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Correlation between the dynamics of the Hyblean
Foreland, Etnean volcanism
Ivan Agostino, Giuseppe Patané, Santo La Delfa
To cite this version:
Accepted Manuscript
Title: Correlation between the dynamics of the Hyblean Foreland, Etnean volcanism
Authors: Ivan Agostino, Giuseppe Patan´e, Santo La Delfa
PII: S0264-3707(08)00083-5
DOI: doi:10.1016/j.jog.2008.11.001
Reference: GEOD 870
To appear in: Journal of Geodynamics
Received date: 12-6-2008 Revised date: 11-11-2008 Accepted date: 11-11-2008
Please cite this article as: Agostino, I., Patan´e, G., La Delfa, S., Correlation between the dynamics of the Hyblean Foreland, Etnean volcanism., Journal of Geodynamics (2008), doi:10.1016/j.jog.2008.11.001
Accepted Manuscript
Correlation between the dynamics of the Hyblean Foreland and
1
Etnean volcanism.
2
Ivan Agostino1*Giuseppe Patané1, Santo La Delfa1
3
1Dipartimento di Scienze Geologiche – University of Catania – Corso Italia 57, 95129, Catania (Italy)
4
*Corresponding author: Corso Italia 57, 95129, Catania (Italy) – Tel & Fax:+390957195707. e-mail: i.agostino@unict.it 5
6 7
Abstract
8
The Hyblean Foreland (South Eastern Sicily) represents the northern extremity of the 9
African Plate in collision with the Euro-Asiatic one. It is mainly made up of quite thick 10
carbonate formations and volcanic units, which are in subduction under the Tertiary 11
allochthonic terrains and Quaternary clays, on which the volcano Etna is formed. In the 12
recent past, researchers have analysed both the historical and instrumental seismicity of 13
the Hyblean Foreland as a phenomenon independent of the dynamics to which Etna is 14
prone; in fact, it was presumed that the particular seismic style of the Hyblean area, 15
characterised by large releases of seismic energy in a brief time span after centuries of 16
relative quiescence, was only related to the dynamics of plate-tectonics in the 17
Mediterranean area. The detailed analyses and the results obtained in this study, based 18
on the instrumental seismicity of the Hyblean area between 1983 and 2002, partially 19
modify this point of view. Although the current convergence between the African and 20
Euro-Asiatic plates does play a significant role in the tectonic activity of south-eastern 21
Sicily, the authors believe that the behaviour of the mantle under Mt Etna does seem to 22
have a controlling influence on the seismicity of the area under study. In particular, on 23
comparing the development between the eruptive activity of Etna and the seismic 24
energy of the Hyblean Foreland in the period between 1983 and 2002, the b-value trend 25
and the characteristics of the stress field in the period 1994 to 2002, the authors deduce 26
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in the Hyblean area. The modifications produced in the stress field trigger earthquakes 28
of a higher magnitude in the areas near the volcanic one. Finally, it is hypothesised that 29
the disastrous seismic phenomena of the last one thousand years have been set off by 30
the interaction between the local dynamics of the mantle and the regional dynamics 31
associated to the plate-tectonics. 32
33
Keywords: Hyblean Foreland, Stress Field, Focal Mechanism, b-value, Plate-tectonics
34 35
1. Introduction
36
While studying the 1990 volcanic tremor of Etna (Patané et al., 1991), it was noticed 37
that the power associated with this phenomenon was growing. It followed the same 38
trend that had been observed the year before when an eruption had occurred on the 39
eastern flank of the volcano starting on 29th September1989. These observations led us 40
to think about the possibility of a new eruption at the end of 1990, but instead there was 41
an earthquake on the 13th December 1990 with Ml=5.4 and with its epicentre in the 42
Ionian Sea, near the North-Eastern edge of the Hyblean Foreland. This is the highest 43
energy earthquake to occur in this area in the last century. On the 13thDecember 1991, a 44
new violent eruption began on the eastern flank of Etna associated with intense 45
fracturing (Patané et al., 2006), and was preceded, in the volcanic area, by seismic 46
phenomena similar to those of 1989 and 1990. These events gave us the idea that some 47
kind of correlation exists between earthquakes in the Hyblean area and Etnean 48
volcanism. At that time the catalogue of earthquakes within the Hyblean area was not 49
sufficient to demonstrate any eventual hypothesis for this correlation. Thus, it was 50
necessary to carry out seismic monitoring for a sufficiently long period of time to 51
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by the Istituto Nazionale Geofisica e Vulcanologia (INGV), and Gruppo Nazionale 54
Difesa Terremoti (GNDT). Between 1983 and 2002 about a thousand seismic events 55
were monitored having epicentres in the Hyblean Foreland and with M>1.0. However, 56
only 750 seismic events were of the quality needed to be considered in drawing up some 57
of the necessary parameters for our study. In particular, the character of the temporal 58
development of the seismic energy was analysed, along with the state of stress of the 59
Hyblean crust using the b-value from the Gutenberg-Richter curve (1954). Later, the 60
time variations of these two parameters were compared to Etna’s dynamism with the 61
aim of highlighting whether a correlation between seismic and eruptive phenomena 62
exists. Our aim was to verify or reject the following hypothesis: that variations in stress 63
intensity to which the Hyblean crust is subjected are also determined by a particularity 64
of dynamism in the mantle, to which the various eruptions which occurred between 65
1984 and 2002 are correlated. Finally, to further the study of this dynamism, the 66
orientations of the principal axes of the stress field in the Hyblean area were determined 67
using the Gephart and Fortsyth (1984) algorithm and Gephart (1990), and the variation 68
