class 2). In fact, most of the activity immediately after the onset and during a flank eruption may be related to a stress re-distribution induced by the eruption itself. 3.2 Results of the pattern recognition analysis We define repose time the time between the end of a flank eruption and the onset of the following one. As a flank eruption is approaching, the number of seismic clusters does not increase. In fact, as shown in Fig. 5, the frequency of the seismic clusters recorded in the data set is approximately independent on the time-to-eruption. In Fig. 5, the frequency is normalized, that is, for each value of time-to-eruption t on the x-axis, the correspondent frequency f (t) is divided by the number of repose times, contained in our flank eruption data set, larger or equal to t. This is done to take into account the fact that a large time-to-eruption is possible only if the repose time is long enough, that is, at least longer than the timeto-eruption. If we do not normalize, we obviously obtain a much larger number of clusters for short times-to-the-eruption, because such short times are possible for any repose time, even for short ones. As we see in Fig. 5, for the case since 1983 (Fig. 5b) the normalized frequency of short times-to-eruption is even lower than the average frequency in the data set. The results of the NSPR analysis, when all the data are used in the learning phase, are shown in Figs 6 and 7 (for BDT) and Figs 8 and 9 (for FIS). For both algorithms and in both cases (since 1974, M c = 3.; since 1983, M c = 2.5) the most important feature is the logarithm of the volume output [Log(VOP)] of the previous flank eruption. Also the logarithm of the time from the previous flank eruption [Log(TSE)] is important. In particular, the pattern found based on both features tells that, for a seismic cluster occurring around Mount Etna, the larger the VOP, the longer the TSE needed in order to consider the cluster as a precursory one (class 1 or type A ). However, the most striking result is that no seismic feature is found with discriminating power between the two classes of seismic clusters. The clusters of type A correctly classified by BDT are 95 per cent and 78 per cent, respectively, for the two cases (since 1974 C 25 The Authors, GJI, 163, 123 1218

128 L. Sandri, W. Marzocchi and P. Gasperini (a).5.45 Since 1974, M c =3. Binary Decision Tree Results, Case since1974, Mc=3..4 normalized frequency normalized frequency (b).35.3.25.2.15.1.5.45.35.25.15.5.5.4.3.2.1 5 5 1 15 2 25 time difference between a cluster and its following eruption (days) 1 15 2 25 time difference between a cluster and its following eruption (days) 3 Since 1983, M c =2.5 Figure 5. Frequency distribution of the time difference between seismic clusters and their next flank eruption (in days). We call this time difference the time-to-eruption. For a flank eruption, we call the time elapsed since its previous flank eruption the interevent time. In this figure, for a given value of time-to-eruption t, its correspondent frequency in the data set is normalized to the number of flank eruptions having an interevent time larger or equal to t. The dashed line represents the average normalized frequency in the data set considered. In panel (a) is the case since 1974 and for M c = 3., in panel (b) since 1983 and for M c = 2.5. M c = 3., and since 1983 M c = 2.5), while those of type C are 56 per cent and 64 per cent. The results obtained by FIS algorithms are 64 per cent, 67 per cent, 83 per cent and 86 per cent, respectively, showing that FIS performs worse than BDT on class A objects, but better on type C. By using a randomly selected subset consisting of 8 per cent of the available data for the learning phase, and voting the remaining 2 per cent of data as new and independent objects, we obtain the results displayed in Table 2. In particular, we notice that: (1) the two algorithms provide consistent results; (2) the pattern found, based on Log(TSE) and Log(VOP), involves the same features as in the case when all the data are used for the learning step, but it is more stable; (3) except for BDT in the case since 1974 (M c = 3.), the fraction of learning objects correctly