Determining the Through-Going Active Fault Geometry of the Western North Anatolian Fault Through Stress Modeling

Abstract:

The North Anatolian Fault (NAF) is a seismically active 1200 km long dextral strike-slip fault part of an east-west trending dextral shear zone (NAF system) between the Anatolian and Eurasian plates. This shear zone widens to the west, complicating potential earthquake rupture paths and highlighting the importance of understanding the geometry of active fault systems. West of the town of Bolu - the NAF bifurcates into the northern and southern strands, which converge and are linked through the Mudurnu Valley, then diverge to border the Marmara Sea. The westward continuation of these two fault traces is marked by further complexities in potential active fault geometry, particularly in the Marmara Sea (northern strand), and the Biga Peninsula (southern strand). We evaluate potential active fault geometries for both strands by comparing stress models of various fault geometries in these regions to a record of focal mechanisms and inferred paleostress from a lineament analysis. For the Marmara region, two of the three possible geometries matched the maximum horizontal stress (σ_H) orientations determined from a record of focal mechanisms; however, only one represented the northern and southern sidewalls associated with the principal zone of deformation of the developing Marmara basin. This suggests that it is the most likely representation of the active through-going fault geometry in the region. In the Biga Peninsula region, the active geometry of the southern strand has the southern component approaching and intersecting the northern component through a linking feature in a narrow topographic valley. This geometry was selected over two others as it overlaps the σ_H orientation determined from focal mechanism data and a lineament analysis. Additionally, this geometry does not develop a prominent mis-oriented NE-SW stress feature observed in the model results of the other two geometries, otherwise absent in the focal mechanism data or inferred from a lineament analysis.