Effects of Source Directivity Observed in the Near-Fault Zones of Large Earthquakes and Accounting for the Effects in Seismic Hazard Analysis and Building Codes

The effects of radiation directivity of finite-fault sources of strong earthquakes have been recognized for a long time. However, only in recent decades, with the development of dense networks of seismic observation, representative data become available, allowing for a detailed study of these effects. Since the 2000s, seismologists have found evidence of super-shear crack propagation in strong earthquakes, resulting in shock fronts (Mach cones) formed by the interference of seismic waves radiated by the crack tip. This phenomenon leads to high peak ground acceleration (PGA) and peak ground velocity (PGV) values recorded in the near-fault zones. In such cases, significant damage occurs in the near-fault zones due to high-amplitude narrow velocity pulses impacting buildings (on fault-parallel components), followed by shaking of another type, caused by trailing Rayleigh ruptures (with dominant fault-normal components). This double punch effect can be particularly destructive to structures. Such phenomena were observed during the earthquakes in Turkey in February 2023. During large subduction earthquakes, due to the geometry of the fault plane, constructive interference of seismic waves and the formation of shock wave fronts are possible without super-shear crack propagation, i.e., with its propagation at normal velocity. Shock fronts were also observed but at considerable distances from the faults, where the resulting damage was relatively minor. This reduced impact is attributed to the high-frequency nature of the strong motions, which pose less risk to buildings. Additionally, the shock fronts, being farther from the sources, were weakened by attenuation mechanisms. Such phenomena are typically observed on large faults with small amounts of asperities, i.e., on faults where earthquakes occur regularly, and asperities are smoothed out, for example, on the North Anatolian and East Anatolian faults in Turkey, on active faults in Tibet, and during large subduction earthquakes.