Crustal structure and seismicity of southern Tyrrhenian basin

PHYSICS OFTHE EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and PlanetaryInteriors 103 (1997) 117-133

Crustal structure and seismicity of southern Tyrrhenian basin G. De Luca a,c,., L. Filippi a, D. Caccamo b, G. Neff b, R. Scarpa

a

a Dipartimento di Fisica, Universith dell'Aquila, 67010-Coppito (L'Aquila), Italy b Dipartimento di Fisica della Materia e Tecnologie Fisiche At,anzate, Unicersith di Messina, 98100-Messina, Italy Laboratori Nazionali del Gran Sasso (LNGS-INFN), 67010-Assergi (L'Aquila), Italy

Received 17 June 1996; accepted 21 April 1997

Abstract In this paper all the travel-time data from crustal earthquakes that occurred in the period 1978-1991 along the southern Tyrrhenian sea have been re-examined. In particular, 410 local earthquakes recorded at a minimum number of seven seismic stations and 52 shots fired on sea have been utilized to infer a tomographic image of the velocity model beneath a region located in the domain 37°41'-39°10'N and 14° 10'-16°44'E. These earthquakes are selected from a database containing about 1000 local earthquakes. The results from the inversion of about 9000 travel-times confirm the strong lateral variations of the velocity model relative to compressional and shear waves. Below the volcanic center of the Aeolian Islands, in the upper crust, several low velocity bodies are found, in agreement with features inferred from seismicity and other geophysical data. The 3-D velocity images also show a thickening of the crust from the center of the Tyrrhenian sea towards the coasts of Sicily and Calabria. Finally hypocenter locations derived from inversion, which appear to be significantly less scattered than using 1-D velocity models, help to identify with some accuracy two notable fault systems of the southern Tyrrhenian basin. © 1997 Elsevier Science B.V. Keywords: Earthquakes; Crustal structure; Tyrrheniansea; Seismicity;Volcanoes

1. Introduction The southern Tyrrhenian basin is one of most interesting regions of the Mediterranean sea for its geodynamic activity. The presence of a Benioff zone led many investigators to hypothesize a variety of models to explain the tectonics of the region. Barberi et al. (1973), on the basis of some geophysical and volcanological evidences, assumed the existence of

* Corresponding author.

an active subduction process and, in this context, the southern Tyrrhenian sea was assumed to be a back arc basin. Gasparini et al. (1982, 1985) and more recently Patacca et al. (1990) have interpreted this Benioff zone as a remnant of a subducted lithosphere. Anderson and Jackson (1987) and Mantovani et al. (1985, 1990) modeled the tectonics of the Tyrrhenian sea as controlled by the rotation of the Adriatic microplate around a pole located in the Western Alps, in the framework of the NE compression by the African plate. Other models have been recently proposed, mainly on the basis of geological, volcanological and structural data by Wezel (1982),

0031-9201/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0031-9201 (97)00026-5

G. De Luca et al. / Physics ~/the Earth and Planetary Interiors 103 (1997) I 17 133

118

Cristofolini et al. (1985), Finetti and Del Ben (1986), Malinverno and Ryan (1986), Rehault et al. (1987), Locardi (1988), Mascle et al. (1988), Savelli (1988), Boccaletti et al. (1990), Lavecchia and Stoppa (1990), Van Dijk and Hokkes (1991), Barberi et al. (1994). The shallow seismic activity is rather moderate and recent earthquakes had magnitudes not exceeding 6 (Del Pezzo et al., 1984). The area features a volcanic islands arc, the Aeolian islands, characterized by two active volcanoes: Vulcano and Stromboll. Close to this region, inland, the Calabrian arc experienced two earthquakes in this century having magnitudes up to 7 (Martini and Scarpa, 1985).

In this paper the shallow seismic activity occurred in the period 1978-1991 has been reanalyzed in order to relocate hypocenters and compute the crustal 3 - D velocity model, with a simultaneous inversion procedure. The need for incorporating the effect of lateral inhomogeneities in the earthquake location problem is quite obvious in this region, since the thickness of the crust and of the lithosphere change strongly from 10 km (30 km for the lithosphere) in the center of Tyrrhenian sea to 4 0 - 5 0 (150 for the lithosphere) beneath Sicily and Calabria. These structural lateral variations are certainly expected to influence the estimates of earthquake source parameters

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but, up to date, their effect has only partially been taken into account in the literature. We attempt here to estimate these effects by using the whole set of local earthquakes recorded in recent years and data from shots (1972, 1984 and 1986) fired in the sea for DSS experiments (Morelli et al., 1975; Milano et al., 1989).