classified is comparable to the one obtained when classifying the voting data, that is, the risk of overfitting the data can be excluded. We perform some control experiments to check the stability of the results obtained. In particular, we repeat four times 3 35 35 NO The clusteris classified as A (Class 1) Log(VOP)>1.78 The cluster is classifiedas C (Class 2) NO YES Log(REE)>3.46 YES The cluster is classified as A (Class 1) Figure 6. Results of the binary decision tree analysis for the case since 1974 and for M c = 3., when all the data are used in the learning phase. TSE is in days, VOP in millions of m 3. Binary Decision Tree Results, Casesince 1983, Mc=2.5 NO The cluster is classified as A (Class 1) Log(VOP)>1.78 YES The cluster is classified as C (Class 2) Figure 7. Results of the binary decision tree analysis for the case since 1983 and for M c = 2.5, when all the data are used in the learning phase. TSE is in days, VOP in millions of m 3. the NSPR analysis, each time considering one of the following variants: (1) a different value of the parameter τ, byusing τ = 4 days; (2) a different cluster duration, by re-extracting clusters of one month (instead of 1 week), from the original seismic catalogues; (3) a different flank eruption catalogue, by excluding all the cluster following the largest eruption listed (the one starting in 1991, see Table 1). The reason why we chose not to consider these clusters is to check whether the pattern found (based on the logarithms of the TSE and of the volume output of the previous lateral event) can be due only to these clusters. (4) a different seismic catalogue, by including only the seismic events in a circle of 3 Km of radius centred on Mount Etna. The C 25 The Authors, GJI, 163, 123 1218

On the occurrence of eruptions of Mount Etna volcano 129.5.45 Fisher Analysis Results, Case since 1974, M c =3. Mean of Class 1 (Type A) Frequency in the seismic cluster dataset.4.35.3.25.2.15.1.5 Mean of Class 2 (Type C) 3.5 3 2.5 2 1.5 1.5.5 1 Direction satisfying Fisher s criterion (Log(VOP[Standardized])) Figure 8. Results of the Fisher analysis for the case since 1974 and for M c = 3., when all the data are used in the learning phase. All the VOP values in millions of m 3. The classification is performed on the basis of Log(VOP), that is the only relevant variable identified by FIS algorithm in this case. The mean of Log(VOP) for the whole set of learning data is 1.8, and the variance is.3. The standardized voting objects should be projected along the Fisher s criterion line, that is, in this case, the standardized Log(VOP) axis. Then, they should be attributed to class 1 if their projected values are closer to the standardized mean of Class 1 (.65), to class 2 if closer to the standardized mean of Class 2 (.15). The red bars is the frequency plot of the Class 1 data projected along the Fisher s criterion line, while the blue bars are for Class 2 data. Fisher Analysis Results, Case since 1983, M c =2.5.45 Frequency in the seismic cluster dataset.4.35.3.25.2.15.1.5 Mean of Class 1 (Type A) Mean of Class 2 (Type C) 5 4 3 2 1 1 2 3 Direction satisfying Fisher s criterion (.72Log(REE[Standardized],1.39Log(VOP[Standardized]) Figure 9. Results of the Fisher analysis for for the case since 1983 and for M c = 2.5, when all the data are used in the learning phase. All the TSE values are in days, and the VOP values in millions of m 3. The classification is performed on the basis of Log(TSE) and Log(VOP), that are the relevant variables identified by FIS algorithm in this case. The mean of Log(TSE) for the whole set of learning data is 2.85, and the variance is.16. The mean of Log(VOP) for the whole set of learning data is 2., and the variance is.3. The standardized voting objects should be projected along the Fisher s criterion line, that is, their standardized Log(TSE) and Log(VOP) measurements should be linearly combined as x =.72 Log(TSE) + 1.39 Log(VOP). Then, they should be attributed to class 1 if their projected values are closer to the mean of Class 1 ( 1.24), to class 2 if closer to the mean of Class 2 (.17). The red bars is the frequency plot of the Class 1 data projected along the Fisher s criterion line, while the blue bars are for Class 2 data. C 25 The Authors, GJI, 163, 123 1218

