These proofs may contain colour figures. Those figures may print black and white in the final printed book if a colour print product has not been planned. The colour figures will appear in colour in all electronic versions of this book. B978-0-12-812064-4.00008-6, 00008 CHAPTER 8 c0040 The Gloria Transform Fault—NE Atlantic: Seismogenic and Tsunamigenic Potential R. Omira*,†, M. Neres*,†, L. Batista*,† *Portuguese Institute for Sea and Atmosphere, IPMA, Lisbon, Portugal †Dom Luiz Institute, IDL, Faculty of Sciences, University of Lisbon, Lisbon, Portugal OUTLINE 1 Introduction 1 5 Discussion 8 2 Geodynamic Setting 3 6 Conclusions 10 3 The Gloria Transform Fault 4 Acknowledgments 10 4 Tsunamigenic Potential of Gloria References 10 Fault 6 s0010 1 INTRODUCTION p0055 The Azores-Gibraltar fracture zone (AGFZ) marks the western plate boundary between the Eurasian and Nubian plates. It extends eastwards from the Mid-Atlantic Ridge in the Azores toward the Strait of Gibraltar (Fig. 1). The central domain of the AGFZ, called Gloria Fault (GF), follows a prominent morphological feature. The GF was first mapped by Laughton et al. (1972) and presents a relatively slow, 5mm/year, interplate motion (Fernandes et al., 2003) dominated by dextral strike-slip faulting. Seismically, it is characterized by low seismic activity, although exhibiting a large variation between its eastern, central and western segments. The lack of seismic activity in the eastern GF segment suggests that the low-magnitude earthquakes are probably not detectable by the available seismic network. Transform Plate Boundaries and Fracture Zones 1 # 2019 Elsevier Inc. All rights reserved. https://doi.org/10.1016/B978-0-12-812064-4.00008-6 Duarte, 978-0-12-812064-4 Comp. by: SATHIVEL.N Stage: Revises1 Chapter No.: 8 Title Name: Duarte Date:28/7/18To protect Time:17:30:50the rights of Pagethe author(s) Number: 1 and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. These proofs may contain colour figures. Those figures may print black and white in the final printed book if a colour print product has not been planned. The colour figures will appear in colour in all electronic versions of this book. B978-0-12-812064-4.00008-6, 00008 2 8. THE GLORIA TRANSFORM FAULT—NE ATLANTIC: SEISMOGENIC AND TSUNAMIGENIC POTENTIAL f0010 FIG. 1 (A) Location of the study area in the Atlantic Ocean. The Azores-Gibraltar fracture zone (AGFZ) is the only fracture-transform zone in the Atlantic Ocean that constitutes a relatively discrete major plate boundary. (B) The AGFZ and its regional context. Red dots are epicenters of M>4 earthquakes from the International Seismological Cen- ter (ISC) for the period 1970–2017. Focal mechanisms from the database compiled by Custo´dio et al. (2016) (M>5). Red line locates the seismic refraction and multichannel seismic reflection profiles by Batista et al. (2017) (Fig. 2). SAFZ: South Azores fracture zone. (C) The Gloria Fault. Red dots:M>4 epicenters. Focal mechanisms (Custo´dio et al., 2016 database) for earthquakes M>5 (in gray) and M>6 (in black, labels indicate date and magnitude). White arrows: vec- tors from the relative NU-EU velocity field as modeled by the neotectonic model of Neres et al. (2016). Yellow labels: Gloria Fault segments and respective slip rates as inferred by Neres et al. (2016) (note that the model allows for fault slip and continuum strain rate). Alternatively, this could indicate that the GF in this area is locked and loading for the next big earthquake. This later hypothesis is supported by the occurrence of past large strike-slip earthquake events within the GF or in its vicinities. p0060 During the 20th century, three large earthquakes (M>7) took place along the GF or in its proximity (Fig. 1C). All have generated tsunamis that were recorded by the regional Duarte, 978-0-12-812064-4 Comp. by: SATHIVEL.N Stage: Revises1 Chapter No.: 8 Title Name: Duarte Date:28/7/18To protect Time:17:30:50the rights of Pagethe author(s) Number: 2 and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. These proofs may contain colour figures. Those figures may print black and white in the final printed book if a colour print product has not been planned. The colour figures will appear in colour in all electronic versions of this book. B978-0-12-812064-4.00008-6, 00008 2 GEODYNAMIC SETTING 3 tide-gauge network. The Ms 7.1, May 