Reassessing seismicity and seismic hazard in offshore areas: The case of the Alboran Sea (western Mediterranean)

8ᵗʰ International Colloquium on Historical Earthquakes, PalaeoMacroseismology and Seismotectonics 17-20 September 2023 - Lixouri, Greece Bulletin of the Geological Society of Greece, Sp. Publ. 11 Ext. Abs. 00031 Reassessing seismicity and seismic hazard in offshore areas: The case of the Alboran Sea (western Mediterranean) Hector Perea1, María José Jímenez2, Lucia Lozano3, José Luis Sánchez Roldán4, Octavi GómezNovell5,6, Ariadna Canari1, Mariano García-Fernández2, Julián García Mayordomo7 (1) ICM, CSIC, Barcelona, Spain, [email protected] (2) MNCN, CSIC, Madrid, Spain (3) Spanish Seismic Network, Instituto Geográfico Nacional, Madrid, Spain (4) Faculta d de Ciencias Geológicas, Universidad Complutense de Madrid, Madrid, Spain (5) Università degli Studi "G. d'Annunzi o" Chieti, Pescara, Italy (6) Facultat de Ciències de la Terra, Universita t de Barcelona, Barcelona, Spain (7) IGME, CSIC, Madrid, Spain. Introduction The potential occurrence of large earthquakes and their capacity to trigger devastating tsunamis poses a significant geohazard that raises crucial societal concerns. Such events possess the ability to disrupt submarine structures, impact coastal regions, and have far-reaching consequences on global economies, thereby posing a risk to local populations (Bilham, 2010). Recent history serves as a stark reminder of the catastrophic nature of these occurrences, exemplified by the colossal Sumatra earthquake and tsunami in 2004 within the Indian Ocean (Mw8.7) or the Tohoku-Oki earthquake and tsunami that struck the northwest region of Japan in 2011 (Mw9.0–9.1). Nonetheless, it is vital to recognize that earthquakes with moderate to large magnitudes (Mw>5.5) in regions characterized by low to moderate tectonic deformation, and featuring long intervals between events, can also exert a substantial impact. The Alboran Sea situated in the Western Mediterranean is one such area of concern. Consequently, in recent decades, there has been a notable expansion in the interest to identify the active faults and to enhance our understanding about their seismic activity and probable aftermaths. The main objective of STRENGTH project is to characterize the structure, development, and associated seismicity of the large fault systems in the Alboran Sea and its implication on seismic hazard and risk on coastal areas. Knowledge on the location and characterization of active structures in this region has been greatly improved in recent studies. Nevertheless, the potential association of offshore seismicity to specific faults has not yet been undertaken in a systematic way. The STRENGTH project aims to link a more detailed and precise characterizations of active faults in the Alboran Sea to offshore earthquakes to understand better the main earthquake sources in the area. To this respect, we are revisiting and analyzing the historical earthquake records and macroseismic information, and relocating the instrumental earthquakes registered in the last two decades, with a non–linear probabilistic approach jointly with high–resolution local/regional velocity models. The study focuses on: a) characterizing the 3D–structure of the fault systems and their evolution, and identifying the active faults that may have generated large–earthquakes; b) associating moderate to low magnitude seismicity and seismicity clusters to specific faults; and c) modeling ground motions in coastal areas based on fault characterization and recorded seismicity. The expected results will contribute to an improved assessment of the seismic hazard and risk in the Alboran Sea Region. 114 Figure 1. Map of Alboran Sea showing the main fault systems and the seismicity (1916–2023) from the IGN catalog. The Eastern Alboran Sea: Seismological and Geological Settings The Alboran Sea is a Neogene basin formed by crustal extension related to the subduction system in the Gibraltar Arc. At present, left-lateral and right-lateral strike-slip faults trending NE-SW and WNW-ESE, respectively, accommodate part of the strain related to the NW-SE convergence (4-5.5 mm/yr) between the African and Eurasian plates (DeMets et al., 2015). Consequently, the Alboran Sea shows a remarkable seismic activity, mainly concentrated along what is known as the Trans-Alboran Shear Zone (De Larouzière et al., 1988), but it does not clearly delineate the present-day European-African plate boundary (Palano et al., 2015). This seismicity (Fig. 1) is mainly characterized by low to moderate magnitude events, usually with Mw<5.5 (e.g., Buforn et al., 1995; Stich et al., 2010). Nevertheless, large and destructive earthquakes have occurred in the region, such as the 1522 Almería (IEMS98 VIII-IX; Spain), the 1790 Oran (IMSK IX-X; Algeria), the 1804 Dalias (IEMS98 VIII-IX; Spain), the 1910 Adra (IEMS98 VIII and Mw6.1; Spain), the 1994 and 2004 Al-Hoceima (Mw6.0 and 6.4, respectively; Morocco), and the 2016 Al–Idrissi (Mw6.4; Morocco) events (Martínez Solares and Mezcua, 2002; Buforn et al., 2017; Gràcia et al., 2019). In previous studies, the main fault systems in the Alboran Sea have been identified and mapped (Fig. 1). Among them, the Carboneras fault is a left-lateral strike-slip fault that trends NE-SW and stretches for about 150 km, including a segment onshore and two offshore segments (Moreno et al., 2016). To the south of Carboneras fault, there is the left-lateral strike-slip Al-Idrissi fault, which is composed of three segments, namely north, central, and south, with an approximate direction of NNE-SSW and a length of 180 km (Gràcia et al., 2019). Finally, the Yusuf and Alboran Ridge faults, potentially the largest structure in the Alboran Sea, extend for approximately 230 km (Perea et al., 2018; Gràcia et al., 2019; Gómez de la Peña et al., 2022). The Yusuf Fault comprises two main segments, which have created a pull-apart basin where they overlap. Initially a right-lateral strike-slip fault, it arches towards its western termination, where a reverse component becomes dominant. This zone of inverse faulting corresponds to the northern termination of the Alboran Ridge thrust fault. 