in the orientation of the σ1 and σ3 axes were analysed, both in the temporal and spatial 69
domains. 70
71
2. Seismotectonic features of Hyblean Foreland
72
The structural framework of Eastern Sicily belongs to the recent tectonic regime that is 73
dominated, in the central Mediterranean area, by the NNW convergence of the African 74
plate towards the Euro-Asiatic plate (Fig.1a) (Finetti and Del Ben, 1986; Ben-Avraham 75
and Grasso, 1990). The Hyblean Foreland in South-Eastern Sicily represents the 76
northern margin of the African continental crust, weakly deformed in the middle 77
Miocene. The Hyblean Maltese Escarpment (HME in the text), a morphotectonic high 78
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Quaternary this area was crossed by normal, reverse and strike-slip faults (Ghisetti and 80
Vezzani, 1980; 1982), which are probably linked to a renewed geodynamic activity. The 81
NNW-SSE faults are located mainly on eastern side of the Foreland and they are linked 82
to the inland evolution of the HME (Adam et al. 2000; Catalano et al., 2008). The 83
Hyblean Foreland has been divided into 4 areas that contain well-defined tectonic 84
trends, as indicated in Fig. 1b with A, B, C, D. 85
In particular, the northern sector of the Foreland is crossed by NE-SW and ENE-WSW 86
fault systems, which form the Simeto Graben and the Lentini Scordia Graben and 87
subsides under the Appenninic-Maghrebian chain and Etna volcano (Cristofolini et al., 88
1980 ; Yellin-Dror et al., 1997). Other important tectonic discontinuities are the N-S 89
Scicli-Monte Lauro line, the NE-SW Acate-Caltagirone-Ponte Barca and Pozzallo-90
Rosolini fault systems that are respectively in the west and south sectors (Fig. 1b, A & 91
D) (Carbone et al., 1987; Lentini et al, 1987; Catalano et al., 2008). Some deep seismic 92
refraction profiles (Cassinis et al. 1969; Contenisio et al., 1997) showed a marked 93
continuity of the Hyblean geological formations beneath the Etnean volcanic structure, 94
where a thin continental crust (17 km thick) marks the upper boundary of a low velocity 95
layer. Panza et al. (2007) and Panza & Raykova (2008) achieved the same results by 96
combining Vs velocity models derived from tomographic inversion with the distribution 97
with depth of hypocentres. 98
According to Sharp et al. (1980) this low velocity layer should probably be correlated 99
with a large crustal volume affected by an intricate net of magma-injected fractures. 100
This magma rising from the mantle may accumulate in this chamber and then, migrating 101
towards the surface, may give rise to various eruptive episodes. The Hyblean Foreland 102
is characterised by relevant releases of seismic energy, sometimes through earthquakes 103
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followed by long periods of time (about 300-500 years) in which earthquakes of lower 105
magnitude, i.e. generally less than magnitude 5, occur (Carbone et al., 1982). Analysis 106
of the historical seismicity shows that the most active zones are those nearest Etna 107
(Carbone et al., 1982; Patanè & Imposa, 1987). The two most disastrous historical 108
earthquakes, in 1169 (MS=7.3) and in 1693 (MS=7.0) (Boschi et al., 1995), were located 109
close to the HME, which also affects the eastern sector of the volcano Etna (Fig. 2a) 110
(Continisio et al, 1997; Hirn et al. 1997). In the north-western sector of the Hyblean 111
area (fig. 2a), the historical earthquakes show lower magnitudes (Azzaro and Barbano, 112
2000). The distribution of the instrumental epicentres, located by the INGV in the 113
period 1983-2002 (Fig. 1b), seems to be consistent with the structural trends that fall 114
into areas A, B, C, and D of Fig. 1b and confirms what was historically observed. 115
The density of epicentres increases from SE to NW and to the East (Fig.2b). In 116
particular, in the N and NE sectors, seismicity is associated with faults orientated NE-117
SW and ENE-WSW, which border the Lentini-Scordia graben, the Simeto graben and 118
the structural alignment, oriented NE-SW, Acate-Caltagirone-Ponte Barca (group A and 119
B in Fig. 1b and Fig 2b). In the eastern sector, the highest number of epicentres are 120
found in correspondence to the HME (group C in Fig.1b and Fig.2b), where the highest 121
magnitude earthquake of the twentieth century occurred, that of 13th December 1990 122
ML= 5.4 (Fig 2a). Lastly, in the southern sector, seismicity is associated with faults 123
oriented NE-SW (Rosolini-Ispica-Pozzallo line) and NW-SE, both outcropping along 124
the eastern margin of the Hyblean Foreland (Fig. 1b, D and 2b, D). 125
126
3. Data and Methods
127
3.1 Energy and b-value
128
The data considered are those recorded by the INGV seismic network and those 129
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in Fig. 2b). The overall number of earthquakes is 750 with magnitudes between 1.0 and 131
5.4, from 1983 to 2002. A preliminary statistical approach to the seismicity of the 132
Hyblean area was carried out through the study of the temporal distribution of the 133
seismic energy. 134
The cumulative energy curve was obtained using the relationship 135
Log Es = 9.9 + 1.9 ML- 0.024ML2 (Richter 1958), 136
as seen in Fig.3a. It shows three periods of higher seismic activity (1st, 3rd, 5th) and two 137
periods of relative quiescence (2nd, 4th). Another statistical approach was carried out by 138
applying the relationship 139
Log N = a - b ML(Gutenberg and Richter, 1954). 140
Many studies performed on similar datasets simply assume that the catalogue is 141
incomplete for any magnitude for which log N in this relationship lies below the best 142
line determined for the higher magnitudes (Caputo, 1987; De Natale et al., 1987; 143
Molchan, 1996). N denotes the number of earthquakes with magnitude greater than or 144
equal to ML, a and b are parameters describing regional seismicity. The constant a, 145
named the "seismic activity" parameter, is related to the total number of earthquakes 146