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G. De Luca et al. / Physics ~?['the Earth and Planeta(v Interiors 103 (1997) I 17 133

for a 3-D velocity structure (e.g. Kissling, 1988). The inhomogeneous spatial distribution of the sources-receivers and the non linearity of this prob-

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Application of travel-time inversion methods in regions of complex geology such as in volcanic areas are reported in relatively few areas around the world (see f.i. Lees and Crosson, 1989; Dawson et al.,

1990; Aster et al., 1992; Iyer, 1992; Lees, 1992; Romero et al., 1993; Rowan and Clayton, 1993; Benz et al., 1996). In addition most of these studies are restricted to imaging the P-wave velocity struc-

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G. De Luca et al. / Physics o['the Earth and Planetary lmeriors 103 (1997) l 17 133

ture only, due to the lack of S-wave readings for the limited diffusion of seismic networks equipped with three component seismographs. The data set available for the southern Tyrrhenian

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G. De Luca et al. / Physics of the Earth and Planetary Interiors 103 (1997) 117-133

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structure. All the first arrival times of earthquake Pand S-waves recorded in the period 1978-1991 at the seismic stations operating along the southern Tyrrhenian sea and arrival times of P-waves from shots have been used for the present work. A map of the seismic stations and the epicentral locations available is given in Fig. 1. Epicenter maps like this are based on the use of the HYP071 program (Lee and Valdes, 1985) and a 1-D velocity model (Fig. 2-Mod. A) extracted from an initial inversion started from a model obtained with a trial-and-error procedure (Neff et al., 1996). The use of I-D velocity models in this region may introduce a bias in the earthquake location, due to the presence of the above mentioned lateral variations of crustal parameters inferred from DSS experiments (Morelli et al., 1975; Boccaletti et al., 1990). The inclusion of station residuals in the location procedure is insufficient to take completely into account for this effect. We have considered all the database provided by the local seismic networks operating in the region in order to infer a 3-D velocity model. A data reduction procedure has been applied, using a fairly rigid selection criterion based on the quality of earthquake

locations. In particular we considered only events with a number of recording stations > 7, RMS < 1.0 s and location errors lower than 10 km. The 410 events reported in Fig. 1 are selected with this criterion from a sample containing more than 1000 earthquakes. We incorporated also about 400 P-wave readings from shots fired in the sea in 1972, 1984 and in 1986 (stars in Fig. 1), attributing to these data a weight higher by a factor 4 as compared to the P arrival times from earthquakes. We started the inversion of available travel-time data by using a set of 1-D velocity models proposed in the literature for parts of the same region. This analysis has been performed by using some general criteria for having as accurate as possible initial models which strongly affect the linearized inversion procedure (Kissling et al., 1994). We decided to test several models (Fig. 2), resulting from the inversion of part of the total travel time data set, by using the Crosson (1976) method, the VELEST algorithm (Kissling, 1988) or trial-and-error such as some models that appeared in the existing literature (Neff et al., 1996). To estimate the 3-D velocity models about 6000 (plus 400 from shots) P-wave and 2400 S-wave

126

G. De Luca et al. / Physics of the Earth and Planetary Interiors 103 (1997) 117-133

artificial explosions for a 3 - D distribution of velocity nodes, by using a linearized approach. The ray tracing is based on an approximate computation with respect to a real 3 - D model, carried out averaging on the adjacent nodes seen by the ray along its path.

readings have been used. The procedure used to invert the travel-times is that developed by Thurber (1983, 1993) with the modifications from Eberhardt Phillips (1993). This method allows to invert separately P and S travel-times of earthquakes a n d / o r

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G. De Luca et al. / Physics of the Earth and Planeta~" Interiors 103 (1997) 117-133

