Multi-millennia slip rate relationships between closely spaced across-strike faults: Temporal earthquake clustering of the Skinos and Pisia Faults, Greece, from in situ ³⁶Cl cosmogenic exposure dating

This study investigates slip behaviour on overlapping, en echelon normal faults by analysing the slip histories of the Skinos and Pisia active normal faults over the past ∼20 kyrs using in situ ³⁶Cl cosmogenic dating. New ³⁶Cl data from the Skinos Fault and published Pisia Fault ³⁶Cl data were modelled, with both sample sites located within an overlap zone and separated by an across-strike distance of 1–2 km. Our analysis reveals fluctuating slip rates, with the two faults alternating between out-of-phase and simultaneous slip. The Pisia Fault exhibited a slip rate of ∼0.5–0.75 mm/yr from ∼20 ka to ∼9.6 ka, increasing to ∼1.25 mm/yr until ∼5.2 ka. It then slowed to ∼0.25 mm/yr or less until ∼2.0 ka, before accelerating again to ∼1.25–1.5 mm/yr to the present day. The Skinos Fault maintained a low slip rate of ∼0.25 mm/yr or less from ∼20 ka to ∼6.4 ka, before accelerating to ∼2.0–3.0 mm/yr, persisting to ∼1.0 ka or possibly the present-day. Comparing their slip histories, the faults show periods of simultaneous slip between ∼6.4 ka to ∼5.2 ka and ∼2.0 ka to ∼1.0–0.0 ka, and out-of-phase slip occurred between ∼9.6 ka and ∼6.4 ka, and from ∼5.2 ka to ∼2.0 ka. Out-of-phase behaviour on faults across strike has now been observed on faults spaced across-strike at distances of 1–2 km, 10–20 km, and ∼100 km, raising the question of why it occurs. Possible mechanism(s), including rheological fluctuations within fault/shear-zone structures linked between the brittle upper crust and viscous lower crust and stress interactions, are discussed to explain the out-of-phase and simultaneous slip behaviour.