with no negative magnitudes. The b-value gives information about the ratio between 147
large and small earthquakes within an area. 148
The 750 earthquakes considered in this study are distributed into the 5 temporal 149
intervals identified on the above mentioned cumulative curve. Each interval contains a 150
variable number of events; for each interval the Gutenberg and Richter curve (1954) 151
was constructed, from which the b-value was calculated (Fig. 3a). To minimise b-value 152
evaluation errors the Magnitude completeness (Mc) was calculated. This represents the 153
minimum magnitude at which 100% of the events can be modelled (Wyss and Wiemer, 154
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reconfigurations by the seismic network, like an increase in the number of seismic 157
stations, changes and improvements in methods of determination of magnitude, etc. 158
However, the best determination of Mc is a necessary condition, to identify the most 159
reliable values of the b-value. Moreover, an accurate estimate of Mc allows the number 160
of events in the catalogue to be maximised and avoids an underestimation of the b-161
value. 162
In this study the calculation of the Mc and of its uncertainty are carried out using a new 163
method, which has the advantage of supplying a stochastic model (Entire Magnitude 164
Range method) of a dataset of earthquakes, integrated through a statistical methodology 165
called "Boot strap" (Woessner & Wiemer, 2005). This is a technique of re-sampling 166
data from an original dataset, described by Chernik (1999), allowing the determination 167
of a parameter of specific interest (in our case Mc) and its evolution through the 168
evaluation of standard errors and the interval of confidence. 169
3.2. Focal Mechanism Stress Inversion
170
With the aim of studying the spatial and temporal distribution of the stress field in the 171
Hyblean area, 138 events which occurred between 1994 and 2002 (ERH < 1.5 Km; ERZ 172
< 2.0 Km, RMS < 0.3 s) were chosen and their focal mechanisms were computed using 173
the FPFIT software (Reasenberg & Oppenheimer, 1985). The stress inversion was 174
carried out using the ZMAP software (Wiemer, 2001), which contains the routine 175
"Focal Mechanism Stress Inversion" (FMSI) developed by Gephart and Forsyth (1984) 176
and Gephart (1990). 177
This method is based on three basic assumptions: stress is uniform in the rock volume 178
of the seismic sample investigated; earthquakes are shear dislocations on pre-existing 179
faults; and slip occurs in the direction of the resolved shear stress on the fault plane. 180
Four stress parameters are calculated: three of them define the orientation of the 181
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R= (σ2-σ1)/ (σ3-σ1). Moreover, a misfit variable (f) is introduced to define 183
discrepancies between the stress tensor and the observed fault plane solutions. For a 184
given stress model, the misfit (f) of a single focal mechanism is the smallest rotation 185
around any arbitrary axis that brings one of the nodal planes, its slip direction and the 186
sense of slip into an orientation that is consistent with the stress model. The size of the 187
average misfit (F) provides a guide of how well the assumption of stress homogeneity is 188
fulfilled relative to the seismic sample submitted to the inversion algorithm (Michael, 189
1987). 190
In the light of the results from a series of tests carried out by Wyss et al. (1992a), Wyss 191
et al. (1992b), and Cocina et al., (1997) for identifying the relationship between FPS 192
uncertainties and average misfit for uniform stress, it is presumed that the condition of a 193
homogeneous stress distribution is fulfilled if the average misfit (F) is smaller than 6° 194
and it is not fulfilled if F>9°. For F values between 6° and 9° the solution is considered 195
acceptable, but may reflect some heterogeneity. This methodology was used to find the 196
orientation of the main stress axes in the spatial-temporal domains and to distinguish 197
between the local stress field and the regional one. 198
199
4. Discussion
200
4.1. Correlation between seismicity and eruptive activity
201
The cumulative curve in Fig. 3a shows the temporal development of seismic energy (E). 202
Three temporal intervals can be identified (1st, 3rd and 5th), in which there is a marked 203
increase in the rate of energy, and two temporal intervals (2ndand 4th) in which there is a 204
decrease in this rate. The b-value is less than or equal to 1.0 in the three intervals where 205
the rate of energy release is higher, whereas it is more than 1.0 in the two periods of 206
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Following Panza & Raykova (2008), who found mantle to be bending in the Hyblean 208
and Etna area, we believe that the seismic character of the Hyblean area is determined 209
by the mantle rheology in eastern Sicily. This would also be responsible for the eruptive 210
phenomena that occurred during the period under study in the Etnean volcanic area. In 211
fact, between 1983 and 1987 (first interval), at medium-high altitudes on Etna there 212
were three eruptions with lateral fractures and lava flows that affected the southern and 213
eastern side of the volcano (Gresta et al., 1986; Clocchiatti et al., 1987; Caltabiano et 214
al., 1987; Gresta et al, 1987; Patané et al., 1987, La Delfa et al., 1999; Tanguy et al., 215
1999). Between September 1989 and March 1993 (3rd interval), two new summit 216
eruptions with lateral fractures, both on the southern and eastern sides, substantially 217
affected the same areas (Montalto et al., 1992; Ferrucci et al., 1993; Patané et al. 1996; 218
Tanguy et al., 1996; Tanguy et al. 1999). Finally, in October 1999, July-August 2001 219
and between October 2002 and January 2003 (5th interval) three strong eruptions 220
occurred in the summit zone of the volcano; the first one was characterised by an 221
