Tectonic stress and seismogenic faulting in the area of the 1908 Messina earthquake, south Italy

GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L10602, doi:10.1029/2004gl019742, 2004 Tectonic stress and seismogenic faulting in the area of the 1908 Messina earthquake, south Italy G. Neri, 1 G. Barberi, 2 G. Oliva, 1 and B. Orecchio 1 Received 17 February 2004; revised 14 April 2004; accepted 19 April 2004; published 18 May 2004. [1] Stress inversion of the twenty best-quality earthquake fault-plane solutions available in the area of the 1908 Messina earthquake showed a nearly uniform extensional regime with s min constrained between N284 E and N312 E, coinciding with the direction of extension derived from geostructural data. The misfits of earthquake nodal planes and related slip vectors to the stress tensor allowed us to identify the fault planes of thirteen of the earthquakes used for inversion. In particular, the fault plane of 1908 earthquake was found in the north-trending east-dipping nodal plane of the focal mechanism. These findings and strain tensor estimates performed with the same dataset lead us to propose that in spite of stress uniformity detected over the study area the seismic strain orientations change significantly in the crustal volume under investigation due to different fault orientations in the different sectors. However, when comparing strong earthquakes with background seismicity in a given sector the strain orientations are found to be similar. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7230 Seismology: Seismicity and seismotectonics; 8164 Tectonophysics: Stresses crust and lithosphere; 9335 Information Related to Geographic Region: Europe. Citation: Neri, G., G. Barberi, G. Oliva, and B. Orecchio (2004), Tectonic stress and seismogenic faulting in the area of the 1908 Messina earthquake, south Italy, Geophys. Res. Lett., 31, L10602, doi:10.1029/2004gl019742. 1. Seismotectonic Scenario of the Investigation [2] The magnitude 7.3 Messina earthquake of December 28, 1908 was the strongest Italian earthquake of the past century and produced destruction and 70,000 casualties in a large area around the Messina Straits (Figure 1) (see Gruppo Nazionale per la Difesa dai Terremoti, http:// emidius.mi.ingv.it/nt). The same area also suffered major damage from other earthquakes of comparable magnitude in the last few centuries, such as the southern Calabria earthquake of February 5, 1783 (Figure 1) (see http:// emidius.mi.ingv.it/nt). Figure 1 shows that the main fault systems strike between N-S and NE-SW near the Straits, ca. ENE-WSW to the south of the Aspromonte chain southern edge, and NNE-SSW along the eastern coast of Calabria (data from Fabbri et al. [1980], Ciaranfi et al. [1983], Ambrosetti et al. [1987], Ghisetti [1992], Lentini et al. [1995], and Tortorici et al. [1995]). Normal faulting is 1 Dipartimento di Scienze della Terra, Università di Messina, Messina, Italy. 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Catania, Italy. Copyright 2004 by the American Geophysical Union. 0094-8276/04/2004GL019742 dominant in this area and is also the primary mechanism associated with seismicity [see, e.g., Valensise and Pantosti, 1992; Amoruso et al., 2002; Neri et al., 2003]. In particular, the 1908 earthquake had a normal faulting mechanism [e.g., Boschi et al., 1989; Bottari et al., 1989; Monaco and Tortorici, 2000; Amoruso et al., 2002], although debate is still open concerning what fault produced it: a west-dipping fault with its top beneath the Calabrian shoreline of the Straits [Monaco and Tortorici, 2000] or a east-dipping fault with its top beneath the Sicilian shoreline [Boschi et al., 1989; Bottari et al., 1989; Amoruso et al., 2002]. The area of 1908 earthquake is a part of the Calabrian Arc (Figure 1 (inset)), a tectonic structure uplifting at a rate of 0.5 1.2 mm/yr in the last 1 0.7 Myrs where internal deformation is mainly accommodated by normal faulting [Monaco and Tortorici, 2000; Catalano and De Guidi, 2003; Catalano et al., 2003]. Seismic strain in this region is one