Traces of the Late Bronze Age Santorini and AD 365 tsunami events in the sedimentary record of Crete : Reconstruction of the regional tectonic geomorphology andpalaeotsunami history
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Abstract
The Eastern Mediterranean belongs to one of the most seismically active regions in Europe. On 21 July AD 365, a strong earthquake occurred offshore, close to southwestern Crete and struck the eastern Mediterranean world. An associated tsunami, confirmed by detailed historical reports, destroyed many coastal settlements and infrastructure between the Levante in the east and the Adriatic Sea in the northwest. Already during the 17th cent. BC, the Late Bronze Age, another disastrous tsunami took place, triggered by the eruption of the Santorini volcano. Based on the results of numeric tsunami simulation and field evidence from the surrounding regions in the eastern Mediterranean, both tsunami events are supposed to have affected the island of Crete. Crete Island, due to its unique position in the middle of the Hellenic fore-arc, is prone to tsunami risk origin from the Hellenic subduction zone and the volcanic arc including the still active Colombo-complex near Santorini.
So far, fine-sedimentary archives along the southwestern and northern coast of Crete have been rarely investigated so that evidence of palaeotsunami deposits has been completely missing. In this study, a multi-proxy approach including sedimentological, geochemical, geochronological and microfaunal methods was conducted in three major study sites in order to search for onshore coastal sedimentary archives that functioned as fine-sediment traps and document Crete’s palaeotsunami history. The main study aims were (I) to systematically look for adequate near-coast fine-sediment archives on Crete, (II) to search for the imprint of the LBA Santorini and the AD 365 tsunami events and of further extreme wave events using a multi-proxy methodical approach, (III) to investigate and reconstruct the palaeogeographical and palaeoenvironmental evolution of the study sites since the mid-Holocene, (IV) to reconstruct the late-Holocene tectono-geomorphological development of the study sites against the background of the seismotectonically induced uplift of western Crete associated with the AD 365 earthquake.
Fine-sediment geoarchives at the study areas of Sougia, Palaiochora, Pirgos and Geropotamos(southwestern and northern Crete) were found to record up to ten extreme wave events (EWE) some of which seem to be associated with known palaeotsunami events that hit Crete. Deposits of the AD 365 tsunami were encountered in the geoarchives of Sougia, Palaiochora and Pirgos indicating, that this event did not only affect the west and southwest of Crete but also hit wide areas along the northern coast of the island.
At the Geropotamos River site, the sediment record contained two tsunamite candidates. These results implicate that tsunami water masses entered the river mouth and propagated at least 1 km upstream triggering mass failures at a presently inactive external bank position. The younger EWE signal appears to have been caused by the AD 365 tsunami event while the older EWE is probably associated with the LBA Santorini eruption. Candidate layers of the LBA Santorini tsunami were also detected near Pirgos. Here, the tsunami layers’s age was reconstructed based on linear regression equations calculated from the Pirgos age-depth model to 1675-1516 cal BC. This age estimate corresponds well with the LBA Santorini eruption.
Combining the Pirgos sedimentary archive with the Geropotamos River valley record revealed evidence of ten major EWE that hit Crete’s northern coast during the past ~ 7500 years. This leads to a statistical recurrence interval of ca. 750 years for large EWE impact. Moreover, the results of this study illustrate that seismically triggered tsunamis, such as the AD 365 tsunami, extended much farther in their range than assumed and the tsunami hazard along the north coast of Crete has so far been underestimated. Depending on the tsunami wave propagation, the local bathymetry and the local geomorphological setting, tsunami waves may also enter river mouths and extend the tsunami risk inland by causing widespread inundation along the river banks.
In summary, the southwestern and northern coasts of Crete offered a variety of adequate Holocene near-coast geoarchives covering information on the coastal evolution over the last ~ 7500 years. From a tectono-geomorphological point of view, the studied sedimentary records revealed that Holocene vertical crust movements on Crete, triggered by the seismic cycle of the Hellenic subduction zone during different periods, had a strong impact on the coastal evolution. During the long-enduring interseismic phase that started in the 7th mill. BC, the study areas have been predominantly subject to subsidence. The AD 365 earthquake, however, induced an abrupt reversion of the direction of the crust movement, and western Crete was coseismically uplifted by several meters.