abundant emission of lava from the Central Crater towards the West, the second and the 222
third ones from eruptive fractures both on the high southern and northern sides (La 223
Delfa et al., 2001; La Delfa et al., 2003; Benckhe and Neri, 2003; Immè et al. 2006). 224
This violent volcanic activity was preceded in 1998 by numerous episodes of 225
Strombolian activity at the summit craters which only lasted briefly (Tanguy et al. 1999; 226
La Delfa et al. 2001). In the periods of relative seismic quiescence (2ndand 4th intervals) 227
there were only rare manifestations of Strombolian activity, which sometimes evolved 228
into lava fountains at the summit craters; these eruptive phenomena, however, are 229
ascribable to low energy level volcanism (Patané et al., 2004). 230
All these results lead us to infer a tight correlation between the seismic energy release 231
modes in the Hyblean area, highlighted by the Energy (E), b-values and the trend in 232
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shows a relative low effective stress, whereas, with a higher effective stress the b-value 234
decreases (Scholz, 1968; Main et al, 1989; Urbancic et al, 1992; Schorlemmer et al, 235
2003). During the two periods of seismic quiescence (2nd and 4th) the Hyblean area is 236
characterised by low effective stress (higher b-values). The stress increases substantially 237
(notably lower b values) in the three periods in which there are more violent eruptive 238
events and the release of energy can occur even with earthquakes with a magnitude as 239
low as ML= 5.4 (Earthquake of 13 December 1990). 240
De Natale et al. (2004) used another statistical methodology to distinguish formally the 241
seismic activity of Vesuvius (Central-Southern Italy) in periods of quiescence, 242
characterised by seismicity background, and in periods of activity, characterised by 243
notably higher energy releases. To assess whether this methodology can also be applied 244
to tectonically active areas, like the Hyblean one, seismicity levels were computed using 245
the criteria suggested by De Natale et al. (2004). The 13th December 1990 earthquake 246
with ML= 5.4 was excluded so that the energy release in the Hyblean area in the period 247
between 1983 and 2002 would be comparable to that of the Vesuvian area. 248
Thus, the relationship E/E2.0 shown by E* was considered, where E represents the 249
seismic energy of an earthquake with magnitude M, E2.0 is the energy released by an 250
earthquake of magnitude Mc= 2.0. This value is the completeness magnitude of the 251
earthquake catalogue considered. The whole catalogue was subdivided into time-252
windows of one year shifted by one month. For each window the value E* was 253
computed. The distribution of E* is shown in figure 3b both in a cumulative and 254
discrete form. During most of the time period under study the energy released is 255
500<E*<2300. For values of E* over 2300 the energy distribution takes into account 256
only 34% of the period considered and undergoes a noteworthy decrease (Fig. 3b); thus 257
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relative quiescence q with L< 2300 and periods of higher activity a with L>2300 259
following the approach of De Natale et al. (2004). 260
Figure 3c shows the normalized monthly energy release (E*) using time windows of 261
one year shifted by one month for the period 1983-2002. The first four time intervals 262
which fall between September 1983 and June 1997 (Fig. 3c: 1, 2, 3, 4) agree well with 263
those in figure 3a (1st, 2nd, 3rd, 4th) although in a few cases their temporal width differs 264
slightly. Indeed, the two quiescence intervals and the two intervals of highest activity 265
almost coincide in the two figures. The 5th interval of figure 3a, considered of higher 266
relevant seismic activity compared to the previous one (4th), in figure 3c corresponds to 267
the intervals 5 and 6 respectively of quiescence (q) and activity (a). However, interval 5 268
of figure 3c shows remarkable fluctuations of E*, which are not found in interval 4 (Fig. 269
3c), even though these fluctuations are always below the threshold value (L), which is 270
clearly passed in the following interval 6 (Fig. 3c). 271
This latter consideration leads us to affirm that E* cannot be the single parameter used 272
to distinguish periods of quiescence from those of activity in active tectonic areas but 273
rather that the energy development modes must also be considered. The cumulative 274
curve in figure 3a shows that, in the 4th interval, the energy develops above all with 275
small magnitude earthquakes, while in the 5th interval there are notably higher 276
magnitude earthquakes. This is highlighted by the b-values, which are lower in the 5th 277
interval than in the 4th interval, the intervals that we believe are ones of quiescence. In 278
our opinion, in active tectonic areas, the cumulative curve of energy release and the b-279
values allow a more significant distinction between periods of quiescence and periods 280
of activity than the approach of De Natale et al. (2004). 281
4.2. Spatial-temporal evolution of Stress
282
The study in the spatial-temporal domain of the evolution of the stress field seems to 283
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volcanism. The focal mechanisms of 138 events with a good quality of the statistical 285
parameters (Tab. 1) were computed, hypothesising double-couple focal sources. 286
However, it is known that in active tectonic areas (Campus et al., 1996; Radulian et al., 287
2000; Aoudia et al. 2003), as well as in volcanic and geothermal areas (Saraò et al., 288
2001; Panza & Saraò, 2000), non-double-couple mechanisms are possible. In our 289
opinion the double-couple focal source hypothesis is supported by conclusions reached 290
in other studies carried out with different methodologies, which fit well with the results 291
of our study of the focal mechanisms. In particular, the velocity vector obtained by the 292
