## 3 Vibration Stability Evaluation Due to Hydrogen Gas Explosion  
Figure 10 shows the simulation results obtained under the scenario of an explosion occurring inside a hydrogen storage facility with a hydrogen concentration of 40%. The results were used to evaluate the magnitude of the vibration velocity over time at observation points A, B, and C located on the ground surface. It is evident that the maximum amplitude of the vibration was significantly higher closer to the explosion point, whereas the amplitude of the vibration decreased as one moved away from the explosion source. Figure 11 shows the field output results of the ground vibration velocity for the same analysis across the entire domain. Immediately after the explosion, the blast stress wave rapidly propagated to the surrounding ground. From a structural perspective, the blast stress was more concentrated in the upper ground of the storage facility, close to the free surface (i.e., the minimum resistance line). Based on these simulation results and the domestic allowable limit of vibration for building structures (Table 5), the vibration stability of aboveground building structures due to gas explosions occurring in underground hydrogen storage was evaluated. Figure 12 illustrates the surface vibration velocity (V) as a function of horizontal distance (L) for various hydrogen concentration explosion scenarios, represented by a power-law-based nonlinear regression analysis. The dashed lines in the graph represent domestically legislated vibration tolerance thresholds. In the range of horizontal distances from 3 to 60 m, the maximum surface vibration velocity due to hydrogen gas explosions was evaluated to be approximately 13 cm/s. With a 10% increase in hydrogen gas concentration, the surface vibration velocity of the surrounding ground increased by approximately 30%. Additionally, the ground vibration velocity showed a sharp decre  
Figure 9: Boundary conditions of the numerical simulation model (a quarter of the field). the horizontal separation distance range of approximately 6-7 m and showed a relatively gentle linear decreasing trend at separation distances of 7 m or more (an inflection point in the slope exists). Although there are some differences in the explosion power depending on the hydrogen gas concentration, the zone with a separation distance within 7 m can be estimated as the fracture zone, where the plastic fracture is concentrated and stress (or fracture energy) is released. At a separation distance of more than 7 m, only simple elastic wave transmission occurred as an elastoplastic region accompanied by a few crack zones. Figure 11: Field output graph of vibration due to hydrogen explosion over time. Figure 10: Predicted vibration results at the ground surface at different distances from the hydrogen storage infrastructure. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type of Structure** & **Cultural Asset** & \begin{tabular}{c} **Masonry Wall** \\ **(Brick, Stone,** \\ **etc.) and Wooden** \\ **Ceiling** \\ \end{tabular} & \begin{tabular}{c} **Structure with** \\ **Underground** \\ **Foundations and** \\ **Concrete Slabs** \\ \end{tabular} & \
\begin{tabular}{c} **Small and Medium** \\ **Building with** \\ **Reinforced Concrete** \\ **Frames and Slabs** \\ \end{tabular} &
\begin{tabular}{c} **Large Building** \\ **with Reinforced** \\ **Concrete/Steel** \\ **Frames and Slabs** \\ \end{tabular} \\ \hline Allowable & & & & & \\ vibration values at & & & & \\ building & & & & \\ foundation (cm/s) & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Allowable limit of vibration suggested by tunnel standard construction specifications [37]. Figure 13 shows the minimum required horizontal separation distance of building structures obtained by applying domestic blast vibration tolerance standards to the simulation results derived in this study. For masonry buildings and wooden structures, the minimum allowable separation distance tends to increase depending on hydrogen concentration.