Ground-Motion Models for 5% Damped PSA for Shallow Crustal Earthquakes in Northern Vietnam

This study presents a ground-motion model (GMM) for Northern Vietnam (NVN). The reference global GMM given by Abrahamson et al. (2014; hereafter, ASK14) is regionalized to NVN using a local data set that contained data recorded at 83 stations from 229 shallow crustal earthquakes (focal depth ≤35 km) in the moment magnitude (Mw) range of 3.0–5.9. The global ASK14 GMM is adjusted to the NVN region by modifying the small-magnitude scaling and distance attenuation but maintaining the large-magnitude and short-distance scaling to constrain the extrapolation of the GMM outside the range of the NVN data. The magnitude-dependent standard deviation was also adjusted based on the local data from NVN. We introduce spatially varying coefficients that adjust the median prediction of the adjusted ASK14 for the NVN region as a function of the earthquake and the station coordinates. The resulting nonergodic GMM increases the accuracy of the estimated ground motion as compared with the regionalized ergodic GMM. The nonergodic GMM has an aleatory standard deviation of 0.72 compared to the ergodic standard deviation of 1.0. The nonergodic GMM can be used to develop design ground motions using either deterministic or probabilistic approaches. The ergodic standard deviation is larger than the values usually seen in other active regions. These large values are likely caused by the variability of the geological conditions within NVN: at sites located near Hanoi, there is a large and deep sediment basin, whereas at sites located in the mountainous area in the northwestern region, there are hard-rock site conditions.