Afterslip following the 2023 Mw7.8 and Mw7.6 Kahramanmaraş earthquakes: observations and modeling

On February 6, 2023, two earthquakes of magnitude Mw7.8 and Mw7.6 struck in South Türkiye. The first mainshock occurred along the East Anatolian fault, at the boundary between the Anatolian and Arabian plates, and was followed 9 hours later by a second one on a secondary fault system to the North. The importance of such continental earthquakes and the relatively good data coverage of the region present an unique opportunity to investigate post-seismic deformation.

To study afterslip, corresponding to post-seismic transient aseismic slip, we use a combination of ground deformation measurements, including Sentinel-1 InSAR timeseries (6 tracks covering almost 2 years after the earthquakes) and GNSS (more than 40 permanent stations and 60 campaign sites). The cities of Hassa and Gölbaşı, located on the East Anatolian fault, are investigated in detail using 8 continuous GNSS stations installed across the fault 6 months after the earthquakes.

While the large surface imprint of the surface deformation, with significant displacements more than 200 km away from the fault, and our inability to model it with fault slip points toward the dominance of a visco-elastic processus, clear markers of shallow afterslip are visible. In the Pütürge segment, located at the tip of the first earthquake’s coseismic rupture, InSAR data reveals a cumulative surface offset 20 months after the earthquake of about 10 cm due to shallow afterslip. Other segments affected with afterslip have been identified in the eastern part of the rupture of the second earthquake, accounting for several centimeters of slip over 20 months. Our local GNSS networks in Hassa and Gölbaşı reveal the smaller scale complexity of post-seismic surface deformation near the fault. In Gölbaşı, subsidence of more than 2 cm/year is highlighted in the pull-apart basin, while horizontal GNSS displacements suggest possible shallow aseismic slip happening at the southern end of the basin.

We model afterslip on the fault by jointly inverting InSAR and GNSS data, minimizing the least squares criterion. Afterslip is concentrated around the coseismic rupture zone, accompanied by important aftershock activity. The Pütürge segment appears as a seismic barrier, having stopped both Mw6.8 2020 Elazığ earthquake to the East and Mw7.8 2023 Kahramanmaraş earthquake to the West, possibly because of the fault geometry and/or heterogeneous coupling. Future efforts will be directed towards the evolution of afterslip with time and its interplay with aftershocks, including visco-elastic relaxation models. These results help us better understand the relationship between the different phases of the seismic cycle.