The slow slip events cycle along the Izmit segment of the North Anatolian Fault 

Abstract

The detection and analysis of transient aseismic slip events is crucial for a better understanding of the seismogenic behavior of major active faults and the associated seismic hazard. The North Anatolian Fault (NAF) in Türkiye has ruptured from east to west since the 1930’s with a Mw 7+ earthquake sequence involving seven events. The two last events ruptured the Izmit and Düzce segments close to Istanbul in 1999, with Mw 7.6 and Mw 7.2 respectively. Postseismic studies of the Izmit earthquake reveal first a rapid and logarithmic decay of the afterslip, followed by a viscoelastic relaxation of the lower crust and upper mantle. Estimates of the mean shallow creep more than 20 years after the rupture using Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data show an average velocity of 2.7 cm/year. An InSAR time series analysis of the aseismic behavior from Aslan et al. [2019], based on 2014-2017 Sentinel 1 data,  enabled the detection of a slow slip event modulating the creep in December 2016. This event was also observed by creepmeters. In this study, we used InSAR data over the extended period 2015-2021, from the Sentinel-1 A and B acquisitions, to analyze the aseismic slip dynamics of the Izmit segment. The InSAR data were processed with the FLATSIM workflow [Thollard et al., 2021] based on the NSBAS processing chain [Doin et al., 2011]. To extract the tectonic signal of the time series, we corrected for the annual seasonal components on each track, we decomposed the residual line of sight signals of the ascending and descending tracks into vertical and east-west (E-W) deformation fields and performed an Independent Component Analysis (ICA) decomposition on the Izmit sedimentary basin to correct for a high frequency signal. In the post-processed time series, we detect three main slow slip events in December 2016, March 2018 and November 2019, the first one corresponding to that  already detected by Aslan et al., [2019]. The average recurrence interval between the three events is 1.5 years. We extracted the E-W surface static displacements associated with each slow slip event and modeled the slip distribution at depth on a 2D-fault interface in a layered elastic half space. The slip associated with the transient slip pulses is localized at a depth of ~ 2 km, at the basis of the sedimentary basin. We estimated that along the central part of the Izmit segment, 50% of the total observed creep is occurring during the three 20 days-long slow slip events, and only 25-30% at the extremities of the segment. The characterization of a succession of slow slip events two decades after a Mw 7.6 earthquake on the Izmit segment and 70 years after a Mw 7.4 earthquakes on the Ismetpasa segment of the NAF [Rousset et al., 2016; Jolivet et al., 2023] question the logarithmic decay of postseismic slip and suggest a more transient and intermittent slip release, at least along some segments of the fault.