## 3 Environmental Vibration Test and Linear-Elastic Modeling
In addition to the frequencies displayed in Table 3, digital recordings were studied in both the time and frequency domains for detailed understanding of the seismic response. Figures 4 and 5 show the respective time-domain signals and average frequency responses \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor** & **Time Frame** & **Frequency (Hz)** \\ \hline  & & 4.62 \\  & During event & 4.90 \\ HVBV04\_RS3D & & 4.37 \\  & Before event & 4.20 \\  & Before event & 3.55 \\ \hline  & & 4.33 \\ \hline  & & 6.53 \\ HVBV05\_RS4D & & 5.54 \\ \hline  & & 5.50 \\ HVBV02\_RS4D & & 4.63 \\ \hline  & & 6.17 \\ HVBV01\_RS4D & & 6.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean fundamental frequency values of velocity components recorded by sensors HVB04, HVB05, HVB02, and HVB01. Figure 3: (**Left**): Map of Orihuela with the instrumented hospital location; (**Right**): sensor placement on the building—four on the top floor (HVB02-HVB05) and one in the basement (first floor, HVB01). for the vertical, north, and east components recorded at sensor HVB04, representing the pre-event ambient noise and the response during the Algorfa event. Figure 4 shows the time and frequency responses during the Algorfa event with mean frequencies of 4.62 Hz, 4.90 Hz, and 4.37 Hz for the vertical, north, and east components, respectively. Figure 5 presents the pre-event ambient noise with mean frequencies of \(4.20\pm 0.14\) Hz, \(3.55\pm 1.68\) Hz, and \(4.33\pm 0.14\) Hz for the corresponding components. Here the blue line represents the mean value, while the red lines indicate the variation. As another check of the results, experimentally recorded frequencies at HVB04 were compared with the first mode frequency obtained by the numerical model, which is shown in Table 4. According to this table, the calibrated numerical model against sensor data possesses a first-mode frequency of about 4.66 Hz close to frequencies developed during the Algorfa event. As a matter of fact, the mean frequency for the vertical component recorded during the event was 4.62 Hz, while that of the north component was 4.90 Hz. The values are within the expected range and confirm the reliability of the numerical model in catching the main dynamic behaviour of the building. However, the most remarkable discrepancies were pointed out concerning pre-event ambient noise, characterised by lower mean recorded frequencies, mainly in the north component (\(3.55\pm 1.68\) Hz); this may be because the building response to low-amplitude vibrations is different compared to seismic excitations, which again points to the relevance of the calibration of numerical models with respect to different excitation scenarios. Figure 4: Time and frequency response of the Algorfa event (10 July 2023) registered at the velocity sensor HVB04. Vertical, north and east components, from up to down, respectively. A refined linear elastic model of the wall-frame building (block 14) was implemented in SeismoStruct [23]. A reinforced concrete rectangular section has been used for modelling the columns and beams. The RC walls and the slabs are modelled by employing linear 3D shear elements. The concrete cracking is taken into account by reducing the moments of inertia of beams and columns by factors 0.4 and 0.7, respectively [24]. The effect of the infill brick masonry walls on the seismic response of the building is modelled using the inelastic infill panel element. The structural model implemented in the SeismoStruct computer program is shown in Figure 6.  
Figure 5: Time and frequency response of the ambient noise registered at the velocity sensor HVB04 before the Algorfa event (10 July 2023). Vertical, north and east components, from up to down, respectively. Figure 6: Model of the hospital Vega de Baja (block 14). The fundamental periods are obtained using the classic linear eigenvalue analysis by discretising the buildings with classical lumped mass models.