VLBI network shows that the Hyblean area moves in the direction NNW-SSE (De Mets 293
et al., 1990), a result confirmed by Montone et al. (2004) and Ragg et al. (1999) from 294
borehole breakout analysis of wells from onshore Sicily. Moreover, the crust in the 295
Hyblean area seems to be of little interest from a geo-thermal point of view (Cataldi et 296
al, 1995), which is confirmed by the particular velocity of the P-waves that are 297
markedly higher compared to the surrounding areas (Di Stefano et al. 1999; Scarfì et al 298
2007). These results support the existence of a relatively “cool” brittle crust, subjected 299
to a maximum principal horizontal stress oriented NNW-SSE, and therefore capable of 300
releasing elastic deformations justifying the assumption of the double-couple 301
mechanism. 302
We have computed a stress tensor inversion in which the misfit shows a high value of 303
11 degrees (Fig. 4). Generally, such a high value implies a very heterogeneous stress 304
field (Cocina et al.1997). To make the analysis more significant, the whole dataset was 305
subdivided into eight sub-sets corresponding to eight areas (Fig. 5 areas A, B, C, D, E, 306
F, G, H) within which the trend of the structural discontinuities are coherent, 307
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The eight areas contain a variable number of focal mechanisms, with a maximum of 35 309
and a minimum of 12. In each area the orientations of the principal stress axes σ1, σ2, 310
and σ3 were determined (Fig.5) and the misfits were calculated. Considering the trend 311
of the stress axes it is possible to note that σ1 is variably oriented and oscillates between 312
NNW-SSE and WNW-ESE; the plunge varies between the horizontal and sub-313
vertical. Also, σ3 shows some variations in trend and in plunge, from NNE-SSW to 314
ENE-WSW and from sub-vertical to sub-horizontal, respectively. In particular, inland, 315
the direction of σ1 remains less variable, oscillating between NNW-SSE and NW-SE 316
(Fig.5 A, B, E, F and G). In contrast, offshore (Fig.5 C, D, and H), near the Hyblean 317
Maltese Escarpment, σ1 shows a marked variability from WNW to NNW (almost 90 318
degrees), due to the instability of the stress field probably related to the complex 319
dynamics to which this escarpment is subjected (Catalano et al., 2008). The plunge of 320
σ1, inland, is generally sub-horizontal and oscillates between 4° and 37°; however, on 321
the western border, it becomes sub-vertical (71°), related to the subduction and 322
consequent faulting of the Hyblean Foreland below some nappes adjacent to the Gela-323
Catania Foredeep (Cogan et al. 1989, Grasso et al., 1995; Billi et al., 2006) (Fig. 1b, 324
Fig. 5, area F & G). 325
On average the trend of σ1 agrees with Musumeci et al. (2005) and would justify a 326
pressure axis oriented in a NNW-SSE direction ascribable to the convergence of the 327
African plate towards the Euro-Asiatic one. However, throughout the Hyblean Foreland 328
(inland and offshore), the trend of σ1 shows, instead, a wide variation from WNW to 329
NNW, even though the latter is more frequent (5 areas out of 8) (Fig. 5, A-H). 330
Therefore, in the Hyblean Foreland, the various stress regimes found over the whole 331
time considered would seem to reflect different causes, one of which, in our opinion, 332
could be that which determines the active volcanism of Etna. This latter consideration is 333
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areas G and H (3,6° an 5,1°), near the Etna area where the stress field is distinctly 335
homogeneous. In the more distant areas, A, B and C, in contrast, the misfit takes on 336
higher values (7.8°; 9.2°; 9.8°), linkable to a stress field that is more heterogeneous. In 337
the remaining areas D, E and F the misfit has intermediate values (6.2°; 7.2°; 7.6°) 338
ascribable to a lower degree of homogeneity compared to that obtained in G and H. 339
The analysis of the stress field in the temporal domains further supports our hypothesis 340
on the correlation which exists between the eruptive dynamism of Etna and the 341
seismicity of the Hyblean Foreland at least in some time intervals. However, 18 342
windows were considered containing 14 focal mechanisms each, which cover an 343
interval of about 8 years, between 1994 and 2002. Between two successive windows the 344
over-step is constant and equal to 7, while the temporal interval which contains each 345
window varies as a function of the frequency of earthquake occurrence. The number of 346
events within each window and the over-step were chosen to highlight any temporal 347
variations of the homogeneity of the stress field. Figure 6a shows that between 1994 and 348
the first few months of 2000 the stress field was homogeneous. Between 2000 and 2002 349
its homogeneity was notably variable and sometimes tended to become more 350
heterogeneous. The analysis of the orientation of the stress axes in the 18 time intervals 351
considered shows that σ1 oscillates around the direction N150E between 1994 and 2000 352
(Fig.6b). Between the end of 2000 and 2002, Etna shows a strong eruptive dynamism, 353
the orientation of σ1 in the Hyblean area becomes more unstable and varies around an 354
average value of N135E; analogous considerations can be made for σ3. Equally 355
interesting is the trend of inclination of σ1 and σ3 in the interval of time under study 356
(Fig.6c). Between 1994 and 1999, σ1 and σ3, generally show a plunge of less than 25° 357
and in only one case do σ1 and σ3 have a value greater than 40°. During the year 2000 358
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20°. Therefore, during the period 1994-1999, the Hyblean Foreland is affected by a 361
mainly tangential pressure which determines focal mechanisms linked to transcurrent 362
faults and occasionally to reverse faults. During the year 2000 the stress field 363
