https://doi.org/10.1177/0959683620950389 The Holocene 1–16 © The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0959683620950389 journals.sagepub.com/home/hol Introduction According to a large number of ancient accounts, in 373 BC, a strong earthquake and accompanying enormous sea waves (tsu- nami) destroyed and submerged Ancient Helike, which was the capital of both the Ionian and the Achaean Dodekapolis in the region of ancient Achaea (Katsonopoulou, 2005). Half a century ago, between 1950 and 1973, the late archaeologist Marinatos made several attempts to locate Ancient Helike with inconclusive results. He considered that an earthquake and a tsunami demol- ished the city, but he suggested, in addition, that the city was probably partly buried by fluvial deposits (Marinatos, 1960). Over the last thirty years, the Helike Project has mounted a cam- paign using a multidisciplinary geoarchaeological methodology and archaeological excavations to locate Ancient Helike and to determine the exact causes which led to its destruction. The capital city of Ancient Helike was located, southeast of Aigion, in the Gulf of Corinth basin (Figure 1), a geotectonic domain well-known for its strong seismicity (Papazachos and Papazachou, 1997, 2003). However, since this historical seismic destruction, over 2500 years ago, there has been a striking lack of a similar catastrophe within the Gulf of Corinth basin. The closest analogue was on 26 December 1861, when a large earthquake damaged more or less the same area (Schmidt, 1879). Thus, a fascinating mystery remains whether the destruction of Ancient Helike was the consequence of an unanticipated event (a ‘black swan’, Aven, 2015) or the result of cascading disasters, when a trigger event led to a series of follow-on impacts (a ‘linear path of events in disasters’, Pescaroli and Alexander, 2015) that over- lapped and interacted during the 373 BC event. We consider as a ‘black swan’ an earthquake larger than seismologists think could happen, followed by a correspondingly enormous tsunami inun- dating the city. In a ‘cascade disaster’ we consider vulnerabilities that overlap and interact, with or without escalation points, and create secondary effects of greater impact than the primary natu- ral hazard (Pescaroli and Alexander, 2016). The ancient city was founded on the distal part of an alluvial fan from the small Katourlas ephemeral river near the modern village of New Keryneia (Figures 1b and 2). It was located on the hanging-wall block of the Helike Fault and the footwall block of the Aigion Fault (Figure 1b; Koukouvelas, 1998). So, uplift and subsidence in the area of the city depended on its proximity to each fault. Several previous papers focused on the hanging wall Earthquake-triggered landslides and mudflows: Was this the wave that engulfed Ancient Helike? Ioannis K Koukouvelas, 1 David JW Piper, 2 Dora Katsonopoulou, 3 Nikolaos Kontopoulos, 1 Sotirios Verroios, 1 Konstantinos Nikolakopoulos 1 and Vasiliki Zygouri 1 Abstract The destruction of Ancient Helike in 373 BC as reported by ancient Greek and Roman writers is inconsistent with modern evidence on the geological context. The classical view of a strong earthquake, similar to the 1817 M = 6.6 earthquake and followed by a giant tsunami wave that permanently inundated the ruined city does not stand up to modern scrutiny. Evidence for co-seismic slip on the Helike Fault at that time and for a corresponding tsunami have already been shown to be lacking, and the archaeological evidence shows that part of the site was reoccupied within 40 years. New observations on outcrops, excavated sites, and boreholes show that at least two mudflow deposits several metres thick of slightly gravelly mud overlie 4th c. BC archaeological remains on the Katourlas fan. Upstream, landslides are common in the 4 km 2 river basin and could have dammed the ephemeral Katourlas River. Relics of this dam are still recognised in the riverbed as a knickpoint. Temporary damming of rivers by earthquake-triggered landslides is a common phenomenon in northern Peloponnese. The destruction of Ancient Helike appears to have been a localised non-linear cascading series of disasters, with a strong earthquake followed by a destructive mudflow generated by breaching of a landslide-dammed lake in the Katourlas basin. Historical records of flooding from the west may record another landslide dam followed by a more watery flood in the Selinous River. We propose that it was mudflows and floods from inland rather than a tsunami from the sea that contributed to the final destruction of Helike. Keywords breaching dam, cascade disaster, engulfment of Ancient Helike, Greece, Gulf of Corinth, Helike area, mudflows Received 6 November 2019; revised manuscript accepted 20 June 2020 1 Department of Geology, University of Patras, Patras, Achaia, Greece 2 Natural Resources Canada, Geological Survey of Canada Atlantic, Bedford Institute of Oceanography, Dartmouth, NS, Canada 3 The Helike Society, Athens, Greece Corresponding author: Ioannis K Koukouvelas, Department of Geology, University of Patras, University campus, Patras, Achaia 26500, Greece. Email: iannis@upatras.gr 950389HOL 0 0 10.1177/0959683620950389The HoloceneKoukouvelas et al. research-article 2020 Research Paper