# Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island
## 2 Ambient Noise Data
Corrections were enacted to the DC components and linear trends prior to calculating the HVSR for each station, aiming to eliminate traffic and other cultural noises contingent on the data. These data comprised ambient acceleration records. In 2016, Kyushu Island in southwest Japan experienced a sequence of 15 earthquakes in the Kumamoto region. The mainshock, registering a magnitude of 7.3, was detected by seismic stations in the Jeju area, located approximately 367-429 km from the epicenter. The earthquakes had a focal depth of 10-20 km, indicative of mid-crustal seismic activity. These seismic events resulted in over 48 fatalities and caused substantial damage to buildings and structures in both the Jeju area and inland cities. Given that ambient noise from the Kumamoto earthquake series was not captured at the JJU and SGP stations nor at the newly installed UDO station, the HVSR data were analyzed using ambient noise from the 2017 Pohang earthquake series (depicted in Figure 1 and Table 1) with magnitudes ranging from 3.5 to 5.4, subsequent to the Kumamoto earthquake series. This methodology is advantageous because it eliminates the need for extensive and expensive recording activities to acquire environmental noise data at each site, thereby resulting in substantial financial savings. Nevertheless, there are challenges associated with acquiring data at precise locations and times. However, this limitation can be mitigated if the National Seismic Network provides comprehensive coverage of the entire Jeju area. The duration of the noise motion recordings was set to be 600 s or more, depending on the specific data, with a ground motion acceleration sampling interval of 0.01 s. In particular, two types of digital sensors, ES-DH and ES-T, were employed. Each seismic station recorded three-component measurements using a permanent acquisition system. The HVSR was calculated by performing the fast Fourier transform on one vertical and two horizontal components of the noise recordings. This involved utilizing time windows of 20-30 s, tapered with a 5% cosine function prior to the spectrogram computation for each noise component. The HVSR curves were then smoothed using the [PERSON] and Ohmachi et al. [33] window, applying a b value of 40. Spectral ordinates relative to the horizontal \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Event Date** & **Latitude** & **Longitude** & **ML** & **Stations** & **Remarks** \\ \hline
22 & 25 December & 36.11 & 129.36 & 3.5 & CJD, JJU, UDO & Pohang Event \\
23 & 11 February & 36.08 & 129.33 & 4.6 & CJD, JJU, UDO & Pohang Event \\
2018 & 05:03:03 & 10 February & 36.16 & 129.90 & 4.1 & CJD, JJU, UDO & Pohang Event \\
24 & 2019 & 12:53:38 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Station** & **Latitude** & **Longitude** & **Sensor** & **Distance [km]** \\ \hline
1 & GOS & 33.3003 & 126.2100 & ES-T & 431.65 \\
2 & HALB & 33.4019 & 126.2729 & ES-DH & 427.20 \\
3 & JJU & 33.4294 & 126.5463 & ES-T & 394.14 \\
4 & SGP & 33.2587 & 126.4983 & ES-T & 411.63 \\
5 & SSP & 33.3873 & 126.8801 & ES-T & 371.16 \\
6 & CJD & 33.9594 & 126.2934 & ES-T & 368.24 \\
7 & UDO & 33.5228 & 126.954 & ES-T & 362.50 \\
8 & MRD & 33.1166 & 126.2659 & ES-T & 424.94 \\
9 & JJB & 33.4515 & 126.6559 & ES-DH & 392.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Seismic stations and their distances from the main event epicenter. components were geometrically averaged and divided by the vertical spectral ordinate to calculate the HVSR function, employing the GEOPSY software ver. 2.1 [34]. The final HVSR was derived by averaging the results obtained from considering 20-25-time windows with a certain overlap between them.