GNGTS – Atti del 19° Convegno Nazionale / 12.09
L. Scarfì (1), H. Langer (1), G. Di Grazia (1), A. Ursino (1) and S. Gresta (1,2)
(1)
(2)
Sistema Poseidon, Sezione di Priolo, C.da Cava Sorciaro, Priolo-Gargallo (SR)
Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia, Catania
ANALYSIS OF TWO SWARMS OF MICROEARTHQUAKES
IN SOUTHEASTERN SICILY: EVIDENCE FOR ACTIVE FAULTS?
Abstract. Two microearthquake swarms, occurring in Southeastern Sicily during November 1999 and
January 2000, were analyzed with respect to their seismotectonic features. Given the low magnitude
of the events fault plane solutions were computed for only four major events, which revealed normal
fault and inverse fault mechanisms. From the comparison of waveforms and the relation of P- and Swave peak amplitudes, two families of multiplets were identified, each one representing a distinct type
of seismic dislocation. Composite fault plane solutions for the two families showed the same trends as
for the single major events. The small number of mismatches of the composite solutions supports the
hypothesis of two distinct types of seismic dislocation during the whole sequence, i.e., a normal fault
mechanism along E-W striking planes and an inverse fault mechanism along NE-SW striking planes.
ANALISI DI DUE SCIAMI DI MICROTERREMOTI NELLA SICILIA SUD-ORIENTALE: EVIDENZA DI
FAGLIE ATTIVE?
Riassunto. Sono state analizzate le caratteristiche sismotettoniche di due sciami di microterremoti
avvenuti nella Sicilia sud-orientale nel novembre 1999 e nel gennaio 2000. Data la bassa magnitudo
degli eventi si è potuto calcolare il meccanismo focale solo dei 4 eventi maggiori, che mostrano sia
meccanismi normali che inversi. Il confronto fra le forme d'onda e la relazione fra le ampiezze delle
onde P ed S hanno evidenziato due famiglie riconducibili a diversi tipi di dislocazione. I meccanismi
focali composti per le due famiglie mostrano le stesse evidenze delle scosse singole, confermando
l'ipotesi di due tipi di dislocazione durante l'intera sequenza: una faglia normale lungo piani orientati EW e una faglia inversa lungo piani diretti NE-SW.
INTRODUCTION
In the early 1990’s a new digital seismic network has been deployed in
Southeastern Sicily in the framework of the “Poseidon Project” for the monitoring of
the whole Eastern Sicily (Ursino et al., 2000).
In this study we present the analyses carried out on two seismic swarms whose
hypocenters are situated some km to north of the border between the Gela-Catania
foredeep and the nappes of the Northern Chain (Fig. 1).
A first swarm of 16 events was recorded on November 19th and 20th, 1999. A
second one (37 events) occurred on January 1 and 2, 2000. Isolated earthquakes
followed on January 3, 4 and 24. Local magnitudes are quite small, i. e. between 0.8
and 2.5.
In the area, significant local seismic activity in history is represented by the
earthquakes of October 3rd, 1624 (Me = 5-6) and December 23, 1959 (MS = 4.5-5),
with epicenters reported both at distances of ca. 10-15 km (see Azzaro and Barbano,
2000) from the earthquakes analized here.
GNGTS – Atti del 19° Convegno Nazionale / 12.09
ANALYSIS OF DATA
Location
The distribution of the epicenters follows grossly a N-S striking trend with a
largest horizontal extension of about 7 km and a width of ca. 3 km (Fig. 2). The
epicenters of the first swarm are concentrated in the southern and central part of the
area, while focal depths range from 14 to 16 km. The epicenters of the second
swarm follow a N-S striking direction and appear slightly shifted to the west with
respect to those of the former swarm. The focal depths scatter in a range between 14
and 24 km. On the other hand the hypocenters of the two events on January 24 th
belong to the cluster of the first swarm (Figs. 2 and 3).
Mt Etna
Epicentral area
ep
ede
For
ent
rpm
sca
ta E
Mal
he
ft
to
n
ro
dF
rie
u
B
e
pp
Na
a
l
Ge
ep
de
re
o
F
Siracusa
Scic
li Lin
e
Hyblean Foreland
Fig. 1 - Simplified geologic map of SE Sicily (based on Carbone et al., 1987; Grasso and Reuther,
1988; Lentini et al., 1996). 1 = Recent-Quaternary deposits; 2 = late Pleistocene-Holocene Etnean
volcanics; 3 = Plio-Pleistocene Hyblean volcanics; 4 = Meso-Cainozoic carbonate sediments; 5 =
normal faults (strike-slip components shown by arrows); 6 = thrusts. Horizontal bars indicate the
compressive domain of Appennine-Maghrebian chain. (modified from Azzaro and Barbano, 2000).
Fault – plane solutions
Fault plane solutions have been computed for four major events of the swarms.
