Neotectonic and Paleoseismic Onshore-Offshore integrated study of the Carboneras Fault (Eastern Betics, SE Iberia) / Estudio integrado tierra-mar de la Neotectonica y Paleosismología de la Falla de Carboneras (Béticas Orientales, SE Península Ibérica)

Abstract

In the Southeastern Iberian Margin, the compression between the African and Eurasian Plates is characterized by a moderate seismicity and a slow NW-SE convergence (4.5-5.6 mm/yr). In southeastern Spain, this shortening is mainly absorbed by a left-lateral strike-slip fault system known as the Eastern Betics Shear Zone (EBSZ) characterized by a long recurrence activity. Preparedness in the face of future earthquakes entails the detection and the characterization of all the possible seismogenic sources in the region, including those that may not have ruptured in the historical period. This work seeks to characterize the most recent tectonic activity of the seismically silent but morphologically expressive Carboneras Fault Zone (CFZ), the longest and southernmost fault of the EBSZ, embracing both onshore and offshore portions of the fault. To this end, a multidisciplinary and multiscale study is carried out resulting in a valuable insight into the seismic potential of the CFZ. The results presented demonstrate that the CFZ has been active at least since the Late Miocene and has been continuously active during the Pliocene and Quaternary. The most recent paleo-earthquake detected is younger than AD 775, and thus, correlates in time with the historical AD 1522 Almería earthquake and tsunami. The CFZ is usually vertical despite showing a variety of structures in the first kilometres below the surface, represented by flower structures, pressure ridges and narrow vertical fault zones. The deformation at the southern end of the CFZ decreases towards the south and is gradually transferred to the Adra Ridge Fault and eventually to the Yussuf Fault, first through a fault shear zone, and then through the pervasive faulting zone at the Yussuf Fault horsetail splay. To the north, the CFZ ends abruptly in a triple fault junction with the Corredor de las Alpujarras Fault Zone and the Palomares Fault, and the deformation seems to be transferred to these structures. Fault segmentation is proposed in line with geomorphological and structural observations. Two first order segments are defined: the N047º/050º trending North Carboneras Fault (NCF) segment and the N059º/050º trending South Carboneras Fault (SCF) segment. This change in the fault trace orientation is thought to be caused by the interaction of the fault zone with an oblique shear zone in the SE part of the SCF. Seven second order sub-segments are differentiated along the NCF segment and 3 along the SCF segment. Maximum magnitudes of each of these segments and sub-segments are estimated on the basis of their length. First order segments yielded maximum magnitudes of Mw 7.4 +/-0.3 for the NCF segment, and Mw 7.0 +/-0.3 for the SCF segment. The worst case scenario would be the complete rupture of the CFZ, which would produce a maximum magnitude of Mw 7.6 +/-0.3. A minimum strike-slip rate of 1.3 mm/yr was estimated for the Northern Carboneras Fault segment along the Quaternary. Dip-slip results strongly suggest that the dip-slip component of the fault is one to two orders of magnitude lower than the strike slip component. The recurrence period is proposed to be about 1.1 ka, according to the slip-rate (>1.3 mm/yr) and the slip per event (≤1.5 m), which is one order of magnitude lower than the recurrence period calculated from the events observed in trenches. The CFZ, which is characterized by a lack of historical and instrumental seismicity, was demonstrated to be a seismogenic structure capable of generating large magnitude earthquakes. Therefore, its seismic potential should be taken into account in the seismic hazard assessment of the Iberian Peninsula.

El SE de la Península Ibérica está caracterizado por la convergencia entre las placas Africana y Euroasiática, con un acercamiento lento de dirección NO-SE (4.5-5.6 mm/a) y una sismicidad moderada. En el SE de la Península Ibérica, esta convergencia es absorbida por un sistema de fallas de salto en dirección sinistrorsas llamado el Sistema de Cizalla de las Béticas Orientales (SCBO). Con tal de estar preparados para futuros terremotos, es imprescindible detectar y caracterizar todas las posibles fuentes sísmicas de la región, incluyendo aquellas que no hayan roto durante el periodo histórico y que por lo tanto no se tengan en cuenta en los catálogos sísmicos.En el presente trabajo se presenta un estudio sobre la actividad tectónica más reciente de la Zona de Falla de Carboneras (ZFC), una estructura sismogénicamente silenciosa pero morfológicamente expresiva, y una de las más largas del SCBO. Para ello, se ha desarrollado un estudio multidisciplinar a diversas escalas, integrando el análisis de la parte emergida y la parte sumergida de la falla. Además de caracterizar la estructura de la falla en los primeros kilómetros y la interacción con otras estructuras de la zona, en este estudio se demuestra que la ZFC, que en un principio no se tenía en cuenta en los catálogos sísmicos por carecer de sismicidad instrumental e histórica, es una estructura sismogenética. Su actividad ha sido continuada durante el Plioceno y el Cuaternario y es capaz de producir terremotos de gran magnitud (w 7.0-7.6 +/-0.3) y por lo tanto, su potencial sísmico debe ser tenido en cuenta en los análisis de peligrosidad sísmica de la región. El estudio integrado tierra-mar llevado a cabo ha permitido obtener un conocimiento más realista del potencial sísmico de la ZFC y calcular sus parámetros sísmicos (1,3 mm/a de tasa de desplazamiento lateral mínima, 1,1 ka de periodo de recuerrencia y paleoterremoto más reciente detectado posterior a AD 775), los cuales contribuirán a las base de datos de estructuras activas permitiendo un análisis de la peligrosidad sísmica más realista para la Península Ibérica.