Did the Mw 7.5 Sürgü-Cardak Event Occur During the 2023 Kahramanmaraș Sequence Without Prior Slip Deficit?

The Sürgü-Cardak fault ruptured ~9 hours after the M_w 7.8 mainshock on the East Anatolian Fault zone (EAFZ) during the 2023 Kahramanmaraș earthquake sequence. With a moment magnitude M_w 7.5, involving up to 11 m slip, this event featured comparable slip magnitudes as the mainshock. Published strain rate fields based on geodetic observations do show strain accumulation around the EAFZ, but strain accumulation around the Sürgü-Cardak fault appears to be absent. We therefore reexamine the GNSS-based interseismic strain rate field to see whether, or not, the Sürgü-Cardak fault accumulated significant slip deficit prior to the earthquake.

We use GNSS data from eastern Anatolia. To estimate strain rates and their uncertainties in regions that experience both fast and slow deformation rates, we employ a tailored stochastic interpolation technique. With this method we show that the strain rate peaks around the Sürgü-Cardak fault. To better interpret 2D strain rate fields around faults, we develop a novel decomposition of the strain rate tensor and its covariance, that allows us to estimate the strain rate in a fault-oriented frame. The decomposition method is analogous to descriptions of deformation in structural geology, and allows for direct comparison with slip types from focal mechanisms. Not only does the strain rate peak around the Sürgü-Cardak fault exceed the uncertainty, the direction of interseismic slip deficit accumulation is also compatible with the coseismic slip direction.

We conclude that interseismic slip deficit accumulation on the Sürgü-Cardak fault was previously missed. Coseismic slip is consistent with the loading history. The Sürgü-Cardak earthquake thus has most likely been triggered by the mainshock.