determines focal mechanisms ascribable both to normal faults and reverse faults with a 364
marked component of horizontal strike-slip. In 2001 and in 2002 focal mechanisms 365
correlated to normal faults prevail although these are sometimes associated with a 366
component of horizontal strike-slip; there are very few focal mechanisms associated 367
with reverse faults. There is, therefore, a substantial variation in the characteristics of 368
the stress field, which during 2001-2002 can be correlated with distension in the 369
Hyblean crust together with a rather intense eruptive dynamism in the Etnean area. 370
371
5. Conclusions
372
The trends of the cumulative curve of the seismic energy, and of the b-value in the 373
period 1983-2002, clearly show that the rate of energy released in the Hyblean area 374
grows markedly in the periods of higher eruptive activity of Etna and also that the b-375
values are low during such activity, thus confirming the increase in the release of energy 376
through higher magnitude earthquakes. 377
Therefore, there is a correlation between the ascent of magma in the volcanic area and 378
the variations of the stress field in the Hyblean Foreland. With the aim of further 379
elucidating this correlation, a spatial-temporal study of the variations of the stress field 380
was developed. Using all the focal mechanisms of the earthquakes a single stress 381
inversion was carried out which showed the existence, in the Hyblean area, of a 382
markedly heterogeneous stress field in the period 1994-2002. Moreover, stress 383
Accepted Manuscript
displays a variable degree of homogeneity: it is markedly homogeneous in the areas 385
near Etna and more heterogeneous in the more distant areas. 386
Between 1994 and 1999 the stress field maintains a good degree of homogeneity. This 387
was when the Hyblean Foreland was subjected to a tangential pressure with a NNW-388
SSE oriented σ1 ascribable, in our opinion, to the convergence between the African and 389
Euro-Asiatic plates. During the year 2000 the stress field became more unstable and 390
took on a clearly distensive character in 2001 and 2002, at the same time as the violent 391
eruption of Etna; moreover, in these three years the stress field is seen to be less 392
homogeneous. In our opinion the distensive character is the effect, on the overlying 393
crust, of the ascent of the mantle, which mainly affects Etna (Patanè et al., 2006), and, 394
probably with a lesser intensity, the central-northern strip of the Hyblean area, at least 395
during the most active volcanic period. 396
It is inferred that, during eruptive phenomena, mantle bending generates a local stress 397
field which upsets the regional one, associated with plate tectonics, superimposed on the 398
latter and producing a rotation of σ1 from NNW towards NW. Therefore, we conclude 399
that the heterogeneity displayed by the stress field, resulting from the inversion of all 400
the focal mechanisms of earthquakes which occurred in the interval 1994-2002, was 401
determined by causes that in reality act in different periods and whose effects overlap 402
each other in the last three years of this time interval. We ascribe these causes to the 403
convergence of the African and Euro-Asiatic plates and to the rise of the mantle in 404
Eastern Sicily, below the crust, which seems to influence locally the regional stress 405
field. 406
This latter consideration is confirmed by Panza & Raykova (2008) through a synoptic 407
representation of the mechanical properties of the uppermost 60 km of the Earth. They 408
Accepted Manuscript
Eastern Sicily and, in particular, in the Hyblean area, the mantle-crust boundary is found 411
at a depth of about 25 km. Finally, it would not seem coincidental that historical and 412
instrumental earthquakes with ML≥ 4.0, are mainly distributed in a direction oriented 413
NE-SW, which affects the North-western sector of the Hyblean Foreland and the 414
northern extremity of the Hyblean-Maltese Escarpment. This zone is adjacent to Etna 415
(Fig. 2a) and shows a particular sensitivity to the pressures determined by the dynamics 416
of the mantle, highlighted by the eruptive events; moreover, the earthquakes of 1169 417
and 1693, historically considered the most disastrous ever, had epicentres very close to 418
the volcano (Fig. 2a). Given this final consideration, we maintain that the earthquakes in 419
South-eastern Sicily, named unequivocally “tectonic” or rather determined only by the 420
release of tension associated with plate-tectonics, are also generated by local 421
movements of the mantle on which the observed volcanic phenomena depend. These 422
movements, which occur when the tensional state of crust varies, may determine the 423
fragile deformation of the crust and therefore may trigger earthquakes, even major ones. 424
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623 624
Figure CAPTIONS
625
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Structural layout of South-eastern Sicily. The black lines represent the main faults and 628
fractures of the Hyblean Foreland. The broken line represents the South-eastern border 629
of the Gela-Catania Nappe. The areas A, B, C, D are the same as in Fig. 2b. 630
Figure 2: a) Map of the main historical and instrumental earthquakes of greatest 631
magnitude in the Hyblean area (Musumeci et al., 2005 modified): b); epicentral 632
distribution of earthquakes in the Hyblean area in the period 1983-2002, located through 633
the PRR seismic network (square), and INGV (triangle); the areas A, B, C, D show the 634
highest density of epicenters located along the structural trends shows in Fig. 1b. 635
Figure 3: a) Cumulative curve of the seismic energy between 1983 and 2002. For each 636
time interval 1st, 2nd, 3rd, 4th, 5th, the b-value and the Magnitude completeness (Mc) were 637
calculated. b) Distribution of the yearly energy release E*(t), estimated for M> 2.0 638
using time windows of one year, shifted by one month. The histogram shows the 639