of the most evident effects of dynamics of the Ionian and Southern Tyrrhenian tectonic units, two of the lithospheric blocks and microplates recognized in the highly fragmented Italian portion of the Africa-Eurasia contact belt [see, e.g., Scandone and Stucchi, 1999; Gvirtzman and Nur, 1999; Nicolich et al., 2000; Doglioni et al., 2001]. [3] The dataset of earthquake fault-plane solutions in the area of Figure 1 has improved in the last few years, both in number and average quality of data. This may allow us to obtain more accurate estimates of seismogenic stress orientations and, possibly, to discriminate between the fault plane and auxiliary plane for each of these earthquakes by comparison of the misfits of the nodal planes and related slip vectors with respect to the stress tensor. The identification of fault planes which were seismically active during the study period may integrate the local geostructural information and, jointly with stress orientation and other geological data, may contribute to improving our knowledge of local tectonics and active deformation in this area of high seismic risk. 2. Data and Methods [4] The dataset used in the present study (Figure 2, Table 1) includes the better quality fault-plane solutions available in the literature relative to the area of Figure 1 and the time period 1988 2001 (earthquakes n. 3 21 in Figure 2, Table 1), and the two fault-plane solutions available for earthquakes of magnitude over 5 in the same area in the past century (n. 1 2). We first selected from the literature all the fault-plane solutions estimated in a 3D local velocity structure with fault-parameter errors less than 20 (earthquakes n. 3 6, 8 10, 15 and 18 21). Joint analysis of fault parameter statistical errors and uncertainty of P and T axes in the FPFIT plot [Reasenberg and Oppenheimer, L10602 1of5

analysis of the individual earthquake misfits is performed for a deeper investigation of stress homogeneity in the study volume. Once the stress distribution is found to be uniform in a given rock volume, the analysis of misfits of nodal planes of earthquakes located in this volume may allow for identification of nodal planes which acted as fault planes. Following Wyss et al. [1992] we assume that the fault plane with its slip vector can be discriminated from the auxiliary plane in a given fault-plane solution if the difference in the two misfits is larger than twice the F value. Finally, Wyss et al. s [1992] algorithm derived from Kostrov [1974] was applied to the same data of Figure 2 in order to estimate seismic strain tensor orientations. Figure 1. Map of the study area with the main fault systems taken from Fabbri et al. [1980], Ciaranfi et al. [1983], Ambrosetti et al. [1987], Ghisetti [1992], Lentini et al. [1995], and Tortorici et al. [1995]. Barbs indicate the downthrown blocks in normal faults. MS stands for Messina Straits. Circles indicate the locations of the earthquakes of 1908 in the Messina Straits and 1783 in Southern Calabria according to Gruppo Nazionale per la Difesa dai Terremoti (see http://emidius.mi.ingv.it/nt). The inset shows the whole region of Calabrian Arc, with the curved line and the square indicating, respectively, the shape of the Arc and the area investigated in the present work. 1985] guided us in the procedure of assigning a weight factor of 2 or 1 to these fault plane solutions for stress inversion (Table 1). Also, we selected from Frepoli and Amato [2000a] the fault-plane solutions estimated with quality factors Q f = A and Q p = A (n. 7, 11 14 and 16 17 in Figure 2 and Table 1). Because these solutions were computed in a 1D velocity model we gave them a weight factor of 1. For the magnitude 7.3 earthquake of 1908 (n. 1) we used the solution estimated from P-onset polarities by Gasparini et al. [1985], which is very similar to that obtained by waveform inversion by Anderson and Jackson [1987]. In the case of the magnitude 5.6 earthquake of 1978 (n. 2) the CMT solution by Dziewonski et al. [1987] was preferred to the solution found by P-onset polarity data inversion [Gasparini et al., 1985] because the latter shows a quite poor level of constraint. [5] We applied the Gephart and Forsyth [1984] inversion method to the data of Figure 2 to estimate stress tensor directions and relative stress amplitudes in the area of interest. The relative stress amplitude is defined as R = (s int s max )/(s min s max ) where s max, s int and s min are stress amplitudes in the directions of principal stress axes. Details on this widely known and used method can be found in Gephart and Forsyth [1984], and in Gephart [1990]. The average misfit F between the estimated stress tensor and the fault-plane solutions gives preliminary information about stress homogeneity in the inversion dataset or, equivalently, in the concerned rock volume. Following Wyss et al. [1992] and Gillard et al. [1996], we assume in the present study that the condition of homogeneous stress distribution is satisfied overall if the F-value is smaller than 6. In this case a detailed 3. Results and Conclusion [6] By application of Gephart and Forsyth s method to the dataset of Figure 2 and to subsets of it corresponding to space partitions of the study area (see previous section and Wyss et al. [1992] for details on the procedure) we found that stress is uniform in the area indicated by the letter A (shaded area) in Figure 2. An average misfit F of 4.4 is found between the stress solution S and focal mechanisms in A, with individual earthquake misfits not exceeding 11.2 (Table 1). Following Wyss et al. s [1992] criteria for evaluation of individual earthquake misfits, we suggest that the misfit of 15.6 between S and the westernmost event in Figure 2 (n.16) may be an evidence of change of stress orientation from the shaded area A to the location of this event. Stress orientations in A are well constrained as can be noted in the stereonet of Figure 3a, where numbers 1, 2 and 3 indicate the orientations of the principal stress axes (s max, s int and s min ) and the black and grey zones are the 95% confidence areas of the s max and s min axes. The histogram shows the 95% confidence limits of the relative stress amplitude R. We may conclude that a uniform extensional regime is present in the crust underneath the shaded area A of Figure 2 with s min oriented N294 E and Figure 2. Fault-plane solutions selected for stress inversion (earthquake dates, focal depths, magnitudes and bibliographic sources are reported in Table 1). Shading indicates the area A where stress inversion indicated uniform stress. Black arrows indicate the nodal planes identified as fault planes by misfit analysis, grey arrows show the nodal planes with the lowest misfit for earthquakes where the fault plane could not be identified. Sub-regions MS and SEC discussed in the text are also shown. 2of5

Table 1. Event Number (N), Date, Origin Time (O.T.), Focal Depth (Z), and Magnitude (M) of Earthquakes Reported in Figure 2 a N Date O.T. Z M W M1 M2 B 1 081228 4.20 10 7.0 1 19.1 1.6 1 2 780311 19.20 15 5.6 1 2.7 32.3 2 3 881107 14.26 16 3.4 1 10.8 14.6 3 4 881108 7.02 16 2.4 2 35.0 6.0 3 5 881108 8.13 22 2.9 1 6.6 17.4 4 6 890123 8.22 22 3.0 2 3.4 6.2 4 7 891121 18.36 10 3.8 1 34.7 8.2 5 8 900915 3.11 15 3.0 1 20.2 0.1 3 9 910907 5.39 8 3.2 2 0.8 20.5 3 10 910925 13.21 8 3.3 2 11.2 11.8 3 11 910925 14.53 10 3.5 1 1.1 25.6 5 12 910925 21.21 9 3.4 1 29.3 3.2 5 13 920628 6.03 14 3.3 1 9.7 5.4 5 14 940819 1.51 16 3.0 1 3.9 7.0 5 15 950308 8.55 5 3.7 2 1.8 25.8 6 16 950724 16:58 11 3.7 1 30.3 15.6 5 17 950803 12:15 4 2.9 1 3.7 1.8 5 18 000208 22.19 26 2.6 1 39.0 9.5 3 19 000308 9.38 13 3.0 2 31.5 5.9 3 20 000317 3.52 9 3.8 1 0.1 4.8 6 21 000317 6.35 11 3.3 2 25.5 2.6 3 a W is the weight assigned to the fault-plane solution for stress inversion. M1 and M2 are the misfits of the two nodal planes and related slip vectors with respect to stress tensor S reported later on in this caption. The lowest value between M1 and M2 (bold) is the misfit of the earthquake with respect to S. B is the bibliographic source: 1 = Gasparini et al. [1985]; 2 = Dziewonski et al. [1987]; 3 = Neri et al. [2003]; 