Surface Indications of Aseismic Creep Along the Gerede Segment of the North Anatolian Fault Zone (Turkey)

The Gerede segment of the North Anatolian Fault (NAF) was reactivated in 1944 with a M 7.3 earthquake. In early 1970s, it was realized that a wall across the fault in Ismetpasa was broken and this deformation was attributed to aseismic creep on this part of the NAF. Previous studies have been mainly concerned with the creep rate and a few researchers recently comment about the length of creeping fault. Despite the abundance of investigation, field evidence for the aseismic creep is limited with the well-known broken wall in Ismetpasa. However, detailed field investigations along the Gerede segment showed other field evidences that could be indications of aseismic creep.

Total length of the 1944 rupture is about 160 km but aseismic creep is taking place in the eastern part. The Gerede town is located almost in the center of the segment and there is a restraining bend about 7 km west of Gerede. Man made structures located on the trace of the fault between this restraining bend and eastern termination of the segment are being deformed in various amounts and types. Wooden poles which were originally aligned vertical in late 1980s are tilted where they cross the fault zone. A concrete wall, built in late 1970s across the fault in Gerede, is dextrally offset up to 15 cm. There are several settlements along the fault and almost all walls of houses are cracked which are located on the trace of the fault. A cemetery is located on the fault zone (about 56 km east of Gerede) and tombs are cracked and rotated in different amounts. These observations suggest that the aseismic creep is going on for a distance of about 85 km on this part of NAF.

The penultimate earthquake was the 1668 event on this part of NAF. A field border which is younger than 1668 but older than 1944 was offset about 6±0.5 m east of the creeping wall in İsmetpaşa. Considering that the coseismic offset on the 1944 surface rupture was about 3.5±0.5 m, this observation suggests that aseismic creep took place before the 1944 event and the present deformation is not post-seismic deformation of the 1944 event. Field studies suggest that the average creep rate is about 6±2 mm/yr along the Gerede segment. Despite this, this segment of the NAF is still capable of generating large earthquakes since at least 70% of the yearly slip (i.e., 20-25 mm/yr) still accumulates on the fault, as was demonstrated by the 1944 event.