121 L. Sandri, W. Marzocchi and P. Gasperini Table 2. Results of the pattern recognition analysis when only a random selected subset consisting of 8 per cent of the available data is used in the learning phase. For both cases, the number of learning (voting) objects is given, together with the variables identified as discriminating and the classification error produced when voting the learning (voting) data. Number of BDT FIS LEARNING (γ (L), (γ (L), (VOTING) objects γ (V ) ) γ (V ) ) Case 1 Class1 ( A ): 32 (7) Log(TSE) Log(TSE) (1974, M c = 3.) Class2 ( C ): 134 (35) Log(VOP) Log(VOP) (36 per cent, (2 per cent, 45 per cent) 26 per cent) Case 2 Class1 ( A ): 35 (9) Log(TSE) Log(TSE) (1983, M c = 2.5) Class2 ( C ): 269 (65) Log(VOP) Log(VOP) SEA (9 per cent, (2 per cent, 9 per cent) 17 per cent) Table 3. Results of the control experiments for the pattern recognition algorithms in the two cases. BDT BDT FIS FIS 1974 1983 1974 1983 M c = 3. M c = 2.5 M c = 3. M c = 2.5 τ = 4 days Log(VOP) Log(VOP) NEV Log(VOP) MXP NEV Clusters Log(TSE) Log(TSE) Log(TSE) Log(VOP) of 14 days Log(VOP) Log(VOP) Log(VOP) NEV NEV SEA Excluding Log(TSE) SEA Log(TSE) eruption 1991 Log(VOP) Log(VOP) SEA SEA Only seismic Log(VOP) Log(TSE) Log(TSE) Log(TSE) events Log(VOP) Log(VOP) Log(VOP) in 3 km SEA NEV LON reason why we do this control experiment is to check whether the trimmed means of the coordinates are biased by the simultaneous occurrence of seismic events close and far away from the volcano. The control experiments (see Table 3) overall confirm the predominant importance of Log(VOP); furthermore, Log(TSE) and NEV show a recurrent discriminant ability. No other recurrent pattern is found. Even though the results obtained in these four control cases are less stable than those obtained in the standard analysis, they confirm that no seismicity feature has a predominant discriminating ability in distinguishing between precursory clusters and clusters not related in time-to-flank activity. Remarkably, our result is different from those previously obtained by Mulargia et al. (1991, 1992) and Vinciguerra et al. (21). All of these studies, in fact, recognized particular seismicity patterns preceding flank activity, linked, in some way, to the tectonic regime 3.3 Multivariate regression fit In this section we analyse the seismic cluster data set with a multivariate regression analysis (Draper & Smith 1998). The use of this statistical technique is particularly indicated in the present case where the TSE seems to be a relevant feature. In fact, in NSPR analysis the continuous variables [for instance, Log(TSE)] are necessarily grouped by the algorithms in order to establish the boundary between the two classes. This grouping does not allow distinguishing between different values of the variable inside the same class; for example, very similar times-from-eruption can belong to different classes if they fall on different sides of the threshold value, while, on the contrary, very different times-from-eruption can belong to the same class. In this case, the use of multivariate regression analysis overcomes this possible drawback because it finds out how the time-to-eruption variable depends on the other variables considered without imposing any grouping. At the same time, the method allows checking the stability of the results obtained with the NSPR analysis, and to further investigate on the pattern found based on Log(VOP) and Log(TSE). Before going through the details of the analysis, it is worth to make some cautionary remarks on this kind of analysis. In particular, when we do regression calculations on unplanned data (that is, data arising from continuing operations and not from a designed experiment), some potentially dangerous possibilities can arise. For example, a false effect (a bias) on a visible variable may be caused by an unmeasured latent variable. Another undesired effect is linked to the case in which the most effective variable is kept within quite a small range, that might lead to the misinterpretation that the variable has no significance. A third problem is that the use of unplanned data often