discrete distribution, the bold line, and the cumulative one. The dotted line indicates the 640
selected threshold L = 2300, corresponding to 34% of the time interval. c) E*(t) 641
determined for events with M > 2.0 using time windows of one year, shifted by one 642
month. For details see text. 643
Figure 4: Stress field obtained through the inversion of 138 focal mechanisms relative to 644
the period 1994-2002 localised in the Hyblean Foreland. 645
Figure 5: Stress regimes obtained through the inversion of focal mechanisms located in 646
the areas A – H, structurally homogeneous (for details see text). 647
Figure 6: a) Temporal trend of the average value misfits. The horizontal bars show the 648
width of the temporal windows. The different length of the bars indicates that the 649
Accepted Manuscript
substantially different in the two periods 1994-2000 and 2000-2002, that is, higher in 651
the latter. b) Trend of the azimuth of the main stress axes σ1 and σ3; in the period 1994-652
2000; σ1 takes on an orientation NNW-SSE, in the period 2000-2002 it takes on, on 653
average, an orientation NW-SE. c) Trend of the plunge of the main stress axes σ1 and 654
σ3; in the period 1994-1999 the stress field mainly favours mechanisms of faults 655
associated with strike-slip and subordinately with thrust, in 2000 the stress field was 656
variable and in 2001-2002 it favoured normal faulting style. 657
Table 1: Parameters of the earthquakes selected for stress inversion 658
Acknowledgements
659
We thank Prof. G.F. Panza of the University of Trieste (Italy), the anonymous referee 660
and the editor Randell Stephenson for the advice and suggestions that improved our 661
Paper. We also thank Dr. Domenico Patané (INGV) and the “Provincia Regionale di 662
Ragusa” for the data support. This research has been funded by “Fondi d’Ateneo” (ex 663
Accepted Manuscript
R: 0.6 Misfit: 11 σ1: trend: 85; plunge: 80 σ2: trend: 346; plunge: 2 σ3: trend: 256; plunge: 10Faulting style: Normal
Accepted Manuscript
No Date Long (E) Lat(N) Depth Magnitude
DD/MM/YYYY H M Degree Degree Km Strike Dip Rake Strike Dip Rake
1 21/06/1994 0 0 14.567 36.936 6.63 3.60 20 60 -35 129 60 -145 2 23/07/1994 5 56 14.988 37.130 18.68 1.00 80 85 170 171 80 5 3 02/08/1994 22 42 14.693 36.850 19.98 2.00 115 55 -175 22 86 -35 4 08/08/1994 14 10 15.158 36.684 23.22 2.20 175 80 -105 52 18 -34 5 13/09/1994 4 39 15.057 37.162 21.97 1.90 210 60 80 49 31 106 6 19/09/1994 1 34 15.143 37.144 7.93 2.40 30 85 -15 121 75 -175 7 29/09/1994 21 44 14.995 37.130 16.45 1.00 180 55 -5 273 86 -145 8 08/10/1994 20 47 15.292 37.244 18.99 1.60 165 65 -5 257 85 -155 9 22/10/1994 17 30 14.967 36.948 22.16 2.40 140 45 5 46 86 135 10 01/11/1994 12 15 15.261 36.993 20.60 1.00 160 30 -55 301 66 -108 11 03/11/1994 4 24 14.752 37.192 18.38 1.60 100 75 170 193 80 15 12 27/11/1994 11 31 14.843 36.863 26.42 1.60 275 65 95 83 25 79 13 09/03/1995 16 32 15.308 37.182 16.48 3.60 295 60 165 33 77 31 14 20/03/1995 7 36 15.296 37.189 17.73 1.90 175 70 10 82 81 160 15 01/05/1995 19 8 14.778 36.875 22.55 1.40 280 60 150 26 64 34 16 10/06/1995 17 27 14.986 37.048 3.50 0.70 300 70 70 167 28 133 17 10/06/1995 13 49 15.376 36.877 16.05 3.30 15 50 40 257 61 133 18 13/06/1995 21 50 14.773 37.432 17.70 1.70 165 85 20 73 70 175 19 16/08/1995 19 2 15.236 37.036 25.00 2.50 255 90 -115 165 25 0 20 10/12/1995 14 17 15.204 37.329 18.08 2.70 110 55 -180 19 89 -35 21 30/07/1997 23 55 14.760 37.246 22.53 4.60 185 80 -5 276 85 -170 22 16/09/1997 16 6 14.609 37.117 17.33 4.20 210 75 25 113 66 163 23 09/10/1997 6 3 14.869 37.347 16.14 1.80 205 65 55 84 42 141 24 18/10/1997 7 50 15.208 37.093 19.24 0.70 275 60 -180 184 89 -30 25 24/10/1997 21 52 14.808 36.827 23.32 1.70 155 75 -120 41 33 -28 26 02/11/1997 23 53 14.813 37.478 21.83 1.80 95 60 -180 4 89 -30 27 04/11/1997 22 17 14.821 37.114 15.58 1.80 350 80 -45 90 46 -166 28 14/11/1997 23 7 14.805 36.821 21.94 2.10 265 75 -155 168 66 -17 29 06/01/1998 12 10 14.657 37.143 8.48 1.30 25 65 -20 124 72 -154 30 16/07/1998 16 34 15.062 37.050 18.10 1.70 260 90 -165 350 75 -180 31 25/01/1999 3 44 14.693 37.234 14.22 1.50 100 85 -150 7 60 -6 Table1 continued
Time 1° Plane Solution 2° Plane Solution
Date, Origin Time, Magnitude, Hypocentral and Focal Parameters of Earthquakes Selected for Stress Inversion
Accepted Manuscript
No Date Long (E) Lat(N) Depth Magnitude
DD/MM/YYYY H M Degree Degree Km Strike Dip Rake Strike Dip Rake
32 11/04/1999 20 43 15.059 37.087 22.35 1.40 35 65 -40 145 54 -148 33 23/04/1999 18 29 14.805 37.422 19.00 2.80 80 10 10 340 88 100 34 28/04/1999 22 50 14.752 37.129 12.50 2.90 10 90 50 280 40 180 35 05/07/1999 4 36 15.178 37.185 20.14 1.10 280 50 -180 189 89 -40 36 10/12/1999 23 51 14.752 37.254 17.50 2.20 280 80 155 15 65 11 37 22/12/1999 2 44 15.362 36.939 26.14 2.10 260 85 -175 170 85 -5 38 23/12/1999 7 31 14.780 37.111 8.84 1.20 95 90 -135 5 45 0 39 25/12/1999 10 7 15.147 37.084 19.48 1.30 205 40 45 78 63 121 40 01/01/2000 4 6 14.746 37.256 11.00 1.70 155 65 -135 42 50 -33 41 10/01/2000 21 6 14.970 36.963 24.05 2.50 290 50 -170 194 82 -40 42 24/01/2000 13 56 14.743 37.445 20.39 2.50 200 35 -70 356 57 -103 43 12/02/2000 16 38 15.358 37.093 20.95 3.00 130 20 -95 315 70 -88 44 09/03/2000 3 57 15.117 37.045 21.97 1.50 355 60 -5 88 86 -150 45 09/03/2000 0 33 15.134 37.084 12.83 1.10 330 80 -175 239 85 -10 46 19/03/2000 0 8 14.973 37.038 9.08 1.10 95 60 -170 360 81 -30 47 27/03/2000 15 33 15.303 37.103 20.11 1.10 205 75 -50 312 42 -157 48 08/08/2000 22 13 14.833 37.176 15.07 1.60 355 80 -25 90 65 -169 49 09/09/2000 3 31 15.206 37.167 21.98 2.30 285 85 150 18 60 6 50 09/09/2000 0 12 15.202 37.165 21.54 2.20 145 55 -55 274 48 -130 51 12/09/2000 15 32 15.175 37.160 20.86 1.90 100 87 -150 8 60 -4 52 13/09/2000 10 43 15.217 37.024 15.91 1.20 289 80 -108 171 21 -30 53 16/09/2000 8 41 15.012 37.058 12.37 1.10 275 77 -180 185 89 -13 54 21/09/2000 11 39 14.981 37.004 20.01 1.20 252 8 -174 156 89 -82 55 28/09/2000 12 2 15.207 37.370 19.17 2.00 281 80 -107 161 20 -31 56 28/09/2000 2 22 15.062 36.954 19.66 1.10 345 58 127 110 47 46 57 01/10/2000 22 43 14.753 37.147 16.08 1.00 317 8 154 73 87 83 58 03/10/2000 5 30 15.225 37.091 24.75 1.70 187 33 92 5 57 89 59 03/10/2000 20 0 15.310 37.002 21.81 1.20 269 81 -142 172 53 -11 60 08/10/2000 22 37 14.999 37.069 18.40 1.30 298 81 -139 200 50 -12 61 08/10/2000 5 6 14.689 37.205 5.23 1.50 194 21 -19 302 83 -110 62 17/10/2000 0 9 15.352 36.881 18.56 1.60 105 81 178 195 88 9
