Palaeoseismicity and pottery: Investigating earthquake and archaeological chronologies on the Hajiarab alluvial fan, Iran

For millennia, humans have lived in locations that are highly vulnerable to large earthquakes, often out of strategic or cultural necessity and/or the proximity of these locations to resources necessary for survival. Despite the often catastrophic effects when large earthquakes occur, recent history reveals that human nature is to rebuild rather than relocate, implying that seismic activity is not a sufficient deterrent of population growth in tectonically vulnerable areas.

In order to investigate whether this was the case for ancient civilisations, and thus perhaps a fundamental tenet of human behaviour, a palaeo-earthquake history was developed for the active Cheskin and Ipak Faults in northwestern Iran, and compared with the well-resolved archaeological history of the nearby ‘Sagzabad cluster’ settlements of Zagheh (7170–6300 BP), Ghabristan (6215–4950 BP) and Sagzabad (4050–2350 BP).

Combining new geologic, geomorphic, and chronologic datasets revealed the presence of a fault-propagated anticline formed by large (Mw ∼ 6.5–7.0) earthquakes on a blind thrust fault that projects to seismogenic depth directly beneath the Sagzabad cluster settlement sites. Large earthquakes with a return period of <1000 y occurred on the Cheskin and Ipak Faults during human occupation of the Sagzabad cluster.

Gaussian cumulative distribution modelling indicates a >90% probability under most faulting scenarios that the energy release from these earthquakes would have been of sufficient magnitude to generate peak horizontal acceleration (PHA) values at the Sagzabad cluster in excess of likely threshold values for complete settlement destruction.

Poisson modelling assuming a time–displacement repeating model for earthquake recurrence indicates a 66 (42) % probability of one (two) earthquakes that would generate PHA ≥ 0.25 g occurring during occupation of Zagheh, a 79 (55) % probability for Ghabristan, and an 88 (65) % probability for Sagzabad.

Despite the near certainty that the residents of these Holocene settlements experienced large destructive earthquakes, the near-continuous history of occupation at this area suggests that early humans were not apt to relocate in response to earthquake activity. Environmental (e.g., alluviation, stream channel avulsion, climate change), cultural and/or political factors may have been more important drivers of settlement shifts and abandonment at the Sagzabad cluster of Iran.