causes large correlations between predictors; this makes it impossible to attribute a causal effect to one specific predictor. From a qualitative graphical point of view, our multivariate regression analysis builds a linear model in which the dependent variable (y) isthe logarithm of the time from the beginning of the cluster to the following flank eruption. The independent variables are the nine variables observed for each cluster. The first step of the analysis consists of selecting the relevant variables of these nine that account for the variability of the dependent variable. Here, we select the most relevant variables by using the procedure called best subset search described by Garside (1971). The procedure first requires the fitting of every possible regression equation, which involves any combination of independent variables, and then selecting the case, which is the best with respect to one criterion. The criterion adopted here is based on the R 2 coefficient, representing the ratio of the variance explained by the regressive model to the total variance. We report the case with the highest R 2 coefficient obtained by using a single independent variable, two independent variables, three independent variables and so on (e.g. see Table 4). The best regression is the one for which the further gain in R 2 is significantly reduced. This method contains a degree of subjectivity (see discussion in Draper & Smith 1998), but straightforwardly identifies the parameters explaining the largest part of variance in the dependent variable. 3.4 Results of the multivariate regression fit Table4and the upper panels of Fig. 1 report the results obtained by using all the available data and, as dependent variable, the logarithm of the time to the next flank eruption. These results show that the best regression is the one obtained when using three independent variables, in particular NEV, Log(TSE) (TSE in days) and Log(VOP) (VOP in millions of m 3 ), with coefficients: Log(y) = (2.6 ±.3) (.6 ±.1)NEV + (.87 ±.9)Log(VOP) (.57 ±.13)Log(TSE), (4) C 25 The Authors, GJI, 163, 123 1218

On the occurrence of eruptions of Mount Etna volcano 1211 Table 4. Results of the multivariate regression analysis when all the data are used in the learning phase. In each row, by using only N (N 9) independent variables, the best regression is obtained if the variables indicated are used, and the correspondent fraction of variability in the data explained by this regression is given (R 2 ). N 1974, M c = 3. 1983, M c = 2.5 variables R 2 variables R 2 1 Log(VOP) 29.2 Log(VOP) 37.9 2 NEV, Log(VOP) 37.5 Log(TSE), Log(VOP) 47.6 3 NEV, Log(TSE), Log(VOP) 43. NEV, Log(TSE), Log(VOP) 52.8 4 NEV, Log(TSE), MXP, Log(VOP) 43.9 NEV, LAT, MXP, Log(VOP) 53.4 5 MXM, NEV, Log(TSE), MXP, Log(VOP) 44.4 NEV, LAT, Log(TPC), Log(TSE), Log(VOP) 53.8 6 MXM, NEV, LAT, 44.6 MXM, NEV, LAT, 53.9 Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), Log(VOP) 7 MXM, NEV, LAT, SEA, 44.7 MXM, NEV, LAT, Log(TPC), 54. Log(TSE), MXP, Log(VOP) Log(TSE), MXP, Log(VOP) 8 MXM, NEV, LAT, LON, SEA, 44.8 MXM, NEV, LAT, SEA, 54.1 Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), MXP, Log(VOP) 9 MXM, NEV, LAT, LON, SEA, 44.8 MXM, NEV, LAT, LON, SEA, 54.2 Log(TPC), Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), MXP, Log(VOP) R 2 R 2.45.4.35.3 Case since 1974, M c =3., all data in learning.25 1 2 3 4 5 6 7 8 9 number of independent variables used.45.4.35.3 Case since 1974, M c =3., 8% data in learning.25 1 2 3 4 5 6 7 8 9 number of independent variables used R 2 R 2.55.5.45.4 Case since 1983, M c =2.5, all data in learning.35 1 2 3 4 5 6 7 8 9 number of independent variables used.6.55.5.45.4.35 Case since 1983, M c =2.5, 8% data in learning 1 2 3 4 5 6 7 8 9 number of independent variables used Figure 1. Plots of R 2 as a function of the number of independent variables used (the best subset) in the multivariate regression. Upper panels are relative to the case in which all the data are used in the regression (cf. Table 4), lower panels relative to the case in which only a randomly chosen subset of 8 per cent of the data is used (cf. Table 5). for the case since 1974, M c = 3., and Log(y) = (2.7 ±.2) (.23 ±.4)NEV + (.9 ±.5)Log(VOP) (.58 ±.7)Log(TSE), (5) for the case since 1983, M c = 2.5. The addition of any other feature improves very little the R 2 coefficient. Note that the regression coefficient for NEV is one order of magnitude smaller than those for Log(TSE) and Log(VOP), and it has the largest percentage error. This means that these latter two parameters explain by far the largest part of the variance, as found in the NSPR analysis. The results obtained on a randomly chosen subset of the data set available, consisting of only 8 per cent of the total amount of data, are shown in Table 5 and in the lower panels of Fig. 1. Here, again, the best subset of variables consists of Log(TSE), Log(VOP) and NEV. In Table 6 the results of the multivariate regression analysis obtained in the same control experiments (as in the NSPR analysis) are displayed. In the majority of the experiments, these three parameters explain the largest part of variance in the dependent variable. In any case, we can never explain more than 5 6 per cent of the variability in the data set (see Tables 4 and 5). This implies that the scatter of the data around the best multivariate regression line is too large to allow to use profitably the multivariate regression line as an efficient forecasting rule. The multivariate regression fit confirm that, except for a minor influence of feature NEV, the discrimination between seismic clusters occurring just before or away from a flank eruptions is mainly due to variables related to the occurrence of the previous flank eruption. In particular, as mentioned above, the larger the volume erupted in a flank eruption, the longer time is needed before having another C 25 The Authors, GJI, 163, 123 1218

1212 L. Sandri, W. Marzocchi and P. Gasperini Table 5. Results of the multivariate regression analysis when only a random selected subset consisting of 8 per cent of the available data is used in the learning phase. In each row, by using only N (N 9) independent variables, the best regression is obtained if the variables indicated are used, and the correspondent fraction of variability in the data explained by this regression is given (R 2 ). N 1974, M c = 3. 1983, M c = 2.5 variables R 2 variables R 2 1 Log(VOP) 28.1 Log(VOP) 39.6 2 NEV, Log(VOP) 36.4 Log(TSE), Log(VOP) 49.6 3 NEV, Log(TSE), Log(VOP) 4.7 NEV, Log(TSE), Log(VOP) 55.3 4 NEV, Log(TSE), MXP, Log(VOP) 42.1 NEV, Log(TPC), Log(TSE), Log(VOP) 55.8 5 NEV, LAT, Log(TSE), MXP, Log(VOP) 42.4 MXM, NEV, Log(TPC), Log(TSE), Log(VOP) 56.2 6 MXM, NEV, LAT, 42.6 MXM, NEV, LAT, 56.6 Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), Log(VOP) 7 MXM, NEV, LAT, Log(TPC), 42.7 MXM, NEV, LAT, Log(TPC), 56.8 Log(TSE), MXP, Log(VOP) Log(TSE), MXP, Log(VOP) 8 MXM, NEV, LAT, SEA, 42.8 MXM, NEV, LAT, LON, 56.8 Log(TPC), Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), MXP, Log(VOP) 9 MXM, NEV, LAT, LON, SEA, 42.8 MXM, NEV, LAT, LON, SEA, 56.9 Log(TPC), Log(TSE), MXP, Log(VOP) Log(TPC), Log(TSE), MXP, Log(VOP) Table 6. Results of the control experiments for the multivariate regression analysis in the two cases. MUL REG MUL REG 1974 1983 M c = 3. M c = 2.5 τ = 4 days Log(VOP) Log(TSE) NEV Log(VOP) NEV Clusters Log(TSE) Log(TSE) of 14 days Log(VOP) Log(VOP) NEV NEV Excluding Log(TSE) Log(TSE) eruption 1991 Log(VOP) Log(VOP) NEV NEV Only seismic Log(VOP) Log(TSE) events NEV Log(VOP) in 3 Km MXM NEV flank eruption. This is a necessary condition in time-predictable systems, and is related to the re-charging of the feeding system. In the next section, we analyse this issue in depth, in order to check whether flank eruption occurrence at Mount Etna is a time-predictable process. 