Earthquake-triggered landslides and mudflows: Was this the wave that engulfed Ancient Helike?

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 km2 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.

Onde Mundoand pseudo‐Aristotle, a geoarchaeological analysis

“…The area uphill, south of the coast (for location see Figure 2), is mainly characterized by Neogene and Quaternary formations highly incised by streams, with a mean spacing of 5 km (see Figure 2 in Soter & Katsonopoulou, 2011). This hydrographic pattern, in combination with nearly E-W-trending normal faults with a moderate length (mostly below 10-15 km, mostly identified in Figure 2 as lineaments), leads to a dissected relief, subject to erosion of weak sediments, combined with localized landslides (see Koukouvelas et al, 2020).…”

“…The advent of geoarchaeological methods in the 1970s triggered an interest in the search for ancient Helike (Engel et al, 2016; Koukouvelas et al, 2020; Soter & Katsonopoulou, 2011), and this study continues today. In addition, the 373 BC earthquake is included in all earthquake catalogs (e.g., Ambraseys, 2009; Guidoboni et al, 1994; Papazachos & Papazachou, 1997), and it has been regarded as a model for coastal slumping (Ferentinos et al, 2015) and as a cause of destruction in archaeological levels, even in relatively remote areas (e.g., Tomlinson, 1996).…”

“…(2) “the great comet rose from south, about the same time with the Achaia earthquake and the wave invasion from the west.” A wave from the west may imply an earthquake and tsunami in the west coast of Achaia, in the Patras area or farther SW; Koukouvelas et al (2020) adopt the interpretation of “flooding from the west,” but do not refer to sea wave ].…”

“…The major stratigraphic anomaly is an about 4.5 m offset in a certain quasi‐isochronic level (Horizon C). This offset has been assigned in the past to fault reactivation in 373 BC, or to the consolidation of deltaic sediments (Engel et al, 2016), but in a more recent article, the hypothesis of any significant coastal effects during the 373 BC earthquake was rejected (Koukouvelas et al, 2020). This is correct, because this stratigraphic anomaly is of an order of amplitude expected even in aseismic active deltas.…”

“…[This ancient text is unclear, and two alternative translations can be proposed: (1) "the great comet rose from SW, about the same time with the Achaia earthquake and the wave invasion," and (2) "the great comet rose from south, about the same time with the Achaia earthquake and the wave invasion from the west." A wave from the west may imply an earthquake and tsunami in the west coast of Achaia, in the Patras area or farther SW; Koukouvelas et al (2020) adopt the interpretation of "flooding from the west," but do not refer to sea wave].…”