Asymmetric Structural Properties Across the 1943 Rupture Zone of the North Anatolian Fault: a Possible Indication for a Preferred Rupture Direction

The North Anatolian Fault (NAF) experienced in the 20th century migration of large ruptures to the west that was attributed to successive failures of fault segments due to stress transfer from failed neighboring segments. Intriguingly, the 1943 earthquake did not nucleate in the region of largest stress increase imposed by the 39 and 42 events from the east. Instead, it nucleated at the opposite end of the rupture, 250 km to the west, and propagated unilaterally back toward the 39 and 42 ruptures. This behavior may reflect a preferred propagation direction associated with contrasting elastic properties across the NAF in the region. A persistent preferred propagation direction should produce, over many large earthquakes, asymmetric damage across the fault. To test whether there is a preferred rupture direction along the 43 segment, we performed systematic field studies of rock damage across the NAF. The results show a consistent asymmetric pattern of rock damage at different scales and forms at 3 localities along the 43 rupture. Near Ladik, the currently active fault is located N of the long term lithological boundary of the NAF. It is stable in its current location and free of geometrical irregularities, as attested by highly localized geomorphic expression. In stream cuts and within a >60 m wide bedrock trench exposure of the fault-zone, structural features such as secondary strike slip faults, gouge-zones and sag-pond deposits are considerably more abundant S of the fault core. The fault core itself correlates well with the geomorphic fault trace and has an asymmetric structure with a 2 m wide belt of highly sheared rocks bound on their N side by a m-wide gouge. The Principal Slip Surface (PSS) at this site is on the northernmost side of the gouge. In Bademci, 140 km west of Ladik, the fault-zone is >140 m wide and includes 3 primary gouge zones. The northernmost gouge correlates with the current geomorphic fault expression. In a trench across the active gouge, we found 5 separate gouge layers, with the northernmost one accommodating the PSS. At a third location 80 km farther west, near Celtikci, we exposed in a trench system a >140 m-wide fault zone composed of two superposed structures. The first includes the long term geologic fault embedded within a fairly symmetric distribution of secondary faults. The second includes the currently active fault 30 m N of the geologic fault, with substantially more active secondary faults cutting young sediments S of the current slip zone. This distribution of secondary faults suggests the development of an asymmetric damage and shear pattern in the recent geologic history. Overall more damage and shear were found S of the active slip zone in all locations. Additional observations of damage asymmetry associated with several geomorphic signals are discussed in a companion work (Yildirim et al., this meeting). An excess of damage on the south side of the fault is compatible with west-to-east preferred rupture direction along this rupture zone, providing a possible explanation to the anomalous rupture behavior during the 43 earthquake.