4 IS THE OCCURRENCE OF FLANK ERUPTIONS AT MOUNT ETNA TIME PREDICTABLE? The pattern found by means of PR and multivariate regression analyses suggests us to check the existence of a time-predictable behaviour in the temporal occurrence of flank eruptions at Mount Etna. Actually, since no seismicity feature has been found as discriminant, we are excluding that the regional tectonic stress is the main feature responsible for the opening of magma-filled fractures along Mount Etna flanks. In this respect, if the re-charging of the magmatic system is the main factor controlling eruption occurrence, the distinction between flank and summit eruptions may be no longer useful. Because of this, in the following we verify the reliability of a time-predictable behaviour both on the flank eruption catalogue (Table 1) and on the joint catalogue of flank and summit eruptions (from now on, flank + summit, reported in Table 7). The latter catalogue has been taken from Behncke et al. (25). Table 7. Catalogue of flank + summit eruptions at Mount Etna volcano. Eruption Onset End Total volume output (1 6 m 3 ) (YYYY MM DD) (YYYY MM DD) (Lava and Tephra) 1 1971 4 5 1971 6 12 47.2 2 1974 1 3 1974 2 17 4.4 3 1974 3 11 1974 3 29 3.2 4 1974 1 1 1975 8 29 17.9 5 1975 9 12 1977 1 8 41.8 6 1977 7 16 1978 3 27 9.6 7 1978 4 29 1978 6 5 27.5 8 1978 8 25 1978 8 3 4. 9 1978 11 23 1978 11 3 11. 1 1979 8 3 1979 8 9 7.5 11 198 9 1 198 9 26.1 12 1981 2 5 1981 2 7.3 13 1981 3 17 1981 3 23 21.3 14 1983 3 28 1983 8 6 79. 15 1984 4 28 1984 1 17 1.2 16 1985 3 1 1985 7 13 3. 17 1985 12 25 1985 12 31.9 18 1986 9 13 1986 9 24 7.4 19 1986 1 3 1987 3 1 6. 2 1989 5 13 1989 1 9 38.5 21 199 1 4 199 2 2 2.2 22 1991 12 14 1993 3 31 235. 23 1995 7 3 1996 8 3 6.4 24 1996 11 6 1999 11 14 53.5 25 2 1 26 2 8 28 46.8 26 21 1 2 21 8 8 44.5 27 22 1 27 23 1 28 85.5 A very simple time-predictable model for volcanic eruptions (cf. Bacon 1982; Burt et al. 1994; Hill et al. 1998) assumes that eruptions always occur when the volume of magma in the storage system reaches a threshold value; if the magma input in the magma storage system is approximately constant (cf. Mulargia et al. 1987a), but the size of eruptions is a random variable (RV) following some kind of statistical distribution, then we have a time-predictable system with longer/shorter interevent times (defined as the time between the onset of two successive eruptions; Mulargia et al. 1987a; Burt et al. 1994) following large/small volume output eruptions. In this respect, the pattern that we have found with PR and multivariate C 25 The Authors, GJI, 163, 123 1218

On the occurrence of eruptions of Mount Etna volcano 1213 regression analyses for flank eruptions at Mount Etna is compatible with a time-predictable behaviour. In order to verify if the occurrence of flank and/or of flank + summit eruptions at Mount Etna really follows a time-predictable pattern, we need to check whether there is a significant linear relationship between the volume erupted during an eruption (flank or flank + summit) and the interevent time following it (the necessary condition for time-predictability to hold). In fact, for a timepredictable system, the time to the next eruption (next interevent time) is determined by the time required for the magma entering the storage system to reach the eruptive level. In this view (cf. Burt et al. 1994), we have: v i r i, (6) where v i is the volume erupted during the ith eruption occurring at time t i, and r i is the interevent time between the onset of ith and the onset of (i + 1)th eruptions (i.e. r i = t i+1 t i ). If we cumulate eq. (6) over the N eruptions occurred, the random noises cancel out. In this case, we obtain a linear plot with t i on the x-axis (where t i,as mentioned above, is the onset time of the ith eruption), and, on the y-axis, the cumulative volume v i erupted during the previous and the present eruptive episodes (i.e. V i = i k=1 v k, i = 1,...,N), as the one shown in Fig. 11 for flank eruptions and Fig. 12 for flank + summit. However, the regression analysis to identify a possible relationship between the interevent times and the erupted volumes must not be carried out on the cumulative curve t i versus V i. This is because a basic assumption of the regression analysis is that the data must be independent, and they are clearly not in case of cumulated data. In this respect, we must analyse the simple v i and r i data. Furthermore, in order to reduce the too high leverage of some of the data (see Draper & Smith 1998), in this work we analyse the log v i and log r i data, although we also display the plot t i vs V i because it represents the standard view when speaking about time-predictable systems. By means of the logarithmic transformation