Seismic potential of Southern Italy

Tectonophysics 415 (2006) 81 – 101 www.elsevier.com/locate/tecto

Seismic potential of Southern Italy S. Jenny a,1 , S. Goes a,⁎, D. Giardini a , H.-G. Kahle b b

a Institute of Geophysics, ETH Zurich, Switzerland Geodesy and Geodynamics Lab., Institute of Geodesy and Photogrammetry, ETH Zurich, Switzerland

Received 22 June 2004; received in revised form 22 November 2005; accepted 20 December 2005 Available online 9 February 2006

Abstract To improve estimates of the long-term average seismic potential of the slowly straining South Central Mediterranean plate boundary zone, we integrate constraints on tectonic style and deformation rates from geodetic and geologic data with the traditional constraints from seismicity catalogs. We express seismic potential (long-term average earthquake recurrence rates as a function of magnitude) in the form of truncated Gutenberg–Richter distributions for seven seismotectonic source zones. Seismic coupling seems to be large or even complete in most zones. An exception is the southern Tyrrhenian thrust zone, where most of the African– European convergence is accommodated. Here aseismic deformation is estimated to range from at least 25% along the western part to almost 100% aseismic slip around the Aeolian Islands. Even so, seismic potential of this zone has previously been significantly underestimated, due to the low levels of recorded past seismicity. By contrast, the series of 19 M6–7 earthquakes that hit Calabria in the 18th and 19th century released tectonic strain rates accumulated over time spans up to several times the catalog duration, and seismic potential is revised downward. The southern Tyrrhenian thrust zone and the extensional Calabrian faults, as well as the northeastern Sicilian transtensional zone between them (which includes the Messina Straits, where a destructive M7 event occurred in 1908), all have a similar seismic potential with minimum recurrence times of M ≥ 6.5 of 150–220 years. This potential is lower than that of the Southern Apennines (M ≥ 6.5 recurring every 60 to 140 years), but higher than that of southeastern Sicily (minimum M ≥ 6.5 recurrence times of 400 years). The high seismicity levels recorded in southeastern Sicily indicate some clustering and are most compatible with a tectonic scenario where the Ionian deforms internally, and motions at the Calabrian Trench are small. The estimated seismic potential for the Calabrian Trench and Central and Western Sicily are the lowest (minimum M ≥ 6.5 recurrence times of 550–800 years). Most zones are probably capable of generating earthquakes up to magnitudes 7–7.5, with the exception of Central and Western Sicily where maximum events sizes most likely do not exceed 7. © 2006 Elsevier B.V. All rights reserved. Keywords: Seismic hazard; Seismicity; Geodesy; Strain rates

1. Introduction

⁎ Corresponding author. Currently at Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ, U.K. Tel.: +44 2075946434; fax: +44 2075947444. E-mail address: [email protected] (S. Goes). 1 Currently at: PartnerRe Zurich Branch, Switzerland. 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.12.003

Seismic hazard assessment in slowly straining regions is a challenge. The South-Central Mediterranean (Fig. 1) deforms at rates of only a few mm/yr (Hollenstein et al., 2003; Serpelloni et al., 2005). Yet, it has a history of earthquakes exceeding magnitude 7. The slow deformation rates mean that even Italy's long catalog, which spans over 2000 years in some areas, is too short for a reliable

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Fig. 1. Topographic map of the study region with the main tectonic and geographic elements marked. The lower-left inset shows the location of the study region (white outline), and the approximate present-day Mediterranean plate boundaries, in bold black. Abbreviations: Pel.M — Peloritani Mountains, TL — Tindari–Letojanni fault zone, Ty — Tyrrhenian, Ae — Aegean.

characterization of large-event recurrence times. This hampers determination of long-term seismic potential, the first step in hazard assessment. Seismic potential is the long-term average frequency of earthquake occurrence as a function of magnitude. Seismic hazard at any location is the integrated effect of all potential nearby earthquakes, weighted by their frequency and magnitude, and convolved with the subsurface-dependent potential for shaking. Traditionally, seismic potential is estimated from seismicity catalogs. Using this approach, the Southern Apennines – where a long record of large events exists –, but also Calabria, the Messina Straits and southeastern Sicily have been identified as regions of high seismic hazard (e. g., Slejko et al., 1998, 1999). Also, the region around the 1968 Belice earthquake in western Sicily is often characterized as an area with an elevated seismic potential. The integral of the seismic potential, i.e., the sum of all earthquake moments according to their recurrence frequency, is the seismic moment rate. Seismic moment

rate is proportional to seismic strain rate, thus allowing a comparison with other types of strain rate data. It has been shown that seismicity catalogs that are much shorter than the largest-event recurrence times generally under represent long-term average seismic moment rates (Ward, 1998; Jenny et al., 2004), even if they are complete. This would imply that Southern Italy's catalog most likely provides a lower bound for longterm average seismic strain rates, and for seismic potential. There is however a small chance that the catalog spans a high-activity seismic period in certain regions. In this case, the short-term seismicity rates documented by the catalog exceed the long-term average rates, and can even exceed the tectonic loading rates. In this study, we add the information that geodetic and geologic data provide on tectonic strain rates to improve seismic potential estimates for Southern Italy. Long-term average seismic strain rates cannot exceed tectonic loading rates, although they can be significantly lower, if part of the deformation occurs as aseismic

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creep. Besides placing an upper bound on the integrated seismic potential, these constraints allow recognition of catalog seismicity rates that are unrepresentatively high because they exceed tectonic strain rates. Some previous work has compared seismic moment rates and geodetically measured deformation rates to estimate the amount of aseismic deformation in Italy. Seismic moment estimates by Westaway (1992) (from macroseismic effects of historic earthquakes) are comparable to geodetically measured strain rates in the Apennines (Hunstad et al., 2003) indicating that deformation there is fully seismic. However, Pondrelli (1999), using regional moment tensors, found that seismicity accounts for only about 30% of the total deformation, inferred from VLBI data, along the Apennines, and for about 30% in Calabria and 10% within Sicily. Only in the Sicily Channel do her estimated ratios reach a higher value (79%). Ward (1998) obtained similarly low percentages in his comparison of seismic and VLBI strain rates. He attributes these low ratios to the short catalogue length compared to the long seismic cycle in this slowly straining region. The rate comparison of Boschi et al. (1995a) yields very low 20-year probability of an M ≥ 6 crustal seismic event in most of Italy, except in northern and southeastern Sicily, where their probabilities reach 65% along the northeastern coast. These previous studies are significantly hampered by the sparse distribution of geodetic data. Recently collected GPS data (Hollenstein et al., 2003; Serpelloni et al., 2005) provide a much more detailed strain rate field for Southern Italy, and led to a modified interpretation of the current tectonics (Goes et al., 2004; Pondrelli et al., 2004). The new geodetic strain rate distribution differs substantially from the distribution of strain inferred from seismicity only. For example, Calabria, which was hit by five M ≥ 6 events in the year 1783 alone, does not show up as a high tectonic deformation rate area. By contrast, in the southern Tyrrhenian, between the north coast of Sicily and Ustica and the Aeolian Islands, the geodetic data document high contractional strain rates. Yet, no historic events have been recorded in this zone, only a number of recent moderate-size earthquakes. Combining the recent GPS information with seismicity data gives us the possibility of improving the estimate of seismic potential of this region. 2. Approach We determine the seismic potential for seven subregions of Southern Italy (Fig. 2). The size of the zones is somewhat larger than generally used for source zones in

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Fig. 2. The seven large-scale seismic source zones for which seismic potential is determined. Each zone comprises a seismotectonic regime.

seismic hazard studies. However, this is the smallest size for which the seismic characteristics can be reasonably constrained from the available geodetic coverage and number of events in the earthquake catalog. Each zone covers a seismotectonic regime, as discussed in Section 4. The region around Etna, which is dominated by volcanic seismicity, is excluded from our analysis. Seismic potential will be expressed in terms of the truncated Gutenberg–Richter magnitude–frequency distribution often used in hazard analyses. The truncated Gutenberg–Richter (TGR) distribution has the form: logN ðM Þ ¼ a−bðM −M0 Þ;

for M VMmax :

ð1Þ

Here N is the number of events of a certain magnitude M per year, the a-value is the recurrence rate of small events with M ≥ reference magnitude M0, the b-value is the slope of the curve, which is usually around 1 (Kanamori and Anderson, 1975; Kagan, 2002a,b), and Mmax is the maximum possible event magnitude. The integral under this curve gives the total seismic moment rate M˙0seis. Although other distributions may better describe magnitude–frequency data in detail (Kagan, 2002a; Koravos et al., 2003), the simpler TGR distribution can often not be rejected by the available data, and the large uncertainties in the Southern Italian seismicity data do not warrant fitting any more complex functions. Tectonic moment rates, M˙0tect, the upper bound for seis M˙0 , are estimated from the strain rates documented by the GPS data of Hollenstein et al. (2003) (Section 6). A good agreement between geodetic strain rates and tectonic deformation rates inferred from geology has been previously demonstrated in several global and