Time 1° Plane Solution 2° Plane Solution
Accepted Manuscript
No Date Long (E) Lat(N) Depth Magnitude
DD/MM/YYYY H M Degree Degree Km Strike Dip Rake Strike Dip Rake
63 11/11/2000 1 31 14.800 36.989 15.71 1.00 270 80 -176 179 86 -10 64 12/11/2000 8 8 14.975 36.841 30.57 2.30 192 68 138 301 52 29 65 13/11/2000 4 20 15.214 37.092 19.76 1.70 282 70 -141 177 54 -25 66 18/11/2000 16 46 15.293 36.961 17.17 1.50 278 81 -158 184 68 -10 67 19/11/2000 13 31 15.229 37.196 22.86 1.30 291 52 -109 140 42 -68 68 22/11/2000 7 37 15.321 37.127 20.97 1.00 159 33 -168 59 83 -58 69 12/12/2000 14 2 14.668 37.440 17.57 2.40 335 81 142 72 53 11 70 06/01/2001 8 23 14.959 37.141 19.97 1.00 111 64 170 205 81 26 71 10/01/2001 2 1 15.042 37.033 17.57 1.00 205 26 -53 345 70 -106 72 19/01/2001 7 53 15.449 37.101 24.26 2.00 102 60 -159 1 72 -32 73 23/01/2001 16 55 15.173 37.146 21.72 3.40 146 26 -57 290 68 -105 74 25/01/2001 15 34 15.510 37.167 26.02 2.70 119 54 -149 10 65 -40 75 27/01/2001 13 49 15.332 37.193 23.50 1.80 356 44 -108 200 49 -74 76 28/01/2001 21 48 15.026 37.011 20.05 1.50 285 73 -169 192 79 -17 77 04/02/2001 12 11 15.211 36.949 22.08 1.60 86 37 -100 278 54 -83 78 07/02/2001 12 58 14.727 36.844 20.71 1.80 247 86 140 340 50 5 79 10/02/2001 17 46 14.934 37.217 9.33 2.40 90 80 176 181 86 10 80 25/02/2001 17 41 14.845 36.913 20.73 1.50 273 42 -139 150 64 -56 81 19/03/2001 2 1 15.225 37.009 25.18 1.40 40 86 159 132 69 4 82 05/04/2001 0 43 14.928 36.954 8.01 1.30 190 17 -126 47 76 -80 83 16/04/2001 12 27 15.092 37.264 11.28 1.60 274 74 -160 178 71 -17 84 04/05/2001 16 49 15.046 37.167 11.63 1.70 317 63 -128 197 45 -39 85 12/05/2001 9 17 14.971 37.073 5.89 1.50 165 90 -132 75 42 0 86 16/05/2001 15 29 15.263 37.140 22.44 2.80 279 41 -133 150 61 -59 87 25/05/2001 18 28 14.558 37.309 25.26 1.70 123 14 144 248 82 79 88 31/05/2001 16 32 14.525 37.175 22.48 1.60 308 18 -127 166 76 -79 89 16/06/2001 19 48 14.886 37.071 22.92 1.70 58 59 -179 327 89 -31 90 17/06/2001 17 41 14.668 37.034 16.72 1.30 271 45 -110 118 48 -71 91 18/06/2001 5 47 14.987 36.911 15.01 1.40 103 88 139 195 49 3 92 20/06/2001 20 25 15.361 37.016 28.08 1.90 245 83 -104 129 16 -27 93 02/07/2001 4 22 14.614 37.150 15.16 1.00 326 28 -161 219 81 -63 continued Table1 (Continued)
Accepted Manuscript
No Date Long (E) Lat(N) Depth Magnitude
DD/MM/YYYY H M Degree Degree Km Strike Dip Rake Strike Dip Rake
94 10/08/2001 16 42 14.607 36.808 0.65 2.30 39 13 153 155 84 78 95 14/08/2001 11 38 14.725 37.162 6.04 1.30 265 84 160 357 70 6 96 15/08/2001 7 9 15.207 37.467 22.15 1.60 83 60 -168 347 80 -31 97 19/08/2001 21 18 15.322 36.972 24.60 1.30 198 88 -135 106 45 -3 98 27/08/2001 11 7 15.086 37.154 25.01 1.60 112 78 -138 11 49 -16 99 30/08/2001 22 9 15.202 37.421 23.81 2.00 128 41 -66 277 53 -110 100 07/09/2001 15 17 14.577 37.350 21.87 2.00 316 87 161 47 71 3 101 16/10/2001 9 20 15.244 37.305 22.69 1.70 118 60 171 213 82 30 102 05/11/2001 10 17 14.717 37.113 18.02 1.20 302 81 127 44 38 15 103 05/11/2001 0 25 14.705 37.090 17.39 2.60 313 87 -160 222 70 -3 104 03/12/2001 0 25 14.620 37.355 14.05 1.30 167 84 -141 72 51 -8 105 19/12/2001 22 5 14.694 37.206 15.98 1.00 237 42 -119 94 54 -66 106 11/01/2002 3 42 14.602 37.252 24.27 1.40 315 87 122 50 32 6 107 13/01/2002 3 57 14.444 37.089 21.18 1.50 282 65 -140 172 54 -32 108 14/02/2002 10 54 15.106 37.341 6.55 1.90 38 76 -153 301 64 -16 109 17/03/2002 14 42 14.627 37.102 19.18 2.40 42 86 -85 171 6 -141 110 10/04/2002 23 21 14.673 37.411 13.76 1.30 150 84 157 243 67 7 111 11/04/2002 20 43 15.171 37.440 12.85 1.30 172 72 144 275 56 22 112 11/05/2002 3 49 15.892 37.294 30.08 0.00 197 51 -110 47 43 -67 113 14/05/2002 17 59 15.190 37.425 16.80 2.80 264 86 -166 173 76 -4 114 14/05/2002 16 18 15.200 37.421 16.15 2.80 93 83 -167 1 77 -7 115 17/05/2002 16 55 15.200 37.414 15.80 1.50 99 52 -131 334 54 -50 116 18/05/2002 19 37 15.225 37.413 13.16 1.60 137 24 -68 293 68 -100 117 19/05/2002 10 13 15.063 37.253 4.90 1.80 182 44 129 314 57 59 118 21/05/2002 6 43 15.152 37.189 21.67 1.30 74 81 111 186 23 24 119 21/05/2002 1 35 15.245 37.444 20.68 1.50 131 23 -155 18 80 -69 120 24/05/2002 2 14 14.704 37.144 21.38 2.70 135 61 -134 18 51 -39 121 02/06/2002 15 43 15.274 36.975 25.31 1.90 243 75 -119 128 32 -29 122 19/06/2002 6 14 15.129 37.364 16.57 1.80 128 25 -89 307 65 -90 123 05/07/2002 8 52 15.052 37.258 7.10 1.70 349 77 160 84 71 14 124 07/07/2002 6 19 14.737 37.222 16.54 2.00 280 45 -172 184 84 -45 Table1 (Continued)
Accepted Manuscript
No Date Long (E) Lat(N) Depth Magnitude
DD/MM/YYYY H M Degree Degree Km Strike Dip Rake Strike Dip Rake
125 13/07/2002 3 57 15.349 37.223 21.37 2.80 293 51 -139 174 59 -47 126 24/07/2002 0 19 14.986 36.909 17.28 2.00 296 32 -119 149 62 -73 127 25/07/2002 14 4 14.761 36.975 21.16 1.40 263 80 162 356 72 10 128 01/08/2002 18 3 15.095 37.009 19.25 1.30 124 49 -99 318 42 -79 129 27/08/2002 2 44 14.662 37.404 13.69 2.20 346 87 170 77 80 3 130 02/09/2002 6 42 14.491 37.040 25.39 1.70 259 87 128 353 38 5 131 05/09/2002 7 21 14.950 36.841 21.67 2.00 306 22 -106 143 69 -84 132 12/09/2002 23 56 15.202 37.471 21.89 2.00 72 78 172 164 82 12 133 13/09/2002 13 3 15.051 37.109 8.39 1.30 175 65 -82 337 26 -106 134 26/09/2002 21 13 15.139 37.227 17.69 1.00 341 35 178 73 89 55 135 14/10/2002 22 46 14.726 37.127 16.37 1.50 245 61 155 348 68 32 136 16/10/2002 13 17 15.076 37.191 18.56 1.40 298 42 -176 205 87 -48 137 17/11/2002 21 42 14.704 37.019 16.00 2.00 278 74 -161 183 72 -17 138 28/11/2002 0 18 15.547 37.206 22.59 4.20 100 45 -150 348 69 -49 Table1 (Continued)
