Postseismic and shallow slow slip events on the Izmit segment of the North Anatolian Fault controlled by depth-dependent frictional variations

Earthquake cycle modeling has enabled to reproduce the full spectrum of slip rates observed along fault segments, and refine our understanding of seismic cycle dynamics. However, key parameters controlling the occurrence of shallow slow slip events (SSE) such as those observed along strike-slip fault segments remain unclear, due to rare worldwide observations and the lack of long-lasting observations covering all phases of the seismic cycle. Here, we apply rate and state friction quasi-dynamic 1D models to explain the ensemble of observations along the Izmit segment of the North Anatolian Fault in Türkiye. This fault segment ruptured in 1999 with the magnitude 7.6 Izmit earthquake, and has been since then widely studied, providing constraints on most of the phases of the seismic cycle, from mainshock amplitude and recurrence times to afterslip logarithmic decay, and the occurrence of shallow SSEs. GNSS, InSAR and creepmeters geodetic data associated with seismological and paleoseismological data enable to describe the cumulative displacement during all phases of the seismic cycle. The comparison between model predictions and the observational time scales led to an optimal set of frictional models. First, the mainshocks maximum slip of ~6 m and return times of about ≥200 yrs are explained by an unstable seismogenic layer below 5.5 km depth with a thickness of 9.5 km and with frictional parameters a-b of about -0.004. The decadal afterslip, well constrained by a pair of campaign GNSS stations located on both sides of the fault, is mostly due to a stable layer located between 5.5 and 1.3 km depth, the lower limit being compatible with the aftershocks sequence limit. We compared model slip predictions and GNSS time series by computing Green's functions for a layered elastic half space medium. Model parameters for this intermediate layer explaining the observed relaxation time have frictional parameters a-b and critical distance of about 0.005 and 8 km, respectively. Finally, a shallow layer from the surface to 1.3 km depth with either a gradient of frictional parameters with depth or constant negative frictional parameters is needed to generate shallow SSEs 20 yrs after the main earthquakes. The shallow layer depth extent being compatible with the Izmit Quaternary sedimentary basin may suggest a key role of the sediments frictional properties to allow a velocity weakening behavior. Models with a gradient of apparent frictional properties throughout the basin may suggest the importance of pore-pressure variations as a function of the fault gouge depth.