of variables, the time-predictable model by Burt et al. (1994) is generalized, because a possible significant linear relationship between log v i and Cumulative Erupted Volume (millions of m 3 ) 8 7 6 5 4 3 2 log r i would imply a power law between v i and r i : log v i = a + b log r i v i = kr b i. (7) If the exponent b is equal to 1, we are in a classical (i.e. as modelled by Burt et al. 1994) time-predictable system; in the other cases, the time derivation of eq. (7) implies non-stationary re-charging process. For Mount Etna, we first check whether there have been changes in the eruptive behaviour. By means of the CHPT algorithm (Mulargia &Tinti 1985), we first analyse the flank eruption catalogue, in particular the time-series of: (1) the erupted magma volume v i, identifying no change point at.5 significance level; (2) the interevent time r i, identifying no change point at.5 significance level; (3) the ratio of the erupted magma volume v i to the following interevent time r i, identifying no change point at a.5 significance level; The third time-series analysed is the one related to the slope of the best fit line in Fig. 13, representing the long-term magma supply rate (Burt et al. 1994). So, we perform a regression analysis on the whole series of log r i and log v i (from 1971 to 22, flank eruption catalogue) obtaining the regression results shown in Fig. 13 and in Table 8.Weconsider as independent variable the interevent times, their uncertainties being much smaller than those on the erupted volumes. As we see from Table 8, the regression model rejects the null hypothesis H : b, (where b is the slope of the model, estimated from the data through the least-squares technique) at a.1 significance level, by means of the F-test. This means that there is a significant linear relationship between the logarithm of the erupted magma volume and the logarithm of the following interevent time. Furthermore, the estimated slope of the linear relationship (ˆb =.9 ±.3, see Table 8) is Cumulative erupted volumes in flank eruptions at Mt Etna 1 1 3 5 7 9 11 13 15 17 Time (labels indicate the occurrence of flank eruptions listed in Table 1, only odd numbers) Figure 11. Plot of the cumulative volume of magma erupted in flank eruptions at Mount Etna, as a function of time. C 25 The Authors, GJI, 163, 123 1218

1214 L. Sandri, W. Marzocchi and P. Gasperini 9 Cumulative erupted volumes in flank+summit eruptions at Mt Etna 8 Cumulative Erupted Volume (millions of m 3 ) 7 6 5 4 3 2 1 1 3 5 79 113 15 1719 21 23 25 27 Time (labels indicate the occurrence of flank+summit eruptions listed in Table 7, only odd numbers) Figure 12. Same as Fig. 11, but for flank + summit catalogue (from 1971 to 22). Log 1 [Erupted Volume (millions of m 3 )] 2.5 2 1.5 1.5.5 1 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 Log [Interevent Time (days)] 1 Figure 13. Linear regression analysis of the whole series of flank eruption data (from 1971 to 22). Each circle represents the logarithm of the erupted volume in a flank eruption as a function of the logarithm of the interevent time following it. The solid line is the best fit line, while the dashed lines represent ±σ, that is, 68 per cent confidence levels for logarithm of the erupted volumes predicted by the model. consistent with a classical time-predictable model as the one proposed by Burt et al. (1994). The regression model explains only a part of the variability in the data (R 2 =.44, see again Table 8). We repeat this analysis on the flank + summit eruption catalogue (Table 7). The results for the searching for change points are that: (1) in the erupted magma volume time-series v i,weidentify one change point at.5 significance level on 1995 July 3, but none at a.1 significance level; (2) the interevent time r i, identifying one change point at.5 significance level on 199 January 4, but none at a.1 significance level; (3) the ratio of the erupted magma volume v i to the following interevent time r i, identifying no change point at a.5 significance level; So, we perform again a regression analysis on the whole series of log r i and log v i from the flank + summit catalogue (from 1971 C 25 The Authors, GJI, 163, 123 1218