Observed H/V Responses Across Thailand from the 2025 Mw 7.7 Myanmar Earthquake: Implications for Geologically Controlled Seismic Hazard

Abstract The 28 March 2025 Mw 7.7 Myanmar earthquake produced moderate-to-strong, long-period ground shaking across Thailand and offers a rare opportunity to evaluate nationwide site-response characteristics. Strong-motion data from 77 Thai Meteorological Department’s seismic stations were analyzed using horizontal-to-vertical spectral ratio to determine fundamental resonance period (T0) and amplification (H/Vmax). Each geological province shows unique signatures in the ground-motion spectrum observed across the country. The northern intermontane basins exhibit a dual-mode resonance pattern (T0≈0.7–2.5 s) and short-period peaks (T00.6 s) caused by both shallow-to-moderate alluvial sediments. The western fault valleys show sharp, short-period peaks caused by thin alluvial layers over hard bedrock, whereas the northeastern plateau shows both short-period peaks due to thin sediments and also behaves as a stable reference domain with flat, rocklike H/V curves. The central Thailand rift basin reveals sharp, short-period peaks (T0≈0.2–0.8 s) that indicate the presence of shallow-to-moderately deep sedimentary layers beneath the sites. The Bangkok basin shows broad, low-frequency amplification (T0≈4–7 s), a direct result of deep soft Quaternary sediments. The southern rift basins and coastal plains show mixed characteristics: sharp, short-period peaks (T0=0.2–0.5 s) and dual or multipeaked H/V spectra (T0=1–1.9 s), due to thin to moderate-to-thin sediment covers. For the first time, this study identifies long-period resonance (T04 s) in multiple provincial basins beyond Bangkok (Chiang Mai, Lampang, Phrae, Pathum Thani, and Surat Thani provinces), indicating that deep sedimentary structures across Thailand can amplify long-period seismic waves that can affect tall and long structures such as high-rise buildings, bridges, and elevated expressways. These results provide the first observation for a countrywide frequency-dependent site response from a real earthquake event, which indicates that both short- and long-period site effects are equally important for seismic hazard evaluation in Thailand.