The results of the fault plane solutions are shown in Fig. 4. Both events on November
19 and January 24 reveal normal faulting mechanisms along E-W striking planes,
with an almost vertical P-axis. On the other hand, the earthquakes on January 1 and
2nd show a thrust mechanism along NE-SW striking planes, with an almost NW
striking P-axis. As stated earlier the hypocenters of the earthquakes on January 24
belong to the cluster of the first swarm on November 19 and 20, whereas the events
of January 1 and 2 should be assigned to the second cluster. Given the limited
number of polarities and the western network gap, one can certainly argue the
GNGTS – Atti del 19° Convegno Nazionale / 12.09
reliability of the fault plane solutions. Nevertheless, it is noteworthy that the faultplane solutions carried out for the representative events of the corresponding clusters
converge fairly well.
Successively, two composite fault plane solutions have been obtained by
combining the P- onset polarities of the two families defined on the basis of waveform
analysis and P/S-wave amplitude ratios (see next paragraph). The composite
solutions fit well the solutions found for the single major events.
L E G E N D
Etna Volcano
N o v. 1 9 9 9
1 -4 th J a n . 0 0
e
G
2 4 th J a n . 0 0
l ta
Esc
o
th
N
Ionian Sea
pe
Ma
Fr
of
nt
a
el
ap
a rp
H y b le a n P la te a u
me
nt
0
5 Km
Fig. 2 - Epicentral map of the data set
Latitude
37.412
37.442
37.472
37.502
37.532
10.00
Focal depth (km)
12.00
14.688
14.728
14.768
10.00
S
N
12.00
14.00
14.00
16.00
16.00
18.00
18.00
20.00
Nov 99
1-4th Jan 00
22.00
W
E
20.00
Nov 99
1-4th Jan 00
22.00
24th Jan 00
24th Jan 00
24.00
a
Longitude
14.808
14 Km
24.00
14 Km
b
Fig. 3 - Vertical cross sections of the events. (a) N-S section; (b) E-W section. Different symbols
correspond to various periods of earthquake occurrence (see the legend).
Waveforms and amplitude ratios
In order to verify our working hypothesis that the two clusters may indeed
represent two different types of seismic dislocation we have been comparing
waveforms and amplitude ratios of P- and S-waves of the clusters.
A crude visual inspection reveals surprising similarities of waveforms for the
events belonging to the same cluster (Fig. 5), despite significant magnitude
GNGTS – Atti del 19° Convegno Nazionale / 12.09
differences and the scatter of hypocentral coordinates. As a simple measure we
calculated the matrix of maximum cross-correlation coefficients for the events. We
considered the vertical component seismograms of three key stations, (i.e., SR1,
SR2, and SR3), and calculated the cross-correlation functions for 8 s windows
starting 1.0 s before the P-wave onset. The correlation matrix is graphically
visualised in Fig. 6 for station SR1. Similar outlines were obtained for matrices of
station SR2 and SR3. The graph confirms the hypothesis of two distinct groups since
high correlation coefficients are observed only among records which belong to the
same families.
ID 36
ID 57
ID Number
5
36
51
57
LEGEND
Nov 99
th
1-4 Jan 00
Date
99/11/19
00/01/01
00/01/02
00/01/24
Magnitude
2.2
1.9
2
2.5
Origin Time
05:22:43.34
07:13:01.69
09:24:40.03
16:38:08.03
b
th
24 Jan 00
Composite Solution
Composite Solution
Family I
Family II
ID 5
ID 51
0
5 Km
C
a
Fig. 4 - a) Epicentral map and focal mechanism (lower hemisphere projection) (+ for compression, O
for dilatation). b) date and relative magnitude of the events, c) Composite fault plane solution of the
two earthquakes families.
1.0E-05
1.0E-05
5.0E-06
5.0E-06
0.0E+00
0.0E+00
-5.0E-06
-5.0E-06
-1.0E-05
ID51
ID16
-1.5E-05
-1.0E-05
0
1
2
3
4
5
6
7
8
1.5E-06
0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
Time (s)
8
6.0E-06
9.0E-07
3.0E-06
3.0E-07
0.0E+00
-3.0E-07
-3.0E-06
-9.0E-07
ID22
ID7
-6.0E-06
-1.5E-06
0
1
2
3
4
5
6
7
8
0
1.8E-05
5.0E-07
9.0E-06
0.0E+00
0.0E+00
V
1.0E-06
-9.0E-06
-5.0E-07
ID8
ID34
-1.8E-05
-1.0E-06
0
1
2
3
4
5
6
7
Time (s)
8
0
Fig. 5 - A comparison between events recorded at SR2 station (vertical component). On the right,
three earthquakes of the first cluster, on the left, events of the second cluster. Note the high waveform
similarity between events of the same cluster.
GNGTS – Atti del 19° Convegno Nazionale / 12.09
Waveforms and in particular the peak amplitudes of P- and S-waves depend
strongly on the focal mechanism. Here we have considered the peak amplitudes of
P-waves measured on the vertical component and the maximum horizontal S-wave
amplitudes. The latter ones were obtained forming the average of the peak
amplitudes on the two horizontal components. We considered again the key stations
SR1, SR2 and SR3. At SR1 the P/S-wave ratios for events of the swarm in
November 1999 tend to be higher than those of the latter swarm. At the stations SR2
and SR3, the P/S amplitude wave ratios of the former swarm are clearly lower than
those of the latter swarm. As before we noted that the events of January 24 show the
same features as the cluster of November 1999.