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regional studies (for example, Argus and Heflin, 1995; DeMets et al., 1994; Ward, 1990; Armijo et al., 1999; Straub et al., 1997; Westaway, 1994). Geodetically measured velocities represent the effects of off-fault elastic loading (which may result in on-fault earthquakes) plus permanent deformation (seismic and aseismic, mostly in fault zones; in our case mainly aseismic, as no significant earthquakes occurred during data collection). Only with dense networks can the offand on-fault deformation be distinguished, and our station distribution is insufficient to separate the two. However the comparison with seismic strain rates provides some constraints on the proportion of seismic versus aseismic slip. We use both continuous mapping and projection on faults to address the incomplete constraints on strain localization. Because slip rates are easier to relate to, we will express the deformation rates in slip rates, as if all motion with a zone is accommodated along a single fault. Bear in mind though that for most regions the deformation is distributed over a number of faults. The seismicity catalogs provide (1) estimates of shortterm seismic moment rates M˙0cat, (2) information on the

seismic style of deformation that can be compared with the geodetic styles, and (3) constraints on the a- and bvalues and Mmax. We merge historical and instrumental catalogs to obtain the longest possible record. Magnitude–frequency distributions are determined for each source zone, taking into account uncertainties in completeness length, in magnitude, and in location of offshore events (Section 6). Integrating these distributions gives a catalog moment rate estimate that can be translated into an equivalent slip rate. The seismic strain style is inferred from focal mechanisms (Section 6). It has been shown in other regions (Amelung and King, 1997; Kreemer et al., 2000; Jenny et al., 2004) that seismic deformation estimated from smaller events provides styles in good agreement with tectonic deformation styles, indicating that the thus obtained strain is representative of long-term seismic deformation style (but not deformation magnitude). Assuming earthquake recurrence times follow a Poissonian distribution (Kagan and Jackson, 1991; Goes, 1996), the catalogs should be long enough to provide a reliable estimate of long-term recurrence statistics for events smaller than about 5, i.e., on a-value. Because of the

Fig. 3. GPS velocities with uncertainties (for some stations too small to see) from Hollenstein et al. (2003) relative to an Eurasian reference frame (Sella et al., 2002). Campaign station positions are marked with white triangles, permanent stations with white circles. Shown for comparison: velocities at two Calabrian stations (TGRC and CATA marked with squares) from Serpelloni et al. (2005), in their Eurasian reference frame. These velocities are not used in our strain rate mapping.

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relative infrequent occurrence of M N 5 events, the Italian catalogs cannot constrain source zone b-value well. The standard value of 1 gives an acceptable representation for most zones. Finally, the long-term average seismic moment rate is constrained (Section 7) by the requirements that (1) it is the integral over a TGR distribution with catalogcompatible a- and b-values, and an Mmax which does not grossly exceed the observed maximum earthquake, and (2) that it is lower than the geodetically constrained loading rates. A similar analysis of seismicity and strain rates in the Eastern Mediterranean showed that it can improve estimates of long-term seismic moment rates (Jenny et al., 2004). 3. Data 3.1. GPS data The tectonic strain rate field is determined from the recently collected GPS data of Hollenstein et al. (2003)

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(Fig. 3). While previous strain rate estimates were mainly constrained by motions of the permanent stations at Matera (MATE), Lampedusa (LAMP and Noto (NOTO), the data set of Hollenstein et al. (2003) provided, for the first time, strain-rate estimates for all the main tectonic elements of Southern Italy. The GPS data were collected during four campaigns between 1994 and 2001 at 16 stations in Southern Italy, Sicily, on the Aeolian Islands and in the Sicily Channel and merged with data from 8 permanent stations. The velocity computation combined these data with data from a large continuous network consisting of up to 54 European IGS and EUREF as well as Greek sites. The reference frame for the whole evaluation was ITRF97 (Boucher et al., 1998) and velocities were rotated into a European reference frame using the pole of Sella et al. (2002). The data processing is described in more detail in Hollenstein et al. (2003). Formal statistical errors were scaled, by 21.25 and 42.5 for campaign and permanent stations, respectively, to obtain the more realistic margin of error shown in Fig. 3 (Hollenstein et

Fig. 4. The distribution of southern Italian seismicity at shallow depths (b40 km). The largest events are shown in white, and the significant events discussed in the text in light grey with bold outlines and marked with their date and estimated moment magnitude. For clusters in Calabria, only the largest events are labelled. The seismicity data are a compilation from the historical CPTI (217 BC–1981) (Working Group CPTI, 1999) and the instrumental CSTI (1981–1996) (Instrumental Catalog Working Group CSTI, 2001). Events are plotted at their catalog location.

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al., 2003). Although the Southern Italian region moves at rates of only a few mm to 1 cm/yr relative to stable Europe, most velocities exceed their estimated uncertainties, except at station SPEC, which was remonumented during the observation period. The reliability of the velocities is further corroborated by two more recent geodetic studies in this area (D'Agostino and Selvaggi, 2004; Serpelloni et al., 2005). No sizable earthquakes occurred near any of the stations in the period of observation, and the volcanic activity at Etna was too distant to affect the stations (Goes et al., 2004). This was confirmed by the station time series and trajectories that showed no sign of shortterm fluctuations that could be due to such earthquake or volcanic movements (Hollenstein et al., 2003), thus increasing our confidence that the motions represent tectonic deformation. 3.2. Seismicity To obtain estimates of catalog moment rates and constraints on TGR parameters, we combined the historical CPTI (Parametric catalogue of Italian earthquakes, http://emidius.mi.ingv.it/CPTI/, Working Group CPTI (1999)) for the time 217 BC–1981 with the instrumental CSTI (Instrumental catalogue of Italian earthquakes, Instrumental Catalog Working Group

CSTI (2001)) for the period 1981–1996. We convert the apparent magnitude Ma of these catalogues into moment M0 (in N m), using the following equation (Working Group CPTI, 1999): log10 M0 ¼ 22:9−0:47  Ma þ 0:14  Ma2 :

ð2Þ

All magnitudes given in this paper correspond to moment magnitude Mw (Hanks and Kanamori, 1979). The distribution of seismicity is shown in Fig. 4, with the main events labeled. Events are plotted at their catalog locations, even though some of these are subject to debate (as discussed later). The focal mechanisms used to constrain the style of seismic strain are shown in Fig. 5. Moment tensor solutions are taken from the Harvard Centroid Moment Tensor (CMT) catalogue (http://www.seismology. harvard.edu, Dziewonski et al., 1981) between January 1977 and September 2003 for events with Mw N 5.5, supplemented by mechanisms for smaller events (4.1 b Mw b 5.2) from the European–Mediterranean Regional Centroid Moment Tensor (RCMT) catalogue (Pondrelli et al., 2002) and moment tensors from the Swiss Seismological Survey (Braunmiller et al., 2002, http://www.seismo.ethz.ch/info/mt.html) and 173 focal mechanisms compiled by Frepoli and Amato (2000) for small earthquakes (2.5 b Mw b 4.4) in Southern Italy and Sicily.

Fig. 5. Southern Italian focal mechanisms for events since 1977 at depths b40 km, compiled from http://www.seismology.harvard.edu — Dziewonski et al. (1981), Pondrelli et al. (2002), Braunmiller et al. (2002), and Frepoli and Amato (2000).

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4. Seismotectonics and source zones Central and Western Mediterranean tectonics over the past 30–35 Myr have been controlled by retreating subduction inside the Africa–European convergence zone (Dewey et al., 1989; Faccenna et al., 2001). In the last 8 m.y., this slab roll-back led to the opening of the Tyrrhenian Sea between the Corsica–Sardinia block and the Italian peninsula. During this retreat, the Calabro–Peloritani fragments detached from the Corsica–Sardinia block and migrated to their present position where they docked 1–0.5 Ma, to become the toe of Italy and the eastern tip of Sicily (Fig. 1). It is often assumed that rollback and the accompanying back-arc extension continue until today. However, recent GPS observations (Hollenstein et al., 2003) in combination with neotectonic data show that a tectonic reorganization must have occurred 1–0.5 Ma (Goes et al., 2004; Pondrelli et al., 2004). Around 0.5 Ma, the compression between Africa and Europe was transferred from within Sicily to its northern margin, and Tyrrhenian back-arc extension as well as roll-back of the Calabrian trench appears to have all but ceased. A complex deformation zone now links the Sicilian back-thrust with the Calabrian part of the plate boundary, and further readjustments may still be occurring. This change in plate motions is most likely the response to the progressing collision with the African margins of Sicily and Apulia and the docking of Calabria–Peloritani (Fig. 6) (Goes et al., 2004).

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Based on this new tectonic interpretation, the following source zones are defined (Fig. 2): SZ 1 — Southern Tyrrhenian back thrust zone. The African motion of Trapani (TRAP) (Fig. 3) implies localization of most of the convergence between Africa and Europe north of Sicily (Hollenstein et al., 2003; Goes et al., 2004; Pondrelli et al., 2004; Serpelloni et al., 2005), probably along the zone marked by a string of at least nine recent M5+ north– south thrust events (Pondrelli et al., 2002, 2004) (Fig. 5). The shallowly southward dipping nodal planes are the most likely fault planes, compatible with thrusting of the oceanic Tyrrhenian lithosphere beneath the Sicilian foreland of the African continent (Goes et al., 2004). No historical earthquakes have been attributed to this zone. It is quite possible, however, that some or all of the events in 1823, 1726 and 1940 (with assigned magnitudes of 5.9, 5.6 and 5.4 Working Group CPTI (1999)), which for hazard analyses have been located along the northern coast (Fig. 4), were actually larger events that occurred offshore, along this back thrust. The intensity data base (DOM4.1, http://emidius.mi.ingv.it/DOM/, Monachesi and Stucchi, 1997) for the islands north of Sicily, where the shaking of such offshore events should have been strongest, contains no records before 1892. But also, no on-land faults have been recognized for any of these events. And most notably, the elongated isoseismals of the 1823 event that span a large region of the Sicilian north

Fig. 6. (top-left) The plate boundary configuration that existed up to 0.8 Ma (left) and two alternative interpretations for the present-day plate boundaries (after Goes et al. (2004) (center and right)). The main plate boundaries are marked with bold lines, other faults with thin lines. Bold white arrows indicate the main direction of motion of the plates and blocks relative to stable Europe, where the dot in the present-day Tyrrhenian denotes the region's fixity w.r.t. Europe. Geodetic data constrain the current motion of most of Sicily with stable Africa, the northward motion of Calabria and the counterclockwise rotation of Adria. These motions require the existence of two additional deformation zones, in the Strait of Otranto and the Tyrrhenian offshore of Calabria, which do not have any well-defined expression and may be very diffuse. There are no observations in the Ionian basin, which leaves open whether it moves with Africa (T1), leading to continued although much slowed down convergence along the Calabrian trench, or the region is internally deforming (T2), with motion changing from African in the southwest to Calabria/Adria-like in the northeast.

