(mediterranean sea). - CSIC-Digital

Resúmenes de la 1ª Reunión Ibérica sobre Fallas Activas y Paleosismología, Sigüenza, España (2010)

STRUCTURE AND POTENTIAL SEISMOGENIC SOURCES OF THE OFFSHORE BAJO SEGURA FAULT ZONE, SE IBERIAN PENINSULA (MEDITERRANEAN SEA). LOOKING FOR THE SOURCE OF THE 1829 TORREVIEJA EARTHQUAKE Estructura y fuentes sismogenéticas potenciales en la cuenca marina del Bajo Segura, SE de la Península Ibérica (Mar Mediterráneo). Buscando la fuente del terremoto de Torrevieja de 1829 H. Perea (1), E. Gràcia (2), Rafael Bartolomé (2), Claudio Lo Iacono (2) and E. Masana (3) (1) Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa. Edif. C6, Piso 2º, Campo Grande, 1746-016 Lisboa. [email protected] (2) Unitat de Tecnologia Marina-CSIC, Centre Mediterrani d’investigacions Marines i Ambientals, Psg. Marítim de la Barceloneta, 37-49, 08003 Barcelona. [email protected], [email protected], [email protected] (3) RISKNAT, Dept. Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona. c/ Martí Franquès, s/n. 08028 Barcelona. [email protected] Resumen: La Zona de Falla del Bajo Segura (BSFZ) corresponde a la parte terminal del al Zona de Cizalla de las Béticas Orientales y se extiende hacia el Mediterráneo. Mientras que la zona emergida de la BSFZ ha sido largamente estudiada, la zona sumergida muestra una casi total falta de información desde el punto de vista de la tectónica. Durante el crucero EVENT-SHELF se obtuvieron diez perfiles de sísmica de alta resolución con el objetivo de cartografiar rupturas superficiales en el fondo marino y las estructuras localizadas en el subsuelo de la zona sumergida de la BSFZ. El análisis de los datos ha revelado la presencia de trece discordancias sedimentarias en la secuencia cuaternaria. Además, esta misma secuencia está afectada por pliegues y fallas, y en algunos casos las fallas deforman la superficie del fondo marino, cosa que implica una actividad muy reciente. Podría ser alguna de estas fallas la fuente del terremoto de Torrevieja del 1829? Palabras clave: fallas activas; tectónica activa; geología marina; Bajo Segura Abstract: The Bajo Segura Fault Zone (BSFZ) is the northern terminal splay of the Eastern Betic Shear Zone and extends further into the Mediterranean Sea. Whereas the onshore zone of the BSFZ has been extensively studied, the offshore zone has almost a complete lack of information from a tectonic point of view. During the EVENT-SHELF Cruise ten high-resolution seismic profiles were obtained with the objective of mapping the seafloor ruptures and the sub-seafloor structures of the offshore area of the BSFZ. The analysis of the data has revealed the presence of thirteen sedimentary unconformities in the Quaternary sequence. Moreover, this sequence is affected by folding and faulting, and in some cases the faults reach the sea-floor, implying their very recent activity. Could some of these faults be the source of the 1829 Torrevieja earthquake? Key words: active faults, active tectonics, marine geology; Bajo Segura.

The present-day crustal deformation of the SE Iberian margin is driven mainly by the NW-SE convergence (4-5 mm/yr) between the African and Eurasian plates (Argus et al., 1989; DeMets et al., 1990; Kiratzi and Papazachos, 1995; McClusky et al., 2003). This convergence is accommodated over a wide deformation zone, composed by the Gulf of Cadiz, the Betics and Rif cordilleras and the Alboran sea. Moreover, all this area show significant seismic activity (Figure 1), being the biggest earthquakes mainly located on the Gulf of Cadiz (1755 Lisbon earthquake, Mw 8.5, or 1969 earthquake, Ms 7.9) and the Rif mountains (1980 El Asnam earthquake, Ms 7.4, or 2004 Alhoceima earthquake, Mw 6.4). Figura 1. Mapas de localización de la Cuenca del Bajo Segura y de las principales estructuras descritas en tierra y de los perfiles de sísmica realizados durante la campaña EVENT-SHELF (líneas amarillas). Figure 1. Location maps of Bajo Segura basin and of the main geological structures onshore and the seismic profiles obtained during the EVENT-SHELF cruise (yellow lines).

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Figura 2. Perfiles de sísmica de alta resolución. Localización de los perfiles en la figura 1. Figure 2. High-resolution seismic profiles. Situation of the profiles on figure 1.

