The Seismic History of the Pisia Fault (Eastern Corinth Rift, Greece) From Fault Plane Weathering Features and Cosmogenic 36Cl Dating

Abstract

The deformation of the eastern Corinth rift (Greece) is distributed along several E-W trending active normal faults. Here the 25-km-long Pisia fault experienced up to 150 cm of coseismic displacement during the 1981 Alkyonides earthquake sequence (M = 6.7, 6.4, 6.3). Using terrestrial laser scanning, coupled with analyses of color changes, lichen colonization, and karstic features, we identify differentially weathered horizontal stripes on the exposed Pisia fault plane. The stripe boundaries occur at scarp heights of 1.10, 2.05, 2.85, 3.60, 4.15, and 5.15 m, with two additional possible boundaries at 3.10 and 4.65 m (~±0.1 m, respectively). This indicates that six to eight paleoearthquakes have exhumed the fault plane in a series of distinct coseismic slip events. A vertical profile of cosmogenic 36Cl measurements is used to constrain age models of the exhumation. The results imply that, in addition to the last earthquake of 1981 (EQ1), exhumation events occurred at ~2.0 kyr (EQ2), ~3.1 kyr (EQ3), ~4.5 kyr (EQ4/4a,b), ~6.0 kyr (EQ5), and ~7.3 kyr (EQ6/6a,b), with modeled age uncertainties of ~0.7 kyr. Bayesian modeling provides a middle and late Holocene slip rate of 0.5–0.6 mm/yr (last 7.3 ± 0.7 kyr), while the upper part of the 8.45-m-high fault plane was exhumed at a higher rate of 0.8–2.3 mm/yr (7.3 ± 0.7 kyr to 10.2 ± 1.9 kyr). This slip rate variability suggests an increased seismicity or larger slip events during the early Holocene.