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coast (Monachesi and Stucchi, 1997) requiring two onshore sources, support the interpretation of one larger offshore event. SZ 2 — Northeastern Sicilian transfer zone. This dextral transtensional zone (Goes et al., 2004) (Fig. 6) connects the Tyrrhenian back thrust with the Calabrian part of the plate boundary. It comprises most of the Aeolian island arc and a complex set of onshore faults. Thrusting mechanisms west of the Aeolian island Salina show that the offshore backthrust continues up to here. East of Salina, the large velocities (for this area) define increased contraction and a clockwise rotation with respect to Africa (Fig. 3, Hollenstein et al., 2003; Serpelloni et al., 2005). This deformation has not been accompanied by any sizable earthquakes (Fig. 4), and hence appears to contribute little to the seismic potential. Onshore earthquakes display a range of mechanisms from dextral to normal (Frepoli and Amato, 2000) (Fig. 5). The strike-slip mechanisms match the orientations of the right-lateral faults of the Tindari–Letojanni system (Ghisetti and Vezzani, 1982). Releasing steps in an offshore extension of this system have been proposed to control the position of Salina, Lipari, and Vulcano, on a line perpendicular to the Aeolian subduction arc (Gioncada et al., 2003; De Rosa et al., 2003; Ventura et al., 1999). A strongly seismically active set of approximately north–south trending extensional faults run through the Messina Straits (Tortorici et al., 1995; Monaco and Tortorici, 2000; Monaco et al., 1997; Stewart et al., 1997). This system is responsible for the destructive M7.2 event in 1908 (Fig. 4). Based on paleoseismic evidence and focal mechanisms, it has been suggested that this system extends at least as far south as off the coast of Mt. Etna (Guidi et al., 2003; Catalano et al., 2003; Argnani et al., 2002; Pondrelli et al., 2004). SZ 3 — Central and Western Sicily. Until about 0.5 Ma, the Sicilian nappes accommodated much of the Africa–Europe convergence. Only minor amounts of deformation are occurring today (Tortorici et al., 2001; Speranza et al., 1999; Lickorish et al., 1999; Goes et al., 2004; Pondrelli et al., 2004). But, the 1968 Belice earthquake sequence (consisting of a main shock M = 6.1, and six other M5+ events) and an older M6.5 event in 361 are evidence of some continued north–south contraction (Monaco et al., 1996). SZ 4 — Southeastern Sicily. Earthquakes in 122 BC, 363, 1169 and 1169 with likely magnitudes around 7 severely damaged most of eastern Sicily and have been associated with tsunamis that were recorded

along the entire Ionian coast of the island (Piatanesi and Tinti, 1998). Both onshore thrust faults and offshore extensional faults have been proposed as responsible fault structures (Mulargia et al., 1985; Boschi et al., 1995b; Postpischl, 1985; Piatanesi and Tinti, 1998; Bianca et al., 1999; Argnani and Bonazzi, 2003). However, given the lack of candidate onshore causative faults, the inconsistency of the isoseismals of the historic events with buried thrust faults of the proposed east–west orientation, and the recorded tsunamis, an (incipient) offshore continuation of transtensional zone 2 appears the most likely candidate for the main seismogenic structure in this zone. Slip along such a structure is controlled by motions in the Ionian, which are not well known. We test two scenarios, discussed under SZ 6. SZ 5 — Internal Calabria. Calabria has been struck by some of the most catastrophic earthquakes in the Mediterranean region, including 19 M6 to 7 events between 1638 and 1908 (Galli and Bosi, 2003). Most events occurred in clusters, spanning several months to years. The most notable series consisted of five events within the single year of 1783; the largest events reaching magnitudes of about 7. In southern Calabria, all major earthquakes have been related to primary NE–SW trending normal faults (Tortorici et al., 1995; Jacques et al., 2001; Galli and Bosi, 2003), consistent with the sparse focal mechanisms (Fig. 4, Pondrelli et al., 2004). Causative faults for the three M6+ earthquakes in eastern Calabria are unidentified (Galli and Bosi, 2003). Since 1908, Calabria has been seismically quiet. Geological estimates of slip rates along the Calabrian normal faults are 0.5–2 mm/yr (Tortorici et al., 1995; Monaco and Tortorici, 2000). Similar rates have been found geodetically (differential motion between PORO, TGRC and CATA, Serpelloni et al. (2005), see Fig. 3). The Calabrian normal faults have often been interpreted as a continuous system with that in the Messina Straits (Monaco and Tortorici, 2000; Meletti et al., 2000), but it seems likely that the normal faulting is a regional response to the almost stalling of slab retreat, rather than constituting a section of the main Africa–Europe plate boundary (Goes et al., 2004; Pondrelli et al., 2004). The stations PORO and TGRC document motions towards Europe (Hollenstein et al., 2003; D'Agostino and Selvaggi, 2004; Serpelloni et al., 2005), allowing no more than minor amounts of back extension in the Southern Tyrrhenian. These motions plus the extension inside Calabria put current trench retreat velocities at less than 2–3 mm/yr.

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SZ 6 — Calabrian Trench. Only minor seismicity has been documented along the Calabrian Trench, and there are no historical earthquakes of any significant magnitude, even with assigned near-shore locations, that are likely to have been trench events. Because of the absence of kinematic constraints within the Ionian we will test two end-member scenarios for motion along the trench: T1 — the Ionian moves with Africa and the trench accommodates Africa–Europe convergence, while only minor slip occurs along approximately north– south trending offshore faults in SZ 4, T2 — the trench is almost inactive and the Ionian is internally deforming, accommodating the change in motion from African (within and south of most of Sicily) to Calabrian, in part along offshore structures in SZ 4. SZ 7 — Southern Apennines. The Southern Apenines are included to provide a comparison with other studies, as it is the best-studied Italian seismotectonic region. This zone accommodates the differential motion between Adria and the Tyrrhenian, which moves with Europe (Fig. 6). Our geodetic data are compatible with the more detailed extension estimates of Hunstad et al. (2003), Serpelloni et al. (2005). The Southern Apennines is the most prominent seismogenic structure in Italy, responsible for significant numbers of past and present extensional events, reaching magnitudes of about 7 (M = 6.9 in 1456 and 1857, M = 6.8 in 1694 and 1980). 5. Tectonic deformation rates 5.1. Deformation modeling methods Although the geodetic data of Hollenstein et al. (2003) significantly improved the spatial resolution of strain, they cannot fully define the localization of deformation. Therefore, we use two different strain rate mapping methods: (1) into a continuous strain rate field, interpolated between stations, (2) into a field where almost all slip is confined to faults embedded in an elastic medium. The continuous strain rate field is derived using the method of Haines and Holt (Haines and Holt, 1993; Holt and Haines, 1995; Haines et al., 1998; Beavan and Haines, 2001), which minimizes the difference between a self-consistent horizontal velocity gradient field, on a curvilinear grid with variable knot point spacing, and strain observations, that can include geodetic velocities, plate motions, seismic moment

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tensors and fault slip rates. The uneven spacing of the GPS stations leads to very smoothed strain rates in areas with few stations and concentrated strain rates only where the distribution of stations is dense (Fig. 7). During the inversion, the continuous strain rate field was smoothed over one neighboring grid cell. In scenario T1 (shown in Fig. 7), the Ionian Basin is prescribed to move rigidly with the rest of Africa, according to the African pole of Sella et al. (2002). In T2, the Ionian Basin moves in the same direction as Calabria, as defined by the GPS velocity at the station of PORO (and Reggio Calabria). Uncertainties in the continuous strain rates are of the order of 20 nanostrain/yr (10− 9/yr). The second method, developed by Lundgren et al. (1995, 1998), computes crustal motions in an elastic spherical shell cut by faults, using a finite element approach. Calculated motions are the result of prescribed nodal displacements and/or relative displacements across faults. Motion along the faults can be unconstrained, prescribed only in style, or also in magnitude. The faults are frictionless, and overlaps or gaps are allowed in response to perpendicular motions. To estimate the slip expected on major fault structures in the region as a result of the geodetically measured velocities, we prescribe GPS motions (as displacements) at the station locations and plate motions at the model boundaries. Motion along all other faults in the area is unconstrained and purely a response to the boundary conditions. The model domain is the same as was used for studying the eastern Mediterranean (Lundgren et al., 1998). It spans the entire Mediterranean and extends from the Azores to Iran, in order to include the stable interiors of the main plates. In the central Mediterranean, we updated the grid and major fault structures (Fig. 8) to match current information, and to allow us to impose the geodetic velocities close enough to the actual station positions to capture the characteristics of the deformation field. Africa–Europe convergence is prescribed using the poles from Sella et al. (2002). Material properties in the central and western Mediterranean are constant (Young's modulus of 70 GPa, Poisson's ration of 0.25). Changing these properties laterally did not affect results in our region of interest. We use this elastic model to test how the geodetically measured motions might be accommodated along main faults by running cases with various combinations of the representative velocities (i.e., we do not try to capture all the details of the Aeolian island deformation). We show the results of two tests where only the fault structures with resolvable motions have been included, and the Ionian is prescribed to either move with Africa or with Calabria

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Fig. 7. Continuous strain rate field derived from the GPS velocities in Fig. 3 using the method of Haines and Holt (1993). Shown are directions of minimum (white arrows) and maximum (black arrows) horizontal contractional strain in each cell of the model grid, for Ionian motions according to scenario T1 (Fig. 6). Strains for a model where the Ionian is constrained to move as in T2 are almost indistinguishable and not shown. White circles and triangles mark the positions of the used permanent and campaign GPS stations, respectively. The African velocity arrow is on the same scale as velocities in Fig. 3.

