Rupture Directivity Effects of Large Seismic Sources, Case of February 6th 2023 Catastrophic Earthquakes in Turkey

An overview of the results obtained by foreign seismologists based on the records of Turkish seismic networks AFAD (State Agency for Disaster Management under the Ministry of Internal Affairs) is presented. The sequence of earthquakes began with the M7.8 main shock and includes thousands of aftershocks. The strongest events occurred in the first twelve hours, with the sources of two M7.0+ events located 100 km apart. Earthquakes have caused ground motions that are destructive to structures, the so-called “pulse-like waveforms,” and epicentral distances, as was previously noted, are not a good indicator of attenuation of waves from earthquakes with extended ruptures.The records of stations in the near-fault zones clearly revealed the directivity effects of seismic radiation. The M7.8 earthquake (main shock) was larger than expected in the current tectonic setting. The near-field records traced an early transition to the super-shear (~1.55V_s) rupture propagation on the Narli lateral fault, where the rupture originated and then passed into the East Anatolian fault. The early transition to the super-shear stage obviously contributed to the further propagation of the rupture and the initiation of slips on the East Anatolian fault. A dynamic fracture model has been constructed that matches the various results of inversions obtained by different authors and reveals spatially inhomogeneous rupture propagation velocities. Super-shear velocities exceeding the shear wave velocity V_s are observed along the Narli lateral fault and at the southwestern end of the East Anatolian fault. Since the late 1990s, seismologists have been working on incorporating the rupture directivity effects of extended sources into the probabilistic seismic hazard analysis procedures, but no consensus has been reached so far, and progress in this area can only be expected with the accumulation of a sufficient amount of observational data.