Broadband Source Model of the 2023 Mw 7.8 Türkiye Earthquake from Strong-Motion Records by Isochrone Backprojection and Empirical Green’s Function Method

The 2023 Mw 7.8 Türkiye earthquake caused severe damage in near-fault regions. The broadband source model, which is important for predicting strong motions in near-fault regions, was estimated. First, high-frequency (3–10 Hz) source imaging was performed through isochrone backprojection using near-field strong-motion records. Four segments were set, consisting of three segments along the East Anatolian fault and one segment along the splay fault where the rupture started. The estimated rupture velocities at the four segments were 2.6–3.3 km/s. The broadband (0.2–10 Hz) source model was then estimated using the empirical Green’s function method. The locations of eight strong-motion generation areas (SMGAs) of the broadband source model were searched with reference to the large brightness area estimated by isochrone backprojection. The source parameters of the SMGAs were estimated to fit the calculated acceleration and velocity envelopes at 21 strong-motion stations to the observed ones. The locations of the SMGAs were mostly consistent with the large slip area estimated by previous studies from long-period waveforms or static data, except for one SMGA with the highest Brune’s stress drop on the splay fault. The highest stress drop caused large ground motions near the splay fault, for which the supershear rupture has been suggested by previous studies. Ground motions were reproduced except for some stations affected by the fling-steps or nonlinear site effects. Although the SMGAs were not located near the southern side of the southwestern segment in Hatay Province, the large ground motions at shorter than about 2 s were mostly simulated. Large empirical site amplification factors estimated in this study must play a role on the large ground motions. The forward rupture directivity effects, with a rupture velocity of 3.3 km/s as large as the S-wave velocity, were also responsible for the large ground motions there.