Initiation of the 2023 Mw 7.8 Kahramanmaraş, Türkiye, Earthquake by the Mw 6.8 Rupture of the Narlı Fault

We investigate the initial stages of the 2023 moment magnitude (Mw) 7.8 Kahramanmaraş, Türkiye, earthquake to understand how the rupture of the Narlı fault (NF) initiated and led to the triggering of the main rupture of the East Anatolian fault (EAF). Using a combination of geodetic and seismic observations, we analyzed the location and slip distribution of the NF rupture to characterize its evolution and the resulting stress change on the EAF. Using both Interferometric Synthetic Aperture Radar and pixel tracking observations, we model slip on both the NF and EAF to obtain an Mw 6.8 for the NF event. The NF rupture initiated at its southern end at 10 km depth and propagated northward to its intersection with the EAF. The NF earthquake had a complex beginning with the first rupture on a blind normal fault that rapidly transitioned to left-lateral slip on the NF. Fault slip increased stepwise from south to north, and remained predominantly below 5 km depth except along a 12 km segment of the central NF where surface displacements measured 3 m. The progression of the rupture from its initiation to the surface rupture was subshear, on average, with the timing documented by two near-fault strong-motion stations. The Mw 6.8 NF earthquake increased the Coulomb failure stress on the EAF where main rupture initiated, just east of their intersection, by approximately 2 MPa. We determine an origin time for the EAF rupture 17.2 s after the nucleation indicating little or no delay between completion of the NF event and initiation of rupture on the EAF.