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Analysis of amplified ground motion in Bangkok from recent earthquakes (2023)

Background The Bangkok Basin has been known from non-instrumental observations of the local population to be subject to ground motion amplification due to the deep alluvial sediments and basin geometry. This study analyzes available seismic data to confirm that basin effects are significant in the Bangkok Basin. The paper contributes to the evaluation of basin effects by characterizing the engineering ground motion parameters and HVSR curves for the Bangkok basin which produce lengthening of ground motion duration with respect to nearby rock sites, albeit with very low ground motions. For this purpose, we analyzed ground motion records from seismic stations located within the Bangkok alluvial basin from 2007 to 2021. Recorded peak horizontal ground acceleration (PGA) for seismic stations inside the basin always exceeded 1 cm/s2 during eight earthquakes with Mw ≥ 5.5. Of these, two were intraslab events and six were shallow crustal earthquakes. These recorded ground motions shook high-rise buildings in Bangkok even though their epicentral distance exceeded 600 km. Methods Several time and frequency domain analyses (such as analysis of residual, HVSR, Hodogram plots, etc.) are used on the ground motion records in the Bangkok basin to determine the frequency content of recorded ground motion and to demonstrate the significance of surface waves induced by the deep basin in altering the engineering ground motion amplitudes. In addition, centerless circular array microtremor analysis is used to determine the depth of sedimentary basin to the bedrock. Results Based on comparisons from those stations located outside the Bangkok basin, we observed the capability of alluvial deposits in the Bangkok basin to amplify ground motion records by about 3 times. We observed that there is a unique site amplification effect between 0.3 and 0.1 Hz due to local surface waves and other moderate amplifications between 2 and 0.5 Hz due to a soft layer like other deep alluvial basins in other metropolitan areas. Conclusion We noticed that there is a unique site amplification effect between 0.1 and 0.3 Hz and smaller peaks around 2 and 0.5 Hz consistent with expectations for site amplification effects associated with deep basins. Moreover, we noticed the presence of low frequencies content of the surface wave generated within the basin which deserved further studies using the 2D/3D ground motion modelling through basin topography and velocity models.

However, there have never been any studies analyzing the set of recorded ground motions in the Bangkok basin similar to those observed in other metropolitan areas (such as in Mexico City (Bard et al. 1988), Gubbio, Central Italy (Pacor et al. 2007), and Tokyo (Yamanaka et al 1989)). The degree of variability of peak value of horizontal ground acceleration for basin stations (i.e. BKSI, PWSA, PTNA, PWNA) records from 2019 and 2021 (Additional file 2: Table S1) could be observed with PGA generally exceeding 1 cm/s 2. Nevertheless, we noticed that for stations located within the central part of Bangkok (i.e. SIRA, PWSA, and PWNA), which have the deepest basement depth, about 600–850 m, the pre-dominant periods vary between 6.5 and 7.5 s. Further studies should investigate the soil amplification effect of these long-period predominant peaks with reliable low-frequency seismometers through large-array observations.

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