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The inversion is performed through the LevenbergMarquardt algorithm and the matricial decomposition proposed by Pavlis and Booker (1980). The advantage of this method with respect to other recent techniques for seismic tomography is that it allows for the computation of exact covariance and resolution matrix. The disadvantage is a remarkable time consumption for large matrix, other than the approximation due to the linearization and the not exact computation of travel-times. The quantity of data available and the estimated model resolution fits well to apply this procedure, with reasonable computer times. We used a Workstation HP-720 for all the calculations. Several inversion runs were performed for models of increasing complexity up to a set of final models having a variable number of nodes in the range 480-624, and by using about 9000 P- and S-wave arrival times from earthquakes and explosions. The main parameters of these inverted models are summarized in Tables ! and 2. In practice the initial 1-D models are characterized by 7 layers, fixing the velocities in the first layer and in the lower two layers. The difference between the model illustrated in Fig. 2 (Mod. A) and in the remaining models

(from Mod. B to Mod. G) takes into account the uncertainties existing in the definition of this model as compared to those existing in the literature (see Neff et al., 1996). A Vp/~ ratio in the range 1.731.78 has also been used, which is close to the average value 1.75 deduced from the Wadati diagram constructed for the best located events (Fig. 3). The network geometry and the approximate linear ray paths, giving a rough estimate of the expected resolution, are illustrated in Fig. 4. It is clear from these figures that velocity properties can be resolved only for cells having size of the order of 20 km or more, due to ray coverage, down to 40 km depth. The expected model resolution, for this reason, is higher only along the Aeolian Islands and the Calabria and Sicily coasts. By using the parametrization illustrated in Fig. 2 (Mod. G), we derived the velocity models for P-waves (Fig. 5), S-waves (Fig. 6) and Vp/V~ ratio (Fig. 7). In these models only data having diagonal resolution elements higher than 0.4 for P-waves and 0.2 for S-waves are represented. The inversion parameters for these models are those for Mod. G-01 in Tables 1 and 2. It is noteworthy to say that the introduction of the velocity models resulting from the simultaneous inversion leads on

128

G. De Luca et al. / Physics q[' the Earth and Planetary Interiors 103 (1997) 117-133

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average to a variance improvement of travel-time residuals amounting to about 60% and for the model represented in Figs. 5 - 7 is larger than 80% (initial unweighed R M S 3.82 s and 0.62 s after inversion). To test the influence of station spacing and geometry on the final results and in general the robust-

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ness of models it has been inverted the available set of travel-times generated with initial hypocenters, based on a velocity model consisting of layers of alternated P-wave velocities bodies. These bodies were characterized by velocity contrasts + 2 0 % smoothed with nearby nodes (this model is illus-

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trated by Fig. 8a for layer 2 at depth of 10 km) and the average velocity for each layer has been derived from the model represented in Fig. 5. The inverted velocity model illustrated in Fig. 8b (of course for layer 2 at 10 km of depth, but this is similar for all depths) shows that the main velocity contrasts are well resolved with the data set utilized, particularly in regions where we expect the highest concentration of rays. This checkboard test is useful to illustrate that the results obtained from the inversion do not depend too much, at least inside the area covered by seismic stations, by the particular geometry hypocenters-stations at disposal.

3. Discussion

The introduction of a 3-D velocity model in the locations of the available seismic events helps to significantly reduce the variance of travel-time residuals, the decrease of which is, on the average, of the order of 60% (or 80% not including Mod. C-01,

Mod. D-01 and Mod. F-01). This gives a measure of the improvement in the earthquake location accuracy and earth structure knowledge. The velocity models illustrated in Figs. 5 - 7 provide a relevant information concerning the strong velocity contrasts in the area. A quite interesting result is the evidence of low velocity bodies in the upper crust along the Aeolian Islands and the submarine volcanoes of this arc. This feature is common to all models inverted, no matter about thickness and size of the blocks, and matches well with the results obtained by Del Pezzo et al. (1979) in their S-wave attenuation study and by Barberi et al. (1994) with gravimetric and magnetic modelling. Again, close to the layer at 25 km depth, high velocity bodies appear in the southern Tyrrhenian sea and along the Messina Straits, which is in agreement to the Moho uprise toward the center of Tyrrhenian sea deduced from DSS experiments (Morelli et al., 1975; Milano et al., 1989). The velocity models resulting from the use of S-waves are less accurate than for P-waves due both

