Spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake

A Pınar,Y Honkura,M Matsushima,Ş Barış,S.B Üçer,S Horiuchi,D Kalafat,N Sezgin,A Ito

doi:10.5047/eps.2009.12.001

Abstract

We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (Mw 7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al. (1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazı, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (σ1) and minimum (σ3) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Çınarcık cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock σ1 around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Gölcük segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20–25° counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.

Highlights

IntroductionAccommodating the translation and counterclockwise rotation between the Anatolian and the Eurasian plates (Fig. 1) is comprised of several fault segments
The 1500-km-long North Anatolian Fault zone (NAFZ)accommodating the translation and counterclockwise rotation between the Anatolian and the Eurasian plates (Fig. 1) is comprised of several fault segments
In the study reported here, we show that the aftershock activity is not in conjunction with the restraining bend; rather, it reflects stress perturbations in a large releasing area bounded in the south by the faults ruptured by the 1967 Mudurnu Valley earthquake and in the north by the faults ruptured by the 1999 Izmit earthquake

Summary

Introduction

Accommodating the translation and counterclockwise rotation between the Anatolian and the Eurasian plates (Fig. 1) is comprised of several fault segments Most of these were ruptured at different times during the twentieth century by major (Mw 7) earthquakes (Barka and Kadinsky-Cade, 1988; Barka, 1992, 1996; Stein et al, 1997; McClusky et al, 2000). The 1992 Landers earthquake (Mw 7.3) ruptured ve distinct fault segments with different geometries separated by releasing and restraining step-overs (Sieh et al, 1993) This multi-segment rupture pattern is a characteristic feature of the large NAFZ earthquakes generating complex waveforms and has been observed in the 1992 Erzincan (Mw 6.8) and 1967 Mudurnu Valley (Mw 7.1) earthquakes (P nar et al, 1994, 1996; Barka, 1996) and, most recently, in the 1999 Izmit and Duzce earthquakes (Gulen et al, 2002; Utkucu et al, 2003).

Objectives

Results

Conclusion