(Fig. 8). Uncertainties in the fault slip rates from the finite element model, due to uncertainties in the geodetic constraints, are about 1 mm/yr. 5.2. Tectonic strain and slip rates We discuss the geodetic strain rate field per source zone. Equivalent tectonic slip rate estimates are summarized in Fig. 9. SZ 1. West of Salina, our geodetic velocities imply a zone of 30–60 nanostrain/yr in the southern Tyrrhenian between the northern coast of Sicily (station TRAP) and Ustica (USTI) and Stromboli (STRM). When all attributed to a single fault, equivalent slip rates are 3–4 mm/yr. For our seismic potential analysis we expand this range to 2–4 mm/yr to include the somewhat lower estimates of Serpelloni et al. (2005). Together with faults further south inside Sicily and the Sicily Channel, slip rates amount to full Africa–Europe convergence. The rest of the Tyrrhenian Sea is a low strain rate area (b10– 20 nanostrain/yr). Strain rates along the southern margin are at the higher end of this range, due to the small velocities of the stations USTI, PORO and STRM relative to Europe.

SZ 2. The strain rate field is dominated by contraction around the Aeolian Islands with strain rates reaching 120 nanostrain/yr (Fig. 7). If localized along one thrust fault this implies slip rates of up to 7–8 mm/yr (Fig. 8). These motions themselves appear to be mainly aseismic. However, because this contraction exceeds the African motions of the other Sicilian stations, extension between northeastern Sicily and Noto is required. If distributed, this extension is on the order of 50 nanostrain/yr. When localized along the faults in the elastic plate model, the Aeolian velocities can add 1–2 mm/yr of dextral to extensional motion along a fault oriented like the Tindari–Letojanni system and rotate the extension along a fault oriented like the Messina Straits. Even without the large Aeolian velocities, 2 ± 1 mm/yr of extension occurs in the Messina Straits due to the jump in the convergent boundary from south of Calabria to north of Sicily. Such estimates are compatible with geological estimates of uplift rates in the Messina Straits of 0.9–2.6 mm/yr (Bordoni and Valensise, 1998; Westaway, 1993; Catalano and De Guidi, 2003; Valensise and Pantosti, 1992), attributed to

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Fig. 8. Fault-localized slip rates computed with an elastic finite-element model (Lundgren et al., 1995, 1998) (grid shown as thin dotted lines), driven by representative GPS observations (in gray with black outlines) and African plate motion (from Sella et al. (2002)). The resulting finite element displacements with uncertainties are shown by the black arrows, the fault slips by the double white (hanging wall) and grey (foot wall) arrowheads with numbers that show the slip rates in mm/yr. The model covers the whole Mediterranean, but only the part covering our region of interest is shown. The rest is unchanged from Lundgren et al. (1998). Two representative models are shown: (a) with Ionian motions according to scenario T1, (b) with Ionian motions according to scenario T2 prescribed by adding two arrows with PORO motions south of the Calabrian trench.

SZ 3.

this extension, and with the rates of Serpelloni et al. (2005). We take the total equivalent tectonic slip rates for the Tindari plus Messina fault systems to be 2–3 mm/yr. The geodetic data imply some low strain rate intraplate deformation (about 20 nanostrain/ yr) south of the Kabilo–Calabrian front. When included in the elastic model, faults within the Sicilian nappes and faults in the Sicily Channel together accommodate slip

rates of up to 1–3 mm/yr. The continuous strain rate field shows that there is resolvable deformation in Sicily's Channel (see also Hollenstein et al., 2003; Serpelloni et al., 2005). Between Malta–Gozo (MLTA, GOZO) and Lampedusa (LAMP) there is a small amount of extension, consistent with NW–SE striking extensional faults within the Channel (Boccaletti et al., 1987; Argnani, 1990), and a small amount of contraction is

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Fig. 9. A comparison of slip rates estimated from the geodetic data (Dtect), the seismic catalogues (Dcat) and our best long-term seismic estimates (Dseis) for the seven source zones in Fig. 2. The uncertainty ranges are shaded.

observed between southeastern Sicily and Malta–Gozo. (Strain around Pantelleria (PANT) is unreliable because of deflation of its caldera (Bonaccorso and Mattia, 2000).) Given this and the geologic information on the cessation of most thrusting inside the Gela Nappes (Tortorici et al., 2001; Speranza et al., 1999; Lickorish et al., 1999), we attribute 0.5–1 mm/yr to Sicily's internal strain. The remaining 0.5–2 mm/yr is probably accommodated in the Sicily Channel (for which the seismicity catalogs cannot constrain rates). SZ 4. A lack of stations south of the trench leads to poor geodetic constraints on the localization of strain in southeastern Sicily and south of Calabria. In the continuous strain rate field, the two end-member cases, T1 and T2, are hardly distinguishable because of the smoothing that occurs in areas of low data distribution. Thus, only case T1 is shown in Fig. 7. In the elastic model without explicitly prescribed internal deformation (T1), the difference in motion between rigid Africa and Noto results in 0–1 mm/yr of left-lateral motion along faults that parallel the southeastern Sicilian coast. Case T2 puts 1–2 mm/ yr of extension between Calabria and Noto on faults of this orientation (Fig. 8). In addition, there could be some 0.5–1 mm/yr of thrusting along faults bounding the Hyblean block to the northwest. SZ 5. In the continuous strain rate model, Calabria is dominated by contraction due to the converging Aegean and African plates. The differential motion between the Apulian stations MATE and SPEC and the Calabrian stations requires some almost east–west extension, possibly accommodated in the Gulf of Taranto and/or along eastern Calabrian faults. The data from Serpelloni et al. (2005) document an additional 10–30 nano-

strain of northwest–southeast extension in Calabria, not represented in our model without CATA. This extension is consistent with the main Calabrian fault motions inferred from geological, geomorphological and structural evidence (Tortorici et al., 1995; Monaco and Tortorici, 2000). The elastic shell model cannot reproduce the Calabrian extension from motions of the surrounding blocks. However, if Calabrian extension or the motion observed in CATA are imposed, the local extension requires compensation by additional compression of a similar amount along the trench. We will use the geologic– geodetic estimates of 0.6–2.0 mm/yr extension for our seismic potential analysis. SZ 6. D˙tect along the Calabrian trench covers the range of almost no motion to substantial loading of a locked trench, depending on the tectonic scenario used. In the elastic model with faults (Fig. 8), scenario T1 gives 2–4 mm/yr of convergence along the sections of the trench that are oriented perpendicular to the African motion. Along the north–south to northwest–southeast oriented eastern end of the trench 2.5–3.5 mm/yr of strike-slip and extension is projected (motion between Calabria and Adria). We use a range of 2.5– 3.5 mm/yr. The internal deformation scenario T2 results in a more north–south directed trench convergence of 0.5–1 mm/yr, depending on how much of the area south of the trench is constrained to move with Calabria. SZ 7. The relative motion between the Apulian and the Sicilian and Tyrrhenian stations documented by the geodetic data of Hollenstein et al. (2003) and Serpelloni et al. (2005) is consistent with the Apenninic triangulation strain rates of up to 100 ± 30 × 10 − 9 /yr (Hunstad et al., 2003), or a slip rate of 3–4 mm/yr in the southern Apennines. The

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westward convergence of the Aegean stations and NE motion of MATE (and SPEC) imply contraction of up to 40 nanostrain/yr in the Strait of Otranto, outside of our source zones. 6. Seismic deformation rates according to the catalog 6.1. Magnitude–frequency distribution and uncertainties To obtain seismic slip rate estimates, we analyze the merged historic–instrumental catalog (Working Group CPTI, 1999; Instrumental Catalog Working Group CSTI, 2001). Uncertainties in the magnitude–frequency distribution are determined from uncertainties in catalog completeness, magnitude and location. The data does not allow unequivocal assessment of completeness, especially for the large magnitudes. Therefore, we define a range of completeness intervals for each magnitude window (Table 1), using the method of Mulargia et al. (1987). The estimated uncertainties include those associated with regional variability in completeness. Magnitude uncertainties are estimated to be ± 0.2 for the time period 1981–1996, ± 0.25 for 1911–1981, ± 0.35 for 1500–1911 and ± 0.5 for pre1500 events. The largest uncertainties in the southern Italian catalogue moment rate M˙0cat are due to the uncertainties in earthquake location for earthquakes close to the shore. Due to the lack of constraints offshore, earthquakes have usually been assigned an onshore location. For northern Sicily and its offshore area and southeastern Calabria and its offshore region, we determine several alternative M˙0cat, based on different assumptions of whether certain events were on- or offshore. When we shift ambiguous epicenters of events offshore, the magnitude of these events is increased using published intensity–attenuation relationships for Italy. The intensity distribution of historical earthquakes is taken from Table 1 Completeness intervals Date (year)

