Deformed Holocene coastal notches reinforce the validity of earthquake slip histories implied by in-situ 36Cl exposure fault scarp dating.

Abstract

We report the first example where the timing of earthquake slip from in situ 36Cl cosmogenic exposure dating of an active normal fault scarp can be verified using independently 14C dated Holocene coastal notches which are deformed along the strike of the fault. We have remodelled 36Cl data from the active Pisia-Skinos normal fault, Greece, published by Mechernich et al. (2018), which indicates that the fault slip rate fluctuated through time. We model the expected coastal uplift and subsidence induced by slip on the fault using elastic half-space models and surface ruptures observed following the 1981 Pisia-Skinos earthquakes. Coastal uplift is constrained by elevation measurements of Holocene coastal notches that have previously been dated using 14C by Pirazzoli et al. (1994) and agree with time periods consistent with Holocene climate stability. We mapped the elevations and numbers of notches along the strike of the Pisia-Skinos fault, including measurements made underwater for locations where fault slip has submerged the notches below the present-day shoreline. We show that the spatial patterns and timing of uplift and subsidence from the notches agrees with the timing of periods of high slip associated with earthquake clusters and quiescence associated with anti-clusters from the slip histories derived from 36Cl data, and with the uplift and subsidence derived from elastic half-space modelling. In particular, where modelled subsidence is highest, Holocene notches that formed between 6-2 ka can be preserved but are submerged. Notches could form at this time because the 36Cl data show that the Pisia fault had entered a period of relative quiescence with a slip-rate of <0.1 mm/yr, accompanied by uplift from the offshore Strava fault. In contrast, rapid slip on the Pisia fault at 1.4 mm/yr between 2 ka and the present-day did not allow notches to form during this time period in the location of highest subsidence. Our example is the first that independently calibrates the timing of slip derived from 36Cl on a fault plane using 14C dates on a deformed coastline, and is consistent with the idea that slip-rate variations can be measured and should be incorporated into seismic hazard assessment.