# Strong Ground Motion Sensor Network for Civil Protection Rapid Decision Support Systems
## 3 Processing Methods  
The site seismic response spectra are a way for the earthquake engineers to understand and study the behavior of structures during earthquakes. They represent the change of spectral acceleration in different periods. To calculate the response spectra, we adopted [PERSON]'s method [21] that has the advantage of not relying on the orientation of the horizontal components. A single rotation of the recorded motions produces one time series of acceleration which is preferable for dynamic structure analysis [21]. According to this method, the largest absolute amplitude of each of the two horizontal component oscillator time series \(\varepsilon_{1},\varepsilon_{2}\) with \(\varphi\) rotation angle is calculated by the equations:  
\[\varepsilon_{1}(t,\varphi)=\varepsilon_{1}(t,0)cos(\varphi)+\varepsilon_{2}(t,0)sin(\varphi), \tag{1}\]  
\[\varepsilon_{2}(t,\varphi)=-\varepsilon_{1}(t,0)sin(\varphi)+\varepsilon_{2}( t,0)cos(\varphi), \tag{2}\]  
The procedure to calculate the ground motion intensity involves the non-overlap rotation of horizontal axis to find the geometric mean \(Geo_{m}\) of the period dependent response spectra \(R_{sp}\):  
\[Geo_{m}=\sqrt{R_{sp}(\varepsilon_{1})R_{sp}(\varepsilon_{2})}, \tag{3}\]  
Then, to calculate the period \(T\) independent response spectra \(R_{sp}I\), a penalty function [14] is minimized based on the rotation angle \(\varphi\):  
\[R_{sp}I(T)=Geo_{m}(\varphi_{min},T), \tag{4}\]  
This procedure requires time series with the instrument response removed. In the initial REF TEK files, the seismic signal was cut few seconds (8-15 s depends on the existence of transient noise) before P waves arrival up to the coda and then the files were transformed into SAC format. The SAC software and the EVALRESP code [22] have been used to prepare the files. A typical signal processing procedure followed for removing trend and offset, if any, and then the waveforms were tapered to avoid the presence of jumps at the time series borders. Finally, to use the [PERSON]'s code [23], the SAC files turned into SMC format, which is the standard time series ASCII type format for strong motion data developed by U.S. Geological Survey [24]. The rotation step angle was set to one degree and the damping to 5% for the full set of periods. The resampling interpolation factor for the FFT was set to 8. Among the results obtained from the software, the median values (50% fractiles) of response spectra for the period-independent rotation angle were adopted and plotted against the period because these circumvent the very small geometric means which are associated to strongly correlated motions [21]. The typical measurements in a strong ground motion analysis were carried out, i.e., the peak ground acceleration (PGA), the peak ground velocity (PGV), the peak ground displacement (PGD). Another measurement provided, which is not based on a maximum value, the Arias Intensity \(I_{A}\), the time-integral of the square of the ground acceleration, \(a(t)\) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & **Date-Time** & **Latitude** & **Longitude** & **Magnitude** & **Depth** & **Distance** \\  & **(yyyy/mm/dd hh:mm)** & **(Degrees)** & **(Degrees)** & **(M\({}_{\text{L}}\))** & **(Km)** & **(Km)** \\ \hline EQ.1 & 2017/04/29 22:39 & 35.5472 & 23.8682 & 3.3 & 18 & 19 \\ EQ.2 & 2018/01/26 22:09 & 35.8970 & 24.1080 & 3.9 & 72 & 44 \\ EQ.3 & 2019/11/27 07:23 & 35.6854 & 23.2599 & 6.1 & 71 & 70 \\ EQ.4 & 2019/12/06 08:45 & 35.1906 & 23.8481 & 4.5 & 8 & 39 \\ EQ.5 & 2019/12/07 14:46 & 35.1773 & 23.8449 & 4.4 & 6 & 39 \\ EQ.6 & 2020/08/17 07:27 & 36.9049 & 23.7250 & 5.2 & 90 & 157 \\ EQ.7 & 2020/10/30 11:51 & 37.9001 & 26.8167 & 6.7 & 12 & 365 \\ EQ.8 & 2021/02/09 18:48 & 35.7491 & 23.7895 & 3.6 & 16 & 33 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The earthquake parameters obtained from NOA and the distance form Chania.