Greece and Turkey Shaken by African tectonic retreat

Earthquakes are a consequence of the motions of the planet’s tectonic plates, yet predicting when and where they may occur, and how to prepare remain some of the shortcomings of using scientific knowledge to protect human life. A devastating Mw 7.0 earthquake on October 30, 2020, offshore Samos Island, Greece was a consequence of the Aegean and Anatolian upper crust being pulled apart by north–south extensional stresses resulting from slab rollback, where the African plate is subducting northwards beneath Eurasia, while the slab is sinking by gravitational forces, causing it to retreat southwards. Since the retreating African slab is coupled with the overriding plate, it tears the upper plate apart as it retreats, breaking it into numerous small plates with frequent earthquakes along their boundaries. Historical earthquake swarms and deformation of the upper plate in the Aegean have been associated with massive volcanism and cataclysmic devastation, such as the Mw 7.7 Amorgos earthquake in July 1956 between the islands of Naxos and Santorini (Thera). Even more notable was the eruption of Santorini 3650 years ago, which contributed to the fall of the Minoan civilization. The Samos earthquake highlights the long historical lack of appreciation of links between deep tectonic processes and upper crustal deformation and geological hazards, and is a harbinger of future earthquakes and volcanic eruptions, establishing a basis for studies to institute better protection of infrastructure and upper plate cultures in the region.

We report fault slip data from the Büyük Menderes and Gediz Grabens that show similar slip systems as those activated during the Mw 7.0 Samos earthquake. Along the Kuşadası fault system where the graben extends into the Aegean Sea towards Samos Island (Figs. 2b, 3b,c), both north and south dipping faults are present. The exhumed footwall rocks are composed mainly of middle to upper Miocene continental carbonate rocks and Jurassic to Cretaceous marble (Fig. 3a), which record past seismogenic slip events along the brecciated fault planes, with well-preserved slickenlines showing the fault-slip directions. These kinematic indicators generally suggest that the nearly N-S extension and can be divided into 2 groups, including a relatively older set with oblique transtensional motions, superposed by a younger, historically active set, whose slip events record nearly dip-slip motions (Figs. 2a, 3d; Table 2). Structural analyses near this fault, including on the Söke fault system and the northern margin along the Büyük Menderes Graben were conducted (Fig. 2a), where we have confirmed similar kinematic features with the main Kuşadası fault strand.
The western extent of Kuşadası Fault strand and its connection with Samos Fault, on the other hand is not well established. There are seismic studies around the area, but they all include the narrow belt confined to the Turkish side of the bay, therefore they are inconclusive in terms of connecting the Kuşadası and Samos Systems. The study of Aksu et al. 47 presents shallow seismic images. Their Fig. 10 shows there is a N dipping E-W normal fault between Samos and Kuşadası faults which is suitable to be the connection between these two. Another offshore study around the area suggests that Samos fault extends towards the Küçük Menderes Graben in the east 48 . Neither of these models present data for the exact linkage. However, based on the consistency between kinematic features of the Kuşadası fault and the focal mechanism solutions of the Samos earthquake, and our perspective on the area, we suggest that the fault responsible for October 31 Mw 7.0 earthquake is very likely to be the western continuation of the Kuşadası Fault, showing that the spectacular continental rift system of the Büyük Menderes graben extends offshore, in the more highly-extended Aegean system.. This connection needs further examination by offshore studies.
It is important to assess the past activity on active faults. By understanding the characteristics of the fault, mitigation of related geological hazards can be done to prevent damage. Mozafari et al. 43 conducted a paleoseismology study on this fault, showing slip events on this system at 15, 8.4 and 3.6 ka, with slip magnitudes of 0.7, 0.9, and 3.1 m on the north dipping Kalafat segment, and events at 7.9, 3.4, 2.0 ka on the south dipping Yavansu strand with vertical displacements of 0.6, 3.5 and 2.6 m (M 6.5-7.1). This yields an approximate overall recurrence interval of 2400 years for slip events of ~ 1.9 m along the western on-land end of the Kuşadası fault system, but that interval is becoming shorter with time.

Historical earthquakes and volcanic eruptions
The Aegean Sea and western Anatolia region represents one of the most seismically active and most rapidly extending continental regions on the planet [3][4][5]19,21,26 (Fig. 4). Earthquakes, volcanic eruptions, and tsunami in the region have repeatedly caused major damage throughout history. If the relationship between plate tectonics, seismic and other geological hazard is not more greatly appreciated, history will repeat itself. Thus, we link the relatively small lesson of the Mw 7.0 Samos earthquake with some similar, yet more devastating events, some of which have changed history.
A large number of ancient cites were built close to the active fault zones, and destroyed by strong historical earthquakes 49 . For instance, the city of Priene (Fig. 4), one of the earliest Ionian settlements within the western Büyük Menderes Graben, was destroyed by a destructive earthquake in 350 BCE and rebuilt 8 km away. However, the new Priene later suffered great damages throughout history 50 . The ancient city of Ephesus (Fig. 4), an ancient Greek city on the coast of Ionia, was built in the tenth century BCE also had destroyed by earthquakes several times, while the earliest destruction can be traced to 17 CE. Relatively good historical earthquake data has been accumulated and compiled by Stucchi et al. 51 (https:// www. emidi us. eu/ SHEEC/), where readers are referred for further information.
Aegean earthquakes are characterized by both normal and transtensional events, with 29 earthquakes of M > 6.0 within 250 km of the Samos event in the past 100 years. These include the M 6.8 1955 Söke earthquake, M 7.7 July 1956 earthquake between Nazos and Santorini, and an associated M 7.2 aftershock. The1967 Skyros (M 6.6), 1969 Alaşehir (M 6.7), and 1970 Gediz (M 6.9) earthquakes are other significant earthquakes of the Aegean and western Anatolia extensional system [52][53][54][55] . The most recent, significant event before the Samos Earthquake was a M 6.6 earthquake on July 20, 2017 near Bodrum, on the southwest coast of Turkey (https:// earth quake. usgs. gov/ earth quakes/ event page/ us200 09ynd/ execu tive). These events show that faults of this area can generate earthquakes of larger magnitude than most other extensional systems around the world.
The South Aegean active volcanic arc formed along the convergent plate boundary of the northward subducting African plate underneath the active margin of the Eurasian plate 6,17 , with some historically volcanoes, such as Methana volcano at the western edge of the volcanic arc, Milos and Santorini volcanos in the central part, and Nisyros at the eastern edge near the Turkish coast, which have caused tremendous catastrophes, perhaps rising to some of the most significant in human history, together with the earthquakes 59,60 .