4 = Barberi et al. [1999]; 5=Frepoli and Amato [2000a]; 6 = Barberi et al. [2003]. Stress tensor S: max. compr. stress (dip 85, str 119 ), interm. compr. stress (0, 24 ), min. compr. stress (5, 294 ), R = 0.4, F (average misfit between stress and faultplane solutions) = 4.4. discriminated from the auxiliary plane in a given fault-plane solution if the difference in the two misfits (M1 and M2 in Table 1) is larger than twice the F value. In this manner, we identified the fault planes of thirteen events, indicated by black arrows in Figure 2. This figure shows that the dislocation surfaces that could be identified by Wyss et al. s [1992] procedure are mainly located in Southeastern Calabria (SEC): here, 6 out of 8 dislocation surfaces trend between N70 E and N100 E (see also Figure 4) and dip preferentially to north (5 cases out of 6). Geostructural maps are not detailed nor recent in this specific sector and show a few faults striking between N70 E and N90 E and dipping to the south (Figure 1). Therefore, our results in Figure 2 for southeastern Calabria earthquakes (focal depths in the range 8 22 km) agree with the geostructural data of Figure 1 concerning fault orientations but disagree with regard to dip. In the Messina Straits (MS in Figure 2) only four dislocation surfaces have been identified, all oriented around north-south (see also Figure 4) and dipping to the east. In this sector, the fault plane of the 1908 earthquake (n.1) was found to correspond to the north-trending eastdipping nodal plane of the focal mechanism (misfit of 1.6 with respect to the stress tensor of Figure 3a) and was discriminated from the NNE-trending west-dipping auxiliary plane characterized by a misfit of 19.1. This result may contribute to the mentioned debate on the 1908 earthquake source, supporting the hypothesis of an east-dipping fault with its top beneath the Sicilian shoreline of the Straits constrained between N284 E and N312 E at the 95% confidence level. The R parameter is found equal to 0.4, i.e., the amplitude of s int is nearly the average of the s max and s min amplitudes. Even considering the uncertainty of this estimate, s min is significantly smaller than s int because the 95% confidence area of R does not include 1.0 in the histogram. The results of Figure 3a mark a significant improvement of seismogenic stress estimates in the study area (see for comparison Frepoli and Amato [2000b, Figure 4]). Using a slightly larger number of fault-plane solutions (20 instead of 16) and having increased the quality of the inversion dataset (i) by inclusion of fault-plane solutions estimated in the 3D local velocity structure and (ii) by exclusion of Frepoli and Amato s [2000a] fault-plane solutions ranked of quality B and C, we obtained better constrained stress orientations (the confidence area of s min is halved) and a smaller value of the average misfit F (4.4 versus 5.9 ). Our estimate of the s min orientation nearly coincides with the estimate by Tortorici et al. [1995] who found extension trending between N112 E and N140 E using geological and structural data collected in the same area (this angular range is shown in Figure 3a outside the stereonet of seismogenic stress). [7] The good constraint of our stress estimate, and the degree of accuracy of the earthquake fault-plane solutions used for inversion, allowed us to apply the Wyss et al. [1992] procedure based on misfit analysis in order to identify which of the nodal planes acted as fault plane in each of these earthquakes. Following Wyss et al. [1992] we assumed that the fault plane with its slip vector can be Figure 3. (a) Stress inversion results obtained for the shaded area A in Figure 2. Numbers 1, 2 and 3 indicate the orientations of the principal stress axes s max, s int and s min, black and grey show the 95% confidence areas of s max and s min. For comparison, the extension direction ED detected in the same area by geostructural investigations [Tortorici et al., 1995] is displayed outside the stereonet. The histogram shows the 95% confidence limits of the relative stress amplitude R. (b) Orientations of the principal strain axes e max, e int and e min estimated for sub-regions SEC and MS of Figure 2: symbols 1, 2 and 3 (SEC) and A, B and C (MS) indicate the orientations of e max, e int and e min derived from analysis of the 1988 2001 seismicity, blue and green show the 95% confidence areas of e max and e min axes in SEC, red and yellow in MS. Squares, circles and rhombs indicate the orientations of e max, e int and e min relative to the strong earthquakes of 1908 in the Messina Straits (empty symbols) and 1978 in southeastern Calabria (full symbols). 3of5

