# Experimental Assessment of the Effect of Vertical Earthquake Motion on Underground Metro Station
## 1 Introduction  
The three-dimensional behavior of underground structures during earthquakes is a matter of interest for improving the seismic analysis and design in civil engineering under extreme loads. Although extensive studies were carried out on the effects of the horizontal component of the groundshaking, only limited work has been done in the analysis of the effect of the vertical component. This topic deserves attention since some field evidence and investigations on the near-field ground motions have demonstrated that vertical components caused destructive consequences, even possible collapse, for some existing structures [1; 2; 3; 4]. The latest developments in the underground vibration or deformation monitoring techniques such as the fiber-optic sensors [5; 6] are expected to facilitate both the acquisition of the field evidence and the research work concerning the vertical earthquake motion. In view of the fact that the ratio of vertical to horizontal peak ground acceleration (RVH) is extremely high in the vicinity of the active faults, it seems essential to study the influence of the RVH on the seismic responses of the underground structures, such as metro stations in the seismic active regions. Through numerical analyses based on the finite element-finite difference method, [PERSON] and [PERSON] [7] discussed the effect of the vertical oscillations and overburden on the damage-concentration mechanism of the columns at several specific underground station sections in the 1995 Hyogo-ken Nanbu (Kobe) earthquake. On the other hand, [PERSON] et al. [8] discussed the effect of the vertical motion on the failure mode and dynamic response of the metro station, and concluded that in this case the vertical ground motion was not the primary cause of the collapse. Research had also been carried out to investigate the influencing factor of RVH. [PERSON] and [PERSON] [9] investigated numerically the dynamic interaction of the twin tunnels subjected to the incident seismic waves. They have shown how the dynamic response of the twin tunnels depended on the incident angle of the earthquake wave, together with the spacing between two adjacent tunnels and the modulus ratio of the tunnel to the surrounding soil. [PERSON] and [PERSON] [10; 11] provided insights into the dynamic responses of the underground infinitely long lined tunnels buried into an elastic or viscoelastic half-space subjected to the obliquely incident waves using the boundary element method (BEM), and proved the influence of the incident directions of the seismic waves on the dynamic responses of the tunnels. [PERSON] and [PERSON] [12] proved that the amplitudes of the stresses and deformations near the tunnel were dependent upon the angle of incidence, which determined the overall trends of the amplitudes. [PERSON] and [PERSON] [13; 14] adopted the weighted residual method to study the scattering and diffraction of the plane SH-waves by an underground cavity of the arbitrary shape in a two-dimensional elastic half-space, and demonstrated same observations as for circular cavities that ground amplifications depended on the orientation of the incident waves. [PERSON] et al. [15] studied the dynamic response of a cylindrical pipe embedded in an elastic semi-infinite medium using hybrid finite element and eigenfunction expansion techniques. They found that dynamic amplification was significantly dependent on the angle of incidence and the depth of embedment. [PERSON] et al. [16] investigated the impact of the incident angles of earthquake shear waves (SV and SH-waves) on the seismic responses of a long lined tunnel, and numerical results indicated that non-linear seismic responses of the long lined tunnels were highly affected by the incident angles of the S-waves, which should be taken into consideration in mapping the seismic risk of the tunnels. [PERSON] and [PERSON] [17] investigated numerically the seismic responses of the underground rectangular tunnel under the vertical seismic excitation.