Mw range

≥0 ± 500 ≥1000 ± 200 ≥1475 ± 125 ≥1625 ± 100 ≥1780 ± 80 ≥1870 ± 40 ≥1885 ± 20 ≥1983 ± 5

≥7.5 ≥7.0 ≥6.5 ≥6.0 ≥5.5 ≥5.0 ≥4.5 ≥3.5

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the intensity data base of damaging earthquakes in the Italian area (DOM4.1, http://emidius.mi.ingv.it/DOM/, Monachesi and Stucchi, 1997). Gasperini (2001) computed the average intensity attenuation relation for all of Italy as: I0 −I ¼ 0:52 þ 0:56  minðD; 45 kmÞ þ 0:0217  maxð0; D−45 kmÞ

ð3Þ

where I0 is the epicentral intensity, I is the intensity at site, D is the hypocentral distance in km (for a source depth fixed at 10 km), and min and max are functions that return the minimum and maximum of their arguments. Carletti and Gasperini (2003) accounted for lateral variability in attenuation. The offshore regions seem the most attenuating, but are too poorly sampled to allow attenuation characterization. By extrapolating their attenuation properties of the shorelines, we get: I0 −I ¼ 0:445 þ 0:530  minðD; 45Þ þ 0:0176  maxð0; D−45Þ

ð4Þ

For events located about 40 km offshore (the approximate offshore distance of the Southern Tyrrhenian back-thrust), the observed intensities along the coast are 2.5–3 lower than the actual epicentral intensity. The moment magnitude for such an event when erroneously located along the coast would then be underestimated by about 1.1–1.3 units (according to the M–intensity relations of http://emidius.mi.ingv.it/ DOM/, Monachesi and Stucchi, 1997). If the macroseismic magnitude is computed using the extent of felt areas (Gasperini and Ferrari, 2000), the underestimate of magnitude is somewhat less. Based on these considerations, we estimate the upward magnitude correction for relocating an event about 40 km off the coast to be 0.6–1.2 magnitude units. A range of M˙0cat is calculated from the historical catalogue taking into account all uncertainties (Table 1). Magnitude and completeness length are randomly chosen within the uncertainty range to generate 10,000 simulated catalogs. The resulting distributions of M˙0cat are close to Gaussian. The listed uncertainty bounds for M˙0cat (Table 2) encompass 2 / 3 of the distributions (i. e., approximately one standard deviation). For SZ 1, 3 and 6 we determine several alternative moment rate estimates (S1–S3) by choosing either on- or offshore locations for near-coastal events (discussed below). The magnitude–frequency data is shown in Fig. 10 (black dots). Events with Mw b 4.5 and with Mw N 4.5 are from the CSTI and CPTI catalogues, respectively. The

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Table 2 L − Equivalent fault length for conversion to slip rate, M˙cat 0 — Seismic-catalogue moment rates, and our best estimate long-term average seismic seis recurrence parameters: a-value for M ≥ 3 events, b-value, and maximum magnitude M seis max and the resulting moment rate M˙0 in each source zone SZ #

Equiv. fault (km)

1

290

2 3

190 290

4

100

5 6

200 200

7

260

(S1) (S2) (S3) (S1) (S2) (S3)

(S1) (S2)

16 M˙cat N m) 0 (·10

a-value

b-value

M seis max

2.27–8.93 0.819–3.93 0.776–1.04 ≤ 19.2 2.14–2.67 2.48–3.12 2.48–3.14 9.60–22.3

0.69–1.01

1.0

7.1–7.6

2.5–22.4

1.31–1.47 1.22–1.42

1.1 1.1

7.1–7.6 6.2–6.7

6.4–26.8 1.9–10.2

0.50–0.90

1.0

7.1–7.6

25.7–36.9 2.16–5.65 0.775–0.965 27.0–40.9

1.24–1.44 0.56–0.88

1.0 1.0

6.9–7.4 6.6–7.1

1.67–1.79

1.0

6.9–7.4

grey area represents the uncertainty range from the simulated catalogues. In several zones, the magnitude– frequency distribution of the data shows a dip for Mw = 4.5–5. Although uncertainties in converting macroseismic intensity to magnitude may contribute to

M˙ seis (·1016 N m) 0

(T1) (T2) (T1) (T2)

2.5–5.3 2.5–10.5 8.7–21.0 1.6–6.0 1.6–5.3 23.5–54.6

an irregular magnitude–frequency distribution, this dip suggests that the CPTI catalogue may, at least regionally, be more incomplete for these intermediate magnitudes than accounted for by our uncertainties in completeness length.

Fig. 10. The log10 of the cumulative number of events versus magnitude for the historical (CPTI) and instrumental (CSTI) seismic catalogues (black dots), with in gray the one-sigma uncertainty range due to uncertainties in event magnitude, location and completeness length of the catalogues. The seis solid lines represent truncated Gutenberg–Richter distributions with upper and lower bounds of M˙ seis ), for our best long-term estimates of 0 (i.e., D˙ (see Table 2). Dotted lines are TGR distributions with the same a-, and b-values, but maximum magnitudes Mtect a-, b-values and Mseis max max required to seis tect make D˙ = D .

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To obtain the average slip rates from M˙0cat, we use a shear modulus μ = 3.5 × 1010 N m− 2, an average 15 km for the thickness of the seismogenic zone, equivalent to the depth above which most of the seismicity is concentrated, and an estimated length of the assumed main fault within the source zone (Table 2). Uncertainties in fault length of ±10% translate into similar uncertainties in slip rate. If rigidity was only 3.0 × 1010 N m− 2, slip rates would change by less than 15%. Seismogenic thickness is the most uncertain parameter. If it would be changed to 10 or 20 km, slip rate would change by 25% to 50%. Intrinsic catalog uncertainties, however, exceed all these parameter uncertainties (Table 2). 6.2. Catalog moment rates For five out of our seven zones D˙ cat lies below D˙tect. D is extremely low for SZ 1 and SZ 6-case T1. While we would expect most zones to have lower D˙cat than D˙tect (Ward, 1998; Jenny et al., 2004), there are two zones where D˙cat actually exceeds D˙tect, Calabria (SZ 5) and SE Sicily (SZ 4), indicative of clustered seismic activity. The D˙cat estimates for each source zone are discussed below. ˙cat

SZ 1. Three end-member scenarios for the locations of the larger events give a large range of moment rates (Table 2), corresponding to slip rates between 0.09 and 0.6 mm/yr. Even in case S1, where all the larger events (1726 M = 5.6, 1823 M = 5.9 and 1940 M = 5.4) were shifted offshore and their magnitude increased accordingly, the seismic historical catalogue reflects only up to 30% of the tectonic deformation estimate (Fig. 12). In case S2, only the oldest and less well localized event of 1726 M = 5.6 is attributed to the active zone offshore, and in S3, only events within the actual southern Tyrrhenian SZ 1 are taken into account. It is well possible that even our upper estimate is below the real seismic rate, due to the incompleteness of the offshore earthquake record. SZ 2. The seismic catalog rates for this zone amount to a substantial fraction (0.9–1.2 mm/yr) of the geodetic estimates for the rates for the dextral Tindari–Letojanni and parallel faults plus the rates for the Messina Straits extensional system (Fig. 9). Very little of this seismic moment rate is released offshore, substantiating the aseismic nature of the strong contraction measured geodetically.