Area Family II
Area Family I
Crosscorrelation Values
ID
Eve
nts
Crosscorrelation
values
ID
0.80-1.00
0.60-0.80
0.40-0.60
0.20-0.40
0.00-0.20
Eve
n
ts
-0.20-0.00
-0.40--0.20
Fig. 6 - Graphical visualization of the correlation matrix obtained for station SR1.
CONCLUSION
Eastern Sicily is a region characterised by a strong regional stress due to the
convergent motion of the European and African plates (Richardson et al., 1979;
Müller et al., 1992; Ward, 1994). It is one of the most seismically active of Italy. It was
struck several times by violent seismic events (Azzaro and Barbano, 2000, Boschi et
al., 1995, Boschi et al., 1997), spaced out by long periods of quiescence, at least
apparently because there are not data of a local seismic network until 1994.
Indeed the destructive earthquakes have not yet been related to well defined
active faults (Bianca et al., 1999; Sirovich and Pettenati, 1999), and only after the last
moderate earthquake occurred in 1990, the first instrumental recordings for this
seismogenic area have been available.
In this work we have investigated a seismic sequence occurred some
kilometres northward from the border the Gela-Catania foredeep, below the thrust
zone (Ragg et al., 1999). The data set is composed of microearthquakes with a
rather irregular distribution in energy. Local magnitudes did not exceed 2.5.
Essentially two swarms occurred: the former in November 1999 and the latter in
January 2000. Hypocenters were concentrated in a relatively small crustal volume.
Nevertheless, two distinct clusters have been recognized. The comparison of the
GNGTS – Atti del 19° Convegno Nazionale / 12.09
waveforms recorded at the three key stations revealed a high degree of similarity
between earthquakes belonging to the same swarm. The cross-correlation
coefficients showed clearly that the two swarms form two distinct families made up by
multiplets. Moving on from the similarities of the waveforms of events belonging to
the same group we conclude that the two families indeed represent two different
types of fault mechanism. This conclusion is confirmed by the comparison of the P/Swave amplitude ratios and composite fault plane solutions. The two mechanisms are
given (i) as a normal fault and (ii) an inverse fault.
REFERENCES
Azzaro R. and Barbano M. S.; 2000: Analysis of the seismicity of Southeastern Sicily: a proposed
tectonic interpretation. Annali di Geofisica, 43 (1), 171-188.
Bianca M., Monaco C., Tortorici L. and Cernobori L.; 1999: Quaternary normal faulting in southeastern
Sicily (Italy): a seismic source for the 1693 large earthquake. Geophys. J. Int., 139, 370-394.
Boschi E., Guidoboni E., and Mariotti D.; 1995: Seismic effects of the strongest historical earthquakes
in the Syracuse area. Annali di Geofisica, 38 (2), 223-253.
Boschi E., Guidoboni E., Ferrari G., Valensise G., & Gasperini P. (Eds.); 1997: Catalogo dei Forti
Terremoti in Italia dal 461 a.c. al 1990. Istituto Nazionale di Geofisica, S.G.A, Bologna, pp. 644.
Carbone S., Grasso M. and Lentini F.; 1987: Lineamenti geologici del plateau Ibleo (Sicilia SE).
Presentazione delle carte geologiche della Sicilia Sud-Orientale. Mem. Soc. Geol. Ital., 38, 127135.
Grasso M. and Reuther C.D.; 1988: The western margin of Hyblean plateau: a neotectonic trasform
system on the SE Sicilian foreland. Ann. Tectonicae, II (2), 107-120.
Lentini F., Carbone S., Catalano S. and Grasso M.; 1996: Elementi per la ricostruzione del quadro
strutturale della Sicilia Orientale. Mem. Soc. Geol. Ital., 38,127-135.
Müller B., L. Zoback M., Fuchs K., Mastin L., Gregersen S., Pavoni N., Stephansson O. and Ljunggren
C.; 1992: Regional patterns of tectonic stress in Europe. Geophys. Res., 97, 11783-11803.
Ragg S., Grasso M., Müller B.; 1999: Patterns of tectonic stress in Sicily from borehole breakout
observations and finite element modeling. Tectonics, 18 (4), 669-685.
Richardson R. M., Solomon S. C. and Sleep N. H.; 1979: Tectonic stress in the plates. Rev. Geophys.,
17, 981-1019.
Sirovich L., and Pettenati F.; 1999: Seismotectonic outline of South-Eastern Sicily: an evaluation of
available options for the earthquake fault rupture scenario. J. Seism., 3, 213-233.
Ursino A., Langer H., Scarfì L., Di Grazia G., Gresta S.; 2000: Discrimination of quarry blasts from
tectonic earthquakes in the Iblean Platform (Southeastern Sicily). Atti del 19° Convegno del
Gruppo Nazionale di Geofisica della Terra Solida, Consiglio Nazionale Delle Ricerche, Roma.
Ward S. N.; 1994: Constraints on the seismotectonics of the central Mediterranean from very long
baseline. Geophys. J. Int., 117, 441-452.