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The Neogene and Quaternary faulting activity in the SE Iberian Margin is dominated by a large left-lateral strike-slip system of sigmoid geometry referred to as the Eastern Betic Shear Zone (EBSZ), stretching over more than 450 km from Alacant to Almería (De Larouzière et al., 1988; Doblas et al., 1991; Silva et al., 1993). The northern terminal splays of the EBSZ correspond to the Bajo Segura fault zone (BSFZ) that extends further into the Mediterranean Sea (Mauffret et al., 1992; Alfaro et al., 2002) (Figure 1). This fault zone has associated some important instrumental seismic activity characterized by small to moderate

earthquakes (Lopez Casado et al., 1987). Even though, moderate to large historical earthquakes have affected the zone, being the 1929 Torrevieja earthquake (IMSK=X) the largest (Muñoz et al., 1984; Muñoz and Udías, 1991; Delgado et al., 1993). The onshore area of the BSFZ (Figure 1) has been extensively studied and it is characterized by active structures (faults and folds) displaying a transpressive behavior since the Plio-Pleistocene and resulting in positive relieves and subsiding zones (Montenat, 1977; Silva et al., 1993; Somoza, 1993; Alfaro, 1995). However, the offshore area shows an almost complete lack of information from the tectonic point of view (Mauffret et al., 1992; Alfaro et al., 2002). In September 2008, the marine geophysical cruise EVENT-SHELF was carried out onboard the Spanish RV Garcia del Cid. The main goal was to map the seafloor ruptures and the sub-seafloor structures of the offshore area of BSFZ using swath bathymetry and high-resolution seismics (Spaker GeoSpark 6kJ). A total of 10 regional high-resolution seismic profiles were acquired along and across the BSFZ (Figure 2). These profiles were acquired in platform areas close to the coast, have a time window of 0.5 s and their longitudes are variable. The majority of the profiles have a NNE-SSW to N-S orientation and two of them have a W-E orientation crossing perpendicularly the other profiles. This fact allows to easily correlate the different discontinuities observed between the different profiles. The analysis of the acoustic and seismic data show that the main structures observed onshore have their continuation offshore, and that some of the faults and folds related to the BSFZ are active. The carefully study and processing of these data will allow us to localize the present active structures and determine its fault parameters and seismogenic behavior. All this information would contribute to a better understanding of the EBSZ kinematics and to improve the seismic hazard studies in the area. To have an idea of the age of the sedimentary sequence crossed by the seismic profiles, the oil exploratory well 410 Torrevieja Marina C-1 has been projected on profile Ev-Shelf_32 (Figures 2 and 3). The well shows that the base of the Quaternary sedimentary succession is located between 505 and 614 m (Figure 3). Considering velocities of 1600 and 1700 m/s, the base of the Quaternary may be located between 0.6 and 0.75 s TWTT. Therefore, taking into account that the time window of the profiles is 0.5 s the sedimentary succession cross by the well and observed on the profile is Quaternary in age. From the analysis of the different high-resolution seismic profiles, thirteen major Quaternary sedimentary unconformities have been identified along the sedimentary sequence. These unconformities are characterized by erosive contacts, downlaps, onlaps and toplaps. Some of these discontinuities may be related to the Quaternary sea level changes, but others could be related to the tectonic activity observed on the zone and expressed by the presence of folds and faults that are affecting the Quaternary sedimentary sequence.

Figura 3. Columna estratigráfica del pozo de exploración petrolera 410-Torrevieja Marina C-1 y su proyección en el perfil Ev-Shelf_32. Ver texto para más información. Figure 3. Stratigraphic sequence corresponding to the oil exploratory well 410-Torrevieja Marina C-1 and its projection in profile ev-shelf_32. See text for more information.

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Generally, the analyzed seismic profiles show the sedimentary basement (late Miocene, Alfaro et al., 2002) intensely folded, but some of them show that the folding is also affecting a number of the Quaternary unconformities identified. However, the folding process seems to be smoothed and in some places sealed by the uppermost unconformities. Therefore, depending on the relation between the sedimentation and strain rates, it could be argued the decrease or termination of the activity in some structures. The sedimentary basement is highly faulted coinciding with the main deformation zones and some of these faults are also affecting the Quaternary unconformities (Figure 4). Nevertheless, an important number of the faults affecting the Quaternary sediments are sealed by an unconformity that seems to be the last (orange in profiles) in a large part of the study area. However, some of these faults are offsetting the reflectors corresponding to the youngest sedimentary deposits and in some cases they are reaching the seafloor (Figure 4). This fact implies a very recent activity of these faults.