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SZ 3. The relocation of several of Sicily's historical earthquakes to offshore zone 1 results in three complementary scenarios for central and western Sicilian seismicity. All scenarios yield very small deformation rates between 0.1 and 0.2 mm/yr. Overall, between 10% and 40% of the tectonic deformation is reflected in the catalogue. SZ 4. The large historical earthquakes recorded in southeastern Sicily add up to moment rates that equal or exceed those expected tectonically (Fig. 9). Assuming a fault length of 100 km, the catalogue moment rate translates to slip rates of 1.8 to 2.9 mm/yr, i.e., to a D˙cat / D˙tect = 180–580% for scenario T1, and 90– 290% for T2 (Fig. 12). This indicates that this zone was overly active for its long-term loading rate, especially if case T1 applies. SZ 5. In Calabria, the observed seismic deformation of 2.4–3.5 mm/yr is 1.2 to 5.8 times larger than the geologically inferred tectonic extension. This can explain the low level of seismicity in Calabria since 1908, as the past seismicity released strain accumulated over a time exceeding the span of the catalogue. SZ 6. As in northern Sicily, locations of the older Calabrian events are not certain enough to discriminate offshore from onshore events. Therefore we test two possibilities: S1) the 1947 M = 5.8 event is shifted 40 km offshore, S2) only events that the catalogues locate within the trench source zone are taken into account. The seismic deformation estimates are all low, with slip rates of 0–0.5 mm/yr. Thus either the motion along the trench is very low, or largely aseismic. The catalog rates overlap with those for scenario T2, but amount only up to 20% of the rates for T1. SZ 7. Southern Apenninic catalog slip rate estimates range from 2 to 3 mm/yr, i.e., half to all of the geodetic slip rates, most consistent with estimates from Westaway (1992). 6.3. Style of seismic strain A Kostrov summation of the moment tensor data (Fig. 5) on the same grid as used in the continuous geodetic strain rate model provides a continuous seismic deformation style field (without strain rate amplitudes). The southern Italian moment–tensor catalogs contain no major events and thus all events were included in the Kostrov summation. The spatially variable smoothing in

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the geodetic strain rate field and the sparseness of focal mechanisms in some parts of Sicily and Calabria increase the uncertainties in this comparison compared to other areas where this was done (California, Indonesia, eastern Mediterranean Amelung and King (1997), Kreemer et al. (2000), Jenny et al. (2004)). However, Figs. 11 and 7 show that geodetic strain rate patterns are generally consistent with seismic extension perpendicular to the Apennines and some strike-slip faulting east of the chain. The Africa–European convergence north of Sicily is consistent with the compressional mechanisms in the offshore belt. Seismic and geodetic directions of compression and strike-slip faulting in central and western Sicily as well as in the Sicily Channel are similar. Focal mechanisms in the Aeolian Islands east of Alicudi and Filicudi reflect both the dextral character of the Tindari–Letojanni system and north–south compression required by the geodetic deformation. Serpelloni et al. (2005) also show consistency of seismic and geodetic strain style, on a grid about as coarse as our source zone grid (but different in geometry). In the Eastern Mediterranean we found that large discrepancies between seismic and geodetic styles of deformation may indicate slip partitioning in which one component of deformation is mostly aseismic (Jenny et

al., 2004). The very strong geodetic contraction with a small strike-slip component around the Aeolian Islands is not clearly represented in the seismicity. Its ensuing north–south extension in northeastern Sicily is reflected in a few smaller events at most (Fig. 11, Pondrelli et al., 2004). But seismic strains mainly reflect strike-slip, consistent with NNW–SSE striking faults of the Tindari–Letojanni system, and east–west extension consistent with the orientation of the Messina Strait faults. The low strength of the warm, wet and therefore most likely thin, southeastern Tyrrhenian lithosphere may cause most of the contractional strain to be released aseismically and may result in a decoupling from deformation further south. Of the events to the southwest of Calabria, some have strike-slip mechanisms with compressional axes approximately parallel to African motion, while a few mechanisms in northeastern Calabria show east–west compression more consistent with influence from the advancing Aegean. 7. Seismic potential estimates 7.1. Recurrence parameter determination With the constraints on geodetic strain rates and catalog magnitude–frequency distributions, we can

Fig. 11. Seismic strain rate style (not amplitudes) shown as the directions of maximum (black arrows) and minimum (white arrows) horizontal contractional strain rate. The styles are obtained by a Kostrov summation of the moment tensors in Fig. 5 within the same grid cells as those used for the continuous GPS strain rate field (Fig. 7), to facilitate a comparison.

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determine best estimate long-term seismic slip rates and the accompanying TGR parameters. The a-value is constrained to approximately twice the standard deviation (i.e., two times the uncertainties shown in Fig. 10), or slightly more if necessary to include the average catalog values (Table 2). The b-value was set to 1.0, with the exception of SZ 2 and 3 where it was increased to 1.1, to be more compatible with the actual magnitude– frequency data (Fig. 10). Increasing the b-value by 0.1 decreases the integrated moment rates for the same range of Mmax and a-value by around 20%, but given the large overall uncertainties, such changes do not alter our main conclusions. seis We determined a plausible range for Mmax , not allowing it to exceed the maximum magnitude observed in the catalog more than 0.5 magnitude unit. The TGR seis lines with the minimum of both a and Mmax give a lower seis seis estimate of M˙0 , while maximum-a, maximum- Mmax lines give an upper estimate. The maximum is capped to not exceed M˙0tect . This results in our best estimates of D˙seis (Fig. 9). For most zones, much larger maximum magnitudes are not expected given the fault zone dimensions, and the fact that it unlikely that faults will link across different tectonic regimes. The dotted TGR lines in Fig. 10 illustrate how large the maximum tect magnitude, Mmax , would need to be if the total tectonic slip rate was accommodated seismically. For SZ 4 and SZ6, we did this for the cases of the highest slip rates, i.e., case T2 and T1, respectively. The ratio of D˙seis over D˙tect (Fig. 12) gives information on how much of the tect deformation might be aseismic. Mmax values that significantly exceed the range that we consider plausible are another indication that part of the deformation occurs aseismically. Finally, we summarize the seismic potential in the recurrence time of M ≥ 6.5 events (Fig. 10). This magnitude threshold provides information on the potential of the largest events, but is far enough away

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from the maximum magnitude for all zones to allow for a meaningful comparison. 7.2. Long-term seismic slip rates and seismic potential SZ 1. In most zones, the average seismic slip rate estimates are lower than the average geodetic slip rates, but full seismic coupling cannot be ruled out (Fig. 12). An exception is SZ 1 where our seismic estimates amount to at most 75% of the geodetic slip rates. The maximum seismic slip rate estimates already require that the catalog for SZ 1 is substantially incomplete and large events have been missed or mislocated (Fig. 10). We cannot rule out that this zone might be capable of generating an infrequent M up to 8 event, but to bring seismic strain release up to the tectonic rates, the maximum magnitude would have to be between 8 and 9.6 (Fig. 10). This is too large for the size of the source zone. Thus, we infer that the southern Tyrrhenian back-thrust deforms for a significant part aseismically. The proportion of aseismic deformation may increase eastwards as the Tyrrhenian lithosphere gets weaker, culminating in the almost 100% aseismic deformation in the Aeolian Islands. Even so, our analysis shows that SZ 1 has a seismic potential which is relatively high for Southern Italy, with M ≥ 6.5 events possibly occurring as frequently as every 200 years (Fig. 13). This is a significantly higher potential than hazard analyses have taken into account so far. SZ 2. Although the Messina Straits are usually recognized as seismically active, the potential of the (onland) strike-slip faults in this region may have been underestimated. The total of 2–

Fig. 12. Ratios of catalog (Dc˙ at) and long-term seismic (D˙seis) over tectonic (D˙seis) slip rates give an indication of whether seismicity was excessive for the loading rates, and whether significant aseismic deformation is likely.

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Fig. 13. Seismic potential is summarized in the average recurrence time of Mw ≥ 6.5 according to the TGR distributions in Fig. 10. On the map, the source zones are shaded according to seismic potential, darkest colors corresponding to the highest potential.

3 mm/yr that accumulates over the many faults in this zone may all be released seismically (Fig. 12). The preference for a b-value somewhat higher than 1 may reflect the fact that the zone comprises many shorter fault segments, and is thus more likely to generate smaller events. We estimate average recurrence times for M ≥ 6.5 events of 200–600 yrs. SZ 3. This region shows the lowest amount of deformation in our study region. All the deformation in this zone could be seismic, but it is also possible that a large part is aseismic (Fig. 12). The low slip rates make this determination particularly uncertain. The preference for a b-value larger than 1 may again reflect distributed faulting. Based on the catalog and the TGR fits we do not really expect M N 7 events, or if they occur they would be extremely infrequent (recurrence

times of thousands of years). M ≥ 6.5 earthquakes probably recur every 800 years or even less often. SZ 4. Catalog uncertainties do not allow us to completely rule out that this area deforms only at the low strain rates of scenario T1. However, the high catalog moment rates would be a much more likely observation if the region deforms at the strain rates for case T2. In either case, the deformation probably occurs predominantly seismically. The minimum recurrence times for M ≥ 6.5 events are estimated to be about 400 years, i.e. a slightly lower potential than in SZ 1, 2 and 5. SZ 5. In our tectonic interpretation (Goes et al., 2004), Calabria experiences internal deformation, and is not cut directly by any of the main plate boundaries in the region. Its normal faulting style of deformation is not compatible with any of the relative motions between the main blocks (Adria, Africa and Europe) in the region. Yet, the high level of past seismicity in this region implies it has a relatively high seismic potential, at least on par with SZ 1 and 2, and only slightly lower than SZ 7 (Fig. 13). The geologic estimates of deformation rates imply the seismicity has been clustered. Some physical earthquake models with heterogeneous faults predict that periods of strong activity, including large earthquakes and spanning several recurrence cycles, alternate with periods of low activity with only small events (Ben-Zion et al., 2003). SZ 6. If much of the Africa–Europe convergence is accommodated along the trench, as scenario T1 assumes, over 80% of these motions must occur aseismically. The Mmax of 8.4–9.3 required for full seismic coupling is implausible for this small subduction zone. However, given the high seismicity rates of SZ 4, we consider it more likely that the Ionian corner of Africa deforms internally (case T2). In this case, the levels of seismicity might be compatible with the rates of tectonic deformation, and the overall seismic potential of this zone is only slightly higher than that of SZ 3, with minimum M ≥ 6.5-event-recurrence time of 550 years. SZ 7. The short recurrence times we estimate for this zone are compatible with the high seismicpotential designation this zone generally receives in Italian hazard studies (Slejko et

