The Seismic History of the Pisia Fault (Eastern Corinth Rift, Greece) From Fault Plane Weathering Features and Cosmogenic³⁶Cl 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 36 Cl 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.The most frequently applied technique for earthquake analysis on limestone bedrock fault scarps is exposure dating using cosmogenic 36 Cl (e.g., Zreda & Noller, 1998). A regular and dense distribution of 36 Cl samples can MECHERNICH ET AL. 4266

Schmidt hammer and terrestrial laser scanning (TLS) used to detect single‐event displacements on the Pleasant Valley fault (Nevada, USA)

“…Similar discoloured bands have been identified on other fault scarps (e.g. Giaccio et al, 2003; Mechernich et al, 2018). It follows that these bands will be more weathered than the subsurface fault scarp and, depending on the length of time (i.e.…”

“…Stewart (1996) measured the surface roughness of the Kaperelli and Pisia faults in the eastern Gulf of Corinth, both of which ruptured to the surface and exposed fresh scarp faces in the 1981 Corinth earthquakes. Mechernich et al (2018) revisited this fault using a multi-proxy approach and identified six to eight weathering bands (or stripes) that could be correlated with earthquakes. Mechernich et al (2018) revisited this fault using a multi-proxy approach and identified six to eight weathering bands (or stripes) that could be correlated with earthquakes.…”

“…Where normal faults rupture to the surface in bedrock, fault scarpsthe surface expression of one or more earthquakesmay be preserved, allowing direct exposure-age dating of the fault surface and modelling of earthquake displacements and recurrence intervals (Bosi et al, 1993;Zreda and Noller, 1998). Zreda and Noller, 1998 ;Benedetti et al, 2002 ;Schlagenhauf et al, 2010 ;Benedetti and van der Woerd, 2014 ;Mouslopoulou et al, 2014 ;Mechernich et al, 2018). Zreda and Noller, 1998 ;Benedetti et al, 2002 ;Schlagenhauf et al, 2010 ;Benedetti and van der Woerd, 2014 ;Mouslopoulou et al, 2014 ;Mechernich et al, 2018).…”

“…The use of cosmogenic 36 Cl on limestone scarps for these objectives is well established and provides some of the most detailed paleoseismic records, with the best age precision, available on any fault (e.g. Zreda and Noller, 1998 ;Benedetti et al, 2002 ;Schlagenhauf et al, 2010 ;Benedetti and van der Woerd, 2014 ;Mouslopoulou et al, 2014 ;Mechernich et al, 2018). The difficulties associated with this technique are that it can be prohibitively time consuming and expensive to prepare and analyse samples at more than one scarp transect and at sampling intervals required to resolve event ages.…”

“…Stewart's (1996) results showed that fault surfaces above the 1981 slip patches were generally rougher than the fresh exposure; however, because of the heteroscedasticity in the data, caution was urged in interpreting paleoseismic histories based on surface roughness beyond historical and MRE events. Mechernich et al (2018) revisited this fault using a multi-proxy approach and identified six to eight weathering bands (or stripes) that could be correlated with earthquakes. Tucker et al (2011) found a positive correlation between pit depths and cosmogenic exposure ages on the Magnola fault scarp in the central Apennines, Italy.…”