Seismotectonic setting of the Karadere–Düzce branch of the North Anatolian Fault Zone between the 1999 Izmit and Düzce ruptures from analysis of Izmit aftershock focal mechanisms

Abstract

We investigate aftershock focal mechanisms along the eastern part of the Izmit M_w = 7.4 August 17, 1999 rupture zone during the time period August 22, 1999–October 17, 1999. Two spatial clusters of aftershock activity are analyzed representing the Karadere Fault (KF) and the Düzce Area (DA). Based on an aftershock hypocenter catalogue restricted to events with horizontal and vertical errors < 2 km, we determine fault plane solutions for 221 events. The high number of focal mechanisms at the eastern Izmit rupture zone could be determined only due to the low magnitude-detection threshold of the seismic network and allows to resolve the local deformation pattern with unprecedented precision. Focal mechanisms along the Karadere Fault allow us to identify dominantly dextral strike–slip mechanisms with normal faulting components on NE–SW trending fault planes. Focal mechanisms in the Düzce Area predominantly exhibit NE–SW extensional normal faulting but also a substantial part of strike–slip faulting. Further subdivision of the data set slightly decreases for the misfit for deeper (z > 10 km) events. North and east of the easternmost end of the Karadere Fault we observe a high variance in stress field orientation correlated with lower b-values. While the Karadere Fault reflects a predominant dextral strike–slip regime with normal faulting components, the Düzce Area further to the East that also hosted the forthcoming M_w = 7.2 mainshock 87 days after the Izmit earthquake can be subdivided into a dominantly NE–SW extensional normal faulting regime below the Düzce Basin (DB) and a first-order strike–slip regime along the western Düzce Fault (DF). We conclude that the Düzce Basin was set under tension by the Izmit rupture and partly released the slip deficit by extensional faulting on Karadere Fault parallel to the coseismic displacement. At the same time this area and in particular the Düzce Fault that bounds the Düzce Basin to the south reflects mostly strike–slip events representing a major asperity along the North Anatolian Fault Zone (NAFZ) before initiating the Düzce rupture 87 days after the Izmit event.

Introduction

The North Anatolian Fault Zone (NAFZ) is one of the seismically most active transform faults worldwide extending along 1600 km from eastern Anatolia to the Aegean Sea (Fig. 1a). The direction of slip corresponds well with the Global Positioning System (GPS) derived 25–30 mm yr^− 1 westward motion of the Anatolian Block with respect to Eurasia (McClusky et al., 2000, Reilinger et al., 2006). The M_w = 7.4 August 17, 1999 Izmit earthquake exhibits a maximum surface displacement of > 5 m at the Sapanca–Akyazi segment (Barka et al., 2002, Fig. 1b). Average coseismic slip obtained from teleseismic waveform inversion is 2.5 m (Tibi et al., 2001) and 2.9 m from regional strong-motion records (Bouchon et al., 2002). Moreover, Synthetic Aperture Radar interferometry (InSAR) data inversion (Wright et al., 2001) and GPS data by Reilinger et al. (2000) show a maximum displacement of 5 m and 3.5 m near the mainshock epicenter, respectively. Delouis et al. (2002) identified four individual segments along the Izmit rupture by using combined GPS, SAR, teleseismic and strong-motion data.

Rupture propagation towards the east ended near Düzce where a large earthquake occurred 87 days after the Izmit event (November 12, 1999 M_w = 7.2). Fault plane solutions of both mainshocks reflect right-lateral strike–slip faulting on a vertical (Izmit) and steeply northward dipping (Düzce) fault plane (Tibi et al., 2001, see Fig. 1b). The easternmost segment of the 1999 Izmit earthquake rupture is right stepping at the Karadere area (1.5 m of co-seismic slip) with respect to the Düzce Area (no co-seismic slip at the surface). Izmit aftershock focal mechanisms for this area for magnitudes > 2 based on recordings from the combined German Task Force (GTF) and SApanca BOlu NETwork (SABONET) are predominantly strike–slip with normal faulting components (Bohnhoff et al., 2006).

The stress field that causes the aftershocks is a combination of the pre-existing stress field and that induced by the mainshock. The apparent dissimilarity between aftershock mechanisms is so strong that it suggests a highly heterogeneous stress field which could be due to either a highly heterogeneous rupture in the mainshock or almost complete stress drop in the mainshock which almost removes the pre-existing stress field (Michael et al., 1990, Michael, 1991).

In this study we determine and analyze a total of 221 Izmit aftershock focal mechanisms for the Karadere–Düzce branch for the entire recording period of the seismic network (August 22, 1999–October 17, 1999) and down to M_l = 0.9. We perform stress tensor inversion to determine the local stress field orientation. Results are compared to the local tectonic setting and discussed in the light of the proceeding Düzce earthquake that occurred on November 12, 1999 extending the Izmit rupture by ~ 50 km to the east.