Normal Faulting Along the Kythira-Antikythira Strait, Southwest Hellenic Forearc, Greece

Abstract

Upper-plate normal faults along forearcs often accumulate slip during >Mw 6 earthquakes. Such normal faults traverse the forearc of the Hellenic Subduction System (HSS) in Greece and are the focus of this study. Here, we use detailed field-mapping and analysis of high-resolution Digital Elevation Models (DEMs) to study 42 active normal faults on the islands of Kythira and Antikythira in the Aegean Sea. Onshore fault kinematic data are complemented by seabed bathymetry mapping of ten offshore faults that extend along the Kythira-Antikythira Strait (KAS). We find that normal faults in the KAS have lengths of ∼1–58 km and scarps ranging in height from 1.5 m to 2.8 km, accommodating, during the Quaternary, trench-orthogonal (NE-SW) extension of ∼2.46 ± 1.53 mm/a. Twenty-eight of these faults have ruptured since the Last Glacial Maximum, with their postglacial (16 ± 2 ka) displacement rates (0.19–1.25 mm/a) exceeding their Quaternary (≤0.7–3 Ma) rates (0.03–0.37 mm/a) by more than one order of magnitude. Rate variability, which is more pronounced on short (<8 km) faults, is thought to arise due to temporally clustered paleoearthquakes on individual KAS faults. When displacement accumulation is considered across the entire onshore fault network, rate variability between the two time-intervals examined decreases significantly (2.79 ± 0.41 vs 1.29 ± 0.99 mm/a), a feature that suggests that earthquake clustering in the KAS may occur over ≤16 ka timescales.

Quaternary E‐W Extension Uplifts Kythira Island and Segments the Hellenic Arc

“…Although we did not carry out systematic measurements, we corroborate the findings of Veliz‐Borel et al. ( 2022 ) that fault scarps generally evidence dip‐slip motion. Compared to the fault mapping of Veliz‐Borel et al.…”

“…We note that in their fault length/displacement analysis the Group A faults of Veliz‐Borel et al. ( 2022 ) generally have low displacements compared to their fault length, which would be consistent with our interpretation that the Group A fault sections with fault scarps are not connected. From the example cross‐section comparing our fault mapping with that of Veliz‐Borel et al.…”

“…Compared to the fault mapping of Veliz‐Borel et al. ( 2022 ) the main difference is that their Group A faults, large NW‐SE striking faults crosscutting the whole island, are much lengthier than we mapped them (Figure S3 in Supporting Information S1 ). The sections with postglacial fault scarps are similar for both maps, but we did not find clear evidence for active faulting in between those sections, despite our 2 m‐resolution DSM and fieldwork.…”

“…Whereas emerged marine sedimentary basins record the island's vertical motions during the Neogene (Van Hinsbergen et al., 2006 ), its landscape archives a recent uplift phase evidenced by coastline abrasion surfaces, marine terraces and steep river gorges. Holocene fault scarps and numerous historical earthquakes evidence active faulting (Papadopoulos & Vassilopoulou, 2001 ; Veliz‐Borel et al., 2022 ), but controversy remains on the processes that drove the initiation of the main fault systems, both on the island itself and within the regional context of the Hellenic Arc. Current propositions for dominant modes of normal faulting include arc‐parallel faulting (e.g., Veliz‐Borel et al., 2022 ), arc‐parallel combined with arc‐perpendicular faulting (e.g., Marsellos & Kidd, 2008 ) and N‐S oriented faulting (e.g., Armijo et al., 1992 ).…”

“…Holocene fault scarps and numerous historical earthquakes evidence active faulting (Papadopoulos & Vassilopoulou, 2001;Veliz-Borel et al, 2022), but controversy remains on the processes that drove the initiation of the main fault systems, both on the island itself and within the regional context of the Hellenic Arc. Current propositions for dominant modes of normal faulting include arc-parallel faulting (e.g., Veliz-Borel et al, 2022), arc-parallel combined with arc-perpendicular faulting (e.g., Marsellos & Kidd, 2008) and N-S oriented faulting (e.g., Armijo et al, 1992). Previous assessment of the vertical motion history of the island suggests a period of rapid Early Pleistocene uplift followed by a period of slow uplift (Van Hinsbergen et al, 2006).…”