Surface Ruptures of the 17 August and 12 November 1999 İzmit and Düzce Earthquakes in Northwestern Anatolia, Turkey: Their Tectonic and Kinematic Significance and the Associated Damage

by Atilla Aydin and Dogan Kalafat

Abstract

The 17 August İzmit (Kocaeli) and 12 November Düzce earthquakes (M_w 7.4 and 7.1, respectively) represent the most recent two of a series of large earthquakes along the North Anatolian Fault, a highly active right-lateral strike-slip fault that crosses the Anatolian Peninsula in Turkey from one end to the other. The first earthquake rupture apparently initiated at a small step-over along a relatively inconspicuous fault strand following the southern coast of the İzmit Bay and propagated to the west and to the east for a distance of 125–150 km. The western segment enters the Bay in both directions after attaining the largest surface slip of about 5 m at the town of Gölcük. The eastern extension of the surface rupture associated with the İzmit earthquake is composed of five major segments with the maximum slip magnitudes from 3 to 4, 4.5, 0.5, 1.4, and 0.1 m from west to east, respectively. The segment with 0.5 m slip trends N70°W, the segment with 1.4-m slip trends N70°E, and the easternmost segment with minor slip trends east–west again.

The surface manifestation of the Düzce earthquake is an extensively segmented rupture trace with dominant transtensional deformation. It has a normal slip component of more than 3.7 m and a right-lateral slip close to 1.4 m along the western part that showed minor rupture during the first earthquake. The central part has a transpressional character with about 1-m reverse slip and about 3-m right-lateral slip. The highest right-lateral slip (nearly 5 m) occurs at the central part. The eastern part of the rupture displays a typical strike-slip deformation with less than about 3 m right-lateral slip and minor local vertical slip that exhibits changing polarity.

The varying orientations of the rupture segments reflect the fault mosaic of the broader region and the interaction between neighboring faults. The temporal and spatial characteristics of the rupture segments, as well as the significance of the nearly 3-month time period between the first and second earthquakes, pose highly challenging mechanical problems regarding the rupture initiation, propagation, and termination processes. The main surface rupture zone, although unusually narrow (2.5–6 m), left a path of death and destruction.

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