8, 1939, earthquake occurred near the Azores (see Fig. 1C) with an epicenter located close to the end of the GF (Buforn et al., 1988; Reis et al., 2017). It caused a small tsunami that was recorded by the Ponta Delgada (Azores) tide gauge (Reis et al., 2017). On November 25, 1941, an M 8.4 earthquake occurred in the central segment of the GF (see Fig. 1C, Udias et al., 1976; Baptista et al., 2016). It was considered the largest strike-slip event ever recorded (Bird and Kagan, 2004) until the April 11, 2012 Sumatra earth- quake, Mw 8.6 (Meng et al., 2012). The 1941 GF earthquake was followed by a tsunami recorded by sea-level stations in Portugal mainland, Morocco, Madeira, and Azores islands, with a maximum wave height of about 0.2m in Ponta Delgada (Azores) (Baptista et al., 2016). Another high-magnitude event, the M 7.9 May 26, 1975, earthquake had its source 100km south from the eastern segment of the GF (see Fig. 1C, Buforn et al., 1988; Kaabouben et al., 2008). This strike-slip earthquake also generated a small tsunami that was recorded by the Portuguese, Spanish, and the Moroccan tide-gauge networks (Kaabouben et al., 2008). The spatial distribution of these large events suggests that there is no unique discrete structure running from the Azores to Gibraltar that can be considered as the locus of seismic slip, but rather some degree of fault-segment delocalization accounting for the observed earthquake dispersion (Baptista et al., 2017). p0065 In this chapter, we provide, as a starting point, an overview of the geodynamic setting of the AGFZ. We then focus on the Gloria transform fault by describing the main geometry and interpreted kinematics of this major tectonic structure, as well as its main associated seismicity. Furthermore, we use numerical modeling of tsunami propagation to provide new insight into the tsunamigenesis of the GF, and discuss the implications of its tsunamigenic potential on the NE Atlantic, the Mediterranean, and Connected Seas Tsunami Warning System (NEAM TWS). s0015 2 GEODYNAMIC SETTING p0070 The AGFZ is a complex structure bordering Nubia and Eurasia in the Atlantic that underwent a complex tectonic evolution (Fig. 1A and B). It runs from the Azores plateau (in the west) to the Strait of Gibraltar (in the east) across three distinct morphotectonic do- mains: the Azores triple junction zone, the GF, and the southwest Iberian Margin (SWIM) (Fig. 1B). Within each domain, different kinematic and stress regimes occur, mostly deter- mined by the Nubia-Eurasia (NU-EU) relative motion (e.g., Fernandes et al., 2003; Serpelloni et al., 2007; Neres et al., 2016). p0075 The Azores triple junction zone is the westernmost domain of the AGFZ where the North American, African (Nubia), and the Eurasian plates meet, and where the NU-EU boundary shows a divergent relative motion (Fig. 1B). This domain comprises a zone of distributed transtensional deformation close to the Mid-Atlantic Ridge that connects to the NW-SE-oriented Terceira spreading ridge (e.g., Miranda et al., 2014, 2015). Along the Terceira Rift, extensional tectonics generate intense normal regime seismicity of low to moderate mag- nitude that accounts for most of the NU-EU relative motion, also suggested by the absence of seismicity at the South Azores fracture zone (SAFZ, see Fig. 1B). p0080 The easternmost domain of the AGFZ runs from the Strait of Gibraltar to the Madeira-Tore Rise across the SWIM (Fig. 1B). Here, Nubia and Eurasia show a convergent movement along Duarte, 978-0-12-812064-4 Comp. by: SATHIVEL.N Stage: Revises1 Chapter No.: 8 Title Name: Duarte Date:28/7/18To protect Time:17:30:50the rights of Pagethe author(s) Number: 3 and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. These proofs may contain colour figures. Those figures may print black and white in the final printed book if a colour print product has not been planned. The colour figures will appear in colour in all electronic versions of this book. B978-0-12-812064-4.00008-6, 00008 4 8. THE GLORIA TRANSFORM FAULT—NE ATLANTIC: SEISMOGENIC AND TSUNAMIGENIC POTENTIAL an apparently seismically diffuse plate boundary, but with some degree of earthquake clus- tering. Deformation is characterized by large-scale strain partitioning along differently orien- tated major dextral strike-slip and thrust faults, as attested by the diversity of focal mechanisms.