Recent Surface Rupturing Earthquakes along the South Flank of the Greater Caucasus near Tbilisi, Georgia

Abstract

ABSTRACT Fault characterization is a critical step toward improving seismic hazard assessment in the Georgian Greater Caucasus but is largely absent from the region. Here, a paleoseismic trench near the capital city of Tbilisi revealed evidence for recurring surface rupture on a shallowly north-dipping thrust fault. The fault has broken through the overturned forelimb of a fault-propagation anticline that folds a sequence of soils and deposits. Stratigraphic relationships and radiocarbon dating of terrestrial gastropod shells corrected for “old carbon” age anomalies loosely constrain three surface-deforming earthquakes on this fault between ∼40 and ∼3 ka, with variable dip-slip displacements ranging between 0.35 and ∼3 m, and a cumulative displacement of 6.5 ± 0.85 m. Single event slips and recurrence intervals (11, 25, and 3 ka open interval) at this site demonstrate apparent slip rate variations of 3−7× over the last two earthquake cycles on the fault, which we attribute to possible rupture complexity involved in crustal thrust fault earthquakes. This study provides a structural and geochronologic template for future paleoseismic investigations in the Greater Caucasus while highlighting some of the challenges of conducting seismic source characterization in this region.