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al., 1999). Seismic moment rates can be compatible with the full tectonic deformation. But also in this zone, the large catalog uncertainties do allow for a significant proportion of aseismic strain, or maximum magnitudes that are slightly larger than the largest M7 events in this zone's catalog. 8. Conclusions By combining geodetic, instrumental and historical seismic deformation rates, one can better quantify the seismic potential of regions where strain rates are low, and historical records are not long enough to span a few cycles of the largest events. In Southern Italy, combining seismic catalog data with a recently published set of GPS data, which significantly improves the spatial resolution of tectonic strain, allows estimating long-term seismic deformation rates in offshore areas where seismic observations are incomplete and location uncertainties are large. It also identifies clustered seismicity that cannot be representative of the long-term average level of seismic activity, because it exceeds the tectonic rates of strain accumulation. We determine seismic potential in terms of a longterm average truncated Gutenberg–Richter distribution described by an a-value, a b-value and a maximum magnitude (Table 2) with uncertainties, for seven source zones (Fig. 13). The southern Apennines have the highest seismic potential in the region, with recurrence times for M ≥ 6.5 events between 60 and 140 years. Previously unrecognized was the high seismic potential north of the Sicilian coast, which may also be associated with a tsunami hazard. This area deforms at rates similar to those in the southern Apennines. Although half or more of this deformation probably occurs asesimically, maximum magnitudes as large as in the Apennines and earthquake recurrence times only about twice as long are expected. Calabrian long-term seismicity rates are probably similar to the Sicilian north coast. This is less than estimated from the catalog alone, due to clustering. Clustered seismicity may also affect the east coast of Sicily, which has a considerable seismic potential but less than Calabria and the Sicilian north coast. Lowest deformation and seismicity rates are found for central and western Sicily and along the Calabrian trench. Overall, Southern Italy displays a strongly timevariable and very diffuse pattern of seismicity in response to relatively low tectonic deformation rates and reorganizing plate boundaries.

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Acknowledgments We thank Bill Holt and Paul Lundgren for making their codes available, and Corné Kreemer and Paul Lundgren for their help in running them. Further, we thank David Jackson, Steven Ward, and Stefan Wiemer for discussions on determining seismic potential and seismicity characteristics, Max Stucchi for comments on the manuscript and P. Gasparini for information on magnitude–intensity relations. Reviews by Marleen Nyst and an anonymous referee were helpful to clarify the manuscript. The work was funded by ETH Zurich and MunichRe. This is contribution number 1365 of the Institute of Geophysics. References Amelung, F., King, G., 1997. Large-scale tectonic deformation inferred from small earthquakes. Nature 386, 702–705. Argnani, A., 1990. The Strait of Sicily rift zone: foreland deformation related to the evolution of a back-arc basin. J. Geodyn. 12 (2–4), 311–331. Argnani, A., Bonazzi, C., 2003. Neotectonics of the Eastern Sicily Offshore: Recent Reactivation of an Old Structure, in Workshop on: Seismogenic Faulting and Seismic Activity in the Calabrian Arc Region. Università degli Studi di Messina, Istituto Nazionale di Geofisica e Vulcanologia, Taormina, Sicily, pp. 12–13. Argnani, A., Bonazzi, C., Crew, M., 2002. Tectonics of eastern Sicily offshore: preliminary results from the MESC 2001 marine seismic cruise. Boll. Geofis. Teor. Appl. 43 (3–4), 177–193. Argus, D.F., Heflin, M.B., 1995. Plate motion and crustal deformation estimated with geodetic data from GPS. Geophys. Res. Lett. 22 (15), 1973–1976. Armijo, R., Hubert, A., Barka, A., 1999. Westward propagation of the North Anatolian fault into the northern Aegean: timing and kinematics. Geology 27 (3), 267–270. Beavan, J., Haines, J., 2001. Contemporary horizontal velocity and strain rate fields of the Pacific–Australian plate boundary zone through New Zealand. J. Geophys. Res. 106, 741–770. Ben-Zion, Y., Eneva, M., Liu, Y.F., 2003. Large earthquake cycles and intermittent criticality on heterogeneous faults due to evolving stress and seismicity. J. Geophys. Res. 108 (B6), 2307. doi:10.1029/2002JB002121. Bianca, M., Monaco, C., Tortorici, L., Cernobori, L., 1999. Quaternary normal faulting in south-eastern Sicily (Italy): a seismic source for the 1693 large earthquake. Geophys. J. Int. 139. Boccaletti, M., Cello, G., Tortorici, L., 1987. Transtensional tectonics in the Sicily channel. J. Struct. Geol. 9 (7), 869–876. Bonaccorso, A., Mattia, M., 2000. Deflation acting on Pantelleria Island (Sicily Channel) inferred through geodetic data. Earth Planet. Sci. Lett. 180 (1–2), 91–101. Bordoni, P., Valensise, G., 1998. Deformation of the 125 Ka marine terraces in Italy: tectonic implications. Coast. Tecton., Geol. Soc. Spec. Publ. 146, 71–110. Boschi, E., Gasperini, P., Mulargia, F., 1995a. Forecasting where larger crustal earthquakes are likely to occur in Italy in the near future. Bull. Seismol. Soc. Am. 85 (5), 1475–1482.

100

S. Jenny et al. / Tectonophysics 415 (2006) 81–101

Boschi, E., Guidoboni, E., Mariotti, D., 1995b. Seismic effects of the strongest historical earthquakes in the Syracuse area. Ann. Geofis. 38, 223–253. Boucher, C., Altamini, Z., Sillard, P., 1998. Results and Analysis of the ITRF96, Technical Note 24. Central Bureau of IERS, Observatoire de Paris, Paris. Braunmiller, J., Kradolfer, U., Baer, M., Giardini, D., 2002. Regional moment tensor determination in the European–Mediterranean area — initial results. Tectonophysics 356, 5–22. Carletti, F., Gasperini, P., 2003. Lateral variations of seismic intensity attenuation in Italy. Geophys. J. Int. 155, 839–856. Catalano, S., De Guidi, G., 2003. Late quaternary uplift of northeastern Sicily: relation with the active normal faulting deformation. J. Geodyn. 36, 445–467. Catalano, S., De Guidi, G., Monaco, C., Tortorici, G., Tortorici, L., 2003. Long-term behaviour of the late Quaternary normal faults in the Straits of Messina area (Calabrian arc): structural and morphological constraints. Quartenary Int. 101–102, 81–91. D'Agostino, N., Selvaggi, G., 2004. Crustal motion along the Eurasia– Nubia plate boundary in the Calabrian arc and Sicily and active extension in the Messina Straits from GPS measurements. J. Geophys. Res. 109 (B11), B11402. De Rosa, R., Guillou, H., Mazzuoli, R., G.V., 2003. New unspiked K– Ar ages of volcanic rocks of the central and western sector of the Aeolian Islands: reconstruction of the volcanic stages. J. Volcanol. Geotherm. Res. 120, 161–178. DeMets, C., Gordon, R., Argus, D., Stein, S., 1994. Current plate motions. Geophys. J. Int. 101, 425–478. Dewey, J.F., Helman, M., Turco, E., Hutton, D., Knott, S., 1989. Kinematics of the western Mediterranean. In: Coward, M., Dietrich, D., Park, R. (Eds.), Alpine Tectonics. Spec. Publ. Geol. Soc., vol. 45. Blackwell Scientific Publications, London, pp. 265–283. Dziewonski, A., Chou, T.-A., Woodhouse, J., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852. Faccenna, C., Becker, T.W., Lucente, F.P., Jolivet, L., Rossetti, F., 2001. History of subduction and back-arc extension in the central Mediterranean. Geophys. J. Int. 145, 809–820. Frepoli, A., Amato, A., 2000. Fault plane solutions of crustal earthquakes in southern Italy (1988–1995): seismotectonic implications. Ann. Geofis. 43 (3), 437–467. Galli, P., Bosi, V., 2003. Catastrophic 1638 earthquakes in Calabria (southern Italy): new insights from paleo-seismological investigation. J. Geophys. Res. 108 (B1), 2004. doi:10.1029/ 2001JB001713. Gasperini, P., 2001. The attenuation of seismic intensity in Italy: a bilinear shape indicates the dominance of deep phases at distances longer than 45 km. Bull. Seismol. Soc. Am. 91 (2), 826–841. Gasperini, P., Ferrari, G., 2000. Deriving numerical estimates from descriptive information: the estimation of earthquake synthetic parameters. Ann. Geofis. 43, 729–746. Ghisetti, F., Vezzani, L., 1982. Different styles of deformation in the Calabrian arc (southern Italy): implications for a seismotectonic zoning. Tectonophysics 85, 149–165. Gioncada, A., Mazzuoli, R., Bisson, M., Pareschi, M.T., 2003. Petrology of volcanic products younger than 42 ka on the Lipari– Vulcano complex (Aeolian Islands, Italy): an example of volcanism controlled by tectonics. J. Volcanol. Geotherm. Res. 122, 191–220. Goes, S., 1996. Irregular recurrence of large earthquakes: an analysis of historic and paleoseismic catalogs. J. Geophys. Res. 101, 5739–5749.

