Evidence for strong ground motion by waves refracted from the Conrad discontinuity

The Conrad discontinuity, a boundary between the upper and the lower crusts, has long been identified in many continental crusts. The influence of the Conrad discontinuity on seismic hazards, however, has been rarely known. Strong regional phases corresponding to the waves refracted from the Conrad discontinuity are observed in the Korean Peninsula. These phases show strong amplitudes of 2-4 times larger than those of the direct waves. The observation is confirmed by numerical modeling of waveforms. These observations not only support the existence of the Conrad discontinuity in the Korean Peninsula but also suggest the potential of seismic hazards by the Conrad phases. Such strong Conrad-refracted phases are typically recorded in the Pn coda portion, which may cause overestimation of Pn body-wave magnitudes for regional events.