## 2 Materials and Methods
### Earthquake Ground Motion Waveforms  
The KIGAM system has recorded over 300 earthquakes during the study period. For this study, only earthquakes with M\({}_{\rm L}\geq\) 3.0 are considered, which results in a total of 100 events. However, one of these events lacks any recorded waveform data, and thus 99 earthquake events are considered. A map showing the distribution of events is shown in Figure 2. The map shows many earthquakes being offshore as well as several originating in North Korea. Epicentral distances ranged from 2 to 500 km, with each earthquake event showing decent distributions across epicentral distances. However, a majority of the usable earthquake data occurs after 2020. All in all, a total of 4318 sets of three-component acceleration-time histories are available for processing. Of these, 783 are from KIGAM surface stations with a measured \(V_{\rm S30}\). Table 1 in the Appendix A includes more data regarding these events. \begin{table}
\begin{tabular}{c c c c} \hline
**Station** & **Latitude (\({}^{\circ}\))** & **Longitude (\({}^{\circ}\))** & \(V_{\rm S30}\) **(m/s)** \\ \hline HDB \({}^{\rm a}\) & 35.73 & 129.40 & 433 \\ HKU & 36.61 & 127.36 & 554 \\ HSB \({}^{\rm a}\) & 36.56 & 126.64 & 1059 \\ HWSB \({}^{\rm a}\) & 34.98 & 126.90 & 1427 \\ IBA & 35.46 & 126.84 & 646 \\ JRB \({}^{\rm a}\) & 35.21 & 127.64 & 724 \\ JUC & 36.09 & 129.41 & 450 \\ KMC & 35.99 & 127.94 & 760 \\ KINUC \({}^{\rm a}\) & 37.87 & 127.74 & 381 \\ MGB \({}^{\rm a}\) & 36.64 & 128.21 & 817 \\ OJR & 36.23 & 128.82 & 1235 \\ PKNU & 35.28 & 129.26 & 748 \\ POHB \({}^{\rm a}\) & 36.08 & 129.35 & 751 \\ SIG & 35.77 & 129.33 & 404 \\ UNI & 35.57 & 129.19 & 570 \\ WDL & 35.10 & 127.37 & 332 \\ YSB \({}^{\rm a}\) & 35.47 & 129.08 & 507 \\ YSUK & 37.38 & 126.67 & 175 \\ YSUM & 37.28 & 127.90 & 1109 \\ \hline \end{tabular} \({}^{\rm a}\) Instrumentation installed at depth.  
\end{table} 
Table 1: _Cont._  
Figure 2: Epicenters of earthquake events with M\({}_{\rm L}\geq\) 3.0 during the study period. The earthquake ground motion waveforms were recorded by broadband instrumentation with a sampling rate of 100 samples per second. Waveforms were downloaded to include 10 s before the recorded event occurrence time and a body of 200 s for a total of 210 s. In cases where events occurred in rapid succession, with overlapping time frames, adjustments were made to ensure that only the ground motion associated with the specific earthquake event was extracted. These waveforms then underwent a pre-processing regime that entailed removing instrument response and baseline correction. Removing instrument response involved calling the Obspy \"remove_response\" function with KIGAM-supplied parameters [46]. These parameters are unique to each station's recording channel. Once instrument response was removed, a baseline correction was needed to minimize noise that would manifest itself as constant drift displacements [49; 50; 51]. Applying a baseline correction involved the following steps:  
1. Remove the trend from the waveform by subtracting the linear function determined from the first and last sample in the data trace;
2. Apply a 5% cosine taper at each end of the data trace;
3. Apply a 4 th order high-pass filter with 0.1 Hz as a minimum frequency;
4. Apply a 5 th order low-pass filter with 49.9 Hz as a maximum frequency. Note that the Nyquist frequency, the highest frequency that a sampling rate can reliably measure, is 50 Hz (note: low-pass filter frequencies down to 25 Hz showed no differences in results);
5. Apply 5% cosine taper again at each end of the data trace;
6. Compute a velocity waveform by integrating the acceleration waveform;
7. Detrend by subtracting a 4 th order polynomial on the velocity waveform;
8. Apply 5% cosine taper again at each end of the data trace;
9. Compute an acceleration waveform by taking the derivative of the velocity waveform. The resultant acceleration ground motion waveforms were used to derive 5% damped linear elastic response spectra.