Abstract Dien Bien basin (DBB), located in the Northwest of Vietnam, is facing a high seismic hazard. To understand the characteristics of the subsurface cover, microtremor measurements were conducted at 452 sites and analysed using the horizontal‐to‐vertical spectral ratio (HVSR) technique. The results indicate that the shape of the HVSR curves reflects local site characteristics, whereas the fundamental frequency ( F 0 ) is associated with the thickness of the subsurface cover. In the plain area, HVSR curves typically display clear peaks, while in the hilly‐mountainous zones (hard soil or exposed rock), they exhibit multiple high‐frequency dominant peaks or appear relatively flat. Seismic microzonation results show that F 0 ranges from 0.50 to 15.87 Hz. Low‐frequency values (7 floors) are more prone to resonance with local site conditions. In contrast, high‐frequency values (>3.5 Hz) are mainly distributed in hilly‐mountainous zones, where low‐rise buildings (<2 floors) are more likely to be affected. Accordingly, the hilly zones are relatively safe zones for high‐rise construction, while the strip along the Nam Rom River represents a seismically vulnerable area. The shallow cover thickness varies between 6 and 157 m, increasing from North to South and from the mountain to the plain zones. The hard rock surface is a depression‐shaped, with a rough‐stepped bottom in the longitudinal direction, smoother in the latitudinal direction and gradually extending southwards. This study demonstrates that the site characteristics of the DBB are primarily controlled by the thickness of the subsurface cover and were well evaluated by microtremor surveys.
Nguyen et al. (Fri,) studied this question.