G. De Luca et al. / Physics of the Earth and Planetary Interiors 103 (1997) 117-133

to the number of S arrival times, which is lower by a factor 2 as compared to that of P arrivals, and to their accuracy. It is noteworthy that P-wave velocity patterns reported in Fig. 5 for layers 1 to 4 allow for a fairly accurate definition of two distinct structural domains (the southern Tyrrhenian and the CalabroSicilian) and of their contact front. This is an additional constraint for the kinematic modelling of the local lithosphere, in agreement with the hypothesis that the southern Tyrrhenian lithosphere is southeast(a)

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ward migrating and sliding with respect to the separate block of Sicily (Finetti and Del Ben, 1986; Rehault et al., 1987; Neri et al., 1996). Unfortunately resolution decreases abruptly in the lower layer, at 45 km depth, and this prevents us from any interpretation concerning the deeper structures. The relocated hypocenters (Figs. 9 and 10) appear less scattered as compared to those estimated without the introduction of a 3-D velocity model (Fig. 1). This is especially evident in the area between the southernmost Aeolian Islands and the northern coast of Sicily where several authors (Finetti and Del Ben, 1986; Patacca et al., 1990, among others) locate with some approximation the transition and dislocation processes between the southern Tyrrhenian and the Sicilian domains. The epicenter distribution in this area may be an indicator of such dislocation processes. The occurrence of earthquakes is basically confined to the upper layers, between 5 and 20 km depth; a few events are located below 20 km, up to 45 km depth.

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The southern Tyrrhenian basin is characterized by the presence of shallow and deep seismicity. In the present paper we focused on the problem of estimating accurately shallow hypocenters through the simultaneous determination of a 3-D velocity model. Such a model reduced significantly (80%) the location errors obtained by a 1-D model and we attribute the unexplained errors as mostly due to the network geometry, which is still inadequate because of the presence of the sea and the lack of ocean bottom seismographs. In spite of these limitations the results obtained from 3-D inversion appear quite stable and confirm the pronounced velocity heterogeneities deduced from other geophysical data. Low velocity bodies are found at shallow depths, close to active volcanic centers of the Aeolian Islands. The P-wave velocities in the lower crust-upper mantle show patterns similar to those evidenced by the DSS experiments (Milano et al., 1989; Boccaletti et al., 1990) and clearly define the main features of crustal thickening from the Tyrrhenian basin to the Calabrian arc.

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G. De Luca et al. / Physics of the Earth and Planetao' Interiors 103 (1997) 117-133

B y u s i n g the i n v e r t e d v e l o c i t y m o d e l the h y p o c e n ters a p p e a r m o s t l y d i s t r i b u t e d in the d e p t h r a n g e 5 - 2 0 km, with a quite r e d u c e d n u m b e r o f e v e n t s o c c u r r i n g at larger depth. E p i c e n t e r l o c a t i o n s c o n firm w i t h a m a j o r p r e c i s i o n t h a n in p r e v i o u s investig a t i o n s (Neri et al., 1996) that r e c e n t s e i s m i c i t y o f A e o l i a n I s l a n d s is m a i n l y d i s t r i b u t e d a l o n g t w o fault s y s t e m s n a m e d Sisifo a n d V u l c a n o , s u p p o r t i n g the h y p o t h e s i s o f d i s l o c a t i o n o f A e o l i a n I s l a n d area in a S E d i r e c t i o n with r e s p e c t to the m a i n b l o c k o f Sicily (Finetti a n d D e l B e n , 1986; R e h a u l t et al., 1987). T h e p r e s e n t results are in a g r e e m e n t with this m o d e l a n d c o n t r i b u t e to its r e f i n i n g b y f u r t h e r e v i d e n c i n g the d e g r e e o f f r a g m e n t a t i o n a n d h e t e r o g e n e i t y o f d i f f e r e n t sectors o f the region.

Acknowledgements R e s e a r c h c a r r i e d o u t w i t h the f i n a n c i a l s u p p o r t o f the N a t i o n a l G r o u p o f V o l c a n o l o g y , Italy, a n d M i n istry o f Scientific R e s e a r c h ( M U R S T 4 0 % ) .

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