Figure 4. Sketch representation of the main findings of the present study. Rose diagrams report the orientations of seismogenic fault-planes in Southeastern Calabria (SEC) and Messina Straits (MS), full arrows of different size indicate the e min orientations estimated in the respective sectors for strong earthquakes and background seismicity, empty arrows show the s min orientation obtained by inversion of earthquake fault-plane solutions in the shaded area of Figure 2. [Boschi et al., 1989; Bottari et al., 1989; Amoruso et al., 2002]. Even considering the low number of seismic dislocation surfaces recognized in the Messina Straits, and consequently the preliminary level of our inferences, we cannot avoid to note in Figure 4 the substantial change in the orientation of dislocation surfaces from Southeastern Calabria (SEC) to the Messina Straits (MS). A similar change is observed in the geostructural data (Figure 1). [8] Finally, we estimated the seismic strain tensor orientations in sectors SEC and MS using the fault-plane solutions available for the period 1988 2001 (n. 3 12, 14 15, 17 18 and 20 21 in Figure 2 and Table 1). The results are shown in Figure 3b where numbers 1, 2 and 3 indicate the orientations of the principal strain axes (e max, e int and e min ) in SEC with the blue and green zones showing the 95% confidence areas of e max and e min. Analogously, letters A, B and C indicate the orientations of e max, e int and e min in MS with the red and yellow zones showing the 95% confidence areas of e max and e min. In this figure we also show the orientations of e max (square), e int (circle) and e min (rhomb) relative to the strong events of 1908 in the Messina Straits (empty symbols) and 1978 in southeastern Calabria (full symbols). Minimum compressive strain derived from earthquakes of 1988 2001 in SEC (symbol 3 in Figure 3b) trends differently from s min (Figure 3a). This is because the faults which were active in southeastern Calabria during 1988 2001 are, in general, far from being orthogonal to the minimum compressive stress. Conversely, the minimum compressive strain derived from 1988 2001 earthquakes in MS (symbol C in Figure 3b) is close to s min (seismogenic faults are nearly orthogonal to s min ). It is worth noting in the same plot of Figure 3b that strain orientations relative to the strong events of 1908 and 1978 match well with strain orientations relative to seismicity recorded during 1988 2001 in the respective sectors of the Messina Straits and Southeastern Calabria. [9] In conclusion, using an improved dataset of focal mechanisms we obtained more accurate estimates of seismogenic stress and seismic strain orientations than available from the previous investigations performed in the same area of Messina Straits. This allowed us to carry out the first investigation in this area for discrimination of fault planes from auxiliary planes in the focal mechanisms and, then, to analyze the still unexplored local relationships between seismogenic stress, faults and seismic strain. A sketch representation of these findings is proposed in Figure 4. Rose diagrams report the orientations of seismogenic faultplanes identified in Southeastern Calabria (SEC) and Messina Straits (MS). The orientations of e min axes estimated in the respective sectors for strong earthquakes and background seismicity are indicated by full arrows of different size (larger for strong earthquakes). 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