Seismic Action   
The seismic  design approach of underground structures is generally characterized by two  different type of earthquake events: the maximum design earthquake (MDE) and  the operating design earthquake (ODE) typically defined as:
In this case, two different type of artificial  seismic accelerograms are taking into account for the evaluation of the seismic  behavior of Mercato Santa Anita Metro Station, according to the study of seismic risk of the project area described in the  previous paragraph and to the values shown in Table D:  one seismic event characterized by a return  period equal to 1000 years in order to represent the ODE (Figure 9) and another  seismic event characterized by a return period equal to 2450 years in order to  represent the MDE (Figure 10), according to GB50909-2014. The recording has been corrected with a low-pass filter at the  cut-off frequency of 15 Hz using [2].