Page 4 - 05 albarello
P. 4
Dario Albarello, Enzo Mantovani and Marcello Viti
As concems the present-day tectonic pat- 1995) whicb indicate sbortening between the
tem, stress and strain field data can be obtained lblean block and Apulian foreland. By assum-
by the analysis of earthquake fault plane solu- ing a thickness of the seismic layer of the order
tions, since few and contrasting indications of l 04 m, as indicated by the distribution of
are provided by in-situ stress measurements available bypocentral depths, an average rigid-
(Grasso et al., 1986; Bousquet et al., 1988). ity of the order of 3 · 1010 Pa and a time inter-
Figure 2 sbows the available fault mecbanisms val of 100 years, Kostrov's approacb gives val-
of sballow earthquakes ($;50 km) in the area ues of the order of I0- 16 s-1 and 10-18 s-I (l0-9
bere considered. The distribution of these data
is not bomogeneous, most eartbquakes are lo- w-and 11 yr1 respectively) for seismic strain
cated inland along the Tyrrbenian coast of Ca-
labria and in Sicily. Figure 2 sbows, for eacb rates in Calabria and Sicily respectively. Some
earthquake, the borizontal projection of P and geodynamic bypotheses bave been advanced to
T directions, whicb can be considered represen- explain the complex deformation pattem dis-
tative of the local maximum and minimum cussed above and, in particular, its most strik-
principal strain axis respectively (Marrett and ing feature, i.e. the presence of tensional
Allmendinger, 1990). In Calabria, earthquakes strains in a small sector, the Sicily Cbannel, of
along the Tyrrbenian coast sbow normal mecb- the Africa collisional boundary in the Mediter-
anisms, while dextral strike-slip mecbanisms ranean region. Some authors (Illies, 1981; Bec-
are observed along tbe Catanzaro fault. Only caluva et al., 1983; Ben Avraham and Grasso,
one fault piane solution, indicating a thrust 1990; Reuther et al., 1993) suggested that the
mecbanism, is available along the Calabrian-
Ionian front. Western Sicily sbows coberent Table I. Orientations (strike and dip in degrees) of
thrust mecbanisms both inland and offsbore. maximum and minimum strain and stress axes in
All these indications are compatible with the Calabria and Sicily deduced from focal mechanisms
quatemary deformation pattem sbown in fig. l. (strike direction is reported with respect to north and
Four out of five available fault piane solutions dip is from the horizontal piane). The two estimates
related to sballow seismicity in the Tyrrbenian (a and b) of principal strain axes for each zone have
basin sbow a compressive cbaracter. Assuming been respectively obtained by the procedures pro-
tbat local faulting is controlled by large scale posed by Marrett and Allmendinger (1990) and by
dynamics, the integrated analysis of seismic Kostrov (1974). For the application of this last tech-
sources belonging to a given crustal block can nique an empirica! relationship between scalar mo-
supply information on tbe regional strain and ment tensor and surface wave magnitude (see, e.g.,
stress fields (see, e.g., Jackson and McKenzie, Jackson and McKenzie, 1988) has been adopted in
1988; Wyss et al., 1992). To this purpose, the cases in which a direct estimate of seismic mo-
some standard numerica! techniques (Kostrov, ment is not available. The two estimates (c and d) of
1974; Gepbart and Forsyth, 1984; Marrett principal stress axes for each zone have been re-
and Allmendinger, 1990; Carey-Gahilardis and spectively obtained by the procedures proposed by
Vergely, 1992) bave been applied to earth- Carey-Gailhardis and Vergely (1992) and by
quakes occurred in the two regions sbown in Gephart and Forsyth (1984). As concems this last
fig. 2, whicb bave been tentatively assumed as approach, the contribution of single fault piane solu-
bomogeneous. The results obtained by this tions has been weighted linearly with magnitude.
analysis (table I and fig. 3) indicate tbat both
Sicily and Calabria are affected by a N-S to Sicily Calabria
NNW-SSE compressive field, associated witb
an E-W to WNW-ESE tensional field, in line Strain el e3 el e3
with neotectonic data (Barbano et al., 1978;
Philip, 1983; Lo Giudice and Rasà, 1986; Van a 228/28 115/38 171152 283/16
Dijk and Okkes, 1991) and VLBI measure-
ments ,(Zarraoa et al., 1994; Lanotte et al., b 193/-5 108/42 184/34 92/1
Stress Oi (}3 Ci] (}3
c 207/17
d 359/9 90/38 207/17 248/33
268/8 321180 87/6
836
