## 5 Conclusions  
In this study, numerical analysis was conducted to assess the impact of hydrogen explosion vibrations on the ground subsurface and nearby underground structures, assuming a disaster scenario involving a hydrogen gas explosion at a low-depth underground hydrogen storage facility. To simulate the dynamic behavior against a hydrogen gas explosion, Dassault Systems' ABAQUS 2023 software was employed, and the JWL-EOS model based on the equivalent TNT was applied. The model examined the impact of ground surface vibrations on buildings and the damage behavior of nearby underground structures in the event of a hydrogen gas explosion at a hydrogen storage facility. The main conclusions drawn from this study are as follows:  
1. To examine the impact of ground vibrations on the surrounding building, ground vibration levels were assessed at various separation distances (the distance between the measurement point and the explosion source) with hydrogen concentrations of 20%, 30%, and 40%. A distinct difference was observed in the tendency of the ground surface vibration to decrease as the separation distance increased based on  
Figure 18: Geological boundary and explosion wave direction according to tunnel location. a separation distance of 7 m. Within a separation distance of 7 m, a sharp decline in the vibration velocity was evident, whereas beyond this range, a relatively gradual decrease in the trend was observed. 2. The surface vibration velocity measured at the range of 3-60 m from the explosion source was approximately 13 cm/s (13 Kine), although there was a deviation depending on the hydrogen gas concentration. However, above a separation distance of approximately 35 m, the surface vibration velocity converged to approximately 1 cm/s. After evaluating the impact of vibrations on ground structures by substituting the predicted vibration velocity values from a simulation model into the blast vibration tolerance standards legislated in Korea, it was determined that safety against ground vibrations can be ensured at a separation distance of approximately 6-7 m or more when targeting large reinforced concrete structures. 3. In the numerical analysis evaluating the impact of a hydrogen gas explosion on nearby underground tunnel structures, safety against explosions could be assured when the minimum separation distance was 10 m or more regardless of the direction (horizontal or vertical). From a conservative perspective, maintaining a minimum separation distance of 10 m or more from the hydrogen storage facility is believed to definitively ensure the damage stability of nearby tunnel structures against a hydrogen gas explosion. 4. The dynamic damage effects on the underground structures were predicted to exhibit substantial variations depending on the dynamic properties of the stratum. The mutual interaction between the stratigraphic surface of the ground layer and the boundary surface of the structure can vary considerably depending on the location of the underground structure. When a stratigraphic surface with significant differences in dynamic characteristics aligns with the boundary of the structure, more damage may occur owing to the bending failure behavior caused by stress deviation. Furthermore, the impact of a hydrogen gas explosion on nearby underground tunnel structures may depend on various factors, including the hydrogen storage infrastructure (storage scale, structural characteristics, and damping structure), as well as the characteristics of the underground tunnel structure. There are still numerous details that require discussion and validation, and these issues will be addressed in our forthcoming research. Conceptualization, Y.K. and G.-H.G. ; methodology, H.-J.C.; software (ABAQUS 2023), V.-H.C. and G.-H.G. ; validation, S.-W.O. ; writing, original draft preparation, G.-H.G. and V.-H.C.; writing, review, and editing, S.-W.O. and M.-J.K.; visualization, V.-H.C., H.-J.C. and S.-W.O. ; supervision, G.-H.G. and Y.K. ; and funding acquisition, Y.K.