LATE HOLOCENE ACTIVITY OF ERZURUM FAULT ZONE IN EASTERN ANATOLIA, TURKEY

The Erzurum Fault Zone (EFZ) is an active NE trending 80 km-long left lateral strike slip fault located near the City of Erzurum, Eastern Turkey. The EFZ is divided into northern and southern parts connected by a thrust zone within a restraining bend. The northern part of the EFZ consists of two subparallel segments: the 35-km-long eastern segment, and the 25-km-long western segment. Tectonic geomorphic features include fault scarps, hanging valleys, springs, and saddles, as well as, offset valleys and cultural features. We completed paleoseismic trenching for the Baku-Tbilisi-Ceyhan pipeline on the eastern and western segments. Three trenches excavated across a sag pond on the eastern segment revealed: (1) Holocene sag pond deposits faulted against bedrock on the east margin of the pond; and (2) faulted Holocene colluvial deposits on the west margin. Stratigraphic and structural relations suggest three or possibly four events in the Late Holocene. The two observed faults display left lateral reverse oblique motion that verge toward one another. The contractional fault geometry and stratigraphic relationships indicate the fault forms a local restraining step-over and that the sag pond may be a topographically controlled feature within the restraining step-over. We also excavated a 205-m-long trench across the western segment. Based on fault terminations, two events were recognized within Holocene colluvium. This fault has a left lateral normal oblique sense of motion. Geological relationships suggest the slip rate on the eastern segment is several times larger than the western segment. The last event on the eastern segment may correlate with the 1850 or 1859 earthquakes.

Paleoseismic data indicate the northern part of the EFZ has produced several late Holocene earthquakes with horizontal displacements of 1.5 to 3.0 m. Based on these displacements and the length of the fault, the northern EFZ is capable of producing damaging earthquakes greater than Mw 7.1 with recurrence intervals of 1000 to 3000 years. Youthful tectonic geomorphology within the central thrust zone (near Ezurum) and along the southern part of the EFZ indicate that these adjacent fault segments may also be active. The location of the EFZ within the City of Erzurum poses a significant seismic hazard to the city.