Figura 4. Detalle del perfil de sísimica multi-canal de alta resolución Ev-Shelf_32 donde se muestran fallas que cortan las formaciones cuaternarias mas recientes y deforman claramente la superficie. Figure 4. Detail of the high-resolution multi-channel seismic profile Ev-Shelf_32 showing that the faults are offsetting the most recent Quaternary formations and clearly deform the surface. DeMets, Ch., Gordon, R.G., Argus, D.F. & Stein, S. (1990). Current plate motions. Geophys.Jour.Int., 101, 425-278. Doblas, M., López, J., Hoyos, M., Martín, C. & Cebriá, J.M. (1991). Late Cenozoic indentation/escape tectonics in the East Betic cordilleras and its consequences on the Iberian Foreland. Estud.Geol., 47, 193-205. Kiratzi A. & Papazachos, C.B. (1995). Active crustal deformation from the Azores triple junction to the Middle East. Tectonophysics, 243, 1-24. López Casado, C., Estévez, A., Pina, J.A. & Sanz de Galdeano, C. (1987). Alineaciones sismotectónicas en el sudeste de España. Ensayo de la delimitación de fuentes sísmicas. Ser. Estud.Geol., 6, 5-39. Mauffret, A., Maldonado & A., Campillo, A.C. (1992). Tectonic framework of the Eastern Alboran and Western Algerian Basins, Western Mediterranean. Geo-Mar.Lett., 12, 104-110. McClusky, S.R., Reilinger, S., Mahmoud, D., Sari, B. & Tealeb, A. (2003). GPS constraints on Africa (Nubia) and Arabia plate motions. Geophys.Jour.Int., 155, 126.138. Montenat, C. (1977). Les bassins Néogènes et Quaternaires du Levant d’Alicante à Murcie (Cordillères Bétiques Orientales, Espagne). Stratigraphie, paléontologie et évoluion dynamique. PhD Thesis, Laboratoire Geologique, University of Lyon, 69, 345 pp. Muñoz, D., Udías, A. & Moreno, E. (1984). Reevaluación de los datos del terremoto de 1829 (Torrevieja). En: Sismicidad de la Península Ibérica. Asociación Española de Ingeniería Sísmica, Madrid, pp.38-41. Muñoz, D. & Udías, A. (1991). Three large historical earthquakes in Southern Spain. En: Seismicity, seismotectonics and seismic risk of the Ibero Maghrebian Region, vol.8. IGN, pp. 175-182. Silva, P.G., Goy, J.L., Somoza, L., Zazo, C. & Bardají, T. (1993). Landscape response to strike-slip faulting linked to collisional settings: Quaternary tectonics and basin formation in the Eastern Betics, southern Spain. Tectonophysics, 224, 289-303. Somoza, L. (1993). Estudio del cuaternario litoral entre Cabo de Palos y Guardamar (Murcia-Alicante). Las variaciones del nivel del mar en relación con el contexto geodinámico. Instituto Español de Oceanografía, 12, 237 pp.

The analysis of the high-resolution seismic profiles obtained on the BSFZ allows as to conclude that the Quarternary sedimentary sequence has been and still is being tectonically deformed. This is corroborated by the presence of folds and faults affecting this sequence, that in some case are deforming the seafloor. The main question that still remains to answer is, could be one of these faults that reach the seafloor the source of the 1829 Torrevieja earthquake? Aknowledgements: EVENT project is funded by the Ministerio de Educación y Ciencia (CGL2006-12861-CO202). Hector Perea is researcher at IDL-UL under contract no. 3/2010/LAB_IDL co-financed by FCT and FEDER.

Referencias bibliográficas Alfaro, P. (1995). Neotectónica en la Cuenca del Bajo Segura (Cordillera Bética oriental). PhD Thesis, University of Alicante, 69, 217 pp. Alfaro, P., Delgado, J., Estévez, A., Soria, J.M. & Yébenes, A. (2002). Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain). Marine Geology, 186, 337-349. Argus, D.F., Gordon, R.G., DeMets, Ch. & Stein, S. (1989). Closure of the Africa-Eurasia-North America plate motion circuit and tectonics of the Gloria fault. Jour.Geophys.Res., 94(B5), 5585-5602. De Larouzière, F.D., Bolze, J., Bordet, P., Hernández, J., Montenat, C. & Ott d’Estevou, P. (1988). The Betic segment of the lithospheric trans-Alboran shear zone during the late Miocene. Tectonophysics, 152, 41-52. Delgado, J., Giner, J.J., López Casado, C. & Auernheimer, C. (1993). Análisis de la respuesta del suelo en intensidades. Aplicación al terremoto de Torrevieja. En: Problemática ambiental y desarrolo (Ortiz Silla, ed.). V Reunión Nacional de Geología Ambiental Ordanación del Territorio, II, pp. 627-636.

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