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In order to better assess earthquake hazards, it is vital to have a better understanding of the spatial and temporal characteristics of fault creep that occur on ruptured faults during the period following major earthquakes. towards this end, we use new far-field GPS velocities from continuous stations (extending similar to 50-70 km from the fault) and updated near-fault GPS survey observations, with high temporal and spatial density, to constrain active deformation along the M-w 7.4,1999 Izmit, Turkey Earthquake fault. We interpret and model deformation as resulting from post-seismic afterslip on the coseismic fault. In the broadest sense, our results demonstrate that logarithmically decaying post-seismic afterslip continues at a significant level 20 yr following 1999 Earthquake. Elastic models indicate substantially shallower apparent locking depths at present than prior to the 1999 Earthquake, consistent with continuing afterslip on the coseismic fault at depth. High-density, near-fault GPS observations indicate shallow creep on the upper 1-2 km of the coseismic fault, with variable rates, the highest and most clearly defined of which reach similar to 12 mm yr(-1) (10-15 mm yr(-1), 95 per cent c.i.) near the epicentre between 2014-2016. This amounts to similar to half the long-term slip deficit rate.

Characterizing the spatiotemporal evolution of creep is essential to constrain fault slip budget and understand creep mechanism. Studies based on interferometric synthetic aperture radar and Global Positioning System (GPS) satellite observations until 2012 have shown that the central segment of the 17 August 1999 M-w 7.4 Izmit earthquake on the North Anatolian Fault began slipping aseismically following the event. In the present study, we combine new interferometric synthetic aperture radar time series, based on TerraSAR-X and Sentinel 1A/B radar images acquired over the period 2011-2017, with near-field GPS measurement campaigns performed every 6months from 2014 to 2016. The mean velocity fields reveal that creep on the central segment of the 1999 Izmit fault rupture continues to decay, more than 19years after the earthquake, in overall agreement with models of postseismic afterslip decaying logarithmically with time for a long period of time. Along the fault section that experienced supershear velocity rupture during the Izmit earthquake creep continues with a rate up to 8mm/year. A significant transient accelerating creep is detected in December 2016 on the Sentinel-1 time series, near the maximum creep rate location, associated with a total surface slip of 10mm released in 1month only. Additional analyses of the vertical velocity fields show a persistent subsidence on the hanging wall block of the Golcuk normal fault that also ruptured during the Izmit earthquake. Our results demonstrate that afterslip processes along the North Anatolian Fault east-southeast of Istanbul are more complex than previously proposed as they vary spatiotemporally along the fault.