Radiation Patterns Control the Near-Source Ground-Motion Saturation Effect

We investigate the feasibility of near-fault ground-motion predictions based on empirical Green's functions (EGFs) in low-to-moderate seismicity areas (i.e., with few available EGFs), and we propose some adjustments to enhance the accuracy of this method. We conduct extended fault ground-motion simulations for a large set of azimuths, based on a kinematic model description according to the k(-2) method combined with the use of numerical Green's functions. We focus on saturation of the ground-motion peak values observed in near-field data for moderate-to-large earthquakes, and we seek to identify the physical mechanisms behind this phenomenon. Based on the simulation performed here for a specific magnitude and focal mechanism, we show that the radiation pattern has a major influence on the near-source ground-motion saturation effect, and that the saturation effect can be seen more strongly for some azimuths compared to others, due to the orientation of the source. We also show that the depth of the source has a role, as it defines the radiation pattern. Finally, we show that unlike previously thought, geometric and anelastic attenuation adjustments are weak, as are the time-shift adjustments due travel-time differences from the different parts of the fault, and these do not account for the near-fault saturation effect.