115 Figure 2. Maps showing the relocated seismicity occurred in the north Alboran Sea between 2000–2021. a) Carboneras fault area with the clusters occurred in 2008, 2010 and 2012. b) North–South faults area. The location of the areas in the Alboran Sea is shown in Figure 1. Hypocenter Relocation of Moderate Seismic Activity An accurate determination of the hypocentral location of this seismicity is a key point for a better knowledge of the active tectonics and may contribute to better trace and characterize active faults and to image their rupture area and, hence, to improve seismic and tsunami hazard assessments. One of the main goals of the STENGTH project is to analyze the recent seismicity in the Alboran domain by performing a high–precision hypocenter relocation of shallow earthquakes recorded in the area during the last two decades. More precise hypocentral locations will allow us to image well–defined lineaments, dominant strike directions and clustering near active structures and to better determine causative faults within the known fault systems. For this purpose, we perform a two–step relocation process: a) an absolute location using a non–linear probabilistic algorithm (NonLinLoc) (Lomax et al., 2014) and a regional 3-D P-wave tomography velocity model for the Alboran-Betic-Rif system (El Moudnib et al., 2015); b) a relative location by means of doubledifference location algorithm (HypoDD) (Waldhauser, 2001) for selected significant seismic sequences. This approach will allow us firstly to account for differences in the propagation of seismic waves in the heterogeneous Alboran domain, providing maximum likelihood hypocentres and complete information on location uncertainties and, secondly, to improve relative hypocentral locations and clustering along specific faults. We applied these methodologies to the North Alboran region, to study the seismicity for the period 20002021. Despite the limitations on the station coverage, we relocated a subset of well-constrained M≥2.0 earthquakes located near the North-South faults (NSF) and the Carboneras Fault offshore segment (CF) (Fig. 2). Results for the CF area, display three clear clusters, corresponding to seismic sequences occurred in 2008, 2010 and 2012, respectively. The northernmost cluster shows a clear NW-SE lineament and shallow-depth distribution with hypocenters grouped up to 6 km depth. The other two clusters, are located in the SW edge of the CF; one lies above the CF in the channel zone and shows a very vertical concentration (up to 10 km depth) that would be in accordance with the known geometry of the fault; and the other cluster, to the SE of the previous one, also shows some clustering and verticality, although it does not appear to be directly related to the CF. On the contrary, the seismicity in the NSF zone is widely dispersed throughout the area and does not show a spatial arrangement indicating the presence of clusters, nor very clear alignments. However, relocated earthquakes are distributed along a NE-SW band about 20 km wide, up to 15 km depth, and slightly dipping to the SE. Further comparison with the known faults in the area would be necessary. Modelling faults for comprehensive seismic hazard assessment The characterization of active faults, although key in earthquake hazard modelling, is based on general broad models. Usually, geological information is essential to delineate areal zones of uniform seismicity, but a detailed understanding of the behavior of active faults is very seldom incorporated in the quantitative seismic 116 hazard assessment (SHA). Based on the results described above, the source model for SHA will be developed by incorporating the precise determination of location of hypocenters, a review on historical seismicity and the characterization of the main fault systems. STRENGTH will develop an updated regional model with both, improved delineation of seismic sources and improved activity parameters of faults (fault slip rates, earthquake rates) based on the newly available geological data. The resulting earthquake rates as modelled through SHERIFS code (Chartier et al., 2019) will be tested against the earthquake catalogue of the region. A seismic hazard assessment methodology based on Monte Carlo simulation (Garcia-Fernandez et al., 2018) and using a Pan-European ground motion model (García-Fernández et al., 2019) validated through recent accelerogram recordings in the region will provide acceleration exceedance probability curves for selected main coastal urban environments and critical coastal infrastructures. 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