Far-field ground motion characteristics of the Bangkok Basin, Thailand, in the 2025 Mw 7.7 Mandalay earthquake: initial insights

Abstract

The 2025 Mw 7.7 Mandalay earthquake occurred on 28 March 2025 along the Sagaing Fault causing severe damage to building structures in Myanmar and large vibrations could be felt in nearby countries. This was the first time in modern history in continental Southeast Asia that an earthquake with magnitude greater than 7.5 was caused by one of the major active faults. The ground motion from the mainshock was recorded by twenty-seven accelerometers across northern and western Thailand. Five of these seismic stations located in the Bangkok basin provided valuable insights into far-field ground motion characteristics for this region, where recorded accelerations are limited. In this work, the recorded ground-motion parameters are assessed and compared with the NGA-West2 Ground Motion Models (GMMs). It was found that the recorded ground motion from the 2025 Mw 7.7 Mandalay earthquake generally provides positive residuals at a long distance, indicating a lower attenuation rate for the observed data than those estimated in the GMMs. The observed acceleration in the deep sedimentary basin indicates significant amplification in long spectral periods, primarily attributed to the thick soft soil layers of the Bangkok basin. This amplification effect is consistent with previous studies highlighting the seismic response of the basin to distant large-magnitude events. The findings underscore the importance of incorporating site-specific amplification in seismic hazard assessments for Bangkok, especially for long-period structures.