Linking deep subduction, slab rollback, and upper plate extension
The African lithosphere is subducting to the north in the Hellenic and Cyprus trenches, but the trench and upper plate are retreating as the slabs roll back to the south. Coupling between the upper and lower plates 16 causes the upper plate to extend (Fig. 5), forming the Aegean extensional province, exposed in the Cycladian Islands of the www.nature.com/scientificreports/ Aegean, and continuous with the Western Anatolia extensional province on the Turkish mainland (Fig. 5). We relate the October 30 earthquake to the link between the subducting and retreating African plate, to the pulling apart and extension of the upper plate (Fig. 5). We show this link through our studies of the active faults of the region, and the historical record of seismicity and volcanism, and note the significance of the link between understanding risks of tectonic processes, and hazards, with historical events including some of the most cataclysmic in recorded history 59,60 . The Samos (Néon Karlovasion) Mw 7.0 earthquake occurred at a critical location for testing the links between deep tectonic subduction, the rollback of the trench system, and how it is coupled with and linked to the fragmentation of the upper plate into numerous small microtectonic blocks. While catastrophic, the earthquake presents an opportunity and demonstrates the societal need to better understand the active surface deformation and driving mechanisms of deformation in the Aegean and western Anatolia. The Aegean and Western Anatolia represent a unique place on Earth to elucidate geologic processes changing from subduction, to the ongoing plate collision, and the transition in between, which in this case has fragmented the upper plate into numerous smaller microplates. It shows that immediately before continental collision, the overriding plate may be broken into small platelets that move laterally with respect to each other, displacing parts of formerly continuous geological features, and that this complication may be difficult to recognize in parts of the Tethyan and other orogenic belts where collision is more advanced. The rollback process of the African plate forms the geometrically complex normal fault systems, and earlier transtensional systems, responsible for the spectacular upper crustal extension and active seismicity in this area. These relationships also explain why in collision orogens, there are often small preserved "pockets" of extensional basins, fault systems, and sedimentary and volcanic deposits. Tectonic escape is moving most of Anatolia westward along the North and East Anatolian and associated fault systems, with predominantly strike-slip tectonics in eastern Anatolia, and extensional tectonics in the west. However, the boundaries between these regimes are not yet well-defined. It is imperative that further studies are conducted in various aspects including the fault systems geometries, paleoseismic records, earthquakes, and tectonic activity evaluation, to better understand the links between tectonic processes, upper crustal deformation, and better protection of society from devastating geological hazards.

Conclusions
• The October 30, 2020 Samos Earthquake (M 7.0) was a major event mainly related to upper plate extension caused by the rollback of the subducting African Plate. • Earthquake solutions and analysis of active fault planes in western Anatolia demonstrate this spectacular extension, and show that the modern extension was preceded on many faults by oblique extension and strikeslip motions, perhaps reflecting a change in tectonic setting from sideways escape from the Africa-Arabia collision with Eurasia, to the pure extension related to slab rollback of the African plate, and the retreat of the Hellenic trench. • The Kuşadası and Samos faults have similar trends, kinematic features and they are formed in the same extensional system, therefore, they are probably linked in offshore areas. Further seismic studies should be conducted to improve this connection. • Extension of the upper plate driven by slab rollback produces relatively large magnitude earthquakes, compared to other areas of continental extension, globally. • The region is prone to continuing earthquake and volcanic hazards from historical times to modern days.
• Further detailed studies are needed in this area to better understand and predict earthquake frequency, possible locations, and to establish better building codes to protect people's lives and property.  Table 2 for details).

Data availability
The datasets analyzed during the current study are available from the corresponding author on reasonable request.

Figure 5.
Tectonic model explaining the forces that triggered the Samos earthquake. The African plate is subducting to the north in the Hellenic and Cyprus trenches, the trench and upper plate are retreating as the slabs roll back to the south, exposed in the Cycladian Islands of the Aegean. The upper plate forearc is coupled to the rolling-back lower plate, which are retreating together, breaking the upper plate into numerous small microplates, forming the Aegean and Western Anatolian extensional provinces in the upper plate.