Goes, S., Giardini, D., Jenny, S., Hollenstein, C., Kahle, H.-G., Geiger, A., 2004. A recent tectonic reorganization in the South-Central Mediterranean. Earth Planet. Sci. Lett. 225, 335–345. Guidi, D.G., Catalano, S., Monaco, C., Tortorici, L., 2003. Morphological evidence of Holocene coseismic deformation in the Taormina region (NE Sicily). J. Geodyn. 36, 193–211. Haines, A.J., Holt, W.E., 1993. A procedure for obtaining the complete horizontal motions within zones of distributed deformation from the inversion of strain rate data. J. Geophys. Res. 98, 12057–12082. Haines, A.J., Jackson, J.A., Holt, W.E., Agnew, D.C., 1998. Representing distributed deformation by continuous velocity fields. Sci. Rept. 98/5, Inst. of Geol. and Nucl. Sci. Wellington, New Zealand. Hanks, T., Kanamori, H., 1979. A moment magnitude scale. J. Geophys. Res. 84, 2348–2350. Hollenstein, C., Kahle, H.-G., Geiger, A., Jenny, S., Goes, S., Giardini, D., 2003. New GPS constraints on the Africa–Eurasia Plate Boundary Zone in southern Italy. Geophys. Res. Lett. 30 (18), 1935. doi:10.1029/2003GL017554. Holt, W.E., Haines, A.J., 1995. The kinematics of northern South Island, New Zealand, determined from geologic strain rates. J. Geophys. Res. 100, 17991–18010. Hunstad, I., Selvaggi, G., D'Agostino, N., England, P., Clarke, P., Pierozzi, M., 2003. Geodetic strain in peninsular Italy between 1875 and 2001. Geophys. Res. Lett. 30 (4), 1181. doi:10.1029/ 2002GL016447. Instrumental Catalog Working Group CSTI, 2001. Catalogo Strumentale dei terremoti italiani dal 1981 al 1996, Versione 1.0, Clueb, Bologna, CD-ROM. Jacques, E., Monaco, C., Tapponnier, P., Tortorici, L., Winter, T., 2001. Faulting and earthquake triggering during the 1783 Calabria seismic sequence. Geophys. J. Int. 147 (3), 499–516. Jenny, S., Goes, S., Giardini, D., Kahle, H.-G., 2004. Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean. Geophys. J. Int. 157, 1331–1347. Kagan, Y.Y., 2002a. Seismic moment distribution revisited: I. Statistical results. Geophys. J. Int. 148, 520–541. Kagan, Y.Y., 2002b. Seismic moment distribution revisited: II. Moment conservation principle. Geophys. J. Int. 149, 731–754. Kagan, Y.Y., Jackson, D., 1991. Long-term earthquake clustering. Geophys. J. Int. 104, 117–133. Kanamori, H., Anderson, D.L., 1975. Theoretical basis of some empirical relations in seismology. Bull. Seismol. Soc. Am. 65, 1073–1096. Koravos, G.C., Main, I.G., Tsapanos, T.M., Musson, R.M.W., 2003. Maximum earthquake magnitudes in the Aegean area constrained by tectonic moment release rates. Geophys. J. Int. 152, 94–112. Kreemer, C., Holt, W.E., Goes, S., Govers, R., 2000. Active deformation in the eastern Indonesia and Philippines from GPS and seismicity data. J. Geophys. Res. 105, 663–680. Lickorish, W.H., Grasso, M., Butler, R.W.H., Argnani, A., Maniscalco, R., 1999. Structural styles and regional tectonic setting of the “Gela Nappe” and frontal part of the Maghrebian thrust belt in Sicily. Tectonics 18 (4), 655–668. Lundgren, P., Saucier, F., Palmer, R., Langon, M., 1995. Alaska crustal deformation: finite element modeling constrained by geologic and very long baseline interferometry data. J. Geophys. Res. 100 (B11), 22033–22045. Lundgren, P., Giardini, D., Russo, R., 1998. A geodynamic framework for eastern Mediterranean kinematics. Geophys. Res. Lett. 25, 4007–4010.

S. Jenny et al. / Tectonophysics 415 (2006) 81–101 Meletti, C., Pattaca, E., Scandone, P., 2000. Construction of a seismotectonic model: the case of Italy. Pageoph 157, 11–35. Monachesi, G., Stucchi, M., 1997. DOM4.1: An Intensity Database of Damaging Earthquakes in the Italian Area. Monaco, C., Tortorici, L., 2000. Active faulting in the Calabrian arc and eastern Sicily. J. Geodyn. 29, 407–424. Monaco, C., Mazzoli, S., Tortorici, L., 1996. Active thrust tectonics in western Sicily (southern Italy): the 1968 Belice earthquake sequence. Terra Nova 8 (4), 372–381. Monaco, C., Tapponnier, P., Tortorici, L., Gillot, P.Y., 1997. Late quaternary slip rates on the Acireale–Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily). Earth Planet. Sci. Lett. 147 (1–4), 125–139. Mulargia, F., Broccio, F., Achilli, V., Baldi, P., 1985. Evaluation of a seismic quiescence pattern in southeastern Sicily. Tectonophysics 116, 335–364. Mulargia, F., Gasperini, P., Tinti, S., 1987. Contour mapping of Italian seismicity. Tectonophysics 142, 203–216. Piatanesi, A., Tinti, S., 1998. A revision of the eastern Sicily earthquake and tsunami. J. Geophys. Res. 103, 2749–2758. Pondrelli, S., 1999. Pattern of seismic deformation in the western Mediterranean. Ann. Geofis. 42 (1), 57–70. Pondrelli, S., Morelli, A., Ekström, G., Mazza, S., Boschi, E., Dziewonski, A.M., 2002. European–Mediterranean regional centroid-moment tensors: 1997–2000. Phys. Earth Planet. Int. 130, 71–101. Pondrelli, S., Piromallo, C., Serpelloni, E., 2004. Convergence vs. retreat in southern Tyrrhenian Sea: insights from kinematics. Geophys. Res. Lett. 31, L06611. doi:10.1029/2003GL019223. Postpischl, D., 1985. Atlas of Isoseismal Maps of Italian Earthquakes, CNR, Progetto Finalizzato Geodinamica. Graficoop, Bologna. Sella, G.F., Dixon, T.H., Mao, A.L., 2002. REVEL: a model for recent plate velocities from space geodesy. J. Geophys. Res. 107 (B4), 2081. Serpelloni, E., Anzidei, M., Baldi, P., Casula, G., Galvani, A., 2005. Crustal velocity and strain rate fields in Italy and surrounding regions: new results from the analysis of permanent and nonpermanent GPS networks. GJI 161 (3), 861–880. Slejko, D., Peruzza, L., Rebez, A., 1998. Seismic hazard maps of Italy. Ann. Geofis. 41, 183–214. Slejko, D., Camassi, R., Cecic, I., Herak, D., Herak, M., Kociu, S., Kouskouna, V., Lapajne, J., Makropoulos, K., Meletti, C., Muco, B., Papaioannou, C., Peruzza, L., Rebez, A., Scandone, P., Sulstarova, E., Voulgaris, N., Zivcic, M., Zupancic, P., 1999.

101

GSHAP seismic hazard assessment for Adria. Ann. Geofis., GSHAP Spec. vol. 42 (6), 1085–1107. Speranza, F., Maniscalco, R., Mattei, M., Di Stefano, A., Butler, R.W. H., Funiciello, R., 1999. Timing and magnitude of rotations in the frontal thrust systems of southwestern Sicily. Tectonics 18 (6), 1178–1197. Stewart, I.S., Cundy, A., Kershaw, S., Firth, C., 1997. Holocene coastal uplift in the Taormina area, northeastern Sicily: implications for the southern prolongation of the Calabrian seismogenic belt. J. Geodyn. 24 (1–4), 37–50. Straub, C., Kahle, H.-G., Schindler, C., 1997. GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia. J. Geophys. Res. 102 (B12), 27587–27601. Tortorici, L., Monaco, C., Carlo, T., Ornella, C., 1995. Recent and active tectonics in the Calabrian arc (southern Italy). Tectonophysics 243, 37–55. Tortorici, L., Monaco, C., Mazzoli, S., Bianca, M., 2001. Timing and modes of deformation in the western Sicilian thrust system, southern Italy. J. Pet. Geol. 24 (2), 191–211. Valensise, G., Pantosti, D., 1992. A 125 Kyr-long geological record of seismic source repeatability: the Messina Straits (southern Italy) and the 1908 earthquake MS 7 1/2. Terra Nova 4, 472–483. Ventura, G., Vilardo, G., Milano, G., Pino, N.A., 1999. Relationships among crustal structure, volcanism and strike-slip tectonics in the Lipari–Vulcano Volcanic Complex (Aeolian Islands, southern Tyrrhenian Sea, Italy). Phys. Earth Planet. Int. 116 (1–4), 31–52. Ward, S.N., 1990. Pacific–North America plate motions: new results from very long baseline interferometry. J. Geophys. Res. 95, 21965–21981. Ward, S.N., 1998. On the consistency of earthquake moment release and space geodetic strain rates: Europe. Geophys. J. Int. 135, 1011–1018. Westaway, R., 1992. Seismic moment summation for historical earthquakes in Italy: tectonic implications. J. Geophys. Res. 97, 15437–15464. Westaway, R., 1993. Quartenary uplift of southern Italy. J. Geophys. Res. 98, 21'741–21'772. Westaway, R., 1994. Present-day kinematics of the Middle East and eastern Mediterranean. J. Geophys. Res. 99 (6), 12071–12090. Working Group CPTI, 1999. Catalogo Parametrico dei Terremoti Italiani, GNDT, SGA, SSN, Bologna. 92 pp.