Field evidences of secondary surface ruptures occurred during the 20 February 1956 Eskisehir earthquake in the NW Anatolia

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OCAKOĞLU F. , Acikalin S.

JOURNAL OF EARTH SYSTEM SCIENCE, vol.119, no.6, pp.841-851, 2010 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 119 Issue: 6
  • Publication Date: 2010
  • Doi Number: 10.1007/s12040-010-0057-y
  • Page Numbers: pp.841-851


Surface rupture and source fault of the 20 February 1956 Eskisehir earthquake have been a matter of debate that potentially contributes towards the understanding of the active deformation and seismic risk in the highly populated NW Anatolia. Field observations on the two fault segments (namely Kavacik and Uludere faults) in the north of the Eskisehir graben revealed evidences of co-seismic surface rupture and mass movements during the Eskisehir earthquake. Surface rupture was observed as a 2.5 m wide, a 100 m long and ca. 20 cm deep asymmetric depression in the Gumuslu valley, 2 km east of the Uludere village. A trench dug on this depression confirms a prominent high-angle south dipping fault (dominantly left lateral strike slip) and two faint north-dipping antithetics as clear noticeable shear zones in organic-rich thick soil. Mass failures are particularly observed in spring depressions filled with loose torrent and carbonaceous material in front of the fault scarp. Some rock slides of several tens of meters in size that obviously require significantly high ground shaking were also developed on steep fault scarps. The orientation of the principal stress tensor as deduced from the surface rupture of the Eskisehir earthquake displays clear inconsistency with the geometry of prominent faults in the area. We concluded that this disagreement may be explained by a curved surface rupture. The western and eastern tips of this rupture are EW trending and the probable NW-running part in the middle would correspond to the bounding zone between two right-stepping faults.