Evolution and complexity of the seismogenic Düzce fault zone (Turkey) depicted by coseismic ruptures, Plio-Quaternary structural pattern and geomorphology

Abstract

We investigated the area struck by the November, 12, 1999, Mw 7.1 earthquake that ruptured the Düzce segment of the North Anatolian Fault Zone (NAFZ) three months after the August, 17, 1999, Mw 7.4 Izmit earthquake. On the basis of 1:25.000 scale field survey and aerial photo interpretation we identified a simple 1999 coseismic fault trace (CFT) and an older complex fault system (PQFS), involving a wider zone of deformation. Overall, we recognized two different sections of the Düzce segment: a western section, where the coseismic fault trace has a staircase trajectory and reactivated part of the older fault system; an eastern section, where the coseismic fault trace shows a straight trajectory and cross-cuts the older and complex fault system. The Düzce fault sections may represent different stages of the segment evolution. In fact, the collected data suggest the tendency of the fault to simplify its trace with time and to evolve from a complex towards a simpler mature trace, as a mechanically more favorable setting. The western section of the Düzce fault segment splays out from a restraining bend of the Izmit (Karadere) fault segment of the NAFZ, and forms a releasing fault wedge. By comparing the coseismic surface deformation field with the observed long-term morphology it is clear that the present landforms and setting are the result of 1999-type coseismic deformation repeating through several seismic cycles. Because of the mechanical interaction of the faults in the release junction, the western section of the Düzce fault undergoes a lower normal stress that may justify its complexities and earlier stage of evolution at the surface. The boundary at the surface between the two portions of the Duzce fault is not only a surface characteristic but it separates at depth a portion of fault plane characterized by a big single asperity, to the east, from a portion of plane with lower slip, to the west. Thus the peculiar arrangement of the Izmit (Karadere) and Düzce fault segments may permanently control the difference in behavior of the two portions of Duzce fault and furthermore control rupture propagation and fault loading. Under this light, the Izmit/Düzce release fault junction (1) may produce an unfavorable setting for the build up of asperities in the western part of the Düzce segment also in the future and (2) could have delayed the propagation of the 1999 August Izmit rupture on the Düzce segment that ruptured on November 1999 along the asperity of its eastern section. These results highlight that the surface geological data contain the potential for integrating and completing the information for imaging structures also at a seismogenic depth.