After a major earthquake, seismometers are often deployed temporarily near the epicenter to monitor aftershock events. These aftershock records are valuable resources for nonergodic site term development, which improves ground motion prediction accuracy. The term ergodic site term refers to temporally and spatially averaged site conditions, while the nonergodic site term accounts for location‐specific variations. A ground motion model (GMM) predicts earthquake intensity measures (IM) at a site. The nonergodic approach reduces prediction uncertainty of the GMM, leading to more accurate IM estimates. Unlike the ergodic GMM method, the nonergodic approach could capture local variability using specific site terms. This study refines the IM map of the M 5.4 Pohang earthquake by incorporating nonergodic site terms derived from aftershock records. The detailed procedure includes: 1) development of nonergodic site terms including residual analysis and interpolation techniques; 2) derivation of site amplification modification factors by coupling a GMM with regional site amplification models; 3) validation using statistical metrics such as RMSE and MAE using two validation stations. As a result, the peak ground acceleration (PGA) shows 30% and 16% reductions in prediction error compared to the PGAs without nonergodic site terms at the two validation sites. The use of aftershock records for the assessment of the nonergodic site terms has a benefit in that the ray paths of earthquake waves to a site are shared among events so that the uncertainty of the path term is minimized. These findings highlight the importance of deploying temporary stations at sites of interest after a main event, which can refine the seismic intensity of the mainshock.
Kwak et al. (Sun,) studied this question.